To
test similar function in the lower extremities, the patient touches their heel
to their shin near the knee and slides it down toward the ankle, and then back
again, repetitively. Rapid, alternating movements are part of speech as well. A
patient is asked to repeat the nonsense consonants “lah-kah-pah” to alternate
movements of the tongue, lips, and palate. All of these rapid alternations
require planning from the cerebrocerebellum to coordinate movement commands
that control the coordination. Posture and Gait Gait can either be considered a
separate part of the neurological exam or a subtest of the coordination exam
that addresses walking and balance. Testing posture and gait addresses
functions of the spinocerebellum and the vestibulocerebellum because both are
part of these activities. A subtest called station begins with the patient
standing in a normal position to check for the placement of the feet and
balance. The patient is asked to hop on one foot to assess the ability to
maintain balance and posture during movement.
Though the station subtest
appears to be similar to Focused In the Romberg test, the difference is that the patient’s
eyes are open during station. The Romberg test has the patient stand still with
the eyes closed. Any changes in posture would be the result of proprioceptive
deficits, and the patient is able to recover when they open their eyes.
Subtests of walking begin with having the patient walk normally for a distance
away from the examiner, and then turn and return to the starting position. The
examiner watches for abnormal placement of the feet and the movement of the
arms relative to the movement. The patient is then asked to walk with a few
different variations. Tandem gait is when the patient places the heel of one
foot against the toe of the other foot and walks in a straight line in that
manner. Walking only on the heels or only on the toes will test additional
aspects of balance. Ataxia A movement disorder of the cerebellum is referred to
as ataxia. It presents as a loss of coordination in voluntary movements. Ataxia
can also refer to sensory deficits that cause balance problems, primarily in
proprioception and equilibrium. When the problem is observed in movement, it is
ascribed to cerebellar damage. Sensory and vestibular ataxia would likely also
present with problems in gait and station. Ataxia is often the result of
exposure to exogenous substances, focal lesions, or a genetic disorder.
Focal
lesions include strokes affecting the cerebellar arteries, tumors that may
impinge on the cerebellum, trauma to the back of the head and neck, or MS.
Alcohol intoxication or drugs such as ketamine cause ataxia, but it is often
reversible. Mercury in fish can cause ataxia as well. Hereditary conditions can
lead to degeneration of the cerebellum or spinal cord, as well as malformation
of the brain, or the abnormal accumulation of copper seen in Wilson’s disease.
The examiner would look for issues with balance, which coordinates
proprioceptive, vestibular, and visual information in the cerebellum. To test
the ability of a subject to maintain balance, asking them to stand or hop on
one foot can be more demanding. The examiner may also push the subject to see
if they can maintain balance. An abnormal finding in the test of station is if
the feet are placed far apart. Why would a wide stance suggest problems with
cerebellar function? The Field Sobriety Test The neurological exam has been
described as a clinical tool throughout this chapter. It is also useful in
other ways. A variation of the coordination exam is the Field Sobriety Test
(FST) used to assess whether drivers are under the influence of alcohol. The
cerebellum is crucial for coordinated movements such as keeping balance while
walking, or moving appendicular musculature on the basis of proprioceptive
feedback.
The cerebellum is also very sensitive to ethanol, the particular type
of alcohol found in beer, wine, and liquor. Walking in a straight line involves
comparing the motor command from the primary motor cortex to the proprioceptive
and vestibular sensory feedback, as well as following the visual guide of the
white line on the side of the road. When the cerebellum is compromised by
alcohol, the cerebellum cannot coordinate these movements effectively, and
maintaining balance becomes difficult. Another common aspect of the FST is to
have the driver extend their arms out wide and touch their fingertip to their
nose, usually with their eyes closed. The point of this is to remove the visual
feedback for the movement and force the driver to rely just on proprioceptive
information about the movement and position of their fingertip relative to
their nose. With eyes open, the corrections to the movement of the arm might be
so small as to be hard to see, but proprioceptive feedback is not as immediate
and broader movements of the arm will probably be needed, particularly if the
cerebellum is affected by alcohol. Reciting the alphabet backwards is not
always a component of the FST, but its relationship to neurological function is
interesting. There is a cognitive aspect to remembering how the alphabet goes
and how to recite it backwards. That is actually a variation of the mental
status subtest of repeating the months backwards. However, the cerebellum is
important because speech production is a coordinated activity.
The speech rapid
alternating movement subtest is specifically using the consonant changes of
“lah-kah-pah” to assess coordinated movements of the lips, tongue, pharynx, and
palate. But the entire alphabet, especially in the nonrehearsed backwards
order, pushes this type of coordinated movement quite far. It is related to the
reason that speech becomes slurred when a person is intoxicated. The cerebellum
is an important part of motor function in the nervous system. It apparently
plays a role in procedural learning, which would include motor skills such as
riding a bike or throwing a football. The basis for these roles is likely to be
tied into the role the cerebellum plays as a comparator for voluntary movement.
The motor commands from the cerebral hemispheres travel along the corticospinal
pathway, which passes through the pons. Collateral branches of these fibers
synapse on neurons in the pons, which then project into the cerebellar cortex
through the middle cerebellar peduncles. Ascending sensory feedback, entering
through the inferior cerebellar peduncles, provides information about motor
performance. The cerebellar cortex compares the command to the actual
performance and can adjust the descending input to compensate for any mismatch.
The output from deep cerebellar nuclei projects through the superior cerebellar
peduncles to initiate descending signals from the red nucleus to the spinal
cord.
The primary role of the cerebellum in relation to the spinal cord is
through the spinocerebellum; it controls posture and gait with significant
input from the vestibular system. Deficits in cerebellar function result in
ataxias, or a specific kind of movement disorder. The root cause of the ataxia
may be the sensory input—either the proprioceptive input from the spinal cord
or the equilibrium input from the vestibular system, or direct damage to the
cerebellum by stroke, trauma, hereditary factors, or toxins. Communication is a
process in which a sender transmits signals to one or more receivers to control
and coordinate actions. In the human body, two major organ systems participate
in relatively “long distance” communication: the nervous system and the
endocrine system. Together, these two systems are primarily responsible for
maintaining homeostasis in the body. The nervous system uses two types of
intercellular communication—electrical and chemical signaling—either by the
direct action of an electrical potential, or in the latter case, through the
action of chemical neurotransmitters such as serotonin or norepinephrine.
Neurotransmitters act locally and rapidly. When an electrical signal in the
form of an action potential arrives at the synaptic terminal, they diffuse
across the synaptic cleft (the gap between a sending neuron and a receiving
neuron or muscle cell).
Once the neurotransmitters interact (bind) with
receptors on the receiving (post-synaptic) cell, the receptor stimulation is
transduced into a response such as continued electrical signaling or modification
of cellular response. The target cell responds within milliseconds of receiving
the chemical “message”; this response then ceases very quickly once the neural
signaling ends. In this way, neural communication enables body functions that
involve quick, brief actions, such as movement, sensation, and cognition.In
contrast, the endocrine system uses just one method of communication: chemical
signaling. These signals are sent by the endocrine organs, which secrete
chemicals—the hormone—into the extracellular fluid. Hormones are transported
primarily via the bloodstream throughout the body, where they bind to receptors
on target cells, inducing a characteristic response. As a result, endocrine
signaling requires more time than neural signaling to prompt a response in
target cells, though the precise amount of time varies with different hormones.
For example, the hormones released when you are confronted with a dangerous or
frightening situation, called the fight-or-flight response, occur by the
release of adrenal hormones—epinephrine and norepinephrine—within seconds. In
contrast, it may take up to 48 hours for target cells to respond to certain
reproductive hormones. In addition, endocrine signaling is typically less
specific than neural signaling.
