Neurones Dendrites Axon Facilitated diffusion Schwann cells Active transport Myelin Na + /K + pump Synapse

Neurones E – Know parts of a neurone and their importance D – Describe the function of some parts of the neurone C – Explain in detail how a resting and action potential are created

Myelinated motor neurone

TASKs Draw label and annotate a myelinated motor neurone

Neurone functions

Recap – Cell membrane Sodium and potassium ions unable to diffuse across phospholipid bilayer Intrinsic proteins span length of bilayer. They contain ion channels. –Some channels remain open at all time and allow both sodium and potassium through –Voltage gated channels are specific to either potassium or sodium and can be open or closed Sodium-potassium pump: When potassium actively transported in and sodium actively transported out of the axon.

Nerve impulse A self-propagating wave of electrical disturbance that travels along the surface of the axon membrane Not a electrical impulse but a temporary reversal of electrical potential across the axon between … Resting potential and action potential

Resting potential Inside of axon negatively charged relative to outside therefore its polarised. Potential difference: mV (usually 65mV)

Establishing resting potential Active pumping of 3 sodium ions (Na+) out of axon to every 2 potassium ions (K+) into the axon. Axon Surrounding tissue 3Na+ 2K+ Sodium-potassium pump negative positive

Active transport

All this makes the axon more negative relative to the surrounding tissue It also creates a chemical gradient More sodium in the surrounding tissue therefore sodium diffuses into the axon More potassium in the axon therefore potassium diffuses into the surrounding tissue Establishing resting potential

Most sodium ion gates are closed while most potassium ion gates are open Therefore the axon membrane is 100 times more permeable to potassium ions Therefore potassium diffuses back out of the axon faster that sodium diffuses back in making the inside of the axon more negative Establishing resting potential

Membrane is more permeable to Na + than K +

All this makes the axon more negative relative to the more positive surrounding tissue Eventually the electrical gradient prevents the potassium ions continuing to diffuse out of the axon. The ions are too strongly repelled by the surrounding tissue and attracted to the positive axon Establishing resting potential

Equilibrium is therefore reached Chemical and electrical gradients are balanced No net movement of ions Establishing resting potential

Nerve impulse A self-propagating wave of electrical disturbance that travels along the surface of the axon membrane Not a electrical impulse but a temporary reversal of electrical potential across the axon between … Resting potential and action potential

Action potential A stimulus received by a receptor/nerve ending provides energy to generate a temporary reverse in the charge across the axon membrane. Axon depolarised -65mV  +40mV

Steps involved in action potential 1) At resting potential some potassium voltage gated are open but the sodium voltage gated channels are closed 2) Energy from stimulus causes some sodium voltage gated channels to open Sodium ions diffuse into axon Influx of sodium ions opens more sodium channels so even greater numbers of sodium ions move in

4) At +40mV the sodium voltage gated channels close More potassium voltage gated channels now open 5) As electrical gradient now reversed potassium can now diffuse out of the axon The causes more potassium voltage gated channels to open increasing the diffusion of potassium out of the axon causing repolarisation of the axon Steps involved in action potential

6) Outward diffusion of potassium causes a temporary overshoot of the electrical gradient (hyperpolerisation). Potassium voltage gated channels close Sodium-potassium pumps re-establish resting potential Axon repolarised Steps involved in action potential

Neurones Dendrites Axon Facilitated diffusion Schwann cells Active transport Myelin Na + /K + pump Synapse