Nerves and Muscles Part 2 - Week 4 Notes

Human Physiology Notes – Week 4
Nerves and Muscles Part 2.


Saltatory Propagation of AP in Myelinated Neurons.

 Because of high membrane insolation by the myelin sheet, changes in membrane charge can
reach out towards the next Ranvier node bringing the sodium channels to the threshold for
opening.
 AP jumps from node to node.

Synapse.

 Presynaptic cell in close contact with the target postsynaptic cell.
 Small space between cells is called the synaptic cleft.
 Transmission is either electrical or chemical.
 Chemical synapses operate by release of neurotransmitter (NT).
 Transmission is only in one direction pre -> post.
 Calcium is the signal for NT release at the synapse.

Excitory Synapses.

 Ligand-gated postsynaptic ion channel opens.
 Increased permeability for Na+ and K+ ions.
 Much more Na+ ions than K+ ions leave.
 Depolarization brings post synaptic membrane potential closer to threshold for AP
generation.

Inhibitory Synapses.

Ligand-gated postsynaptic ion channels open.
 Increased permeability for K+ and Cl- ions.
 Cl- ions enter, and some K+ ions leave.
 Hyperpolarization drives postsynaptic membrane potential away from the threshold for AP
generation.

Transmitter Lifecycle.

 Acetylcholine (ACh) (nicotinic receptors).
 Release leads to excitation in postsynaptic membranes.
 ACh opens specific ligand gated channels and sodium influx (depolarization).
 ACh is degraded and chlorine fragments taken up again into synapse for reuse in transmitter
synthesis.

Turning Chemical Signal On.

 Release of transmitter into synaptic cleft in response to AP.
 Transmitter binds to receptor channels.
 Receptor channels open for specific ions.

Turning Chemical Signal Off.

 Released transmitter continuously removed from the synaptic cleft.