Vt

Iā€” E. Nitric oxide (NO) as a transmitter substance

Messenger, e.g., acetylcholine

Messenger,

Messenger, e.g., acetylcholine

Messenger,

calmodulin J i J complex cGMP

Cell 2

(e.g., vascular myocyte)

Arginine

Cell 1

(e.g., endothelial cell)

calmodulin J i J complex

O2,NADPH Citrulline cGMP

Cell 2

(e.g., vascular myocyte)

Arginine

O2,NADPH Citrulline

Cell 1

(e.g., endothelial cell)

ai T3 C

Hypothalamic-Pituitary System

In the hypothalamus, (1) humoral signals from the periphery (e.g., from circulating cortisol) can be converted to efferent neuronal signals, and (2) afferent neuronal signals can be converted to endocrine messengers (neurosecretion).

The first case is possible because the hypothalamus is situated near circumventricular organs like the or-ganum vasculosum laminae terminalis (OVLT), sub-fornical organ, the median eminence of the hypothalamus, and the neurohypophysis. Since there is no blood-brain barrier there, hydrophilic peptide hormones can also enter.

The hypothalamus is closely connected to other parts of the CNS (^ p. 330). It controls many autonomous regulatory functions and its neuropeptides influence higher brain functions. The hypothalamus is related to the sleeping-waking rhythm (^ p.334) and to psychogenic factors. Stress, for example, stimulates the release of cortisol (via CRH, ACTH) and can lead to the cessation of hormone-controlled menstruation (amenorrhea).

Neurosecretion. Hypothalamic neurons synthesize hormones, incorporate them in granules that are transported to the ends of the axons (axoplasmic transport ^ p. 42), and secrete them into the bloodstream. In this way, oxytocin and ADH are carried from magno-cellular hypothalamic nuclei to the neurohy-pophysis, and RHs and IHs (and ADH) reach the median eminence of the hypothalamus (^ A). The action potential-triggered exocytotic release of the hormones into the bloodstream results in Ca2+ influx into the nerve endings (^ p. 50ff.).

Oxytocin (Ocytocin) and antidiuretic hormone (ADH) are two posterior pituitary hormones that enter the systemic circulation directly. ADH induces water retention in the renal collecting ducts (V2-rec.; ^ p. 166) and induces vasoconstriction (endothelial V1 rec.) by stimulating the secretion of endothelin-1 (^ p.212ff.). ADH-bearing neurons also secrete ADH into the portal venous circulation (see below). The ADH and CRH molecules regulate the secretion of ACTH by the adenohy-pophysis. Oxytocin promotes uterine contractions and milk ejection (^ p. 304). In nursing mothers, suckling stimulates nerve endings in the nipples, triggering the secretion of oxy-tocin (and prolactin, ^ p. 303) via neurohumoral reflexes.

Releasing hormones (RH) or Mberins that stimulate hormone release from the adenohy-pophysis (Gn-RH, TRH, SRH, CRH; ^ p.270ff.) are secreted by hypothalamic neurons into a kind of portal venous system and travel only a short distance to the anterior lobe (^ A). Once in its vascular network, they trigger the release of anterior pituitary hormones into the systemic circulation (^ A). Some anterior pituitary hormones are regulated by release-inhibiting hormones (IH) or statins, such as SIH and PIH = dopamine (^ p. 270ff.). Peripheral hormones, ADH (see above) and various neu-rotransmitters such as neuropeptide Y (NPY), norepinephrine (NE), dopamine, VIP and opioids also help to regulate anterior pituitary functions (^ p. 272).

The four glandotropic hormones (ACTH, TSH, FSH and LH) and the aglandotropic hormones (prolactin and GH) are secreted from the anterior pituitary (^ A). The secretion of growth hormone (GH = somatotropic hormone, STH) is subject to control by GH-RH, SIH and IGF-1. GH stimulates protein synthesis (anabolic action) and skeletal growth with the aid of somatomedins (growth factors formed in the liver), which play a role in sulfate uptake by cartilage. Somatomedin C = insulin-like growth factor-1 (IGF-1) inhibits the release of GH by the anterior pituitary via negative feedback control. GH has lipolytic and glycogeno-lytic actions that are independent of soma-tomedin activity.

Proopiomelanocortin (POMC) is a peptide precursor not only of ACTH, but (inside or outside the anterior pituitary) also of p-endorphin and a-melano-cyte-stimulating hormone (a-MSH = a-melanocor-tin). p-endorphin has analgesic effects in the CNS and immunomodulatory effects, while a-MSH in the hypothalamus helps to regulate the body weight (^ p. 230) and stimulates peripheral melanocytes.

|ā€” A. Hypothalamic-pituitary hormone secretion (schematic)

Nucleus ventromedials

Nucleus dorsomedialis

Nucleus infundibularis

Hypothalamus

Nucleus ^ supraopticus Nucleus paraventricularis

Chiasma opticum

Superior hypo-physial artery

I RH induce release of anterior pituitary hormones hormones IH inhibit their release

Anterior pituitary (adenohypophysis)

Nucleus ^ supraopticus Nucleus paraventricularis

Chiasma opticum

Superior hypo-physial artery

I RH induce release of anterior pituitary hormones hormones IH inhibit their release

Release of RHs, IHs, ADH, NE, NPY and other transmitters

Nucleus Paraventricularis Crh

Posterior pituitary (neurohypophysis)

Anterior pituitary hormones

Posterior pituitary hormones

ADH, oxytocin

Release of RHs, IHs, ADH, NE, NPY and other transmitters

Posterior pituitary (neurohypophysis)

Anterior pituitary hormones

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Responses

  • mhret
    How oxytocin causes milk ejection mechanism?
    7 years ago

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