12. Electromyography (EMG) : defined as the study of electrical potentials generated by depolarization of muscle
a. Motor unit: functional unit in EMG
b. Motor unit: described as a single motor neuron and the muscle fibers it innervates; comprises a single motor neuron and the muscle fibers it innervates
c. Motor unit action potential: the recorded depolarization of muscle fibers that results from activation of a single anterior horn cell; muscle action potentials are detected by the following:
(1) Surface electrode
(c) Determines gross motor activity
(d) Cannot detect individual EMG potentials
(e) Represents a total net record of anal or urethral sphinc-teric activity
(2) Needle electrode
(b) Invasive and uncomfortable
(c) Produces an “injury potential”
(d) Must be placed directly into the striated sphincter
(e) Simultaneously records urethral, bladder, and rectal pressure changes and flow
d. In a normal EMG tracing, with the bladder empty and at rest, there is a flat “quiet” baseline of activity with few random potentials.
e. As bladder volume increases to 150 to 350 mL, more muscles depolarize and contract to maintain continence.
f. As the bladder capacity is reached, maximal activation of perineal floor muscles occurs, producing a full interference pattern. With voiding there is silence.
g. The progression from minimal activity to maximal activity with bladder filling is called the guarding reflex.
h. Abnormal findings can be subdivided into the following categories7:
(1) Upper motor neuron lesions
(a) Loss of ability to contract or relax the sphincter voluntarily
(b) Example: detrusor sphincter dyssynergia
(2) Lower motor neuron lesions
(a) Complete lesion: lesions of the spinal cord or cauda equina; total denervation of the striated sphincter
(b) Incomplete lesion: partial denervation of the striated sphincter with consequent sphincteric contractions of poor magnitude
(3) Mixed neuron lesions
(a) Demonstrate features of both upper and lower motor neuron lesions
(b) Examples: myelodysplasia and multiple sclerosis
Normal pressure EMG tracing.