Spinal cord anatomy and PNS - Dermatomes, Myotomes

Anatomy

• Spinal cord encased in veretebral column
• Spinal cord divided into segments
• Each segment has exiting dorsal (sensory) and ventral (motor) nerve root
• Each segments supplies a specific area of skin (cutaneous) sensation or gross movement at a joint.
• Roots recombine into a spinal nerve where they exit the vertebrae then re-divide into dorsal and ventral rami (each carrying both motor and sensory functon) and later joined by autonomic contributions from the sympathetic chain
• Vertebral at which spinal nerve exits gives name to the corresponding vertebral e.g C5 nerve root exists C5 vertebrae
• Cord ends horizontal to L1 vertebrae (i.e. each segment is not exactly opposite its named vertebra)
• Cervical (7 segments) - supplies scalp, upper limb (arm)
• Thoracic (12) - inner arm/thorax/abdomen (torso)
• Lumbar (5) - lower limb (leg)
• Sacral (5) - sole of foot, posterior leg,buttock/perineum/genital

Brachial and lumbar plexus

• Cervical and lumbar spinal nerve combine and divide in a complex manner before supplying the upper and lower limbs.
• The resulting peripheral nerves are contributed to by different cord segments

Neuroanatomy Components

• Motor (corticospinal / pyramidal tract)
• Sensory (spinothalamic tract, dorsal columns)
• Autonomic (thoraco-lumbar sympathetic nuclei) - chain lies outside vertebral column
  + misc extrapyramidal tracts (olivo-, rubro-, vestibulo-, reticulo-spinal tracts)

Clinical relevance

• Lesions involving cord segments or spinal nerve lead to specific patterns of motor (myotome) or sensory (dermatome) loss
• Lesions of the brachial or lumbar plexus lead to complex patterns of loss
• Lesions of specific peripheral nerve have their own patterns
  
  The diagnosis of the location of a lesions is determined by the specific pattern of motor or sensory loss.