Cremaster muscle motor unit action potentials

Area of Science:

• Neurology
• Human Physiology
• Muscle Electrophysiology

Background:

• The human cremaster muscle (CM) is a unique striated muscle with limited prior electrophysiological analysis.
• Its function is largely involuntary, making its motor unit action potentials (MUAPs) an area of interest for physiological understanding.

Purpose of the Study:

• To analyze the motor unit action potentials (MUAPs) of the human cremaster muscle (CM).
• To gain insights into the physiology of this unusual striated muscle, which operates primarily without voluntary control.

Main Methods:

• Investigated MUAPs in 31 healthy subjects via voluntary contraction or reflex response to tactile stimulation.
• Performed concentric needle jitter studies in 10 subjects.

Main Results:

• Analyzed 289 MUAPs: 40% simple (mean duration 4.7 ms), 60% polyphasic (mean duration 10.1 ms).
• Jitter study showed increased jitter (mean 81.9 μs) and intermittent blocking.
• Long MUAPs (>20 ms) exhibited grouping or subgrouping.

Conclusions:

• Electromyography (EMG) results resembled those in chronic neurogenic disorders, despite normal CMs.
• Suggests CM is an "embryogenic" muscle with polyneural innervation.
• CM exhibits weak cortical control, primarily excited by cutaneous input.