Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
When receptors are stimulated they are depolarised.
If the stimulus is very weak, the cells are not depolarised
very much and the sensory neurone is not activated to
send impulses (Figure 15.19). If the stimulus is stronger,
then the sensory neurone is activated and transmits
impulses to the CNS. If the receptor potential is below
a certain threshold, the stimulus only causes local
depolarisation of the receptor cell. If the receptor potential
is above the threshold, then the receptor cell stimulates the
sensory neurone to send impulses. Above this threshold,
action potentials are initiated in the sensory neurone.
This is an example of the all-or-nothing law: neurones
either transmit impulses from one end to the other or
they do not. As we have already seen, the action potentials
always have the same amplitude. As the stimuli increase
in intensity, the action potentials are produced more
frequently. The action potentials do not become bigger;
they have the same amplitudes, not larger ones.
Threshold levels in receptors rarely stay constant all
the time. With continued stimulation, they often increase
so that it requires a greater stimulus before receptors send
impulses along sensory neurones.
QUESTION
15.4 Use Figure 15.19 to answer these questions.
a Explain what is meant by the terms:
i receptor potential
ii threshold receptor potential
iii the all-or-nothing law.
b Describe the relationship between the strength of
the stimulus and the size of the receptor potential
that is generated.
c Describe the relationship between the strength of
the stimulus applied and the frequency of action
potentials generated in the sensory neurone.
d What determines the maximum frequency of
action potentials in a neurone?
e Threshold potentials in receptor cells can
increase and decrease. Suggest the likely
advantages of this.
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High frequency of
impulses generated in
the sensory neurone
when receptor is
given a strong
stimulus.
Receptor potential
Low frequency of
impulses produced
when receptor is
given a weak stimulus.
threshold receptor potential
No impulses
produced when
receptor is given
a very weak stimulus.
Strength of stimulus
Figure 15.19 As the strength of a stimulus increases, the receptor potential also increases. If the receptor potential reaches the
threshold, then impulses are sent along the sensory neurone at low frequency. Increasing the strength of the stimulus above the
threshold increases the frequency of the impulses; it does not change their amplitude.