Cerebellar cortex {cerebellum}| is for smooth, continuous, and rapid movement. Cerebellar activity is never conscious. Peripheral vision, cerebellum, and vestibular system find body positions.
functions
Cerebellum maintains balance, posture, equilibrium, and muscle tone. It sets appropriate voluntary-muscle motor control, rates, forces compared to resistance, movement directions, and coordination.
functions: comparator
Cerebellum works as comparator. Motor cortex sends to spinal cord to initiate voluntary actions and to cerebellum to inform about intended movements. Proprioceptive nerve input goes to cerebral cortex and then to cerebellum to report actual movements. Cerebellum sends to motor cortex and spinal cord to correct movements.
functions: damping
Damping involves inhibiting agonist and antagonist contractions to eliminate muscle tremor, for smooth movement.
functions: error control
Error control involves initial strong muscle contraction and subsequent antagonist-muscle contraction.
functions: feedforward
Sense delays prevent feedback alone from controlling fast and accurate biological movements. Cerebellum uses predictive, feedforward control.
functions: gain
Perhaps, cerebellum controls amplification gain in spinal and brainstem reflexes. Cerebellum can subtract adjustable signal from fixed-gain saccadic circuit.
functions: precision
Cerebellum compares sense stimuli about actual performance with movement program received from cerebrum, measures error, and corrects movement. For example, it regulates smooth eye movements by tuning reflexes using Purkinje cells.
It regulates premotor networks by inhibiting and disinhibiting motor-control actions that begin in brainstem, sensorimotor-cortex, and spinal-cord premotor networks. To control movement, Purkinje cells first exert increased inhibition on deep nuclei excited by cerebral cortex and sense information. Then, inhibition decreases, and deep nuclei send excitatory output to pons and red nuclei, which send to motor cortex.
functions: prediction
Prediction involves comparing information received from eyes, body, and cerebrum, to calculate when to slow and/or stop motion.
functions: progression
Progression involves muscle contraction in sequence, to coordinate and time.
functions: sensation
Cerebellum coordinates information from different senses. Skin touch receptors send to separate cerebellum areas. Cerebellum reacts more quickly to auditory stimulus than visual stimulus. Cerebellum reacts faster to higher intensity and multiple sensory stimuli.
Cerebellum affects sense accuracy, sense quickness, sense timing, short-term memory, attention, emotions, and planning. Lateral cerebellum affects perception, pattern recognition, and cognition.
functions: timing
Perhaps, cerebellum is for timing. Perhaps, parallel fibers are delay lines, and climbing fibers are clock read-out mechanisms. When parallel fiber and climbing fiber activation coincide, Purkinje cells fire to activate antagonist muscles and stop movements at intended targets.
One climbing fiber synapses on one Purkinje cell. Perhaps, cerebellar clock activates proper Purkinje-cell assemblies at right time.
learning
Cerebellum learns movement timing and guides learning in deep nuclei. It stores learned-skill model or memory within six hours, so skill becomes automatic.
learning: long-term depression
Increased dendritic calcium concentration induces cerebellar long-term depression (LTD). Cerebellar LTD reduces excitatory input to Purkinje cell. LTD can decrease synaptic weights, using climbing fiber input as training signals, Purkinje cell firing as postsynaptic factor, and/or parallel fiber synaptic activity as presynaptic factor. LTD at basket-cell and stellate-cell spiny synapses maintain excitatory input to Purkinje cells [Eccles et al., 1976].
cells: basket cell
Basket cells lie in Purkinje cell dendrites.
cells: flocculus neuron
Flocculus neurons send corrective signals for movements.
cells: Golgi cell
Golgi cells lie in middle layer between Purkinje cells.
cells: granule cell and parallel fiber
Granule cells are small neurons, have high density, and are the most common. Granule-cell-axon parallel fibers pass through middle layer, contacting one Purkinje cell many times, to outer layer, where they split, form straight line, and extend horizontally through outer layer. Parallel fiber is perpendicular to hundreds of Purkinje cell dendrite trees and contacts each once.
cells: Purkinje cell
Purkinje cells are large neurons that have tree-shaped flat dendrite planes, which converge onto one trunk into Purkinje cell. Purkinje cell membrane has 150,000 synapses, ten times more than other neuron types, mostly from granule cells. Purkinje cell axons inhibit Golgi cells and granule cells in cerebellar nuclei, which send axons to brain pyramidal and extrapyramidal tracts.
cells: stellate cell
Stellate cells lie beside Purkinje cell dendrites and have fibers that run horizontally through outer layer, mainly through one Purkinje cell dendrite tree.
biology
Cerebellum anatomy is the same in all vertebrates. Cerebellum has surface area equal to one cerebral hemisphere. It has more than half of all brain neurons. It has more folding than cerebrum.
Low-threshold cerebellar receptive fields and neurons align with nociceptive punishment signal fields and neurons.
biology: damage
Cerebellum damage decreases muscle tone, causes slowing and trembling, and fails to stop movements on time. One-side damage causes flexion on one side and extension on other. Within 45 minutes, cutting spinal cord stops flexions and extensions. Cutting after 45 minutes does not stop flexions and extensions. Cerebellum damage in early life does not affect behavior.
biology: evolution
During human evolution, cerebellum expanded at same rate as cerebrum.
biology: input
Sensory cerebellum receives tactile, visual, and auditory nerves. Cerebellum lobes have body-surface tactile representations.
biology: output
Motor cerebellum has reverberatory circuit to higher motor centers, regulates voluntary movements, organizes somatotopically, and has archicerebellum and neocerebellum. Motor cerebellum maintains muscle tonus, posture, and equilibrium.
biology: waves
Cerebellum has electrical waves lasting 150 to 200 milliseconds, at 0.02 mV to 0.12 mV.
biology: layers
Inner deep granule-cell layer has closely packed granule cells, as well as scattered Golgi cells that inhibit nearby granule cells. Middle Purkinje-cell layer has one Purkinje-cell row, surrounded by smaller basket cells. Wide outer-molecular layer has Purkinje cell dendrites that spread in plane and stellate cells that contact dendrites.
biology: pathways
Cerebellum receives excitatory input from vestibular-system mossy fibers or pons climbing fibers. Mossy fibers from vestibular system synapse with Golgi cells and granule cells. Mossy fibers from Golgi cells and granule cells send to Purkinje cells and process intersensory information. Pons climbing fibers synapse with granule, Golgi, Purkinje, basket, and stellate cells. Purkinje cells inhibit Golgi cells and granule cells.
biology: peduncles
Cerebellum attaches to posterior brainstem by three pairs of stalks {cerebellar peduncle}, which contain both afferent and efferent nerve fibers. Superior, middle, and inferior tracts join cerebellum to midbrain. Tract {superior peduncle} comes from neocortex. Tract {middle peduncle} {brachium pontis} comes from pons. Tract {inferior peduncle} comes from inner-ear vestibular apparatus.
biology: hemispheres
Cerebellum has two lateral parts {cerebellar hemisphere}, which have many small folds {folia}, connected by thin central worm-shaped part {vermis}. Vermis has inferior part that controls gross motor coordination and superior part that controls fine motor coordination. Gray-matter outer cover {cerebellar cortex} is over white matter {medullary body}.
biology: nuclei
Four deep nuclei are in cerebellar white matter. All vertebrates have the oldest cerebellum part {archicerebellum} {vestibulocerebellum}, in center {flocculus} {nodule, cerebellum}, which has afferent and efferent connections in inferior peduncle, mainly with inner-ear vestibular semicircular canals {maculae}. Cerebellum has small inferior portion {flocculonodular lobe}, for balance, position, head position changes, acceleration, deceleration, and angular movements. Fibers from retina, eye movement nuclei, and cortex terminate in vestibulocerebellum. The second oldest part {paleocerebellum} {spinocerebellum} corresponds to anterior lobe and posterior vermis and receives touch, pressure, thermal, and proprioceptive input from inferior-peduncle ascending spinal-cord and brainstem pathways. Skin, muscle, and tendon receptors send performance information about rate, force, and movement direction, especially propulsive movements such as walking and swimming.
Perhaps, cerebellum has static associative memories {Cerebellar Model Articulation Controller} (CMAC) that implement locally generalizing non-linear maps between mossy-fiber input and Purkinje-cell output. Granule and Golgi cell-network association layer generates sparse expanded mossy-fiber-input representations. Adjustable weights couple large parallel fiber vector to Purkinje-cell output units with graded properties.
pattern
Adjustable pattern generator (APG) model can generate elemental burst command with adjustable intensity and duration. It models positive feedback between cerebellar nucleus cell and motor cortical cell.
Excitatory input {climbing fiber} from inferior olive goes to one Purkinje cell, making 300 synapses, to fire Purkinje cell. Perhaps, climbing fiber makes error-and-training signals to adjust parallel-fiber synaptic weights, teaching Purkinje cells to recognize patterns signaled by input vectors and to select movements that reduce errors. Perhaps, cerebral cortex activates climbing fiber input, to train cerebellum to recognize appropriate contexts for generating same movements more automatically.
Inferior-olive small neuron clusters, with similar receptive fields, stimulate parasagittally-oriented cerebellar Purkinje-cell strips, which send to cerebellar nuclear-cell common cluster {microzone}.
Spinal-cord and brainstem excitatory axons {mossy fiber} synapse on more than 40 granule-cell glomeruli and deep-cerebellar nuclei. Mossy fiber also directly contacts 250 Purkinje cells but cannot fire them. Mossy fiber influences 200,000 Purkinje cells. Mossy fibers have sensory properties, but Purkinje and nuclear cells do not respond to somatosensory stimulation.
The newest and largest cerebellum part {neocerebellum} {pontocerebellum} is anterior and posterior cerebellar lobes, for skilled or complex movements and intentions. Neocerebellum receives from pons and sends through superior peduncle.
Excitatory axons {parallel fiber} from cerebellum send to Purkinje cell dendrites. Perhaps, parallel fibers provide input vectors. Parallel fibers form sequential-activity lines, with one synapse per Purkinje cell. Parallel fibers can induce long-term depression.
Cerebellum has seven million large neurons {Purkinje cell}, which receive 200,000 synapses on planar dendritic spines and send inhibitory GABA output to cerebellar deep nuclei.
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Date Modified: 2022.0225