Hormones

Quick, short-lasting and localised:

• Autocrine communication - releases a substance into the extracellular environment, targetting the cell that secreted it eg. noradrenalin 
• Paracrine communication - releases a substance into the extracellular environment, targetting cells nearby eg. histamine

Slow, long-lasting and blood borne:

• Endocrine communication - releases a sbstance into the bloodstream, without ducts eg. andrenaline
• Nueroendocrine communication - releases a substance into the bloodstream, occuring only in dendritic cells eg. ADH

• Cells that secrete similar substances are often localised together in glands
• Exocrine glands:
  • Secretion cells surround the cavity with the substance being released into the cavity
  • Substance travels along a duct where it is secreted into bodily cells
  • Bodily cells can be either an internal or external surface
  • Can secrete saliva (mammals), slime (fish), silk (silkworms) and pheromones (insects)
• Endocrine glands:
  • No duct
  • Secretion cells are located near capillaries and release hormones directly into the bloodstream
  • Hormones can either be lipid soluble (diffuse directly across the membrane) or water solubule (must undergo exocytosis)

• Released by neurosecretory cells
• Similar to  neurone - a stimulus results in an action potential which travels down the cell and in turn causes the release of neurohormones into the bloodstream
• The end of the neurosecretory cell is against a capillary
• The action potential is more prolonged than in a normal action potential
• Example = corticotrophin releasing hormone (CRH)

• Chemical compound
• Produced and released by endocrine and neurosecretory cells
• Exerts regulatory influences
• Blood borne
• Effective at low concentrations

In Mammals

• At least 67 hormones
• Can be lipid soluble: amine and steroid
• Can be lipid insoluble: peptide and protein

• Requires receptors
• Lipid soluble receptors require cytoplasmic receptors - interacts with DNA to make changes in the genome, resulting in long-lasting effects
• Lipid insoluble hormones require cell surface receptors

Negtive Feedback

• Stabilises its own secretion and hormone concentration
• Doesn't stop the hormone from being produced, just decreases the amount
• Example = thyroxin

Positive Feedback

• Self-amplifying cycle - acts to increase its own secretion
• Rare
• Example = oxytocin in labour

Neuroendocrine Reflexes

• Hormone secretion in response to specific stimuli
• Example = oxytocin in suckling

Biological Rhythms

• Regular patterns 
• Example = cortisol
  • Peaks in the morning so you can get going
  • Decreases at night so you can relax
  • Can be disrupted if there's an increase in adrenalin 
• Cycles can vary from 24 hours long to a year long

Autonomous Hormones

• Calcium homeostasis - parathyroid activity increases and calcitonin decreases
• Glucose homeostasis - insulin and glucagon
• All present in some form in all vertebrates

Regulated Hormones

• Reproduction - GnRH ---> FHS/LH
• Lactation - prolactin
• Thyroid - TSH-RH ---> TSH ---> thyroxin
• Stress - CRH ---> ACTH ---> cortisol
• Stress - adrenalin and noradrenalin

Insulin

• Secreted by beta-cells in the prancreatic islets of Langerhans in response to high blood sugars
• Insulin effects carbohydrate metabolism
• Stimulates glycogenesis (glucose ---> glycogen)

Glucagon

• Secreted by alpha-cells in the pancreatic islets of Langerhans in response to low blood sugars
• Opposite effect to insulin
• Stimulates glucogenesis (glycogen ---> glucose)

Diabetes

• Condition characterised by disrupted glucose homeostasis (increased blood glucose)
• Can be either type I (natural) or type II (envirnonmental)

Consists of the pituitary gland (hypophysis - connected to the hypothalamus by the infundibulum), the posterior pituitary (neurohypophysis - nervous tissue) and the anterior pituitary (adenohypophysis - glandular epithelial tissue). The intermediate lobe separates the anterior and posterior pituitary, and is involved with cryptic colouring in fish. It is not present in birds and cetaceans, and is rudimentary in humans after birth.

The Posterior Pituitary

• Direct neural connection to the hypothalamus
• Stores hormones produced by the hypothalamus
• Stimulation ---> results in the release of hormones into the blood in the posterior pituitary
• Examples = vasopressin (ADH):
  • Peptide neurohormone
  • Released in response to thirst
  • Helps maintain blood pressure which stops you from being thirsty

The Anterior Pituitary

• Vascular connection with the hypothalamus
• Shorter neurosecretoy cells
• Hormones produced in the hypothalamus
• Stimulation ---> relase of hormones into the blood in the hypothalamus
• Hormones carried to anterior pituitary via bloodstream
• Stimulate/inhibit anterior pituitary to release hormones
• Released into the general circulation