Question

A impulse conduction, explain how a nerve impulse was initiated and transmitted along the axon. Illustrate answer with an action potential diagram to explain the membrane potential changes due to movement of ions in the gated channels.

Answer 1

Lets see some terms first

Membrane potential – The charge inside and outside the cell membrane of axon is not same and membrane potential is the difference in the total charge between the inside of the cell and the outside of the cell.

Resting membrane potential – whene the nerve is not stimulated, the nerve will be in resting state.  Resting membrane potential is the difference in voltage across the cell membrane in a resting state. At this stage, the axonal membrane of the neuron is more permeable to the potassium ions and not permeable to the sodium ions.

Action potential – It is a short-term change in the electrical potential that travels across the neuron cell when the neuron encounters a stimulus.

Diagramatic Representation of the Action potential

When a strong stimuls encounter a neuron an impulse is generated. This stimulus will brong about electrical and chemical changes in the neuron. There are different ions on either side of the cell membrane. The exterior side has excess of  sodium ions that are positively charged as the cell membrane is impermeable to sodium in resting state. The interior side of the cell is negatively charged with more potassium ions. Due to this difference in the charges, there is an electrochemical difference.

Didnt got the concept of why inner of the membrane has negative charge during resting state?? let us be little elaborate. Listen carefully

When the membrane is at rest, K+ ions accumulate inside the cell due to a net movement with the concentration gradient. The negative resting membrane potential is created and maintained by increasing the concentration of cations outside the cell relative to inside the cell. The negative charge within the cell is created by the cell membrane being more permeable to potassium ion movement than sodium ion movement. In neurons, potassium ions are maintained at high concentrations within the cell while sodium ions are maintained at high concentrations outside of the cell. The cell possesses potassium and sodium leakage channels that allow the two cations to diffuse down their concentration gradient.

But the neurons have far more potassium leakage channels than sodium leakage channels. Therefore, potassium diffuses out of the cell at a much faster rate than sodium leaks in. Because more cations are leaving the cell than are entering, this causes the interior of the cell to be negatively charged relative to the outside of the cell. The actions of the sodium potassium pump help to maintain the resting potential, once established. Sodium potassium pumps brings two K+ ions into the cell while removing three Na+ ions per ATP consumed. As more cations are expelled from the cell than taken in, the inside of the cell remains negatively charged relative to the extracellular fluid.

Now back to the Action Potential

Transmission of a signal within a neuron from dendrite to axon terminal is carried by a reversal of the resting membrane potential called an action potential. When a strong stimulus encounters an axon excitatory sodium ion channels open allowing more sodium to to move in reducing the negative charge inside . thus difference in electric potential reduces. when it reaches the threshold potential -55mv voltage gated sodium channels open allowing a rapid influx of more sodium resulting in generation of action potential. The inner of cell become further reduced in its negativity. This process is called depolarization.

Once opening up of the voltage gated sodium channels causes the neuron completely depolarizes to a membrane potential of about +40 mV. Action potentials are considered an “all-or nothing” event, in that, once the threshold potential is reached, the neuron always completely depolarizes.

Once depolarization is complete, the cell must now “reset” its membrane voltage back to the resting potential. To accomplish this, the Na+channels close and cannot be opened. This begins the neuron’s refractory period, in which it cannot produce another action potential because its sodium channels will not open.

At the same time, voltage-gated K+ channels open, allowing K+ to leave the cell. As K+ ions leave the cell, the membrane potential once again becomes negative. The diffusion of K+ out of the cell actually hyperpolarizes the cell, in that the membrane potential becomes more negative than the cell’s normal resting potential. At this point, the sodium channels will return to their resting state, meaning they are ready to open again if the membrane potential again exceeds the threshold potential.

Propogation of action potential

Action potentials are propagated along the axons of neurons via local currents. Local current flow occurs due to depolarisation resultin in depolarisation of the adjacent axonal membrane and where this reaches threshold, further action potentials are generated. The areas of membrane that have recently depolarised will not depolarise again due to the refractory period – meaning that the action potential will only travel in one direction.

Myelinated Axons

There are periodic gaps along a myelinate axon where there is no myelin and the axonal membrane is exposed. This gaps are called Nodes of Ranvier. Myelinated sections of the axon lack voltage gated ion channels where as there is a high density of ion channels in the Nodes of Ranvier. So in myelinated nerves action potential can only occur at the nodes.

The myelin sheath acts as good insulator so the action potential is able to propagate along the neurone at a higher rate than would be possible in unmyelinated neurons. The electrical signals are rapidly conducted from one node to the next node, where it causes depolarisation of the membrane above the threshold and initiates another action potential which is conducted to the next node. In this manner an action potential is rapidly conducted down a neuron. This is known as saltatory conduction.