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The Nerve Impulse
Junction between the terminal branches of a meuron and the membrane of another cell is
called the sysnapse.
A microscopic gap between the end of the terminal brnaceh and the adjoining cell. Impulses
are transmitted across the gap.
Each axon may synapse with as many as 1000 other neurons and may form many synapses
with each of these neurons.
Interconnections and impulse pathways of a typical nervous system are enormously complex.
The Nerve Impulse
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nerve impulse is an impulse from another nerve or a stimulus from a nerve receptor.
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positive and outside negative).
This reversal occurs in a small area of the membrane and results in a flow of electrical
current that affects the permeability of the adjacent areas of the membrane.
The reversal of polarization is the nerve impulse and it travels the length of the axon.
High permeability of the membrane to sodium ions last only a fraction of a second and
then returns to normal.
The sodium pump and potassium diffusion allow normal distribution of ions to be
restored.
A brief recovery period occurs during which the nerve cell membrane cannot be
stimulated to carry impulses. This refractory period lasts a few thousandths of a
second.
The rate at which an impulse travels depends on the size of the nerve and wether or
not it is myelinated (unmyel
In myelinated fibers the signal jumps from one node of Ranvier to the next.
This is saltatory conduction ans occurs because the membrane at the node is highly
sensitive and this uses less energy due to polarization only at the nodes.
For a nerve impulse to be transmitted, the stimulus must be at least a certain minimum
strength or must reach a threshold.
The impulses transmitted by a given neuron are all alike, a neuron operates on all or
none basis.
The strength of the stimulus os measured by two effects.
a. A stronger stimulus causes more impulses to be transmitted each second.
b. Different neurons have different thresholds.
A large number of neurons fire when a stimulus is stronger.
Recognition of the type of stimulus is determined by the particular pathways that carry
the nerve impulses. For example, the light sensitive receptors in the retina of the eye
transmit nerve impulses only when light strikes them.
Transmission at the Synapse
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the transmission of the impulse across the synaptic cleft is a chemical process.
Within the synaptic knob, the synaptic vesicles contain neurotransmitters or
neurohumors which are chemicals such as acetylcholine and norepinephrine.
When an impulse reaches the synaptic knob, the synaptic vesicles fuse with the
membrane of the synaptic knob and release their contents into the synaptic cleft.
Special receptor proteins in the membrane of the neighboring dendrite sense these
neurotransmitters.
When the impulses are arriving at a faster rate (representing a stronger initial
stimulus), more neurotransmitter is released into the synaptic cleft and more impulses
per second are sent.
When the neurotransmitter has done its work, it is removed from the synaptic cleft by
an enzyme that breaks down the molecules.
acetylcholine, norepinephrine, histamine, and glutamic acid
serotonin, epinephrine, and glycine
If the overall results are excitatory, impulses are transmitted down the axon to the
next set of synapses. If the results are inhibitory, no impulses are transmitted.
 Much of the complex behaviour of an organism results from the great number and
variety of synaptic circuits formed when neurons are switched on and off.
Neuromuscular Junctions
 he passage of impulses from motor neurons to muscles occur at special points of
contact called neuromuscular junctions.
 The motor end plates contain synaptic vesicles which release acetylcholine which
combine with receptors molecules on the muscle cell membrane, thus sending an impulse
to the muscle.
 The acetylcholine causes muscle cell membrane to become more permeable to sodium,
causing an impulse to travel the membrane and the muscle cell to contract.
Drugs and the Synapses
Many poisons and drugs affect the activity of chemical neurotransmitters at the synapses.
Nerve gas, curare, botulin toxin, and some poisonous insecticides can interfere with the functioning of
acetylcholine and cause muscle paralysis (death for respiratory paralysis).
Stimulants cause a feeling of well-being, alertness, and excitement such as amphetamines (mimic
norepinephrine by binding to receptors) and caffeine (aids in synaptic transmissions).
Depressants slow the body activity or cause depression such as barbiturates (block the formation of
norepinephrine).
Some mind altering or hallucinatory drugs such as LSD or mescaline interfere with the effect of the
inhibitory transmitter serotonin.