Chemical Coordination

Hormones are chemical messengers, which regulate and control a number of activities of the body. Hormones are produced by structures called endocrine glands. These are ductless glands, which pour their secretion directly into blood. The study of endocrine glands and hormonal control is called endocrinology.

Hormones

A hormone is a regulatory chemical. It has the following properties:

(a) It travels in the blood.  
(b) It has its effect at a site different from the site where it is made, called the target, hence the term messenger.  
(c) It fits precisely into receptor molecules in the target, like a key in a lock. It is, therefore, specific for a particular target.  
(d) It is a small soluble organic molecule.  
(e) It is effective in low concentration.

Types of Hormones

The two main types of hormone are:

(1) Steroid hormones which are lipids made from cholesterol, e.g. estrogen, testosterone, cortisone.

(2) Non-steroid hormones are all synthesized from amino acids and there are three main classes of these substances:

(a) The amine hormones are modified versions of single amino acids e.g., thyroxin, epinephrine and norepinephrine.

(b) The peptide hormones are short chains of amino acid as few as three, antidiuretic hormones or vasopressin, oxytocin, thyrotropin.

(c) The protein hormones are made of polypeptide having as many as 200 amino acids e.g. insulin and glucagon.

Endocrine Glands

Some of the endocrine glands are exclusively endocrine in function and some have both endocrine and exocrine function. The figure shows the locations of most of the human endocrine glands.

The following endocrine glands are present in man

(1) Thyroid
(2) Parathyroid
(3) Pancreas
(4) Adrenal Gland
(5) Gonads
(6) Glandular Epithelium
(7) Pineal
(8) Thymus
(9) Pituitary Gland




Thyroid Gland

The thyroid gland is a large gland located in the neck where it attaches to the trachea below the larynx. The thyroid gland has two lobes one on each side of the junction between the larynx and trachea. A bridge of thyroid tissue called isthmus connects the two lobes. The thyroid gland is made of spherical thyroid cells filled with stored thyroid hormone.
Thyroid Gland

The hormone produced by thyroid called thyroxin (thyroxin) which has two forms. Thyroxin is usually secreted as T4 (tetraiodothyronine) which contains four iodine atoms, but eventually this    form is converted to T3 (triiodothyronine), the active form of the hormone. The two hormones act essentially the same way.      Another hormone secreted by thyroid, is called calcitonin.

The thyroid is active continuously but produces higher levels of secretions during period of rapid growth and sexual maturation and in stress situations such as cold and hunger. Thyroxin (T4 and T3) increases the metabolic rate. It does not have one target organ; instead, it stimulates all organs of body to metabolize at a faster rate. More glucose is broken down with the release of heat and more ATF is generated. They also act in conjunctions with somatotropin (growth hormone secreted by anterior pituitary) in bringing about growth and act directly on brain cells causing them to differentiate (in amphibians, they affect the process of metamorphosis. If secretion of thyroid is deficient, the tadpole of frog does not metamorphose to develop into frog, but instead grows into a large size tadpole.

Hypothyroidism

It is a general term for the condition in which too little thyroxin is produced. It can cause the following:

Cretinism

If there is a great deficiency of thyroxin in early childhood, a disease called cretinism occurs. The child develops into a midget called cretin. Cretinism results in mental retardation and irregular development of bones and muscles. The skin is dry, eyelids are puffy, hair is brittle and the shoulders sag. There is also failure to develop sexuality. Ordinarily cretinism cannot be cured but early diagnosis and treatment with thyroxin may arrest the disease before the nervous system is damaged.

Myxedema

The occurrence of hypothyroidism in the adult produces the condition known as Myxedema (myxoedema). It is characterized by lethargy, weight gain, loss of hair, slower pulse rate, lower body temperature and thickness and puffiness of hand and skin.

Goiter

The disease goiter is the enlargement of the thyroid gland itself. Goiter cause an increase in the size of the entire throat and neck. The thyroid has to work harder to produce a sufficient amount of thyroxin. Thus, it enlarges. It is common in mountainous areas due to lack of iodine in soil and water.

Hyperthyroidism

It is also known as Graves' diseases. It is a condition in which too much thyroxin is produced.

Symptoms: These include nervousness, irritability, increased heart rate and blood pressure, weakness, weight loss, rapid use of oxygen at rest and bulging eyes (exophthalmic goiter) causes in part by increased fluid behind eyes. This can lead to cardiac failure if prolonged.

Cause: The cause of Graves' disease is the production of an abnormal body protein, which continuously stimulates the thyroid to excessive secretions.

Treatment: Drug therapy that inhibits thyroxin production and the administration of radioactive iodine has successfully replaced surgery as treatment for hyperthyroidism.

Calcitonin

High calcium ions concentration in the blood causes stimulation of the synthesis and release of calcitonin, low level of calcium ions suppress its manufacture, it helps to regulate the blood calcium level in the blood and opposes the action of parathyroid hormone. Calcitonin lowers blood calcium (Ca++) by increasing the buildup of bone. Excess or deficiency leads to a disturbance of calcium metabolism with its associated effects on nerve, skeleton, muscles, blood etc.

Parathyroid

Located on the thyroid gland are two pairs of very small structures called the parathyroid glands? They secrete the hormone parathormone. Low levels of blood calcium ions stimulate the parathyroid directly to increase parathormone production whereas high levels of calcium ions suppress its release. It controls balance of calcium ions and phosphate in body.
Parathyroid Gland 

Deficiency: Too much Ca++ ions move from the blood to the bones, the muscles and nerves do not receive enough calcium ions. As a result, they become very sensitive to stimuli. Spasm (contractions and relaxations of muscles) and even death may occur, in case of severe deficiency.
Overproduction: It causes Ca++ ions to leave the bones to the blood and causes weakening of the skeletal system similar to rickets. Nerve and muscle do not respond well to stimuli. Over activity also causes massive formation of kidney stone. Both conditions may be fatal.

Removal: Small as the parathyroid glands are, their removal invariably causes death.

Pancreas

Pancreas
Pancreas is composed of two types of tissue. Exocrine tissue produces and secretes digestive juices that go by way of ducts to the small intestine. Endocrine tissue called pancreatic Islets (island) of Langerhans named after Paul Langerhans who discovered them. These cells are under the control of pituitary hormones STH (Somatotropin hormone) and ACTH (adrenocorticotrophic hormone) and responds directly to the level of blood glucose. Islets of Langerhans secrete two hormones Insulin and glucagon. Both are proteins. Insulin is secreted by the alpha cells, which are large in number, and glucagon is secreted by cells, which are less in number.

Insulin

Insulin is a small protein composed of 51 amino acids. Insulin is secreted when the level of blood sugar rises, such as right after a meal. The most important affect of insulin is to facilitate glucose transport across cell membrane. Insulin depresses blood glucose level in various ways
(a) Increases the rate of conversion of glucose to glycogen, called glycogenosis, which takes place mainly in the liver and muscle.  
(b) Increase in the rate of uptake of glucose by cells.
(c) Increase in the use of glucose rather than other substance.
(d) Increase in the conversion of glucose into lipids and proteins.
(e) Decrease production of glucose.

Diabetes Mellitus

A deficiency in insulin production leads to the metabolic disease known as diabetes mellitus. The symptoms include:

(1) Sugar in Urine.
(2) Frequent Copious Urination.
(3) Abnormal Thirst.
(4) Rapid Weight Loss
(5) General Weakness
(6) Drowsiness and Fatigue.

Hypoglycemia

If excess of insulin is produced, the utilization of sugar is too great and its level falls in the blood that upsets nerve and muscle functioning.

Glucagon

It is a protein composed of 29 amino acids. Its role is to increase blood glucose level. Its main target is the liver. Glucagon stimulates the conversion of glycogen to glucose (glucogenesis). It also stimulates the breakdown of proteins and fats to glucose and conversion of lactic acid to glucose (gluconeogenesis). Glucose abnormalities seem rare as endocrine disorders. Tumors on beta cells will cause excess glucagon secretions and consequent high blood glucose level.

Adrenal Glands

Each of the two adrenal glands rests on a kidney. Each adrenal gland is composed of an inner portion called the medulla and an outer portion the cortex.

Adrenal Glands
The hypothalamus exerts control over the activity of both portions of the adrenal glands. The hypothalamus, by means of ACTH (adrenocorticotrophic hormone) releasing hormone controls the anterior pituitary's secretion of ACTH, which in turn, stimulates the adrenal cortex.

Adrenal Cortex

The two major types of hormones produced by the adrenal cortex arc glucocorticoids, which help to regulate the blood glucose level, and mineralocorticoids, which help to regulate the level of minerals in the blood.

Adrenal Medulla    

Epinephrine (adrenaline) and norepinephrine are produced by the adrenal medulla. Epinephrine and norepinephrine are involved in the body's immediate response to stress. They bring about all the bodily changes that occur when an individual reacts to an emergency. Both the hormones contribute to the short-term stress response. Both hormones stimulate liver cells to release glucose, thus making more fuel for cellular energy. They also prepare the body for action by increasing the blood pressure, the breathing rate, and the metabolic rate.

In addition, epinephrine and norepinephrine change blood flow patterns making some organs more active and other less so e.g. epinephrine dilates blood vessels in the brain and skeletal muscles, thus increasing alertness and the muscles ability to react to stress. At the same time, epinephrine and norepinephrine constrict blood vessels elsewhere, thereby reducing activities that are not immediately involved in the stress response, such as digestion. The short-term stress response is rapid and usually subsides shortly after we first encounter stress.

The Following Hormones Are Being Discussed:

Cortisol

It is the glucocorticoid that promotes the hydrolysis of muscle protein to amino acid and the liver converts these amino acids to glucose. Cortisol also favors metabolism of fatty acids rather than carbohydrates, It also counteracts the Inflammatory response that leads to the pain and the swelling of joints In arthritis etc.

Corticosterone

It is both glucocorticoid and a mineralocorticoid. It Increases blood glucose levels and regulate mineral Ion balance.

Aldosterone

It is a mineralocorticoid. It promotes renal absorption of sodium (Na+) and renal excretion of potassium (K+).

When there is less production of cortical hormone e.g. destruction of the adrenal cortex, such as occurs in Addison's disease, will lead to general metabolic disturbance, in particular, weakness of muscle action and loss of salts. Stress situation, such as cold, which would normally be overcome, lead to collapse and death. When too much cortical hormone is produced such as in Cushing's Disease, symptoms are an excessive protein breakdown resulting muscular and bone weakness. The high blood sugar disturbs the metabolism as in diabetes.

Sex Hormones

Adrenal cortex secretes a small amount of male sex hormones (androgen) and a small amount of female sex hormones in both male and female.

Gut

Many parts of the gut function as endocrine tissue. The important hormones produced are:

(a) Gastrin: It is produced by the mucosa of the pyloric region of the stomach. It stimulates the production of gastric juice rich in hydrochloric acid. It is produced under the influence of protein food in the stomach after it is partially digested.

(b) Secretin: It is produced by the mucosa of duodenum, when acidic food touches its lining. It stimulates production of pancreatic juice by the pancreas and also affects the rate of bile production in the liver, and inhibits secretion of gastric juice.

(c) Cholecytokinin (CCK): It is produced by mucosa of duodenum. The stimulus for secretion is fatty food and protein in the duodenum. CCK stimulates increased secretion of pancreatic juice rich in enzymes.

Gonads

The gonads are the ovaries in the females and testes in the males.

Ovary: Ovaries are located in the abdominal cavity. The ovaries secrete female sex hormones (a) estrogen (oestrogen) and (b) progesterone.
Gonads 

Estrogen

This hormone is secreted by ripening follicles whose development has been initiated by FSH from the anterior pituitary gland. Estrogen performs the following functions

(1) Estrogen secreted at the time of puberty is responsible for secondary sex characteristics in females. The secondary sex characteristics include a more rounded appearance in female than males because of a greater accumulation of fats beneath the skin. Also the pelvic girdles extend more in females than the males, resulting in females having a larger pelvic cavity and wider hips. Estrogen stimulates enlargement of various accessory organs, including (internal vagina, uterus, uterine tubes, and ovaries) and external reproductive structures.

(2) Estrogen is necessary for egg maturation.

(3) At a point during estrous or menstrual cycle exert a positive feedback, which results in a sharp rise in LH output by the pituitary.

(4) Estrogen aids in healing and repair of uterine wall after menstruation.

(5) Under influence of estrogen, some of the cells of uterine wall become glandular and start secreting proteinaceous secretions which are taken up by the embryo during early stages of development.

Deficiency: It leads in the young to failure to mature sexually and sterile in the adult.

Progesterone

This hormone is produced by the ruptured follicles in response to LH (luteinizing hormone) from the pituitary.
The functions performed by progesterone are:
(1) It inhibits further FSH secretion from the pituitary, thus preventing any more follicles from ripening.
(2) It causes further thickening and vascularization of the uterus wall and other areas of the female body preparing it for maintaining state of pregnancy.
(3) Progesterone suppresses ovulation, so it is a major constituent of birth control pill.
(4) Progesterone affects the mammary glands and help to regulate the secretion of gonadotropin from the anterior pituitary gland.

Testes

The testes are located in the scrotum. The testes consist of many coiled seminiferous tubules, where sperms develop. The regions between the tubules consist of interstitial cells, which produce gonad hormones called testosterone and 17-beta hydroxytestosterone.
Testes

The male sex hormone testosterone has many functions

(1) It is essential for the normal development and functioning of the sex organs in males.

(2) It is necessary for the maturation of sperms.

(3) In the fetus, it initiates the development of the sex organs.

(4) Testosterone brings about and maintains the secondary sex characteristics in males that develop at the time of puberty. Testosterone causes growth of beard, axillary hair, and pubic hair. It prompts the larynx and the vocal cords to enlarge, causing the voice to change.

(5) Testosterone also stimulates oil and sweat glands in the skin.

Thymus

The thymus is a lobular gland that lies inside the thorax on the ventral side. This organ reaches its largest size and is most active during childhood. Thymus produces various hormones thymosin. Certain lymphocytes that originate in the bone marrow and then pass through the thymus are transformed into T lymphocytes.
Thymus 
Pineal Gland

It is located deep between the cerebral hemisphere, and is attached to hypothalamus, produces hormone called melatonin, primarily at night Melatonin is involved in a daily cycle called a circadian rhythm.
Pineal Gland

Hypothalamus and Pituitary Gland

The hypothalamus is a portion of the brain that regulates the internal environment. For example, it helps to control heart rate, body temperature, and water balance, as well as the glandular secretion of the pituitary gland. The pituitary, a small gland about 1 cm in diameter, is connected to the hypothalamus by a stalk like structure. The pituitary has two portions the posterior pituitary and anterior pituitary.
Hypothalamus and Pituitary Gland

Posterior Lobe

The axon endings in the posterior pituitary store antidiuretic hormone (ADH, sometimes called vasopressin) and oxytocin.

Vasopressin

ADH promotes the re-absorption of water from the collecting ducts within the kidney. Increase levels cause increase water re-absorption in distal part of kidney. Inability to produce ADH causes Diabetes Insipidus (watery urine) in which a person produces copious (plentiful, overflowing) amounts of urine with a resultant loss of ions from the blood and great thirst.

Oxytocin is the other hormone that is made in the hypothalamus and stored in the posterior pituitary. Its release is stimulated by distention of cervix, decrease in progesterone level in blood, and neural stimuli during parturition (childbirth) and suckling. Oxytocin causes the smooth muscles of the uterus to contract and is used to artificially induce labor. It also stimulates the release of milk from the mammary glands when the baby is nursing.

Anterior Pituitary - The Master Gland

It is called master gland because it produces tropic hormones, which stimulate other endocrine glands to release their hormone.

The anterior pituitary produces at least six different types of hormones, each by a distinct cell type.
Anterior Pituitary - The Master Gland

Somatotropin (STH)       

It is also known as growth hormone (GH). Somatotropin releasing factor (SRF) is secreted from hypothalamus throughout the life. It promotes cell division, protein synthesis, and bone growth. Evidence suggests that the effects on cartilage and bone may actually be due to hormone called somatomedins, which are released by the liver in response to GH, when growth has mostly ceased after adolescence, the hormone continues to promote protein synthesis throughout the body.

During childhood if too little GH is produced, the individual becomes a pituitary dwarf, and if too much GH is secreted, a person can become a giant.

If there is overproduction of growth hormone in adult, a condition called acromegaly results. Some cartilage and bone becomes thickened. The feet, hands, and face particularly the chin, nose, and eyebrow ridges become very large.

Thyroid Stimulating Hormone (TSH)      

The hormone produced by thyroid is thyroxin. The levels of thyroxin in the blood control the release of thyrotrophin releasing factor from hypothalamus. If level of thyroxin is low, there is more production of TSH and vice versa. TSH is secreted throughout life, and reaches high levels during the period of rapid growth and development, TSH acts directly on the cells of thyroid gland. It increases the number of cells and secretory activity of the thyroid gland.

Adrenocorticotrophic Hormone (ACTH)

It stimulates the release of corticotrophin releasing factor from hypothalamus. This is controlled by steroid level in the blood and by direct nervous stimulation of the hypothalamus because of stress, e.g. cold, heat, pain, fright, infections.

Gonadotropin Hormones

These are follicle stimulating hormone (FSH), luteinizing hormone (LH, also called interstitial cell stimulating hormone, ICSH in the male), protecting (sometimes inappropriately called luteotrophic hormone, (LTH). FSH and LH/ICH share a common hypothalamus releasing factor. The FSH acts upon an ovary to stimulate maturation of a follicle and during its development, the follicular cells produce increasing amounts of estrogen and some progesterone. In males, FSH stimulates development of the germinal epithelium of the testes and sperm production.

Prolactin (PRL) is produced in quantity only after childbirth. It causes mammary glands in the breasts to develop and produce milk. It also plays a role in carbohydrate and fat metabolism. Prolactin is continuously produced from the pituitary and is inhibited by protecting inhibiting factor (PIH) from the hypothalamus.

LH works with FSH to stimulate estrogen secretion and rupture of mature follicles to release ovum. It also causes the luteinization (turning yellow) of the rupture follicles. Under the influence of LH, the follicular cells enlarge to form a temporary glandular structure called Corpus Luteum.

ICHS in the male stimulates the interstitial cells of the testes (singular, testis) to secrete testosterone.

Median Lobe

It secretes Melanocyte Stimulating Hormone (MSH). The concentration of this hormone in human is very low. MSH causes inhibition of secretion controlled by hypothalamus. External light governs its secretion. More secretion in pregnancy stimulates melanocytes in skin to produce brown pigment, melanin, which darkens the skin. Excess MSH is secreted in Addison's disease (It is the peculiar bronzing of the skin. It is due to adrenal cortex hypo-secretion), and its one of the symptoms is darkening of the skin.

Feedback Mechanism

The general principle is that the product of a series of reactions controls its own production by turning off the pathway when it reaches a certain level. This is comparable with a thermostat where the product, heat, switches off its own production when a certain temperature is reached. If there is too little of the product, its production is switched on again.

Commonly, the control of hormonal secretions involves a negative feed back system. An endocrine gland is sensitive either to the concentration of a substance it regulates or the concentration of a product from a process it controls. Whenever this concentration reaches a certain level, the endocrine gland is inhibited (a negative effect) and its secretory activity decreases. Then, as the concentration of the gland's hormone drops, the concentration of the regulated substance drops also, and inhibition of the gland ceases. When the gland is no longer inhibited, it begins to secrete its hormone again. Because of such negative feedback system the concentration of some hormones remain relatively stable, although they may fluctuate slightly within normal range. Example of negative feed back are secretion of thyroxin, cortisol, sex hormones etc.

Example: control of thyroxin release. 

1. Neurosecretory cells of the hypothalamus are stimulated by stress or low body temperature and release hormones (TSH).

2. TRH (Thyroid releasing hormone) stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH)

3. a) The level of TSH exerts feedback control over the hypothalamus to release thyroxin (thyroxin),

b) The level of thyroxin exerts feedback control over the anterior pituitary.

c) The level of thyroxin exerts feed back control over the hypothalamus.


Thyroxin causes increase in the metabolic activity of the body, generating ATP and heat. The higher body temperature and higher thyroxin levels in the blood inhibits production of thyroxin. In this way, thyroxin controls its own secretion.

Effects of Drugs on Coordination

Drugs that affect the nervous system have two general effects:

They affect the limbic system.
They either promote or decrease the action of a particular neurotransmitter.

NICOTINE

It is an alkaloid derived from tobacco. In the CNS, nicotine causes neurons to release dopamine, a neurotransmitter. In the PNS, nicotine stimulates the same post-synaptic receptors as acetylcholine and leads to increased activity. It also increases the heartbeat rate and blood pressure and digestive tract mobility, Nicotine may even occasionally induce vomiting also causes water retention in the kidney.

Nervous Disorders

The common nervous disorders are:

(1) Parkinson’s disease (2) Epilepsy (3) Alzheimer's disease

Parkinson's disease

Symptoms: The three symptoms of this nervous disorder ere slowness of movement, tremors, and rigidity cause; the disease is caused by the death of cells in the brain that produce dopamine. This disease usually appears at the age of 50 to 60, but the patient may live for many years as the disease progress slowly. The disease may result by head trauma. Treatment: Effective drugs are L-dopa and carbidopa etc and glial derived growth neutrotrophic factor (GDNF) which has shown to boost uptake of dopamine and is being considered for treatment of Parkinson’s disease.

Epilepsy

Symptoms: This disease is one of the convulsive disorders of nerves. These are characterized by abrupt transient symptoms of motor, sensory, psychic or autonomic nature* frequently associated with changes in consciousness.

Cause: These changes are believed to be secondary to sudden transient (short duration) alternations in brain function, associated with excessive rapid electric discharges in gray matter Epilepsy usually appears before the age of 30. In some patients, emotional disturbances play a significant "trigger" role. Electroencephalography is most important test in the study of epilepsy.

Treatment: Anticonvulsant drugs are used. Parson suffering from epilepsy should avoid alcohol, as it aggravates epilepsy.

 Al-Zheimer's disease

This disease was first discovered by Alois Alzheimer in 1907.

Symptoms: This disorder is characterized by gradual loss of reasoning that begins with memory lapses and ends with an inability to perform any type of daily activity.

Cause: The brain tissue under microscope has protein 'plaques' accumulating outside brain cells and tangled deposits of protein appearing inside the cells. Two key parts of the brain affected are the cortex of the cerebral hemisphere (the conscious part of the brain) and hippocampus (involved with memory). Both plaques and tangles are caused by accumulation of abnormal proteins. Alzheimer's disease is difficult to diagnose because its symptoms are similar to those of other diseases that cause dementia (memory loss). There is also evidence that high levels of aluminum may contribute to the onset of the disease. Nevertheless, it appears that the neurotransmitter acetylcholine may be in short supply in the brain of a patient of this disease.


Treatment: Drugs that enhance acetylcholine production are now available for the patient suffering from this disease.

Human Nervous System

The human nervous system consists of central nervous system (CNS) and peripheral nervous system (PNS). The CNS has central location. They lie in the midline of the body. The PNS, which is further divided into the somatic division and the autonomic division, includes all the cranial nerves and spinal nerves. The two systems work together and are connected to one another.

Central Nervous System or CNS

It consists of brain and spinal cord, and both are hollow. The brain and spinal cord are covered with three protective membranes called meninges (singular: meninx).

Brain

Human Brain 
It is enclosed within the cranium. The three meninges are, duramatter (next to the bone of cranium), arachnoid matter (middle membrane), and pia matter (next to the nervous tissue). Between the arachnoid arid pia matter there is a fluid, the cerebro-spinai fluid, which helps to cushion the brain from shock. The brain is divided into three parts, forebrain, midbrain, and hindbrain.

Forebrain

It consists of cerebrum, thalamus and hypothalamus. Cerebrum is the largest part of the human brain. The surface of the cerebrum is called cerebral cortex containing 10 billion neurons. The cerebrum consists of two layers. The first layer is composed of gray matter. Beneath the cerebral cortex is the thick white matter, of the cerebrum, which constitutes the second layer. Cerebral cortex has many folds or convolutions forming ridges or gyri (singular, gyrus) which are separated by grooves. A shallow groove is called a sulcus (plural, sulci) and a deep groove is called a fissure. The cortex covers and overlaps most of the other brain area. The cerebrum is differentiated into gray and white matter. Cerebrum is divided into two cerebral hemispheres. Two hemispheres are separated by longitudinal fissure. At the base of the longitudinal fissure, there is a tough band of axon called corpus callosum. It connects two hemispheres. Each hemisphere contains four surface lobes, which corresponds roughly with the bones of the cranium, frontal, parietal, temporal and occipital lobe. Their cavity is called lateral ventricle. 

Functions of Cerebrum

The regions of the cerebral cortex that perform specific functions have been identified. The cortex can be divided into three areas:

MOTOR AREAS                       SENSORY AREAS                     ASSOCIATION AREAS

1- Motor Areas

The primary motor areas of the cerebral cortex lie in the frontal lobe. Most of the nerve fibers of this area cross over from one side of the brain to the other within the brain stem. As a result, the motor area of the right cerebral hemisphere generally controls skeletal muscles on the left side of the body and vice versa. In addition to the primary motor areas, certain other regions of the frontal lobe are involved with motor functions. It coordinates the complex muscular actions of the mouth, tongue and larynx, which make the speech possible. Another area controls the voluntary movement of the eyes and eyelids. Another region just in front of the primary motor area controls the muscular movements of the hands and fingers that make skills such as writing.

2- Sensory Areas

These areas occur in several lobes of the cerebrum function in interpreting impulses that arrived from various sensory receptors (see figure). The centre of the right cerebral hemisphere interprets impulses originating from the left side of the brain and vice versa.


3- Association Areas

These areas occupy the anterior portions of the frontal lobes and are widespread in the lateral portions of the parietal, temporal and occipital lobes. Association areas function in the analysis and interpretation of sensory experiences and involved with memory, reasoning, judgment and feelings.

According to research work of Nobel Prize winner Roger Sperry, our two cerebral hemispheres look alike superficially. Their primary motor and sensory areas also function in the same way, however, the left and right association function so differently that we could almost say we have two brains in one, the left hemisphere house our language centers. It also has association areas for logic and mathematical abilities. In contrast, the right hemisphere lack language, logic, and math centers, but has association areas that underline our Imagination, spatial perceptions, artistic and emotional abilities.

Thalamus

Thalamus
It is below the cerebrum and is covered by the cerebral hemisphere. The thalamus consists of gray mater located in the sides of the Ventricles, which Is the cavity of Thalamus, It receives all sensory Impulses (except sense of smell) and channels them to limbic system and to appropriate regions of the cortex for Interpretation.     

Hypothalamus

Hypothalamus 
On the ventral side of the thalamus is the hypothalamus, which forms the floor of the third ventricle. It maintains homeostasis or the constancy of the internal environment, and contains centers for regulating hunger, sleep, thirst, body temperature, water balance, and blood pressure, menstrual cycle and the sleep wake cycle. The hypothalamus also controls the pituitary gland and thereby serves as a link between the nervous and endocrine system.

Limbic System

The limbic system involves portions of both the subconscious and conscious brain. It lies just beneath the cerebral cortex and contains neural pathways that connect portions of the frontal lobes, the temporal lobes, the thalamus and the hypothalamus. It Is a two partial rings formed by portions of cerebral cortex around thalamus and hypothalamus. One cerebral structure is amygdala, or amygdaloid (a -mig' da-loyd; meaning almond shaped) nucleus, which is a cluster of neurons producing sensation of pleasure, punishment or sexual arousal when stimulated. It also involves in the feeling of fear and rage. The other cerebral structure is hippocampus (hip-o-kampus; meaning shaped like a seahorse). It interacts closely with another part of the cerebral cortex, (the prefrontal cortex), which is involved in complex learning, reasoning and personality. Limbic system is essential to both short term and long-term memory.

Midbrain

It is reduced in humans and acts as a relay station for tracts passing between the cerebrum and the spinal cord or cerebellum, and it also has reflex centers for visual, auditory and tactile (touch) responses. Midbrain contains reticular formation, which is a relay centre connecting hindbrain with the forebrain. Reticular formation is very important in screening the input information, before they reach higher brain centre.

Hindbrain

The hindbrain consists of:

(a) Cerebellum (b) Medulla Oblongata (c) Pons

Cerebellum

It is a bulb like structure extending upwards from the pons and is situated under the cerebrum. It is the second largest part of the brain. It consists of two lateral hemispheres. It also shows folds. Its interior is made of neuron cell body.

Cerebellum controls equilibrium i.e. body position and co-ordination of the actions of individual muscles to produce complex activities such as walking, running, ridding bicycles, doing delicate work with hand. The cerebellum is also involved in learning and memory storage for behavior, (it is best developed in birds and helps in complex activity of flight.)

Medulla Oblongata

It forms a swelling below the pons at the base of the brain along the spinal cord. Medulla consists mostly of ascending and descending tracts of white matter, with some gray matter in the interior. Medulla controls the automatic functions of the body, such as heartbeat, blood pressure, contraction and dilation of blood vessels, respiration, sweating, swallowing.

Pons

Pons is a small lobe like structure lying just above the medulla. It is composed of largely white matter. It acts as a bridge between the cerebellum, medulla and cerebrum. Pons appears to influence transitions between sleep and wakefulness and the rate of pattern of breathing.

Brain Stem

It maintains life support system, consists of the medulla oblongata, pons and midbrain.

Ventricles

Within the cerebral hemispheres and brain stem is a series of interconnected cavities called ventricles. These spaces are continuous with the central canal of the spinal cord and like it; they are filled with cerebrospinal fluid. The largest ventricles are the lateral ventricles (first and second ventricles) which extend into the cerebral hemisphere. A narrow space that constitutes the third ventricle is located in the midline of the brain, beneath the corpus callosum. The fourth ventricle is located in the brain stem just in front of the cerebellum.

Spinal Cord

An oval shaped hollow cylinder continues with medulla oblongata and runs below the skull down the neural canal of the backbone. The spinal cord in man ends at the third lumber vertebrae. Below this pointy the nerve travel down the neural canal rather like a horsetail.

Cross section the spinal cord shows an inner gray matter containing a central canal. The outer portion is composed of white matter. As in the other part of the nervous system, the gray matter consists of neuron cell bodies and non-myelinated parts of the fibers. The white matter is made up of bundles of myelinated nerve fibers.
Spinal Cord

Function: The Spinal cord carries out the reflex actions, conduct impulses to and from the brain and helps in better function of brain.

Peripheral Nervous System

The peripheral nervous system is made up of nerves. Nerves are the bundle of fibers i.e. axons, or dendrites bounded by connective tissues. The cell bodies of neurons are found in CNS. Ganglia (sing. ganglion) are collection of cell bodies of neurons within the PNS. Only the fibers of neurons (axon or dendrites) are found in the nerves of the PNS.

Cranial Nerves

Human have 12 pairs of cerebral or cranial nerves arising from the brain. Some are sensory, some are motor and some are mixed nerves that contains both type of fibers. Cranial nerves are largely concerned with head, neck and facial regions of the body, the vagus nerves have branches to the pharynx and larynx and to most of the internal organs.
Cranial Nerves

Spinal Nerves

Humans have 31 pairs of spinal nerves. Each spinal nerve emerges from the spinal cord by two short branches or roots, which lie within the vertebral column... The dorsal root contains fibers of sensory neuron and the ventral root contains the fibers of motor neurons. The two roots join just before a spinal nerve leaves the vertebral column. Therefore, all spinal nerves are mixed nerves. Each spinal nerve serves the particular region of the body in which it is located.
Spinal Nerves

Somatic Nervous System

The somatic nervous system includes the mixed nerves that control the voluntary movement of the musculoskeletal system and the exterior sense organs including those in the skin.

Autonomic Nervous System

The motor neurons that control the internal organs function automatically and usually without need for conscious intervention. The autonomic system is divided into (a) sympathetic (b) parasympathetic system. Both of these systems (1) function automatically and usually subconsciously In an Involuntary manner (2) Innervate (to supply nerve impulse) all internal organs (3) utilize two neurons and one ganglion for each impulse. The first of these two neurons has a cell body within the CNS and a preganglionic fiber. The second neuron has a cell body within the ganglion and a post ganglion fiber.

Sympathetic Nervous System

Most preganglionic fibers of the sympathetic system arise from the middle or thoracic lumbar portion of the spinal cord and almost immediately terminate in ganglia that lie near the cord; the sympathetic system is specialty important during emergencies and is associated with fight or flight, as active muscle requires a ready supply of glucose and oxygen.

The system accelerates heartbeat, dilates bronchi, and inhibits the digestive tract. The neurotransmitter released by the postganglionic axon is primarily norepinephrine.

Parasympathetic Nervous System

A few cranial nerves, including the vagus nerve, together with fibers that arise from the sacral (bottom) portion of the spinal cord, form the parasympathetic nervous system. This system promotes all the internal responses that are associated with relaxed state; for example, it causes the pupil of the eye to contract, promotes digestion of food and retards heartbeat. The neurotransmitter utilized by parasympathetic system is acetylcholine.