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Spinal Cord and Nerve Anatomy - Anatomy & Physiology

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  • Length and width of the spinal cord
    The spinal cord is about 18 inches (45 cm) long and 1/2 inch (14 mm) wide.
  • End of the spinal cord
    The spinal cord ends between vertebrae L1 and L2 at the conus medullaris.
  • Number of spinal nerve pairs
    There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
  • Spinal cord enlargements
    Cervical enlargement supplies nerves to upper limbs; lumbar enlargement supplies nerves to pelvic region and lower limbs.
  • Cauda equina
    Bundle of nerve roots from L2 to S5 occupying the vertebral canal below the conus medullaris.
  • Gray matter in spinal cord
    Central butterfly-shaped area containing neuron cell bodies, with posterior (sensory) and anterior (motor) horns.
  • White matter in spinal cord
    Surrounds gray matter; consists of myelinated axons organized into posterior, lateral, and anterior columns.
  • Function of gray commissure
    Connects right and left sides of gray matter and contains the central canal filled with cerebrospinal fluid.
  • Ascending tracts
    Carry sensory information from the body to the brain via first-, second-, and third-order neurons.
  • Gracile fasciculus
    Carries signals for vibration, visceral pain, deep touch, and proprioception from lower limbs and trunk.
  • Cuneate fasciculus
    Carries similar sensory signals as gracile fasciculus but from upper limbs and chest.
  • Spinothalamic tract
    Carries pain, pressure, temperature, light touch, tickle, and itch sensations to the brain.
  • Spinoreticular tract
    Transmits pain signals from tissue injury to the reticular formation and then to the cerebral cortex.
  • Spinocerebellar tracts
    Carry proprioceptive signals from limbs and trunk to the cerebellum for coordination of movement.
  • Descending tracts
    Carry motor commands from the brain to the spinal cord, involving upper and lower motor neurons.
  • Corticospinal tracts
    Control precise, finely coordinated movements; most fibers decussate in the medulla forming lateral corticospinal tract.
  • Anatomy of a spinal nerve
    Formed by the union of dorsal (sensory) and ventral (motor) roots; spinal nerves are mixed nerves.
  • Connective tissue layers of spinal nerves
    Epineurium (outer), perineurium (around fascicles), and endoneurium (around individual axons).
  • Spinal meninges layers
    Pia mater (inner), arachnoid mater (middle), and dura mater (outer) protect and isolate the spinal cord.
  • Function of muscle spindles
    Stretch receptors in skeletal muscles that monitor muscle length and help coordinate movement and posture.
  • Monosynaptic reflex
    Simplest reflex arc with sensory neuron synapsing directly on motor neuron, e.g., patellar tendon reflex.
  • Polysynaptic reflex
    Reflex involving one or more interneurons between sensory and motor neurons, allowing complex responses.
  • Reciprocal inhibition
    Inhibition of antagonistic muscles during a reflex to allow smooth movement, e.g., flexor reflex inhibits extensor muscles.
  • Crossed extensor reflex
    Contralateral reflex that supports body weight by extending the opposite limb during a withdrawal reflex.
  • Dermatome
    Bilateral region of skin monitored by a specific pair of spinal nerves; important for diagnosing nerve damage.