R.NCS.T. Exam key terms

The summed electrical response recorded from a muscle when its supplying nerve is stimulated. Its amplitude reflects the number of muscle fibers activated, and its latency reflects conduction along the fastest motor fibers.

The summed response recorded from a sensory nerve or its distal branches after electrical stimulation. Because sensory fibers are smaller, the SNAP is low in amplitude and is easily lost when conduction slows or axons are lost.

The time in milliseconds from the stimulus artifact to the onset of the CMAP. It measures conduction along the fastest motor fibers between the stimulus site and the recording electrode, including neuromuscular junction and muscle activation time.

The interval from the stimulus to the first deflection of the recorded response, marking when the fastest conducting fibers arrive. It is used for motor studies and, when a response is small, for sensory studies.

The time from the stimulus to the highest point (peak) of a sensory response. It is easier to mark than onset when the waveform is small or dispersed, but it reflects conduction in fibers of average rather than fastest velocity.

The speed of nerve conduction, calculated by dividing the distance between two stimulus sites by the difference in their latencies. It is expressed in meters per second and reflects the fastest surviving fibers.

The height of a recorded response, measured from baseline to peak or from peak to trough. It estimates the number of axons or muscle fibers contributing to the response and falls when axons are lost.

A sensory technique in which the nerve is stimulated distally and the response is recorded proximally, so the impulse travels opposite to the normal physiologic direction. It usually produces larger, easier-to-record sensory responses.

A sensory technique in which the nerve is stimulated proximally and recorded distally, so the impulse travels in the normal physiologic direction. Responses are smaller than with antidromic recording but avoid volume-conducted motor artifact.

A late motor response produced when an antidromic impulse travels back to the anterior horn cell and then returns along motor axons. It assesses proximal conduction and is judged mainly by its minimum latency and persistence.

A late response produced by a submaximal stimulus that activates sensory fibers, which in turn excite motor neurons through a monosynaptic spinal reflex. It is used mainly to study the tibial nerve and S1 root pathway.

Loss of waveform sharpness and prolonged duration of a response caused by unequal conduction speeds among fibers. It is typical of acquired demyelinating disease and can reduce amplitude even when axons are intact.

A significant drop in CMAP amplitude when a nerve is stimulated proximal to a lesion compared with distal stimulation, without proportional dispersion. It indicates failure of impulses to cross a focal demyelinating or compressive lesion.

Loss or damage to the myelin sheath that surrounds axons. It slows conduction, prolongs latencies, may cause conduction block, and often leaves the response amplitude relatively preserved when recorded close to the stimulus site.

Degeneration or death of nerve axons. It reduces the amplitude of sensory and motor responses in proportion to the number of fibers lost, while conduction velocity in surviving fibers remains near normal or only mildly slowed.

Breakdown of the axon and its myelin distal to a nerve injury, occurring over days after the lesion. It explains why amplitude loss may not appear on testing until several days after acute nerve damage.

A reversible conduction failure across a nerve segment without structural degeneration of the axon. Conduction returns when the cause, such as compression or stretch, resolves, and findings may include focal slowing or conduction block.

A stimulus intensity above the level needed to activate all fibers in the nerve being tested. It is achieved by increasing current until the response stops growing, then adding a small safety margin.

The brief electrical deflection recorded at the moment of stimulation, before the biologic response arrives. It marks the start of the latency measurement and must be kept separate from the response waveform.

The surface distance between stimulation and recording sites, or between two stimulation sites, measured along the path of the nerve. Accurate measurement is essential because errors directly change calculated conduction velocity.

The insulating layers of lipid-rich membrane wrapped around large axons by Schwann cells in the peripheral nervous system. It speeds conduction by restricting current flow to gaps between segments.

The jumping of action potentials from one node of Ranvier to the next along a myelinated axon. It greatly increases conduction velocity compared with continuous conduction in unmyelinated fibers.

A short gap in the myelin sheath where the axon membrane is exposed and dense with voltage-gated sodium channels. Action potentials are regenerated at these nodes during saltatory conduction.

The glial cell of the peripheral nervous system that forms myelin around a single axon segment. Schwann cells also support axonal repair after injury and guide regenerating sprouts.

The lower motor neuron whose cell body lies in the gray matter of the spinal cord and whose axon travels in a peripheral nerve to skeletal muscle. Its health determines the motor response recorded in nerve conduction studies.

A cluster of sensory neuron cell bodies located beside the spinal cord, outside the blood-nerve barrier. Because the cell body is spared in many root injuries, the distal sensory axon can still degenerate and produce an abnormal SNAP.

A single motor neuron together with all the muscle fibers it activates. The number and size of motor units, and their firing pattern, shape the electrical response recorded from muscle.

The synapse where a motor nerve terminal releases acetylcholine onto receptors on the muscle fiber membrane. Transmission across it adds a brief delay to motor latency and can fail in disorders such as myasthenia gravis.

The network of nerve fibers formed by the ventral rami of spinal nerves C5 through T1 that supplies the upper limb. It is divided into roots, trunks, divisions, cords, and terminal branches.

A major nerve of the upper limb formed from the medial and lateral cords of the brachial plexus. It is commonly studied at the wrist and elbow, including for carpal tunnel syndrome.

A major nerve of the upper limb arising from the medial cord of the brachial plexus. It is often tested across the elbow, where it is superficial and vulnerable to compression.

A surface electrode placed between the stimulating and recording electrodes to provide a common reference and reduce electrical noise and shock hazard. It does not record the nerve response itself.

The opposition to alternating current flow at the electrode-skin interface. High impedance increases noise and artifact, so skin preparation and good electrode contact are needed before recording.

The rate at which the display traces across the screen, usually expressed in milliseconds per division. Adjusting it lets the examiner see the whole response and mark latencies accurately.

The display gain, usually expressed in millivolts or microvolts per division, that sets how large the waveform appears on screen. It must be adjusted so the response fills the display without being clipped.

Repeatedly delivering a stimulus and summing the resulting responses to cancel random noise while preserving the consistent biologic signal. It is used mainly to reveal small sensory responses that are otherwise hidden.

The process of explaining the purpose, methods, risks, and alternatives of a procedure and confirming that the patient voluntarily agrees. It must be obtained before testing and documented according to policy.

Practices that prevent injury from the stimulating current, such as checking equipment, using appropriate stimulus limits, and avoiding current paths through the heart in patients with pacemakers or central lines.

The professional duty to protect a patient's personal and medical information from unauthorized disclosure. It applies to records, conversations, and any discussion of test results outside the care team.

The set of duties and limits defined by law, regulation, and employer policy for a technologist's role. It determines which procedures may be performed independently and when physician supervision or interpretation is required.