UofR: Neural Basis of Cognition

Lecture 3Motor Control

Motor Control

• Carrying out an action such as a purposeful arm movement is a complex process

• Muscle fiber contraction is caused by a nerve impulse

• This nerve impulse originates in the motor cortex

• How are these signals chosen and planned?

Motor Tracts

• Corticospinal pathway - links the cortex to the spinal cord; cell bodies are located mainly in the motor cortex– Lateral corticospinal tract: control of distal muscles

• Damage results in difficulty in reaching for and grasping objects in contralateral side

• Crosses midline entirely in medulla

– Ventral corticospinal tract: control of muscles of the trunk and upper legs• Damage results in difficulty walking and maintaining posture• Projects both ipsi- and contralaterally

Motor Tracts

• Corticobulbular pathway: most cell bodies are loacted in the cortex but synapse in the pons on the 5th, 7th, 10th, 12th, cranial nerves

• Some portions project only contralaterally, some project ipsilaterally as well

• The regions of the motor cortex that control movements of the upper part of the face project both ipsi- and contralaterally. Not true for the lower face

Motor Tracts• Ventromedial pathway

– Primarily controls movements of the trunk and proximal muscles. Contributes to posture

– Involved in coordination of eye movements with those of trunk and head (these cell bodies lie in the superior colliculus)

– Involved in autonomic functions such as sneezing, breathing, muscle tone, as well as walking

• Rubrospinal pathway– Originates mainly in the red nucleus of the midbrain– Receives input from and projects to the motor cortex and cerebellum– Controls distal muscles: hands (not fingers), feet, forearms, lower legs– Allows movement of forearms, hands independently of the trunk

• Both:– Cell bodies lie in the brain stem and project to both the spinal cord and

indirectly to the primary motor cortex

Cerebellum

• Essential role in motor control

• Modulates motor movements and is important in the learning of motor skills

• Modulates ipsilateral muscles

Cerebellum

• Vermis– Input: spinal cord regarding somatosensory and

kinesthetic information– Output: fastigial nucleus (influences some

ventromedial tracts)– Damage: difficulty with postural adjustments and

movements such as walking

Cerebellum

• Intermediate zone:– Input: Red nucleus (and so, indirectly, the motor

cortex) and somatosensory information from the spinal cord

– Output: interpositus nucleus (which projects to the red nucleus)

– Damage: rigidity in and difficulty in limb movement, tremors that are observed most often when reaching for something, inability to make smooth movement to a target location (action tremor / intention tremor)

Cerebellum

• Lateral zone– Input: Motor and association cortices via the pons– Output: dentate nucleus, which projects to the primary motor

and premotor cortices through the red nucleus and ventrolateral thalamus

– Damage:• Rapid and smooth ballistic movement and overshooting• Poor coordination of multijoint movement (leads to decomposition of

movement)• Hampered learning of new movements• Impaired ability to make simple but precisely timed tapping

movements, make judgements about the temporal duration of events (i.e. timing)

Cerebellum

• May have important cognitive roles• Decreased cerebellar size has been observed

in ADHD, autism• Neuroimaging studies show activation of

cerebellum during higher-level cognitive tasks– Ask for references if you are interested

Basal Ganglia

• Complex collection of nuclei that form loops with cortical regions– Caudate, putamen, nucleus accumbens, globus

pallidus, substantia nigra, subthalamic nucleus• Vast majority of input goes to caudate,

putamen, together called the striatum• Output is from the globus pallidus to the

thalamus, which projects to the cortex

Basal Ganglia

• Modulates “internally guided” motor activity• Damage, depending on which region is affected:– Akinesia: inability to initiate spontaneous movement such

as in Parkinson’s, where there is death of dopaminergic neurons in the substantia nigra

– Bradykinesia: slowness of movement– Tremors– Hyperkinesias: involuntary undesired movements, such as in

Huntington’s, where there is a selective loss of striatal neurons that bind GABA

• See “Striatal Connections” reading

Cortical Regions

• Primary Motor Cortex (M1)– Provides the command signal to drive motor

neurons to make muscles move– Population vote (Georgopoulos, Scwartz, and

Kettner, 1986)

Cortical Regions

“The outcome for one of the movement directions tested is shown in Fig. 3. The yellow line indicates the movement direction M. The cluster of light purple lines represents the 224 cell vectors (that is, the vectors Ni(M), i = 1 to 224) for movement direction M. The direction of the population vector P(M) yielded by the vectorial summation of these cell vectors is orange.”

Motor Plan

• M1 codes movement; damage to M1 results in the inability to control some portion of the body

• Planning of a motor action is planned in the supplementary and premotor areas (motor program)

• The brain generates an entire plan of action before movement commences rather than creating the plan as actions are being performed.

Motor Plan

• The supplementary motor area (SMA) constructs the motor plan at the most abstract level (sequencing of the critical components of an action)

• Premotor area codes for details of each action• Primary motor areas code exactly how the muscles would

be controlled to implement the required grasp on the bottle• Timing:

– Neuroimage. 1998 Aug;8(2):214-20. Origin of human motor readiness field linked to left middle frontal gyrus by MEG and PET. Pedersen JR, Johannsen P, Bak CK, Kofoed B, Saermark K, Gjedde A.

Motor Plan

• SMA activates only during planning of complex motion; activity in the SMA is observed before any electrical activity in the limbs begins

• SMA is active when subject is asked to imagine but not perform a complex task without M1 activation

• SMA projects to both ipsi- and contralateral motor cortex as well as the contralateral SMA

Motor Plan

• Premotor area: projects to M1• Mirror neurons

Motor Control

• Anterior cingulate cortex:– Topographic organization• Caudal: manual movements• Rostral: occulomotor• In between: speech

– Most active when a novel response is required in task

– Determines when a response is incorrect

Motor Control

• Frontal eye field: controls voluntary eye movements via the cranial nerves– Precedence of superior colliculus

• Parietal lobe– Interface between sensory and motor information– Contributes to ability to produce complex, well-learned

motor acts– Sensitive to proprioceptive and kinesthetic information– Damage results in loss of ability to guide limbs in a

well-controlled manner