Speed of the nerve impulse


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Factors affecting the speed of a nerve impulse


  • The myelin sheath. The myelin sheath that forms around a neurone acts as an electrical insulator, stopping an action potentials forming on the surface of the neurone. It is only at regular breaks in the myelin sheath, called the nodes ranvier, where action potentials can form, and the impulse will jump from node to node by saltatory conduction. This can increase the speed of the impulse from 30ms-1 to 90ms-1.   (The size of the action potential remains the same all the way along.)
  • Axon diameter. The larger the diameter of the axon, the faster the speed of conductance down the axon. This is due to less ions leaking out of the axon. Ion leakage can make membrane potentials harder to maintain.
  • Temperature. Temperature affects the rate of diffusion of sodium and potassium ions across the membrane. The higher the temperature, the faster ions diffuse, the faster the axon membrane becomes depolarised and the faster the nerve impulse travels. In restoring and maintaining resting potentials, ATP is required to operate the sodium-potassium pumps. This ATP comes from respiration, and like most biochemical processes, it is catalysed by enzymes, which work faster at higher temperatures.Above a certain temperature however, enzymes denature and cell membranes break down, reflecting the importance of temperature maintenance in complex organisms
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The Refractory Period

After an action potential has been generated at a point on the axon membrane, there is a small period of time where the membrane cannot be depolarised again. This is because the sodium voltage gates are closed, and any stimulus cannot cause sodium ions to diffuse inward and cause an action potential. This is called the refractory period. The benefits of the refractory period include:

  • Ensuring the action potential propagates in one direction. An action potential can only pass from a depolarised section of the membrane to a section at resting potential. When a region is refractory, it is incapable of passing on the action potential. The only other direction is the opposite direction, meaning that an action potential will always pass in one direction.
  • Discrete impulses. The refractory period prevents a new impulse forming directly behind it, and therefore prevents a continuous stream of impulse being sent to the brain. This allows the brain to analyse the frequency of impulses to perceive intensity.
  • Limiting the number of impulses. The refractory period stops continuous streams of impulses to the brain, stopping it from becoming overloaded with incoherent messages.
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All-o-nothing principle

Nerve impulses are described as an all-or-nothing response. The threshold value of a neurone is the level of stimulation that is needed to depolarise it and create an action potential. If a stimulus falls short, then no impulse is generated at all. If a stimulus exceeds the threshold value, then an action potential is generated, which is independent of how much the stimulus exceeded the threshold.
The brain can interpret the size of a stimulus by analysing the Frequency of impulses sent along a neurone, as a stronger stimuli will generate a higher frequency. The body can also have different neurones with different threshold values, allowing the body to gauge the intensity of a stimulus.

Any stimulus, of whatever strength, that is below the thresholdvalue will fail to generate an action potential- this is the nothing part. Any stimulus above the threshold value will succeed in generating an actino potential. It doesnot matter how much aove the threshold a stimulus is, it will still only generate one action potential - this is the 'all' part. How then can an orgnaism percieve the size of the stimulus? This is acheived in 2 ways 
- by the number of impulses passing in a given time. The larger the stimulus, the more impulses that are given
- by having different neurones with different threshold values.  The brain interprets the number and tpe of neurones that pass impulses as a result of a given stimulus and thereby determines its size. 

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