Endocrinology

• The endocrine system:

  • important communication and regulation system involved in homeostasis
  • works alongside nervous system in feedback loops maintaining homeostasis
• Feedback systems:

  • self regulating
  • cycle of events - status of a condition is continually monitored, and reported to control region, sends instructor to effector, alters status of condition
• Types of hormones in endocrine system:

  • circulating hormones
  • local hormones
• types of hormones: circulatory hormones

  • chemical messengers secreted by endocrine glands
  • diffuse into blood and
  • carried to target cells
• types of hormones: local hormones

  • produced by many tissues
  • serve as means of local control
  • paracrines
  • autocrines
• the endocrine and nervous system work together to co-ordinate body systems
• responses of endocrine system are slower but last longer than nervous system
• endocrine system:

  • use hormones to directly and indirectly influence neuronal activity and behaviour
  • hormones work like neurotransmitters
• similarities between hormones and neurotransmitters:

  • hormones stored in endocrine cells
  • hormones bind with specific receptors
  • hormones bind with receptors on surface or inside cells
  • binding triggers activities within the cell
• differences between neurotransmitter and hormonal actions:

  • NTs travel over synaptic space to receiving neuron, hormones travel in blood to different locations
  • hormonal communication (seconds - minutes) is slower than neurotransmission (milliseconds)
• paracrine control

  • local regulators produce change in cells
• functions of endocrine system:

  • maintain constant internal environment in face of external challenges
  • regulate metabolism and energy balance
  • help regulate involuntary muscle contractions and secretions by glands
  • guide growth and development
  • contribute to control of reproductive function
• hormone activity:

  • hormones only act on cells that have receptors for that hormone
  • hormone conc and number of receptors determine level of activity
  • hormones bind to receptors inside or on cell membrane
• what happens when hormone binds on cell membrane ?

  1. hormone binds, activates G-proteins which activate adenylate cyclase
  2. ATP converted to cAMP, activates protein kinase
  3. protein kinase phosphorylates other enzymes
  4. phosphorylated enzymes catalyse reactions that produce physiological responses
• what happens when hormone binds inside the cell?

  1. lipid soluble hormone diffuses into cell and binds to receptor
  2. alters gene expression
  3. newly formed mRNA directs synthesis of specific proteins on ribosomes
  4. new proteins alter cells activity
• hormone secretion:

  • released in short bursts (pulsatile release pattern)
• hormone secretion can be controlled by...

  negative feedback
• the hypothalamus:

  master control centre of the endocrine system
• the hypothalamus: what and where?

  • regulates posterior and anterior pituitary gland
  • in the bone cavity
• posterior pituitary:

  • composed of nervous tissue
  • secretes hormones made in hypothalamus
• anterior pituitary:

  • composed of glandular tissue
  • hypothalamus exerts control over anterior pituitary by releasing or inhibiting hormones
• oxytocin release at parturition causes:

  • a neuroendocrine reflex
  • a positive feedback loop
• Thyroid hormones (T3 & T4) regulate:

  • metabolic rate
  • cellular metabolism
  • growth and development
• thyroid hormones regulate: metabolic rate:

  • increase basal metabolic rate
  • use extra oxygen to produce ATP and heat
• thyroid hormones regulate: cellular metabolism:

  • stimulate protein synthesis
  • increase use of glucose
• thyroid hormones regulate: Growth & development

  • accelerate body growth
• adrenal gland hormones:

  cortisol ( a glucocorticoid)

  • stimulates glucose production to provide resistance to stress
  • depresses immune response and decreases inflammation
• cortex produces...

  • mineralcorticoids - aldosterone
  • involved in regulation of filtration in the kidney as part of renin - angiotensin negative feedback system
• adrenal gland hormones: medulla produces:

  • epinephrine an norepinephrine in response to stress
  • as part of the 3Fs response
  • increase blood pressure, dilate airways to lungs, increase blood glucose, decrease digestion
• calcitonin regulates:

  • calcium
  • phosphate
  • homeostasis
• endocrine vs exocrine

  • endocrine glands = ductless glands, secretion pass into blood
  • exocrine glands = open to outside medium through duct
• glucagon and insulin secretion:

  • low blood glucose (hypoglycaemia) stimulates release of:
  • glucagon by alpha cells
  • high blood glucose stimulates release of:
  • insulin by beta cells
• response to low blood glucose (hypoglycaemia):

  • glucagon released by alpha cells, acts on liver cells to:
  • convert glycogen - glucose (glycogenolysis)
  • form glucose from lactic acid and amino acids (gluconeogenesis)
  • this glucose is released by hepatocytes - raises blood glucose level to normal
  • if it continues to rise, hyperglycaemia inhibits release of glucagon
• Response to high blood glucose (hyperglycaemia)

  • insulin released by beta cells, acts on various body cells to:
  • accelerate facilitated diffusion of glucose into cells
  • speed conversion of glucose to glycogen (glycogenesis)
  • increase uptake of amino acids and increase protein synthesis
  • speed synthesis of fatty acids (lipogenesis)
  • slow glycogenolysis
  • slow gluconeogenesis
  • so blood glucose level falls
  • if it continues to fall hypoglycaemia inhibits release of insulin
• growth hormone:

  • an anterior pituitary hormone
  • no specific target gland
  • also called somatotrophin
• growth hormone function:

  • stimulate body growth - muscle + bone
  • regulate metabolism - stimulate lyposis + provide nutrients
  • promote synthesis of IGF's - stimulate cell growth, division and protein synthesis
  • maintains tissues and promotes repair in adults
• Endocrine glands involved in control of G & D (growth hormone)

  • hypothalamus
  • pituitary
  • thyroid
  • gonads (testes + ovaries)
  • pancreas
  • adrenal gland
• hypothalamus:

  • centre of bodys metabolic regulation:
  • appetite
  • maintenance of body temp
  • metabolic rate
• major hormones in hypothalamus:

  all four act on pituitary gland

  • GHRH - growth hormone releasing hormone
  • TRH - thyrotropin releasing hormone
  • GnRH - Gonadotropin releasing hormone
  • CTRH - corticotropin releasing hormone
• endocrine glands in G & D: Thyroid gland

  • stimulated by TSH to produce thyroxine
  • affects metabolism & physical development
  • provide nutrients to generate energy for protein synthesis