Nerve Conduction

Nerve Conduction


NERVE CONDUCTION

  • Neuron – Basic unit & functional unit of nervous tissue.
  • Specialized for function of reception, integration, & transmission of information within body.

Structure: 

  • Nerve cell has cell body “Soma” with 5-7 small processes called ‘Dendrites’.
  • Continuing as “Axon hillock” – Thickened area of cell body.
  • Origination point for long process “Axon”.
  • First portion of axon is called “Initial segment”.
  • Nerve fibers may be myelinated/unmyelinated.
  • Schwann cells:
  • Found both in myelinated & non-myelinated nerve fibers of PNS.
  • ln myelinated nerves – Provide structural support & form myelin sheath.
  • In non-myelinated nerves – Provide only structural support.

Functional division of neuron:

  • 4 zones –

1. Receptor zone:

  • Dendrites & cell body – Soma.
2. Transmitter zone:
  • Transmits nerve impulse – Axon.

3. Generator area:

  • Point of impulse origination/generation.
  • Mainly at Axon hillock of body & initial segment of axon.
  • Due to their lowest excitation threshold.
  • Also contain higher density of voltage-gated sodium channels.
4. Release zone:
  • Release neurotransmitters – Nerve terminals.

Orthodromic” vs “Antidromic” conduction:

  • Experimentally, an axon can conduct impulse in either direction.
  • When an AP initiated in middle of axon, impulse shall travel in both direction.
  • One along axon towards its terminal knobs.
  • Another (opposite) towards cell body soma & dendrites.
“Orthodromic conduction”:
  • In Natural situation(intact body), impulses conducted unidirectionally only.
  • I.e. From synaptic junction/receptors along axons to their termination.

“Antidromic conduction”:

  • Conduction opposite to physiological direction.
  • Very rare & seen only in muscle tissue.

NERVE CONDUCTION PROCESS:

  • In nerve fibers, AP propagation is unidirectional (orthodromic) to synapse/NM junction.

Reason:

  • Presence of neurotransmitters at presynaptic terminal producing required effect.

Factors favoring nerve conduction: 

  • Low axoplasmic resistance (Ri).
  • Low external longitudinal resistance (Ro).
  • High membrane resistance (Rm).
  • Low membrane capacitance (Cm).
  • Thick (large) nerve Linear relation with conduction.
  • Myelination of nerve.
  • High space & time constant.

Myelination & nerve conduction:

Main purpose of myelin sheath:

1. To increase impulse conduction speed.

  • Conduction is faster in myelinated nerve than in unmyelinated.
  • Impulses jump from one node of Ranvier to next nodeHence, faster.
  • Referred as “Saltatory conduction” or “Propagation by Saltation”.
  • Note: In unmyelinated fibers, impulses move continuously as waves.

2. Nerve myelination decreases membrane capacitance.

  • Allows faster depolarization  es AP speed propagation.
  • Membrane capacitance & nerve conduction:
  • Measure of quantity of charge that must be moved across a unit area of membrane producing unit change in membrane potential.

Variations:

  • Membrane with high capacitance  Ions crossing membrane is high  Slower AP conduction.
  • With low capacitance Ions crossing is less faster AP conduction.
Exam Question
 

NERVE CONDUCTION

Point of impulse origination:
  • In a motor neuron, axon hillock & initial segment of axon have lowest excitation threshold.
  • Because they have a much higher density of voltage-gated sodium channels.
  • Axon hillock of body & initial segment of axon→ Generator area (Nerve impulse is generated).
  • Schwann cells are found both in myelinated & non-myelinated nerve fibers of peripheral nervous system.
  • ln myelinated nerves Schwann cells provide structural support & form myelin sheath.
  • Schwann cells are derived from neuroectoderm 

CONDUCTION PROCESS:

  • Experimentally, an axon can conduct impulse in either direction.
  • In Natural situation, impulses are conducted in one direction only (in an intact body)
  • I.e. From synaptic junction or receptors along axons to their termination.
  • Such conduction is called “Orthodromic conduction”.
“Antidromic conduction”:
  • Conduction in opposite direction (i.e., opposite to physiological direction).

Direction of flow in nerve fiber:

  • In nerve fibers, action potential propagation is unidirectional (orthodromic)
  • I.e., In natural situation, impulses travel only orthodomically.

Reason for unidirectional flow:

  • Conduction is unidirectional at synapses or in NM junction
  • Because, transmission across synapses & NM function is unidirectional
  • Inturn due to presence of neurotransmitters at terminal end of axon (Presynaptic terminal)
  • FACTORS AFFECTING NERVE CONDUCTION:
  • Propagation of action potential (nerve conduction) is favored by, 
  • Low axoplasmic resistance (Ri).
  • High membrane resistance (Rm).
  • Low membrane capacitance (Cm).
  • Thick (large) nerve →  Linear relation with conduction.
  • Myelination of nerve.

1. Effect of myelination on conduction of AP:

  • The rate of propagation of nerve impulse is faster in a myelinated nerve fiber than in unmyelinated nerve fiber.

Main purpose of myelin sheath:

  • Is to increase in speed at which impulses propagate along myelinated fiber.
  • In myelinated fibers, they jump from one node of Ranvier to next node.
  • So propagation is much faster.
  • This type of conduction of nerve impulse in myelinated nerves is called “Saltatory conduction” or “Propagation by Saltation”.
  • Myelination of nerve decreases membrane capacitance.
  • Allows depolarization to occur very fast. 
  • So, myelination increase speed of action potential propagation.

2. Membrane capacitance:

  • Measure of quantity of charge that must be moved across a unit area of membrane to produce a unit change in membrane potential.
  • If membrane has low capacitance → Number of ions (charge) crossing membrane is less.
  • Hence, faster AP conduction.
Don’t Forget to Solve all the previous Year Question asked on Nerve Conduction

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