hypothalamic pituitary axis and feedback control

hypothalamus and pituitary gland

hypothalamus: key regulator of the endocrine system. It integrates signals from the brain and the body, using this information to control hormonal secretions. regulates the function of thyroid, adrenal gland, reproductive glands

pituitary gland (master gland): releases hormones that regulate many bodily functions. responsible for somatic growth lactation, milk secretion

hypothalamus

represents less than 1% of the brain mass

connected with many areas of the brain and spinal cord

acts as an interface between internal and external environment

external responses - emotional, instinctual, appetite

internal homeostatic responses are produced through ANS and endocrine system - endocrine link in through the hypothalamic pituitary axis

pituitary gland (hypophysis)

small gland

physiologically divisible in two distinct portions: anterior pituitary (adenohypophysis) and posterior pituitary (neurohypophysis)

embryologically these two portions originate from different sources

hypothalamus and pituitary relationship

release of hormones from the pituitary gland is regulated by the hypothalamus by 2 different types of signals

• posterior pituitary (neural pathway): direct neural signals stimulate the release of hormone. instant messages to release stored hormones

• anterior pituitary (vascular link): specialized blood vessel system, the hypophyseal portal system, transports hormonal signals from the hypothalamus

hypothalamus and posterior pituitary

certain nerve cells in the hypothalamus (called neurosecretory neurons) make hormones that are released into the blood to affect organs far away

supraoptic nucleus (SON)

• produces mostly vasopressin (ADH)

• contributes to oxytocin production but to a lesser extent

paraventricular nucleus (PVN)

• er focus on oxytocin produces a mix of oxytocin and vasopressin with a slightly high

• also releases corticotropin releasing hormone (CRH) for stress responses

pituitary hormone synthesis and secretion

hormones (vasopressin/ADH and oxytocin) are first made in their inactive form (pro-hormones)

transport to posterior pituitary

• hormones are carried down nerve fibres to the posterior pituitary with the help of special proteins called neurophysins

• this takes several days

release into blood

• when nerve signals arrive, the hormone are released from the nerve endings into the nearby blood vessels

• both the hormone and the carrier protein are released but they separate quickly, and the hormone enters the bloodstream

oxytocin function

helps release milk from the breast during breastfeeding

makes muscles in the mammary gland squeeze milk into ducts

milk is then removed by the infant while suckling

triggers uterine contractions, which help during childbirth

sometimes used medically to start or aid labour

its role in men is not clearly understood

role of vasopressin/ antidiuretic hormone (ADH)

water retention

• ADH helps the kidneys save water by making certain parts more absorbent.

• this concentrates urine, reduces water loss, and keeps the body hydrated, especially during dehydration

blood vessel constriction

• ADH can tighten blood vessels, which helps increase blood pressure

• this is especially important during blood loss or low blood pressure (hypotension)

control of ADH secretion

dehydration

• less water in the body makes blood more concentrated (higher osmolality)

• special cells in the brain sense this change and signal for more ADH to be released

• ADH helps the kidneys save water, reducing urine and fixing the imbalance

after drinking

• extra water dilutes the blood (lower osmolality)

  • osmoreceptors stop signaling and less ADG is released

• the kidneys remove the extra water by producing more urine

factors affecting ADH release

stimulators of ADH release

• low blood volume or pressure (after bleeding)

• dehydration or increased blood concentration (higher osmolality)

• physical stress (pain) and emotional stress

• aging: older individuals often secrete more ADH than younger ones

inhibitors of ADH release

• high blood volume or pressure

• drinking alcohol

how ADH responds

• at normal levels, ADH responds balance water in the body

• mild dehydration: ADH secretion can increase 3-5times

• severe cases: ADH release can surge up to 50 times to help conserve water and maintain blood pressure

hormone secretion by anterior pituitary

anterior pituitary releases hormone through a process starting with special hormones in the hypothalamus (releasing and inhibiting hormones)

these hormones travel through small blood vessels (hypothalamic-hypophyseal portal system) to the anterior pituitary

once there, they tell the anterior pituitary cells to either release or stop releasing specific hormones

hypothalamic hypophyseal portal system

specialized blood vessel network that directly links the hypothalamus and anterior pituitary

1. hormones from the hypothalamus: special hormones (releasing and inhibiting) are collected by capillaries in the hypothalamus

2. travel to the anterior pituitary: these capillaries merge into portal vessels, which carry hormones down the pituitary stalk

3. action on pituitary cells the portal vessels branch into a second network of capillaries in the anterior pituitary, delivering the hormones directly to pituitary cells

4. release to the body: after acting on the pituitary, the blood flows into the rest of the body

hypothalamus and anterior pituitary gland

anterior pituitary gland contains several different cell types that synthesize and secrete these hormones (peptide hormones)

• gonadotropes: produce FSH (follicles stimulating hormone) and LH (luteinizing hormone) which control reproduction

• corticotropes: produce (ACTH) (adrenocorticotropic hormone) which helps the body respond to stress

• thyrotropes: produce TSH ( thyroid stimulating hormone) which regulates the thyroid gland

• lactotropes: produce prolactin )PRL) which is important for milk production

• somatotropes: produce growth hormone (GH) essential for growth and metabolism

FLAT PiG

each pituitary hormone is controlled by at least one hypothalamic hormone

hypothalamic hypophysiotropic hormones

thyroidotropin-releasing hormone (TRH) - stimulates release of TSH (thyrotropin) and PRL. targets the thyroid glands for thyroid hormone production

corticotropin-releasing hormone (CRH) - stimulates release of ACTH (corticotropin). targets adrenal glands for glucocorticoid production

gonadotropin-releasing hormone (GnRH) - stimulates release of FSH and LH (gonadotropins). targets reproductive system for sex hormone/gamete production

growth hormone-releasing hormone - stimulates release GH (growth hormone). targets liver, bone, muscles for IGF production

growth hormone-inhibiting hormone (GHIH/somatostatin) - inhibits release of GH and TSH. targets liver, bone, muscles for IGF production

prolactin-inhibiting hormone (PIH/dopamine) - inhibits release of PRL. targets mammary glands for milk production

prolactin-releasing hormone (PRH) - stimulates release of PRL. targets mammary glands for milk production

feedback types

long loop feedback: hormones from target glands (e.g. thyroid hormones) travel back to the hypothalamus or pituitary to reduce their own stimulation

short loop feedback: hormones from the pituitary act on the hypothalamus to adjust its releasing hormone

ultra short loop feedback: hypothalamus controls its own hormone release through very quick feedback mechanisms

feedback control in stress response

stress starts process: hypothalamus releases corticotropin releasing hormone (CRH)

pituitary response: CRH signals the anterior pituitary to release adrenocorticotropic hormone (ACTH)

adrenal gland action: ACTH stimulates the adrenal glands to produce cortisol

feedback regulation: high cortisol levels send a signal back to the hypothalamus and pituitary to reduce CRH and ACTH production, balancing the system

thyroid feedback system

initiation: hypothalamus releases thyrotrophin-releasing hormone (TRH) which signals to the anterior pituitary to release TSH

action: TSH (thyroid stimulating hormone) tells the thyroid gland to produce thyroid hormones (T3 and T4)

feedback: when T3 and T4 levels are high, they signal the hypothalamus and pituitary to reduce TRH and TSH production, creating a negative feedback loop