HOMEOSTASIS

homeostasis = an active and dynamic process to maintain a stable internal environment to sustain life, despite external changes, managed by feedback systems

 e.g. blood pressure, thermoregulation, acid base, respiration, blood, hydration, glucose levels ect


the four components

  1. stimulus

  2. receptor

  3. control centre

  4. effector


nervous system 

  • uses electrical impulses and neurotransmitters

  • targets particular cells 

  • has a rapid responses

endocrine system

  • uses blood-borne hormones as chemical messengers

  • secreted by cells into extracellular fluids and travel via the bloodstream to target cells 

  • slower response 


NEURONES = highly specialised nerve cells that respond to stimuli and transmit electrical impulses 

  • found only in the brain, spinal cord and nerves

  • sensory neurones detect a change and send signals to the control centre 

  • the control centre interprets the information and initiates a response via motor / efferent pathways 


afferent (sensory) neurons carry sensory information from receptors to the central nervous system - their cell bodies are located outside of the spinal cord

efferent (motor) neurons carry information from the brain to the peripheral nervous system - their cell bodies are located in the ventral horn of the spinal cord


describe the automatic/voluntary reflex action 

  • a reflex arc begins with a stimulus 

  • a receptor detects the stimulus

  • a sensory neurone sends electrical impulse to a relay neurone which is located in the spinal cord of the CNS

  • the relay neurons connect sensory neurones to motor neurones

  • the spinal cord acts as the integration centre bypassing the brain for faster response 

  • the motor neurone sends electrical impulses to an effector 

  • the effector produces a response


describe a negative feedback 

  • the stimulus produces a change in variable 

  • a receptor detects change 

  • the information is sent along the afferent pathway to the control centre 

  • information is sent along the efferent pathway to the effector 

  • the effector feeds back to reduce the effect of the stimulus and returns variable to the homeostatic level 


positive feedback loop - change is enhanced not reversed like blood clotting and labour 



major glands in the endocrine system

  • hypothalamus

  • thyroid

  • adrenal glands

  • pituitary glands

  • pancreas

  • uterus

  • ovaries 

  • testes


HYPOTHALAMUS AND PITUITARY GLAND

HYPOTHALAMIC-PITUITARY CONNECTIONS

  1. TRH = thyrotropin-releasing hormone 

  2. CRH = corticotropin-releasing hormone

  3. GHRH = growth hormone-releasing hormone

  4. GnRH = gonadotropin-releasing hormone 

  5. Somatostatin = GHIH = growth hormone-inhibiting hormone

  6. Dopamine = PIH = prolactin-inhibiting hormone


(ANTERIOR) PITUIRARY HORMONES RELEASED

  1. TSH = thyroid stimulating hormone

  2. ACTH = adrenocorticotropic hormone 

  3. GH = growth hormone

  4. LH = luteinising hormone

  5. FHS = follicle stimulating hormone

  6. inhibits GH release

  7. inhibits prolactin release 


(POSTERIOR) PITUITARY RELEASES

  1. oxytocin - which stimulates uterine contractions and milk ejection 

  • uterine stretch during labour 

  • suckling of infant as it stimulates milk ejection

  • social/psychological cues like bonding, trust, emotional touch


  1. ADH = antidiuretic hormone - regulates water reabsorption in the kidneys and blood pressure 

  • water reabsorption in kidneys, concentrates urine, conserves water

  • vasoconstriction helps maintain blood pressure 

  • brain/behaviour which modulated thirst and some social/behavioural functions 


THYROID GLAND

produces

  • T3 (triiodothyronine)

  • T4 (thyroxine)

  • calcitonin

main functions 

  • to enhance metabolism, protein synthesis and growth

  • increases basal metabolic rate, oxygen consumption and energy expenditure

  • reduces blood calcium levels by inhibiting bone resorption


PARATHYROID GLAND


  • released PTH = parathyroid hormone

  • primary function to regulate blood calcium and phosphate levels 

  • raises blood calcium by 

    • stimulating osteoclasts - bone respiration

    • increasing kidney calcium reabsorption 

    • stimulating calcitriol production - intestinal calcium absorption 


ADRENAL GLAND 

region and main hormones 

  • cortex 

    • outer = aldosterone 

    • middle = cortisol 

    • androgens 

  • medulla = catecholomines 

main functions 

  • aldosterone - regulates sodium and water balance which affects blood pressure 

  • cortisol - controls metabolism and stress response

  • androgens - contribute to secondary sex characteristics 

  • medulla - fight or flight response, increases heart rate, blood flow and energy availability 


GROWTH HORMONE

IGF-1 = INSULIN LIKE GROWTH FACTOR 1 

promotes tissue growth and provides negative feedback to the hypothalamus and pituitary to control GH release 


DOPAMINE

a neurotransmitter in the nervous system and a neurohormone in the hypothalamic-pituitary system 

normal state 

dopamine released by the hypothalamus binds to D2 receptors in anterior pituitary gland which inhibits prolactin secretion 

suckling or stress 

neural input supresses hypothalamic dopamine which causes prolactin levels rise which stimulated milk production and secretion 

feedback loop 

elevated prolactin stimulated dopamine release which restored inhibition 


GONADS HYPOTHALAMIC PITUITARY GONADAL (HPG) AXIS 

LH = LUTEINISING HORMONE 

acts on leydig cells in the testes 

stimulates testosterone production 

FSH = FOLLICLE-STIMULATING HORMONE 

acts on sertoli cells in the testes 

stimulates spermatogenesis

promotes secretion of androgen-binding protein which binds testosterone to maintain high local concentrations needed for sperm development  



THREE TYPES OF ENDOCRINE GLAND STIMULI 

  1. humeral stimulus 

  2. neural stimulus 

  3. hormonal stimulus 


a target cell responds to a hormone because it bears receptors for the hormone 

hormones travel through the circulatory system to reach target cells through

  1. endocrine signalling 

distance = far

duration = long lasting 

  1. paracrine signalling 

distance = effects are localized to adjacent or nearby cells 

duration = short term and immediate 

  1. autocrine signalling 

distance = effects on the same cell that released them 

duration = immediate and self-regulating