Unit
- Resting membrane potential: The inside of a nerve axon is maintained at approximately -70 millivolts relative to the extracellular space due to the sodium-potassium pump and selective permeability of the cell membrane. This electrochemical gradient is essential for generating action potentials.
- Action potential propagation: Depolarization occurs when voltage-gated sodium channels open, allowing sodium ions to rush in. This wave of depolarization travels along the axon in a self-propagating manner, forming the basis of nerve conduction.
- Saltatory conduction: In myelinated fibers, action potentials jump between nodes of Ranvier, significantly increasing conduction velocity. This mechanism conserves energy and allows rapid signal transmission over long distances.
- Compound muscle action potential (CMAP): When a motor nerve is stimulated supramaximally, the resulting electrical activity recorded from a muscle represents the sum of all motor unit action potentials in that muscle. The amplitude reflects the number of functional motor units.
- Sensory nerve action potential (SNAP): Recording from a sensory nerve after distal stimulation produces a small, biphasic waveform. The amplitude is typically much lower than CMAP and is influenced by temporal dispersion and phase cancellation.
- Conduction velocity calculation: Velocity is determined by dividing the distance between stimulation and recording sites by the latency difference. Normal values vary by nerve and age, but generally exceed 40 meters per second for motor nerves in the upper limbs.
- Factors affecting conduction: Temperature, age, and limb length influence conduction velocity. Cooling slows conduction, while warming increases it. Neonates have slower velocities that increase with myelination through early childhood.
- Axonal versus demyelinating pathology: Axonal loss primarily reduces amplitude, whereas demyelination prolongs latency and slows conduction velocity. Distinguishing these patterns aids in diagnosing peripheral neuropathies.
- Late responses (F-wave and H-reflex): F-waves result from antidromic activation of motor neurons followed by orthodromic backfiring, providing information about proximal motor nerve segments. H-reflexes are monosynaptic spinal reflexes elicited by submaximal stimulation of mixed nerves, useful for assessing S1 radiculopathy or polyneuropathy. Both require specific stimulation parameters and recording windows.
- Technical considerations for recording: Proper electrode placement, skin preparation, and impedance verification are critical for artifact-free recordings. Ground electrodes should be placed between stimulating and recording sites to reduce stimulus artifact. Filter settings typically range from 2 Hz to 10 kHz for motor studies and 20 Hz to 2 kHz for sensory studies to optimize signal quality.
- Temperature control and its impact: Limb temperature should be maintained at or above 32 degrees Celsius for reliable results. For each degree below 32 deg C, conduction velocity decreases by approximately 2 meters per second, and distal latencies prolong by about 0.2 milliseconds. Warming the limb with warm packs or soaking in warm water can standardize conditions.
Scope of Practice
Role: Neurodiagnostic Technologist Performing Nerve Conduction Studies
Within scope:
- Setting up and calibrating the electromyography (EMG) machine
- Measuring limb temperature and adjusting recording parameters
- Applying surface electrodes for motor and sensory studies
- Delivering electrical stimuli at appropriate intensities
- Documenting waveforms and labeling traces for physician review
Outside scope:
- Interpreting clinical significance of findings
- Diagnosing specific neuropathies or radiculopathies
- Adjusting protocol based on preliminary interpretation
- Communicating diagnostic impressions to patients
- Prescribing treatment based on study results
Quick Revision
- Nerve conduction relies on resting membrane potential and action potential propagation.
- Saltatory conduction in myelinated fibers increases velocity.
- CMAP and SNAP amplitudes reflect motor and sensory unit integrity.
- Conduction velocity is calculated from distance and latency difference.
- Age, temperature, and pathology affect conduction parameters.