Question

Explain how nervous signal transmission is propagated along the axon and what happens when the signal reaches its effector organ or tissue. In your discussion include a brief explanation of the refractory period. How can a signal be made stronger?

Answer 1

The transmission of a nerve impulse along a neuron from one end to the other occurs as a result of electrical changes across the membrane of the neuron .A neuron can receive input from other neurons via a chemical called a neurotransmitter. If this input is strong enough, the neuron will pass the signal to downstream neurons. As an action potential travels down the axon, the polarity changes across the membrane. Once the signal reaches the axon terminal, it stimulates other neurons. Steps involved are as follows,

Depolarization : When neurotransmitter molecules bind to receptors located on a neuron’s dendrites, voltage-gated ion channels open, the voltage gated sodium and calcium channel that had been closed while the cell was at resting potential are opened, because of this sodium ions rushes into the cell, the entry of sodium ions adds to the positivity of the cell as a result the cell membrane becomes positive.

Repolarization : Follwing depolarization the voltage gated sodium ion channel that had been opened while the cell was undergoing depolarization closes again, the increased positive charge inside the cell causes the positive potassium ion to leave the cell through the potassium channels as a result potential within the cell decreases and approaches resting potential again.

Hyperpolarization : The process of repolarization causes an overshoot in the potential of the cell, potassium ions continue to move out of the cell, so that resting potential is exceeded and the new cell potential becomes more negative than the resting potential.

This hyperpolarization is refered to as Refractory period, this ensures an action potential will only travel forward down the axon and not backwards, during refractory period another action potenial cannot be triggered.

The signal reaches the target cell through a synapse and than releases a neurotransmitter at the synapse, this neurotransmitter binds to the receptors on the postsynaptic cell membrane of the target cell to cause an action.