Efferents from the deep cerebellar nuclei leave mainlythrough the SCP and influ ence allupper motoneurons. In particular, axons from the dentate and interposed nuclei leave through the SCP, cross the midline, and terminate in the ventrolat eral (VL) nucleus ofthe thalamus.
The VL nucleus ofthe thalamus projects to primary motor cortex and influences the firing ofcorticospinal and corticobulbar neurons.
Axons from other deep cerebellar nuclei influence uppermotoneurons in the red nucleus and in the reticular formation and vestibular nuclei.
Cerebellar Lesions
The hallmark of cerebellar dysfunction is a tremor with intended movement without paralysis or paresis. Symptoms associated with cerebellar lesions are ex pressed ipsilaterally because the major outflow ofthe cerebellum projects to the contralateral motor cortex, and then the corticospinal fibers cross on their wayto the spinal cord. Thus, unilateral lesions ofthe cerebellum willresult in a patient falling toward the side ofthe lesion.
Lesions that include the hemisphere
Lesions that include the hemisphere produce a number of dysfunctions, mostly involving distal musculature.
An intention tremor is seen whenvoluntarymovements are performed. Forexam ple, ifa patient with a cerebellar lesion is asked to pick up a penny, a slight tremor of the fingers is evident and increases as the penny is approached. The tremor is barelynoticeable or is absent at rest.
Dysmetria (past pointing) is the inabilityto stop a movement at the proper place. The patient has difficulty performing the finger-to-nose test.
Dysdiadochokinesia (adiadochokinesia) is the reduced ability to perform alter nating movements, such as pronation and supination of the forearm, at a mod erately quickpace.
Scanning dysarthria is causedby asynergy ofthe muscles responsible for speech. In scanning dysarthria, patients divide words into syllables, thereby disrupting the melody ofspeech.
Gaze dysfunction occurs when the eyes try to fix on a point: They may pass it or stop too soon and then oscillate a few times before they settle on the target. A nystagmus may be present, particularly with acute cerebellar damage. The nys tagmus is often coarse, with the fast component usually directed toward the in volved cerebellar hemisphere.
Hypotonia usually occurs with an acute cerebellar insult that includes the deep cerebellar nuclei. The muscles feel flabby on palpation, and deep tendon reflexes are usually diminished.
Lesions to the vermal region
Verma! lesions result in difficultymaintaining posture, gait, or balance (an ataxic gait). Patients with vermal damage maybe differentiated from those with a lesion of the dorsal columns by the Romberg sign. In cerebellar lesions, patients will swayor lose their balancewith their eyes open; in dorsal column lesions, patients sway with their eyes closed.
