Nerve Conduction

NERVE CONDUCTION

Q. 1

Lowest threshold potential in a motor nerve fibre is at:

 A

Dendrite

 B

Body

 C

Axon hillock

 D

Axon

Q. 1

Lowest threshold potential in a motor nerve fibre is at:

 A

Dendrite

 B

Body

 C

Axon hillock

 D

Axon

Ans. C

Explanation:

C i.e. Axon hillock


Q. 2

Which is TRUE regarding nerve conduction:

 A

All or none phenomenon

 B

Conduction independent of amplitude

 C

Propagated action potential is generated in dendrites

 D

Faster in unmyelinated fibres

Q. 2

Which is TRUE regarding nerve conduction:

 A

All or none phenomenon

 B

Conduction independent of amplitude

 C

Propagated action potential is generated in dendrites

 D

Faster in unmyelinated fibres

Ans. A

Explanation:

A i.e. All or none phenomenon

All or none law: Action potential will not develop unless the stimulus searches a threshold intensity. Once a threshold in tensity is reached, a full fledged action potential is produced.

Further increase in the intensity of stimulus produce no increment or other change in action potential as long as other experimental condition remain constant and is said to obey “all or none law”.

Action Potential

develops in the initial segment of axonQ and the impulse is transmitted along the axon to its termination. – follow ‘all or none’ phenomenon

– conduction of nerve is an active self propagating process and impulse moves at a constant amplitude & velocity.Q conduction is faster in myelinated axonsQ.


Q. 3

Which of the following of is TRUE about propagated nerve action potential is:

 A

Decremental

 B

Not affected by hypoxia

 C

Fastest in C fibres

 D

None

Q. 3

Which of the following of is TRUE about propagated nerve action potential is:

 A

Decremental

 B

Not affected by hypoxia

 C

Fastest in C fibres

 D

None

Ans. D

Explanation:

None


Q. 4

Action potential is initiated at the Axon Hillock – Initial segment of the neuron because

 A

Threshold for Excitation is lowest

 B

Neurotransmitter is released at this site

 C

It is an unmyelinated segment

 D

Has lowest concentration of voltage gated sodium channels

Q. 4

Action potential is initiated at the Axon Hillock – Initial segment of the neuron because

 A

Threshold for Excitation is lowest

 B

Neurotransmitter is released at this site

 C

It is an unmyelinated segment

 D

Has lowest concentration of voltage gated sodium channels

Ans. A

Explanation:

A i.e. Threshold for Excitation is lowest

Action potential in motor neurons is initiated at axon hillock’ (or initial segment) because it has the lowest threshold for excitationQ due to presence of high density of voltage gated sodium (Na+) channels.

  • The axon (devoid of Nissl granules) starts from a short pyramidal shaped part of cell body k/a axon hillock. In myelinated axons, the part of axon between axon hillock and beginning of myelination is k/a initial segment. So initial segment is the portion of axon at and just beyond the axon hillock.
  • In myelinated motor neurons, the umnylinated initial segment is the site of initiation of production of action potential (full fledged) and propogation of nerve impulseQ. In cutaneous sensory nerves, the site of initiation of nerve impulse is 1st node of Ranvier.
  • Action potential begins in initial segment and not adjacent to excitatory synapses of soma because the initial segment membrane has 7 times greater concentration of voltage gated sodium channels as does the soma. So the initial segment has lowest threshold of excitationQ. The excitatory post synaptic potential (EPSP) that will elicit an action potential in initial segment is + 10 to + 20 milivolts whereas > + 30 to + 40 mV is required for soma.

When the 10 – 15 mV of depolarization sufficient to reach firing level is reached and impulse is propogated in two directions: down the axon & back into the stoma. Retrograde firing of soma has value in wiping the state clean for subsequent renewal of interplay of excitatory & inhibitory synaptic knobs.



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