Physiology: Hypothalamus and Pituitary Gland (Lecture 14)

Learning Objectives:

**Detailed Learning Objectives**

What is the hypothalamus?

- connects the brain to the rest of the body

- regulates hormonal output to the rest of the body

- releases chemical as hormones through the rest of the body

- controls the pituitary gland and systemic hormonal processes

Describe the anatomical relationships between the hypothalamus and the anterior/posterior pituitary glands. (come back to this)

- hypothalamus: hypo = below thalamus

- pituitary = "stalk"; extends below brain

- anterior = true glandular tissue; true endocrine cells;

- posterior pituitary = extension of the brain; axons that are coming down from cell bodies in the hypothalamus terminate and comprise the posterior back half of pituitary

- congenital changes in hormone release; car accidents, TBIs, etc.

- delicate part of brain where patients can develop endocrine disorders based on injury or dysregulation

What are the four steps in which the hypothalamus controls the anterior pituitary?

1. hypothalamic neurons release a hormone into the median eminence/portal system

2. hormone binds to pituitary target cells

3. pituitary target cells ("trophs") release a second hormone

4. second hormone is released into the pituitary bloodstream

What is Prolactin? What does it do?

What are Luteinizing Hormone and Follicle-Stimulating Hormone (LH and FSH)?

- important in male/female reproduction

What is the posterior pituitary? What does it do?

- releases oxytocin and vasopressin

Which hypothalamic factor stimulates corticotrophs, and what do they release?

- hypothalamic factor CRH → stimulates corticotrophs → releases ACTH (adrenocorticotropin) and β-endorphin

1. hormone 1 (CRH) travels from hypothalamus to the anterior pituitary and acts on the anterior pituitary cells (corticotrophs); corticotrophs have a CRH receptor and can respond to that

2. corticotrophs make hormone 2 (ACTH and β-endorphin)

Which hypothalamic factor stimulates thyrotrophs, and what do they release?

hypothalamic factor TRH → stimulates thyrotrophs → releases TSH (thyroid-stimulating hormone)

Which hypothalamic factor stimulates gonadotrophs, and what do they release?

hypothalamic factor GnRH → stimulates gonadotrophs → releases LH (luteinizing hormone) and FSH (follicle-stimulating hormone)

Which hypothalamic factor stimulates somatotrophs, and what do they release?

hypothalamic factor GHRH → stimulates somatotrophs → releases Growth Hormone

Which hypothalamic factor regulates lactotrophs, and what do they release?

hypothalamic factor Dopamine (inhibits) → stimulates lactotrophs → releases Prolactin (normally kept turned off by dopamine)

1. Dopamine is released from the hypothalamus and travels to the anterior pituitary and acts on lactotrophs (associated with lactation)

2. Normally turned off

What are the three steps of hormone release in the hypothalamic–pituitary–endocrine axis?

1. Hypothalamus (Hormone 1) →

2. Anterior Pituitary (Hormone 2) →

3. Endocrine Gland (Hormone 3)

What is short-loop feedback in the hypothalamic–pituitary axis?

- hormone 2 (released from the anterior pituitary) goes back and inhibits the hypothalamus

What is long-loop feedback in the hypothalamic–pituitary axis?

- hormone 3 (released from the endocrine gland) inhibits both the hypothalamus and anterior pituitary

In the endocrine system, why is negative feedback important?

- hormones secreted by the third endocrine gland go back to the anterior pituitary and hypothalamus

- prevents overproduction of hormones ("sculpts the response" so that it's not active forever); this sets the homeostatic range of hormones

What are the two steps of hypothalamic control on the posterior pituitary?

1. hypothalamic neurons release hormones into the pituitary bloodstream

2. acts on a target cell/tissue to restore homeostasis

- in response to a deviation from homeostasis

What is oxytocin? Where is it made? What does it do?

- hormone made in the hypothalamus

- released in the posterior pituitary

- oxytocin receptors are located in the uterus and mammary myoepithelial cells

- causes uterine contraction and milk ejection

- oxytocin release is very important during the moment of childbirth

- the cervix stretches at the end of pregnancy (mechanical stimulus) sends signals back to hypothalamus which causes the hypothalamus to release oxytocin

- true positive feedback loop

- breastfeeding (mechanical stimulus); hormone releases oxytocin (milk ejection reflex)

What is vasopressin? Where is it made? What does it do?

- hormone made in the hypothalamus

- released in the posterior pituitary

- V1R: smooth muscle cells

- V2R - located in kidneys; collecting ducts

- important for vasoconstriction and water retention

- vasopressin = arginine vasopressin (AVP) = anti-diuretic hormone (ADH)

What are the three steps in which oxytocin causes muscle contraction in smooth muscle?

1. oxytocin binds to its receptor; this causes an increase of intracellular calcium levels

2. calcium binds to calmodulin; this activates myosin light chain kinase (MLCK)

3. myosin light chain kinase (MLCK) phosphorylates myosin; this activates it myosin and causes muscle contraction

What psychological factors increase oxytocin release?

- feeling secure

- privacy

- dim lighting

- people you know

- touch

- comfortable environment

- relaxation

What psychological factors inhibit oxytocin release?

- feeling frightened/watched

- bright lights

- strangers

- questions

- uncomfortable environment

- thinking

What are the two ways vasopressin works in the body?

1. detects low blood pressure

2. detects dehydration

What are the four steps of the baroreceptor reflex?

1. baroreceptors located in vasculature (blood vessels) detect low blood pressure

2. baroreceptors stimulate the hypothalamus and release vasopressin

3. vasopressin is released from the posterior pituitary and goes everywhere

4. causes vasoconstriction and restores normal blood pressure (restores homeostasis)

*uses the same smooth muscle contraction pathway as oxytocin

What are the four steps of the hypothalamic response to dehydration?

*dehydration is initially caused by an increase in extracellular Na+

1. elevated extracellular Na+ stimulates osmoreceptors located in the hypothalamus (osmoreceptors sense blood osmolarity)

2. vasopressin is released to the kidneys; this decreases urinary water loss (water retention) and thirst increases water gain

3. additional water dilutes the extracellular fluid and increases volume

4. this restores extracellular Na+ concentration (homeostasis restored)

In water retention, how does vasopressin affect aquaporins?

- aquaporins collect water/retain water

- vasopressin increases the levels of aquaporins in kidneys to retain water

- this is to dilute the extracellular Na+ levels and restore the correct concentration