Endocrine System Overview

Endocrine System Overview

• The endocrine system uses chemical messengers (hormones) to control body functions.
• Hormones are secreted by glands and affect target cells.
• Endocrine responses are slower than nervous system responses but can last longer.

Nervous vs. Endocrine Systems

• Nervous system uses electrochemical impulses through neurons.
• Nervous system primarily affects excitable tissues (e.g., muscle, glands).
• Nervous system responses are fast, happening in milliseconds.
• Nervous system responses are less intense over time as adaptation occurs.
• Endocrine system uses chemical messengers (hormones) to affect various cells in the body.
• Endocrine responses occur in seconds to several minutes or hours, varying based on hormones and their targets.
• The effects of an endocrine response can last longer compared to nerve impulses.

Exocrine vs Endocrine Glands

• Exocrine glands secrete substances through ducts (e.g., digestive glands, sweat glands).
• Endocrine glands secrete directly into the bloodstream. These glands lack ducts.
• Endocrine secretions are hormones.
• Endocrine glands are highly vascularized.
• Examples of endocrine glands include the pituitary, thyroid, parathyroid, adrenals, and pancreas.

Chemical Messengers

• Hormones: Chemical messengers that travel long distances in the body (e.g., insulin, thyroid hormone).
• Autocrines: Chemicals affecting the same cell that produced them (e.g., cytokines).
• Paracrines: Chemicals affecting neighboring cells (e.g., histamine).
• Pheromones: Chemicals secreted by one individual that affect a response in another individual (e.g., sex attractants).

Hormone Action

• Hormones exert their effects on target cells exhibiting receptors for the specific hormones.
• Hormones can elicit various generalized effects, including growth, metabolism, reproduction, and development.
• Hormone effects vary depending on the type of target cell.

Hormonal Effects

• Open/close ion channels (change cell permeability).
• Stimulate protein synthesis (produce specific protein types).
• Activate/deactivate enzymes.
• Promote secretion (influence gland/cell function).
• Stimulate mitosis/differentiation, influencing growth and development.
• Hormones cause growth and repair.

Chemical Classifications of Hormones

• Amino-acid based (nonsteroid): water-soluble hormones using secondary messenger systems (e.g., insulin, epinephrine).
• Steroid: lipid-soluble (derived from cholesterol) hormones requiring receptor sites inside the target cell (e.g., testosterone, estrogen).

Mechanisms of Hormone Action

• Amino Acid-based hormones typically use a second messenger system.
• Steroid and thyroid hormones use intracellular receptors.

Hormones and Body Regulation

• Concentrations of hormones in the bloodstream control hormonal effects.
• Up regulation and down regulation modulate receptor number and hormone responsiveness on target cells.
• Permissiveness, synergism, and antagonism describe the interactions of different hormones on the same target cell.

Hormone Release Regulation

• Humoral regulation: Blood concentration/related changes stimulate gland hormone release.
• Neural regulation: Nervous system impulses can stimulate hormone release.
• Hormonal regulation: One hormone stimulates or inhibits release of another hormone.

Major Endocrine Glands and Locations

• Pituitary, thyroid, parathyroid, adrenals, pancreas, gonads, pineal, thymus

Neurohypophysis (Posterior Pituitary)

• Made of nervous tissue.
• Secretion of oxytocin and antidiuretic hormone (ADH).

Adenohypophysis (Anterior Pituitary)

• Composed of glandular tissue.
• Secretion of various hormones regulating other endocrine glands.

Thyroid Gland

• Secretion of thyroxine (T4) and triiodothyronine (T3) regulating metabolic rate and other functions.
• Secretion of calcitonin.

Parathyroid Glands

• Secretion of parathyroid hormone (PTH) regulating calcium homeostasis.

Adrenal Glands

• Adrenal cortex secretes mineralocorticoids (e.g., aldosterone; regulates sodium and potassium balance) and glucocorticoids (e.g., cortisol; for stress response and metabolism), and gonadocorticoids.
• Adrenal medulla secretes epinephrine and norepinephrine.

Pancreas

• Secretion of insulin and glucagon regulating blood glucose levels.

Gonads

• Testes secrete testosterone.
• Ovaries secrete estrogen and progesterone.

Pineal Gland

• Secretion of melatonin.

Thymus

• Plays a role in immune development (in childhood).

Reproduction Summary

• Testes produce testosterone, and ovaries produce estrogen and progesterone.
• Spermatogenesis (sperm production) and oogenesis (egg production) are described in terms of hormone influence and regulation
• Key structures such as the seminiferous tubules, corpus luteum, uterus, and vagina are described.
• The processes of sperm maturation, fertilization, and sexual differentiation/hormonal regulation are detailed.

Hormones in Other Organs

• List of hormones from tissues/organs not endocrine in nature and influence on the body.
• Role of other organs regulating the endocrine system, for example the heart, in terms of hormone modulation and release.