Endocrine System Overview

Anterior Pituitary Gland (Adenohypophysis)

The anterior pituitary gland is often referred to as the master endocrine gland because it regulates many other endocrine glands through hormone secretion.

• Produces several hormones:

  • TSH (Thyroid-Stimulating Hormone)

  • ACTH (Adrenocorticotropic Hormone)

  • FSH and LH (Gonadotropins)

  • GH (Growth Hormone)

  • PRL (Prolactin)

Hypothalamus

The hypothalamus controls the pituitary gland by releasing and inhibiting hormones, acting as a major regulatory center for the endocrine system.

• Produces TRH (Thyrotropin-Releasing Hormone), which signals the anterior pituitary to release TSH.

• Regulates the HPA Axis and overall endocrine function.

Posterior Pituitary

The posterior pituitary stores and releases hormones made by the hypothalamus.

• ADH (Antidiuretic Hormone)

• Oxytocin

Hormonal Pathways and Glands

Thyroid Gland

The thyroid gland produces hormones that regulate metabolism and are essential for growth and development.

• Produces T3 and T4 (thyroid hormones)—the primary hormones that regulate metabolism.

• Stimulated by TSH from the anterior pituitary.

• Made in thyroid follicle cells, not the thymus.

Pancreas

• Maintains blood glucose homeostasis by producing insulin and glucagon.

Adrenal Glands

• Adrenal cortex: Releases cortisol (stress hormone, long-term stress).

  • Part of the HPA axis: Hypothalamus → Anterior Pituitary → Adrenal Cortex.

• Adrenal medulla: Releases epinephrine and norepinephrine (fight or flight response).

Pineal Gland

• Secretes melatonin, which regulates circadian rhythms.

Hormone Classification

Hormones are classified by their chemical structure, which determines their solubility, receptor location, and mechanism of action.

• Steroid and thyroid hormones (lipid-soluble) cross cell membranes and bind to intracellular receptors, causing gene expression changes.

• Peptide hormones need post-translational modification to become active.

Hormone Transport & Action

Hormones travel via the cardiovascular system and act on target cell receptors to produce specific effects.

• Specificity is based on target cell receptors.

• Hormones can:

  • Alter enzyme activity.

  • Modify membrane transport.

  • Regulate gene expression/protein synthesis.

Quick Reference - Key Hormones and Functions

This table summarizes the origin and function of major hormones in the endocrine system.

Important Terms to Review

• Hypothalamic-hypophyseal portal system – Connects hypothalamus to anterior pituitary.

• Gonadotropins – FSH and LH.

• Hyperthyroidism – Excess thyroid hormones.

• Peptide hormone – Insulin, ADH.

• Steroid hormone – Cortisol, testosterone.

Nervous System Cell Types

The nervous system is composed of specialized cells that transmit and support neural impulses.

• Neurons: Transmit electrical impulses.

• Glial cells: Support and protect neurons (e.g., astrocytes, oligodendrocytes).

Divisions of the Nervous System

The nervous system is divided into central and peripheral components, each with specific functions.

• Central Nervous System (CNS): Brain and spinal cord.

• Peripheral Nervous System (PNS): Nerves outside CNS.

• Autonomic Nervous System (ANS): Controls involuntary body functions; includes:

  • Sympathetic division: "Fight or flight"

  • Parasympathetic division: "Rest and digest"

Brain Anatomy and Function

The brain contains distinct regions and tissue types, each with specialized roles in processing and regulating body functions.

• Gray matter: Unmyelinated cell bodies, found in the outer cortex of the brain.

• White matter: Myelinated axons.

Brain Regions

• Hypothalamus: Regulates body temperature, hunger, thirst, circadian rhythms, and autonomic functions.

• Pons: Regulates breathing and REM sleep.

• Medulla oblongata: Controls heart rate, BP, respiration, and reflexes.

• Cerebellum: Coordinates movement (not directly involved in autonomic control).

Neuroglial Cells

• Astrocytes: Form blood-brain barrier, CNS support.

• Oligodendrocytes: Myelinate CNS neurons.

• Schwann cells: Myelinate PNS neurons.

Neurotransmission and Membrane Potential

Neurons communicate via changes in membrane potential, which are essential for signal transmission.

• Resting Membrane Potential: (more K+ inside, more Na+ outside).

• Na+/K+ Pump: Restores resting potential by pumping 3 Na+ out and 2 K+ in.

• Depolarization: Na+ channels open—cell becomes more positive.

• Repolarization: K+ channels open—cell becomes more negative.

• Hyperpolarization: Excess K+ exits, making membrane more negative than resting.

Neurotransmitter Pathways

Neurotransmitters are released at synapses to transmit signals between neurons and to target tissues.

• Somatic motor (skeletal muscle): Acetylcholine

• Autonomic ganglia (both sympathetic and parasympathetic): Acetylcholine

• Sympathetic target (most organs): Norepinephrine

• Parasympathetic target: Acetylcholine

• Neuromuscular Junction: Acetylcholine triggers muscle contraction.

Muscle Cell Structures

Muscle cells contain specialized structures for contraction and signal transmission.

• Sarcoplasmic Reticulum: Specialized ER that stores calcium for muscle contraction.

• Sarcolemma: Muscle cell membrane.

• Axolemma: Neuron membrane.

Autonomic Nervous System Pathways

• Preganglionic neuron: Cell body in CNS, releases ACh.

• Postganglionic neuron: Cell body in autonomic ganglion; releases ACh (parasympathetic) or NE (sympathetic).

Key Concepts Summary

• Sensory receptor cells: Convert stimuli into graded potentials.

• Not all sensory receptor cells are neurons.

• Autonomic Ganglion: Relay point where pre- and postganglionic neurons synapse.

Example: The HPA axis is a classic example of endocrine regulation: Hypothalamus → CRH → Anterior Pituitary → ACTH → Adrenal Cortex → Cortisol.

Additional info: For exam preparation, review figures on neuron anatomy and muscle cell structures, and be able to identify all major components.