A & P Exam #2

What is the difference between a chemically gated channel and a voltage-gated channel?

Both types of channels are normally closed. Chemically gated channels open in response to binding of a neurotransmitter. Voltage-gated channels open in response to changes in electrical charge (potential) across the plasma membrane.

How does conduction of an action potential in an unmyelinated axon and myelinated axon differ?

Conduction along a myelinated neuron (saltatory conduction) is significantly faster than in an unmyelinated neuron because an action potential is generated only in unmyelinated nodes.

How does depolarization and repolarization occur in the conductive segment?

Depolarization occurs when the threshold voltage of −55 mV is reached; voltage-gated Na+ channels open and Na+ rapidly enters the cell, reversing the polarity from negative to positive. Repolarization occurs due to closure of the voltage-gated Na+ channels and opening of voltage-gated K+ channels. K+ moves out of the cell and polarity is reversed from positive to negative.

Which functional segment of a neuron contains chemically gated channels? Which contains voltage-gated channels?

The receptive segment of a neuron (dendrites and cell body), and the initial segment (the axon hillock), contain chemically gated ion channels. The conductive segment (axon and telodendria) and transmissive segment (synaptic knobs) contain only voltage-gated ion channels.

What is the significance of the threshold membrane potential in the initial segment of a neuron?

The threshold membrane potential is the voltage difference that must be achieved in order to generate an action potential within the initial segment of a neuron. Typically this is at −55 mV, +15 mV above the resting membrane potential. If the ratio of IPSPs and EPSPs reaches the threshold membrane potential, only then will an action potential be generated and propagated along the neuron.