Neurones can generate nerve impulses over a wide range of frequencies. The frequency
is limited by the absolute refractory period, during which the neurone is completely
inexcitable, and the relative refractory period, during which it is less excitable than normal
During the absolute refractory period the sodium ion channels are open.
During this period, which lasts about a millisecond or less, a second stilumuls
will not trigger a second action potential no matter how strong the stimulus is.
The relatve refractory period follows the absoulte refractory period. It
corresponds to the time when the extra potassium channels open to
repolarise the membrane and potassium ions flood out of the axoplasm
causing the membrane to become briefly more negative than the normal
resting potential. During this period which lasts several milliseconds, a
greater than normal stimulus is needed to indicate an action potential.
However, the membrane becomes progressively easier to stimulate as
the relative refractory period proceeds
By limiting the maximum frequency of nerve impulses, the refractory
period enables the nervous system to distinguish seperate stimuli and
make coordination possible. IT ensures each nerve impulse is
sperated from the next, with no overlapping signals, an essential
feature for a system conveying frequency code information
Only vertebrates have a myelin sheath surrounding neurones.
Saltatory conduction increase the speed of propagation dramatically.
Unmyelinated neurones transmit impulses at a maximum speed of
about 1ms, while myelinated neurones have speeds of up to 100ms.
Nerve fibres vary in diameter from about 0.5 to 1000 micrometers. Unmyelinated axons with wide
diameters can transmit nerve impulses faster that those with small diameters. The conduction speeds of
myelinated fibres also increase with axon diameter, but the advantage of myelination means there is no
need for giant axons. The greatest advantage of mylinated axons comes from their small size, which
allows a highly complex nervous system with high conduction speeds that does not take up much space.
The conduction speed of a nerve impulse is strongly affected by temperature. Within limits, the higher the
temperature, the faster the speed. This is mainly because the propogation of an impulse involves diffusion
of ions, and the rate of diffusion increases with temperature as a result of the increased kinectic activity of
the ions. Temperature also affects the integrity of membrances and the actions of enzymes involved in the
active transport required to maintain the resting potential. At very high temperatures, membrances may be
damaged and enzymes denatured, resulting in disruption of the nerve conduction.
An important consequence of the relationship between temperature and conduction
speed is that homoiotherms generally have fast responses irrespective of the
enviromental temperature, while poikilotherms such as snakes can only respond
quickly when their bodies have been warmed by, for example, the sun.