The structure of a myelinated motor neurone. The establishment of a resting potential in terms of differential membrane permeability, electrochemical gradients and the movement of sodium and potassium ions. Changes in membrane permeability lead to depolarisation and the generation of an action potential. The all-or-nothing principle. The passage of an action potential along non-myelinated and myelinated axons, resulting in nerve impulses. The nature and importance of the refractory period in producing discrete impulses. Factors affecting the speed of conductance: myelination and saltatory conduction; axon diameter; temperature.

Structure

A motor neurone is a nerve that carries an impulse for a response from the CNS to an effector.

A mylinated neurone is one which is partially covered by a myelin sheaths. A myelin sheath is actually the membrane of a Schwann cell- the cell wraps itself around the axon of a neurone many times creating the sheath. The sheaths are not conductive because they contain the lipid myelin and therefore keep a signal inside the axon.

In between the myelin sheaths are nodes of Ranvier. These are gaps which are not insulated by the myelin allow signals to leave the axon, how ever they are attracted back in further along- at the next node of Ranvier. They therefore speed up a signal by letting it jump sections- called saltatory conduction.

Signals arrive at the dendrites and are transmitted at the axon terminals.

Resting potential maintenance

When the neurone is not transmitting a signal, it is in a state called resting potential (also called being polarised). In this state the potential difference (between the charges inside and outside the axon) is around -65 mV.

This is maintained by moving positive ions out of the axon- making the outside more positive than the inside. The ions that are moved out are sodium ions (I remember this because the symbol for sodium is Na+ which I pretend stands for Not Allowed (in the axon)).

To move these ions out there is a protein embedded in the membrane which transports 3 Na+ out at a time; as it does this, it also moves potassium ions (K+) in. Potassium ions are positive, however, because only two are moved in for every three Na+ moved out it means that the overall charge stays more positive outside than inside.

There are other ion channels in the membrane, ones K+ and Na+ separately. Many more of the K+ channels are open during resting potential making the membrane around 100x more permeable to potassium. Sometimes potassium moves out through the channel because it is moving down the concentration gradient away from the high concentration of K+ ions inside the axon. Sometimes it is in through the channel because out side is positive which repels its positive charge. The inward and outward movements find an equilibrium.

Action potential

Many of the sodium ion channels are voltage gated and will open due to a positive charge in the axon. A stimulus or signal from a synapse will cause sodium to be released into the axon at the dendrites this increases the positivity, causing the voltage gated sodium channels around to open and the charge inside to be more positive than the outside (known as depolarisation). These then make the area near them more positive and cause the voltage gated sodium channels to open further along the axon (represented by the channel after the dotted line). In this way a wave of positivity moves down the nerve.

When the potential difference of the axon is 40mV, the sodium channels close, and the potassium channels open.

1. Depolarisation occurs as Na+ channels open, it moves in down its chemical and electrical gradients
2. When the mV reaches 40 the Na+ gates close stopping further influx, the K+ gates open
3. Too much K+ leaves the axon, making the potential difference more negative that normal (when it reaches -80 the K+ gates close)
4. K+ moves back in down the chemical and electrical gradients through permanently open K+ channels (then resting potential maintenance resumes)

The all or nothing principle

The potential difference must reach a certain voltage before a voltage gated ion channel will open. Voltage gated sodium ion channels in the axon hillock open at -55mV. These channels opening causes an action potential in the axon.

So if there is enough Na+ coming from the dendrites to make the potential difference -55mV, the channels will open and a action potential will start. If there are some Na+ coming from the dendrites, but not enough to make the potential difference -55mV then no action potential will be started and no signal will be transmitted.

Passage of a signal in a mylinated neurone

 

Localised circuits form between areas where there is an action potential and areas where there is not. In a unmylinated neurone this happens with an adjacent area. In a mylinated neurone the majority of the axon is insulated by the fat, so circuits cannot form with adjacent areas, however there are gaps (nodes of Ranvier) every 1-3mms and circuits form between these areas- this allows the signal to jump between the nodes. Know as salutatory conduction this speeds up the passage of a signal.

 

Refractory period

 

A circuit does not form to both sides of an action potential- it only forms in front of it, not backwards. The reason for this is because in an area where there has just been, there is hyperpolarization, meaning the inside of the axon has an even lower potential difference than usual. So the positive ions, which would usually be enough to open ion channels can't bring the potential difference up high enough to. This is beneficial because it means that signals only travel in one direction down a neurone.

 

The refectory period stops new signals forming immediately in the same way, which enables the body to transmit discrete signals

Speed of signals

 

An increase in temperature increases energy. This means that ions move more and so diffuse more quickly- speeding up an action potential. Also ion pumps work using active transport using ATP which is created during respiration; so enzymes are involved and they work faster with heat until they denature.

 

A larger axon has a smaller surface area ratio- this means that comparatively less ions are lost through leaky channels, speeding up the signal.

 

If the axon is mylinated (which it is in humans) then there is salutatory conduction which speeds up the signal.