Membrane Potential and Action Potentials in Excitable Cells

 
M
embrane potential
 
R
esting potential
A
ction potential
 
M
embrane 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)
 
A
ction 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 (-
7
0 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
 
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
 
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
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Membrane potential, resting potential, and action potentials play crucial roles in the functioning of excitable cells like neurons, muscle cells, and endocrine cells. Voltage-gated channels, depolarization, and repolarization are key processes involved in generating and propagating action potentials. The all-or-none principle dictates that action potentials are generated with specific stimuli, and the stages of action potentials involve sequential changes in ion channel permeability. Explore the fascinating world of cell excitability through membrane potential dynamics and action potential mechanisms.

  • Membrane Potential
  • Action Potentials
  • Excitable Cells
  • Voltage-Gated Channels
  • Depolarization

Uploaded on Sep 18, 2024 | 1 Views


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  1. Membrane potential Resting potential Action potential

  2. 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.

  3. 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.

  4. 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

  5. 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)

  6. Action potential

  7. 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

  8. 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)

  9. Stages of action potential 2. Repolarization (+40 to -70) Opening of voltage gated K+channels Closure of voltage gated Na+channels

  10. 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

  11. 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)

  12. 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.

  13. 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

  14. 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

  15. 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

  16. Conduction of nerve impulse Nerve impulse conduction is always unidirectional Chemical synapses are unidirectional Ensure one way transmission of nerve impulse

  17. 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

  18. Myelin sheath 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

  19. 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

  20. 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.

  21. Types of conduction 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

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