Endocrine System and Behavioural Regulation

Introduction to Body Control Systems

The human body maintains internal balance and coordinates behaviour through two major regulatory systems: the nervous system and the endocrine system. Both systems are essential for homeostasis, but they differ in their mechanisms and effects.

• Nervous System: Uses electrical impulses transmitted by neurons for rapid, short-term control of tissues.

• Endocrine System: Uses hormones transported via blood and tissue fluids for slower, longer-lasting regulation.

Additional info: The nervous system is responsible for immediate responses, such as reflexes, while the endocrine system manages processes like growth, metabolism, and reproduction.

Hormones and Endocrine Glands

General Properties and Functions of Hormones

Hormones are chemical messengers secreted by endocrine glands directly into the bloodstream. They act on specific target tissues, influencing metabolism and physiological processes.

• Regulate: Extracellular fluid (ECF), metabolism, biological clock, muscle contraction, glandular secretion, growth, development, reproduction, and some immune functions.

• Target Specificity: Only cells with appropriate receptors respond to a given hormone.

Example: Insulin lowers blood glucose by acting on liver, muscle, and fat cells.

Types of Glands

• Exocrine Glands: Secrete substances to body surfaces via ducts (e.g., sweat glands).

• Endocrine Glands: Secrete hormones directly into blood or body fluids (e.g., pituitary, thyroid).

Major Endocrine Glands

• Pituitary gland

• Thyroid gland

• Adrenal glands

• Pancreas

• Ovaries

• Testes

• Parathyroid gland

Pituitary Gland (Hypophysis)

Structure and Function

The pituitary gland is a small, pea-sized gland located at the base of the brain, connected to the hypothalamus by the infundibulum. It consists of two distinct lobes: anterior and posterior.

• Anterior Lobe (Adenohypophysis): Secretes tropic hormones that regulate other endocrine glands.

• Posterior Lobe (Neurohypophysis): Releases neurohormones produced by the hypothalamus.

Anterior Pituitary Hormones

• Follicle Stimulating Hormone (FSH): Stimulates ovarian follicle growth and estrogen release in females; sperm production in males.

• Luteinizing Hormone (LH): Triggers ovulation and hormone release in females; testosterone production in males.

• Thyroid Stimulating Hormone (TSH): Stimulates thyroid hormone release.

• Adrenocorticotropic Hormone (ACTH): Stimulates adrenal cortex to release steroid hormones.

• Growth Hormone (GH): Stimulates growth in most body cells (not a tropic hormone).

• Prolactin: Stimulates breast development and milk production (not a tropic hormone).

Posterior Pituitary Hormones

• Antidiuretic Hormone (ADH, Vasopressin): Promotes water conservation in kidneys and vasoconstriction in blood vessels.

• Oxytocin: Stimulates uterine contractions and milk ejection.

Thyroid and Parathyroid Glands

Thyroid Gland

The thyroid gland is located anterior to the trachea and produces thyroid hormones (e.g., thyroxine) and calcitonin.

• Thyroxine: Increases basal metabolic rate, body temperature, CNS activity, and protein synthesis.

• Calcitonin: Lowers blood calcium by acting on bone and kidneys.

Parathyroid Gland

Embedded in the thyroid gland, the parathyroid glands secrete parathyroid hormone (PTH).

• Parathyroid Hormone (PTH): Increases blood calcium by acting on bone and kidneys.

Pancreas

Structure and Function

The pancreas is a mixed gland with both exocrine (digestive enzyme secretion) and endocrine (hormone secretion) functions. The endocrine portion consists of pancreatic islets.

• Acini: Secrete pancreatic juice (exocrine).

• Pancreatic Islets: Secrete hormones (endocrine).

• Beta Cells: Secrete insulin, which lowers blood sugar.

• Alpha Cells: Secrete glucagon, which raises blood sugar.

Example: After a meal, insulin is released to promote glucose uptake by cells, reducing blood sugar levels.

Adrenal Glands

Structure and Function

The adrenal glands are located on the superior poles of the kidneys and consist of two functionally distinct regions: the cortex and the medulla.

• Adrenal Cortex: Produces steroid hormones (glucocorticoids, mineralocorticoids, sex hormones).

• Adrenal Medulla: Produces catecholamines (epinephrine and norepinephrine).

Adrenal Cortex Hormones

• Glucocorticoids (e.g., cortisol): Increase blood glucose, have anti-inflammatory effects, and suppress the immune system.

• Mineralocorticoids (e.g., aldosterone): Conserve sodium and increase potassium loss in the kidneys.

• Sex Hormones: Small amounts contribute to secondary sex characteristics.

Adrenal Medulla Hormones

• Catecholamines (epinephrine and norepinephrine): Mimic sympathetic nervous system effects, preparing the body for 'fight or flight' responses.

Gonads: Ovaries and Testes

Ovaries

The ovaries produce estrogen and progesterone, which regulate the ovarian cycle and development of female secondary sex characteristics.

Testes

The testes produce testosterone, which regulates sperm production and development of male secondary sex characteristics.

Summary Table: Major Endocrine Glands and Hormones