Physiology Lecture 23 – Endocrinology I

Endocrine Systems and Hormones

Endocrine Tissues/Organs and Functions

  • Pineal Gland

    • Hormone: Melatonin
    • Function: Regulates circadian rhythms.

  • Posterior Pituitary Gland

    • Hormones:
    • Oxytocin: Milk ejection, labor and delivery, social behavior.
    • Vasopressin (also known as Antidiuretic Hormone): Promotes water reabsorption in the kidneys.

  • Anterior Pituitary Gland

    • Hormones:
    • Prolactin: Stimulates milk production.
    • Growth Hormone (GH): Influences growth, metabolism, and secretion of growth factors.
    • Corticotropin (Adrenocorticotropic Hormone, ACTH): Stimulates cortisol release from the adrenal cortex.
    • Thyrotropin (Thyroid Stimulating Hormone, TSH): Promotes thyroid hormone synthesis.
    • Gonadotropins:
      • Follicle-Stimulating Hormone (FSH): Gamete production, stimulates growth of ovarian follicles or spermatogenesis.
      • Luteinizing Hormone (LH): Triggers sex hormone production from gonads (estrogen, progesterone, testosterone).

  • Thyroid Gland

    • Hormones:
    • Thyroid Hormones (T3 and T4): Regulate metabolism, growth, and development.
    • Calcitonin: Lowers blood calcium levels.

  • Parathyroid Gland

    • Hormone: Parathyroid hormone (PTH)
    • Function: Increases blood calcium levels.

  • Liver

    • Hormone: Angiotensinogen
    • Function: Stimulates secretion of aldosterone, increasing blood pressure.
    • Hormone: Insulin-like growth factors (IGF-1)
    • Function: Promotes growth.

  • Pancreas

    • Overall Function: Manages metabolism of glucose and nutrients.
    • Hormones:
    • Insulin: Lowers blood glucose levels.
    • Glucagon: Raises blood glucose levels.
    • Somatostatin: Inhibits gastric acid secretion.

  • Adrenal Cortex

    • Hormones:
    • Aldosterone: Regulates sodium and potassium homeostasis.
    • Cortisol: Mediates stress responses.
    • Androgens: Influence female sex drive.

  • Adrenal Medulla

    • Hormones: Epinephrine and norepinephrine
    • Function: Mediate fight-or-flight responses.

  • Testes (Males)

    • Hormones:
    • Androgens (Testosterone and Dihydrotestosterone (DHT)): Facilitate sperm production, development of secondary sex characteristics.
    • Inhibin: Inhibits FSH secretion from anterior pituitary.

  • Ovaries (Females)

    • Hormones:
    • Estrogen and Progesterone: Responsible for egg production and development of secondary sex characteristics.
    • Inhibin: Inhibits FSH secretion from anterior pituitary.
    • Relaxin: Aids muscle relaxation during pregnancy.

  • Adipose Tissue

    • Hormones: Leptin, adiponectin, resistin
    • Function: Regulates food intake, metabolism, and reproduction.

  • Hypothalamus

    • Hormone: Not applicable for now.
    • Function: Secretes neurohormones for regulation of anterior pituitary or directly into the bloodstream via posterior pituitary.

  • Thymus Gland

    • Hormone: Not applicable.
    • Function: Development of lymphocytes.

True/False Statements Regarding Genetic Mutations

  • a. True - A genetic mutation in the gene for insulin is possible.
  • b. False - A genetic mutation in the gene for cortisol is not possible.
  • c. False - A genetic mutation in the gene for epinephrine is not possible.

Comparisons Between Hydrophilic Hormones and Hydrophobic Hormones

  • Hydrophilic Hormones (e.g., peptide hormones & catecholamines)

    • How They Exit Cells: Utilize exocytosis.
    • Transport in Blood: Dissolved in plasma.
    • Simple Diffusion into Target Cells: Do NOT use simple diffusion; typically require endocytosis to enter target cells.
    • Location of Receptors: Found on the cell membrane.

  • Hydrophobic Hormones (e.g., steroid hormones & thyroid hormones)

    • How They Exit Cells: Use simple diffusion (steroid hormones) and require a transport protein (thyroid hormones).
    • Transport in Blood: Bound to carrier proteins.
    • Simple Diffusion into Target Cells: YES, they use simple diffusion.
    • Location of Receptors: Found in the nucleus and cytosol (occasionally on cell membrane).

Hormone Synthesis and Storage

  • Made On Demand: Steroid hormones.
  • Made in Advance and Stored in Secretory Vesicles: Amine hormones and peptide hormones.

Examples of Hormones by Category

  • Peptide Hormones:

    • Insulin
    • Parathyroid hormone (PTH)

  • Steroid Hormones:

    • Androgens (Testosterone and DHT)
    • Estrogen
    • Cortisol

  • Amine Hormones:

    • Catecholamines:
    • Epinephrine
    • Norepinephrine
    • Thyroid Hormones:
    • T4: Thyroxine
    • T3: Triiodothyronine

Hypothalamus and Pituitary Relationships

  • Hypothalamus and Posterior Pituitary:

    • Synthesizes neurohormones oxytocin and vasopressin, transported in neurons that terminate in the posterior pituitary, where they are secreted into circulation.

  • Hypothalamus and Anterior Pituitary:

    • Synthesizes trophic neurohormones transported via a capillary portal system to the anterior pituitary, which secretes its own tropic hormones into circulation that act on other glands.

Hormones from Hypothalamus and Anterior Pituitary

  • Hypothalamic Neurohormones:

    • Dopamine: Inhibits prolactin secretion.
    • Thyrotropin-Releasing Hormone (TRH): Stimulates TSH release.
    • Corticotropin-Releasing Hormone (CRH): Stimulates ACTH release.
    • Growth Hormone-Releasing Hormone (GHRH): Stimulates GH release.
    • Gonadotropin-Releasing Hormone (GnRH): Stimulates FSH and LH release.

  • Posterior Pituitary Hormones: Oxytocin and vasopressin.

  • Anterior Pituitary Targets: Produce hormones affecting mammary glands, thyroid gland, adrenal cortex, liver, and gonads.

Pathologies of Cortisol and Thyroid Hormone

  • Hypercortisolism: Excess cortisol, often caused by adrenal tumors, pituitary tumors, or is iatrogenic.
  • Hypocortisolism: Insufficient cortisol, commonly due to Addison's disease or adrenal cortex defects.
  • Hyperthyroidism: Excess thyroid hormone, related to Grave's disease or tumors.
  • Hypothyroidism: Insufficient thyroid hormone, often due to iodine deficiency or surgical removal of the thyroid gland.

Thyroid Enlargement Causes

  • Both hyperthyroidism and hypothyroidism lead to goiter (enlargement) due to overstimulation:
    • Hyperthyroidism: Overstimulation with autoantibodies.
    • Hypothyroidism: Overstimulation due to lack of T3 and T4 (no feedback mechanism).

Effects of Cortisol

  • Cortisol's impacts include:
    • Suppressing the immune system.
    • Stimulating glucose production in the liver.
    • Promoting protein breakdown in muscles.
    • Encouraging lipid breakdown from adipose tissue.

Anabolic Nature of Growth Hormone

  • Anabolic Hormones: Growth hormone promotes tissue building (cartilage, bone, etc.), can increase blood glucose. Other anabolic hormones include testosterone.

Reflex Pathways and Endocrine Hormones Production

  • Cortisol Production Pathway:
    • HPA Axis: Hypothalamus (IC1), CRH (H1), Anterior Pituitary (IC2), ACTH (H2), Adrenal Cortex (IC3), Cortisol (H3).
  • Thyroid Hormones Production Pathway (HPT Axis):
    • Hypothalamus (IC1), TRH (H1), Anterior Pituitary (IC2), TSH (H2), Thyroid Gland (IC3), T3/T4 (H3).

Resting Membrane Potential and Action Potentials

  • Definitions: Resting membrane potential is the electrical gradient across cell membranes, typically around -70 mV in neurons.

  • Hyperpolarization: Membrane potential becomes more negative (e.g., -80 mV, -90 mV), resulting from ion flow.

  • Depolarization: Membrane potential becomes less negative (e.g., -60 mV, -50 mV), typically caused by Na+ inflow.

  • Repolarization: The return to resting potential following depolarization, marked by K+ exit.

Mechanism to Maintain Resting Membrane Potential

  • Maintained by the sodium/potassium pump (Na⁺/K⁺-ATPase) using ATP.
  • Leak channels for K+ (more permeable at rest) lead to a negative potential.
  • Pump restores gradients of Na+ (high outside) and K+ (high inside) to maintain stability at -70 mV.

Components of the Nervous System

  • Central Nervous System:
    • Brain
    • Spinal Cord
  • Peripheral Nervous System:
    • Sensory Division (afferent)
    • Efferent Division:
    • Somatic motor neurons
    • Autonomic neurons (sympathetic and parasympathetic)

Anatomy of a Neuron

  • Dendrites: Receives signals, increasing surface area.
  • Cell Body: Contains nucleus, acts as the control center.
  • Axon Hillock: Initiates action potentials.
  • Axon: Transmits outgoing signals.
  • Myelin Sheath: Insulates axon, preventing current leakage, supports saltatory conduction.
  • Axon Terminal: Releases neurotransmitter signals to other neurons or target cells.

Changes in Permeability and Action Potentials

  • Increased sodium permeability leads to depolarization (Na+ entry).
  • Increased potassium permeability results in hyperpolarization (K+ exit).
  • Movement along the axon occurs via voltage-gated channels responding to depolarization.

Steps Leading to Action Potentials

  1. Resting Membrane: -70 mV with closed Na+ and K+ channels.
  2. Depolarizing Stimulus: Triggers potential towards -55 mV (threshold).
  3. Na+ Entry: Rapid influx depolarizes the cell.
  4. Na+ Channels Close, K+ Channels Open: Leading to repolarization.
  5. K+ Exits: Resulting in hyperpolarization.
  6. Restoration: Via sodium-potassium pump to re-establish resting potential.

All-or-Nothing Principle of Action Potentials

  • The axon hillock must meet the -55 mV threshold to fire an action potential; all potentials below that will not generate an AP.
  • Once the threshold is surpassed, an action potential occurs uniformly or not at all.

Comparison Between Action and Graded Potentials

Graded Potentials

  • Type: Input signal.
  • Location: Dendrites and cell body.
  • Channels: Mechanically or chemically gated.
  • Ions: Na⁺, K⁺, Ca²⁺.
  • Response: Depolarizing or hyperpolarizing, can sum.

Action Potentials

  • Type: Regenerating conduction signal.
  • Location: Axon.
  • Channels: Voltage-gated only.
  • Ions: Na⁺ and K⁺.
  • Response: Only depolarizing, all-or-nothing, cannot sum.

Effects of Ion Concentration Changes on Neurons

  • Hyperkalemia: Increased K⁺ in blood makes neurons more excitable, subthreshold stimuli can trigger AP.
  • Hypokalemia: Decreased K⁺ makes neurons less excitable; stronger stimuli needed to fire AP.

Absolute vs. Relative Refractory Periods

  • Absolute Refractory Period (ARP): No second AP can occur due to closed VGNa channels.
  • Relative Refractory Period (RRP): AP can occur only with a stronger than normal stimulus due to partially reset channels.

Directional Conductance of Action Potentials

  • Action potentials usually conduct forward due to the inactivation of previous VGNa channels in the ARP, preventing backward propagation.

Myelination and Action Potential Conduction

  • Myelin Sheath Role: Prevents current leakage; facilitates faster signal transmission through saltatory conduction.
  • Nodes of Ranvier: Unmyelinated sections containing VGNa and VK channels enable rapid AP propagation.
  • Demyelination Effects: Slows conduction, as seen in multiple sclerosis (MS).

Synapse and Neurotransmitter Release

  • EPSP (Excitatory Post Synaptic Potential): Depolarization response.
  • IPSP (Inhibitory Post Synaptic Potential): Hyperpolarization response.

Summation Types

  • Spatial Summation: Multiple stimuli received simultaneously from different locations.
  • Temporal Summation: Consecutive stimuli from the same source close in time.

Fate of Signals at Axon Hillock

  • If a neuron's axon hillock receives multiple signals, the net resultant potential determines whether an action potential occurs based on the cumulative effect of EPSPs and IPSPs.

Comparison of Autonomic Nervous System Pathways

Sympathetic Pathways

  • Overall function: Fight-or-flight responses.
  • Origin: Thoracic and lumbar spinal cord.
  • Ganglion location: Close to the spinal cord.
  • Neurotransmitters: Postganglionic neurons secrete norepinephrine (NE).

Parasympathetic Pathways

  • Overall function: Rest-and-digest responses.
  • Origin: Brainstem and sacral spinal cord.
  • Ganglion location: Close to or on target organs.
  • Neurotransmitters: Both pre- and postganglionic neurons secrete acetylcholine (ACh).

Norepinephrine vs. Epinephrine

  • Norepinephrine: Secreted from sympathetic postganglionic neurons.
  • Epinephrine: Released from adrenal medulla chromaffin cells; considered a neurohormone as it travels via the bloodstream.

Sensory Receptors and Neural Pathways

  • Mechanoreceptors: Respond to mechanical events; examples include pressure and sound.

  • Chemoreceptors: Initiate upon binding to chemical ligands; examples are taste and smell.

  • Photoreceptors: Detect light for vision.

  • Thermoreceptors: Detect temperature changes.

  • Neural Sensory Receptors: Directly convey stimuli as action potentials; receptor potentials dictate functionality.

  • Non-Neural Sensory Receptors: Indirectly influence afferent neurons to initiate action potentials via neurotransmitter release.

Vision Sensing: Rods vs. Cones

  • Rods: Specialized for low-light environments.
  • Cones: Enable color vision in bright environments.

Auditory Sensing Mechanism

  • Sound waves enter through the ear canal, triggering mechanical changes that lead to neural signaling in the auditory cortex.

Taste Sensations

  • Five primary taste sensations include sweet, sour, salty, bitter, and umami.

Olfactory Pathway

  • Odorants enter the nasal cavity, stimulating olfactory receptors that transmit signals to the olfactory cortex for perception.