Motor Functions of the Cerebellum Flashcards
What is the function of the cerebellum?
- helps to sequence motor activities
- monitors and makes corrective adjustments in the body’s motor activities while they are being executed so that they will conform to the motor signals directed by the cerebral motor cortex and other parts of the brain
- It is especially vital during rapid muscular activities such as running, typing, playing the piano, and even talking.
> Loss of this area of the brain can cause almost total incoordination of these activities even though its loss causes paralysis of no muscles
Describe the functional anatomy of the cerebellum?
The cerebellum is divided into three lobes by two deep fissures:
(1) the anterior lobe
(2) the posterior lobe
(3) the flocculonodular lobe
Name the 3 layers of the cerebellar cortex?
the molecular layer, the Purkinje cell layer, the granular cell layer
What is the functional unit of the cerebellum?
The functional unit centres on a single very large Purkinje cell (30 million in the cerebellar cortex), and on a corresponding deep nuclear cell (output)
> dendrites of Purkinje cells can receive up to 200,000 synapses
What are the input fibers of the cerebellum?
- Climbing fibres
- Mossy fibers
Describe how the functional unit of the cerebellum works?
Purkinje cells and deep nuclear cells fire continually
The output activity of these cells can be modulated either downwards or upwards
A decrease in the firing of the deep nuclear cells below the normal level would provide an inhibitory output signal to the motor system
The opposite would provide excitatory output signal
Describe how the balance between excitation and inhibition of deep cerebellar nuclei is kept?
Normally, the balance between excitation and inhibition of deep nuclear cells is slightly in favour of excitation
During fast movement, the cerebellum can exert or initiate breaking impulses to slow down or dampen a movement.
Ordinarily, the motor cortex transmits far more impulses than are needed to perform a movement and the cerebellum inhibits the motor cortex at the appropriate time
Prevents overshooting of muscle movement
Name other inhibitory cells in the cerebellar cortex?
Basket cells
Stellate cells
Golgi cells
Describe the motor loop through the lateral cerebellum?
Axons arising from layer V pyramidal cells in the sensorimotor cortex form a massive projection to clusters of cells in the pons, the pontine nuclei, which in turn feed the cerebellum (corticopontocerebellar pathway)
Once the cerebellum has received the signal for movement intent, its activity appears to instruct the primary motor cortex with respect to movement direction, timing, and force
For ballistic movements, these instructions are based entirely on predictions about their outcome
such movements are too fast for sensory feedback to be of much immediate use
Such predictions are based on past experience i.e., they are learned.
Therefore, the cerebellum is another important site for motor learning
it is a place where what is intended is compared with what has happened
Describe the inputs to the cerebellum from the brainstem?
Corticopontocerebellar pathway which terminates in the lateral cerebellum
The olivocerebellar tract which passes from the inferior olive to all parts of the cerebellum
excited in the olive by fibers from the cerebral motor cortex, basal ganglia, widespread areas of the reticular formation, and spinal cord
Vestibulocerebellar fibers which originate in the vestibular apparatus and from the brain stem vestibular nuclei
almost all of these terminate in the flocculonodular lobe and fastigial nucleus of the cerebellum
Reticulocerebellar fibers which originate in different portions of the brain stem reticular formation and terminate in the midline cerebellar areas
Describe the inputs from the cerebellum from the periphery?
Through the spinocerebellar tracts
from muscle spindles, Golgi tendon organs, large tactile receptors of the skin, and joint receptors
Through the spinal dorsal columns to the dorsal column nuclei of the medulla
Through the spinal cord through the spinoreticular pathway to the reticular formation of the brain stem
Through the spino-olivary pathway to the inferior olivary nucleus.
Then signals are relayed from these three areas to the cerebellum
Describe the cerebellar output?
Tracts that link the cerebellum with the brain stem, cerebrum, and spinal cord leave the cerebellar hemispheres as the superior, middle, and inferior cerebellar peduncles.
SCP: to the cerebral cortex via midbrain and thalamus
MCP: to pontine nuclei
ICP: to the medulla oblongata
Describe the function of the cerebellum in voluntary muscle control?
Equilibrium in a person with cerebellar dysfunction is more disturbed during performance of rapid motion than during stasis
More pronounced when movements involve changes in direction that stimulate the semicircular canals (vestibular system)
This suggests that the cerebellum is important in controlling balance between agonist and antagonist muscle contraction
> An independent feedback circuitry exists between the motor cortex of the cerebrum and the cerebellum
What are the 2 principal functions of circuitry between the motor cortex of the cerebrum and the cerebellum?
- Helps the cerebral cortex to coordinate patterns of movement involving distal parts of the limbs especially the hands, fingers, feet
- Helps the cerebral cortex plan the timing and sequencing of the next successive movement that will be performed after the present is completed
Describe the cerebellar feedback control of distal limb movements?
Part of the cerebellum involved in this function is mainly the intermediate zone of the cerebellar cortex and its associated nucleus interpositus
Receives two types of information when a movement is performed:
1. Direct info from the motor cortex and red nucleus telling the cerebellum the sequential intended plan of movement for the next few fractions of a second
2. Feedback info from the peripheral parts of the body especially distal parts of the limb, telling the cerebellum what actual movements result
