Question

Discuss the steps in generating an action potential in a neuron. (Make sure to include which ion channels are activated and how, changes in membrane potential as a result of the movement of those ions, the different phases seen in an action potential, and the movement of the action potential in relation to the anatomy of a neuron.).

Answer 1

Action potential -

Definition - The brief sequence of changes which occur in the membrane potential following excitation is called as action potential.

Origin - Excitable cells ( e.g. nerve and muscle cells) generate action potentials when they are stimulated by a change in membrane potential. It is due to changes in the ionic equilibrium across the cell membrane i.e changes in the conduction of ions across the cell membrane that are produced by alternations in the ion channels.

+ membrane potential (mv) → O Resting state © Reaching threshold * Depolarization O Repolarization © Hyper polarization - 100 Time (m see) EVENTS DURING ACTION POTENTIAL

Phases of action potential -

1) resting phase

2)depolarization(rising) phase

3)peak phase

4)repolarization (falling) phase

5)Hyperpolarization(undershoot) phase

Mechanism of development of action potential -

1)Resting phase

Resting membrane potential is negative inside the nerve, due to presence of more negative ions than positive ions.

In resting state, potassium ions permeability to the outside of nerve is greater than than sodium ions permeability into the cell. Some non diffusable ions(e.g. Proteins) remain inside the cell. So there is more negative ions inside the nerve causing negative resting membrane potential.

The resting membrane potential of nerve ranges from - 40 to - 90 mV. A typical value of - 70 mV.

2)Depolarization phase

During this phase, voltage gates of sodium ion channels opens causing more permeability to sodium ions than potassium ions as voltage gates potassium ion channels closes.

Due to more permeability, more sodium ions moves into the cell. So, more positive ions present inside the nerve.

Therefore,the membrane potential changes from its resting value due to presence of more sodium ions inside the nerve. So inside the nerve, the membrane potential becomes more positive or less negative causing depolarization phase.

3)peak phase

The rate of depolarization increases the point at which this change in rate occurs is called firing phase.

4)Repolarization phase

During this phase, voltage gate of potassium ion channels opens causing more potassium efflux(potassium ions moves from inside to outside of the nerve).There is less positive ions present inside the nerve causing membrane potential to become more negative causing repolarization phase.

5)Hyperpolarization phase

The repolarization typically goes more negative than the resting potential. This is called Hyperpolarization. This occurs because of the increased potassium permeability as voltage gates of potassium ion channels open for more time and decreased sodium ion movement into the nerve as voltage gates of sodium ion channels closed.

Movement of the action potential -

An action potential (nerve impulse) is transmitted from one neuron to another through junction called synapse.

A synapse is formed by the membranes of pre-synaptic neuron and post-synaptic neuron, which are separated by the synaptic cleft.

Neurotransmitters are involved in the transmission of action potentials at these synapses.

The axon terminal of neuron contain vesicles filled with neurotransmitters. When an action potential arrives at the axon terminal, it depolarizes the membrane opening voltage gated calcium channels.

Calcium ions stimulate the vesicles to release neurotransmitters into the synaptic cleft.

The released neurotransmitters bind to their specific receptors present on the post-synaptic membrane.

The post synaptic membrane has ligand gated ion channels and responds causes entry of ions.

Entry of ions generates a action potential in post-synpatic neuron.