synapses and receptors

action potential is ..

process of depolarisation and repolarisation - all or nothing

where is an action potential initiated

at base of neuron, in region called axon hillock

nodes of ranvier

small gaps in myelin - allow ion movement across the axon membrane at these sites

saltatory conduction

fast signal transmission when action potential can jump from one node to another

how do electrical synapses communicate with other cells

using gap junctions that directly connect the cytoplasm, between 2 cells

how do chemical synapses communicate with other cells

involves neurotransmitter release from a presynaptic neuron that diffuses across synaptic cleft and binds to postsynaptic neuron

steps after action potential arrives at a chemical synapse

voltage gated Ca2+ channels open - Ca2+ influx causing vesicles to fuse with membrane - release of neurotransmitter into synaptic cleft - diffuses across synapse and binds to postsynaptic receptor

2 ways that synaptic communication can stop

uptake of transmitter into axon terminal/glial cells. catabolism - degradation of neurotransmitter by enzymes

quantum

amount of neurotransmitter in one vesicle

excitatory neurons

raise membrane potential towards critical threshold, becomes less negative

inhibitory neurons

lower membrane potential away from critical threshold, becomes more negative

EPSP

excitatory postsynaptic potential

IPSP

inhibitory postsynaptic potential

summation

process where neuron sums up all the excitatory and inhibitory signals it receives over a period

criteria for transmitter substance

made in neuron - present at presynaptic terminals, drug at reasonable concentration mimics exactly action of endogenous neurotransmitter, specific mechanism exists to remove it from synapse

class and secretion site of acetylcholine

excitatory at skeletal muscle, either at other site, secreted at CNS/PNS/ neuromuscular junction

class and secretion site of norepinephrine

excitatory or inhibitory- CNS&PNS

class and secretion site of epinephrine

excitatory or inhibitory- CNS & PNS

class and secretion site of dopamine

generally excitatory - CNS & PNS

class and secretion site of serotonin 5-HT

generally inhibitory - CNS

class and secretion site of GABA

inhibitory - CNS & neuromuscular junction in invertebrates

class and secretion site of glycine

inhibitory - CNS

class and secretion site of glutamate

excitatory - CNS & neuromuscular junction in invertebrates

class and secretion site of aspartate

excitatory - CNS

class and secretion site of substance P

excitatory - CNS & PNS

class and secretion site of met-enkephalin

generally inhibitory - CNS

ionotropic receptors

transmitter binding = ion channel opens directly, always stimulatory - fast effect

ionotropic receptor at rest

channel pore closed, no ion movement

when neurotransmitter binds to ionotropic receptor …

channel opens, ions flow down conc gradient, channel is permeable to anions/cations

metabotropic receptors

transmitter binding = indirect activation of G protein. can trigger opening/closing of a separate ion channel and downstream signalling cascade

slow effect

binding of neurotransmitter to metabotropic receptor …

causes G protein activation

either directly causes ion pore to open or activates a second messenger. second messenger can open ion channel or initiate a signalling cascade

structure of an ionotropic receptor

made of 4/5 subunits arranged around central pore in membrane

metabotropic receptor structure

made of 1 protein with 7 transmembrane regions (7 alpha helices)

steps in G protein activation

transmitter binds to receptor - GTP exchanges for GDP on alpha subunit - G protein dissociates from receptor - 3 subunits also dissociate - alpha subunit activates ion channel then is inactivated by GTP hydrolysis - alpha subunit recombines with others and attaches to receptor

3 most common targets for second messengers

adenylate cyclase -> cAMP. guanylate cyclase ->cGMP. phospholipase C -> IP3 & diacylglycerol

agonists

drugs that mimic actions of neurotransmitter. binding to receptor = activation

antagonists

drugs that block action of neurotransmitter. binding to receptor = no activation