Transmission of nerve impulses

Course: B.Sc.(Micro./Biotech) sem II

Subject: Animal & Plant Physiology

Unit 4.1

1

Transmission of nerve impulses Neurones transmit

impulses as electrical signals

These signals pass along the cell surface membrane of the axon as a nerve impulse

2

Transmission of nerve impulses It is NOT the same as

an electric current passing down a wire (which is much faster)

The mechanisms is the same throughout the animal kingdom

3

Transmission of nerve impulses Experiments have been carried out using squid

axons which are big enough to have electrodes inserted in them.

One electrode can be placed inside the axon and one on its surface.

Transmission of nerve impulses When at rest, the inside of the membrane

has a negative electrical potential compared to the outside.

This difference in potential is called the resting potential and is typically about between -70 mV to -80 mV

In this resting state the axon is said to be polarised.

Transmission of nerve impulses This is maintained because the neurone has

an internal composition which is different to the outside.

Sodium ions and potassium ions are transported across the membrane against their concentration gradients by active transport

Transmission of nerve impulses Carrier proteins pick up

Na+ ions and transport them to the outside.

At the same time K+ ions are transported into the axon.

This is known as the sodium-potassium pump and relies on ATP from respiration

4

Transmission of nerve impulses Inside the axon there

are large numbers of negatively charged organic ions which can not move out of the axon.

Transmission of nerve impulses The Na+ ions are passed out faster than the K+ ions

are bought in. Approx. three Na+ ions leave for every two K+

ions that enter. K+ ions can also diffuse back out quicker than Na+

ions can diffuse back in. Net result is that the outside of the membrane is

positive compared to the inside.

The action potential A nerve impulse can be initiated by mechanical,

chemical, thermal or electrical stimulation When the axon is stimulated the resting potential

changes. It changes from –70 mV inside the membrane to

+40 mV For a very brief period the inside of the axon

becomes positive and the outside negative

The action potential This change in potential is called the

action potential and lasts about 3 milliseconds

When an action potential occurs, the axon is said to be depolarised.

When the resting potential is re-established the axon membrane is said to be repolarised

depolarisation

depolarisation

reploarisation

depolarisation

reploarisation

‘overshoot’

depolarisation

reploarisation

‘overshoot’

direction of impulse

Depolarisation When the membrane

depolarises changes occur in the membrane to the permeability of both Na+ ions and K+ ions

Depolarisation When the axon is stimulated, channels

open on its cell surface which allow Na+ ions to pass through.

Na+ ions flood in by diffusion The Na+ ions create a positive charge of

+40 mV inside the membrane, reversing the resting potential and causing the action potential

Repolarisation Potassium channels open in the membrane

and K+ ions diffuse out along their concentration gradient.

This starts of repolarisation At the same time, sodium channels in the

membrane close preventing any further influx of Na+ ions.

5

Repolarisation The resting potential is re-established as

the outside of the membrane becomes positive again compared to the inside.

So many K+ ions leave that the charge inside becomes more negative that it was originally.

This shows up as an ‘overshoot’.

resting potential

(no net ion movement)

resting potential

(no net ion movement)

Na+ start to move in

resting potential

(no net ion movement)

Na+ start to move in

Na+ ions diffuse in rapidly

resting potential

(no net ion movement)

Na+ start to move in

Na+ ions diffuse in rapidly

K+ ions diffuse out rapidly

resting potential

(no net ion movement)

Na+ start to move in

Na+ ions diffuse in rapidly

K+ ions diffuse out rapidly

Sodium ions pumped out potassium ions pumped in

Repolarisation The potassium channels close and the

sodium-potassium pump starts again. Normal concentrations of sodium and

potassium ions is re-established. The membrane is once again at its resting

potential

6

a) In the resting axon, there is a high conc. of Na+ ions outside and a high conc. of K+ ions inside. But the net effect is that the outside is positive compared to the inside giving the resting potential

b) The axon is stimulated producing an action potential, setting up local circuits on the axon membrane

c) Sodium ions rush into the axon along a diffusion gradient depolarising the membrane causing an action potential

d) As the action potential passes along the axon potassium ions diffuse out along a concentration gradient, starting off the process of repolarisation

e) The sodium-potassium pump is re-established, fully repolarising the membrane

References:

Image 1: http://universe-review.ca/I10-75-impulse.jpg

Image 2: http://postimg.org/image/ibu6ul45b/

Image 3: http://postimg.org/image/k4x3iwpbz/

Image 4: http://postimg.org/image/klmwj32gv/

Image 5: http://postimg.org/image/9krre2a7z/

Image 6: http://www.biologydiscussion.com/neural-control/mechanism-of-transmission-of-nerve-impulse-explained-with-diagram/5013

Book: Human Physiology 4th edition by Lauralee Sherwood