Endocrine System Overview

Endocrine System

• The endocrine system uses chemical messengers (hormones) that affect any tissue with a receptor for that specific chemical.
• Response is slower compared to the nervous system, taking days for action to occur.
• The nervous system uses neurons to send electrical signals (action potentials).
• This system stimulates muscles and glands, responding to stimuli very quickly, and has a short-lived effect.

Exocrine and Endocrine Glands

• Exocrine glands secrete hormones into ducts.
• Endocrine glands are ductless, secreting hormones directly into the bloodstream.
• Some organs have both endocrine and exocrine functions.

Neuroendocrine Link

• The neuroendocrine link involves stimulation by the hypothalamus, the major regulator for endocrine activity.
• The hypothalamus sends nervous and precursor signals to activate other endocrine glands.

Chemical Messengers

• Hormones are chemical messengers that affect another part of the body.
• Autocrines are chemicals produced by a cell to change the activity of that same cell.
• Paracrines affect neighboring cells.
• Local regulators act on nearby cells or tissues (e.g., prostaglandins, leukotrienes).
• Pheromones are produced by one individual, affecting another's behavior (sexual or repulsive).

Hormone Actions- Cellular Effects

• Hormones target cells with specific receptors on their cell surface.
• Hormones can have various effects, including opening or closing ion channels, changing membrane potentials, stimulating protein synthesis, and activating/deactivating enzymes.
• Hormones can also promote secretion of other substances.

Hormone Chemical Classifications

• Amino acid-based hormones are derived from proteins, usually water-soluble and cannot cross plasma membranes.
• Steroid hormones are lipid-soluble, derived from cholesterol, with receptors inside the cell.
• Sources of steroid hormones are the adrenal cortex and gonads.
• Eicosanoids are biologically active lipids that function locally but indirectly.

Mechanisms of Hormone Action

• Steroid hormones directly activate genes.
• Non-steroid hormones (amino acid-based) use an indirect mechanism involving secondary messengers like cyclic AMP (cAMP) and calcium ions (PIP-Calcium mechanism).

Hormone Action Regulation

• Hormone levels in the bloodstream, receptor quantity, and receptor affinity affect hormone action.
• Upregulation increases the number of receptors, while downregulation decreases them, often in response to stress.
• Three types of hormone interactions:
  • Permissiveness: one hormone enhances the effect of another.
  • Synergism: two or more hormones have a combined effect.
  • Antagonism: two hormones have opposite effects.
• Hormone release is regulated by humoral (blood concentration), neural (nervous stimulation), and hormonal stimuli (hormones produced from another gland).

Major Endocrine Organs and Locations

• Pituitary gland- brain extension.
• Thyroid gland- surrounds the trachea.
• Parathyroid glands- embedded in the thyroid.
• Adrenal glands- on top of the kidneys.
• Pancreas (endocrine portion)- abdominal cavity.
• Gonads (testes/ovaries)- male/female reproductive organs.
• Pineal gland- brain.
• Thymus- active in young individuals.

Posterior Pituitary (Neurohypophysis)

• Composed of neural tissue, does not synthesize hormones but secretes them.
• Releases oxytocin (stimulates uterine contractions and milk ejection) and antidiuretic hormone (ADH) (regulates water balance).

Anterior Pituitary (Adenohypophysis)

• Composed of glandular tissue, secretes tropic hormones that stimulate other endocrine glands.

Growth Hormone (GH)

• Stimulates growth, cell division, protein synthesis, fat metabolism, and glucose conservation.
• Overactive GH leads to gigantism in children and acromegaly in adults.
• Insufficient GH causes pituitary dwarfism.

Thyroid Stimulating Hormone (TSH) Cascade

• Involved in thyroid hormone production and its regulation.

Adrenocorticotropic Hormone (ACTH) Cascade

• Involved in the production of corticosteroids by the adrenal cortex and the body's response to stress.

Gonadotropins

• Stimulate the function of gonads (testes & ovaries).
• Include follicle-stimulating hormone (FSH) and luteinizing hormone (LH), affecting sperm and egg production, and sex hormone production.

Prolactin

• Stimulates milk production and enhances testosterone production in males.
• High estrogen levels stimulate prolactin release, and prolactin production is brief due to ovarian hormone cycling.
• Suckling promotes sustained prolactin production.

Pro-opiomelanocortin (POMC)

• A precursor hormone involved in creating other hormones.