Lecture 08: Contributions of Cerebellum and Basal Nuclei to Motor Function Flashcards
Describe the function of the Primary motor cortex (area 4)
- Signals motor neurons to contract skeletal muscle fibers
- Signals via the corticospinal (pyramidal) tract
- Execution of commands is preceded by extensive processing by cerebellum and basal nuclei
Describe the function of the cortex (Area 6):
Plans movements based on sensory and visual cues
Describe the function of the Supplementary motor area (Area 6):
Retrieves and coordinates memorized motor sequences
Functions of motor cortex system:
• Provides most of activating signals to spinal cord
• Issues sequential and parallel commands that initiate various cord
patterns
• Cortical patterns are usually complex and can be learned.
• Cord patterns are mainly determined by heredity and are “hard wired.”
Cerebellum
The cerebellum plays major roles in the timing of motor activities and in rapid, smooth progression from one muscle movement to the next.
What are the specific responsibilities of the cerebellum?
- Not essential for locomotion
- Helps sequence motor activities
- Monitors and makes corrective adjustments to motor activities while they are being executed
- Compares actual movements with intended movements
- Aids cortex in planning next sequential movement
- Learns by its mistakes
More functions of Cerebellum
- Functions with spinal cord to enhance the stretch reflex
- Functions with brain stem to make postural movements
- Functions with cerebral cortex to provide accessory motor functions.
- Turns on antagonist at appropriate time
- Helps program muscle contraction in advance
- Functions mainly when muscle movements have to be rapid
What do the Basal nuclei do?
The basal nuclei help to plan and control complex patterns of muscle movement, controlling relative intensities of the separate movements, directions of movements, and sequencing of multiple successive and parallel movements.
More functions of the basal nuclei:
- Plans and executes motor commands in concert with cerebral cortex; helps cortex execute subconscious but learned pattern
- Helps plan multiple parallel sequential patterns
- Controls complex patterns of motor activity
General Functions of the Cerebellum
- Electrical excitation of the cerebellum does not cause any conscious sensation and rarely causes any motor movement.
- Removal of the cerebellum causes body movements to become highly abnormal.
Explain how the cerebellum is organized anatomically:
Two hemispheres separated by vermis:
• Each hemisphere is divided into an intermediate zone and a lateral zone.
Anatomically divided into three lobes (Anterior →Posterior):
• Anterior lobe:
• Posterior lobe:
• Flocculonodular lobe:
Associated with vestibular system
The Flocculonodular lobe is:
Associated with vestibular system
Vermis:
• Location for control functions for muscle movements of the axial body, neck, shoulders, and hips
Intermediate zone:
• Concerned with controlling muscle contractions in the distal portions of the upper and lower limbs, esp. hands, feet, fingers, and toes
Lateral zone:
• Associated with cerebral cortex with planning of sequential motor movements
Cortex:
• Consists of transversely arranged narrow gyri called folia
Intracerebellar nuclei:
- Dentate
- Emboliform
- Globose
- Fastigial
Dentate nuclei, Emboliform nuclei, Globose nuclei:
- Lesions in these nuclei → extremity ataxia
- These fibers project to the red nucleus
- Related to limb musculature and fine manipulative movement
Fastigial nuclei:
- Lesion in these nuclei → trunk ataxia
- Fibers project to reticular formation and vestibular nuclei
- Related to postural activity and limb movements via reticulospinal and vestibulospinal tracts
Granular cells:
• Axons form parallel fibers in cortex (+)
Golgi cells:
• Project from parallel fibers to granular cell bodies (-)
Basket cells:
• Project from parallel fibers to Purkinje axon hillock (-)
Stellate cells:
• Project from parallel fibers to Purkinje dendrites (-)
What cells provide lateral inhibition on purkinje fibers?
Note that basket cells and stellate cells provide lateral inhibition on adjacent Purkinje cells to provide damping.
Purkinje Cells:
• Extensive dendritic branching • Receive input from parallel fibers (20,000 synapses between parallel fibers and one Purkinje cell. • Project to intracerebellar nuclei (-) • ONLY output from cortex
Functional Unit of Cerebellar Cortex
• Each functional unit is centered on a Purkinje cell and a corresponding deep nuclear cell.
• Output from a functional unit is from a deep nuclear cell.
• Afferent inputs to the cerebellum are mainly from the
climbing and mossy fibers.
• All climbing fibers originate from the inferior olives.
• Mossy fibers enter cerebellum from a variety of sources.
• Send excitatory collaterals to deep nuclear cells and then synapse in granular layer with thousands of granule cells.
What do Mossy Fibers do?
• Send excitatory collaterals to deep nuclear cells and then synapse in granular layer with thousands of granule cells.
Granule Cells:
send axons to outer cerebellar surface; axons branch in two directions parallel to folia.
Dendrites of Purkinje cells
project parallel fibers
climbing and mossy fibers excites
Direct stimulation by climbing and mossy fibers excites deep nuclear cells. Purkinje cell signals inhibit deep nuclear cells.
Basket cells and stellate cells also function
as inhibitory fibers
Nervous system uses cerebellum to coordinate motor control functions at three levels:
- Vestibulocerebellum
- Spinocerebellum
- Cerebrocerebellum
Vestibulocerebellum:
• Consists of flocculonodular lobes and vermis
• Evolved at about the same time as vestibular system
• Receives fibers from: Vestibular system
Oculomotor system (pontocerebellar fibers)
• Sends output primarily to vestibular system.
• Loss of flocculonodular lobes → extreme disturbance of equilibrium and postural movements.
Vestibulocerebellum:
•Relationship of vestibulocerebellum to pendular
movements:
Most body movements are pendular (swing back and forth)
All pendular movements have tendency to overshoot (WHY?)
Appropriate learned subconscious signals from intact cerebellum can stop movement precisely at intended point ( = damping system).
Changes that occur when cerebellum is removed: Movements are slow to develop.
Force developed is weak.
Movements are slow to turn off.
Spinocerebellum:
• Receives:
Information from motor cortex and red nucleus telling cerebellum intended sequential plan of movement for the next few fractions of a second.
Feedback information from periphery telling cerebellum what actual movements result
• Compares two sources of information and sends corrections to:
Motor cortex via thalamus
Magnocellular portion of red nucleus
Cerebrocerebellum:
• Consists of lateral parts of hemispheres
• Mostly associated with the premotor and the primary and
association somatosensory areas of the cerebral cortex
• Receives corticopontocerebellar projections
• Involved in coordination of skilled movement and speech
• Plans as much as tenths of a second in advance of actual movements:
Referred to as “motor imagery”
Dysmetria and Ataxia
Two of the most important symptoms of cerebellar disease are dysmetria and ataxia. As pointed out previously, in the absence of the cerebellum, the subconscious motor control system cannot predict how far movements will go. Therefore, the movements ordinarily overshoot their intended mark; then the conscious portion of the brain overcompensates in the opposite direction for the succeeding compensatory movement. This effect is called dysmetria, and it results in uncoordinated movements that are called ataxia. Dysmetria and ataxia can also result from lesions in the spinocerebellar tracts because feedback information from the moving parts of the body to the cerebellum is essential for cerebellar timing of movement termination.
Past pointing
Past pointing means that in the absence of the cerebellum, a person ordinarily moves the hand or some other moving part of the body considerably beyond the point of intention. This results from the fact that normally the cerebellum initiates most of the motor signal that turns off a movement after it is begun; if the cerebellum is not available to do this, the movement ordinarily goes beyond the intended mark. Therefore, past pointing is actually a manifestation of dysmetria.
Dysdiadochokinesia
When the motor control system fails to predict where the different parts of the body will be at a given time, it “loses” perception of the parts during rapid motor movements. As a result, the suc- ceeding movement may begin much too early or much too late, so that no orderly “progression of movement” can occur. One can demonstrate this readily by having a patient with cerebellar damage turn one hand upward and downward at a rapid rate. The patient rapidly “loses” all perception of the instantaneous position of the hand during any portion of the movement. As a result, a series of stalled attempted but jumbled move- ments occurs instead of the normal coordinate upward and downward motions. This is called dysdiadochokinesia.
Dysarthria
Another example in which failure of progression occurs is in talking because the formation of words depends on rapid and orderly succession of indi- vidual muscle movements in the larynx, mouth, and respiratory system. Lack of coordination among these and inability to adjust in advance either the intensity of sound or duration of each successive sound causes jumbled vocalization, with some syllables loud, some weak, some held for long intervals, some held for short intervals, and resultant speech that is often unintelligi- ble. This is called dysarthria.
Cerebellar nystagmus
Cerebellar nystagmus is tremor of the eyeballs that occurs usually when one attempts to fixate the eyes on a scene to one side of the head. This off-center type of fixation results in rapid, tremulous movements of the eyes rather than steady fixation, and it is another manifestation of failure of damping by the cerebellum. It occurs especially when the flocculonodu- lar lobes of the cerebellum are damaged; in this instance it is also associated with loss of equilibrium because of dysfunction of the pathways through the flocculonodular cerebellum from the semicircular ducts.
Hypotonia
Loss of the deep cerebellar nuclei, particu- larly of the dentate and interposed nuclei, causes decreased tone of the peripheral body musculature on the side of the cerebellar lesion. The hypotonia results from loss of cerebellar facilitation of the motor cortex and brain stem motor nuclei by tonic signals from the deep cerebellar nuclei.
Trace the pathway of the Corticopontocerebellar path:
Motor and premotor corHces/Somatosensory cortex → PonHne nuclei → Lateral divisions of cerebellum
Where does the Vestibulocerebellar pathway terminate?
terminates in flocculonodular lobes
Where does the Reticulocerebellar pathway terminate?
Terminates primarily in vermis
Spinocerebellar:
- Dorsal and ventral
* Transmit signals at 120 m/sec.
where do mossy fibers travel?
All the above tracts form the mossy fibers that terminate on the granule cells in the cerebellar cortex (+)
Dorsal spinocerebellar:
- Muscle spindles → ipsilaterally in vermis and intermediate zones
- Apprise cerebellum of momentary status of:
- Muscle contractions
- Degree of tension on the muscle spindles
- Positions and rates of movement of parts of the body
- Forces acting on surfaces of the body
Afferent: Ventral spinocerebellar:
- Terminates both ipsilaterally and contralaterally
- Excited by signals coming from:
- Cortex via corticospinal and rubrospinal tracts
- Internal motor pattern generators within spinal cord
- Tells cerebellum:
* Which motor signals have arrived at the anterior horns - This feedback = efference copy of the anterior horn motor drive
Olivocerebellar:
- Neurons project from inferior olivary nuclei (in medulla) to Purkinje cell dendrites (+) and to intracerebellar nuclei.
- Axons form climbing fibers
- Climbing fiber causes a single, prolonged (up to one second) action potential on each Purkinje cell with which it connects (one climbing fiber per 5-10 Purkinje cells.
- Each signal starts out as a strong spike and is followed by a series of weak secondary spikes (= complex spike)
- Note that mossy fibers (discussed previously) send (+) signals to granule cells.
Efferent Tracts of Cerebellum:
• Cerebelloreticular:
Fastigial nuclei → reHcular nuclei in pons and medulla •Cerebellothalamocortical:
Dentate, emboliform, globose nuclei → thalamus → motor cortex
• Cerebellorubral:
Dentate, emboliform, globose nuclei → red nucleus
• Cerebellovestibular:
Cerebellum → vestibular nuclei
What are pendular movements? How does the cerebellum affect pendular movements?
Almost all movements of the body are “pendular.” For instance, when an arm is moved, momentum develops, and the momentum must be overcome before the movement can be stopped. Because of momentum, all pendular move- ments have a tendency to overshoot. If overshooting does occur in a person whose cerebellum has been destroyed, the conscious centers of the cerebrum eventually recognize this and initiate a movement in the reverse direction attempting to bring the arm to its intended position. But the arm, by virtue of its momentum, overshoots once more in the opposite direction, and appropriate corrective signals must again be insti- tuted. Thus, the arm oscillates back and forth past its intended point for several cycles before it finally fixes on its mark. This effect is called an action tremor, or intention tremor.
But, if the cerebellum is intact, appropriate learned, subconscious signals stop the movement precisely at the intended point, thereby preventing the overshoot as well as the tremor. This is the basic characteristic of a damping system. All control systems regulating pendular elements that have inertia must have damping circuits built into the mechanisms. For motor control by the nervous system, the cerebellum provides most of this damping function.
What are ballistic movements?
Most rapid movements of the body, such as the movements of the fingers in typing, occur so rapidly that it is not possible to receive feedback information either from the periphery to the cerebellum or from the cerebellum back to the motor cortex before the movements are over. These movements are called ballistic movements, meaning that the entire movement is preplanned and set into motion to go a specific distance and then to stop. Another important example is the saccadic movements of the eyes, in which the eyes jump from one position to the next when reading or when looking at successive points along a road as a person is moving in a car.
How does removal of the cerebellum affect movements of
the body?
(1) The movements are slow to develop and do not have the extra onset surge that the cerebellum usually provides, (2) the force developed is weak, and (3) the movements are slow to turn off, usually allowing the movement to go well beyond the intended mark. Therefore, in the absence of the cerebellar circuit, the motor cortex has to think extra hard to turn ballistic movements on and again has to think hard and take extra time to turn the movement off. Thus, the automatism of ballistic movements is lost.
Basal nuclei
• Basal nuclei receive most of their input from the cerebral cortex and return most of their output to the cerebral cortex. • Principal role is to work with corticospinal system to control complex patterns of motor activity. • Basal nuclei consist of paired: Caudate nucleus Putamen Globus pallidus Substantia nigra Subthalamic nucleus
Two major basal nuclei circuits:
Putamen circuit
Caudate nucleus circuit
Putamen circuit:
For subconscious execution of learned patterns of movement
Cerebral cortex (premotor, supplementary motor, somatosensory) → Putamen →
(Note that it bypasses caudate nucleus)
Globus pallidus (internal) →
Thalamic relay nuclei (ventroanterior & ventrolateral nuclei) → Primary motor cortex (and premotor/supplementary)
Lesions in globus pallidus:
• Result in continuous spontaneous writhing movements of a hand, arm, neck or face = athetosis
Lesions in subthalamus:
• Result in sudden, flailing movements of an entire limb = hemiballismus
Lesions in putamen:
• Results in flicking movements in hands, face, or elsewhere = chorea
Lesions in substantia nigra:
• Results in rigidity, akinesia, and tremors = Parkinson’s disease
Caudate Circuit:
For cognitive planning of sequential and parallel motor patterns.
Plays major role in cognitive control of motor activity
Cerebral cortex (premotor, supplementary motor, somatosensory) → Caudate nucleus →
Globus pallidus (internal) →
Thalamic relay nuclei (ventroanterior & ventrolateral nuclei) → Premotor and supplementary motor cortex
Substantia nigra → caudate nucleus and putamen:
Dopamine (inhibitory)
Caudate nucleus and putamen → globus pallidus and substantia nigra:
GABA (Inhibitory)
Cortex → caudate nucleus and putamen:
Acetylcholine
Multiple pathways from brain stem:
Norepinephrine, serotonin (inhibitory), encephalin
Multiple glutamate pathways:
Provide most of the excitatory signals
Parkinson’s
• AKA: Paralysis agitans
• Results from widespread destruction of pars compacta of substantia nigra that sends dopaminergic fibers to caudate nucleus and putamen
• Characteristics:
Rigidity of much of body musculature
Involuntary tremors of involved areas even at rest at a fixed rate Serious difficulty in initiating movement (akinesia)
Postural instability
Dysphagia, speech disorders, gait disturbances, fatigue
Huntington’s
• Usually becomes symptomatic around 30-40 years of age
• Characteristics:
Flicking movements of individual muscles
Progressive severe distortional movements of entire body Severe dementia
Motor dysfunctions
• Abnormal movements probably caused by loss of most of cell bodies of GABA-secreting neurons of caudate nucleus and putamen and of Ach neurons in other parts of the brain.
• GABA neurons normally inhibit parts of the globus pallidus and substantia nigra.
Athetosis
lesions in the globus pallidus frequently lead to spon- taneous and often continuous writhing movements of a hand, an arm, the neck, or the face—movements called athetosis.
hemiballismus
A lesion in the subthalamus often leads to sudden flailing movements of an entire limb, a condition called hemiballismus.
chorea
Multiple small lesions in the putamen lead to flick- ing movements in the hands, face, and other parts of the body, called chorea.
What is agnosia? With what lesion is it associated?
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What is personal neglect syndrome?
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What two major functions are provided by the hindbrain for general motor control of the body?
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Summarize the functions of the cerebellum (from text).
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What are the two most important functions of the basal nuclei (ganglia)?
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