Biomedical Science - Action potentials

How do impulses travel along neurons?

using electrical impulses, known as action potentials

saltatory conduction

from node to node

Na+/K+ pump

-protein found in all cell membranes

-carries out active transport, so uses ATP

-pumps 3 Na+ OUT and 2K+ INTO cell, keeps the cell with more Na+ outside than in so more + on outside more - on inside

resting potential inside cell

-70mV

stimulus process

-a neuron receives a stimulus eg. touch

-there are sodium ion channels in the membrane which get disturbed by the stimulus and open up

-some sodium ions flow into the neuron, creating a more positive voltage inside the cell

Threshold potential

for an impulse to be initiated down a neuron, the initial stimulus must cause enough sodium ions to flow into the cells to raise the membrane potential to atleast -55mV

failed initiations

-when stimulus is too small to reach threshold of -55mV

-the stimulus only caused a small amount of depolarisation (positive charges) not enough to trigger full depolarisation

-ALL OR NOTHING PRINCIPLE

depolarisation

positive charges flowing into cell cause membrane potential to increase, voltage goes up

repolarisation

Once full depolarisation occurs this changes the inside of the cell to ~+40V, then neuron has to return to polarised/resting state rapidly so it can recieve another stimulation, to do this the K+ ions move out making it negative again

hyperpolarisation (refractory period)

K+ ions continue to move out, inside becomes more negative than resting creating an undershoot HYPERPOLARISATION, then K+ and Na+ are returned to resting, The Na/K+ ATPase pump helps to do this quickly, a new action potential cannot be fired during this time

propagating the action potential

-sodium channels along the axon are voltage gated, sense a change in voltage on the inside of the cell

-as the voltage changes inside the cell this causes nearby V gated Na+ channels to open

-this now allows another rush of Na+ ions into the cell (positive feedback)

-the impulse is therefore propagated along the axon

-Wave of action potentials then move along the axon membrane

factors affecting speed of impulses

axon diameter - greater diameter, greater speed

myelin - myelin acts as an insulator, the depolarisation spreads more rapidly up the neuron when myelinated, the activation of the V gated ion channels 'jumps' from 1 Node of Ranvier to the next