Generation and conduction of impulse

Nerve cell have polarised membrane i.e., have electrical potential difference or membrane potential.

This is because of a variety of ion channels (pores formed by proteins) specific for a particular type of ions.

Some remain open while most ion channels open under one condition, but closes under other condition.

Because of such regulated or voltage gated channel, membrane become excitable as these channels respond to different type of stimuli e.g., Light, touch, sound etc.

When a neuron is not sending any signal, it is said to be at rest and its membrane has resting membrane potential.

Resting Membrane Potential

In the resting nerve fibre, the cytoplasm just beneath its membrane is electronegative relative to the layer of extracellular fluid (ECF) just outside the membrane.

If the two sides of the membrane are connected by galvanometer (double beam cathode ray oscilloscope) the inner side is seen to possess a negative potential of about 70 mV relative to the outerside.

This is called the resting membrane potential. This results from two factors:

(i) The resting membrane has a poor permeability for Na+ although it has a higher permeability for K+. Therefore, K+ can cross more easily while Cl and Na+ have more difficulty in crossing.

(ii) Negatively charged protein molecule inside the neuron cannot cross the plasma membrane because of its semipermeability.

The differential flow of the positively charged ions and the fact that the negatively charged organic ions within the nerve fibre cannot pass out cause an increasing positive charge on the outside of the membrane and negative charge on the inside of the membrane.

This makes the membrane of the resting nerve fibre polarized (i.e., its outside being positively charged with respect to the inside.)

Such electrochemical gradients are maintained by the active transport of ions involving Na+ – K+ ion transmembrane pump.

It pumps out 3Na+ for every 2K+ ions passed inwardly.

K+ concentration is 30 times more inside neuron than outside and Na+ concentration is 10 times more in interstitial fluid as compared to inside of neuron.

Conduction of Nerve Impulse

It involves initiation of impulse followed by conduction along the axon so as to be transferred to target muscle/tissue.

Initiation of Impulse:

When stimulated, voltage gated Na+ channel open which causes a rapid, very localised, temporary inflow of Na+ into the cell which causes development of net positive charge on the inner side of membrane in that area.

This is called depolarisation.

It occurs at a particular region of neuron called trigger zone.

Voltage gated ion channels are clustered in the area of trigger zone.

Stimulus of threshold value causes stoppage of Na+ – K+ ATP-ase pump.

Continued passage of Na+ ions into inside of neuron creates a reverse potential of +20 mV to +30 mV, rarely to +60 mV.

The total change occurs in spike-like fashion which is also called spike potential.

Na+ ion channels open for about 0.5 m sec.

It creates a potential that sets in a wave of depolarisation through the nerve fibre.

The membrane potential which sets in a wave of depolarisation is called action potential.

For most excitable cells, the threshold is about -55 mV to -60 mV.

Conduction of Impulse:

In the area of depolarisation, the potential difference across the membrane is small while its nearby region has large difference in membrane potential.

This produces a small local current in the area.

The local current becomes a stimulus and causes the gated Na+ channels of next region to open and depolarise the area to produce fresh action potential.

The process will continue till the impulse reaches the end of neuron.

Repolarisation:

As the Na+ channels close after 0.5 m sec the membrane becomes extra permeable to K+ ions due to opening of K+ ion gates.

With the pumping out of K+ ions, the neuron interior becomes negative and the potential falls back to resting potential.

The phenomenon of change of membrane potential from excited state to resting state is called repolarisation.

However, K+ ion channels remain open for a bit longer period so that the membrane potential becomes more negative than -70 mV.

It is called hyperpolarisation.

It takes about 1 -5 m sec for repolarisation.

 

Figure: Diagrammatic representation of impulse conduction through an axon (at points A and B)