The Neurobiology of Motor Control

motor control systems in the brain

• organised hierarchically

  • Premotor/supplementary motor cortex & parietal cortex

    • responsible for actions plans & goals

  • Primary motor cortex (basal ganglia connects to top)

    • which translate the motor plans/goals into specific actions

  • brain stem (cerebellum connects to top)

  • spinal cord

    • provides direct control of muscles via motor neurons & interneurons

muscles

• Motor control is carried out by muscles

• composed of elastic fibers that can change length & tension

• arranged in antagonist pairs

  • e.g. biceps & triceps

spinal cord

• Muscles are controlled by motor neurons in the spinal cord

  • Motor neurons originate in spinal cord, exit through the ventral root and terminate in the muscle fibers.  

• Action potential in a motor neuron triggers the release of acetylcholine

  • neurotransmitter that makes muscle fibers contract

• number/frequency of action potentials + number of muscle fibers determine the force the muscle can generate

motor structures

• muscles

• spinal cord

Subcortical motor structures

• brainstems

• cerebellum

• basal ganglia

brain stem

• subcortical motor structure

• 12 cranial nerves control essential reflexes for keeping us alive e.g eating, breathing, facial expressions

• Extrapyramidal tracts → Direct pathways from brainstem nuclei, including substantia nigra, down spinal cord to control posture, muscle tone, movement speed.

cerebellum

• subcortical motor structure

• Contains more neurons than rest of CNS combined

• Controls balance & eye/body coordination

• Lesions result in balance/gait problems, ataxia (fine coordination) & attentional, planning and language problems

basal ganglia

• subcortical motor structure

• The basal ganglia are a key node in the subcortical motor control system

• Contains 5 nuclei

  • caudate, putamen, globus pallidus, subthalamic nucleus & substantia nigra

• Critical role in selection & initiation of actions

• lesions cause parkisons’s disease

cortical motor regions

• primary motor area

• secondary motor areas

• association motor areas

Primary Motor Cortex (M1)

• receives input from almost all cortical motor regions

• regulates the activity of spinal motor neurons.

• Crossed hemispheric control

• Corticospinal → axons that project directly from the cortex to the spinal cord

• has Somatotopic organisation → different regions represent different body parts

• Can elicit predictable twitches in different regions using TMS. 

• Lesions to M1 produce hemiplegia, loss of voluntary movements on contralesional side of body

Secondary motor areas

• Premotor cortex & supplementary motor area (SMA)

  • highest parts of the hierarchy

  • involved in planning & control of movement, either sensory guided or internally guided.

• Planning and control of movement

• Lesions result in apraxia – patients can produce simple gestures but cannot link them into meaningful actions, e.g. brushing hair.

central pattern generators

• how regions enable motor control

• neurons in the spinal cord

  • able to hold a representation of entire pattern of movements required to produce a complex motor act

• enables higher level regions to send an simple signal that triggers one of these central pattern commands

  • no need for higher-level regions to hold the entire representation themselves

• Probably evolved to enable actions essential for survival, e.g. running

Brown & Sherrington (1947)

• severed spinal cord of cats & placed them on treadmill

• even without descending commands from (sub)cortex → produced rhythmic alternating limb movements required to walk

representations of movement plans

• neurons can represent the direction of a movement

• Coding of movement direction in primary motor cortex

  • Georgopoulos et al (1995)

    • require monkeys to move a lever to 1of 8 targets, starting from the centre

    • Neurons preferred a direction (fire strongly when movement is in that direction)

      • neurons had same selectivity movement even though the target location changed

• neurons prefer multiple directions → population vectors

  • neuron’s response tuned to broad range of directions

Population vectors

• Vector = neuron’s preferred direction & information about strength of firing

  • Tuning of neurons is broad → prefer several directions

• Hard to predict direction of movement from activity of a single neuron

  • Population vector (sum of individual neuron vectors)

    • Population vector provides the most accurate estimate of the planned direction of movement

      • used to predict direction of movement before initiation of movement

brain-machine interfaces

• Chapin trained rats to press a lever for reward

  • measured multiple neuron responses in motor cortex

  • neural networks learnt the patterns of neuronal activation predicting forces exerted on the lever

• switched input to reward delivery system from the lever to the neuronal population vector

  • mice stopped pressing the lever → learnt about lack of precise correlation between the force exerted & reward

  • Mice continued to produce cortical signals necessary for moving the lever

Visuomotor

• the coordination of neuronal activity between visual-related & motor-related parts of the brain in order to influence behavior and perception.

Visuomotor adaptation

• process by which individuals adjust their motor actions in response to changes in visual feedback or environmental conditions

• involves learning to modify motor behaviors to compensate for discrepancies between expected & actual sensory information

Effects of transcranial direct current stimulation (tDCS) on visuomotor adaptation

• tDCS → electrical currents are applied to the scalp producing changes in the excitability of neurons under the electrode

• Galea et al → investigate the different roles of cerebellum & the primary motor cortex

  • tDCS of the cerebellum led to a faster rate of adapation

  • tDCS of the motor cortex led to increased retention of adaptation

    • more error for a longer period of time after the end of adaptation

Patients with lesions in cerebellum, prefrontal cortex & parietal cortex

• have deficits in learning to move in novel environments

Cerebellum is important for learning new mapping

• involved in the generation of forward models

• Time lag between generation of motor commands and movement initiation

• generates prediction of sensory consequences of motor command

  • are essential in visuomotor adaptation – errors used to correct future predictions.

Primary motor cortex (M1) important for consolidating newly learnt mapping

• plays a less flexible role and more of an instructive role, passing on motor plans to spinal cord motor neurona

• plays a different role in visuomotor adaptation

Effect of transcranial magnetic stimulation of the cerebellum on forward models

Miall et al. (2007)

• P were instructed to move their arm to the right when they heard a tone, signaling them to move to a visual target.

• task required predicting where the hand would be in the future due to a delay between hearing the tone & initiating movement

• Simply generating a motor command when hearing the tone would likely result in missing the target because the trajectory of movement would be outdated by the time the movement was initiated.

• under normal conditions, P were generally accurate in hitting the target

role of the cerebellum in motor prediction

• transcranial magnetic stimulation (TMS) was applied to disrupt cerebellar function.

• When the cerebellum was temporarily disrupted with TMS → path of the hand matched what would be expected if the motor command had been issued 138 milliseconds earlier than when the movement was initiated.

• suggests that the cerebellum provides predictions of where the hand will be when the movement is initiated & adjusts the motor command accordingly

key points of motor control

• Many different brain regions involved in motor control

• Coding of movement direction by population vectors &their use in brain-machine interfaces

• Visuomotor adaptation → different roles of cerebellum & M1

• Cerebellum: Forward models

• M1: consolidation of learning