A nerve impulse is an electro-chemical message that travels along the length of a nerve fibre.

Process of a Nerve Impulse

The threshold is the minimum voltage required to pass a signal along an axon following a stimulus, where this is -55mV. It stimulates voltage-gated channels. An action potential is simply the rapid depolarisation and repolarisation of the neuron cell membrane.

The process of an axon potential is as follows:

1. the stimulus: some sodium ligand-gated channels open following stimulation, increasing the flow of sodium ions into the cell and increasing the potential difference.

2. depolarisation: the sodium voltage-gated channels are opened, leading to a rapid increase in the membrane’s polarity, reaching approximately +40mV.

3. repolarisation: the sodium voltage-gated channels close and the potassium channels open, leading to a decreased membrane potential due to the interior being more negative.

4. hyperpolarisation: the potassium channels are opened for longer than needed, leading to the membrane potential dropping lower than the resting membrane potential at -75mV.

Refractory Period

A neuron becomes unresponsive to an additional stimulus following the initial firing of an action potential as a result of the sodium voltage-gated channels inactivating. This lasts from when -55mV to back at -70mV.

Synaptic Transmission

A synapse is a small gap between neurons.

The process of transmitting a nerve impulse between neurons include:

1. the activation of calcium voltage-gated channels upon reaching the axon terminal.

2. calcium ions flow into the axon terminal due to being in high concentration in the extracellular fluid.

3. the synaptic vesicles fuse with the cell membrane and release the neurotransmitters via exocytosis.

4. neurotransmitters diffuse across the synapse and attach to post-synaptic cell receptors.

5. ligand-gated channels are stimulated on the post-synaptic neuron.

Role of Neurotransmitters

Neurotransmitters are chemicals that enable activity across the synapse.

The main neurotransmitters of concern include:

• dopamine is involved with learning, attention, motivation, pleasure and reward, where a deficiency can lead to a lack of focus or Parkinson’s disease - characterised by muscle tremors.

• serotonin is involved with the constriction of smooth muscle, sleep, carbohydrate cravings, pain control, digestion and mood, where a deficiency can lead to depression.

• nor-adrenaline functions to provide stimulatory body responses, being associated with concentration, recall, obsession and determination - works with alpha receptors to increase blood pressure.

  • it is mainly produced in the nerves, but sometimes synthesised in the adrenal medulla.

  • dysfunctions associated with nor-adrenaline can lead to depression and dementia.

• acetylcholine is found at the neuromuscular junction to connect motor neurons to muscles as well as contract smooth muscle, dilate blood vessels and increase bodily secretions.

• endorphins work to regulate feelings and perceptions of pain.

• adrenaline is similar to nor-adrenaline but is only released during stress instead of continuously in small amounts, and is predominantly synthesised by the adrenal medulla.