Nerve Impulse &Its' transmission
outside our
about what is happening both inside and
system receives information
the
addition,
In
information.
to this
body.It also directs the way in which our body responds
T h e nervous
nervous system helps maintain homeostasis.
After
can make an organism react.
signal in the environment that A RESPONSE is
U A STIMULUS is any change
does
body
our
what
it causes a response.
our nervous system analyzes a stimulus,
are called
information through your nervous system
that
cells
carry
stimulus.The
in reaction to a
IMPULSE.
NERVE
that a neuron carries is called a
NEURONS, orNERVE CELLS. The message
or
the nerve fibre membrane that is
is that electrochemical change in
its termination.
is carried along the nerve fibre to
aroused/excited by a stimulus and
chemical
overall physiological changes (physical,
Physiologically a nerve impulse is the
nerve
stimulated.Thus,
is
it
nerve fibre when
and electrical changes) that occur in the
mechanical,
that occur in a neuron due
impulse is the overall physiological changes
stimulus.
chemical or electrical disturbance created bya
Nerve
impulse
from
one
end to the other
occurs as a
ofa nerveimpulse along a
neuron_ Its propagation through
result of electrical changes across the membrane of the IMPULSE CONDUCTION.
neuron
T h e transmission
is called NERVE
axon, synapse and neuromuscular junction
Characteristics of nerveimpulse
stimulus.
a)
b)
c)
d)
e)
a threshold
a nerve fibre on receiving
A nerve impulse is generated by
direction.
the nerve fibre usually in one
The nerve impulse is conducted along
fibre
nerve
(the thicker fibres
the
the diameter of
The conduction velocity depends upon
to a
shows higher velocity) and temperature.
a maximum response
nerve fibre will always give
law'
single
n
o
n
e
(a
or
no
It follows 'all
stimulus be further increased,
or duration of the
the
strength
suitable stimulus; if
will occur).
and involves the consumption
change in the response
nerve
impulse is a metabolic process
of
The transmission
heat and movement of ions.
of carbon-di-oxide and
when a
production
oxygen,
PERIOD for proper functioning (i.e.,
a brief REFRACTORY
requires
fibre
nerve
brief
a
period.
A
stimulus after
it will respond to a 2nd
once stimulated,
nerve fibre is
NERVE IMPULSE:
MECHANISM
OThe transmission
ionic theory of
OF TRANSMISSION
of nerve impulse
nervous
has been explained
conduction as
proposed by
by
the so-called
membrane or
&Andrew
Lloyd Hodgkin
of nerve impulse depends
Alan
conduction
to it, the
Fielding Huxley. According
AXOLEMA to Na+ and K+ ions
permeability of
differential
The
membrane
outside the axon
gradient in and
the
transport of ions against
electrochemical ionic
electrochemical
on
are
gradient by Na+
Na+ & K+
by
million or so ATP powered
neuron has a
Each
trans-membrane pumps.
action potential
K+ ion
it to keep conducting
membrane to enable
its
into
pumps built
maintained
the active
indefinitely
POLARIZED-that
T h e membrane
difference
in
of
an
electrical
UNSTIMULATED NEURON_is
and inside of the
between the outside
charge
theoutside
insideisnegative with respect to
is, there is a
The
membrane.
A) Transmission through a NON-MYELINATED NERVE fibre (continuous
conduction):
Transmission of nerve impulse along nerve fibre can be summarized in three steps:
a) Polarization (Resting Potential)
b) Depolarization (Action Potential)
c)Repolarization
a) Polarization (Resting Potential):
A neuron at resting is electrically charged
but not conducting (i.e., when a
neuron is not actively transmitting a nerve impulse, it is in a resting state, ready to
transmit a nerve impulse).
there is a
The membrane of an unstimulated neuron is polarized-that is,
difference in electrical charge between the outside and inside of the membrane. The
the AXOPLASM
or
plasma
membrane
inside is negative with respect to the outside.Thus,
fluid.
of a resting neuron is NEGATIVELY CHARGED as compared to the interstitial
a+ ion which is about 16
The interstitial fluid has high concentration of
times
higher outside the neuron than inside the neuron. Similarly, the axoplasm has high
Concentration of K+ ion which is about 25 times higher inside than in outer interstitial
fluids.Due to difference in concentration of ions, Na+ ion tends to diffuse into the
axoplasmand K+ ion tends to diffuse outside the axoplasm.
to K+ ion than
Thus, the membrane of neuron at resting is more permeable
difference in
neuron.The
the
enter
Na+
Na+ ion. So, K+ leaves the n e u r o n faster than
permeability results in accumulation of high concentration of cation(+ve charged
ion) outside the neuron
compared
inside.This state of
to the concentration of cation
charged.The
is called POLARIZED STATEand it is electro-negatively
POTENTIALwhich is
at this stage is called RESTING
measured
difference
potential
about-70mV to -90mV.
pump maintains a
[During the RESTING STATE, the sodium-potassium
resting
neuron
difference in charge across the cell membrane. It uses ener8y in ATP to pump positive
cell. The sodium:
the cell and potassium ions (K*) into the
sodium ions
(Na*)
out of
(electrical,
potassium pumpstops operating when a stimulus
is applied to a membrane of a nerve fibre.]
b)
chemical or
mechanical)
Depolarization (Action Potential):
such stimulus
the threshold can initiate an impulse.When
Any stimulus beyond
the permeability of
it opens the sodium channel. Now
is applied in the resting neuron,
DEPOLARIZATION.The
stimulus
causing
increases at the point of
Na+ ion suddenly
outside to inside. As a result the
increases by 10 times from
diffusion of Na+ ion
which is exact
axoplasm become positively charges,
depolarized stateor
reverse
opposite to polarized state, so called
polarized state.
(in one direction only,from dendrite
ion channels,
carried out by the opening and closing ofvoltage-gated
to axon terminal) is
an action
create
potential to
the resting membranethe
across the
which causesa brief reversal oftravels
changes
polarity
the
axon,
down
potential.As an action potential
neurons.
stimulates other
as
Thus,
transmission
membrane. Once the
Thus, a
ofa signal within a
neuron
it
signal reaches the axon terminal,
nerve
acrossthe
charge
of theelectrical
impulseisa sudden reversal
is called an action potential.In
of
reversal
charge
the
along
travelling
potential
The
membrane of a resting neuron.
other words, the action
+30 mV.
impulse. It is around
membrane
is called the
nerve
Thus,
action
potentials are Jormed
whena stimulus
causes
the cell membrane to
depolarize past the threshold of excitation, causing all sodium_ jon_channels to open.The
depolarization of the membrane stimulates the adjacent voltage channel, so the action potential
the membrane is
wave along the lengthof neuron. The action potential travelling along
passes
called as
thea nerve
impulse. It is around + 30 mV. The sodium-potassium pump starts to operate
once the action potential is completed].
c)Repolarization:
When the concentrationof Nat ion inside axoplasm increases, the permeability
and the Na channel starts to CLOSE, The Na-K pump activates, so that
toNa+decreases
Nat are pumped out and K+ inside until the originalresting potentialis restored. The
and it starts from the same point from where
starts.Thus, the sodium-potassium pump starts to operate once the
process is known as
depolarization
repolarization
action potential is completed. As a result, the axon membrane will obtain a resting
potential by repolarization.
REFRACTORY PERIOD: With the passage of the action potential, the cell membrane is in
an unusualstate of affairs. The membrane is polarized,
but the NA and K*are
on
the
Wrong sides of the membrane.During this refractory period, the axon will not respond
re-establish
stimulustoTotheir
a newreturned
to
K*are
resting
the original distribution of these ions, the Na* and
potential location by Na /K pumps in the cell
membrane.
Once these ions are completely returned to their resting potential location, the neuron
is ready for another stimulus.
T h e entire process of POLARIZATION, DEPOLARIZATION and REPOLARIZATION
occur within fraction of seconds. Now, again the neuron is ready for another impulse.
Sona
Axcn ermnal
AxOn
Drection of uUavel of acton potential
a. In response to a signal, the
Soma end of the axon becomes
depolarized
Depolarized
4+
++4
b. The depolarzation spreads don
the axon Meanwhle, the hirst pa't of
the membiane repolatiZes Because
Na' channels are inactwaled and
addhttornal K channels have ofperned,
the membrane canret tepolarnze again
- - - -
-
+t+4t++
Rpolarizung
Rastirg
Depokrzetd
--- t + t
C.The action potentia contnues to
tavel dowrI the axorn
Resung
Repolarvirg
+*---
Depclarized
CONDUCTION OF NERVE
Na
High Na
K
IMPULSSE
K
Na
High K
Negative organic ions
BYJU'Ss
Na- K Pump
K
Polarized
Resting Axon
membrane
(a)
Na
K
K
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * -* * * * * * * * * * * * * * * * * *
K
Na
Stimulus
applied
Polarised
Excited Axon
(b)
****
K
****
Na
De-polarised
********
********
K
Na*
Na
K
Stimuluss
applied
Polarised
Re-polarised
Excited Axon
De-polarised
(c)
K' Na
Na
K