The Spinal Cord : the Brain's Link to the Body

• The spinal cord emerges from the skull at the foramen magnum, where it merges imperceptibly with the medulla
• The cord ends at the first lumbar vertebra (L₁), where it splits into the cauda equina
• It contains motor and sensory tracts, plus reflex centers and alternating flexor/extensor patterns (such as walking)
• Like the brain, the cord is protected by the three layers of meninges, bone and CSF
• The dura mater of the cord is single layered and typically called the spinal dural sheath
• The dura mater is not connected directly to the vertebrae; instead, there is a space between the membrane and the bone called the epidural space which is filled with fat and a venous network. The fat acts as a shock absorber
• At the cauda equina, the dural and arachnoid layers extend to S₂, forming a meningeal "pouch" of sorts
  • Because the spinal cord ends above this pouch, it is an ideal place to perform a spinal tap to collect CSF for testing
• The spinal cord is anchored to the vertebrae in two ways:
• The filum terminale, an extension of the pia mater, attaches to the posterior surface of the coccyx

• Denticulate ligaments attach the cord throughout its length to the interior portion of the vertebral foramina
• 31 pairs of spinal nerves branch off from the cord and exit through intervertebral foramina
• the cord is defined by these 31 segments, named for the vertebrae inferior to the extension
• the spinal cord is not circular in cross section; it is more oval, with two grooves (anterior median fissure and posterior median sulcus) that split it into two "halves"; gray matter is located on the inside, white on the out
• the gray matter of the cord is arranged in an H pattern.
• The cross-bar of the H is the gray commissure, and the two posterior portions of the gray matter are the posterior or dorsal horns, while the anterior pair are the anterior or ventral horns. Two small "horns", the lateral horns, extend off to either side
• The anterior horns are somatic motor neurons, which send axonal fibers out through the ventral root to skeletal muscles
  • Because there is more innervation of skeletal muscles in the limbs, there are larger anterior horns in the cervical and lumbar regions, resulting in the cervical enlargment and the lumbar enlargements of the spinal cord
• The lateral horns are autonomic motor neurons, but their fibers leave via the ventral root as well
• The dorsal roots are formed by sensory nerve fibers from peripheral receptors
  • An enlarged area of the dorsal root, the dorsal root ganglion, is where the nerve cell bodies of those neurons are found
• The dorsal and ventral roots join laterally to form the actual spinal nerve

• Polio is a disease caused by destruction of the anterior horn motor neurons, causing weakness, muscle pain, and eventually paralysis and atrophy. Death eventually occurs if the medulla is affected
• The white matter of the spinal cord runs in three directions, ascending sensory tracts, descending motor tracts and commissural tracts
  • Vertical tracts are arranged into three columns on each side of the cord &endash; the posterior, anterior and lateral funiculi
  • The funiculi are composed of several fiber tracts, each with neurons of similar destination and function
  • The posterior funiculi are primarily sensory
  • The anterior and lateral funiculi are primarily motor
  • Lou Gehrig's disease (amyotrophic lateral sclerosis) affects the anterior horn motor neurons and pyrimidal (motor) tracts
• Damage to the spinal cord always results in some kind of functional loss, typically paralysis
• Damage to the ventral root or anterior horn cells causes flaccid paralysis of the involved skeletal muscles; atrophy results

• Spastic paralysis occurs when the primary motor cortex is damaged, so the reflex reactions in the spinal cord can still reach the innervated muscles; atrophy does not result, but the muscles are no longer under voluntary control
• Transection of the spinal cord anywhere along its length results in paraplegia or quadraplegia, depending on where the transection occurs
  • A cut between T₁ and L₁ results in paraplegia
  • A cervical region injury results in quadraplegia
• Spinal shock is a transient period of functional loss that follows injury to the spinal cord; it is typically seen in whiplash
  • Results in depression of reflex activity below the site of injury &endash; bowel and bladder reflexes stop, blood pressure falls, all muscles below the injury are paralyzed, and there is no perspiration in the affected area, so fever may develop
  • Function returns in a few hours; if not, the damage is probably permanent

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