Yuste C14: Motor Control/Cerebellum Flashcards
What happens if you remove the cerebellum
You’ll have a variety of symptoms, including
instability in your stance and gait, loss of precise and fine
skill movements, and learning new movements also
becomes very difficult.
Position of the basal ganglia and the cerebellum in the motor system
as if they were modulating, controlling, or instructing the decending pyramidal pathway.
Similarities in connectivity of basal ganglia and cerebellum
a major pathway that goes from the cerebral cortex through intermediate steps to the cerebellum, to the thalamus, and back up to the cortex. Another loop.
What is unique about the connectivity of the cerebellum
It also sends outputs to the brain steam and spinal cord.
Inputs/outputs of the cerebellum
The cerebellum receives inputs from the motor cortex and sends outputs to the thalamus, brainstem and spinal cord.
Date/location of cerebellum
part of the hindbrain near the pons and medulla oblongata; relatively ancient brain region.
What type of cells is the cerebellum packed with
Granule cells. About 50 billion. There are more neurons in the cerebellum than the rest of the brain put together.
What is the inner core of the cerebellum composed of
Its inner core is formed by several nuclei connected to the brain stem. The surface of the cerebellum is similar to the cerebral cortex in appearance and name.
The cerebellum has an internal core of nuclei and an external cortex composed of highly folded lobes, maximizing its surface area.
Cerebellar cortex
highly folded into little lobules called folia. Deep invaginations that are even deeper than those found in the cerebral cortex. Same folding principle.
Three parts of the cerebellar cortex
vestibulocerebellum (in the midline), the spinocerebellum (next to it) and the cerebrocerebellum (lateral to the sides); describe three different loops.
Vestibulocerebellum
connected with the vestibular nuclei and controls our balance. vestibular sense = the part of the inner ear where the sense in position with respect to gravity is computed.
Lesions in the vestibulocerebellum
you have equilibrium problems. You’ll tend to tumble.
Produce loss of equilibrium, alterations in gait and unstable eye movements.
Spinocerebellum
spinocerebellum has to do with the spinal cord.
Cerebrocerebellum
The parts of the cortex that project to the cerebellum include a large part of the frontal cortex and also parts of the parietal cortex
What is the cerebellum involved in overall?
involved in motor function, but also in computing some representation of the world, which speaks to the cerebellum’s role in motor learning.
Motor map in the cerebellar cortex
Specific parts of the cerebellar cortex are activated when the person is performing particular movements. It looks like a homunculus, with different distortions of the body proportions. The lower limbs occupy more space; The hands and both upper limbs are represented, but there’s not so clear representation of individual digits.
The cerebellar cortex has mapped representations of the body.
What does the representation in the motor map of the cerebellar cortex tell us about the cerebellum
the cerebellum has a more evolutionarily ancient homunculus — one that is more similar to a lower vertebrate than a primate.
What comes into the cerebellum - overall
the cerebellar cortex receives information about our balance and position with respect to gravity (from the vestibular nuclei), eye movements (from the brain stem), skeletal muscle (from the spinal cord), and voluntary movement (from the cerebral cortex). Looks like it is tapping into all the information you need related to moving the body and changing its posture.
Upward and downward projection of cerebellum
The cerebellum is essentially receiving the information from several areas, processing that information and relaying it back to the cortex, brainstem, and spinal cord. The cerebellum has an “upward” projection into primary motor cortex and premotor cortex via the deep cerebellar nuclei and the ventral lateral complex thalamus. And there is a “downward” projection into the brainstem and spinal cord, although through the deep cerebellar nuclei.
Cerebellum outputs
The cerebellum sends outputs to the thalamus and motor cortex and also to the brainstem and spinal cord.
Purkinje cells
large, flat neurons that look like trees and receive hundreds of thousands of afferent connections from different neurons. Probably the most connected neurons in the brain. Have flat dendrites.
Purkinje cells, which happen to be inhibitory, collect these inputs and then generate the sole output of the cerebellar cortex. Their axon forms inhibitory synapses onto the cerebellar nuclei, which then send excitatory axons outside the cerebellum.
Inputs to Purkinje cells
The vast majority of inputs to Purkinje cells come from granule cells.
Granule cells
excitatory, are made up of more than half of the neurons in the entire brain, send axons, which are called parallel fibers. Each parallel fiber axon bifurcates, forming a T, and each of the branches of the T makes contact with tens of thousands of Purkinje cells. Bundles of parallel fibres are intersected by Purkinje cells, which have flat dendrites.
Geometry of Purkinje cells and granule cells
Minimises the number of contacts between the wires and the supporting poles while maximizing the number of contacts with different poles (Purkinje cells in this case). In fact, not only are Purkinje cells flat and stacked, but parallel fibers cross them at exactly 90 degrees.
Inputs to granule cells
Mossy fibres
Mossy fibres
have their origins in the pontine nuclei, the spinal
cord, and the brainstem reticular formation. Each mossy
fiber synapses on hundreds of granule cells.
Second input to Purkinje cells
climbing fibres.
Climbing fibres
wrap themselves around the dendrites of Purkinje cells —
forming hundreds of synapses — and stimulate the Purkinje
cells so strongly that every time a climbing cell fires, a
Purkinje cell fires.
The two opposing connectivity principles demonstrated by climbing fibres and parallel fibres (of the granule cells)
In the case of the climbing fibers, we see a single fiber going to a single Purkinje cell to form many synapses so that the information is funneled into one cell more effectively. Parallel fibers, on the other hand, represent a single fiber going to many neurons to make a single synapse so that the information is spread out into a whole population.
Basket cells
inhibitory neurons that surround the Purkinje cells.
Cerebellar microcircuit
This is a loop, but with two input branches: inputs into the cerebellum activate the Purkinje cells in tandem, through the climbing fibers and the parallel fibers, which are both excitatory. Purkinje cells do some magic and send an inhibitory output, which gets relayed on to the motor part of the brain - have to do with learning.
Cerebellar microcircuit - disinhibitory loop
Purkinje cells compare the two inputs they receive, and, if they arrive at the same time, they weaken (depress) that particular parallel fiber synapse. This would make the Purkinje cell fire less the next time around, if it receives the exact same input. But because the Purkinje cell is inhibitory, this will activate whatever part of the brain is receiving this input, reinforcing that motor action or program.
Like the striatum of the basal ganglia.
Purkinje cells and control theory
Purkinje cells are comparing movement prediction with actual measurement of the movement, computing an error, a motor error, and sending this information back. ==> compute future movements.
Motor learning - example with distorting glasses and throwing darts
Then she puts on glasses that shift her vision to the right (Don’t try this at home), finding that her darts are off target, falling to the side of the target. But, as she continues to wears these glasses, her throws become more accurate until they all land on target. Looks like she has learned to compensate for the shift in vision! Then, she removes her glasses, and her throws are off again, but in the opposite direction now, as if this compensation is still going on. After a few more throws without glasses, she is back to
normal.
illustrates the learning, or adaptation, of the connection between vision and motor programs.
Motor learning example of drinking and darts and distortion glasses
no adaptation! The cerebellum is necessary for the adaptation: patients with lesions in the cerebellar cortex cannot adapt
Lesions in spinocerebellum
you end up with loss in muscle tone, muscle weakness, and dysmetria, over or under-reaching when you have a voluntary movement like grabbing a glass.
Lesions in cerebellocerebellum
Lesions in the cerebellocerebellum generate incoordination of movement, and also impairment in volitional and highly skilled movements.
What these lesions’ impacts tell us about the cerebellum’s function
the cerebellum is involved in the coordination of movements and motor learning, perhaps by computing a motor error so postural responses can be continuously adjusted.
fMRI evidence and function of cerebellum
suggests that the cerebellum is involved in memory, which may relate to the ancient role the motor system may play in cognitive functions. This may be related to another fascinating idea: that the cerebellum, by generating oscillations, is the clock (or metronome) of the brain, to synchronize movements of different parts of the body.
Oscillation of cerebellum
all responses of the cerebellum are oscillatory! The cerebellum oscillates at frequencies of 8Hz, which is the same frequency of the physiological tremor and occurs throughout the body.
