Question

1) a. Define the threshold potential for a neuron. Describe what will occur in a neuron when the membrane reaches potentials lower (more negative) and higher (less negative) than the threshold, and why.

b. Describe why action potential propagation is unidirectional, and why myelin increases the speed of action potential propagation.

Answer 1

1.A)Threshold potential

• Action potential or impulse is a sequence of rapidly upcoming events that decreases and reverse the membrane potential and then eventually restore it to the resting state.
• Action potential has two main phases- a depolarizing phase and repolarizing
• So when action potential occurs in the membrane of the action of a neuron when depolarization reaches a certain level about -55 MV in many neurons is termed as an threshold potential.

· A graded potential in such small deviations from the membrane potential that makes the membrane either more polarized( inside more negative )or less polarized (inside less negative).

· When the response makes the membrane more polarized inside more negative it is termed as hyperpolarizing graded potential

· When the response makes the membrane leads polarized inside less negative it is termed as depolarizing graded potential

· A graded potential occurs when a stimulus causes mechanically gated or ligand gated channels to the open or close in and excitable cells plasma membrane

· So, mechanically gated channels and ligand gated channels can be present in the dendrites of sensory neurons and ligand gated channels are neurones in the dendrites and cell bodies of inter neurons and motor neurons

· Electrical signals are graded means that they vary in amplitude depending on the strength of the stimulus.

· They are larger or smaller depending on how ligand gated or mechanical gated channels have opened and how long it remains open .

· The opening and closing of these Ion channels alter the flow of specific ions across the membrane producing a flow of current that is localised which means that is spread to the adjacent region along the plasma membrane in a direction from this to stimulus source for a short distance and then gradually dies out as the charges are lots across the membrane through leakage channels.

· This mode of travel by which gradient potential die out at they spread along the membrane is known as decremental condition.

B)

Action potential propagation is unidirectional because to communicate information from one part of the body to another, action potential in a neurone must travel from where they arise at the trigger zone of the axon to the Axon terminal

In contrast to the graded potential an action potential is not decremental.

Instead an action potential keep its strength as it spreads along the membrane.

This mode of conduction is called propagation ,and it depends upon the positive feedback.

In actually it is not the same action potential that propagate along the entire axon.

Instead action potential re-generate over and over at adjacent region of the membrane from the trigger zone to the Axon terminals.

In a neuron,the myelin sheath increases action potential that can propagate in this direction only -it cannot propagate back toward the cell body because any region of the membrane that has just undergone an action potential is temporaily in absolute refractory period and cannot generate another action potential