Membrane potential
Resting potential
Action potential
Membrane potential
• Membrane potential ( transmembrane
potential or membrane voltage) is the
difference in electrical potential between the
interior and the exterior of a biological cell.
• Typical values of membrane potential range
from –40 mV to –100 mV.
Excitable cells
• Action potentials occur in several types of
animal cells, called excitable cells, which
include neurons, muscle cells, and endocrine
cells, as well as in some plant cells.
• Action potentials in neurons are also known as
"nerve impulses“.
• It sends the messages from our muscles to our
brains and back, as well as all the thought
processes in our brain.
• We could stimulate an excitable cell
chemically, electrically, or mechanically.
Voltage gated channels
• Action potentials are generated by special
types of voltage-gated ion channels
embedded in a cell's plasma membrane.
• Two types of channels are present:
• 1. Voltage gated Na+channels
• 2. Voltage gated K+channels
Resting membrane potential to
threshold level (-70 to -50 mv)
• 1. Opening of voltage gated Na+channels
(electrical stimulus)
• 2. Opening of mechanically gated
Na+channels (mechanical stimulus)
• 3. Opening of ligand gated Na+channels
(chemical stimulus)
Action potential
All or none principle
►Action potential will either be generated or
not…no gradations or intensities or possible
►Suprathreshold stimulus will elicit same
action potential as elicited by threshold
stimulus
►Subthreshold stimulus will not elicit action
potential
Stages of action potential
• 1. Depolarization (-50 to +40 mv)
►Opening of voltage gated Na+channels
►About 5000 fold increase in Na+permeability
►Voltage rises and crosses zero (overshoot)
Stages of action potential
• 2. Repolarization (+40 to -70)
• ►Opening of voltage gated K+channels
• ►Closure of voltage gated Na+channels
Stages of action potential
• 3. Hyperpolarization
• ►Some voltage gated K+channels remain
open even after RMP (-70 mv) is restored
• ►Potential decreased more than resting level
• ►Na+ -K+pump restores RMP from
hyperpolarization
Re-establishment of ionic gradients
• During action potential Na+& K+ionic
gradients reverse. In this condition cells
contain:
►Large amount of Na+(due to massive
Na+influx)
►Too less amount of K+(due to massive K+
efflux)
• Na+ -K+pump re-establishes ionic gradients
(recharges the nerve fiber)
Refractory period
1. Absolutely refractory period
• Period during which a 2nd action potential can not be
generated. This can be elicited:
►From start of depolarization to initial 1/3 of repolarization
►After closure, the inactivation gates do not reopen until RMP
is restored
• It is mostly of 0.4 ms in large myelinated nerve fibers.
2. Relative refractory period
• Period during which 2nd action potential can be generated
but with stronger than normally required stimulus. This can
be elicited:
►From end of initial 1/3 of repolarization to start of after
depolarization (middle 1/3rd)
►Some voltage gated Na+channels regain their resting
configuration
• During this period K+efflux continues.
Refractory period
• limits frequency of action potentials.
►Longer the refractory period, less will be the
frequency
►Absolutely refractory period of large
myelinated nerve fiber is 0.4 ms, therefore,
frequency of action potential is 2500/second
►Determine direction of action potential
►Action potentials can not be summated
Local anesthetics
• Procaine, Tetracaine etc block voltage gated
Na+channels, thus
►No action potential occurs
►No nerve signal from periphery to brain
►No sensation of pain
Propagation (conduction) of action
potential
• Propagates along nerve fiber as nerve signal
or nerve impulse
►Means of communication between neurons
or nerves and muscles.
► Causes muscle contraction
Conduction of nerve impulse
►Nerve impulse conduction is always
unidirectional
►Chemical synapses are unidirectional
►Ensure one way transmission of nerve impulse
Types of nerve fibers (based upon
myelination)
• Myelinated fibers
►Covered by myelin sheath
►Large diameter fibers (A fibers) carrying touch
and pressure sensations to CNS
►Somatic motor fiber to skeletal muscle
• Unmyelinated fibers
►Not covered by myelin sheath
►Small diameter fibers (C fibers) carrying dull pain
sensation to CNS
►Postganglionic autonomic fibers
Myelin sheath
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►Fatty material
►Produced by Schwann cells
►Wraps around the axon in multiple layers
►Insulates the nerve fiber
►Ionic exchange can not take place through
myelin sheath
Nodes of Ranvier
• ►Parts of myelinated nerve fiber devoid of
myelin sheath
• ►Present after every1-3 mm of myelinated
part of nerve fiber
• ►Are in contact with ECF
• ►Have abundance of voltage gated
Na+channels
• ►Sites of action potential generation
Propagation
• The action potential generated at the axon
hillock propagates as a wave along the axon.
The currents flowing inwards at a point on the
axon during an action potential spread out
along the axon, and depolarize the adjacent
sections of its membrane. If sufficiently
strong, this depolarization provokes a similar
action potential at the neighboring membrane
patches.
Types of conduction
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Contiguous conduction
►Occurs in unmyelinated fibers
►Every part of nerve fiber undergoes depolarization
►Slow speed of impulse conduction
►More energy consumption
Saltatory conduction
►In myelinated nerve fibers
►Depolarization occurs only at nodes of ranvier
►Myelinated parts do not depolarize
►Activation ‘jumps’ from node to node