Core of the method and physiological basics
Needle electromyography is a precise method for investigating the bioelectrical activity of muscle tissue. During the procedure, a special needle electrode is inserted into the muscle under study. It is directly connected to an electromyograph, an apparatus that detects, amplifies many times over, and graphically records the resulting electrical activity.
A key concept in EMG is the motor unit action potential (MUAP). A motor unit is not a single fiber, but an entire complex. It includes one motor neuron (whose cell body is located either in the anterior horn of the spinal cord or in the motor cranial nerve nuclei) and a group of muscle fibers innervated by this neuron. Consequently, the MUAP reflects the total electrical discharge of all these muscle fibers firing upon the command of a single nerve impulse.
Normal parameters
The evaluation of electromyography results is always based on comparing the obtained data—MUAP duration and amplitude—with established age-adjusted norms for each specific human muscle.
- At rest: when the muscle is completely relaxed, no bioelectrical activity is normally recorded. The electromyograph shows a straight line ("electrical silence").
- During muscle contraction: as soon as the patient begins to tense the muscle, specific waves appear on the screen—these are the biopotentials of individual motor units (MUAPs). The stronger the contraction, the more units are recruited into work.
Changes in peripheral neuron lesions
The peripheral motor neuron can be affected at various levels: from damage to the cell body itself (in the anterior horns or nuclei) to damage to its long process (axon) in the spinal roots, nerve plexuses, or peripheral nerves.
If the connection between the nerve and the muscle is disrupted, a characteristic pattern emerges:
- At rest, pathological spontaneous activity appears.
- Fibrillation potentials are recorded.
- Positive sharp waves are detected.
The pathogenesis of these changes lies in the fact that muscle fibers deprived of control from the nervous system (denervated) become pathologically excitable. Their intrinsic spontaneous activity increases dramatically, which is detected by the needle electrode.
Specifics of anterior horn and nuclear lesions
A special EMG pattern is observed when the pathological process selectively affects the neuron cell bodies in the anterior horns of the spinal cord or the motor cranial nerve nuclei (and somewhat less frequently when the roots themselves are damaged).
| Muscle state | Characteristic EMG changes |
|---|---|
| At rest | Fasciculation potentials appear. Unlike fibrillations, these represent spontaneous discharges not of the muscle fibers themselves, but of the affected motor neurons. |
| During contraction | A small number of giant MUAPs are recorded. Both the amplitude and duration of the potential increase significantly. |
Such enlargement of MUAPs is associated with deep mechanisms of motor unit reorganization upon the death of a portion of motor neurons.
Briefly: answers to topical questions
What main deep (tendon and periosteal) reflexes are examined in neurology, and at what spinal cord level are they integrated?
The main deep reflexes include tendon and periosteal reflexes of the upper and lower extremities.
- Biceps brachii reflex — integrated at the C5–C6 level.
- Triceps brachii reflex — integrated at the C6–C7 level.
- Brachioradialis reflex — a periosteal reflex of the upper extremity.
- Knee-jerk reflex (patellar reflex) — integrated at the L2–L4 level.
- Achilles reflex (calcaneal reflex) — integrated at the S1–S2 level.
Which pathological extensor plantar reflexes are elicited in pyramidal tract lesions?
In pyramidal tract lesions, the following extensor pathological reflexes are elicited in the lower extremities:
- Babinski reflex (Babinski reflex) — slow tonic extension of the great toe with fan-like spreading of the other toes upon stroking the sole of the foot.
- Oppenheim sign — extension of the great toe when stroking down the anterior surface of the tibia.
- Gordon sign — reflex extension of the great toe upon squeezing the calf muscle.
Which pathological flexor plantar reflexes are examined in neurological status?
The neurological status includes the examination of the following pathological flexor reflex in the lower extremity:
- Rossolimo reflex (Rossolimo reflex) — rapid plantar flexion of the toes in response to brief taps on the plantar surface of the distal phalanges of digits II–V.
Which reflexes of oral automatism are classified as pathological, and in what syndrome do they occur?
Reflexes of oral automatism are signs of subcortical structure disinhibition and occur in pseudobulbar syndrome. They include:
- Palmomental reflex (Marinesco–Radovici reflex) — contraction of the mentalis muscle (m. mentalis) upon stroking the palm.
- Snout reflex — protrusion of the lips into a pout due to contraction of the orbicularis oris muscle (m. orbicularis oris) upon tapping the upper lip.
- Grasp reflex.
Which pathological hand reflexes indicate central motor neuron lesions?
Pathological hand reflexes include:
- Trömner reflex (Trömner reflex) — elicited by a quick tap (flick) from bottom to top on the palmar surface of the distal phalanx of the III (middle) finger.
- Hoffmann sign — a pathological hand reflex characteristic of pyramidal tract lesions.
Which superficial (cutaneous) reflexes are examined in neurology, and what are their integration segments?
Neurology examines the following superficial (cutaneous) reflexes:
- Abdominal reflexes (upper, middle, lower) — integrated at the level of thoracic segments Th6–Th12.
- Cremasteric reflex — integrated at the level of segments L1–L2.
- Plantar reflex — integrated at the level of segments L5–S2.
- Anal reflex — integrated at the level of sacral segments S1–S5.
What five links make up a simple somatic reflex arc?
A simple somatic reflex arc consists of the following five components:
- Receptor — perceives the stimulus (exteroceptors, proprioceptors, interoceptors).
- Afferent limb — a sensory neuron that transmits the impulse to the CNS.
- Interneuron — the central component in the CNS (spinal cord) that performs switching.
- Efferent limb — a motor neuron that directs its axon to the effector organ.
- Effector — the executive organ (muscle) that carries out the response.
How do changes in tendon and superficial (skin) reflexes differ in central versus peripheral paralysis?
Reflex changes depend on the level of the nervous system lesion.
| Sign | Central paralysis | Peripheral paralysis |
|---|---|---|
| Tendon reflexes | Increased (hyperreflexia, briskness), expansion of reflexogenic zones | Diminished (hyporeflexia) or completely absent (areflexia) |
| Superficial reflexes | Decreased or completely absent | Decreased or absent (when the corresponding reflex arc is damaged) |
What are foot and patellar clonus, and what structure's lesion are they a sign of?
Clonus is a series of rapid, rhythmic muscle contractions manifested by repeated oscillatory movements. It serves as a sign of upper motor neuron (pyramidal tract) lesion.
- Patellar clonus — rhythmic upward and downward movement of the patella due to repeated contraction of the quadriceps femoris muscle.
- Ankle clonus — rhythmic plantar flexion of the foot due to repeated contraction of the gastrocnemius and soleus muscles.
What spinal automatism protective reflexes are elicited during spinal cord compression or transverse lesion?
In transverse spinal cord lesions, pathological reflex responses known as spinal automatisms are elicited.
- Protective flexion reflex (triple shortening reflex) — the most common type of spinal automatism. When a stimulus is applied below the level of the lesion, involuntary flexion of the lower extremity occurs in three joints: the hip joint (articulatio coxae), knee joint (articulatio genus), and ankle joint (articulatio talocruralis).
To avoid confusing terms regarding resting pathology: "Fibrillations" are the spontaneous activity of the muscle fibers themselves left without a nerve. "Fasciculations" are spontaneous discharges of the neuron itself (in the anterior horns).
Class and exam questions
What does needle EMG show in a healthy muscle at rest?
Normally, with complete muscle relaxation, bioelectrical activity is completely absent.
What is a MUAP?
This is a motor unit action potential — the total electrical charge of a group of muscle fibers that receive a command from a single common motor neuron.
Where do fibrillation potentials come from?
They arise due to pathologically increased spontaneous activity of single muscle fibers that have lost their normal nerve supply.
What else to study on this topic
- Pathogenesis of MUAP enlargement (why remaining MUAPs become larger when some neurons die)
- Age-related norms for MUAP duration and amplitude in various muscles
- Differences in EMG patterns between plexus lesions and peripheral nerve lesions
- Stimulation electromyography (in addition to needle EMG)
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