endocrine system pt 2 extra notes

Posterior Pituitary Gland Physiology

Hypothalamic Control of Posterior Pituitary

• hormones are produced in neurons that are in the hypothalamus

  • hypothalamic nuclei are located in the hypothalamus

    • hormones are transported along axon of hypothalamo-hypophyseal tract

      • hypothalamo-hypophyseal tract: tract of nerve fibers (axons) that transports antidiuretic hormone and oxytocin from the hypothalamus to the posterior pituitary.

• hormones are released from synaptic knob into capillary bed, entering the blood stream in the posterior pituitary

  • neurons axons extend into posterior pituitary gland, releasing hormones directly into capillary bed

• although the messenger signal is made in a neuron, it is called hormones (instead of neurotransmitter) because released directly into blood

  • would be called neurotransmitter if it was released directly on synapse

    

2 Hormones Released by the Posterior Pituitary Lobe:

1. Antidiuretic Hormone (ADH)

• Target: Kidneys and blood vessels.

• Effects:

  • stimulation of water retention in kidneys

    • less water is secreted in the urine

    • what gives the hormone its name

  • vasoconstriction of blood vessels.

    • rise in blood pressure due to vasoconstriction

  • Secreted when hydration levels are low (dehydration).

• Regulation:

  • secretion of ADH is stimulated by hypothalamic osmosensors and angiotensin II

    • osmoreceptors neurons: A sensory neuron in hypothalamus that responds to changes in the osmotic pressure of the surrounding fluid.

  • secretion of ADH is inhibited by Atrial baroreceptors

    • baroreceptors is a type of mechanoreceptor that is a stretch receptor

    • located in left atrium of the heart

    • stimulated when there is a rise in blood volume, causing inhibition

2. Oxytocin

• Target: Mammary glands and uterus.

• Effects:

  • Triggers milk ejection during lactation

    • stimulates contraction of mammary gland alveoli and ducts

  • stimulates uterine contractions during childbirth.

• Regulation: More complex regulation, particularly in reproductive scenarios.

Anterior Pituitary Gland Physiology

Hypothalamic Control of Anterior Pituitary

• Axons are only in hypothalamus, do not enter anterior pituitary lobe

• Neurons secrete regulatory hormones into hypophyseal portal system

  • hypophyseal portal system: vascular system with contains two capillary beds, transporting hormones from the hypothalamus to the anterior pituitary

• regulatory hormones are secreted into first capillary bed at the base of the hypothalamus

  • then delivered to another capillary bed in anterior pituitary gland

• Regulatory hormones either stimulate or inhibit release of hormones

  • Negative feedback loop can either work at the top in the hypothalamus or at the bottom in the anterior anterior pituitary lobe

    

Hypothalamic Regulatory Hormones: hormones released in the hypothalamus

1. Growth Hormone-Releasing Hormone (GHRH)

2. Growth Hormone-Inhibiting Hormone (GHIH)

3. Thyroid-Releasing Hormone (TRH)

4. Corticotropin-Releasing Hormone (ACTH)

5. Prolactin-Inhibiting Hormone (PIH)

6. Gonadotropin-Releasing Hormone (GRH)

Anterior Pituitary Hormones: hormones released at anterior pituitary gland

1. Growth Hormone (GH)

2. Thyroid-stimulating Hormone (TSH)

3. Adrenocorticotropic Hormone (ACTH)

4. Follicle-stimulating Hormone (FSH)

5. Luteinizing Hormone (LH)

6. Prolactin (PRL)

1. Growth Hormone (GH): released at anterior pituitary gland

• Targets: Bone, muscle, adipose tissue, cartilage, and many others

• Effects:

  • Stimulates growth in bone, cartilage, and muscle

  • increase lipolysis and lipid immobilization

    • break down of lipids (fats) and mobilizes it

• Hypothalamic Regulatory Hormones:

  • Growth hormone-releasing hormone (GHRH): stimulates release of growth hormone

  • Growth hormone-inhibiting hormone (GHRH): inhibits release of growth hormone

• Hypothalamic-Pituitary-Somatotropic Axis - Hypothalamic Regulation of Growth Hormone:

  • Negative feedback loop

  • Pituitary gland secretes growth hormone (GH)

    • increase of growth hormone (GH) causes increase in somatomedins from the liver

      • increase of somatomedins levels blocks growth hormone-release hormone AND stimulates growth hormone-inhibiting hormone in the hypothalamus

  • NOT negative product inhibition

    

2. Adrenocorticotropic Hormone (ACTH): released at the pituitary gland

• adreno: target of hormone

• cortico: cortisol

• tropic: ACTH is a tropic hormone

  • Tropic Hormone: target is another endocrine gland

    • ACTH is released from pituitary gland (endocrine gland) and targets adrenal gland (another endocrine gland)

  • Trophic Hormone: causes growth in target

• Target: Adrenal cortex.

  • (adrenal medulla is stimulated by Sympathetic Nervous System from hypothalamus)

• Effects: Stimulates the secretion of glucocorticoids (e.g., cortisol)

  • glucocorticoids: hormones whose effects include glucose metabolism

    • also stress hormones

  1. increases gluconeogenesis

     • gluconeogenesis: production of glucose from a non-carbohydrate

  2. increases lipolysis

  3. increase protein degeneration

     • protein degeneration: protein digestion

  4. depresses immune/inflammatory response

     • all stress hormones have same functions on immune system

• Hypothalamic Regulatory Hormones:

  • Corticotropin-Releasing Hormone (CRH)

    • tropic hormone

    • targets pituitary gland to stimulate release of Adrenocorticotropic Hormone (ACTH), causing release of cortisol

      • stress stimulates higher brain centers $\rightarrow$ higher brain centers stimulate hypothalamus to release CRH $\rightarrow$ CRH stimulates anterior pituitary gland to release ACTH $\rightarrow$ ACTH targets adrenal cortex $\rightarrow$ adrenal cortex increases cortisol levels

• Hypothalamic-Pituitary-Adrenal Axis:

  • anterior pituitary often has negative feedback inhibitions at two levels:

    1. hypothalamus

       • increase of cortisol inhibits hypothalamus’ secretion of Corticotropin Releasing Hormone (CRH)

         • hypothalamus has cortisol receptors that detect elevated cortisol levels

           • hypothalamus acts as sensor and integrating center

         • leads to a decrease in CRH production

           • adrenal cortex acts as effector, decreasing amount of cortisol being secreted

    2. Pituitary

       • increase in cortisol inhibits anterior pituitary’s responsiveness of Corticotropin Releasing Hormone (CRH)

         • anterior pituitary has cortisol receptors detect elevated cortisol levels, decreasing responsiveness to CRH, leading to a reduction in Adrenocorticotropic Hormone (ACTH) secretion and thus lower cortisol production from the adrenal glands.

           • anterior pituitary acts as sensor, integrating center and effector

             

• Adrenal Insufficiency Disease: disease involving hypo-secretion (not enough secretion) of glucocorticoids

  • Two Types:

    • Primary Adrenal Insufficiency

      • disease of adrenal cortex

        • adrenal cortex is stimulated by ACTH but is unable to produce cortisol

      • most common type: Addison’s disease

        • autoimmune disease of the adrenal cortex

        • other causes include congenital, drug-related, infections

        • common symptoms: hair loss, blurred vision, abdominal pain, decreased appetite, darkening of skin, shaking or tremors, depression

    • Secondary Adrenal Insufficiency

      • disease at the anterior pituitary where it does not produce enough ACTH, leading to inadequate stimulation of the adrenal cortex and resulting in decreased cortisol production.

        

  • Testing to diagnose primary or secondary adrenal insufficiency: injection of ACTH

    • if nothing happens after injection, diagnosis is primary adrenal insufficiency

      • problem is in the adrenal cortex because adrenal cortex does not increase levels of cortisol

    • if injection causes an production of cortisol, diagnosis is secondary adrenal insufficiency

      • problem is in the anterior pituitary because it fails to produce adequate amounts of adrenocorticotropic hormone (ACTH), which stimulates the adrenal cortex to release cortisol.

• Cushing's Syndrome: hyper-secretion (excessive secretion) of glucocorticoids causing increased lipolysis and redistribution of fat

  • Two types:

    • Primary Cushing Syndrome: typically from tumor in adrenal cortex

      • excessive cell division of adrenal cortex cells, causing more cells to secrete cortisol

    • Secondary Cushing Syndrome: typically from tumor in pituitary gland

      • excessive cell division of pituitary gland cells, causing more cells to secrete ACTH, increasing cortisol levels

        

        
        

3. Thyroid-Stimulating Hormone (TSH): released at anterior pituitary gland

• Target: Thyroid gland.

  • tropic hormone AND Trophic hormone

    • tropic: released by anterior pituitary gland

    • trophic: increase size of thyroid

• Effects: Stimulates the secretion of thyroid hormones, Triiodothyronine (T3) and Thyroxine (T4)

  • increases basal metabolic rate and body temperature

  • increases catabolism of carbohydrates and proteins

• Hypothalamic Regulatory Hormones:

  • Thyrotropin Releasing Hormone (TRH)

    • hypothalamus secretes Thyrotropin Releasing Hormone (TRH) $\rightarrow$ TRH triggers anterior pituitary gland to produce thyroid-stimulating hormone (TSH) $\rightarrow$ TSH causes thyroid to secrete Triiodothyronine (T3) and Thyroxine (T4) AND increases growth of thyroid

• Hypothalamic-Pituitary- Thyroid Axis:

  • Negative Feedback Inhibition at two levels:

    • hypothalamus

      • increase of T3 and T4 inhibits hypothalamus’ secretion of Thyrotropin Releasing Hormone (TRH)

        • hypothalamus has receptors that detect elevated T3 and T4 levels

          • hypothalamus acts as sensor and integrating center

        • leads to a decrease in TRH production

          • thyroid acts as effector, decreasing amount of T3 and T4 being secreted

    • Pituitary

      • increase in T3 and T4 inhibits anterior pituitary’s responsiveness of Thyrotropin Releasing Hormone (TRH)

        • anterior pituitary has T3 and T4 receptors detect elevated T3 and T4 levels, decreasing responsiveness to TRH, leading to a reduction in TSH secretion and thus lower T3 and T4 production from the thyroid glands

          • anterior pituitary acts as sensor, integrating center and effector

            

• Synthesis of thyroid hormone requires iodide (iodine)

  • iodide must be in diet

    • if there is not enough iodide in diet, there is no production of Triiodothyronine (T3) and Thyroxine (T4)

  • Goiter Disease: insufficient dietary iodide leads to hypertrophy of thyroid gland

    • excessive growth of thyroid gland

    • if not enough iodide, leads to low levels of T3 and T4

      • results in lack of negative feedback inhibition causing anterior pituitary gland to secrete excessive TSH

        • excessive TSH causes abnormal growth of thyroid

          

• Graves’ Disease: an autoimmune disease, caused by auto-antibodies binding to TSH receptors, which stimulates overproduction of thyroid hormones

  

Hypothalamic Regulation of Adrenal Medulla

Sympathetic Nervous System Control of Adrenal Medulla

• adrenal medulla is stimulated by hypothalamus through sympathetic innervation

• Effects: stimulate secretion of epinephrine and norepinephrine, which target other organs:

  • increase heart rate

  • dilate bronchioles

  • vasoconstriction in skin

  • vasodilation in muscle

“Fight or Flight” Response

Hormonal Regulation of Blood Calcium Levels

Calcitonin and Parathyroid Hormone (PTH)

• Calcitonin: produced by thyroid glands, inhibits dissolution of Ca2+ from bone and stimulates Ca2+ excretion

  • if blood calcium is too high, thyroid gland produces calcitonin

    • Lowers blood calcium by causing calcium deposition in bone (inhibiting osteoclast activity) and increasing renal excretion of calcium

      • leads to blood calcium levels decreasing

• Parathyroid Hormone. (PTH): Produced by parathyroid glands, promotes dissolution of Ca2+ from bone and inhibits Ca2+ excretion

  • if blood calcium is low, parathyroid gland produces parathyroid hormone

    • raises blood calcium by releasing stored calcium from bone (stimulating osteoclasts and inhibiting osteoblast activity) and enhancing reabsorption of calcium in kidneys (decrease calcium excretion in kidneys)

      • increases blood calcium levels

• Calcitonin and Parathyroid Hormone work in antagonist effects

Therapeutic Use of Cortisol Mimics

• Cortisol Mimics: Synthetic molecules that act like cortisol, binding to cortisol receptors, causing same physiological response as cortisol

  • most often used for immunosuppressive effects

    • depresses immune’s systems response and reduces inflammation

• Common Medications:

  • Prednisone

  • Prednisolone

  • Dexamethasone

• Caution: Long-term use can lead to adrenal suppression

  • careful dosing and tapering are necessary to prevent withdrawal symptoms.