Dermatomes, Myotomes & Spinal Nerve Root Anatomy

The same reflex that localises a root also flags a cord lesion — by whether it is reduced or exaggerated, the single most important interpretive step examiners test:

• Radiculopathy = a LOWER motor neuron pattern at the level: the affected reflex is diminished or absent, with hypotonia, segmental wasting and fasciculations and weakness in that myotome (e.g. an absent Achilles in S1, an absent biceps jerk in C5-6).
• Myelopathy / cord compression = an UPPER motor neuron pattern BELOW the level: reflexes are brisk/exaggerated, with clonus, spasticity, an up-going plantar (Babinski), a positive Hoffmann sign (flicking the middle-finger nail flexes the thumb/index), and Lhermitte's phenomenon — while a single root at the level of the cord lesion may still be LMN (a wasted, areflexic segment with brisk reflexes below it).
• The inverted supinator (radial) reflex is the classic crossover sign: tapping brachioradialis gives little brachioradialis contraction but brisk finger flexion, localising a C5-C6 cord/myelopathic level (the C5-6 LMN arc is interrupted while C8 below is disinhibited) — a peripheral musculocutaneous/C6 lesion does NOT do this.

So a "depressed reflex" is a root sign, a "brisk reflex with clonus/Babinski/Hoffmann" is a cord (UMN) sign, and the two can coexist at the compressed cervical level — distinguishing radiculopathy, myelopathy and myeloradiculopathy from the same examination.

"Incomplete" (AIS B-D) is not one thing: the dermatome/myotome map plus the three long tracts (corticospinal = motor, dorsal columns = proprioception/vibration/light touch, spinothalamic = pain/temperature) define recognisable syndromes the exam expects you to name:

• Central cord syndrome — the commonest incomplete injury; a hyperextension injury in an older patient with cervical spondylosis (often without fracture). Upper limbs (especially the hands) are weaker than the lower limbs (medial-to-lateral somatotopy of the corticospinal tract), with variable "cape" sensory loss and usual sacral sparing. Generally managed without emergent surgery; most regain ambulation, hand recovery is least.
• Brown-Séquard (cord hemisection) — penetrating/lateral mass injury: ipsilateral loss of motor (corticospinal) and dorsal-column (proprioception/vibration) function, with contralateral loss of pain and temperature (spinothalamic) beginning a few levels below (fibres decussate within 1-2 segments). Best prognosis of the incomplete syndromes.
• Anterior cord syndrome — anterior spinal artery territory / flexion-compression: loss of motor and pain/temperature with PRESERVED proprioception, vibration and light touch (dorsal columns spared). Worst prognosis.
• Posterior cord syndrome — rare: isolated dorsal-column loss (proprioception/vibration → sensory ataxia and a positive Romberg) with motor and pain/temperature preserved.

Conus medullaris vs cauda equina (both involve the lumbosacral roots/segments and saddle anaesthesia, but localise differently): the conus (cord segments around L1-L2) gives an early, symmetrical, severe bladder/bowel and saddle deficit with mixed UMN + LMN signs (an up-going plantar can persist), relatively symmetric and less radicular pain; the cauda equina (roots below the conus) gives an asymmetric, LMN picture with prominent radicular pain, areflexia and a later but progressive bladder dysfunction. Both demand urgent MRI; cauda equina compression is the classic decompression emergency.