NS4 - Synaptic Transmission

Intro

process allows either excitatory or inhibitory neurotransmission

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)

Nernst Equation

calculates equilibrium potential for any ion

E = 61 x log(Co/Ci)

Co = ion conc outside cell

Ci = ion conc inside cell

of K+ = -87mV (close to RMP)

of Na+ = 60mV

depol: potential is moving +ve

hyperpol: potential is moving -ve

caused by inhibitory postsynaptic potential (K+/Cl-)

caused by excitatory postsynaptic potential (Na+)

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

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

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)

Refractory period

0.001-0.002s after stimulus

means neurons can fire 500-1000 APs/s

No APs can be fired