Generation and Conduction of Nerve Impulse
நரம்புத் தூண்டல் உருவாக்கமும் கடத்தலும்
At rest the inside of the axon is negative because the membrane lets K+ through but almost blocks Na+ — and the Na+/K+ pump sends out 3 Na+ for every 2 K+ it brings in. Remember 3-out, 2-in.
The idea
Neurons are excitable cells because their membranes are in a polarised state. Different types of ion channels are present on the neural membrane; these channels are selectively permeable to different ions. When a neuron is not conducting any impulse (resting), the axonal membrane is comparatively more permeable to potassium ions (K+) and nearly impermeable to sodium ions (Na+); it is also impermeable to the negatively charged proteins present in the axoplasm. Consequently, the axoplasm inside the axon contains a high concentration of K+ and negatively charged proteins and a low concentration of Na+, whereas the fluid outside contains a low concentration of K+, a high concentration of Na+, and thus forms a concentration gradient. These ionic gradients across the resting membrane are maintained by the active transport of ions by the sodium-potassium pump, which transports 3 Na+ outwards for 2 K+ into the cell. As a result the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged; the electrical potential difference across the resting plasma membrane is called the resting potential, and the membrane is said to be polarised. When a stimulus is applied at a site on the polarised membrane, the membrane at that site becomes freely permeable to Na+, leading to a rapid influx of Na+ followed by reversal of the polarity at that site — the outer surface becomes negatively charged and the inner side positively charged; the membrane is depolarised, and the electrical potential difference at that site is called the action potential, which is in fact termed a nerve impulse. At sites immediately ahead the membrane still has a positive charge on the outer surface, so current flows on the inner surface from the depolarised site to the next site, and the impulse (action potential) is conducted along the axon. The rise in the stimulus-induced permeability to Na+ is extremely short-lived; it is quickly followed by a rise in permeability to K+, K+ diffuses outside the membrane and restores the resting potential of the membrane at the site of excitation, and the fibre becomes once more responsive to further stimulation.
Why it matters
Keeping a charge difference ready across the membrane lets a neuron fire instantly; the impulse regenerates at each point, so the signal does not fade over a long axon.
NEET lens
Resting: membrane more permeable to K+, nearly impermeable to Na+; inside negative (high K+, negative proteins), outside positive (high Na+); Na+/K+ pump = 3 Na+ out, 2 K+ in. Stimulus → Na+ influx → depolarisation (outside negative, inside positive) = action potential. Repolarisation by K+ efflux. Current flows on the inner surface from depolarised to next site.
Clinical significance
Disturbed blood levels of Na+ and K+ (for example from severe vomiting, diarrhoea or kidney disease) alter membrane potentials and can cause muscle weakness, confusion or dangerous heart rhythms.
Introduction
A nerve impulse is a wave of reversed electrical charge (action potential) that travels along the axon membrane.
Core concept
Resting potential: axonal membrane more permeable to K+, nearly impermeable to Na+ and to negatively charged axoplasmic proteins. Inside: high K+, negative proteins, low Na+; outside: low K+, high Na+. Na+/K+ pump: 3 Na+ out for 2 K+ in (active transport). Outer surface positive, inner negative = polarised membrane. Stimulus: membrane at that site becomes freely permeable to Na+ → rapid Na+ influx → polarity reversed (outer negative, inner positive) = depolarised; potential = action potential = nerve impulse. Conduction: current flows on the inner surface from site A (depolarised) to site B; on the outer surface from B to A → impulse moves along the axon. Recovery: Na+ permeability short-lived → K+ permeability rises → K+ diffuses out → resting potential restored (repolarisation).
Go deeper
Because the inner surface at the depolarised site is positive and the site just ahead is still negative inside, current flows forward and depolarises the next patch — the impulse is self-propagating.
From Class 10
Class 10 called the nerve signal an electrical impulse. This concept shows which ions make it happen.