Nerve Impulse

1
Nerve Impulse -
A
occurs when
an action potential in one region of a nerve
fiber membrane causes an electric current to flow
to adjacent portions of the membrane. It then
stimulates the membrane to units threshold level
and triggers other action potentials which
stimulate other potentials and travel to the end
of fibers.
2
Impulse Conduction
Key . 's
Myelin serves an insulator that prevents almost
all of the flow of ions through a membrane. An
unmyelinated nerve fiber (a fiber lacking myelin)
conducts an impulse over its entire surface.
However, a myelinated fiber does not prevent the
conduction of a nerve impulse altogether because
they contain constrictions called nodes of
Ranvier, which occur between adjacent Schwann
cells. At these nodes the fiber membrane can
become permeable to sodium and potassium ions and
a nerve impulse can appear to jump from node to
node. This type of impulse conduction is called
Saltatory Conduction and is many times faster
than conduction in a myelinated fiber.
3
the
All-or-None Response
Nerve impulse conduction is an all-or-none
response. It either responds completely or not at
all. Therefore, a nerve impulse is conducted
whenever a stimulus of threshold intensity is
applied to a nerve fiber and all impulses carried
on that fiber are of the same strength. A greater
intensity does not produce a stronger impulse.
4
"FYI"
procaine cocaine
These drugs produce effects by decreasing
membrane permeability to sodium ions. When one
drug is present in the surround nerve fiber
fluids, impulses are prevented from passing
through the region. These drugs are used as local
anesthetics because they help keep impulses from
reaching the brain and prevent sensations of
touch and pain.
5
S
ynapse

• Nerve impulses travel from neuron to neuron
  along complex nerve pathways.
• The junction between these neurons is called the
  synapse
• The neurons are never in direct contact they are
  separated by a gap called the synaptic cleft.
• For an impulse to continue along a nerve pathway
  it must cross the gap.

6
Synaptic Transmission
A nerve impulse within a neuron travels from a
dendrite to its cell body and then moves along
the axon to its end where it crosses a synapse
and continues to a dendrite or cell body of
another neuron. This process is known as synaptic
transmission.
Axons have several rounded synaptic knobs at
their ends. Dendrites lack these synaptic knobs.
The synaptic knobs contain numerous membranous
sacs called synaptic vesicles. When a nerve
impulse reaches a knob, some of the vesicles
respond by releasing a neurotransmitter.
The neurotransmitter diffuses across the cleft
and reacts with specific receptors of the neuron
membrane. If enough of the transmitters are
released, the membrane is stimulated and a nerve
impulse is triggered.
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Neurotransmitter
Neurotransmitters Acetylcholine- stimulates
skeletal muscle contractions, decomposed by
enzyme cholinesterase Monoamines-epinephrine,
norepinephrine, dopamine and serotonin which are
all formed from modified amino acid molecules.
Inactivated by enzyme monoamine oxidase found in
the mitochondria. Amino Acids- glycine, glutamic
acid, aspartic acid, and GABA Peptides- consists
of a relatively short chain of amino acids.
These substances are usually synthesized in the
cytoplasm of the synaptic knobs and stored in the
synaptic vesicles.
Substances
8
A
fter the release of some neurotransmitters into
the synaptic cleft, some are destroyed by enzymes
present in the cleft. Others are removed from the
cleft by being transported back into the synaptic
knob that released them or into nearby neurons or
neurological cells. The destruction or removal
of neurotransmitter substances is important in
preventing a continuous stimulation of the
postsynaptic neurons.
9
E

xcitatory
I
nhibitory
actions

• Neurotransmitters that cause a decrease in
  membrane permeability to Na ions causing the
  threshold stimulus to be raised.
• Lessens the chance of a nerve impulse transfer to
  the adjoining neuron.
• - Include amino acids GABA and glycine.

Neurotransmitters that cause an increased
membrane permeability to Na ions and trigger
nerve impulses. - Include serotonin, dopamine,
and norepinephrine.
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Synaptic knobs of a thousand or more neurons may
communicate with the dendrites and cell body of a
particular neuron. Neurotransmitters released by
some of these knobs probably have an excitatory
action. Those from other knobs more likely have
an inhibitory action. If more excitatory
neurotransmitters are released, the neurons
threshold may be exceeded and a nerve impulse
will trigger to pass over its surface. If more
inhibitory neurotransmitters are released no
impulse will be conducted.