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NS4 - Synaptic Transmission (RMP (generated due to selective permeability…
NS4 - Synaptic Transmission
Intro
process allows either excitatory or inhibitory neurotransmission
excitatory + inhibitory signals cancel each other out
A strong stimulus does not cause a stronger AP (all or nothing response), but dies increase AP frequency (more neuronal firing)
RMP
extracellular ions: Na+, Cl-, Ca2+
intracellular ions: K+
unequal charge distribution: if chargers are separated across a membrane, the membrane has potential + is excitable
of neuron = -70mV (polarised, i.e not 0)
generated due to selective permeability to K+
Na+/K+ ATPase pump (3 Na out, 2 K in)
establishes chemical gradient that drives K+ out of cell, the residual -ve charge draws it back in (= EQUILIBRIUM POTENTIAL, electrical force=chemical force, no net transport)
depol: potential is moving +ve
caused by excitatory postsynaptic potential (Na+)
hyperpol: potential is moving -ve
caused by inhibitory postsynaptic potential (K+/Cl-)
Nernst Equation
calculates equilibrium potential for any ion
of K+ = -87mV (close to RMP)
of Na+ = 60mV
E = 61 x log(Co/Ci)
Co = ion conc outside cell
Ci = ion conc inside cell
NT Release
1) AP reaches + depots axon terminal
2) Voltage-gated Ca2+ channels (N+P type) open
3) Ca2+ influx activates Ca2+ sensitive fusion proteins that cause vesicles to fuse with presynaptic membrane + release NT
4) NTs diffuse across synaptic cleft + activate postsynaptic Rs
may also activate presynaptic Rs as feedback for recycling/upcycling (e.g. glutamine synthetase)
NT R types
ionotropic
fast
ligand-gated
increased ion permeability
e.g. AMPA (Na+ in), GABAa (Cl-)
metabotropic
slower
GPCRs with 2nd messenger
short + long term effects
altered gene expression
NTs
excitatory
glutamate
ACh
NA
inhibitory
GABA
Glycine
dopamine
serotonin
2 NTs can be released from same neuron (COTRANSMISSION)
2nd one is usually a peptide
neuromodulatory
inhibitory
induces slow response
e.g. neuropeptide Y, Substance P, VIP
Summation
Temporal
occurs when interval between EPSPs < 15msec (1 EPSP lasts 15 msec)
Spatial
occurs when there is inputs from multiple presynaptic neutrons @ multiple dendrites on 1 postsynaptic neuron
Refractory period
0.001-0.002s after stimulus
means neurons can fire 500-1000 APs/s
No APs can be fired