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Nerve impulse transmission

The fundamental function of neurones (nerve cells) is the transmission of signals from sense organs such as the skin and eyes to the CNS (Central Nervous System – brain and spinal cord) and from the CNS to effector organs.

Each neuron is made up out of a cell body, from which small projections called dendrites protrude. One specially lengthened dendrite is called the axon and this is usually coated in an electro-insulating compound called myelin that speeds the signal. At the end this divides into many branches, meaning that the signal can be passed onto many target cells at once.

The signal carried by the neuron is transmitted in the form of an electrical current, carried by the changes in potential difference (voltage) across the membrane of the axon.

At rest the neurone membrane is polarised, and has a potential difference of around -70mV. This is caused by the action of a co-transporter, which carries 2 potassium ions into the axon for every 3 sodium ions that it transports out. This creates a high potassium concentration inside the axon, and a high sodium concentration outside, but the greater concentration of sodium means that the outside of the membrane is more highly charged than the inside.

When an action potential is propagated in the axon sodium channels that are sensitive to the membrane potential difference (described as being voltage gated) open suddenly and allow sodium ions to move inside the membrane. This is referred to as depolarisation, and when a threshold value of -40mV is reached the rest of the sodium channels in the membrane open and full depolarisation occurs, raising the potential difference to 40mV.

Before another action potential can be propagated it is necessary for the axon to become repolarised. This is achieved by closing the sodium channels, the activation of voltage-gated potassium channels and restarting the co-transporter. Rather than returning straight to -70mV hyperpolarisation occurs, in which the membrane becomes more polarised than it needs to be, leading to a membrane potential difference of -75mV. The potential is then subsequently reduced to -70mV by the refractory period.

Synapses

Between one neuron and the next there exists a gap known as the synapse or synaptic cleft (around 20nm across), which the signal carried by the neuron must cross. A high concentration of a neurotransmitter such as acetylcholine is released into the synapse and binds to receptors on the post-synaptic membrane, triggering the action potential in this neuron. The neurotransmitter is stored in vesicles near to the edge of the pre-synaptic membrane, and is only released by exocytosis when the action potential reaches the synapse.

The depolarisation of the membrane opens voltage gated calcium channels, which allow calcium ions from outside the cell to enter the cytosol. The calcium concentration outside the cell is over 1000 times that inside it so the effect upon the membrane potential is massive. This rise in internal polarity triggers the fusion of the synaptic vesicles with the plasma membrane.

Comments

• Suitable for: Biology A Level.
• Written by: Bekaboo.
• From this post.