Neurons, Drugs

membrane potential - difference in charge across the membrane 

• cytoplasm = negative, extracellular matrix = positive
• fixed anions - negatively charged molecules too large to diffuse out of the cell
• leak channels and sodium-potassium pump keep positively charged ions out of the cell
• equilibrium potential - point where electrical/chemical forces balance out for a certain ion

graded potentials - small changes in membrane potentials 

• casued by activation of gated ion channels (can open in response to stimuli like hormones)
• chemical (ligand) gated channel - open when chemicals bind to them
  • channels open >> change in membrane permeability >> different ions can get in/out
• depolarization >> membrane potential becomes less negative
• hyperpolarization >> membrane potential becomes more negative
• summation - ability of graded potentials to combine
• threshold - amount of depolarization needed to create action potential

action potential - nerve impulse once voltage-gated ion channels open 

• voltage-gated ion channel - opens/closes depending on membrane potential
• Na+ gates open first, before K+ gates
• Na+ enters cell (depolarization) >> K+ exits cell (repolarization) >> possible undershoot if K+ channels stay open (hyperpolarization)
• cannot combine w/ other action potentials
• either occurs completely or none at all
• can depolarize another area of the membrane, starting a chain of action potentials
• saltatory connection - action potentials jumping from node to node in myelinated axons
  • speeds up nervous transmissions
• myelinated + larger axon diamter >> fast action potential transmission

synapse - intercellular junction between dendrites and soma 

• electrical synapse - uses direct cytoplasmic connections
  • usually found in invertebrate systems
• chemical synapse - accounts for majority of synapses
  • synaptic cleft - narrow space that separates 2 cells
  • synaptic vesicles - contains neurotransmitters
  • action potential at end of axon >> Ca++ channels open >> depolarization >> vesicles bind to membrane >> neurotransmitters released through exocytosis, bind to receptor proteins on other cell
• neurotransmitters recycled into cell by transporters, but most go back to cell body before being used again by vesicles
• excitatory postsynaptic potential (EPSP) - depolarization
• inhibitory postsynaptic potential (IPSP) - hyperpolarization
• synaptic integration - EPSP’s and IPSP’s working together to bring about overall effect on cell

neurotransmitters - 

• dopamine - used to control body mov’ts
  • deficiency causes Parkinson’s disease
  • excess causes schizophrenia
• norepinephrine - adds on to the effect of epinephrine, secreted by adrenal gland
• serotonin - regulates sleep/emotion
  • deficiency can cause depression
  • drug LSD blocks serotonin receptors >> depression
• substance P - neuropetide that responds to pain stimuli
  • pain won’t be felt w/o it
• nitric oxide - 1st gas discovered to act as regulatory molecule
  • cannot be stored (diffuses through membranes)
  • causes smooth muscles to relax

drugs - decreases the sensitivity of receptors, mimics the effects of neurotransmitters 

• habituation - receptors lost ability to respond if exposed to constant stimulus for long time
  • number of receptor proteins decrease
• blocks transporters >> excess of neurotransmitters in synapse cleft >> # of receptors decrease due to over-stimulation >> addiction
• body adjusts to conditions when drug is present >> withdrawal symptoms occur when drug no longer used
• agonist - acts like the neurotransmitter
• antagonist - blocks the receptor for a neurotransmitter