An Action Potential (impulse)
The impulse, in the form of this reversal of charge, then runs the length of the neurone fibre:
The transmission of an impulse
The Refractory Period
For a brief period following the passage of an action potential, the axon is no longer excitable. This phase is called the refractory period.
Initially the block on the conduction of a second impulse is absolute; that is to say, no stimulus will generate an impulse. This brief period is known as the absolute refractory period. At this stage there is a huge excess of sodium ions inside the nerve fibre, and the membrane is temporarily impermeable to the passage of ions.
Subsequently, in the relative refractory period, the resting potential is progressively restored. The first stage of restoration is due to the outward diffusion of potassium ions. In the next stage, the continuing actions of the potassium/sodium ion pumps and the differing rates of diffusion of sodium and potassium ions re-establish the potential difference across the membrane. During this period it becomes increasingly possible for an action potential to be generated.
The Threshold Of Stimulation
A stimulus must be at or above a certain minimum strength, known as the threshold of stimulation, in order to initiate the transmission of an action potential.
Thus a stimulus evokes either a full response or no response at all. When a stimulus is too weak, the influx of sodium ions into the neurone is slight, and normal polarity is very quickly re-established. Such a stimulus is said to be sub-threshold.
With very strong stimulation, the frequency of action potentials increases, up to the maximum rate permitted by the refractory period.
The size of the action potentials is more or less constant. However, it does not depend upon the strength of the stimulus.
The Speed Of Transmission
Two structural features of axons influence the speed of conduction of an action potential:
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The diameter of the axon. The amount of resistance that an action potential experiences is related to the diameter of the axon. The narrower the axon, the greater it's resistance and the lower the speed of conduction of the action potential. The large the diameter, the faster the axon will conduct action potentials. For example, the Squid has 'giant' nerve fibres in order to allow it to jet propel itself away from danger very quickly.
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Myelin sheaths. Myelin consists largely of lipid, and has a high electrical resistance. At certain points along the myelinated axon, the axon membrane is exposed. These points, known as 'nodes of ranvier' are the junctions between the sheath cells (Schwann cells). In myelinated axons, the action potential jumps from node to node (as depolarisation is restricted to nodes) at high speed. This is referred to as saltatory conduction (as the impulse leaps along). By contrast, depolarisation of an un myelinated axon is referred to as continuous conduction and is much slower.
Transmission of an action potential across a myelinated axon
Saltatory conduction has two advantages:
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In humans, un myelinated fibre nerve impulses travel at 1 to 3 metres per second, while myelinated fibres conduct at speeds of up to 120 metres per second.
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Metabolically, saltatory conduction is economical, because fewer ions move across the membrane, so the ion pumps need less energy to restore the ionic balance.
The speed of transmission of a nerve impulse is also determined by the number of synapses involved. Communication between neurones across the minute gaps at the synapses involves chemical release and a brief time delay. Therefore, the greater the number of synapses (or number of neurones) in a series of neurones, the slower the conduction velocity.
