Page 237 - Biology_F5

Biology for Advanced Level Secondary Schools
Functions of synapses
(a) They maintain the unidirection of
impulses in the nerve fibre. Such
impulses are transmitted at the
presynaptic membrane and are received
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by the postsynaptic membrane, which
ensures one direction flow along a
given pathway.
(b) They amplify impulses from weak
stimuli provided that they reach a
thre-shold-level of action potential.
Repeated low level stimuli can be
amplified as each impulse arrives at
the synapse, causing the release of
more neurotransmitter, resulting in
one larger impulse in the postsynaptic
neurone. Therefore, this allows the
body to respond to the stimuli more
effectively.
(c) They act as junctions that transmit
electric nerve impulses between
neurones, or between neurones and
effector cells. A synaptic connection
between a neurone and muscle cell is
known as neuromuscular junction.
(d) They filter out low level stimuli. For
the impulses to be generated in the
postsynaptic neurone, it must reach
the action potential of about +40
mV. If the impulse has not reached a
threeshold level it is filtered out.
(e) They allow adaptation to intense
stimulation and fatigue. The amount
of transmitted substance, which is
released by synapse steadily falls off
in response to a constant stimulation
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until the supply of the transmitted
substance is exhausted.
(f) They allow convergence, spatial
summation and integration of the
stimuli. In convergence of the stimuli,
the postsynaptic nerves receive
impulses from a large number of
excitatory presynaptic neurones. In
a spatial summation postsynaptic
neurone is able to sum-up the stimuli
from all presynaptic neurones,
where the synapse acts as a centre
for the integration of stimuli from
different sources; hence it produces a
coordinated response.
Exercise 4.2
1. Why are action potential, resting
potential, polarisation and
depolarisation of nerve cells
necessary in the transmission of
impulses in the nerve cells?
2. How will the formation and
conduction of nerve impulses occur
in your hand when you pick up a
ball pen from the table?
3. Each neurone is unique though
impulses are transmitted from
one neurone to the next through
their gaps. How does impulses get
transmitted across the gaps?
4. Why are synapses important in the
transmission of impulses along the
neuro arc?
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