hormone - regulatory chemical secreted by endocrine gland 

• only target cell can respond to hormone (but blood carries hormone throughout the body)
• neurotransmitter - only diffuse a short distance, but may be chemically similar to hormones
  • effects the postsynaptic neuron
  • neurohormone - chemicals secreted from neurons into blood
  • some molecules work as both neurotransmitters and hormones
• targets ligand-gated receptors on target cells
  • messages sometimes relayed through 2nd messenger, magnified through enzyme cascade
• paracrine regulation - using chemicals as local regulators in an organ
• pheromone - chemical released into the environment
  • used as communication between animals

types of hormones - 

• polypeptides - chains of amino acids (never more than 100)
  • insulin, ADH
• glycoproteins - polypeptides (w/ over 100 amino acids) attached to carbohydrate
  • FSH, LH
• amines - derived from tyrosine/tryptophan amino acids
  • catecholamines - secreted by adrenal medulla, includes adrenaline/noradrenaline
  • melatonin, thyroid hormone
• steroids - lipids derived form cholesterol
  • sex steroids - secreted by testes, ovaries, placenta, adrenal cortex
  • corticosteroids - secreted only by adrenal gland
• lilophilic hormones - fat soluble, includes steroids/thyroxine
• lilophobic hormones - water soluble, all other hormones
• most releases of hormones controlled by the brain

hormones that enter cells - lipophilic hormones easily enter cells (can pass plasma membrane) 

• either finds receptor in cytoplasm or inside nucleus (either way, eventually reaches nucleus)
• hormone response elements - DNA segments binding to hormones

hormones that do not enter cells - lipophobic hormones can’t pass plasma membrane 

• have to bind to receptors
• 2nd messenger needed within the cell to get the reactions started