Upper Motor Neuron Lesion

Comparison of Upper Motor Neuron Lesion and Lower Motor Neuron Lesion Syndromes:

Location of lesion; Structures involved	Central nervous system- cortex, brainstem, corticospinal tracts, spinal cord	Cranial nerve nuclei/nerves and Spinal cord: anterior horn cell, spinal roots, peripheral nerve
Diagnosis/Pathology	Stroke, traumatic brain injury, spinal cord injury	Polio, Guillain Barre, Peripheral nerve injury, Peripheral neuropathy, Radiculopathy
Tone	Increased: Hypertonia, velocity dependent	Decreased or absent: hypotonia, flaccidity, not velocity dependent
Reflexes	Increased: hyperreflexia, clonus, exaggerated cutaneous and autonomic reflexes, +Babinski	Decreased or absent: hyporeflexia, cutaneous reflexes decreased or absent
Involuntary movements	Muscle spasms: flexor or extensor	With denervation: fasciculations
Strength	Weakness or Paralysis: ipsilateral(stroke) or bilateral(SCI) Corticospinal: contralateral if above decussation in medulla; ipsilateral if below distribution: never focal	Ipsilateral weakness or paralysis; Limited distribution: segmental or focal pattern, root innervated pattern
Muscle Bulk	Disuse atrophy: variable, widespread distribution, especially of antigravity muscles	Neurogenic atrophy: rapid, focal distribution, severe wasting
Voluntary Movements	Impaired or absent: dyssenergic patterns, obligatory mass synergies	Weak or absent if nerve interrupted

Spinal Cord:

It is a continuation of medulla and runs within the vertebral canal extending from foramen magnum to conus medullaris (cone shaped) which is approximately at the level of second lumbar vertebra.

Below the level of second lumbar vertebra there is a collection of nerve roots which runs down from the spinal cord resembling a horse's tail and hence is known as CAUDA EQUINA. This is made up of nerve roots from L₂ to S₅. The length of spinal cord is approximately 17 inches.

The passage for spinal cord in spine is the vertebral foramen which is surrounded and protected by the bony structures of each individual vertebra. The opening through which the spinal nerve roots exit, the vertebral canal is known as INTERVERTEBRAL FORAMEN. It is formed by the superior vertebral notch of the vertebra below and the inferior vertebral notch of the vertebra above. This foramen is located on the sides of the vertebral column.

The cross sectional view of the spinal cord reveals white matter on the periphery and grey matter at its center. The grey matter in the center of the spinal cord is H-shaped or butterfly shaped. The top portion of this H is the posterior horn and the lower portion is the anterior horn. Motor impulses travel from the brain down the spinal cord through the anterior horn and out towards the periphery via spinal nerves. While the sensory impulses from the periphery travel up the nerves, enter the spinal cord through the posterior horn and then goes up the cord upto the level of the brain.

The white matter of the spinal cord contain ascending and descending (motor) fibre pathways. Each of these pathways carry a particular type of impulse. The pathway, which is of particular significance to musle control is the CORTICOSPINAL TRACT. It runs from the motor area of the cerebral cortex to the spinal cord and crosses over at about the lower part of the brain stem.

Motor neurons that synapse above this level are called as UPPER MOTOR NEURONS and those that synapse at or below the level of the anterior horn cells are called LOWER MOTOR NEURONS. If an injury/lesion occur between the brain and the spinal cord i.e proximal to anterior horn, it will be called or considered as an UPPER MOTOR NEURON LESION. Whereas if an injury or lesion occurs between the anterior horn of the spinal cord and the peripheral part, it will be considered as a LOWER MOTOR NEURON LESION.

Examples of upper motor neuron disease are spinal cord injuries, multiple sclerosis, parkinsonism, CVA etc. Examples of lower motor neuron disease are muscular dystrophies, poliomyelitis, myasthenia gravis and peripheral nerve injuries. In either case of lower motor neuron or upper motor neuron lesion, paralysis usually results, however, the clinical signs differ greatly.

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