THE MOTOR SYSTEM, part II

THE MOTOR SYSTEM

The brain influences activity of the spinal cord in order to generate voluntary movements

Hierarchy of controls

Highest level: Strategy, the goal of the movement and best way to achieve it. Associated to neocortex and basal ganglia

Middle level: Tactics, the sequence of muscle contraction to achieve the goal. Associate to motor cortex and cerebellum

Lowest level: Execution, activation of motor neurons that generate the movement. Associated to brain stem and spinal cord

DESCENDING SPINAL TRACTS

Axons from brain descend along two major pathways Lateral Pathways: involved in voluntary of distal musculature movement under cortical controlVentromedial Pathways: involved in control of posture and locomotion, under brain stem control

THE LATERAL PATHWAYS

Base of cerebral peducle

Medullarypyramid

Corticospinaltract

Rubrospinaltract

pyramidal decussation

midbrain

Right red nucleus

THE VENTROMEDIAL PATHWAYS

Vestibular nucleus

Spinal cord

Vestibulospinaltract

Tectospinaltract

Vestibulospinal tract: information from vestibular system. Control neck and back muscles. Guide head movements

Tectospinal tract: information from retina and visual system. Guide control eye movements.

THE VENTROMEDIAL PATHWAYS

Cerebellum

Spinal cord

Reticulospinaltract

pons

Medullaryreticular formation

Pontine reticular formation

Pontine reticulospinal tract: enhance antigravity reflexs, helps maintaining a standing posture

Medullary reticulospinal tract: opposite function

THE MOTOR CORTEXArea 4 = “Primary motor cortex” or “M1”Area 6 = “Higher motor area”Lateral region Premotor area (PMA), controls distal motor unitsMedial region Supplementary motor area (SMA), controls proximal motor units

THE MOTOR CORTEXThe Contributions of Posterior Parietal and Prefrontal CortexRepresent highest levels of motor control. Help in deciding about actions and their outcome, by integrating many source of informationArea 5: Inputs from areas 3, 1, and 2Area 7: Inputs from higher-order visual cortical areas. They both project to Area 6

Instruction

Trigger

APs of PMA neuron

THE BASAL GANGLIA

Basal ganglia Project to the ventral lateral (VLo) nucleusProvides major input to area 6

Cortex Projects back to basal gangliaForms a “loop” in order to select and initiatiate willed movements

THE BASAL GANGLIAAnatomy of the Basal GangliaCaudate nucleus, putamen, globus pallidus, subthalamic nucleusSubstantia nigra: Connected to basal ganglia

THE BASAL GANGLIA

The Motor Loop: Selection and initiation of willed movementsExcitatory connection from the cortex to cells in putamenCortical activation excites putamen neurons. Inhibits globus pallidus neurons.Release cells in VLo from inhibition. Activity in VLo influences activity in SMA

THE BASAL GANGLIA

Basal Ganglia Disorders: Hypokinesia and hyperkinesia

Parkinson’s diseaseSymptoms: Bradykinesia, akinesia, rigidity and tremors of hand and jawOrganic basis: Degeneration of substantia nigra inputs to striatumDopa treatment: Facilitates production of dopamine to increase SMA activity

Huntington’s diseaseSymptoms: Hyperkinesia, dyskinesia, dementia, impaired cognitive disability, personality disorder

HemiballismusViolent, flinging movement on one side of the body

Some examples….

http://www.youtube.com/watch?v=ECkPVTZlfP8&feature=related PARKINSON

THE CEREBELLUM

Function: Sequence of muscle contractionsLesion: Ataxia, characterized by uncoordinated and inaccurate movements. Dysynergia, dysmetricAnatomy: Folia and lobules, Deep cerebellar nuclei (relay cerebellar cortical output to brain stem structures) Vermis (contributes to ventromedial pathways) Cerebellarhemispheres (contributes to lateral pathways)

THE CEREBELLUM

THE CEREBELLUMThe Motor Loop Through the Lateral CerebellumAxons from layer V pyramidal cells in the sensorimotor cortex form massive projections to ponsCorticopontocerebellar projection are 20 times larger than pyramidal tractFunction: Execution of planned, voluntary, multijoint movements