dr thouraya. motor unit consists of a motor neuron and all the muscle fibers it innervates. when...
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Dr Thouraya
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Motor UnitMotor Unit
Consists of a motor neuron and all the muscle fibers it innervates.
When an action potential occurs in a motor neuron, all the muscle fibers in its MU are stimulated to contract.
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EMG is the recording of electrical activity of a muscle at rest & during contraction:
(to evaluate the electrophysiology of a MU)
Activity is amplified and displayed on an oscilloscope.
Instrument : Electromyograph
Record: Electromyogram
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A concentric needle Ede inserted into the belly of the muscle.
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Needle EMG does not introduce any electrical stimulation instead it records the intrinsic electrical activity of skeletal muscle fibers.
Normally a muscle is silent at rest after insertional activity has ceased.
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• Then the patient is asked to contract the muscle smoothly.
With muscle contraction, MUs are activated and MUAPs appear on the screen:
Motor unit potential : represents the summation of the potentials generated by muscle fibers belonging to the MU.
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Normal MUPsNormal MUPs• Bi – Triphasic• Duration : 3 – 16mSec.• Amplitude : 300μV – 5mV (5000μV).
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With increasing strength of contracto
→recruitment of MUs →↑number & size of MUAPs.
At full contraction separate MUAPs will be indistinguishable resulting in a complete recruitment = interference pattern.
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AnalysisAnalysisThe EMG is used to investigate both
neuropathic and myopathic disorders (weakness, numbness,pain )
• The size, duration, frequency of the electrical signals generated by muscle cells help determine if there is damage to the muscle or to the nerve leading to that muscle .
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Some diseases that cause alterations Some diseases that cause alterations in EMG MUPsin EMG MUPs
• Myopathy: progressive degeneration of skeletal muscle fibers.
Eg: Duchenne Muscular dystrophy
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•Neuropathy :Damage to the distal part of the nerve. Peripheral neuropathy mainly affects feet & legs.
Most common etiologies:
Guillain Barré syndrome Diabetes mellitus Alcohol abuse
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LMN lesions: interrupt the spinal
reflex arc ( α motor N) →Partial or complete loss of voluntary contraction , muscle wasting,↓reflexes, fasciculations.
Example: Polyomyelitis
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In neurogenic lesion or in active myositis, spontaneous activity is noted:
Positive sharp wavesFibrillationsGiant motor unit potentials
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Fibrillation potentials:
Low amplitude, short duration byphasic potentials: correspond to the spontaneous discharge of a denervated single muscle fiber due to denervato hypersensitivity to acetylcholine.
Fine invisible,irregular contractions of individual muscle fibers.
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Positive sharp wavesSmall fibrillation APs (50 to 100 µV, 5 to 10 msec duration) whose propagation is blocked at the level of the recording electrode.
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Fasciculation potentials spontaneous discharge of a MU at rest, can be
seen and felt by the patient.
• Partial re-innervation of denervated muscle, by sprouting of the remaining nerve terminals, produces abnormally large, long polyphasic potentials (giant potentials)
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REINNERVATION by COLLATERAL SPROUTING
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Myopathic alteration of the EMGMyopathic alteration of the EMG
PolyphasiaPolyphasia , ,short durationshort duration , ,reduced reduced voltage voltage MUPs MUPs
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Neuropathic alteration of the EMGNeuropathic alteration of the EMG
PolyphasiaPolyphasia, , long durationlong duration, , high high voltagevoltage MUPs MUPs
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MUP NORMAL NEUROGENIC MYOPATHIC
Duration msec.
Amplitude
Phases
RestingActivity
Interferencepattern
3 – 16 msec
300 – 5000 µV
Biphasic / triphasic
Absent
full
> 16 msec
> 5 mV
Polyphasic
Present
partial
< 3 msec
< 300 µV
May be polyphasic
Present
full
Analysis of MUP
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Nerve Conduction studies
A nerve conduction study (NCS) is a test commonly used to evaluate the function, especially the ability of electrical conduction, of the motor and sensory nerves of the human body.
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Motor Nerve Conduction VelocityMotor Nerve Conduction Velocity• Stimulato of the median
nerve at two points until visible muscle contracto is seen and a reproducible Compound Muscle Action Potential is recorded.
CMAP: summated potentials from all Motor Units in a muscle.
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MOTOR NERVE CONDUCTION MOTOR NERVE CONDUCTION VELOCITY (MNCV)VELOCITY (MNCV)
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MNCV=
l1 = latency at elbow.
l2 = latency at wrist.
Distance between the two stimulating
electrodes
abNl if < 40 m/sec
Conduction distance
Conduction time: l1- l2
(m/sec)
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Normal values for conduction Normal values for conduction velocityvelocity
In arm 50 to 70 m/sec.
In leg 40 to 60 m/sec.
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Conduction is faster in myelinated fibres.
Diseases which produce demyelinated peripheral nerves (diabetes,Gillain Barré)slow the conducto greatly(20-30 m/s).
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