Neurones and Action Potentials

The function of neurones is to transmit an action potential from one part of the body to another. There are different types of neurones including motor, sensory and relay neurones but they all have a similar basic structure.

• Many neurones are long as to transmit the action potential over a long distance.
• They have sodium/potassium pumps that use ATP to actively transport sodium ions out and potassium ions into the cell.
• The cell surface membrane has gated ion channels which control the amount of sodium/potassium/calcium ions that across the membrane in/out of the cell.
• The axon is surrounded by a fatty myelin sheath,which is actually a series of schwann cells, that insulate the neurone. The gaps in between the myelin sheath at intervals are called nodes of ranvier.

Sensory Neurones:

Sensory neurones carry an action potential from a sensory receptor to the central nervous system (CNS).

• The cell body is in the middle of the neurone.
• They have a long dendron carrying the action potential from a sensory receptor to the cell body which is positioned just outside the central nervous system.
• There are dendrites at the end of the dendron
• An axon carrying a nerve impulse from the cell body to the CNS.

Motor neurones:

Motor neurones carry an action potential from the CNS to an effector such as a muscle or gland.

• They have a cell body at the end of the neurone which is in the CNS.
• Many short dendrites that carry impulses towards the cell body.
• A long axon which carries the impulse away from the cell body and ends in a motor endplate.

Relay neurones connect sensory and motor neurones.

A resting potential is when a neurone is not transmitting an action potential. The neurone will actively transport three sodium ions out for every two potassium ions into the cell using sodium/potassium pumps.The plasma membrane is a lot more permiable to potassium ions and therefore potassium ions may diffuse out again. This maintains the inside of the neurone at a negative potential compared to the outside (-60mV) and the cell membrane is said to be polarised.

An action potential consists of the following stages:

• The membrane starts in it's resting state (as described above), where the potential difference is -60mV.
• Sodium ion channels open and some sodium ions diffuse into the neurone.
• The membrane then depolarises and reaches the threshold value of -50mV.
• Voltage-gated sodium ion channels open and many more sodium ions flood into the neurone.
• The potential difference across the membrane then reaches +40mV.
• The sodium ion channels close and the potassium ion channels open.
• Potassium ions diffuse out of the cell and bring the potential difference back to negative (repolarised), however the potential differences overshoots slightly making the cell hyperpolarised and then the original potential difference is restored and the cell returns to it's resting state.

Local currents are the movement of ions across the neurone caused by the diffusion of ions away from a region of higher concentration. This is outlined in the following steps:

• When an action potential occurs the sodium ion channels open at a particular point.
• This allows sodium ions to diffuse across the membrane from the region of higher concentration to lower concentration i.e. from outside the neurone to inside the neurone.
• This increases the concentration of sodium ions inside the neurone at the point where the channels open.
• This causes the sodium ions to diffuse sideways away from the region of high concentration which causes a local current.