Page 231 - Biology_F5

Biology for Advanced Level Secondary Schools

Characteristics of nerve impulses in homoeothermic organsms is greater than
Nerve impulses have the following in poikilothermic organisms.
characteristics:
(ii) Axon diameter
(a) Transmission speed The greater the axon diameter, the higher
Impulses are always transmitted at a very high the transmission speed is. This is because
speed. Depending on the nature of a nerve increased axon diameter minimizes the
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cell, the speed of transmission varies from 0.5 resistance of the axoplasm.
to 100 metres per millisecond. The speed of
impulse transmission is determined by body (iii) Presence of myelin sheaths
temperature, axon diameter, and presence of Axons with myelin sheaths conduct
myelin sheaths. impulses at higher speed than those

without sheaths. The myelin sheath
(i) Body temperature allows impulses to leap from one node
The speed of impulse transmission depends of Ranvier to another, thereby increasing
on the body temperature of the organism, transmission speed (Figure 4.14). This is
such that, the speed of impulse transmission the saltatory conduction.

Nerve impulse propagation

K Na + K Na +
+
+

Axon Myelin sheath cells Nodes of Ranvier

Figure 4.14: Leap of impulse from one node of Ranvier to another in a myelinated axon

(b) Unidirectionality divided into two phases, namely absolute
Impulses always flow in only one direction, refractory period and relative refractory
that is, from cell body to terminal dendrites period. Absolute refractory period lasts

in the neurone, or from pre-synaptic to for 1 ms. In this period, a part of an axon
post-synaptic neurone across a synapse. is unable to propagate an action potential
regardless of the strength of the stimulus.

(c) Refractory period The relative refractory period lasts for 5
During this period, a part of an axon is unable ms. During this period, the impulse can
to conduct a new impulse immediately after be propagated if its strength exceeds a
propagation of an action potential. The threshold value. The significance of the
resting potential is restored by outward refractory period is to ensure unidirectional
movement of K ions and prevention of flow of impulse and to separate one action
+
inward movement of Na ions. potential from the next.
+
Furthermore, the refractory period is

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