6. Disorders of the Motor System
Part 3 of 3
James Knierim, Ph.D.
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Disorders of the Cerebellum
Like the basal ganglia, the cerebellum has historically been considered part of the motor system because damage to it produces motor disturbances. Unlike the basal ganglia, damage to the cerebellum does not result in lack of movement or poverty of movement. Instead, cerebellar dysfunction is characterized by a lack of movement coordination. Also unlike basal ganglia (and motor cortex), damage to the cerebellum causes impairments on the ipsilateral side of the body.
1. Ataxia is a general term used to describe the general impairments in movement coordination and accuracy that accompany cerebellar damage. There are two major forms of cerebellar ataxia.
- Disturbances of posture or gait result from lesions to the vestibulocerebellum. Patients have difficulty maintaining posture because of the loss of the fine-control mechanisms programmed by cerebellar circuits that translate vestibular signals into precise, well-timed muscle contractions to counter small sways in the body. As a result, patients often develop abnormal gait and stances to compensate. For example, the feet are often spaced widely apart when the patient stands still, as this provides a more stable base to maintain balance. In addition, patients display a staggering gait, with a tendency to fall toward the side of the lesion. This gait resembles that of a drunken individual; indeed, alcohol is known to affect the firing of Purkinje cells, which may explain the loss of coordination that accompanies inebriation.
- Decomposition of movement results from the loss of the cerebellum’s ability to coordinate the activity and timing of many muscle groups to produce smooth, fluid movements. Instead, cerebellar patient decompose each movement into its component parts, performing them in serial, rather than all at once in a coordinated fashion.
2. Dysmetria refers to the inappropriate force and distance that characterizes target-directed movements of cerebellar patients. For example, in attempting to grab a cup, they may move their hand outward with too much force or may move it too far, with the result of knocking over the cup instead of grabbing it.
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3. Dysdiadochokinesia refers to the inability of cerebellar patients to perform rapidly alternating movements, such as rapidly pronating and supinating the hands and forearms (Figure 6.6). This diagnostic sign results from the lack of the cerebellum’s ability to coordinate the timing of muscle groups, alternately contracting and inhibiting antagonistic muscles, to produce the rhythmic movements.
4. Scanning speech refers to the often staccato nature of speech of cerebellar patients. The production of speech is a motor act, as muscles of the jaw, tongue, and larynx need to work in unison to produce words and sounds. Cerebellar patients have difficulty in coordinating these muscle groups appropriately, and therefore their speech tends to be slow and disjointed.
5. Hypotonia is another symptom of cerebellar damage. There is a decreased, pendulous myotatic reflex, as the decreased muscle resistance tends to cause the limb to swing back and forth after the initial reflex contraction.
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6. Intention tremor refers to the increasingly oscillatory trajectory of a cerebellar patient’s limb in a target-directed movement (Figure 6.7). For example, the hand will start out on a straight path toward the target, but as it gets closer, the hand begins to move back and forth, and the patient must slow down the movement and very carefully approach the target. Note that this tremor contrasts with the resting tremor of Parkinson’s disease, which disappears when the movement is made. Intention tremor is absent when the hand is still, but appears toward the end of a target-directed movement.
7. Nystagmus is an oscillatory movement of the eyes, resulting from damage to the vestibulocerebellum. Recall that one function of the cerebellum is to fine-tune the gain of the vestibuloocular response. Damage to the cerebellum can disrupt this circuitry, resulting in a continuing oscillation of the eyes.
8. Delay in initiating movements. Cerebellar patients take longer to initiate movements, often because they must actively plan sequences of movements that are performed effortlessly by normal individuals.
9. In addition to movement disorders, cerebellar patients also demonstrate subtle cognitive deficits, such as an impaired ability to estimate time intervals.
Test Your Knowledge
- Question 1
- A
- B
- C
- D
- E
Following a strenuous workout with his neighborhood team, a right-handed, 52-year-old former professional basketball player awoke the next morning with paralysis of the right lower extremity. A neurological exam revealed an exaggerated stretch reflex. There was no disturbance of position sense, pain sensation or tactile discrimination. Where is the problem localized?
A. Anterior (ventral) horn, right side.
B. Cerebellum, right side.
C. Posterior (dorsal) columns of spinal cord, right side.
D. Left motor cortex, lateral (inferior) portion of motor map.
E. Left motor cortex, medial (superior) portion of motor map.
Following a strenuous workout with his neighborhood team, a right-handed, 52-year-old former professional basketball player awoke the next morning with paralysis of the right lower extremity. A neurological exam revealed an exaggerated stretch reflex. There was no disturbance of position sense, pain sensation or tactile discrimination. Where is the problem localized?
A. Anterior (ventral) horn, right side. This answer is INCORRECT. An exaggerated stretch reflex is an upper motor neuron symptom.
B. Cerebellum, right side.
C. Posterior (dorsal) columns of spinal cord, right side.
D. Left motor cortex, lateral (inferior) portion of motor map.
E. Left motor cortex, medial (superior) portion of motor map.
Following a strenuous workout with his neighborhood team, a right-handed, 52-year-old former professional basketball player awoke the next morning with paralysis of the right lower extremity. A neurological exam revealed an exaggerated stretch reflex. There was no disturbance of position sense, pain sensation or tactile discrimination. Where is the problem localized?
A. Anterior (ventral) horn, right side.
B. Cerebellum, right side. This answer is INCORRECT. Cerebellar lesions do not produce paralysis.
C. Posterior (dorsal) columns of spinal cord, right side.
D. Left motor cortex, lateral (inferior) portion of motor map.
E. Left motor cortex, medial (superior) portion of motor map.
Following a strenuous workout with his neighborhood team, a right-handed, 52-year-old former professional basketball player awoke the next morning with paralysis of the right lower extremity. A neurological exam revealed an exaggerated stretch reflex. There was no disturbance of position sense, pain sensation or tactile discrimination. Where is the problem localized?
A. Anterior (ventral) horn, right side.
B. Cerebellum, right side.
C. Posterior (dorsal) columns of spinal cord, right side. This answer is INCORRECT. Lesions of the posterior columns of the spinal cord produce sensory deficits, not paralysis.
D. Left motor cortex, lateral (inferior) portion of motor map.
E. Left motor cortex, medial (superior) portion of motor map.
Following a strenuous workout with his neighborhood team, a right-handed, 52-year-old former professional basketball player awoke the next morning with paralysis of the right lower extremity. A neurological exam revealed an exaggerated stretch reflex. There was no disturbance of position sense, pain sensation or tactile discrimination. Where is the problem localized?
A. Anterior (ventral) horn, right side.
B. Cerebellum, right side.
C. Posterior (dorsal) columns of spinal cord, right side.
D. Left motor cortex, lateral (inferior) portion of motor map. This answer is INCORRECT. The lateral portion of the motor map controls face muscles.
E. Left motor cortex, medial (superior) portion of motor map.
Following a strenuous workout with his neighborhood team, a right-handed, 52-year-old former professional basketball player awoke the next morning with paralysis of the right lower extremity. A neurological exam revealed an exaggerated stretch reflex. There was no disturbance of position sense, pain sensation or tactile discrimination. Where is the problem localized?
A. Anterior (ventral) horn, right side.
B. Cerebellum, right side.
C. Posterior (dorsal) columns of spinal cord, right side.
D. Left motor cortex, lateral (inferior) portion of motor map.
E. Left motor cortex, medial (superior) portion of motor map. This answer is CORRECT! Lesions to the medial portion of the motor map produce contralateral paralysis of the lower parts of hte body.
- Question 2
- A
- B
- C
- D
- E
All of the following are examples of dyskinesia EXCEPT:
A. Athetosis
B. Chorea
C. Tremors
D. Rigidity
E. Ballismus
All of the following are examples of dyskinesia EXCEPT:
A. Athetosis This answer is INCORRECT. Atheosis is an involuntary, abnormal movement.
B. Chorea
C. Tremors
D. Rigidity
E. Ballismus
All of the following are examples of dyskinesia EXCEPT:
A. Athetosis
B. Chorea This answer is INCORRECT. Chorea is an involuntary, abnormal movement.
C. Tremors
D. Rigidity
E. Ballismus
All of the following are examples of dyskinesia EXCEPT:
A. Athetosis
B. Chorea
C. Tremors This answer is INCORRECT. Tremors are involuntary, abnormal movements.
D. Rigidity
E. Ballismus
All of the following are examples of dyskinesia EXCEPT:
A. Athetosis
B. Chorea
C. Tremors
D. Rigidity This answer is CORRECT! Rigidity is not an involuntary movement.
E. Ballismus
All of the following are examples of dyskinesia EXCEPT:
A. Athetosis
B. Chorea
C. Tremors
D. Rigidity
E. Ballismus This answer is INCORRECT. Ballismus is an involuntary, abnormal movement.
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