The interaction between hormonal and nervous systems

Both nervous system and endocrine system control  various  physiological  activities in organisms through coordination. The process of coordination in an organism is achieved by the two systems acting together. Although the nervous system and the endocrine system are two di ft erent systems, both release chemical substances as a means of communication between cells. The major centres for linking the two systems are the pituitary gland which is the control centre for endocrine glands and the hypothalamus which is the control centre for the nervous system. The hypothalamus is the gland which receives information from the brain and blood vessels and sends it to the pituitary gland.

The pituitary gland is located at the base of the brain just below the hypothalamus. This gland directly or indirectly controls the secretions of other glands. It is therefore, known as the master gland. The pituitary gland is divided into two distinct regions called the anterior and posterior lobes. Anterior lobe is the one that is connected by blood vessel called portal system to the hypothalamus. It secretes various hormones under the influence of the hypothalamus. The anterior lobe secretes two groups of hormones namely:

(a) Tropic  hormones
These are the hormones that stimulate other glands to secrete hormones. Examples of tropic hormones include gonadotropic hormones which stimulate the secretion of luteinizing  hormone  that  causes  ovulation and the follicle stimulating hormone which controls the development of graflian follicles in the ovary. Another example of tropic hormone is the thyroid stimulating hormone (TSH) which acts on thyroid gland and stimulates secretion of thyroxine hormone which controls metabolism in cells.

(b) Growth hormones
Growth hormones arc the hormones that stimulate  the  growth  and  development of an organism by controlling the tissues responsible for growth and development. When there is under secretion of the growth hormones the result is a disorder called dwarfism. Over secretion of this hormone results in abnormally large size of the body, a condition known as giantism. When over secretion occurs in adults it results into acromegaly. This is a situation whereby some body parts such as face, hands and feet become large in terms of thickness rather than length.

(c) Prolactin
Prolactin is another hormone secreted by the anterior lobe. This hormone stimulates the production of milk in lactating mothers.

Posterior lobe is the extension of the brain which facilitates the secretion of antidiuretic hormone (ADH) or vasopressin and oxytocin hormone which are produced by neurosecretory cell bodies,

(a) Antidiuretic hormone (ADH (vasopressin))\

This is the hormone that acts on the kidneys and regulates the amount of w ater reabsorbed  in  the  kidney  tubules.  The hormone is released in response to a fall in the water content in blood plasma and leads to an increase in the permeability to water at distal convoluted tubule in the nephron of the kidney. The deficiency of this hormone results to a condition called diabetes insipidus. Diabetes insipidus is associated with the production of large quantities of dilute urine.

(b) Oxytocin
This is another hormone secreted by the posterior lobe which stimulates the contraction of the muscles of the uterus during child birth. The hormone causes the muscles of the uterus to contract and push the baby out. Oxytocin hormone also stimulates ejection of milk from nipples by contraction of the ducts of the mammary gland in lactating animals.

Adrenal   hormones
These are hormones secreted by adrenal glands. These glands arc located slightly above each kidney. Structurally each gland is made up by two types of cells that have different origins. The glands have two regions, the outer region called the cortex and the inner region called the medulla. The hormones secreted by these glands include adrenaline, aldosterone and cortisol. Adrenaline prepares the body for emergency during fight or flight action. It prepares the body wherever there is a frightening or an anxious situation. This hormone is produced and released into the blood stream causing an increase in blood pressure. Thus, the rate of heartbeat, blood sugar level and supply of blood to the muscles increases. Adrenaline can also be produced during passion, anxiety, pain, excitement and shock. Aldosterone is die hormone that controls the amount of water and salts content in the body. The hormone stimulates the conservation of sodium ions and chloride ions and removal of potassium ions. It also prevents the loss of sodium ions through urine and saliva, thus maintains osmotic concentration.

Cortisol is another hormone produced by the adrenal gland. Its function is to stimulate the metabolism of carbohydrates and proteins  by  promoting  gluconeogenesis and breakdown of plasma proteins. It also helps in the prevention of inflammation and allergic  reactions.

Thyroid   hormones
These are hormones produced and released by the thyroid gland. Thyroid gland is found in the neck region. It is divided into two lobes which arc surrounded by collagenous connective tissues. Thyroxine is an iodine containing hormone which is secreted by this gland. The role of this hormone is to regulate the basal metabolic rate of different food substances such as carbohydrates, proteins and lipids. Also, thyroxine promotes normal skeletal growth and it is essential for normal growth and development of die brain. Deficiency of this hormone during infancy causes a condition called cretinism which is accompanied by thickened facial features, retarded and stunted growth, abnormal bone development and mental retardation. Undersecretion  of  thyroxine in adulthood leads to a condition called myxoedema. This condition is characterised by low metabolic rate, increase in body fluid and fatigue. Insufficient iodine in diet leads to abnormal development of the thyroid gland, a condition is called colloid goitre. This happens because the thyroid gland enlarges to create more space so that it can extract much iodine from the blood. On the other hand, when there is over secretion of thyroxine, it results into a condition called exophthalmic goitre. Calcitonin is another hormone secreted by thyroid gland, it regulates the level of Calcium in the blood by decreasing the level of Calcium in blood.

Parathyroid: This gland is found within the thyroid gland, it secretes the hormone called Parathyroid hormone (PTH). Like calcitonin, this hormone increases the level of calcium in the blood.

Pancreatic hormones
These are the hormones produced by pancreas. Pancreas gland that contains a cluster of cells called islets of Langerhans. It is located in the abdominal cavity. This gland is surrounded by different organs such as small intestine, liver and spleen. The pancreas performs two main functions which arc exocrine and endocrine functions. Through exocrine function, pancreas produces different enzymes that are used in the digestion of food. As an endocrine gland the pancreas produces two important hormones called insulin and glucagon. These hormones are directly released into the blood stream. Insulin hormone is released when there is raise in blood sugar. It accelerates the rate through which glucose is converted to glycogen in the liver. It also facilitates the uptake of glucose in muscle cells and the liver. Apart from that, insulin promotes the rate of protein synthesis in the cell. When pancreas fails to produce sufficient amount of insulin in the body, it results into a disorder called diabetes mellitus. This is the condition caused by the increase in amount of sugar in the blood. Excess blood sugar is excreted through urine. It is characterized by frequent urination and excretion of sugar in urine. Glucagon hormone is secreted when there is a fall in blood sugar. It converts glycogen in the liver cells to glucose, resulting into an increase of the level of glucose in the blood.

Gonadal hormones
These are hormones produced by gonads. The gonads include ovaries and testes. These organs are also called sex organs. They produce both hormones and gametes. The testes produce testosterone hormone which facilitates the development of secondary sexual characteristics and the production of sperms. Testosterone is one of the important androgen hormones secreted in males. It controls the development of secondary sexual characteristics in males such as deepening of voice, growth of beard and growth of pubic hair. On the other hand, the ovary produces oestrogens and progesterone hormones. These hormones control the development of secondary sexual characterises in females such as development of mammary gland, pubic- hair. high pitched voice and broaderning of the hips. It also causes thickening of the lining of the uterus. Apart from that, the progesterone hormone prepares the lining of the uterus for implantation. Table 3.4 gives a summary of the components and functions of the endocrine system.