The Endocrine System

Overview of Endocrine and Exocrine Glands

The human body uses two main types of glands to regulate physiological processes: endocrine and exocrine glands. Understanding their differences is fundamental to the study of the endocrine system.

• Endocrine glands: Ductless glands that secrete hormones directly into the bloodstream. These hormones regulate various body functions.

• Exocrine glands: Glands that release their secretions through ducts to the surface of an organ or tissue or into a body cavity (e.g., sweat, salivary glands).

Major Endocrine Glands:

• Pituitary gland (anterior and posterior)

• Thyroid gland

• Parathyroid glands

• Adrenal glands (cortex and medulla)

• Pancreas (islets of Langerhans)

• Gonads (testes and ovaries)

• Pineal gland

• Hypothalamus (neuroendocrine organ)

Hormone Classes and Receptor Location

Hormones are classified based on their chemical structure, which determines their solubility and receptor location.

• Amino acid-based hormones: Hydrophilic; include peptides, proteins, and amines. Receptors are located on the cell membrane.

• Steroid hormones: Hydrophobic; derived from cholesterol. Receptors are intracellular (cytoplasmic or nuclear).

Hydrophilic hormones cannot cross the plasma membrane and bind to surface receptors. Hydrophobic hormones diffuse through the membrane and bind to intracellular receptors.

Adenylate Cyclase cAMP Second Messenger System

This is a common mechanism for hydrophilic hormone action, amplifying the signal inside the cell.

• Step 1: Hormone (first messenger) binds to its receptor on the cell membrane.

• Step 2: Receptor activates a G-protein.

• Step 3: G-protein activates adenylate cyclase enzyme.

• Step 4: Adenylate cyclase converts ATP to cyclic AMP (cAMP) (second messenger).

• Step 5: cAMP activates protein kinases, which phosphorylate target proteins, leading to a cellular response.

Signal amplification occurs because one hormone can generate many cAMP molecules, each activating multiple kinases.

Equation:

Hydrophobic Hormone Mechanism: Direct Gene Activation

Hydrophobic (steroid and thyroid) hormones act via direct gene activation.

• Hormone diffuses through the plasma membrane.

• Binds to an intracellular receptor, forming a hormone-receptor complex.

• The complex enters the nucleus and binds to specific DNA regions.

• This binding initiates transcription of target genes, leading to protein synthesis.

Example: Cortisol binding to its receptor to increase gluconeogenesis enzymes in the liver.

Regulation of Hormone Secretion

Hormone release is tightly regulated by three main types of stimuli:

• Hormonal stimuli: Hormones stimulate other endocrine glands to release hormones (e.g., hypothalamic hormones stimulate pituitary hormones).

• Humoral stimuli: Changes in blood levels of ions or nutrients trigger hormone release (e.g., low blood calcium stimulates parathyroid hormone release).

• Neural stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic stimulation of adrenal medulla to release epinephrine).

Relationship Between Nervous and Endocrine Systems

The hypothalamus is the key link between the nervous and endocrine systems, controlling the pituitary gland via two main pathways:

• Infundibulum: The stalk connecting hypothalamus to pituitary.

• Hypothalamic-hypophyseal tract: Neural connection to the posterior pituitary (stores and releases hypothalamic hormones).

• Hypothalamic-hypophyseal portal system: Blood vessel network carrying releasing/inhibiting hormones to the anterior pituitary.

Hormones of Major Endocrine Glands

Summary Table: Major Endocrine Glands, Hormones, Stimuli, Targets, and Effects

Examples of Hormonal Imbalances

• Gigantism, Acromegaly: Hypersecretion of growth hormone (GH) in children (gigantism) or adults (acromegaly); leads to abnormal growth.

• Pituitary dwarfism: Hyposecretion of GH in children; results in short stature.

• Graves’ disease: Hypersecretion of thyroid hormones; causes hyperthyroidism, weight loss, and exophthalmos.

• Hypothyroidism: Hyposecretion of thyroid hormones; leads to fatigue, weight gain, and cold intolerance.

• Hyperparathyroidism: Excess PTH; causes hypercalcemia, bone weakness.

• Hypoparathyroidism: Deficient PTH; leads to hypocalcemia, muscle spasms.

• Hyperaldosteronism: Excess aldosterone; results in hypertension, hypokalemia.

• Cushing’s disease/syndrome: Hypersecretion of cortisol; causes obesity, moon face, muscle weakness.

• Addison’s disease: Hyposecretion of adrenal cortex hormones; leads to weight loss, hypotension, hyperpigmentation.

• Hypoglycemia: Low blood glucose, often due to excess insulin.

• Hyperglycemia (Diabetes Mellitus): High blood glucose due to insulin deficiency (Type I) or resistance (Type II).

Additional info: In diabetes mellitus, Type I is an autoimmune destruction of beta cells (insulin deficiency), while Type II involves insulin resistance in target tissues.