Zoology Assignment Help >> Co-Ordination, Endocrine System, Pituitary Gland, Thyroid, Adrenal, Pancreas

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Co-Ordination - Endocrine System

Exocrine and Endocrine glands: The glands are secretory sites of the body and are of two kinds-the exocrine glands and the endocrine glands. The exocrine glands convey their secretion to the place of their action via ducts whereas the endocrine glands secrete the active chemical messengers the hormones and pour their secretion into the blood which takes these substances to their place of action. But pancreas and gonads are of mixed nature,

Hormones: These are informational molecules. These are inhibitory or stimulatory in nature. The hormones co¬ordinate the activities of different organs and tissues. They may be made up of proteins, amino acids, peptides, catecholamines or steroids.

Certain cells are situated in the hypothalamus and they secrete neurohormones. These neurohormones (such as thyrotropin releasing hormone, gonadotropin releasing hormone somatostatin and corticotropin releasing hormone) are carried to the anterior pituitary or adenohypophysis by portal veins and regulate secretion of its hormones thyrotropin, gonadotropin, STH, cortricotropin respectively.

Certain neurons are modified into glandular cells. These cells situated on hypothalamus are known as neurosecretory cells and their secretion is called as neurosecretory material (NSM). They are carried to the posterior pituitary or neurohypophysis. This part of pituitary stores the hormones vasopressin and oxytocin.

PITUITARY GLAND

At present the pituitary gland is divided on the basis of its histology and nature into adenohypophysis and neurohypophysis.

Adenohypophys: The adenohypophysis secretes its own synthesised hormone. It includes anterior and middle pituitary.

The hormones of anterior pituitary are -

(1) Growth hormone (GH) or somatotropic hormone (STH): It stimulates the body growth.

(2) Thyroid stimulating hormone (TSH): It stimulates the thyroid for hormonal secretion.

(3) Adreno-corticotropic hormone (ACTH): It stimulates the adrenal cortex to secrete glucocorticoids.

(4) Follicle stimulating hormone (FSH)

(5) Luteinizing hormone (LH or ICSH): It controls the male and female gonadal- development and their hormone secretion.

(6) Prolactin hormone (PRL or MTH or LTH): It controls milk secretion in females.

The secretion of anterior pituitary is controlled by the age, external stimuli and feedback system of hormone.

Middle pituitary: It secretes the melanocyte stimulating hormone (MSH) or intermedin which controls the pigmentation of skin.

Neurohypophysis : The neurohypophysis stores the hormones secreted by the neurosecretory cells of hypothalamus and includes posterior pituitary only.

The hormones of neurohypophysis or posterior pituitary are-

(1) Oxytocin or pitocin: It contracts the smooth muscle of uterus and mammary glands.

(2) Antidiuretic hormone (ADH) or vasopressin or pitressin: It regulates the renal water reabsorption.

THYROID

Thyroid gland secretes the thyroxin and tri-iodothyronin which control the basal metabolic rate, body growth, tissue differentiation and metamorphosis of tadpoles.

PARATHYROID

Parathyroid secretes the parathormone and calcitonin which increase the blood calcium level by increasing the bone calcium and renal reabsorption of calcium from the urine.

ADRENAL

Adrenal cortex: It secretes the following steriod hormones.

(1) Adrenalin or epinephrin: Its function is the vasodilation of arterioles of skeletal muscles and vasoconstriction of arterioles of skin, mucous membrane and viscera. It increases the heart beat rate and blood pressure.

(2) Nor-adrenalin or nor epinephrin: Its function is antagonistic to the above hormone.

Adrenal medulla and sympathetic nervous system together act in the integrated and co-ordinated way and form a sympathetico-adrenal system.

PANCREAS

Pancreas is exocrine and endocrine gland. Its islets of Langerhans tissue has two types of cells-alpha and beta cells.

(1) Alpha cells: They secrete the glucagon hormone. This converts the liver glycogen into glucose. Thus its function is opposite to that of insulin.

(2) Beta cells: They secrete the insulin (hypoglycemic antidiabetic factor). It stimulates the glycogenesis and opposes the gluconeogenesis.

GONADS

INTERSTITIAL CELLS OF THE TESTIS -secrete the testosterone. It stimulates the growth and functions of secondary sexual characters.

GRAAFIAN FOLLICLE AND CORFUS LUTEUM: of ovary secrete the estrogen and progesterone respectively. The former stimulates the growth of female secondary sexual characters; whereas the pregnancy changes are controlled by the progesterone.

Testosterone is excreted in the urine in the form of 17 ketosteroid, androsterone and eitocholanole. The estogen is excreted in the form of estron or estradiol; whereas the pregnanediol is the main excretory product of progesterone.

PINEAL BODY is situated on the dorsal surface of the diencephalon and it secretes melatonin and seratonin hormone. These regulate (probably) the hormonal monthly cycle, reproductive cycle and related physiology.

MUCOUS MEMBRANE OF STOMACH AND DUODENUN OF

Secrete the gastrin, secretin, pancredzymin, cholecystokinin, enterokinin and enterogastrin.

Gastrin: It stimulates the gastric glands to secrete the gastric juice.llt is secreted by the gastric mucosa by the mechanical stimulation.

Secretin: It stimulates the pancreas to produce pancreatic juice. It is secreted by the duodenal mucosa.

Pancreozymin: It controls the composition of pancreatic juice.

Cholecystokinin: It contracts the gall bladder.

Enterokinin: It stimulates the intestinal mucosa for succus entericus secretion.

Enterogastrin: It checks the gastric secretion while stomach is empty.