Visual
cortex information is also part of the processing that occurs in the
cerebrocerebellum while it is involved in guiding movements of the finger or
toe. Rapid, alternating movements are tested for the upper and lower
extremities. The patient is asked to touch each finger to their thumb, or to
pat the palm of one hand on the back of the other, and then flip that hand over
and alternate back-and-forth. 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 Max Synapse 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 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.
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