Nervous System : Organ or Gland Failure

Nervous System : Organ or Gland Failure

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If an organ was to start failing, not total failure, such as the thyroid gland, is it possible that the fault may not lie in the failure of the organ but in the nerve signals coming from the brain?

In many hormonrs, the hypothalamus produces a hormone which acts on the pituitary gland which subsequently acts on an end organ. Taking your example of the thyroid gland let's take a look at it.

The hypothalamus produces TRH which acts on the anterior pituitary causing it to release TSH which in turn stimulates the thyroid gland to make thyroid hormones T3 and T4.

Abnormal levels of thyroid hormone may be caused by problems in any one of the three stages. Problems on the end organ are termed primary, in the pituitary gland are termed secondary and in the hypothalamus tertiary.

For simplicity let's look at abnormally high levels of thyroid hormones called hyperthyroidism. In a patient with a primary or thyroid gland problem we expect high levels of thyroid hormones but low levels of TSH. The pituitary gland is trying its best to tell the thyroid gland, "No more!" but it won't listen. In secondary or pituitary disease, the pituitary gland is the bully pushing the thyroid gland to make lots of thyroid hormone by over production of TSH. Usually there are other hormone disturbances too in this case as the pituitary gland has a range of hormones it secretes. Also the swollen pituitary gland puts pressure on the nerves carrying vision leading to visual loss (specifically in the temporal fields). In this case the hypothalamus will be secreting low levels of TRH (which isn't routinely measured) trying to get the pituitary gland to stop acting out. The rarest situation is when the hypothalamus has the problem where every hormone is increased. However again this will be causes by something or the other detectable usually on imaging.

So the short answer is yes, but there will be other features. Primary or end organ dysfunction is most common and will cause characteristic findings in blood tests and examination (e.g. swollen thyroid gland) but secondary and tertiary disease can be figured out when other features are found in association.

Sense Organs and Endocrine System of Human Body (with diagram)

Sight, hearing and balance, smell and taste are called the special senses. There are separate organs located on various body parts for particular senses.

These organs are called sense organs e.g., for sight, there is a pair of eyes for hearing and balancing, a pair of ears for smell, one muscular structure, the nose and for taste another muscular structure in the mouth, the tongue. The sense of touch is detected by the skin.

The eyes (Fig. 7.1) are delicate organs, located in deep sockets of orbits on the front side of the head. The eye ball can be rotated in the eye orbit with the help of muscles. The eyebrows shade them from bright light, and also prevent the entry of water and dust into the eye from the above. The outer surface of the eye is protected by upper and lower eyelids which can be moved. Tear glands are present at the upper lateral portion of the orbit. A watery fluid with salt is secreted by the tear glands.

The eye ball is hollow and its wall is made up of sclera, choroid and retina. The sclera is tough, non-elastic provides a fibrous coat around the eye ball. The choroid a layer of tissue lining, the sclera, is richly supplied with blood vessels for the eye. The retina is a layer consisting of light sensitive cells.

These light cells are rods and cones. Rods are cylindrical, have rhodopsin pigment and are associated with dim light-vision (scotopic vision). The cones have pyramidal shape and contain iodopsin pigment and are associated with bright light vision (photopic vision) and colour vision. Pigment cell layer of retina is composed of single layer at hexagonal epithelial cells, having dark brown pigment called fuchin.

The optic nerve is attached to the retina. The light rays coming from an object cross the aqueous humour, a thick fluid, and pass through the crystalline lens and the vitreous humour, (a jelly like fluid) to form an image of the object on the retina. This sensation is carried to the brain by the optic nerve. The pupil regulates the amount of light entering in the eye (Fig 7.2 a & b). There is a yellow spot on the retina, which is more sensitive to light. This is the region of brightest vision. Just below the yellow spot there is a blind spot which has no sensory cells. This is the point of no vision.

There are some common eye defects such as Myopia, (Fig. 7.3) in this, near vision is clear for a person while distant vision is blurred. The lens becomes too convex or due to elongation or lengthing of the eye ball is the cause of this defect. Concave lens is used to correct this disorder. Astigmatism, this is due to the irregular curvature of cornea and formation of image is prevented on retina, defect can be removed by using cylindrical lens.

Retina detachment, Retina detaches from the choroid, due to this, retina billow out towards the vitrous body, this leads to distorted vision and blindness in the corresponding field of vision. Hypermetropia (Fig. 7.4) (long-sight) in this distant vision is clear while near vision is blurred. The lens is less convex or eyeball is too short from front to back, in this defect, biconvex lens is used.

Cataract, in this, transparency of the lens is lost it is corrected by replacing the lens. Nightblindness, when a person is having difficulty to see at night or dim-light, due to the failure in the formation of visual purple of the rod cells, this is due to lack of vitamin A. Colour blindness, when an individual cannot distinguish some colours from others, this is due to the missing of some cone cells from retina.

The sensation of sound is received by the ears (Fig. 7.5). The external part of the ear is called pinna which has a cartilaginous structure. The outer ear is a tube which opens on the side of the head and leads inward to the eardrum. The middle ear contains three small bones, called hammer, anvil and stirrup, and an eustachian tube which connects the cavity of the middle ear with the throat. These three bones are called ear ossicles. The inner ear consists of cochlea and semicircular canals.

The cochlea is filled with a fluid, semicircular canals help to provide balance and a sense of position to the body. The external ear, pinna, collects sound waves and conducts them through the auditory canal. These waves strike on the ear drum, producing vibrations of the ear drum. These vibrations are carried to the brain by the auditory nerve.

The nose is a muscular part situated on the face, just above the lips. There are sensory cells of smell in the mucous membrane of the upper part of the nose, nerve fibers from these cells pass into the brain. (Fig. 7.6) These cells are stimulated by chemical substances which arrive in the air, breathed in through the nose, and get dissolved in the mucous covering of the mucous membrane of the nasal cavity. The sense of smell is carried to the brain by the olfactory nerve.

The tongue is a muscular structure located in the mouth (Fig. 7.7). It is sensitive to taste. The taste buds are situated on the upper surface of the tongue. These are connected to the taste centres of the brain through nerves. Taste buds for sweetness and saltiness are present at the tip of the tongue, sourness is detected by the sides, and bitterness is detected at the back.

Skin the largest organ of human body. The skin is a sense organ of touch. The skin is mainly divided into two parts, epidermis and dermis.

Epidermis is the outer layer made up of epithelial tissue (Fig. 7.8). At some places epidermis is thick and hard as on the palms, soles and on the heals. It is devoid of blood supply at all the places. The thinnest skin in human body is conjunctiva. Colouration of the skin is due to the pigment melanin present in epidermis.

The inner layer is dermis which is made up of connective tissue. Dermis contains hair follicle and erector muscles. Dermis contains blood supply and nerve endings. The nerve endings and sense organs here are concerned with sensations of touch and pain.

Dermis part of skin contains sebaceous glands and sweat glands. Sebaceous glands produce oil secretion called sebum which keeps hair and skin soft. Sweat glands help to excrete out sweat from the body. The modified sweat glands are called mammary glands in the females. Skin regulates body temperature and helps in excretion. The skin synthesis vitamin D. The skin protects our body against excess ultraviolet radiations also.

Taking care of sense organs:

We should be very much careful regarding the cleanliness and proper maintenance of our sense organs since we perceive all the changes around us and respond to them.

1. We should splash our eyes with water in the morning and also after a long journey.

2. We should avoid watching the television on short distance, proper distance should be maintained to watch the television to avoid over straining the eyes.

3. Sufficient light should come on books or copies during study time.

4. Keep sharp objects away from the eyes.

5. Wear sunglasses of good quality on bright sunny days.

6. If you feel that Iron dust or Iron filings has fallen in your eyes do not rub your eyes, take the help of your adults to take it out with the help of magnet.

7. We should avoid reading in moving vehicles.

8. If you have any problem in reading on board in your class you should visit to ophthalmologist.

9. You should eat vegetables & fruits rich in vitamin A.

10. Reading materials should be kept at comfortable distance from the eyes to avoid strain.

1. We should avoid sharp objects to clean our ears, it can injure our ear drum.

2. Earwax should clean by soft cotton buds like Johnson buds.

3. We should avoid our ears to expose to loud noise.

4. When you sleep on floor, you should close the openings of your ears by cotton to avoid entry of any (poisonous) worm.

5. Never put oil into the ear.

Skin, Nose and Tongue:

We should take bath regularly so that the pores of the skin do not get closed. For bathing purpose we can mix liquid Dettol or savlon in water or can use any germicidal soap. Unclean skin may lead to skin infections. Time to time nose should be cleaned and avoid inserting any object like pencil or pen in the nostrils as the damage of inner lining of nasal chamber may take place. Tongue should be cleaned regularly by tongue cleaner.

The Endocrine System:

Nervous system controls and keeps co-ordination between the systems of human body. Similarly some functions are controlled by the chemicals released by some glands. These glands are called endocrine glands, and chemicals are called hormones. Hormones regulate the growth and development of the body. Bayliss and Starling two British scientists gave the term ‘hormone’ in 1902. The branch of biology concerned with the study of structure and functions of endocrine glands is called endocrinology. Thomas Addison is father of endocrinology.

Some important endocrine glands are as follows:

This gland is bilobed, and is situated in front of the neck below the larynx. This gland secretes a hormone, known as thyroxin, which regulates the rate of metabolism in the body and also effects the general growth of the body. If this gland is under active (Hypo-secretion) it results in the formation of swelling in the neck, which is called goitre (Fig. 7.9). Over secretion (Hyper-secretion) of this gland may also cause another type of goitre called Exopthalmic goitre and rapid heartbeat with shortness of breath.

This gland hangs (Fig. 7.10) from the base of the mid­brain. It is also called master gland. It has two Fig. 7.9. Goitre, parts an anterior lobe and a posterior lobe. The anterior lobe produces the growth hormone which stimulates the growth of the body.

The posterior lobe produces oxytocin hormone which stimulates contraction of the uterus in a pregnant women. The deficiency of the growth hormone leads to dwarfism in children. Over activity of the growth hormone leads to gigantism in the young and acromegaly in the adults (gorilla-like appearance).

This gland is cap-like structure located on each kidney. This gland produces adrenalin hormone which regulates the blood pressure, and also prepares the body to meet emergencies and to be ready to fight. It also regulates salt and water balance in the body. Hyper secretion of adrenalin leads to prolonged fight symptoms which wears down the individual.

Pancreas (Fig. 7.10) is ductless gland as well as a duct gland. Pancreas contains special cells called the Islets of langerhans. These are of two types- Alphacells and Betacells. Alpha cells secrete glucagon, which stimulates the break-down of glycogen into glucose in the liver. The betacells secrete the insulin hormone which controls absorption of glucose by the body cells. Hypo secretion of insulin leads to diabetes mellitus and hyper secretion of insulin leads to insulin shock.

Sex glands are (Fig. 7.10) the testis in the male and ovary in the female. The testis produce testosterone hormone which controls the development of the male sex organs and secondary sexual characteristics, ovaries produce progesterone hormone, which controls the enlargement of uterus walls and enlargement of mammary glands during pregnancy.

Neurons and Glial Cells

The nervous system of the common laboratory fly, Drosophila melanogaster, contains around 100,000 neurons, the same number as a lobster. This number compares to 75 million in the mouse and 300 million in the octopus. A human brain contains around 86 billion neurons. Despite these very different numbers, the nervous systems of these animals control many of the same behaviors&mdashfrom basic reflexes to more complicated behaviors like finding food and courting mates. The ability of neurons to communicate with each other as well as with other types of cells underlies all of these behaviors.

Most neurons share the same cellular components. But neurons are also highly specialized&mdashdifferent types of neurons have different sizes and shapes that relate to their functional roles.

Like other cells, each neuron has a cell body (or soma) that contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components. Neurons also contain unique structures for receiving and sending the electrical signals that make communication between neurons possible (Figure 16.6.1). Dendrites are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called synapses. Although some neurons do not have any dendrites, most have one or many dendrites.

The bilayer lipid membrane that surrounds a neuron is impermeable to ions. To enter or exit the neuron, ions must pass through ion channels that span the membrane. Some ion channels need to be activated to open and allow ions to pass into or out of the cell. These ion channels are sensitive to the environment and can change their shape accordingly. Ion channels that change their structure in response to voltage changes are called voltage-gated ion channels. The difference in total charge between the inside and outside of the cell is called the membrane potential.

A neuron at rest is negatively charged: the inside of a cell is approximately 70 millivolts more negative than the outside (&ndash70 mV). This voltage is called the resting membrane potential it is caused by differences in the concentrations of ions inside and outside the cell and the selective permeability created by ion channels. Sodium-potassium pumps in the membrane produce the different ion concentrations inside and outside of the cell by bringing in two K + ions and removing three Na + ions. The actions of this pump are costly: one molecule of ATP is used up for each turn. Up to 50 percent of a neuron&rsquos ATP is used in maintaining its membrane resting potential. Potassium ions (K + ), which are higher inside the cell, move fairly freely out of the neuron through potassium channels this loss of positive charge produces a net negative charge inside the cell. Sodium ions (Na + ), which are low inside, have a driving force to enter but move less freely. Their channels are voltage dependent and will open when a slight change in the membrane potential triggers them.

A neuron can receive input from other neurons and, if this input is strong enough, send the signal to downstream neurons. Transmission of a signal between neurons is generally carried by a chemical, called a neurotransmitter, which diffuses from the axon of one neuron to the dendrite of a second neuron. When neurotransmitter molecules bind to receptors located on a neuron&rsquos dendrites, the neurotransmitter opens ion channels in the dendrite&rsquos plasma membrane. This opening allows sodium ions to enter the neuron and results in depolarizationof the membrane&mdasha decrease in the voltage across the neuron membrane. Once a signal is received by the dendrite, it then travels passively to the cell body. A large enough signal from neurotransmitters will reach the axon. If it is strong enough (that is, if the threshold of excitation, a depolarization to around &ndash60mV is reached), then depolarization creates a positive feedback loop: as more Na + ions enter the cell, the axon becomes further depolarized, opening even more sodium channels at further distances from the cell body. This will cause voltage dependent Na + channels further down the axon to open and more positive ions to enter the cell. In the axon, this &ldquosignal&rdquo will become a self-propagating brief reversal of the resting membrane potential called an action potential.

An action potential is an all-or-nothing event it either happens or it does not. The threshold of excitation must be reached for the neuron to &ldquofire&rdquo an action potential. As sodium ions rush into the cell, depolarization actually reverses the charge across the membrane form -70mv to +30mV. This change in the membrane potential causes voltage-gated K + channels to open, and K + begins to leave the cell, repolarizing it. At the same time, Na + channels inactivate so no more Na + enters the cell. K + ions continue to leave the cell and the membrane potential returns to the resting potential. At the resting potential, the K + channels close and Na + channels reset. The depolarization of the membrane proceeds in a wave down the length of the axon. It travels in only one direction because the sodium channels have been inactivated and unavailable until the membrane potential is near the resting potential again at this point they are reset to closed and can be opened again.

An axon is a tube-like structure that propagates the signal from the cell body to specialized endings called axon terminals. These terminals in turn then synapse with other neurons, muscle, or target organs. When the action potential reaches the axon terminal, this causes the release of neurotransmitter onto the dendrite of another neuron. Neurotransmitters released at axon terminals allow signals to be communicated to these other cells, and the process begins again. Neurons usually have one or two axons, but some neurons do not contain any axons.

Some axons are covered with a special structure called a myelin sheath, which acts as an insulator to keep the electrical signal from dissipating as it travels down the axon. This insulation is important, as the axon from a human motor neuron can be as long as a meter (3.2 ft)&mdashfrom the base of the spine to the toes. The myelin sheath is produced by glial cells. Along the axon there are periodic gaps in the myelin sheath. These gaps are called nodes of Ranvier and are sites where the signal is &ldquorecharged&rdquo as it travels along the axon.

It is important to note that a single neuron does not act alone&mdashneuronal communication depends on the connections that neurons make with one another (as well as with other cells, like muscle cells). Dendrites from a single neuron may receive synaptic contact from many other neurons. For example, dendrites from a Purkinje cell in the cerebellum are thought to receive contact from as many as 200,000 other neurons.

Figure 16.6.1: Neurons contain organelles common to other cells, such as a nucleus and mitochondria. They also have more specialized structures, including dendrites and axons.


At one time, scientists believed that people were born with all the neurons they would ever have. Research performed during the last few decades indicates that neurogenesis, the birth of new neurons, continues into adulthood. Neurogenesis was first discovered in songbirds that produce new neurons while learning songs. For mammals, new neurons also play an important role in learning: about 1,000 new neurons develop in the hippocampus (a brain structure involved in learning and memory) each day. While most of the new neurons will die, researchers found that an increase in the number of surviving new neurons in the hippocampus correlated with how well rats learned a new task. Interestingly, both exercise and some antidepressant medications also promote neurogenesis in the hippocampus. Stress has the opposite effect. While neurogenesis is quite limited compared to regeneration in other tissues, research in this area may lead to new treatments for disorders such as Alzheimer&rsquos, stroke, and epilepsy.

How do scientists identify new neurons? A researcher can inject a compound called bromodeoxyuridine (BrdU) into the brain of an animal. While all cells will be exposed to BrdU, BrdU will only be incorporated into the DNA of newly generated cells that are in S phase. A technique called immunohistochemistry can be used to attach a fluorescent label to the incorporated BrdU, and a researcher can use fluorescent microscopy to visualize the presence of BrdU, and thus new neurons, in brain tissue (Figure 16.6.2).

Figure 16.6.2: This image shows new neurons in a rat hippocampus. New neurons tagged with BrdU glow red in this micrograph. (credit: modification of work by Dr. Maryam Faiz, University of Barcelona)

Visit this link interactive lab to see more information about neurogenesis, including an interactive laboratory simulation and a video that explains how BrdU labels new cells.

Study the Endocrine System, its Organs and its Functions

Endocrine glands are glands whose secretions (called hormones) are collected by the blood and reach tissues through circulation. The hypophysis (pituitary gland) and the adrenal glands are examples of endocrine glands. Exocrine glands are a glands whose secretions are released externally through ducts (into the skin, the intestinal lumen, the mouth, etc.). The sebaceous glands and the salivary glands are examples of exocrine glands.

Endocrine Glands and Hormones

More Bite-Sized Q&As Below

2. What are the components of the endocrine system?

The endocrine system is composed of the endocrine glands and the hormones they secrete.

3. What is the histological nature of glands? How are they formed?

Glands are epithelial tissue. They are made of epithelium that during the embryonic development invaginated into other tissues during embryonic development..

In exocrine glands, the invagination contains preserved secretion ducts. In endocrine glands, the invagination is complete and there are no secretion ducts.

4. Why is the endocrine system considered one of the integrative systems of the body? What other physiological system also has this function?

The endocrine system is considered to be of an integrative nature, since the hormones produced by endocrine glands are substances that act at a distance and many of them act in different organs of the body. therefore, endocrine glands receive information from certain regions of the body and can produce effects in other regions, providing functional integration for the body.

In addition to the endocrine system, the other physiological system that also has integrative function is the nervous system. The nervous system integrates the body through a network of nerves connected to central and peripheral neurons. The endocrine system integrates the body through hormones that travel through circulation.

5. What are hormones?

Hormones are substances secreted by endocrine glands and collected by circulation. They produce effects on specific organs and tissues.

Hormones are the effectors of the endocrine system.

6. What are the target organs of hormones?

Target organs, target tissues and target cells are the specific organs, tissues and cells on which each hormone acts and produces its effects. Hormones selectively act on their targets due to the specific receptor proteins present in these targets.

7. How does the circulatory system participate in the function of the endocrine system?

The circulatory system is fundamental for the functioning of the endocrine system. Blood collects hormones produced by endocrine glands and these hormones reach their targets through circulation. Without the circulatory system, the "action at distance" feature of the endocrine system would not be possible.

8. Are hormones only proteins?

Some hormones are proteins, such as insulin, glucagon and ADH, others are derived from proteins (modified amino acids), such as adrenaline and noradrenaline.  Others are steroids, such as corticosteroids and estrogen.

9. What are the main endocrine glands of the human body?

The main endocrine glands of the human body are the pineal gland (or pineal body), the hypophysis (or pituitary gland), the thyroid, the parathyroids, the endocrine part of the pancreas, the adrenal glands and the gonads (the testicles or ovaries).

Other organs such as the kidneys, the heart and the placenta also play a role in the endocrine system.

The Pineal Gland

10. What is the pineal gland?

The pineal gland, also known as the pineal body or epiphysis, is located in the center of the head. It secretes the hormone melatonin, a hormone produced at night and related to the regulation of circadian rhythm (or the circadian cycle, the wakefulness-sleep cycle). Melatonin may also regulate many body functions related to the night-day cycle.

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The Hypophysis

11. In which bone cavity is the pituitary gland located?

The pituitary gland, or hypophysis, is located in the sella turcica of the sphenoid bone (one of the bones at the base of the skull). Therefore, this gland is located within the head.

12. What are the main divisions of the hypophysis? What are their functions?

The hypophysis is divided into two portions: the adenohypophysis, or anterior hypophysis, and the neurohypophysis, or posterior hypophysis.

The adenohypophysis produces two hormones that act directly, growth hormone (GH) and prolactin. It also produces four tropic hormones, that is, hormones that regulate other endocrine glands: adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), luteinizing hormone (LH) and follicle-stimulating hormone (FSH).

The neurohypophysis stores and releases two hormones produced in the hypothalamus, oxytocin and antidiuretic hormone (ADH, or vasopressin).

13. What is the relationship between the hypothalamus and the hypophysis?

The hypothalamus is a part of the brain located just above the hypophysis. The hypothalamus receives peripheral and central neural impulses that trigger the response of its neurosecretory cells. The axons of these cells descend into the adenohypophysis to regulate hypophyseal secretions by means of negative feedback. When the levels of adenohypophyseal hormones in the plasma are too high, the hypothalamus detects this information and commands the stoppage of the production of the hormone. When the blood level of an adenohypophyseal hormone is low, the hypothalamus stimulates the secretion of the hormone.

Hypothalamic cells produce the hormones released by the neurohypophysis. These hormones are transported by their axons to the hypophysis and are then released into the circulation.

The Adenohypophysis

14. What hormones are secreted by the adenohypophysis? What are their respective functions?

The adenohypophysis secretes GH (growth hormone), prolactin, ACTH (adrenocorticotropic hormone), TSH (thyroid-stimulating hormone), FSH (follicle-stimulating hormone) and LH (luteinizing hormone).

GH, also known as somatotropic hormone (STH), acts on bones, cartilage and muscles to promote the growth of these tissues. Prolactin is the hormone that stimulates the production and secretion of milk by the mammary glands in women. ACTH is the hormone that stimulates the cortical portion of the adrenal gland to produce and secrete cortical hormones (glucocorticoids). TSH is the hormone that stimulates the activity of the thyroid gland, increasing the production and secretion of its hormones T3 and T4. FSH is a gonadotropic hormone, meaning that it stimulates the gonads and, in women, it acts on the ovaries to induce the growth of follicles and, in men, it stimulates spermatogenesis. LH is also a gonadotropic hormone it acts upon the ovaries of women to stimulate ovulation and the formation of the corpus luteum (which secretes estrogen) in men, it acts on the testicles to stimulate the production of testosterone.

15. What is the relationship between the thyroid and the hypophysis?

The hypophysis secretes TSH, thyroid-stimulating hormone. This hormone stimulates the secretion of thyroid hormones (triiodothyronine and thyroxine, or T3 and T4).

When the plasma concentration of thyroid hormones is high, this information is detected by the hypothalamus and the hypophysis, and the latter reduces the TSH secretion. When thyroid hormone levels are low, TSH secretion increases. This is therefore an example of negative feedback.

Injuries to the hypophysis that cause TSH hyposecretion (for example, in the case of tissue destruction) or hypersecretion (for example, excessive cell proliferation or cancer) can change the functioning of the thyroid gland completely.

16. What are some diseases caused by abnormal GH secretion by the hypophysis?

During childhood, GH secretion deficiencies may lead to delayed growth and in severe cases to nanism (dwarfism). Excessive production of GH in children may cause exaggerated bone growth and gigantism. In adults, excess GH (for example, in hypophyseal cancer or in people that wrongly mistakenly ingest GH as a nutritional supplement) may lead to acromegaly, which is excessive and disproportional growth of bone extremities, such as the skull, the maxillaries, the hands and the feet.

17. What are the target tissues and target organs of each adenohypophyseal hormone?

GH: bones, cartilage and muscles. Prolactin: the mammary glands. ACTH: the cortical portion of the adrenal glands. TSH: the thyroid gland. FSH and LH: the ovaries and testicles.


18. What hormones are secreted by the neurohypophysis? What are their respective functions?

The neurohypophysis secretes oxytocin and antidiuretic hormone (ADH).

Oxytocin is secreted in women during delivery to increase the strength and frequency of uterine contractions and therefore to help the baby’s birth. During the lactation period, the infant’s sucking action on the mother’s nipples stimulates the production of oxytocin, which then increases the secretion of milk by the mammary glands.

Vasopressin, or ADH, participates in the regulation of water in the body and therefore in the control of blood pressure, since it allows the reabsorption of free water through the renal tubules. As water goes back into circulation, the volume of blood increases.

19. What is the difference between diabetes mellitus and diabetes insipidus? What are the characteristic signs of diabetes insipidus?

Diabetes mellitus is the disease caused by deficient insulin secretion by the pancreas or by the impaired capture of this hormone by cells. Diabetes insipidus is the disease caused by deficient ADH secretion by the pituitary gland (hypophysis) or also by an impaired sensitivity to this hormone in the kidneys.

In diabetes insipidus, blood lacks ADH and, as a result, the reabsorption of water by the tubules in the kidneys is reduced, and a large volume of urine is produced. The patient urinates in large volumes and many times a day, a symptom which is also accompanied by polydipsia (increased thirst and an exaggerated ingestion of water) and sometimes by dehydration.

20. Why does the volume of urine increase when alcoholic beverages are ingested?

Alcohol inhibits ADH (antidiuretic hormone) secretion by the hypophysis. Low ADH reduces the tubular reabsorption of water in the kidneys and therefore urinary volume increases.

21. What are the target organs and target tissues of the neurohypophysis?

The target organs of oxytocin are the uterus and the mammary glands. The target organs of ADH are the kidneys.

The Thyroid Gland

22. Where in the body is the thyroid gland located?

The thyroid is located in the anterior cervical region (frontal neck), in front of the trachea and just below the larynx. It is a਋ilobed mass below the Adam’sਊpple.

23. What hormones are secreted by the thyroid gland? What are their functions?

The thyroid secretes the hormones thyroxine (T4), triiodothyronine (T3) and calcitonin.

T3 and T4 are iodinated substances derived from the amino acid tyrosine. They act to increase the cellular metabolic rate of the body (cellular respiration, metabolism of proteins and lipids, etc.). Calcitonin inhibits the release of calcium cations by bones, thus controlling the level of calcium in the blood.

24. Why is the ingestion of dietary iodine so important for thyroid function?

Obtaining iodine from your diet is important for the thyroid because this chemical element is necessary for the synthesis of the thyroid hormones T3 and T4. Iodine supply often comes from the diet.

25. What is goiter? What is endemic goiter? How is this problem socially solved?

Goiter is the abnormal enlargement of the thyroid gland. Goiter appears as a tumor in the anterior neck. It may or may not be visible but is often palpable. Goiter can occur as a result of hypothyroidism or hyperthyroidism.

Endemic goiter is goiter caused by a deficiency in iodine consumption (a deficiency of iodine in the diet). The endemic character of the disease is explained because dietary iodine is often a social or cultural condition affecting many people in certain geographical regions. The hypothyroidism caused by deficient iodine ingestion is more frequent in regions far from the coast (since sea food is rich in iodine).

Nowadays, the problem is often solved by the obligatory addition of iodine to table salt. As table salt is a widely used condiment, the supply of iodine in the diet is almost always assured by this method.

26. What happens to the level of TSH (thyroid-stimulating hormone) in the blood during hypothyroidism? Why is the thyroid enlarged in the endemic goiter?

When there is a low level of T3 and T4 secretion by the thyroid, TSH secretion by the hypophysis is very stimulated and the level of TSH in the blood level. The increase in the availability of TSH promotes the enlargement of the thyroid gland.

Thyroid enlargement is the reaction of a tissue that tries to compensate for the functional deficiency by making the gland increase in size.

27. What are some signs and symptoms found in patients with hyperthyroidism?

The hormones made by the thyroid gland stimulate the basal metabolism of the body. In hyperthyroidism, there is an abnormally high production and secretion of T3 and T4 and, as a result, the basal metabolic rate is increased. The signs of this condition may be tachycardia (an abnormally high heart rate), weight loss, excessive heat sensation, excessive sweating, anxiety, etc. One of the typical signs of hyperthyroidism is exophthalmos (protrusion of the eyeballs). Generally the patient also presents goiter.

28. What are some signs and symptoms found in patients with hypothyroidism?

In hypothyroidism, the production and secretion of T3 and T4 are impaired. Since these thyroid hormones stimulate the basal metabolism of the body (cellular respiration, fatty acid and protein metabolism, etc.), a patient with hypothyroidism may present bradycardia (a low heart rate), a low respiratory rate, excessive tiredness, depression, cold intolerance and weight gain. Hypothyroidism is normally accompanied by goiter (the enlargement of the thyroid in the neck).

29. What is the physiological cause of the syndrome known as cretinism?

Cretinism is caused by a chronic deficiency of thyroid hormones (T3 and T4) during childhood. Chronic hypothyroidism during childhood may cause retardation and a low stature due to the low basal metabolic rate during a period of life when growth and the development of mental faculty occur.


30. What are the parathyroids? Where are they located and what hormones are secreted by these glands?

The parathyroids are four small glands, two of which are embedded in each posterior face of one lobe of the thyroid. The parathyroids secrete parathormone, a hormone that, along with calcitonin and vitamin D, regulates calcium levels in the blood.

31. What is the relationship between the secretion of parathormone and the level of calcium in the blood?

Parathormone increases the level of calcium in the blood, since it stimulates the reabsorption (remodeling) of the bone tissue. When osteoclasts remodel bones, calcium is released in the circulation.

Parathormone is also involved in increasing calcium absorption in the intestines via vitamin D activation. It also plays a role in the kidneys, promoting the tubular reabsorption of calcium.

The Pancreas

32. What is a mixed gland? Why is the pancreas considered a mixed gland?

A mixed gland is a gland that produces endocrine and exocrine secretions.

The pancreas is an example of a mixed gland because it secretes hormones into circulation, such as insulin and glucagon, while also releasing an exocrine secretion, pancreatic juice.

33. What pancreatic tissues are involved in exocrine and endocrine secretions? What are their respective hormones and enzymes?

Exocrine secretions of the pancreas are produced in the pancreatic acini, aggregates of secretory cells that surround small exocrine ducts. The exocrine pancreas secretes the digestive enzymes of pancreatic juice: amylase, lipase, trypsin, chymotrypsin, carboxypeptidase, ribonuclease, deoxyribonuclease, elastase and gelatinase.

Endocrine secretions of the pancreas are produced and secreted by small groups of cells dispersed throughout the organ called islets of Langerhans. The pancreatic islets make insulin, glucagon and somatostatin.

Hormonal Glucose Regulation

34. What is the importance of blood glucose levels for human health?

Blood glucose levels (glycemia) must be maintained normal. If they are abnormally low, there will not be enough glucose to supply the energy metabolism of cells. If they are abnormally and chronically high, it causes severe harm to peripheral nerves, the skin, the retina, the kidneys and other important organs, and may predispose the person to cardiovascular diseases (acute myocardial infarction, strokes, thrombosis, etc). If they are acutely in excess, medical emergencies such as diabetic ketoacidosis and a hyperglycemic hyperosmolar state may occur.

35. How are insulin and glucagon involved in blood glucose control?

Glucagon increases glycemia and insulin reduces it. They are antagonistic pancreatic hormones. Glucagon stimulates glycogenolysis, thus forming glucose from the breakdown of glycogen. Insulin is the hormone responsible for the entrance of glucose from blood into cells.

When glycemia is low, for example, during fasting, glucagon is secreted and insulin is inhibited. When glycemia is high, like after meals, glucagon is inhibited and insulin secretion is increased.

36. What are the target organs of insulin and glucagon?

Glucagon mainly acts on the liver. In general, insulin acts on all cells. Both also act on the adipose tissue, stimulating (glucagon) and inhibiting (insulin) the use of fatty acids by the energy metabolism (an alternate path of energy metabolism is activated when there is a shortage of glucose).

37. What are the effects of somatostatin on pancreatic hormonal secretions?

Somatostatin inhibits both insulin and glucagon secretions.

Diabetes Mellitus Explained

38. What is diabetes mellitus?

Diabetes mellitus is the disease caused by the deficient production or action of insulin and, as a result, characterized by a low glucose uptake by cells and a high blood glucose level.

39. What are the three main signs of diabetes?

The three main signs of diabetes mellitus are known as the diabetic triad: polyuria, polydipsia and polyphagia.

Polyuria is the excessive elimination of urine in diabetes, it is caused by reduced water reabsorption in the renal tubules due to the increased osmolarity of glomerular filtrate (caused by excessive glucose). Polydipsia is the exaggerated ingestion of water the thirst is due to excessive water loss in the urine. Polyphagia is the exaggerated ingestion of food caused by a deficiency in energy generation by glucose-deficient cells.

40. Why do diabetic patients often undergo dietary sugar restriction? What are the main complications of diabetes mellitus?

Diabetic patients are often advised to ingest less carbohydrates since these substances are broken down into glucose and this molecule is absorbed in the intestines. The goal of dietary sugar restriction is to control glycemia and to maintain it at normal levels.

The main complications of diabetes are tissue injuries that occur in various organs caused by chronic high blood osmolarity: in the peripheral nerves (diabetic neuropathy), resulting in sensitivity loss, increased wounds (the person does not feel that the tissue is being wounded and the wound expands) and muscle fatigue in the kidneys (diabetic nephropathy), causing glomerular lesions that may lead to renal failure in the retina (diabetic retinopathy), leading to vision impairment and blindness and in the skin, as a consequence of the neuropathy. Diabetes mellitus is also one of the major risk factors for cardiovascular diseases such as embolism, myocardial infarction and stroke.

41. What is the difference between type I diabetes mellitus and type II diabetes mellitus?

Type I diabetes, also known as juvenile diabetes, or insulin-dependent diabetes (this name is not adequate, since type II diabetes may become insulin-dependent), is the impaired production of insulin by the pancreas, and is believed to be caused by the destruction of the cells of the islets of Langerhans by autoantibodies (autoimmunity).

Type II diabetes occurs adults and it is often diagnosed in older people. In type II diabetes, the pancreas secretes normal or low levels of insulin,਋ut the main cause of the high glycemia is the peripheral resistance of the cells to the action of the hormone.

42. In ancient Greece, the father of Medicine, Hippocrates, described a method of diagnosing diabetes mellitus by tasting the patient's urine. What is the physiological explanation for this archaic method?

Under normal conditions, the glucose filtered by renal glomeruli is almost entirely reabsorbed in the nephron tubules and is not excreted in urine. With elevated blood glucose levels, the renal tubules cannot reabsorb all the filtered glucose and a certain amount of the substance appears in the urine. This amount is enough to provide the sweet taste that helped Hippocrates diagnose diabetes and differentiate it from other diseases򠫌ompanied by polyuria. Nowadays,  this method is not used due to the danger of contaminating the tester with disease agents possibly present in the patient's urine.

43. What are the main treatments for diabetes mellitus?

The general goal of diabetes treatment is to maintain normal glycemic levels.

Type I diabetes is treated with the parenteral administration of insulin. Insulin must be administered intravenously or intramuscularly because, as a protein, it will be digested if ingested orally. In type II diabetes, treatment is done with oral drugs that regulate glucose metabolism or, in more severe cases, with parenteral insulin administration. The moderation of carbohydrate ingestion is an important aid in diabetes treatment.

Diabetes treatment with the use of hypoglycemic agents, such as insulin or oral medicines, must be carefully and medically supervised, since if wrongly used, these drugs may abruptly decrease the blood glucose levels, causing hypoglycemia and even death.

Many other forms of diabetes treatment are being researched worldwide.

44. How can bacteria produce human insulin on an industrial scale? What are other forms of insulin are made available by the pharmaceutical industry?

Bacteria do not naturally synthesize insulin. However, it is possible to implant human genetic material containing the insulin gene into bacterial DNA. The mutant bacteria then multiply and produce human insulin. The insulin is isolated and purified for subsequent sale. This biotechnology is known as recombinant DNA technology.

In addition to human insulin, the pharmaceutical industry also produces insulin to be used by humans made from the pancreas of pigs and cows.

The Adrenal Glands

45. Where are the adrenal glands located? How many are there and into which parts are they divided?

Each adrenal gland is located on the top of each kidney (forming a hat-like structure on the top of the kidneys) therefore, there are two glands. The adrenal parenchymal structure is divided into two parts: the most outlying part is the cortical portion, or the adrenal cortex, and the central part is the medullary portion, or the adrenal medulla.

The Endocrine System Review - Image Diversity: the adrenal glands

46. What hormones are secreted by the adrenal medulla? What are their respective functions?

The medullary portion of the adrenal glands secretes hormones of the catecholamine group: adrenaline (also known as epinephrine) and noradrenaline (also known as norepinephrine). Besides their hormonal function, adrenaline and noradrenaline also act as neurotransmitters. The neurons that use them as neurotransmitters are called adrenergic neurons.

Adrenaline increases the breakdown of glycogen into glucose (glycogenolysis), thus increasing glycemia and the basal metabolic rate of the body. Adrenaline and noradrenaline are released during situations of danger (fight or flight response) and they intensify the strength and rate of the heartbeat and selectively modulate blood irrigation in some tissues via selective vasodilation and vasoconstriction. Through vasodilation, they increase the supply of blood to the brain, the muscles and the heart and, through vasoconstriction, they reduce the supply of blood to the kidneys, the skin and the gastrointestinal tract.

Substances that promote vasodilation or vasoconstriction, such as adrenaline and noradrenaline, are called vasoactive substances.

47. What hormones are secreted by the adrenal cortex? What are their respective functions?

The cortical portion of the adrenal glands secretes hormones of the corticoid (or corticosteroid) group, which are derived from cholesterol: glucocorticoids, mineralocorticoids and cortical sex hormones.

The glucocorticoids secreted are cortisol and cortisone. Glucocorticoids stimulate the formation of glucose from the degradation of proteins of muscle tissue (gluconeogenesis) and, as a result, help to increase glycemia. These hormones play an important immunosuppressive role, meaning that they reduce the action of the immune system and for this reason are used as medicine to treat inflammatory and autoimmune diseases and the rejection of transplanted organs.

The mineralocorticoids aldosterone and deoxycorticosterone regulate the concentration of sodium and potassium in the blood and, as a result, control the water level in the extracellular space. Aldosterone increases sodium reabsorption and therefore water reabsorption in the renal tubules, and also stimulates the renal excretion of potassium and hydrogen.

The adrenal cortical sex hormones are androgens, male sex hormones present in both men and women. In men, their main site of production is the testicle and they promote the appearance of secondary male sex characteristics, such as body hair and a beard, a deep voice, the male pattern of fat distribution and the maturation of the genitalia. If abnormally high in women, they cause an inhibited maturation of the female genitalia and disturbances in the menstrual cycle.

48. Why are glucocorticoids used in transplant patients?

Patients with transplanted organs are prone to host versus graft rejection, since their own immune system tends to attack the grafted organ because it recognizes the grafted tissue as foreign material. In the prevention and treatment of this common problem, patients are given glucocorticoids or other immunosuppressants. Glucocorticoids have an immunosuppressant�t and, as a result, reduce the aggression of the immune system against the graft.

However, immune action is also very important for the individual. The immune system defends the body against invasion and infection by pathogenic agents (viruses, bacteria, toxins) in addition to being necessary for the elimination of modified cells that may proliferate and cause cancer. Patients receiving immunosuppressants such as glucocorticoids therefore have an increased risk of infectious and neoplastic diseases.

Reproductive Hormones

49. What hormones are produced by the testicles and the ovaries?

The testicles produce androgenic hormones, the main hormone of which is testosterone. The ovaries produce estrogen and progesterone.

50. What is the endocrine function of the placenta?

The placenta is not a permanent gland of the endocrine system but it nonetheless has an endocrine function. The placenta produces estrogen and progesterone. It also secretes human chorionic gonadotropin (HCG, which has a function similar to that of hypophyseal LH), human placental lactogen, similar to prolactin and a mammary gland stimulant, and a series of hormonal peptides similar to the hormones of the hypothalamus-hypophysis axis.

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The study of plant organs is covered in plant morphology. Organs of plants can be divided into vegetative and reproductive. Vegetative plant organs include roots, stems, and leaves. The reproductive organs are variable. In flowering plants, they are represented by the flower, seed and fruit. [6] In conifers, the organ that bears the reproductive structures is called a cone. In other divisions (phyla) of plants, the reproductive organs are called strobili, in Lycopodiophyta, or simply gametophores in mosses. Common organ system designations in plants include the differentiation of shoot and root. All parts of the plant above ground (in non-epiphytes), including the functionally distinct leaf and flower organs, may be classified together as the shoot organ system. [7]

The vegetative organs are essential for maintaining the life of a plant. While there can be 11 organ systems in animals, there are far fewer in plants, where some perform the vital functions, such as photosynthesis, while the reproductive organs are essential in reproduction. However, if there is asexual vegetative reproduction, the vegetative organs are those that create the new generation of plants (see clonal colony).

Non-placozoan animals such as humans have a variety of organ systems. These specific systems are also widely studied in human anatomy. The functions of these organ systems often share significant overlap. For instance, the nervous and endocrine system both operate via a shared organ, the hypothalamus. For this reason, the two systems are combined and studied as the neuroendocrine system. The same is true for the musculoskeletal system because of the relationship between the muscular and skeletal systems.

    : pumping and channeling blood to and from the body and lungs with heart, blood and blood vessels. : digestion and processing food with salivary glands, esophagus, stomach, liver, gallbladder, pancreas, intestines, colon, rectum and anus. : communication within the body using hormones made by endocrine glands such as the hypothalamus, pituitary gland, pineal body or pineal gland, thyroid, parathyroids and adrenals, i.e., adrenal glands. : kidneys, ureters, bladder and urethra involved in fluid balance, electrolyte balance and excretion of urine. : structures involved in the transfer of lymph between tissues and the blood stream, the lymph and the nodes and vessels that transport it including the Immune system: defending against disease-causing agents with leukocytes, tonsils, adenoids, thymus and spleen. : skin, hair and nails of mammals. Also scales of fish, reptiles, and birds, and feathers of birds. : movement with muscles. : collecting, transferring and processing information with brain, spinal cord and nerves. : the sex organs, such as ovaries, fallopian tubes, uterus, vulva, vagina, testes, vas deferens, seminal vesicles, prostate and penis. : the organs used for breathing, the pharynx, larynx, trachea, bronchi, lungs and diaphragm. : structural support and protection with bones, cartilage, ligaments and tendons.

Origin and evolution Edit

The organ level of organisation in animals can be first detected in flatworms and the more derived phyla. The less-advanced taxa (like Placozoa, Sponges and Radiata) do not show consolidation of their tissues into organs.

More complex animals are composed of different organs, which have evolved over time. For example, the liver evolved in the stem vertebrates more than 500 million years ago, while the gut and brain are even more ancient, arising in the ancestor of vertebrates, insects, and worms more than 600 million years ago.

Given the ancient origin of most vertebrate organs, researchers have looked for model systems, where organs have evolved more recently, and ideally have evolved multiple times independently. An outstanding model for this kind of research is the placenta, which has evolved more than 100 times independently in vertebrates, has evolved relatively recently in some lineages, and exists in intermediate forms in extant taxa. [8] Studies on the evolution of the placenta have identified a variety of genetic and physiological processes that contribute to the origin and evolution of organs, these include the re-purposing of existing animal tissues, the acquisition of new functional properties by these tissues, and novel interactions of distinct tissue types. [8]

Many societies have a system for organ donation, in which a living or deceased donor's organ is transplanted into a person with a failing organ. The transplantation of larger solid organs often requires immunosuppression to prevent organ rejection or graft-versus-host disease.

There is considerable interest throughout the world in creating laboratory-grown or artificial organs. [ citation needed ]

The English word "organ" dates back to the twelfth century and refers to any musical instrument. By the late 14th century, the musical term's meaning had narrowed to refer specifically to the keyboard-based instrument. At the same time, a second meaning arose, in reference to a "body part adapted to a certain function". [9]

Plant organs are made from tissue composed of different types of tissue. The three tissue types are ground, vascular, and dermal. [10] When three or more organs are present, it is called an organ system. [11]

The adjective visceral, also splanchnic, is used for anything pertaining to the internal organs. Historically, viscera of animals were examined by Roman pagan priests like the haruspices or the augurs in order to divine the future by their shape, dimensions or other factors. [12] This practice remains an important ritual in some remote, tribal societies.

The term "visceral" is contrasted with the term "parietal", meaning "of or relating to the wall of a body part, organ or cavity" [13] The two terms are often used in describing a membrane or piece of connective tissue, referring to the opposing sides. [14]

Antiquity Edit

Aristotle used the word frequently in his philosophy, both to describe the organs of plants or animals (e.g. the roots of a tree, the heart or liver of an animal), and to describe more abstract "parts" of an interconnected whole (e.g. his logical works, taken as a whole, are referred to as the "organon"). [15]

Some alchemists (e.g. Paracelsus) adopted the Hermetic Qabalah assignment between the seven vital organs and the seven classical planets as follows: [16]

Planet Organ
Sun Heart
Moon Brain
Mercury Lungs
Venus Kidneys
Mars Gall bladder
Jupiter Liver
Saturn Spleen

Modern times Edit

The variations in natural language definitions of what constitutes an organ, their degree of precision, and the variations in how they map to ontologies and taxonomies in information science (for example, to count how many organs exist in a typical human body) are topics explored by writer Carl Engelking of Discover magazine in 2017 as he analyzed the science journalism coverage of the evolving scientific understanding of the mesentery. [17] He explored a challenge now faced by anatomists: as human understanding of ontology generally (that is, how things are defined, and how the relationship of one thing to another is defined) meets applied ontology and ontology engineering, unification of varying views is in higher demand. [17] However, such unification always faces epistemologic frontiers, as humans can only declare computer ontologies with certainty and finality to the extent that their own cognitive taxonomy (that is, science's understanding of the universe) is certain and final. For example, the fact that the tissues of the mesentery are continuous was something that was simply not known for sure until it was demonstrated with microscopy. [18] Because humans cannot predict all future scientific discoveries, they cannot build a unified ontology that is totally certain and will never again change. However, one of the points made by an anatomist interviewed by Engelking is that, finality aside, much more could be done even now to represent existing human knowledge more clearly for computing purposes.

Organ Procedures Edit

Beginning in the 20th century [19] transplants began to occur as scientists knew more about the anatomy of organs. These came later in time as procedures were often dangerous and difficult. [20] Both the source and method of obtaining the organ to transplant are major ethical issues to consider, and because organs as resources for transplant are always more limited than demand for them, various notions of justice, including distributive justice, are developed in the ethical analysis. This situation continues as long as transplantation relies upon organ donors rather than technological innovation, testing, and industrial manufacturing. [ citation needed ]

Directionally the pineal gland is situated between the cerebral hemispheres and attached to the third ventricle. It is located in the center of the brain.

Melatonin is produced within the pineal gland and synthesized from the neurotransmitter serotonin. It is secreted into the cerbrospinal fluid of the third ventricle and is directed from there into the blood. Upon entering the bloodstream, melatonin can be circulated throughout the body. Melatonin is also produced by other body cells and organs including retinal cells, white blood cells, gonads, and skin.

Melatonin production is vital to the regulation of sleep-wake cycles (circadian rhythm) and its production is determined by light and dark detection. The retina sends signals about light and dark detection to an area of the brain called the hypothalamus. These signals are eventually relayed to the pineal gland. The more light detected, the less melatonin produced and released into the blood. Melatonin levels are at their highest during the night and this promotes changes in the body that help us to sleep. Low levels of melatonin during daylight hours help us to stay awake. Melatonin has been used in the treatment of sleep-related disorders including jet lag and shift-work sleep disorder. In both of these cases, a person's circadian rhythm is disrupted either due to travel across multiple time zones or due to working night shifts or rotating shifts. Melatonin has also been used in the treatment of insomnia and depressive disorder.

Melatonin influences the development of reproductive system structures as well. It inhibits the release of certain reproductive hormones from the pituitary gland that affect male and female reproductive organs. These pituitary hormones, known as gonadotropins, stimulate gonads to release sex hormones. Melatonin, therefore, regulates sexual development. In animals, melatonin plays a role in regulating mating seasons.

Week 4 to Week 8

Week 4 - Stage 13

Week 4 - Stage 16

Week 8 - Stage 23

The above MRI scan movie shows the structure of the central nervous system at the end of the embryonic period. Note the relative size and position of the CNS parts, the flexures, the size of the ventricular spaces and chord plexus within this space. There are additional Stage 23 movies available in the links below.

Week: 1 2 3 4 5 6 7 8
Carnegie stage: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

The Peripheral Nervous System

The peripheral system (PNS) is composed of nerves that extend outside of the central nervous system. The nerves and nerve networks that make up the PNS are actually bundles of axons from neuron cells. The nerve bundles can be relatively small or large enough to be easily seen by the human eye.

The PNS is further divided into two different systems: the somatic nervous system and the autonomic nervous system.

Somatic Nervous System

The somatic nervous system transmits sensory communications. It is responsible for voluntary movement and action. It is composed of sensory (afferent) neurons and motor (efferent) neurons.

Sensory neurons carry information from the nerves to the brain and spinal cord while motor neurons transmit information from the central nervous system to the muscle fibers.

Autonomic Nervous System

The autonomic nervous system is responsible for controlling involuntary functions such as heartbeat, respiration, digestion, and blood pressure. The system is also involved in human emotional responses such as sweating and crying.

The autonomic nervous system is subdivided into the sympathetic nervous system and parasympathetic nervous system.

  • Sympathetic nervous system: The sympathetic nervous system controls the body’s response to an emergency. When the system is aroused, your heart and breathing rates increase, digestion slows or stops, the pupils dilate and you begin to sweat. Also known as the fight-or-flight response, the system is preparing your body to either fight the danger or flee.
  • Parasympathetic nervous system: The parasympathetic nervous system counters the sympathetic system. After a crisis or danger has passed, the system helps to calm the body by slowing heart and breathing rates, resuming digestion, contracting the pupils, and stopping sweating.

Concept in Action

Visit the following website to learn more about split-brain patients and to play a game where you can model split-brain experiments yourself.

Each hemisphere contains regions called lobes that are involved in different functions. Each hemisphere of the mammalian cerebral cortex can be broken down into four functionally and spatially defined lobes: frontal, parietal, temporal, and occipital (Figure 11.33).

Figure 11.33 The human cerebral cortex includes the frontal, parietal, temporal, and occipital lobes.

The frontal lobe is located at the front of the brain, over the eyes. This lobe contains the olfactory bulb, which processes smells. The frontal lobe also contains the motor cortex, which is important for planning and implementing movement. Areas within the motor cortex map to different muscle groups. Neurons in the frontal lobe also control cognitive functions like maintaining attention, speech, and decision-making. Studies of humans who have damaged their frontal lobes show that parts of this area are involved in personality, socialization, and assessing risk. The parietal lobe is located at the top of the brain. Neurons in the parietal lobe are involved in speech and also reading. Two of the parietal lobe’s main functions are processing somatosensation—touch sensations like pressure, pain, heat, cold—and processing proprioception—the sense of how parts of the body are oriented in space. The parietal lobe contains a somatosensory map of the body similar to the motor cortex. The occipital lobe is located at the back of the brain. It is primarily involved in vision—seeing, recognizing, and identifying the visual world. The temporal lobe is located at the base of the brain and is primarily involved in processing and interpreting sounds. It also contains the hippocampus (named from the Greek for “seahorse,” which it resembles in shape) a structure that processes memory formation. The role of the hippocampus in memory was partially determined by studying one famous epileptic patient, HM, who had both sides of his hippocampus removed in an attempt to cure his epilepsy. His seizures went away, but he could no longer form new memories (although he could remember some facts from before his surgery and could learn new motor tasks).

Interconnected brain areas called the basal ganglia play important roles in movement control and posture. The basal ganglia also regulate motivation.

The thalamus acts as a gateway to and from the cortex. It receives sensory and motor inputs from the body and also receives feedback from the cortex. This feedback mechanism can modulate conscious awareness of sensory and motor inputs depending on the attention and arousal state of the animal. The thalamus helps regulate consciousness, arousal, and sleep states.

Below the thalamus is the hypothalamus. The hypothalamus controls the endocrine system by sending signals to the pituitary gland. Among other functions, the hypothalamus is the body’s thermostat—it makes sure the body temperature is kept at appropriate levels. Neurons within the hypothalamus also regulate circadian rhythms, sometimes called sleep cycles.

The limbic system is a connected set of structures that regulates emotion, as well as behaviors related to fear and motivation. It plays a role in memory formation and includes parts of the thalamus and hypothalamus as well as the hippocampus. One important structure within the limbic system is a temporal lobe structure called the amygdala. The two amygdala (one on each side) are important both for the sensation of fear and for recognizing fearful faces.

The cerebellum (cerebellum = “little brain”) sits at the base of the brain on top of the brainstem. The cerebellum controls balance and aids in coordinating movement and learning new motor tasks. The cerebellum of birds is large compared to other vertebrates because of the coordination required by flight.

The brainstem connects the rest of the brain with the spinal cord and regulates some of the most important and basic functions of the nervous system including breathing, swallowing, digestion, sleeping, walking, and sensory and motor information integration.

Sensory-Somatic Nervous System

The sensory-somatic nervous system transmits sensory information from the body to the brain and motor movements from the brain to the body.

Learning Objectives

Explain the role of the cranial and spinal nerves in the sensory-somatic nervous system

Key Takeaways

Key Points

  • The sensory and motor neurons of the sensory-somatic system have only one synapse between the organ and a neuron of the CNS these synapses utilize acetylcholine to transmit signals across this synapse.
  • The twelve cranial nerves either enter or exit from the skull some transmit only sensory information, some transmit only motor information, and some transmit both.
  • There are 31 spinal nerves that convey both sensory and motor signals between the spinal cord and the rest of the body.

Key Terms

  • cranial nerve: any of the twelve paired nerves that originate from the brainstem instead of the spinal cord
  • spinal nerve: one of 31 pairs of nerves that carry motor, sensory, and autonomic signals between the spinal cord and the body
  • acetylcholine: a neurotransmitter in humans and other animals, which is an ester of acetic acid and choline

Sensory-Somatic Nervous System

The sensory-somatic nervous system is composed of cranial and spinal nerves and contains both sensory and motor neurons. Sensory neurons transmit sensory information from the skin, skeletal muscle, and sensory organs to the central nervous system (CNS). Motor neurons transmit messages about desired movement from the CNS to the muscles, causing them to contract. Without its sensory-somatic nervous system, an animal would be unable to process any information about its environment (what it sees, feels, hears, etc. ) and could not control motor movements. Unlike the autonomic nervous system, which has two synapses between the CNS and the target organ, sensory and motor neurons have only one synapse: one ending of the neuron is at the organ and the other directly contacts a CNS neuron. Acetylcholine is the main neurotransmitter released at these synapses.

Cranial Nerves

Humans have 12 cranial nerves, nerves that emerge from or enter the skull (cranium), as opposed to the spinal nerves, which emerge from the vertebral column. Each cranial nerve has a name. Some cranial nerves transmit only sensory information. For example, the olfactory nerve transmits information about smells from the nose to the brainstem. Other cranial nerves transmit almost solely motor information. The oculomotor nerve controls the opening and closing of the eyelid and some eye movements. Other cranial nerves contain a mix of sensory and motor fibers. For example, the glossopharyngeal nerve has a role in both taste (sensory) and swallowing (motor).

Cranial nerves: The human brain contains 12 cranial nerves that receive sensory input and control motor output for the head and neck.

Spinal Nerves

Spinal nerves transmit sensory and motor information between the spinal cord and the rest of the body. Each of the 31 spinal nerves (in humans) contains both sensory and motor axons. The sensory neuron cell bodies are grouped in structures called dorsal root ganglia. Each sensory neuron has one projection with a sensory receptor ending in skin, muscle, or sensory organs, and another that synapses with a neuron in the dorsal spinal cord. Motor neurons have cell bodies in the ventral gray matter of the spinal cord that project to muscle through the ventral root. These neurons are usually stimulated by interneurons within the spinal cord, but are sometimes directly stimulated by sensory neurons.

Spinal nerves: Spinal nerves contain both sensory and motor axons. The cell bodies of sensory neurons are located in dorsal root ganglia. The cell bodies of motor neurons are found in the ventral portion of the gray matter of the spinal cord.