Motor System 1

The motor control system Overview

The Motor system 1 • Cortex • The Corticospinal tract • Alpha motor neuron • Muscles

Motor Control Motor Cortex UMN

Alpha motor neuron axon, LMN Muscle

Corticospinal tract (UMN) Alpha motor neuron, LMN

Four Hierarchical Components that Control Movements • Motor systems consist of separate neural circuits that are linked. • Ultimately, whether directly or indirectly distributed, all motor processing is focused on a single target ‘the motor neuron’ constituting the ‘final common pathway’ of motor system.

Four Hierarchical Components that Control Movements  Spinal cord  Brainstem  Subcortical (basal nuclei, thalamus, cerebellum)  Cortical –(primary motor cortex, premotor and supplementary motor areas)

Motor system 2 • • • •

Cortex corticospinal tract Alpha motor neuron Muscles

• Two control circuits that influence the activity of corticospinal tract – Cerebellum – Basal Ganglia

Motor system 2 two control circuits

Motor system 3 • • • • • •

Cortex corticospinal tract Alpha motor neuron Muscles + two control circuits influence the corticospinal tract Cerebellum and BG

• The Indirect brainstem motor control centers and pathways which tonically activate the Lower Motor Neurons especially those that innervate the Axial and Antigravity muscles

Motor system 3

Upper Motor Neuron • The corticospinal tract has its main influence on LMN that innervate the muscles of the distal extremities, i.e., the hand and the foot • The corticospinal tract has collaterals that modulate the control of indirect brainstem motor centers, so that we are not as a statue opposing gravity and can move at will and have the right amount of supporting tone • When there is lesion of UMN, clinical findings are a combination of both direct + indirect effects

Premotor and supplementary motor areas

Cortical level

Subcortical level

Brain stem level

Spinal cord level

Sensory areas of cortex

Basal nuclei

Figur e 8.24 Page 285

Primary motor cortex

Thalamus

Cerebellum

Brain stem nuclei

Afferent neuron terminals

Motor neurons

Muscle fibers Periphery Movement

“To move things is all that mankind can do… for such the sole executant is muscle, whether in whispering a syllable or in felling a forest”.. Charles Sherrington • The spinal cord contains certain motor programs for the generation of coordinated movements and that these programs are accessed, executed, and modified by descending commands from the brain.

Types of Movements • Involuntary motor acts – Reflex: the most automatic behaviors (such as reflexes-organized at spinal cord level)

• Voluntary motor acts – The maintenance of position (posture) – Goal directed movements- skilled voluntary movements- organized at higher centers

Somatic musculature in relation to the joint they act on • Axial muscles: – For movements of the trunk

• Proximal muscles (or girdle muscles) – For movements of the shoulder, elbow, pelvis and knee

• Distal muscles – That move the hands, feet, and digits (fingers and toes)

Important aspects of hierarchical organization: • Somatotopic maps – preserved in interconnections at different levels • each hierarchical level receives information from periphery so that sensory input can modify the action of descending commands • The higher levels have capacity to control the information that reaches them, allowing or suppressing the transmission of afferent volleys through sensory relays.

Important aspects of hierarchical organization: • The various motor control levels are also organized in parallel: so that each level can act independently on the final common pathway. • This allows commands from higher levels either to modify or to supersede lower order reflex behavior.

Upper Motor Neuron Lesion

UMNL  loss of direct effect of UMN

UMNL  loss of indirect effect of UMN

UMNL is a combination of Loss of regulation of indirect brainstem motor control centers

Loss of direct CST control of LM neurons

Upper Motor Neuron Lesion • Loss of distal extremity strength • Loss of distal extremity dexterity • Babinski sign • Increased tone • Hyperreflexia • Clasp-knife phenomenon

Loss of direct effect

Loss of indirect effect

UMNL on opposite side of clinical findings if lesion is above the decussation

UMNL on same side of clinical findings if lesion in the spinal cord after decussation

Figure 9: The brain of a recovered stroke patient relies on a compensatory neural pathway (dark blue) as substitution for the damaged neuralpathway (blue dashed). The cerebello-thalamo -cortical pathway (green) is “teaching” the supplementary motor area its new function, which is indicated by abnormal activity in the cerebellum and thalamus. (Freely adapted from Azari & Seitz, 2000)