Motor Systems Vocabulary

Sensory Motor Cortex

Motor Cortex Topography

• Charles Sherrington confirmed that stimulating adjacent regions of the precentral gyrus in apes elicits movements of adjacent contralateral body parts.
• Wilder Penfield, Sherrington's student, verified this in humans by systematically stimulating different brain regions in patients that were awake.
• Motor maps, revealed through direct electrical stimulation, are also evident in BOLD response during movements.
• These maps show distorted body representations, with disproportionately large areas for lips, tongue, and hands, and smaller areas for lower extremities and genitalia. This highlights that cortical space allocation reflects the capability for fine motor control.

Motor Maps and Movement Generation

• Debate exists whether motor map activity specifies what to do (motor command) or how to do it (muscle activity patterns).
• Early studies suggested a map of individual muscles in the primary motor cortex.
• Later studies, including those by Penfield and Sherrington, showed that stimulating portions of the primary motor cortex evokes coordinated, multijoint movements, aligning with the primary motor cortex targeting local circuits in the brainstem and spinal cord.

Neural Representation in Motor Cortex

• Electrophysiological recordings support the conclusion that movements, not muscles, are represented in motor maps.
• Edward Evarts pioneered implanting electrodes in monkeys' motor cortex to study reaching movements.
• Recordings showed neurons fired action potentials related to specific movements, with the firing rate corresponding to force changes during movement.
• Many neurons discharged before movement initiation, suggesting their activation does not directly cause movement.
• Commands for eye movements are similarly generated in frontal eye fields and the superior colliculus, projecting to the brainstem reticular formation.
• Higher motor centers, such as the primary motor cortex and frontal eye fields, provide motor command signals and signals indicating movement forcefulness, specifying both what to do and how to do it.

Neuronal Populations and Movement Coding

• Motor maps are gross anatomical descriptions; how maps relate to coordinated movements isn't entirely clear.
• Neurons in the primary motor cortex are coarsely tuned, making it difficult to predict movement direction/amplitude from single neuron activity.
• The averaging activity of numerous neurons computes the desired movement.
• Eye movement maps provide a model for understanding complex movements due to oculomotor system simplicity.
• Electrical stimulation in the superior colliculus produces coordinated gaze shifts; single neurons fire before saccades.
• The superior colliculus contains a topographical map of eye movements; neurons vote for range of movements, weighted by firing strength.
• The weighted votes are averaged to determine the desired movement vector.
• Inactivation experiments in monkeys showed eye movements biased away from the inactivated portion of the collicular map, supporting the idea that movements are specified by averaging activity of neuron populations.
• Precise movements are encoded by averaging activity of many coarsely tuned neurons in primary motor cortex.
• A vector representing weighted average activity across the neuronal population specifies the impending movement well in advance of muscle activity, encoding intended movements.

Planning Movements

• Some actions are planned in advance; neurons in the primary motor cortex become active before movement initiation is cued.
• Anticipatory activation during arm movement planning occurs in premotor areas, including premotor, supplementary motor, dorsolateral prefrontal, and parietal cortices.
• Planning-related activity persists when cues are removed and ceases when monkeys are cued to stop planning.
• Premotor areas are hierarchically organized; they provide abstract planning information related to behavioral goals, translated into intention to perform specific movements in the primary motor cortex.
• Higher motor areas specify the motor program, translated by local circuits into muscle contraction patterns.
• EEG recordings from humans show a readiness potential—a negative wave—that begins before voluntary movement, originating in premotor areas before enhancing over the contralateral primary motor cortex.
• Neuroimaging identified the readiness potential with activation in premotor areas, particularly the supplementary motor area.
• Damage to premotor areas along with the primary motor cortex leads to anosognosia (loss of awareness of motor deficits), suggesting the premotor cortex is the source of motor planning and intentional awareness.

BENJAMIN LIBET'S EXPERIMENT

• Subjects were asked to produce an uncued voluntary movement, and then estimate the time at which they became aware of the intention to move.
• EEG recordings suggested estimates of the intention to move by about 200 milliseconds.
• Readiness potentials over premotor areas distinctly proceded awarenes of the intention to move. This may imply that conscious awareness follows the intention to move (or urge), and may influence this outcome, rather than preceded and thus causing it.

Selecting Goals for Action

• Multiple behavioral goals may be options for motor planning; neural processes linking sensory information to motor output are crucial.
• Neurons in premotor areas respond to sensory cues guiding movements; responses can be enhanced or diminished if the stimulus is more or less likely to be the movement target.
• Sensory-motor linkage is graded by the quality of information guiding movement integrated over time.
• Neuronal responses in premotor areas are systematically related to the weight of sensory evidence favoring a particular movement.
• Neurons in several premotor areas specify movements in a graded manner as sensory evidence accumulates, with microstimulation biasing movement endpoints based on the pattern of motion viewed.
• Motor preparation is a dynamic, competitive process linking sensory information to the intention to move, entailing graded activation of neurons in higher-order premotor cortical areas.

Motivational Control of Goal Selection

• Goal selection is often based on complex stimuli and stored information, requiring an understanding of motivational control.
• Neurons in the posterior parietal cortex are sensitive to the reward value of shifting gaze to a particular target, dependent on the probability and magnitude of the juice reward associated with that target.
• Neurons in premotor areas are sensitive to movement value, with fMRI studies showing similar scaling of activation by movement value.
• Selecting a movement goal involves scaling neuronal responses associated with each possible movement by that movement’s value, biasing the motor system towards movements satisfying biological motivations.

Sequential Movements and the Supplementary Motor Area

• Human behavior consists of sequences of movements, with regions of the frontal cortex specialized to support their production.
• The supplementary motor area (SMA) is crucial for generating movements without explicit sensory cues; the premotor cortex is important for cued movements.
• The SMA is critical in the production of movement sequences.
• SMA neurons are selectively activated when a particular action embedded within a sequence is performed, responding only for a particular action sequence, irrespective of the type of movement.
• Deactivation of the SMA impairs performing sequences from memory.
• Neuroimaging in humans demonstrates preferential activation of the supplementary motor cortex during self-initiated movements. Activation of the SMA neurons in monkeys by internally generated sequences of action is supported by neuroimaging studies in humans.
• The supplementary motor area provides relatively abstract motor intention signals controlling internally guided action sequences; prefrontal cortex plays a more important role in the termination/initiation of movement sequences.
• The primary motor cortex issues sequences of commands activating motor units in the brainstem and spinal cord.

Sensory-Motor Coordination

• Translate the spatial and textural information to appropriate motor commands.
• Reaching toward a visible target requires retinal location translation into a location anchored to the position of the hand. Coordinate transformations are needed.
• The parietal cortex is crucial for sensory-motor coordination; damage can disrupt reaching and saccades (optic ataxia), reflecting integration failure.
• Parietal lesions cause difficulty grasping objects accurately across their centers of mass.
• The dorsal visual stream helps to guide movement, whereas the ventral visual stream is specialized for object identification, following the lateral temporal damage model showing that damage disrupts object identification/naming.
• Lesion data from human patients support the idea that the parietal cortex integrates visual, eye position, and limb position data necessary to produce coordinated movements of the eyes and hands.

Initiation of Movement by the Basal Ganglia

• The basal ganglia serve as a gating mechanism, inhibiting movements until appropriate and coordinating movement timing, and may play an important role in one's subjective sense of time.
• The basal ganglia are associated with: substantia nigra, caudate, and putamen (known collectively as the striatum), globus pallidus, subthalamic nucleus and the substantia nigra pars compacta—make important contributions.
• Nearly all cortical areas project to the basal ganglia, principally through the caudate and putamen; the globus pallidus modulates cortical activity via a thalamic relay.
• Activation of the caudate and putamen inhibits the globus pallidus, releasing the thalamus and its cortical targets from tonic inhibition.
• The net effect of basal ganglia activation results in excitation of cortical neurons, releasing and coordinating desired movements.
• The substantia nigra pars reticulata circuits that go to the superior colliculus serve a “braking” function to saccades.
• The basal ganglia inhibit undesired movements and permit desired ones by “releasing the breaks".

PARKINSON'S DISEASE

• Results in the death of neurons in the Substantia Nigra Pars Compacta.
• Results in an increases excitatory tone of the direct pathway through the basal ganglia.
• A Major symptom results in a marked disruption (inability) of initiating voluntary movement.

HUNTINGTONS DISEASE

• Involves hereditary atrophy of the caduate nucleus.
• Exhibit the signs and symptoms opposite to the that in Parkinson's.
• Exhibits choreiform (dancelike) movements of the trunk and extremities the patients are unable to control. They also suffer a gradual onset of psychotic thought patterns and eventually dementia.
• Damage to the pathway causes movements to release from inhibition thus resulting in unwanted actions.

Basal Ganglia and Cognition

• Motor deficits associated with basal ganglia dysfunction indicate a crucial role in gating movement. In addition, there are explicit cognitive signals at play.
• Separate nonmotor pathways through the basal ganglia exist, including a limbic/emotional channel and an associative/cognitive channel.
• The same principles governing motor disinhibition apply to emotional/cognitive processing in the limbic/associative channels.
• Animals with basal ganglia lesions struggle to acquire rewards or avoid punishments through movement. Parkinson’s patients have impairments in probabilistic classification tasks.
• Learning and memory impairments follow damage to prefrontal regions projecting to the associative basal ganglia pathway.
• New movement sequences and adaptations show that a dysfunction during motor learning was strongest in the putamen.
  *Basal ganglia also supports the theory that neurons are modulated by anticipation of reward.

Error Correction and Motor Coordination by the Cerebellum

• Brainstem circuits coordinate lower-level reflex circuits in the spinal cord and brainstem motor nuclei, supporting anticipatory postural adjustments and complex multijoint movements.
• The cerebellum is responsible for correcting ongoing errors to coordinate smooth, skilled movements.
• Specialized neurons in the cerebellum compute the net error between motor commands and actual movements, relaying error signals to the frontal/parietal cortices via the dentate nucleus and thalamus.
• Cerebellar damage impairs smooth movement performance
  *The lesions result in uncoordinated disorganized movements because a disconnect from the lateral cerebellum is also apparent.
• Cerebellum's role in correcting ongoing errors extends to motor learning; lesions disrupt the ability to learn new motor skills.

Cerebellar Contributions to Cognitive Behavior

• Neuroimaging studies highlight that the cerebellum also contributes to cognitive/nonmotor aspects including Learning, Attention, verbal time/working memory.
• Cerebellum's computational power as an error correction device may be harnessed to serve cognitive functions.
• The cerebellum cellular architecture could perform the same type of computation on any inputs-The motor cortex may send signals of output to be compared (error)
  *Cognitive operations instantiated in prefrontal circuits feeding to the cerebellum might rely on the highly stereotyped cellular architecture of the cerebellum to provide fast, accurate, and automatic “simulations” of the output of these cognitive operations.
• This allows fast and accurate simulations of cognitive operation.