Endocrinology

• A hormone is defined as a substance which may be released at one site and act on another site to change function or production - They are produced by specialised gland cells and are released into tissue fluids and the brain to be distributed throughout the body
• The primary endocrine organs of the body are the pituitary gland, parathyroid glands, thyroid gland, islets of Langerhans (pancreas), adrenal glands and gonads
• the nervous system provides fast, all-or-nothing responses using electrical and chemical messengers. the endocrine system provides slow but long lasting, graded responses using chemical messengers
• Paracrine: localised acting on cell of another type through the extracellular space
• Autocrine: acting on the cell of secretion (itself) through the extracellular space
• Endocrine: long-distance hormones which travel through the circulation to its target area
• Neuroendocrine: neurosecretory cells which release neurohormones. Found in the brain and have longer effects than co-released transmitters.
• Neurocrine: neurotransmitter release over a short distance (into the synaptic space)
• True hormones are chemical messengers which are synthesised in endocrine glands and are secreted into the blood stream. They act on specific cell receptors and exert specific physiological regulatory actions

  Neurohormones are produced and released from neurosecretory cells (modified nerve cell). They require an action potential to be released
• Pheromones are chemical messengers which are synthesised in exocrine glands and are secreted into the environment. they are small, volatile and easily dispersed. they act on other individuals through olfaction or taste to alter behaviour or the neuroendocrine system
• prostaglandins and histamine are examples of parahormones, which are not produced in an endocrine gland but have the characteristics of a true hormone
• Prohormones are large peptide molecules which may be processed into single or multiple hormones. POMC is cleaved into ACTH, alpha-MSH and beta-endorphin. Angiotensinogen is converted to angiotensin
• Growth factors are hormone-like substances which promote growth of body or brain tissue.
• Cytokines are hormone-like substances released from lymphocytes, macrophages or other immune system cells. they regulate the immune or neuroendocrine systems
• Hormones are produced in response to external signals such as season, temperature and stress, and internal signals such as changes in osmolality, electrolytes and glucose concentration
• sex steroids stimulate genitalia and secondary sex organ maturation at puberty. Growth hormone is produced via IGF-1 from the liver. simultaneously there is a growth of bones and muscles to achieve mature body conformation
• Behaviour is a coordinated action between hormones and the central nervous system
• hormone response by a cell is determined by receptor distribution, both spatial (where) and temporal (how many)
• the efficacy and potency of hormone response may be affected by several different molecules (ligands) binding to the same receptor type, altering magnitude of activation. This may be changed by other ligands binding to other sites (allosteric effect) e.g progesterone being enhanced by GABA
• receptor population can be changed by up or down regulation by negative feedback response to receptor activation. e.g if few receptors are activated, more will be produced to increase binding chance and vice versa
• some hormones are metabolised at their target tissue to increase or decrease their effect or affinity
• Controlling hormones can either directly influence the cells producing them in an inhibitory negative-feedback loop or act downstream and the effect they have results in regulation and inhibition

  Examples include noradrenaline and insulin
• The anterior and posterior pituitary lobes have different embryological origins, reflecting their different functions

  The posterior lobe (pars nervosa) is a downgrowth of brain tissue from the hypothalamus and is therefore of neural origin.

  The anterior lobe (pars distalis) is formed of epithelial tissue from the oral cavity which divides into pars distalis and pars intermedia.

  Rathke’s pouch is the remnant space left from embryonic development
• The pituitary gland receives blood from the internal carotid artery via the Circle of Willis.

  The Posterior pituitary is supplied directly with arterial blood from the Inferior Hypophysial Artery (a branch of the internal carotid artery)
• The capillary bed (primary plexus) surrounding the infundibulum and Median Eminence of the pituitary gland is supplied directly from the Superior Hypophysial Artery (a branch of the internal carotid artery)
• The anterior lobe of the pituitary gland is supplied indirectly by the hypophysial portal veins. The hypophysial portal veins run into the secondary capillary plexus of the hypothalmo-hypophysial portal system
• Both lobes of the pituitary gland have blood supplies which drain into the hypophysial veins to the venous sinuses and back to the systemic circulation.
• the paraventricular nucleus and supraoptic nucleus are the origins for the neuroendocrine signals in the posterior pituitary. they produce vasopressin and oxytocin
• Magnocellular oxytocin and vasopressin cells have one axon, that projects directly to the posterior pituitary. magnocellular neurones release hormone both into the blood and the brain
• oxytocin originates in the posterior pituitary gland. it affects partuition, the milk ejection reflex and ejaculation. it has central effects on maternal behaviour, memory, social and sexual behaviours
• vasopressin (ADH) originates in the posterior pituitary and effects plasma osmolality via its effects on the kidney. centrally it effects egression, social behaviour including pair bonding and memory
• adrenocorticotrophic hormone (ACTH) originates in the anterior pituitary. it is released in response to stressors such as fear and pain. it acts on the adrenal cortex to release glucocorticoid hormones such as cortisol. Under control of corticotrophin releasing hormone
• Thyroid stimulating hormone (TSH) originates in the anterior pituitary and is released in response to prolonged cold. Acts on the thyroid gland to cause release of T4 and T3 which stimulate oxidative metabolism, raising basal metabolic rate. Under the control of TRH
• Follicle stimulating hormone (FSH) originates in the anterior pituitary. It promotes the development of gametes and the secretion of gonadal hormones (follicles and oestrogen in females, spermatogenesis and Inhibin in males) . Is under control of GnRH and GnIH (releasing and inhibiting hormones)
• Luteinising hormone (LH) originates in the anterior pituitary. Is released during reproductive cycles. Stimulates ovulation in females and the formation of progesterone-secreting luteal cells. Stimulates leydig cells in males to secrete testosterone. Under control of GnIH and GnRH
• Growth hormone (GH) originates in the anterior pituitary. Released during growth and starvation. It acts directly on the liver and adipocytes. Promotes growth in all cells by stimulating relaese of IGF-1 from the liver. Has an anti-insulin effect. Inhibited by somatostatin
• Prolactin (PRL) originates in the anterior pituitary. it is concerned with milk production, acting on the mammary glands and parental behaviour. Has essential functions related to growth, osmoregulation, metabolism and reproduction. Is under the control of dopamine, PRF and PIF
• Melanocyte Stimulating Hormone (MSH) originates in the intermediate pituitary.  Acts on the melanophores of amphibians, fish and reptiles to change their skin colour to match background.  MSH Acts on melanocytes in mammals and birds.   Central effect:  Influences learning and memory and sexual behaviour in mammals
• diabetes insipidus shows clinical signs including excessive thirst and excretion of large amounts of severely diluted urine. reducing fluid intake does not affect urine concentration as the patient has a vasopressin deficiency. the kidneys are insensitive to vasopressin.
• the median eminence (base of the hypothalamus) serves an an interface between the neural and peripheral endocrine systems.