Download Transmission at the Synapse and the

This document was created by Alex Yartsev ([email protected]); if I have used your data or images and forgot to reference you, please email me.
Transmission at the Synapse and the Neuromuscular Junction
First, an orgy of nomenclature
- Inhibition can be POST-SYNAPTIC or PRE-SYNAPTIC
- DIRECT inhibition: inhibition after an inhibitory postsynaptic potential- “direct” because it is inhibition by virtue of being
-
-
hyperpolarized by an arriving inhibitory stimulus, not because of any previous discharges of the post—synaptic cell
INDIRECT inhibition is the result of previous postsynaptic neuron discharges, eg. when the postsynaptic cell is
refractory to excitation because it just fired
PRESYNAPTIC INHIBITION AND FACILITATION happens when an inhibitory neuron sends a nerve ending to an
excitatory synapse on another neuron, and the two nerve endings form an axoaxonal synapse.
o There are 3 mechanisms of presynaptic inhibition:
 Activation of chloride channels in the PRE-synaptic neuron – that hyperpolarizes the
excitatory nerve ending and thus reduced the magnitude of excitatory action potential; and that in
turn reduces the amount of calcium that enters the excitatory nerve ending, reducing the amount of
excitatory neurotransmitter released
 Voltage-gated potassium channels can open, thus hyperpolarizing the membrane by allowing
a stream of potassium to exit, and thusa decreasing the inward calcium stream upon the arrival of
the action potential
 Direct inhibition of neurotransmitter release independent of calcium influx
GABA is a model presynaptic inhibitory neurotransmitter
o GABAA receptors are Chloride channels
o GABAB receptors are Potassium channels
Pre-synaptic facilitation also occurs andusually features a prolongation of the action potential, and
INCREASED calcium release into the cell (thus increased release of neurotransmitter)
o for example: SEROTONIN acts a presynaptic facilitator; it increases cAMP activity, which results in
phosphorylation of potassium channels (which become closed in the phosphorylated state). The result is
delayed repolarization, and thus a prolonged action potential.
ORGANISATION OF INHIBITORY CIRCUITS
A typical RENSHAW CELL: inhibitory interneuron of the spinal cord
The Renshaw cell receives input from a collateral axon of a spinal
motor neuron; it then sends a post-synaptic inhibitory signal to both
the same neuron that stimulated it ( thus exerting a negative
feedback) as well as a neighboring neuron
SUMMATION AND OCCLUSION
-
SUBLIMINAL FRINGE: if neurons A and B both receive an excitatory input from the same network of endings, and A
reaches firing threshold by spatial summation (it has more excitatory endings contacfting it) then B, which is excited
but not yet at threshold, is said to be in the SUBLIMINAL FRINGE of neuron A
Neurons are said to be in the subliminal fringe if they are affected by excitatory input, but not brought to
firing thereshold ( the “discharge zone”)
OCCLUSION is the result of presynaptic endings sharing postsynaptic neurons. There is a decrease in expected
response from any given single stimulation