Physiology 1, Fall 2008, LPC

Chapter 7- Introduction to Endocrine System

I-General
A. Definition of Hormone – a chemical secreted by a cell or group of cells into the blood for transport to a distant target, where it exerts its effect at a very low concentration

Discrete glands, isolated cells (diffuse system), neurohormones, cytokines (from immune system)

Act at nanomolar (10-9 M) to picomolar (10-12 M) concentrations

B. Receptor Binding – bind to target cell receptors and initiate biochemical responses

Variable responsiveness of a cell to a hormone depends on cell’s receptor and signal transduction pathways

C. Termination of Action – response must be limited so that body can respond to a variety of conditions

Circulating hormone can become inactivated via liver (-> bile) or kidneys (-> urine)

Half-life = time for half of circulating hormone to be degraded

Membrane receptor-bound hormone can be degraded by:

Enzymes
Endocytosis, then digestion in lysosomes

Figure 7-2 – learn location, gland or cell, chemical class, hormones, targets, and main effects of hypothalamus, posterior pituitary, anterior pituitary, thyroid, pancreas, adrenal cortex, adrenal medulla, testes, ovaries

 

II-Classification

  1. Peptide or Proteins – most common

Synthesis – ribosomes -> preprohormones with signal sequence
Rough endoplasmic reticulum
Golgi – remove signal sequence (-> prohormone), then peptide fragments -> active hormone

Secretory vesicles containing peptides stored in cytoplasm, secreted upon signal
Exocytosis

Peptide hormones – water soluble, short half-life in serum

Mechanism of action

Bind to surface membrane receptor
Hormone-receptor complex initiates response via signal transduction system

  1. Steroids

Made in adrenal cortex, gonads, placenta (female)
Lipophilic, so diffuse through membranes
Made when needed
Bound to protein carriers in blood
Only unbound hormone can diffuse into target cell. Minute amount unbound in plasma but only takes minute amount for effect.

Steroid binds to receptor, usually in cytoplasm or nucleus.
Steroid-hormone complex to nucleus, acts as transcription factor -> represses or activates transcription
If activates transcription, -> translation -> new proteins
Takes time (minutes)

  1. Amines

Catecholamines – epinephrine, norepinephrine, dopamine

Neurohormones act like peptide hormones

Thyroid hormone

Behave as steroid hormone to activate genes

 

III-Control of Release

  1. Reflex Pathways

Stimulus, input signal, integration of the signal, output signal, response
Output signal = hormone or neurohormone
Negative feedback signal turns off reflex

  1. Simple

Endocrine cell directly senses a stimulus and responds by secreting its hormone. Endocrine cells acts as both sensor and integrating center. Ex = parathyroid and calcium

  1. Complex

More than one stimulus for endocrine response. Ex = insulin secretion. Increased blood sugar or stretch receptor in digestive tract -> CNS -> pancreas -> release insulin

  1. Reflexes Involving the Nervous System – nervous system & endocrine system overlap in structure & function

E.Neurohormones

Chemical signals released in to the blood by a neuron
Three major groups:

  1. catecholamines from modified neurons in adrenal medulla
  2. hypothalamic neurohormones from posterior pituitary
  3. hypothalamic neurohormones that control hormone release from the anterior pituitary
  1. Pituitary

Two fused glands.
Posterior (neurohypophysis) = extension of neural tissue of the brain, secretes neurohormones made in the hypothalamus.
Anterior (adenohypophysis) = pouch formed from roof of mouth, secretes 5 trophic hormones and prolactin.

    1. Posterior Pituitary

    Neurohormones made in hypothalamus, travel in vesicles though cell extension to be stored in cell ending in posterior pituitary.
    Hormones released into vein in posterior pituitary to blood circulation.

    1. Anterior Pituitary

    Neurons in hypothalamus secrete hormones into blood vessels which form portal system to anterior pituitary.
    Signal received by specific endocrine cells in anterior pituitary make hormones, all of which are trophic, except prolactin.
    Anterior pituitary hormones signal target glands or organ to make and release their specific hormone.

G.Hypothalamic-hypophpyseal portal system

Hypothalamic hormones dumped into closed circulatory system to anterior pituitary.
A little dab (of hormone) will do it.

  1. Hypothalamic-pituitary pathway feedback loops

Negative feedback loops.
Short-loop = hormone has negative feedback to gland that directly controls it.
Long-loop = hormone has negative feedback to all glands that control it.

  1. Anterior Pituitary hormones control growth, metabolism and reproduction.  More later when study organ systems.

 

IV-Interactions

  1. Synergism– effects more than additive
  1. Permissiveness – hormone necessary for another hormone to produce its effect

Ex – thyroid hormone necessary for tissues to develop and function

  1. Antagonistic – two hormones with opposing effects

Ex – insulin (decreases blood sugar) & glucagon (increases blood sugar)

 

V-Pathology

A. Hypersecretion – too much hormone

Caused by too much secretion by last endocrine gland in reflex (primary) OR too much trophic hormone or trophic hormone mimic (secondary)

Down-regulation – target tissue decreases number of receptors to try to diminish responsiveness

B. Hyposecretion – too little hormone

Caused by too little secretion by last endocrine gland in reflex (primary OR too little trophic hormone or trophic hormone mimic (secondary)

Up-regulation – target tissue increases number of receptors to try to increase responsiveness

C.Receptor or 2nd Messenger problems

Hormone may be normal but receptor non-functional, -> abnormal or no response
2nd messenger may not function properly, -> symptoms of hormone excess or deficiency