02. Hypothalamus & Pituitary Hormones

Label:

A: hypothalamus

B: infundibulum

C: anterior pituitary

D: posterior pituitary

Define: Hypophysiotropic Hormones

hypothalamic hormones that regulate anterior pituitary fxn

How do hypophysiotropic hormones reach the anterior pituitary gland?

hypothalamo-hypophyseal portal vessels

neurons synthesize trophic neurohormones → release into capillaries of portal system → portal veins carry to anterior pituitary → act on endocrine cells → endocrine cells release peptide hormones into second set of capillaries for distribution to body

Basic Format of Three-Hormone Sequence

stimulus → hypothalamus → anterior pituitary → third endocrine gland → target

Three-Hormone Sequences

Which anterior pituitary hormones are tropic?

What does this mean?

FSH, LH, TSH, & ACTH → target endocrine glands & stimulate release of other hormones

Two Mechanisms to Regulate Hypophysiotropic Hormones

1. Neural control → stimulatory/inhibitory input from CNS

2. Negative feedback control → high levels of final hormone in pathway stops its own production (inhibit secretion from hypothalamus & anterior pituitary) → hormone three not in excess

Negative Feedback Control: Short-Loop vs. Long-Loop Feedback

Short: hormone 2 inhibits release of hormone 1

Long: hormone 3 inhibits release of hormones 1 & 2

Feedback Loop for CRH

Production of Posterior Pituitary Hormones

1. produced & packaged in cell body of neuron in hypothalamus

2. vesicles transported down cell → stored & released into the blood in/from posterior pituitary

What are the two posterior pituitary hormones?

What nucleus is responsible for secreting each?

oxytocin → from supraoptic nucleus

vasopressin (antidiuretic hormone) → from paraventricular nucleus

Three Primary Functions of Oxytocin

• promotes milk production & ejection reflex of lactation

• uterine contractions during childbirth

• emotional/social bonding

Three Primary Functions of Vasopressin (ADH)

• regulation of blood pressure (via vessel constriction)

• regulation of water balance

• regulation of osmolarity

Positive Feedback Loops of Oxytocin

Childbirth

• oxytocin prompts uterine contractions → pushes head twd cervix → causes cervical stretching (sensed by mechanoreceptors) → stimulates more oxytocin synthesis/release

Breastfeeding

• suckling activates nipple mechanoreceptors → triggers oxytocin release → cause milk ducts to contract → milk ejected → stimulate further oxytocin release

What is the effect of ADH in the kidneys? in the blood vessels?

Kidneys → inc. water reabs. (reduce urine production & conserves water) → helps maintain blood osmolarity

Blood vessels → lead to vasoconstriction → inc. peripheral resistance & inc. BP

Regulation of ADH Release

Neg. feedback

1. blood osmolarity inc.

2. inc. ADH secretion

3. inc. water reabs. in kidneys (helps maintain blood volume/pressure)

4. blood osmolarity dec. to normal

5. dec. ADH secretion

inc. BP → detected by stretch receptors → inhibit ADH secretion

dec. BP → reduce inhibition of ADH secretion → promote ADH release

Vasopressin (ADH) Effect at Kidneys

ADH Abnormalities: Alcohol

inhibits ADH release → inc. urine production, dehydration, & hangoverA

ADH Abnormalities: Diabetes Insipidus

(symptoms similar to diabetes but not related to blood sugar)

chronic underproduction of ADH → excessive urination (polyuria) & chronic dehydration

ADH Abnormalities: Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)

excess ADH release/action, causing…

• abnormal water retention

• diluted blood Na+ levels (hyponatremia)

• blood hypo-osmolality (dilute blood)

• high urine osmolality

Four Influences on Normal Growth

1. growth hormone + other hormones → IGF-1, IGF-2, T3, insulin, sex hormones, cortisol

2. adequate diet → enough protein, fatty acids, vitamins, minerals

3. absence of chronic stress

4. genetics

Release & Delivery of Growth Hormone (GH)

release stimulated by growth hormone-releasing hormone (GHRH)

release inhibited by growth hormone-inhibiting hormone/somatostatin (GHIH/SST)

GH binds growth hormone-binding protein (GHBP) → very important so bind to ensure proper delivery as peptides have shorter life → stimulate IGF secretion

Function of GH & IGF Together

• stimulate protein synthesis

• inc. lipolysis (fat breakdown) & gluconeogenesis (new glucose production)

• dec. glucose uptake → inc. blood glucose

• inc. bone growth

Feedback Control of GH Secretion

Define: Epiphyseal Growth Plate

site of growth in length of bone → composed of hyaline cartilages & located b/w epiphysis & diaphysis (shaft)

Define: Chondrocytes

cartilage cells in the epiphyseal plate → are target cells for GH & IGF-1

Label: Cross Section of Bone

Define: Osteoblasts

bone makers → secrete bone matrix which later mineralize to form bone

opp. of osteoclasts

Define: Osteoclasts

bone breakers → responsible for bone resorption → break down bone & releasing minerals (e.g. Ca2+) into blood

opp. of osteoblasts

Define: Osteocytes

bone maintainers → regulate bone remodeling by directing -blast & -clast activity → maintain mineral content of surrounding matrix

Define: Epiphysis vs. Diaphysis

epiphysis: end of long bone

diaphysis: shaft of long bone

separated by epiphyseal plate → site of growth

Composition of Bone

mostly calcified extracellular matrix

by weight…

• ~60-65% inorganic material → primarily hydroxyapatite (calcium phosphate minerals) → provides hard structure

• ~30-35% organic material → mainly collagen & other proteins

• ~5-10% water

Steps of Bone Growth

1. chondrocytes produce new cartilage (by dividing) in the epiphyseal plate

2. epiphyseal plate widens → bone lengthens

3. chondrocytes undergo hypertrophy  then die

4. osteoblasts create calcium phosphate crystals to replace cartilage (chondrocytes) → lay down bone

Epiphyseal Plate Closure / Growth Plate Fusion

senescence of growth plate near end of puberty → chondrocytes die & cartilage replaced by mature bone

end of bone's longitudinal growth

GH Disorders: Gigantism

in children before growth plate fusion

abnormally rapid growth, excessive height, large hands/feet, delayed puberty

caused by benign pituitary tumor (adenoma) → producing excess GH

GH Disorders: Acromegaly

in adults after growth plate fusion

gradual enlargement of bones/soft tissues, enlarged extremities, coarsened facial features, thickened skin

caused by benign pituitary tumor (adenoma) → producing excess GH