The strength-duration curve is a graphical representation of the relationship between the intensity of an electrical stimulus at the motor point of a muscleand the length of time taken to elicit a minimal contraction in that muscle.
2. STRENGTHDURATIONCURVE
• Strength duration/Intensity duration curve shows the relationship
between the magnitude of the change of stimulus and the duration of
the stimulus.
• The curve provides valuable information regarding the state of
excitability of nerve lesion.
• It should be done only after 21 days following nerve injury.
3. • Wallerian degeneration: Nerve degenerates proximally to nearest
node of Ranvier and distally throughout whole length.
• Debris is cleared by macrophagic activity.
• Process takes up to 21 days to complete and is a preparation for
regeneration.
4. Types of Injury- Seddon’s classification of injury
Neuropraxia:
• Loss of conduction without degeneration
• Nerve conduction possible below lesion
• Sensory part frequently least affected than motor.
Axonotmesis:
• Disruption of axon, but nerve sheath intact
• Wallerian degeneration is followed by axons regrowing to own end organs.
Neurotmesis:
• Disruption of axon and nerve sheath.
• Surgery required approximating nerve sheaths and enabling growing axon
to reach correct end organ
5.
6. Shape of the Curve
Normal innervation
• When all the nerve fibers supplying the muscles are intact, the
strength duration curve has a shape characteristic of normally
innervated muscle.
• The curve is of this typical shape because the same strength of
stimulus is required to produce a response with all the impulses of
longer duration, while those of shorter duration require an increase in
the strengths of the stimulus each time the duration is reduced.
8. Complete Denervation
• When all the nerve fibers supplying a muscle have degenerated, the
strength duration produced is characteristic of complete denervation.
• For all impulses with duration of 100 ms or less the strength of the
stimulus must be increased each time the duration is reduced and no
response is obtained to impulses of very short duration.
• curve rises steeply and is further to the right than that of a normally
innervated muscle
10. Partial Denervation
• 1. As impulses shortened—denervated fibers respond less readily. So that a
stronger stimulation is required.
• 2. With impulse of shorter duration
• innervated fibers responses When some of the nerve fibers supplying a
muscle have degenerated while others are intact, the characteristic curve
obtained clearly indicates partial denervation.
• The right hand part of the curve clearly resembles that of denervated
muscle, the left hand part that of innervated muscle, and a kink is seen at
the point where the two parts meet.
11.
12. Rheobase
• The rheobase is the smallest current that produces a muscle
contraction if the stimulus is of infinite duration.
• In practice an impulse of 100 ms (0.1 s) is used.
Chronaxie
• The chronaxie is the duration of shortest impulse that will produce a
response with a current of double the rheobase.
• The chronaxie of the innervated muscle is appreciably less than that
of denervated muscle, the former being less and the latter more than
1 ms if the constant-voltage stimulator is used.
