Describe nerve impulse, resting membrane potential, action potential and its transmission between cells
Nerve impulse:
Electrochemical
signal developed by a neuron for communication is called nerve impulse. Most of
the neurons develop an impulse in resting membrane potential and action
potential.
(1) Resting
membrane potential (RMP): It was first studied in squids that unstimulated
inactive neurolemma (Plasma membrane of neuron) maintains a charge of -65 mv.
This electrical potential is called resting membrane potential. This polarity
is due to unequal distribution of ions across neurolemma. It was found that
there is greater concentration of sodium ions outside than inside the membrane.
Similarly Potassium ions are concentrated much inside than outside the
membrane. This is attributed to the activity of ATP driven Sodium Potassium
Pump in neurolemma. Towards the inner surface of the membrane, negative organic
ions such as proteins, organic acids, are more concentrated which make the
polarity of neurolemma negative.
(2) Action
Potential: It is temporary and local reversal of the polarity of neurolemma
which arises when neurolemma is stimulated (e.g. electric shock, touch, sound,
vibrations etc.). During the action potential, the polarity of neurolemma first
changes to +40 mv (i.e. depolarization) and then restores to -65 mv again i.e.
repolarization.
This action
potential is extremely rapid as it takes only few milliseconds to happen.
The change in
potential across membrane is due to the presence of Sodium and Potential channels
in the neurolemma. Upon stimulation sodium channels open to allow the pushing
in of Na+ ions inside the cell, so the membrane potential changes
from negative to zero and then proceeds upon +40 m. At this stage sodium
channels are automatically shut down but simultaneous potential channels open
to allow their outward movement. As a consequence the + vehy charged membrane
gradually restores its negative charge of -65 mv. Since sodium ions accumulate
inside and potassium outside at this stage sodium potassium pump becomes
operational to restore the initial accumulation of sodium outside and potassium
inside.
Action potential
transmission between cells:
After an action
potential travels along an axon it reaches the end of a branching axon terminal
called the end bulb. The synapse in the junction between axon of one neuron and
he dendrite of another neuron or effector cell. The space between the end bulb
and the dendrite of the next neuron is the synaptic deft. The neuron carrying
the action potential toward a synapse is presynaptic neuron. It initiates a
response in the receptive segment of post synaptic neuron leading away from the
synapse. The presynaptic cell is always a neuron but the post synaptic cell can
be a neuron muscle cell or gland cell.
Types:
(1) In an
electrical synapse, nerve impulses transmit directly from neuron to neuron when
positively charged ions move from one neuron to the next. These ions depolarize
post synaptic membrane as two neurons were electrically coupled. An electrical
synapse can rapidly transmit impulses in both directions. Electrical synapses
are common in fishes and partially account for their ability to dart swiftly
away from a threatening predator.
(2) In chemical
synapse, two cells communicate by means of chemical agent called
neurotransmitter. While the presynaptic neuron releases. A neurotransmitter
changes the resting potential in the plasma membrane of post synaptic cell,
creating an action potential in that cell continues the transmission of the
impulse when a nerve impulse reaches an end bulb, it causes storage vesicles to
fuse with the plasma membrane. The vesicles release the neurotransmitter by
exocytosis into synaptic cleft acetyl chlorine or norepinorphrine.
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