M2(1) The Endocrine System - Intro + Hypothalamus + Pituitary Glands

Nervous System Comparison

Initiates responses rapidly

Short-duration responses

Acts via action potentials and neurotransmitters

Acts at specific locations determined by axon pathways

Neurotransmitters act over very short distances

Endocrine System Comparison

Initiates responses slowly

Long-durations responses

Acts via hormones released in the blood

Acts at diffuse locations (targets anywhere blood reaches)

Hormones act over long distances

Endocrine Organs

• Pineal gland

• Hypothalamus

• Pituitary gland

• Thyroid gland

• Parathyroid glands (located on dorsal aspect of thyroid gland)

• Thymus

• Adrenal glands

• Pancreas

• Gonads

Fat Soluble Hormones

Nonpolar / hydrophobic

• Can cross the cell mb easily

Receptor located inside cell

Fat Soluble Hormone Steps

1. Fat Soluble Hormone Diffusion
   Fat soluble hormone (e.g. steroid) diffuses through the plasma mb and binds to an intracellular receptor

2. Intracellular Receptor Binding
   Steroid binds to a receptor inside the cell, forming a hormone-receptor complex

3. Nuclear Translocation
   If the receptor is in the cytoplasm, the hormone-receptor complex moves into the nucleus

4. DNA Binding
   The complex binds to a specific section of the DNA

5. Gene Transcription
   Binding of the complex activates transcription of the target gene → mRNA is produced

6. Protein Synthesis
   mRNA directs protein synthesis

Water Soluble Hormones

Polar / hydrophilic

• Cannot cross the cell mb easily

Receptor located on the cell mb

Water Soluble Hormone Steps

1. Water-Soluble Hormone (first messenger)
   Hormone (e.g. glucagon) binds to receptor on the cell membrane, causing a conformational change

2. G protein activation
   The activated receptor causes the G protein to exchange GDP for GTP, activating the G protein.

3. Adenylate cyclase activation
   The activated G protein stimulates adenylate cyclase in the membrane.

4. cAMP (second messenger)
   Adenylate cyclase converts ATP → cAMP

5. Protein kinase activation
   cAMP activates protein kinase

6. Cellular response
   Activated PKA phosphorylates target proteins, leading to a physiological response
   → e.g., glycogen breakdown and glucose release from liver cells.

Fat Soluble Hormone Comparison

Includes: All steroid hormones and thyroid hormone

Sources: Adrenal cortex, gonads, thyroid gland

Stored in Secretory Vesicles: No

Transport in Blood: Bound to plasma proteins

Half-Life in Blood: Long (most need to be metabolized by liver)

Location of Receptors: Typically inside cell

Mechanism of Action in Target Cell: Activate genes, causing synthesis of new proteins

Water Soluble Hormone Comparison

Includes: All AA-based hormones except thyroid hormone

Sources: All other endocrine glands

Stored in Secretory Vesicles: Yes (exit cell via exocytosis)

Transport in Blood: Typically free in plasma

Half-Life in Blood: Short (most can be removed by kidneys)

Location of Receptors: On plasma mb

Mechanism of Action in Target Cell: Typically acts through second-messenger systems

Endocrine Gland Stimuli

3 types

• Humoral Stimulus

• Neural Stimulus

• Hormonal Stimulus

Humoral Stimulus

Hormone release is triggered by changes in blood levels of ions or nutrients.

Example:

↓Blood calcium → parathyroid gland releases PTH (parathyroid hormone) → raises blood calcium

Neural Stimulus

Hormone release is triggered by direct neural input from the nervous system.

Example:

Sympathetic nervous system stimulates adrenal medulla → releases epinephrine/norepinephrine during stress (“fight or flight”)

Hormonal Stimulus

Hormone release is triggered by another hormone.

Example:

• Hypothalamus releases hormone → anterior pituitary releases hormone → thyroid releases hormone

Tropic Hormone

A hormone that primarily stimulates another endocrine gland to release its hormone

• Rather than directly causing an effect on target tissues

Hypothalamus

Hypothalamic neurons synthesize oxytocin and ADH

Controls the pituitary gland

• Anterior pituitary: via releasing/inhibiting hormones (GHRH, GHIH, TRH, CRH, GnRH, PIH)

• Posterior pituitary: stores and releases hormones made by the hypothalamus

Posterior Pituitary

Does not make hormones itself

• Stores hormones made in the hypothalamus (oxytocin, ADH)

Hypothalamus communicates via neurons that deliver hormones

Oxytocin

Stimulus: Impulses from hypothalamic neurons in response to infant suckling, stretching of uterus during labour

Inhibited: Lack of appropriate neural stimuli

Target Organ: Uterus, Breast

Effects: Stimulates uterine contractions, Milk ejection

Hyposecretion Effects: Unknown

Hypersecretion Effects: Unknown

Antidiuretic Hormone (ADH)

Stimulus: Impulses from hypothalamic neurons in response to increased blood solute concentration or decreased blood volume

Inhibited: Adequate hydration of the body or alcohol

Target Organ: Kidneys

Effects: Stimulates kidney tubule cells to reabsorb water from forming urine back into blood

Hyposecretion Effects: Syndrome of inappropriate ADH secretion

Hypersecretion Effects: Diabetes Insipidus

Anterior Pituitary

Produces hormones

• GH (growth hormone),

• TSH (thyroid-stimulating hormone),

• ACTH (adrenocorticotropic hormone),

• FSH (follicle-stimulating hormone),

• LH (luteinizing hormone)

• PRL (prolactin)

Hypothalamus communicates via blood vessels (by releasing hormones)

Growth Hormone (GH)

Stimulus: GHRH release (triggered by ↓GH in blood)

• Secondary Triggers: deep sleep, hypoglecemia,↑ AAs in blood, ↓ FFAs, exercise, other stressors

Inhibited: Feedback inhibition of GH and insulin-like growth factors (IGFs)

• Hyperglycemia, hyperlipidemia, obesity, emotional deprivation (either increased GHIH or decreased GHRH release)

Target Organ: Liver, muscle, bone cartilage, other tissues

Effects: Stimulates somatic growth, mobilizes fats, spares glucose

• Indirect growth-promoting effect (by IGFs)

Hyposecretion Effects: Pituitary dwarfism in children

Hypersecretion Effects: Gigantism in children; acromegaly in adults

Thyroid-Stimulating Hormone (TSH)

Stimulus: TRH

• Indirectly in infants (by cold temperature)

Inhibited: Feedback inhibition by thyroid hormones on anterior pituitary gland and GHIH

Target Organ: Thyroid Gland

Effects: Stimulates thyroid gland to release thyroid hormones

Hyposecretion Effects: Hypothyroidism (effects similar to Graves’ disease, antibodies

Hypersecretion Effects: Hyperthyroidism (may cause myxedema)

Adrenocorticotropic Hormone (ACTH)

Stimulus: CRH

• Secondary Triggers: fever, hypoglycemia, other stressors

Inhibited: Feedback inhibition by glucocorticoids

Target Organ: Adrenal Cortex

Effects: Stimulates adrenal cortex to promote release of glucocorticoids and androgens

Hyposecretion Effects: Rare

Hypersecretion Effects: Cushing’s Disease

Follicle Stimulating Hormone (FSH)

Stimulus: GnRH

Inhibited: Feedback inhibition by inhibin

• Estrogen (females)

• Testosterone (males)

Target Organ: Ovaries / Testes

Effects:

• Stimulate ovarian follicle maturation and production of estrogen (females)

• Stimulate sperm production (males)

Hyposecretion Effects: Failure of sexual maturation

Hypersecretion Effects: No important effects

Luteinizing Hormone (LH)

Stimulus: GnRH

Inhibited: Feedback inhibition by

• Estrogen and progesterone (females)

• Testosterone (males)

Target Organ: Ovaries / Testes

Effects:

• Triggers ovulation, stimulates ovarian production of estrogens and progesterone (females)

• Promotes testosterone production (males)

Hyposecretion Effects: Failure of sexual maturation

Hypersecretion Effects: No important effects

Prolactin (PRL)

Stimulus: ↓PIH

• release enhanced by estrogens, birth control pills, breast feeding, dopamine-blocking drugs

Inhibited: Feedback inhibition by PIH (dopamine)

Target Organ: Breast Secretory Tissue

Effects: Promotes lactation

Hyposecretion Effects: Poor milk production in nursing women

Hypersecretion Effects: Galactorrhea (inappropriate milk production), cessation of menses (females), impotence (males)