The
patient then must indicate whether one or two stimuli are in contact with the
skin. Why is the distance between the caliper points closer on the fingertips
as opposed to the palm of the hand? And what do you think the distance would be
on the arm, or the shoulder? Muscle Strength and Voluntary Movement The skeletomotor
system is largely based on the simple, two-cell projection from the precentral
gyrus of the frontal lobe to the skeletal muscles. The corticospinal tract
represents the neurons that send output from the primary motor cortex. These
fibers travel through the deep white matter of the cerebrum, then through the
midbrain and pons, into the medulla where most of them decussate, and finally
through the spinal cord white matter in the lateral (crossed fibers) or
anterior (uncrossed fibers) columns. These fibers synapse on motor neurons in
the ventral horn. The ventral horn motor neurons then project to skeletal
muscle and cause contraction.
These two cells are termed the upper motor Focused In neuron
(UMN) and the lower motor neuron (LMN). Voluntary movements require these two
cells to be active. The motor exam tests the function of these neurons and the
muscles they control. First, the muscles are inspected and palpated for signs
of structural irregularities. Movement disorders may be the result of changes
to the muscle tissue, such as scarring, and these possibilities need to be
ruled out before testing function. Along with this inspection, muscle tone is
assessed by moving the muscles through a passive range of motion. The arm is
moved at the elbow and wrist, and the leg is moved at the knee and ankle.
Skeletal muscle should have a resting tension representing a slight contraction
of the fibers. The lack of muscle tone, known as hypotonicity or flaccidity,
may indicate that the LMN is not conducting action potentials that will keep a
basal level of acetylcholine in the neuromuscular junction. If muscle tone is
present, muscle strength is tested by having the patient contract muscles
against resistance. The examiner will ask the patient to lift the arm, for
example, while the examiner is pushing down on it. This is done for both limbs,
including shrugging the shoulders. Lateral differences in strength—being able
to push against resistance with the right arm but not the left—would indicate a
deficit in one corticospinal tract versus the other. An overall loss of
strength, without laterality, could indicate a global problem with the motor
system.
Diseases that result in UMN lesions include cerebral palsy or MS, or it
may be the result of a stroke. A sign of UMN lesion is a negative result in the
subtest for pronator drift. The patient is asked to extend both arms in front
of the body with the palms facing up. While keeping the eyes closed, if the
patient unconsciously allows one or the other arm to slowly relax, toward the
pronated position, this could indicate a failure of the motor system to
maintain the supinated position. Reflexes Reflexes combine the spinal sensory
and motor components with a sensory input that directly generates a motor
response. The reflexes that are tested in the neurological exam are classified
into two groups. A deep tendon reflex is commonly known as a stretch reflex,
and is elicited by a strong tap to a tendon, such as in the knee-jerk reflex. A
superficial reflex is elicited through gentle stimulation of the skin and
causes contraction of the associated muscles. For the arm, the common reflexes
to test are of the biceps, brachioradialis, triceps, and flexors for the
digits. For the leg, the knee-jerk reflex of the quadriceps is common, as is
the ankle reflex for the gastrocnemius and soleus. The tendon at the insertion
for each of these muscles is struck with a rubber mallet.
The muscle is quickly
stretched, resulting in activation of the muscle spindle that sends a signal
into the spinal cord through the dorsal root. The fiber synapses directly on
the ventral horn motor neuron that activates the muscle, causing contraction.
The reflexes are physiologically useful for stability. If a muscle is
stretched, it reflexively contracts to return the muscle to compensate for the
change in length. In the context of the neurological exam, reflexes indicate
that the LMN is functioning properly. The most common superficial reflex in the
neurological exam is the plantar reflex that tests for the Babinski sign on the
basis of the extension or flexion of the toes at the plantar surface of the
foot. The plantar reflex is commonly tested in newborn infants to establish the
presence of neuromuscular function. To elicit this reflex, an examiner brushes
a stimulus, usually the examiner’s fingertip, along the plantar surface of the
infant’s foot. An infant would present a positive Babinski sign, meaning the
foot dorsiflexes and the toes extend and splay out. As a person learns to walk,
the plantar reflex changes to cause curling of the toes and a moderate plantar
flexion.
If superficial stimulation of the sole of the foot caused extension of
the foot, keeping one’s balance would be harder. The descending input of the
corticospinal tract modifies the response of the plantar reflex, meaning that a
negative Babinski sign is the expected response in testing the reflex. Other
superficial reflexes are not commonly tested, though a series of abdominal
reflexes can target function in the lower thoracic spinal segments. Testing
reflexes of the trunk is not commonly performed in the neurological exam, but
if findings suggest a problem with the thoracic segments of the spinal cord, a
series of superficial reflexes of the abdomen can localize function to those
segments. If contraction is not observed when the skin lateral to the umbilicus
(belly button) is stimulated, what level of the spinal cord may be damaged?
Comparison of Upper and Lower Motor Neuron Damage Many of the tests of motor
function can indicate differences that will address whether damage to the motor
system is in the upper or lower motor neurons. Signs that suggest a UMN lesion
include muscle weakness, strong deep tendon reflexes, decreased control of
movement or slowness, pronator drift, a positive Babinski sign, spasticity, and
the clasp-knife response. Spasticity is an excess contraction in resistance to
stretch. It can result in hyperflexia, which is when joints are overly flexed.
The clasp-knife response occurs when the patient initially resists movement,
but then releases, and the joint will quickly flex like a pocket knife closing.
