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The Spinal Cord

The Spinal Cord | The Brain Stem And Cerebellum | The Diencephalon | The Basal Ganglia | The Cerebral Cortex | The Peripheral Nervous System, Neuromuscular Junction, and Muscle

The spinal cord is the simplest portion of the central nervous system, with the same general structure (with variations that we will ignore) at every segmental level.

Anatomy

It is important to understand the distinction between segmental and long tract findings.

  1. The segmental level:
    1. At each segment, sensory neurons contained in a dorsal root enter the spinal cord. The territory of skin innervated by a segment (for example, C6) is called a dermatome.
    2. Motor neuron cell bodies are located in the anterior horns; for each segment, their axons collect to form an anterior spinal root, and project (via plexus and peripheral nerves) to groups of muscles (the myotome).
    3. Large sensory fibers from muscle spindles enter at each level and synapse with motor neurons that project to the same muscle. The monosynaptic stretch reflex arc is an important aspect of segmental organization.
  2. Long tracts: of the 10 or more long fiber tracts coursing longitudinally in the spinal cord, only three are of prime importance in clinical practice:(see Figure 1)
    1. The lateral corticospinal tract contains axons from neurons in the motor cortex that project directly or through interneurons to motor neurons at the segmental levels
    2. Sensory fibers subserving pain and temperature (and crude touch) enter at each segment through the dorsal roots, synapse, and the second order neuron crosses to join the spinothalamic tract.
    3. Sensory fibers subserving position, vibration and disciminative touch enter through the dorsal roots, and directly (without a synapse and without crossing) join the posterior (or dorsal) columns
    4. Autonomic function: Autonomic fibers descend and synapse with cell bodies in the intermediolateral columns. Sympathetic fibers exit between T1 and L2, and parasympathetic between S2 and S4.

Localization

Spinal cord lesions are suspected when there are long tract signs below a certain spinal level, with or without segmental signs at that level

  1. Segmental signs:
    1. Motor: weakness and atrophy in a myotomal pattern.
    2. Sensory:
      1. Sensory loss in a dermatomal distribution.
      2. With central cord lesions: bilateral dissociated sensory loss (loss of pain and temperature, with preservation of position, vibration and touch.
      3. Reflexes: loss of tendon reflexes at the level of the lesion.
  2. Long tract signs:
    1. Motor: upper motor neuron dysfunction is characterized by weakness, spasticity, increased tendon reflexes, and Babinski responses. Bilateral leg weakness (paraparesis) is the commonest presentation of spinal cord dysfunction, but quadriparesis, monoparesis or any combination of limb weakness can be seen. Acute transection (or similarly severe lesion) can cause Aspinal shock,@ with flaccid paralysis and diminished tendon reflexes. This is temporary: spastic paralysis will usually supervene.
    2. Sensory: The characteristic finding is that of bilateral sensory loss below the level of the lesion. When spinal cord pathology is suspected, the physical examination should be designed to detect sensory levels in the limbs and on the trunk. The modalities lost depend upon the tracts involved.
    3. Autonomic: Many autonomic functions can be affected, but clinically the most useful symptoms relate to bladder control. Loss of descending inhibition of segmental reflex control leads to urinary urgency and incontinence. Acutely, however, lesions may be associated with a flaccid bladder and urinary retention, be segmental reflexes become active.
  3. Classic syndromes:
    The Brown-Sequard syndrome of spinal cord hemisection (see figure 2)
    Segmental findings with spinal cord hemisection will depend upon the level of the lesion and its rostro-caudal extent: they may be totally inapparent, for example, with a small lesion in the thoracic cord. Long tract findings, however, are prominent. Posterior column sensory loss (position, vibration) and long tract motor signs are found ipsilateral to the lesion, whereas pain and temperature are lost contralaterally. Bladder function may be spared since bilateral lesions are required to interfere with bladder function.
    Extrinsic compression
    Extrinsic spinal cord compression from neoplasms or other masses affects the spinal cord by direct compression and by interference with blood supply. Segmental findings are variable, and again depend on the level and extent of the lesion. If nerve roots are affected, segmental motor, sensory, or reflex changes may be apparent at the level of the lesion (see number 4 below, spondylotic myelopathy). Sometimes, these are minimal. The long tract findings are clinically more important, since they indicate that there is spinal cord, rather than just nerve root, involvement. The lateral columns and intermediolateral columns are particularly prone to damage, so urinary urgency and lower extremity long tract motor signs are often the earliest manifestation of spinal cord compression. Loss of pain and temperature sensation may begin in the sacral region, and slowly ascend as the compression gets more severe, because the spinothalamic tract is laminated with the fibers from the sacral regions being most lateral and thus most vulnerable to compression.
    Central cord syndrome (see figure 3)
    If a lesion in the center of the spinal cord extends over many segmental levels, segmental findings are prominent. Intrinsic spinal cord neoplasms and cysts (syringomyelia) may present with segmental loss of pain and temperature sensation, loss of tendon reflexes, segmental atrophy and weakness, and, below the level of the lesion, long tract signs (spasticity, brisk reflexes, Babinski response, weakness, urinary urgency, and, later, long tract sensory abnormalities). If the process begins in the middle of the cord, the first manifestation will be a dissociated sensory loss (loss of pain and temperature without loss of position and vibration) resulting from interruption of second order sensory neurons decussating in the center of the cord on their way to the lateral spinothalamic tracts. With cervical syringomyelia (a cyst in the cervical spinal cord), the dissociated sensory loss is usually bilateral, and may assume a cape distribution.
    Cervical spondylotic myelopathy
    Degenerative changes in the cervical spine (disc degeneration, formation of new bone) may impinge on spinal roots and may also compress the spinal cord. A common presentation is with C6 segmental findings (particularly decreased biceps and brachioradialis reflexes) and long tract abnormalities below that level (with increased triceps and lower extremity reflexes, spasticity in the legs, Babinski responses, and urinary urgency). Sensory findings, both segmental and long tract, may be present, but tend to be a later complication.


      Updated: December 23, 2003
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