The
paralysis observed in LMN diseases is referred to as flaccid paralysis,
referring to a complete or partial loss of muscle tone, in contrast to the loss
of control in UMN lesions in which tone is retained and spasticity is
exhibited. Other signs of an LMN lesion are fibrillation, fasciculation, and
compromised or lost reflexes resulting from the denervation of the muscle
fibers. DISORDERS OF THE… Spinal Cord In certain situations, such as a
motorcycle accident, only half of the spinal cord may be damaged in what is
known as a hemisection. Forceful trauma to the trunk may cause ribs or
vertebrae to fracture, and debris can crush or section through part of the
spinal cord. The full section of a spinal cord would result in paraplegia, or
loss of voluntary motor control of the lower body, as well as loss of
sensations from that point down. A hemisection, however, will leave spinal cord
tracts intact on one side. The resulting condition would be hemiplegia on the
side of the trauma—one leg would be paralyzed.
The sensory results are more
complicated. The Focused In ascending tracts in the spinal cord are segregated between the
dorsal column and spinothalamic pathways. This means that the sensory deficits
will be based on the particular sensory information each pathway conveys.
Sensory discrimination between touch and painful stimuli will illustrate the
difference in how these pathways divide these functions. On the paralyzed leg,
a patient will acknowledge painful stimuli, but not fine touch or
proprioceptive sensations. On the functional leg, the opposite is true. The
reason for this is that the dorsal column pathway ascends ipsilateral to the
sensation, so it would be damaged the same way as the lateral corticospinal
tract. The spinothalamic pathway decussates immediately upon entering the
spinal cord and ascends contralateral to the source; it would therefore bypass
the hemisection. The motor system can indicate the loss of input to the ventral
horn in the lumbar enlargement where motor neurons to the leg are found, but
motor function in the trunk is less clear. The left and right anterior
corticospinal tracts are directly adjacent to each other.
The likelihood of
trauma to the spinal cord resulting in a hemisection that affects one anterior
column, but not the other, is very unlikely. Either the axial musculature will
not be affected at all, or there will be bilateral losses in the trunk. Sensory
discrimination can pinpoint the level of damage in the spinal cord. Below the
hemisection, pain stimuli will be perceived in the damaged side, but not fine
touch. The opposite is true on the other side. The pain fibers on the side with
motor function cross the midline in the spinal cord and ascend in the
contralateral lateral column as far as the hemisection. The dorsal column will
be intact ipsilateral to the source on the intact side and reach the brain for
conscious perception. The trauma would be at the level just before sensory
discrimination returns to normal, helping to pinpoint the trauma. Whereas
imaging technology, like magnetic resonance imaging (MRI) or computed
tomography (CT) scanning, could localize the injury as well, nothing more
complicated than a cotton-tipped applicator can localize the damage. That may
be all that is available on the scene when moving the victim requires crucial
decisions be made. The sensory and motor exams assess function related to the
spinal cord and the nerves connected to it. Sensory functions are associated
with the dorsal regions of the spinal cord, whereas motor function is
associated with the ventral side.
Localizing damage to the spinal cord is
related to assessments of the peripheral projections mapped to dermatomes.
Sensory tests address the various submodalities of the somatic senses: touch,
temperature, vibration, pain, and proprioception. Results of the subtests can
point to trauma in the spinal cord gray matter, white matter, or even in
connections to the cerebral cortex. Motor tests focus on the function of the
muscles and the connections of the descending motor pathway. Muscle tone and
strength are tested for upper and lower extremities. Input to the muscles comes
from the descending cortical input of upper motor neurons and the direct
innervation of lower motor neurons. Reflexes can either be based on deep
stimulation of tendons or superficial stimulation of the skin. The presence of
reflexive contractions helps to differentiate motor disorders between the upper
and lower motor neurons. The specific signs associated with motor disorders can
establish the difference further, based on the type of paralysis, the state of
muscle tone, and specific indicators such as pronator drift or the Babinski
sign. The role of the cerebellum is a subject of debate. There is an obvious
connection to motor function based on the clinical implications of cerebellar
damage.
There is also strong evidence of the cerebellar role in procedural
memory. The two are not incompatible; in fact, procedural memory is motor
memory, such as learning to ride a bicycle. Significant work has been performed
to describe the connections within the cerebellum that result in learning. A
model for this learning is classical conditioning, as shown by the famous dogs
from the physiologist Ivan Pavlov’s work. This classical conditioning, which
can be related to motor learning, fits with the neural connections of the
cerebellum. The cerebellum is 10 percent of the mass of the brain and has
varied functions that all point to a role in the motor system. The cerebellum
is located in apposition to the dorsal surface of the brain stem, centered on
the pons. The name of the pons is derived from its connection to the
cerebellum. The word means “bridge” and refers to the thick bundle of
myelinated axons that form a bulge on its ventral surface. Those fibers are
axons that project from the gray matter of the pons into the contralateral
cerebellar cortex. These fibers make up the middle cerebellar peduncle (MCP)
and are the major physical connection of the cerebellum to the brain stem . Two other white matter bundles connect the cerebellum to the other
regions of the brain stem. The superior cerebellar peduncle is the connection
of the cerebellum to the midbrain and forebrain. The inferior cerebellar
peduncle is the connection to the medulla.
The connections to the cerebellum
are the three cerebellar peduncles, which are close to each other. The ICP arises
from the medulla—specifically from the inferior olive, which is visible as a
bulge on the ventral surface of the brain stem. The MCP is the ventral surface
of the pons.