Nerve Cells PDF

Nerve cells Stuart Cruickshank  Neurons Neurons receive, conduct and transmit signals sense muscles CNS organs...

Nerve cells Stuart Cruickshank  Neurons Neurons receive, conduct and transmit signals sense muscles CNS organs glands nerve terminal cell body dendrites axon terminal (1mm1m) branches Signals transmitted as changes in electrical potential across neuronal plasma membrane  Action potentials Neuron stimulated by signal at localized site on membrane surface  change in membrane potential at site Change in membrane potential must travel along axon to nerve terminal to transmit information But signal rapidly weakens with distance from initiation site (termed passive spread) Requires mechanism to maintain/boost signal if information to be transmitted over long distances (>1m) Neurones employ action potentials to transmit electrical signals Action potential can carry message without weakening over any distance at speeds up to 100 m/s (>200 mph) Action potential production due to properties of voltage-gated ion channels  Role of Na+ in action potential mechanism Action potential triggered by local depolarisation of neuron plasma membrane (ie membrane potential shifts to a more positive/less negative potential) If depolarisation moves membrane potential above threshold potential  temporary activation of voltage-gated Na+ channels at stimulation site. Na+ moves into neuron down concentration gradient  further depolarisation  opens more voltage-gated Na+ channels  further Na+ entry  further depolarisation  etc etc - POSITIVE FEEDBACK - lasts ~ 1 ms Membrane potential shifts rapidly from -60mV at rest to +40mV (close to Na+ equilibrium potential) Na+ channels adopt inactive state (can’t open for several milliseconds)  Role of K+ in action potential mechanism Voltage-gated K+ channels also present in membrane K+ channels also open in response to membrane depolarisation but threshold potential for activation is higher than for Na+ channels K+ channels stay open while membrane is depolarised K+ flow out of cell down electrochemical gradient (enhanced by positive membrane potential)  membrane potential becomes more negative  resting state  Action potential +40 membrane potential (mV) action 0 potential -40 threshold potential -60 resting membrane potential state of Na+ 0 1 2 time (ms) channels closed open inactive closed  Propagation of action potential Positive-feedback activation of voltage-gated Na+ channels  “all or nothing” nature of action potential ALL action potentials are of same magnitude and duration Ensures adjacent region of membrane is depolarised beyond threshold  action potential  adjacent region depolarised beyond threshold  action potential  etc etc… Electrical signal transmitted along entire length of axon (even > 1m) WITHOUT AN Y reduction in intensity NB: action potential can only travel away from site of depolarisation because of temporary Na+ channel inactivation (refractory period)  Signal transmission at nerve terminals When action potential reaches nerve terminal, signal has to be transmitted to target cell (neurone or muscle) Signal transmission takes place at synapses At synapse, pre and post synaptic cells separated from each other by synaptic cleft (~ 20 nm) Electrical signal cannot cross cleft  converted into chemical signal - neurotransmitter Neurotransmitter molecules packaged in membrane bound synaptic vesicles within nerve terminals When action potential reaches nerve terminal, voltage-gated Ca2+ channels activated in nerve terminal  Ca2+ influx down electrochemical gradient  fusion of vesicles with plasma membrane  neurotransmitters released into synaptic cleft by exocytosis  Synapses presynaptic nerve terminal postsynaptic membrane synaptic cleft dendrite of postsynaptic neurone presynaptic membrane synaptic vesicles 2 m  Conversion of electrical signal into chemical signal resting nerve terminal activated nerve terminal presynaptic voltage-gated Ca 2+ nerve terminal action channel voltage-gated potential (closed) Ca2+ channel neurotransmitter (open) synaptic vesicle Ca2+ synaptic neurotransmitter neurotransmitter cleft receptor released neurotransmitter receptor postsynaptic cell  Synaptic Transmission An AP reaches the axon terminal of the presynaptic cell and causes V-gated Ca2+ channels to open. Ca2+ influx, binds to regulatory proteins & initiates NT exocytosis.  Conversion of chemical signal to electrical signal Neurotransmitter diffuses across synaptic cleft and binds to neurotransmitter receptors concentrated on postsynaptic membrane of target cell Binding of neurotransmitter to receptors  change in membrane potential If membrane potential depolarises above threshold  action potential Neurotransmitter rapidly removed from synaptic cleft by enzyme degradation or reuptake into terminal  when presynaptic cell stops firing  postsynaptic cells stop firing Neurotransmitter receptors of various types but most commonly transmitter (ligand)-gated ion channels  rapid response - milliseconds  Excitatory and inhibitory synapses Excitatory neurotransmitters cause postsynaptic cell to fire action potentials Excitatory neurotransmitters (eg acetylcholine, glutamate) e.g. act on ion channel receptors selective for Na+ and Ca2+ Inhibitory neurotransmitters prevent postsynaptic cell from firing Inhibitory neurotransmitters (eg -aminobutyric acid (GABA) and glycine) e.g. act on Cl- channels