Page 232 - Biology_F5

Coordination and Irritability

(d) All-or-nothing law across a synapse is carried out by chemical
According to this law, for the action substances called neurotransmitters;
potential to be propagated, the stimulus which are stored in the synaptic vesicles.
applied should exceed a threshold value. The presynaptic membrane is modified
The threshold value is the minimum
energy level, and when reached, the action for the attachment of synaptic vesicles
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potential will be generated. However, and the release of transmitter substance
the size of the action potential will not into the synaptic cleft. The postsynaptic
decrease as it is transmitted along the membrane contains large protein
neurone, but it will always remain the molecules, which act as receptor sites for
same. In other words, the action potential the transmitter substances and numerous
is both generated and kept the same or channels and pores for the movement of
not generated if the threshold value is not ions into the postsynaptic neurone (Figure
exceeded.
4.15).

(e) Propagation (Conduction)
A nerve impulse is conducted as a wave The neurotransmitter substance is either
of depolarisation that moves along the produced by the cell body of the neurone
surface of the nerve cell. This means or synaptic knob. These substances
that progressive depolarisation of the are synthesised by enzymes stored in
axon membrane leads into impulse the cell body. The neurotransmitters
transmission. allow the transmission of signals
from one neurone to the next across
Structure of a chemical synapse synapses. There are many types of
The point where the axon of one neurone neurotransmitters, and these include
joins the dendrite or cell body of another
neurone is known as a synapse. The acetylcholine, norepinephrine, serotonin,
membrane of the first neurone connecting dopamine, and glutamate. The two
to the synapse is called a presynaptic common neurotransmitters in vertebrates
membrane while the other membrane of are acetylcholine (Ach; an ammonium
the next neurone is called a postsynaptic compound) and norepinephrine (also
membrane. These membranes of the two called noradrenaline). Neurones using

neurones are separated by a gap of about acetylcholine as a neurotransmitter
20 nm called the synaptic cleft. At the are described as cholinergic neurones,
end of the presynaptic neurone, there is a while those using norepinephrine
bulge called synaptic knob. (noradrenaline) are called adrenergic
neurones. Norepinephrine is released in
The cytoplasm of the synaptic knob the sympathetic nervous system by some
contains numerous mitochondria and nerves, while acetylcholine is released by
small synaptic vesicles. Transmission all nerves except some nerves in the brain.

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