Chap 56 Phys
Cerebellum and Its Motor Functions
 Cerebellum helps sequence motor activities and monitors and makes corrective adjustments in body’s motor
activities while they are being executed so they will conform to motor signals directed by cerebral motor cortex
and other parts of brain
o Receives info about desired sequence of muscle contraction from brain motor control areas and
continuous sensory info from peripheral parts of body
o Compares actual movements as depicted by peripheral sensory feedback info with movements intended
by motor system; if 2 don’t match, instantaneous subconscious corrective signals transmitted back into
motor system to increase or decrease levels of activation of specific muscles
o Aids cerebral cortex in planning next sequential movement fraction of a second in advance while current
movement being executed, progressing smoothly from one movement to next
o If movement doesn’t occur as intended, cerebellar circuit learns to make stronger or weaker movement
next time; changes occur in excitability of appropriate cerebellar neurons
 Cerebellum divided into 3 lobes by 2 deep fissures: anterior lobe, posterior
lob, and flocculonodular lobe
o Flocculonodular lobe developed along with and functions with
vestibular system in controlling body equilibrium
o Vermis controls most cerebellar functions for muscle movements
of axial body, neck, shoulders, and hips
o Intermediate zone of hemisphere concerned with controlling
muscle contractions in distal portions of upper and lower limbs
o Lateral zone of hemisphere joins with cerebral cortex in overall
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planning of sequential motor movements (coordination)
Vermis and intermediate zones of cerebellum have topographical
representation of different parts of body; topographical representations
receive afferent nerve signals from all respective parts of body, as well as
from corresponding topographical motor areas in cerebral cortex and
brainstem
o Send motor signals back to same respective topographical areas of
cerebral cortex, red nucleus, and reticular formation
Lateral areas of hemispheres receive input signals almost exclusively from
cerebral cortex, especially premotor areas of frontal cortex and
somatosensory and other sensory association areas of parietal cortex
o Connectivity with cerebral cortex helps lateral portions play role in planning and coordinating body’s
rapid sequential muscular activities
Each fold of cerebellum called a folium; lying deep beneath folia are deep cerebellar nuclei
Corticopontocerebellar pathway originates in cerebral motor and premotor cortices and cerebral somatosensory
cortex, passes by way of pontile nuclei and pontocerebellar tracts in
lateral divisions of cerebellar hemispheres on opposite side of brain from
cerebral areas
Olivocerebellar tract passes from inferior olive to all parts of cerebellum
and is excited in olive by fibers from cerebral motor cortex, basal ganglia,
widespread areas of reticular formation, and spinal cord
Vestibulocerebellar fibers – some originate in vestibular apparatus, some
originate in vestibular nuclei of brainstem; almost all terminate in
flocculonodular lobe and fastigial nucleus of cerebellum
Reticulocerebellar fibers originate in different portions of brainstem
reticular formation and terminate in midline cerebellar areas (mainly
vermis)
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Dorsal spinocerebellar tract enters cerebellum through inferior cerebellar peduncle and terminates in vermis
and intermediate zones of cerebellum on same side as origin; transmits signals mainly from muscle spindles and
to lesser extent from other somatic receptors throughout body (i.e., Golgi tendon organs, joint receptors)
o Signals apprise cerebellum of momentary status of muscle contraction, degree of tension on muscle
tendons, positions and rates of movement of parts of body, and forces acting on surface of body
Ventral spinocerebellar tract enters cerebellum through superior cerebellar peduncle, but terminates on both
sides of cerebellum; excited mainly by motor signals arriving in anterior horns of spinal cord from brain (through
corticospinal and rubrospinal tracts) and internal motor pattern generators in cord itself
o Tells cerebellum which motor signals have arrived at anterior horns (efference copy)
Spinocerebellar pathways have most rapid conduction in any pathway of CNS
Signals transmitted to cerebellum from periphery through spinal dorsal columns to dorsal column nuclei of
medulla and relayed to cerebellum
Signals transmitted up spinal cord through spinoreticular pathway to reticular formation of brainstem and
through spino-olivary pathway to inferior olivary nucleus; these signals relayed to cerebellum, collecting info
about subconscious movements and positions of body
All deep cerebellar nuclei receive signals from cerebellar cortex and deep sensory afferent tracts to cerebellum
Each time input signal arrives in cerebellum, it divides and goes directly to one of cerebellar deep nuclei and to
corresponding area of cerebellar cortex overlying deep nucleus
o Cerebellar cortex then relays inhibitory output signal to deep nucleus
o All input signals that enter cerebellum eventually end in deep nuclei in
form of initial excitatory signals followed by inhibitory signals
o From deep nuclei, output signals leave cerebellum and are distributed
to other parts of brain
Major efferent pathways leading out of cerebellum consist of
o Pathway that originates in vermis and passes through fastigial nuclei
into medullary and pontile regions of brainstem; functions in close
association with equilibrium apparatus and brainstem vestibular nuclei
to control equilibrium; functions in association with reticular formation of brainstem to control posture
o Pathway that originates in intermediate zone of cerebellar hemisphere; passes through interposed
nucleus to ventrolateral and ventroanterior nuclei of thalamus; then to cerebral cortex, several midline
structures of thalamus, and basal ganglia; red nucleus and reticular formation of upper portion of
brainstem; helps coordinate mainly reciprocal contractions of agonist and antagonist muscles in
peripheral portions of limbs, especially hands and fingers
o Pathway that begins in cerebellar cortex of lateral zone of cerebellar hemisphere and passes to dentate
nucleus, then ventrolateral and ventroanterior nuclei of thalamus, then to cerebral cortex; helps
coordinate sequential motor activities initiated by cerebral cortex
Cerebellum has many functional units; each centers on single, very large Purkinje cell and corresponding deep
nuclear cell
o Major layers of cerebellar cortex are molecular layer (outside), Purkinje cell layer, and granule cell layer;
beneath cortical layers are deep cerebellar nuclei
o Output from functional unit from deep nuclear cell continually under both excitatory and inhibitory
influences: excitatory influences arise from direct connections with afferent fibers that enter cerebellum
from brain or periphery; inhibitory influence arises entirely from Purkinje cell in cortex of cerebellum
o Afferent inputs to cerebellum mainly of climbing fiber type or mossy fiber type
 Climbing fibers all originate from inferior olives of medulla; 1 fiber for every 5-10 Purkinje cells
 Single impulse in climbing fiber always causes single, prolonged AP in each Purkinje cell
with which it connects, beginning with strong spike and followed by tail of weakening
secondary spikes (complex spike AP)
 Mossy fibers are all other fibers that enter cerebellum from multiple sources; send collaterals to
excite deep nuclear cells; proceed to granule cell layer of cortex, where they synapse with
granule cells
 Granule cells send small axons up to molecular layer of cerebellar cortex
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In molecular layer, axons divide into 2 branches that extend parallel to folia (parallel
nerve fibers; many per Purkinje cell)
 Dendrites of Purkinje cells project into molecular layer
 Synaptic connections with Purkinje cells weak, so large numbers of fibers must be
stimulated simultaneously to excite Purkinje cell; activation usually short-duration
 Purkinje cell AP is simple spike
Purkinje cell fires continuously (slower than deep nuclear cells); output activity of both Purkinje cells and deep
nuclear cells can be modulated upward or downward
Direct stimulation of deep nuclear cells by both climbing and mossy fibers excites them; signals arriving from
Purkinje cells inhibit them; normally balance slightly favors excitation
In execution of rapid motor movement, initiating signal from cerebral motor cortex or brain stem at first greatly
increases deep nuclear cell excitation; then feedback inhibitory signals from Purkinje cell arrive
o Inhibitory signal resembles delay-line negative feedback signal effective in providing damping; stops
muscle movement from overshooting its mark
Basket cells and stellate cells in cerebellum are inhibitory cells with short axons; located in molecular layer, lying
among and stimulated by small parallel fibers
o Send axons at right angles across parallel fibers and cause lateral inhibition of adjacent Purkinje cells
Cerebellum provides rapid turn-on signals for agonist muscles and simultaneous reciprocal turn-off signals for
antagonist muscles at onset of movement
o On approaching termination of movement, cerebellum mainly responsible for timing and executing
turn-off signals to agonists and turn-on signals to antagonists
o Signals from cerebral cortex for movement pass through non-cerebellar brainstem and cord pathways
directly to agonist muscle to begin initial contraction; at same time, parallel signals sent via pontile
mossy fibers into cerebellum
 One branch of each mossy fiber goes directly to deep nuclear cells in deep cerebellar nuclei
 Instantly sends excitatory signal back into cerebral corticospinal motor system either by return
signals through thalamus to cerebral cortex or by neuronal circuitry in brainstem to support
muscle contraction signal already begun in cerebral cortex
 Turn-on signal becomes even more powerful than at start because it becomes sum of both
cortical and cerebellar signals (makes turn-on muscle contraction stronger
o All mossy fibers have second branch that transmits signals via granule cells to cerebellar cortex and
eventually by parallel fibers to Purkinje cells
 Purkinje cells inhibit deep nuclear cells; axons small and signals weak, so it requires time to build
up enough excitation in dendrites to excite Purkinje cell
 Once Purkinje cell excited, it sends strong inhibitory signal to same deep nuclear cell that
originally turned on movement; helps turn off movement after short time
Degree to which cerebellum supports onset and offset of muscle contractions, as well as timing of contractions,
must be learned by cerebellum
o When person performs new act, degree of motor enhancement by cerebellum at onset of contraction,
degree or inhibition at end of contraction, and timing almost always incorrect for precise performance
o After act has been performed many times, individual events become progressively more precise
o Sensitivity levels of cerebellar circuits themselves progressively adapt during training process, especially
sensitivity of Purkinje cells to respond to granule cell excitation; sensitivity change brought about by
signals from climbing fibers entering cerebellum from inferior olivary complex
o When person performs new act, feedback signals from muscle and joint proprioceptors usually denote
to cerebellum how much actual movement fails to match intended movement; climbing fiber signals
alter long-term sensitivity of Purkinje cells
o Once person experienced enough, climbing fibers no longer need to send error signals to cerebellum to
cause further change
Vestibulocerebellum – consists principally of flocculonodular lobes and adjacent portions of vermis; provides
neural circuits for most of body’s equilibrium movements
o Loss of vestibulocerebellum – equilibrium far more disturbed during performance of rapid motions,
especially when motions involve change in direction of movement and stimulate semicircular ducts
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Important in controlling balance between agonist and antagonist muscle contractions of spine, hips, and
shoulder during rapid changes in body positions as required by vestibular apparatus
o Signals from periphery tell brain how rapidly and in which directions body parts moving, and
vestibulocerebellum calculates in advance where different parts will be, resulting in advance planning
Spinocerebellum – consists of most of vermis of posterior and anterior cerebellum plus adjacent intermediate
zones; provides circuitry for coordinating smooth movements of distal portions of limbs
o Receives info from cerebral motor cortex and from midbrain red nucleus, telling cerebellum intended
sequential plan of movement
o Receives feedback info from peripheral parts of body, telling cerebellum what actual movements result
o Sends corrective output signals back to cerebral motor cortex (through relay nuclei in thalamus) and
magnocellular portion (lower portion of red nucleus that gives rise to rubrospinal tract)
o Rubrospinal tract joins corticospinal tract in innervating lateral motor neurons (neurons that control
distal parts of limbs)
o Ventral spinocerebellar tract transmits back to cerebellum efference copy of actual motor control
signals that reach anterior motor neurons; this info integrated with signals arriving from muscle spindles
o Similar comparison signals go to inferior olivary complex; if signals don’t compare well, olivary-Purkinjhe
cell system eventually corrects motions until they perform as desired
o In person with cerebellum destroyed, cerebrum can learn to prevent overshoot, but often overshoots in
opposite direction (oscillation; action tremor; intention tremor)
o Most rapid movements of body occur so rapidly, not possible to receive feedback info either from
periphery to cerebellum or from cerebellum back to motor cortex before movements over (ballistic
movements; entire movement preplanned and set into motion to go specific distance and stop)
 Also saccades of eyes
o When cerebellum removed, ballistic movements slow to develop and don’t have extra onset surge,
force developed is weak, and movements slow to turn off, usually allowing movement to overshoot
Cerebrocerebellum – consists of large lateral zones; receives virtually all input from cerebral motor cortex and
adjacent premotor and somatosensory cortices of cerebrum; transmits output info in upward direction back to
brain, functioning in feedback manner with cerebral cortical sensorimotor system to plan sequential voluntary
body and limb movements in advance of actual movement; called development of motor imagery
o Almost all communication between this area and cerebral cortex through premotor area and primary
and association somatosensory areas (not primary cerebral motor cortex)
o Destruction of this area with their deep nuclei (dentate nuclei) can lead to extreme incoordination of
complex purposeful movements of hands, fingers, feet, and speech
o Planning of sequential movements requires lateral zones of hemispheres to communicate with both
premotor and sensory portions of cerebral cortex; requires 2-way communication between cerebral
cortex areas with corresponding areas of basal ganglia
o Plan begins in sensory and premotor areas of cerebral cortex; from there plan transmitted to lateral
zones of cerebellar hemispheres
o Through 2-way traffic between cerebellum and cerebral cortex, appropriate motor signals provide
transition from one sequence of movement to next
o Provides appropriate timing for each succeeding movement; in absence of cerebrocerebellum, person
loses subconscious ability to predict how far different parts of body will move in given time; without this
capability, person unable to determine when next sequential movement needs to begin
o Helps time events other than movements of body (rates of progression of both auditory and visual
phenomena can be predicted by brain, requiring participation of cerebrocerebellum)
 Visual cues can tell you when you are approaching a wall, for example
o If deep cerebellar nuclei not damaged by lesion affecting up to half lateral cerebellar cortex, motor
functions appear almost normal if person moves slowly; remaining portions of motor contro system
capable of compensating for loss of parts of cerebellum
o To cause serious and continuing dysfunction of cerebellum, cerebellar lesion must involve one or more
deep cerebellar nuclei
Dysmetria – subconscious motor control system can’t predict how far movements will go, so overshoot, then
conscious portion of brain overcompensates in other direction, resulting in oscillation and ataxia
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Can also result from lesions in spinocerebellar tracts because feedback info from moving parts of body
to cerebellum essential for cerebellar timing of movement termination
 Past pointing – moving past point of intention of movement because of lack of cerebellar influence
 Dysdiadochokinesia – inability to perform rapid alternating movements; person loses perception of parts during
rapid motor movements and thus begins next movement at inappropriate time, so no orderly progression of
movement can occur; jumbled movements occurring instead of normal coordinate movement
 Dysarthria – failure of progression in talking because formation of words depends on rapid and orderly
succession of individual muscle movements in larynx, mouth, and respiratory system; results in lack of
coordination of intensity of sound or duration of each successive sound
 Cerebellar nystagmus – tremor of eyeballs that occurs usually when one attempts to fixate eyes on scene to one
side of head; off-center fixation results in rapid tremulous movements of eyes ratherthan steady fixation
o Occurs especially when flocculonodular lobes damaged (damage of pathways through flocculomotor
cerebellum from semicircular ducts)
 Loss of deep cerebellar nuclei (particularly dentate and interposed nuclei) causes decreased tone of peripheral
body musculature on side of cerebellar lesion
o Hypotonia results from loss of cerebellar facilitation of motor cortex and brain stem motor nuclei by
tonic signals from deep cerebellar nuclei
Basal Ganglia – Their Motor Functions
 Functions in close association with cerebral cortex and corticospinal motor control system
 Receive most of input signals from cerebral cortex itself and
return almost all output signals back to cortex
 Basal ganglia are caudate nucleus, putamen, globus pallidus,
substantia nigra, and subthalamic nucleus
 Almost all motor and sensory nerve fibers connecting
cerebral cortex and spinal cord pass between major masses
of basal ganglia (caudate nucleus and putamen)
o Space is internal capsule of brain
 Basal ganglia function in association with corticospinal
system to control complex patterns of motor activity
o When there is serious damage to basal ganglia,
cortical system of motor control no longer provide
Putamen Circuit
patterns and motions become crude (as if trying for first time)
 Putamen circuit has inputs mainly from parts of brain adjacent to primary
motor cortex, but not much from primary motor cortex
o Outputs go mainly back to primary motor cortex or closely
associated premotor and supplementary cortex
o Ancillary circuits pass from putamen through external globus
pallidus, subthalamus, and substnatia nigra, returning to motor
cortex via thalamus
o Lesions in globus pallidus lead to spontaneous writhing
movements of hand, arm, neck, or face (athetosis)
o Lesion in subthalamus often leads to sudden flailing movements of
entire limb (hemiballismus)
o Multiple small lesions in putamen lead to flicking movements of
hand, face, and other parts (chorea)
o Lesions of substantia nigra lead to rigidity, akinesia, and tremors (Parkinson’s disease)
 Cognition – thinking processes of brain, using both sensory input to brain plus info already stored in memory
o Most motor actions occur as consequence of thoughts generated in mind (cognitive control)
o Caudate nucleus plays major role in cognitive control of motor activity
o Caudate nucleus extends into all lobes of cerebrum and receives large amounts of input from
association areas of cerebral cortex overlying caudate nucleus, mainly areas that integrate different
types of sensory and motor info into usable thought patterns
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After signals pass from cerebral cortex to caudate nucleus, they are transmitted to internal globus
pallidus, then to relay nuclei of ventroanterior and ventrolateral thalamus, and back to prefrontal,
premotor, and supplementary motor areas of cerebral cortex
o Almost no returning signals pass directly to primary motor cortex; go to accessory motor regions in
areas concerned with putting together sequential patterns of movement instead of those exciting
individual muscle movements
Caudate Circuit
o Without cognitive functions, person might not have instinctive
knowledge to react to danger without thinking for too long
o Cognitive control determines subconsciously which patterns of
movement will be used together to achieve complex goal
In patients with severe lesions of basal ganglia, timing and scaling (big
sweeping motion or small subtle motion) functions poor; function in close
association with cerebral cortex
o Posterior parietal cortex – locus of spatial coordinate for motor
control of all parts of body as well as for relation of body and its
parts to all surroundings
 Lesions produce inability to accurately perceive objects
through normally functioning sensory mechanisms (agnosia;
patient forgets to draw half of picture to copy, etc.)
 Patient with lesions will avoid using one side of body and possibly forget to clean that side
o Timing and scaling of movements part of caudate cognitive motor control circuit
Multiple glutamate pathways provide most of excitatory signals that balance out large numbers of inhibitory
signals transmitted by dopamine, GABA, and serotonin
o GABA always functions as inhibitory agent, so GABA
neurons in feedback loops from cortex through basal
ganglia and back make virtually all loops negative feedback
loops, lending stability to motor control systems
o Dopamine functions as inhibitory neurotransmitter in most
parts of brain, so functions as stabilizer
Parkinson’s disease (paralysis agitans) results from destruction of
portion of substantia nigra that sends dopamine-secreting nerve
fibers to caudate nucleus and putamen (pars compacta)
o Characterized by rigidity of much of musculature,
involuntary tremor of involved areas, serious difficulty in
initiating movement (akinesia), postural instability caused
by impaired postural reflexes, dysphagia, speech disorders,
gait disturbances, and fatigue
o Destruction of dopaminergic neurons allows caudate nucleus and putamen to become overly active and
cause continuous output of excitatory signals to corticospinal motor control system (rigidity)
o Some feedback circuits easily oscillate because of high feedback gains after loss of inhibition (tremor)
 Involuntary tremor that happens during all waking hours, not only when person tries to move
(intention tremor of cerebellar lesion)
o Dopamine secretion in limbic system, especially nucleus accumbens, often decreased along with
decrease in basal ganglia, reducing psychic drive for motor activity so greatly akinesia results
o L-dopa administration can ameliorate symptoms; converted in brain to dopamine, which restores
normal balance between inhibition and excitation in caudate nucleus and putamen; can’t use actual
dopamine because that can’t cross BBB
o L-deprenyl inhibits monoamine oxidase, which is responsible for destruction of most dopamine after
secretion; any dopamine released remains in basal ganglial tissues for longer time; helps slow
destruction of dopamine-secreting neurons in substantia nigra
o Transplantation of dopamine-secreting cells from brains of aborted fetuses into caudate nuclei and
putamen has been used with some short-term success to treat Parkinson’s; cells don’t live more than a
few months
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Variable degrees of success achieved with surgical lesion in ventrolateral and ventroanterior nuclei of
thalamus to block abnormal signals from basal ganglia to motor cortex
 Huntington’s disease – hereditary disorder that usually begins causing symptoms at age 30-40; characterized by
flicking movements in individual muscles and progressive severe distortional movements of entire body
o Severe dementia develops with motor dysfunctions
o Caused by loss of most of cell bodies of GABA-secreting neurons in caudate nucleus and putamen; loss
of ACh-secreting neurons in many parts of brain
o Axon terminals of GABA neurons normally inhibit portions of globus pallidus and substantia nigra; loss
of inhibition allows spontaneous outbursts of globus pallidus and substantia nigra activity that cause
distortional movements
o Dementia results from loss of ACh-secreting neurons, especially in thinking areas of cerebral cortex
o Caused by many repeating glutamines in Huntington protein
Integration of Many Parts of Total Motor Control System
 Spinal cord has programmed local patterns of movement for all muscle areas of body (withdrawal reflex)
o Has complex patterns of rhythmical motions (walking)
o All programs of cord can be commanded into action by higher levels of motor control or can be inhibited
while higher levels take over control
 Hindbrain provides maintenance of axial tone of body for standing and continuous modification of degrees of
tone in different muscles in response to info from vestibular apparatuses for maintaining equilibrium
 Motor cortex provides most of activating motor signals to spinal cord; functions by issuing sequential and
parallel commands that set into motion various cord patterns of motor action
o Can change intensities of different patterns or modify timing or other characteristics
o When needed, corticospinal system can bypass cord patterns, replacing them with higher-level patterns
from brainstem or cerebral cortex
o These patterns learned (versus cord patterns that are hard wired/instinctual)
 Cerebellum functions with spinal cord to enhance stretch reflex
o Functions with brainstem to make postural movements of body, especially rapid movements required
by equilibrium system, smooth and continuous without abnormal oscillations
o Helps program in advance muscle contractions required for smooth progression
o Functions mainly when muscle movements rapid
 Basal ganglia help cortex execute subconscious but learned patterns of movement and help plan multiple
parallel and sequential patterns of movement that mind puts together to accomplish purposeful task
o Required to modify patterns for difference in size of movement
o Provides overall sequential steps of action for responding to each new situation
 Hypothalamus, amygdala, hippocampus, septal region anterior to hypothalamus and thalamus, and old regions
of thalamus and cerebral cortex initiate most motor and other functional activities of brain (limbic system)