Nerve impulse Conduction:
The propagation of action potential
in nerve is called nerve impulse Conduction. The nerve Impulse (electrical
impulses) propagates in the form of wave of depolarisation and repolarisation.
The cell membrane of neuron (nerve
Cell) consists of pump and Channels. They are:
• Na-k pump: also called Na-k
ATPase. It is an enzyme that moves sodium and potassium ions across Cell
membrane. This pump uses energy from ATP to move 3 sodium ions out of Cell and
two potassium ions into the Cell. This helps to maintain concentration gradient
of sodium and potassium across cell membrane. This gradient helps to generate
resting membrane potential. @ NOKIA
⇒ Voltage gated Channels: Remains closed in general. Are regulated by voltage i.e. open or Close in response to Change in membrane potential.
It opens at the level of Certain voltage. It
is of two types; voltage gated sodium Channel (opens earlier than that of K.
opens at -55mv and closes at +30mv) and voltage gated potassium Channel (opens
late. opens at +30mv and closes at -85mv).These channels participate in generation
and Conduction of action action potentials.
⇒ Mechanical Channels: Remains closed in general. Are
regulated by vibrations, touch, tissue stretching, temperature, pressure etc.
Eg:_ Auditory receptors in ear, stretch receptors in internal organs, touch/pressure/temperature receptors in skin etc.
⇒ Ligand gated channels: Remains closed in general. open or close when a
specific Chemical molecule (ligand) binds to them. when Chemical messenger like
neurotransmitter binds to the Channel , it gives conformational change which
opens the Channel. The Channels maybe of sodium, potassium, Chloride, Calcium
etc. They are present in Synapse and neuromuscular junction.
⇒ Leaky Channels: They always
remain open and ions move through them based on their electrochemical gradient.
They help to maintain the resting membrane potential in neuron.In nerve cell,
K+ leaky channels are more active while Na+ leaky Channels are less or
nonactive. Allows sodium to move inside and potassium to Pass outside.
Resting membrane potential(RMP) : The nerve fiber is covered by neurilemma. Cytoplasm just beneath the neurilemma is electronegative while extracellular fluid is electropositive.The potential difference developed across membrane (Axolemma) at rest is called RMP. In neurons, The RMP is about - 70 m (-40 to -90 mv ). The minus sign indicates that the inside of cell is negative relative to outside. A cell that exhibits a membrane potential is Called polarised.
At normal Condition more sodium ions
(About 10 times )are present outside while more potassium ions ( About 20
times) are present inside.
The RMP arises due to 3 major
factors:
• unequal distribution of ions in
the ECF and Axoplasm: It is the major factor. ECF is rich in Na+ and CI-.
In Axoplasm, phosphate and negatively Charged proteins, anions are present which cannot Cross the Axolemma.
• Na-K
pump :- uses ATP and throws out 3 Na+ ions and sends in 2 K+ ions.
This electrogenic nature of Na-k pump helps to maintain RMP as these pumps
remove more positive Charges from all than they bring into the Cell. However,
their Contribution is very low: only -3mV of the -70mv RMP of a typical neuron.
• More K+ leaky channels (functional) are present in Axolemma in comparision to leaky Na+ Channels(non-functional). Thus, number of potassium ions that diffuse out from axoplasm is more. As more potassium Ions exit, the inside of the membrane become more negative while outside of the membrane become more positive.
⇒ Excitability and Conductivity:
• when a nerve fiber is stimulated by
a Stimulus of adequate Strength, the nerve fiber come in a State of local
exicitation. The Stimulus maybe physical (heat, cold, pressure etc. ),
mechanical (injury), Chemical ( acid, base) or electrical.
• When nerve fiber get stimulated by
Stimulus of adequate strength then its polarity is reversed and new potential
difference Called action potential is developed. This depolarisation is self
propagated and transmitted in Particular direction in a non-decremental manner.
This property of Conduction is Called Conductivity.
⇒ Mechanism of conduction of nerve impulse:
The most accepted theory for nerve
impulse Conduction is ionic theory Proposed by Hodgkin and Huxley. This
theory states that nerve impulse is an electrochemical event regulated by
differential permeability of neurilemma to Na and K.
• Generation of nerve impulse or
action potential or Depolarisation:
When nerve fiber is Stimulated by
stimulus of adequate Strength , the Stimulated area comes into local state of
excitation. This is due to inward movement of sodium ions as mechanical
channels of Sodium open. In this area ,the membrane potential increases and
when the membrane potential is about -55 mv i.e. thresold potential ,
the voltage gated sodium channels open then sodium ions rapidly move inward
into cytoplasm of nerve from ECF i.e. influx of sodium ions and action
potential is generated.Due to this influx of Na ions ,the potential difference
across membrane Increases and become zero and then become positive. This is
Called depolarisation. the
membrane potential reaches upto +30mV from -55 mV.this potential difference is
Called action potential. once the event of depolarisation have occured , a
nerve impulse or spike is initiated.
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• Repolarisation: At about +30mv potential difference ,the voltage gated Na-Channels closes and voltage gated K-channels open. As voltage gated K- Channels open, the K ions move outward i.e. eflux of k ions which lowers the number of positive ions inside the neuron. Thus, potential difference across membrane decreases and is back towards RMP i.e. from +30mV to -70mV.This is Called repolarisation. As voltage gated K- channels remain open for more time the membrane potential may reach up to -80 to -85 mv. This is Called hyperpolarisation.
The time taken for restoration of
resting membrane potential is called refractory period. During
it, the membrane is incapable of receiving another impulse.During this period
the voltage gated sodium channels are closed and will not respond to any
changes in voltage, regardless of the strength of the stimulus. It is about 1
to 2 miIIi seconds. It ensures that action potential is discrete which means
action potential Can travel only in one direction.
• Propagation of nerve impulse
along non_ medullated fiber:-
The depolarised area Causes depolarisation of next adjacent area and so on. During it, negative Charge present on outer Surface of a depolarised area attract positive Charge from outer surface of next polarised area, while positive Charge present on inner surface of depolarised area in attracted by negative Charge on inner surface of next polarised area. so, the depolarised area becomes repolarised and the next polarised area becomes depolarised. This Process is repeated and the action potential propagates as wave of depolarisation and repolarisation.
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• propagation of nerve impulse along myellinated fiber or Saltatory conduction:
In myellinated nerve fiber, the myelline sheath is impermeable to ions. Thus, ionic exchange and depolarisation occurs only at node of Ranvier which has more number of ionic Channels and Na-K Pumps. when impulse reaches a node of Ranvier, the depolarisation occurs which is enough to generate depolarisation to next node and so on. Thus, action potential is Conducted from node to node in a jumping manner. This is Called Saltatory conduction.
The speed of Conduction of nerve
impulse is about 20 times faster than in non_medullated fiber. Less energy is
required as depolarisation and repolarisation takes only at nodes. so, is
energy efficient process.
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Differences between the impulse conduction in a myelinated nerve fibre and unmyelinated nerve fibre-
1. In myelinated nerve fibre, impulse conduction is carried from node to node.
In unmyelinated nerve fibre impulse conduction carried along the
length of axon.
2. In myelinated nerve fibre,
action potential propagation requires activation of voltage gated Sodium
channels only in nodal spaces.
In unmyelinated nerve fibres, action potential propagation
requires activation of voltage gated sodium channels along the entire length of
axon.
3. Action potential propagation in
myelinated nerve fibres is much faster (upto 150 m/s) due to presence of
Nodes of Ranvier.
Action potential propagation in
unmyelinated nerve fibres is much slower (upto 10m/s) due to the absence of
Nodes of Ranvier.
4. Due to the presence of myelin
sheath myelinated nerve fibres do not lose the impulse during conduction. Unmyelinated nerve fibres can lose
the nerve impulse during conduction.
Some terms related to nerve Impulse
Conduction:
• subthresold potential: A stimulus or membrane potential
which is not enough to generate action potential is known as subthreshold
potential. It is also Called as graded potential or local potential. A series
of subthreshold Stimuli applied to a nerve fiber In rapid succession add up and
may Set up an impulse. This additive effect of Several subthresold stimuli in
the generation of a nerve Impulse is Called summation.
• Thresold potential: The
minimum level of depolarisation which is required to trigger an action
potential. It is the minimum stimulus which generates action potential. Its
value is about -55mv (membrane potential).
• All or none principle: The
stimuli below thresold value do not generate any action potential (nerve impulse)
while all stimuli having intensity above the thresold generate the Same action
potential regardless of the intensity of Stimulus above thresold level. The
size or magnitude of the all action potential is Same, no matter how strong the
stimulus become.
• The strength of stimulus is
encoded by neurons through the frequency of action potential generated by the
Sensory receptor and also by the number of neurons activated by stimulus.
• The whole process of depolarisation and repolarisation is very fast and lasts for about 1-5 milli seconds.
Absolute vs Relative Refractory Period
The absolute refractory period occurs
Immediately after an action potential and lasts for about 1-2 milliseconds. It
is due to closure of voltage gated Na - channels.
The relative refractory period occurs
after absolute refractory period and lasts for about 4 milliseconds. It is due
to hyperpolarisation.
The absolute refractory period refers to the time span in which the
Sodium channels remain inactive.
The relative refractory period is the phenomenon in which the
Sodium gated channels transit from its inactive status to the closed status
that prepares the channels to be activated.
During the absolute refractory
period, the stimulus will not produce a second action potential.
During the relative refractory
period, the stimulus must be stronger than the usual to produce the action
potential.
The sodium ion channels are completely
inactive during the absolute refractory period.The potassium ion
channels are active, and flow of potassium out of the cell takes place during
the relative refractory period.
• Speed of nerve impulse
Conduction depends upon:
⇒ Myelination: speed is more in myelinated nerve fiber
(about 120 m/s than in non- myelinated nerve
fiber( about 10-20 m/s).
⇒ Thickness of nerve fiber: If thickness increases speed also increases.
⇒ Temperature: If temperature increases speed also increases.