Question

2. Where along the cell does a presynaptic input (synapse) have the greatest effect on determining whether a postsynaptic neuron fires an action potential or not?

3.At the resting potential, which ion is a cell most permeable to? Why is the cell most permeable to that ion?

4. Explain why an action potential normally does not flow backwards, toward the cell body. Would it be different if you injected Na+ into the middle of the axon? Why?

Answer 1

2. There are 3 main synaptic connections: Axo-dendritic, Axo-somatic, Axo-axonic.

Axo-dendritic is most common and it may be excitatory or inhibitory. Axo-somatic is less common which is also either excitatory or inhibitory. Axo-axonic is mostly inhibitory and it is a synapse between two axons.

Axo-dendritic is better as the impulses are carried fully over the neuron and help in impulse conduction. So presynaptic input must be close to soma to dendrites for complete effect.

3. At the resting state potassium ion is the most permeable ion because of its equilibrium potential of -90 mV which is close to the the resting membrane potential of -70 mV whereas sodium ion has equilibrium potential of +65 mV which has high permeability during depolarised state of membrane.

4. The action potential never flow backward regardless of site of stimulation. Because of the the sodium channels present behind which will not open as they are in resting state. The impulse given cause the neuron to depolarize and the impulse is carried along the axon in uni-direction.