NEURO: Neurohormones Flashcards
Describe neurohormones.
Neurohormones are produced by specialised nerve cells called neurosecretory cells and are released into the blood. Because they are defined as hormones, they are secreted into the blood and have their effect on cells some distance away.
The same compounds can also act as neurotransmitters or as autocrine (self) or paracrine (local) messengers.
Describe the two main control systems of the body (compare and contrast).
The body has two main control system: the endocrine system and the nervous system.
ENDOCRINE SYSTEM:
- mediators travel within blood vessels
- utilises chemical mediators (hormones)
- slow communication
- effects can be long-lasting
NERVOUS SYSTEM:
- signalling along the nerve fibres
- transmission of electrical impulses
- fast communication
- effects are generally short-acting
Describe the different types of hormones.
PROTEIN & PEPTIDE HORMONES:
- vary considerably in size
- can be synthesised as a large precursor and processed prior to secretion (eg. GH, somatostatin, insulin)
- can be post-translationally modified (eg. glycosylation)
- can have multiple subunits synthesised independently and assembled (eg. FSH, LH, TSH)
AMINO ACID DERIVATIVES:
- mostly tyrosine-derived
- neurotransmitter that can also act as a hormone
- eg. epinephrine, norepinephrine, dopamine
STEROID HORMONES:
- steroids are a class of lipids derived from cholesterol
- includes cortisol, aldosterone, testosterone, progesterone, oestradiol
Describe endocrine rhythms.
Most, if not all, bodily activities show periodic rhythms or cyclic changes. Many of the hormones show periodicity.
CIRCADIAN RHYTHMS: based on a 24-hour cycle (eg. secretion of cortisol, GH, PRL)
PULSATILE (ULTRADIAN) RHYTHMS: periodicity of fewer than 24 hours (usually every 1/2 to 2 hours) (eg. secretion of gonadotrophin in adults)
INFRADIAN RHYTHMS: periodicity of longer than 24 hours (eg. menstrual cycle)
List some of the principal endocrine organs in the body.
- hypothalamus
- pituitary gland
- thyroid gland
- parathyroid glands
- adrenal gland
- pancreas
- ovary (females)
- testes (males)
How are hormone signals sent to both parts of the pituitary?
With the anterior pituitary, we have the hypophyseal portal circulation. Hormones are released into these blood vessels, which transport them to the anterior pituitary, where they act.
With the posterior pituitary, we have neurones called magnocellular neurons, which project from the hypothalamus directly into your posterior pituitary. The hormones are released into the PP, where they travel through the capillary network and get released straight into the blood circulation.
List the hormones that control the pituitary, and what effect they have.
ANTERIOR PITUITARY:
- corticotrophin releasing hormone (CRH):
a peptide that controls the release of adrenocorticotrophin (ACTH)
- thyrotrophin releasing hormone (TRH):
a peptide that controls the release of thryoid stimulating hormone (TSH) and prolactin (PRL) - gonadotrophin releasing hormone (GnRH):
a peptide that controls the release of luteinising hormone (LH) and follicle-stimulating hormone (FSH) - growth hormone releasing hormone (GHRH):
a peptide that controls the release of growth hormone (GH) - growth hormone inhibitin hormone (somatostatin):
a petide that inhibits the release of GH, gastrin vasoactive intestinal polypeptide (VIP), glucagon, insulin, TSH and PRL - dopamine (DA):
a monoamine that inhibits the release of PRL
POSTERIOR PITUITARY:
The posterior pituitary releases vasopressin (which has an antidiuretic effect) and oxytocin (which acts on the uterus to induce uterine contraction, and acts on the mammary glands to induce milk ejection).
List the specialised cells of the anterior pituitary and what they release.
GONADOTROPH cells that secrete LH and FSH in response to GnRH.
SOMATOTROPHS that control GH secretion in response to GHRH.
CORTICOTROPHS that control ACTH secretion in response to CRH.
THYROTROPHS that regulate TSH secretion in response to TRH.
LACTOTROPHS that control the secretion of prolactin in response to TRH, somatostatin and dopamine.
As a recap, describe the role of ACTH in the release of cortisol.
Hypothalamic neurones release corticotrophin-releasing hormone (CRH) to stimulate pituitary corticotrophs to release ACTH into the circulation.
ACTH stimulates the production of glucocorticoid (cortisol) and sex hormone from the zona fasciculata of the adrenal cortex.
Cortisol provides negative feedback to the hypothalamus and pituitary, reducing the amount of CRH and ACTH released.
As a recap, describe the regulation of TSH and thyroid secretion by negative feedback.
Thyrotropic releasing hormone (TRH) from the hypothalamus stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH).
TSH acts on the thyroid to increase T3/T4 secretion, T3 is the most potent thyroid hormone, and targets the tissues containing a deiodinase enzyme (DI) to convert T4 to T3.
The pituitary also expresses deiodinase to convert T4 to T3 to facilitate negative feedback.
As a recap, what happens when you have too much or too little thyroxine?
If you have too much thyroxine, it can lead to hyperthyroidism, tachycardia, anxiety, etc.
If you have too little thyroxine, it can lead to weight gain, low energy, cognitive impairment, etc.
As a recap, describe prolactin and its regulation.
It is released by the lactotrophs in the anterior pituitary.
It stimulates mammary gland development during puberty. It also maintains lactation (synergised by glucocorticoids, inhibited by oestrogen and progesterone - we get a decrease in both after parturition).
It sregulation is under the dominant negative control of dopamine. It’s increased during pregnancy and lactation.
Describe the synthesis and transport of the neurohormones of the posterior pituitary.
The neurohormones of the posterior pituitary are vasopressin and oxytocin.
They are synthesised in the supraoptic and paraventricular nuclei in the hypothalamus.
They’re transported to the terminals of the nerve fibres located in the posterior pituitary.
Structurally, they’re quite similar (only a 2 amino acid difference in a 9 amino acid structure), yet they have very different functions.
As a recap, describe vasopressin.
It is also known as the anti-diuretic hormone (ADH). Its release is stimulated by changes in the activity of the osmoreceptor complex in the hypothalamus.
It controls plasma osmolality by regulating water excretion and drinking behaviour.
It stimulates vascular smooth muscle contraction in the distal tubules of the kidney to reduce loss of water and raise blood pressure.
As a recap, describe oxytocin.
Normally, it’s undetectable, but its levels are elevated during parturition, lactation and mating.
It is released in response to peripheral stimuli of the cervical stretch receptors and suckling at the breast. It may also be involved in responses to stroking, caressing, grooming, etc.
It regulates the contraction of smooth muscles (e.g. uterus during labour, myoepithelial cells lining the mammary duct, contraction of reproductive tract during sperm ejaculation).
