MovementPart II

Brain Control of Movement

Motor Control Hierarchy

• The brain influences the spinal cord to command voluntary movements

• Central motor system is a hierarchy of control levels:– Top STRATEGY

Association areas of neocortex, basal ganglia

– Middle TACTICS Motor cortex, cerebellum

– Bottom EXECUTION Brain stem, spinal cord

Strategy• The cerebral neocortex has information

about where the body is in space based on vision, hearing, somatic sensation, proprioception

• Strategies must be devised to move the body from the current position to the desired state

• Options for moving are filtered through the basal ganglia and back to the cortex

• A decision is made based in part on experience (memory)

Tactics & Execution• Motor areas of the cortex and cerebellum

make the tactical decision• Send instructions to the brain stem & spinal

cord• Activation of neurons in the brain stem &

spinal cord cause movement• Activation of motor neurons must be

properly timed to produce coordinated movement

• Brain stem dictates simultaneous postural adjustments to the spinal cord

A Sensorimotor System• Proper function of the motor hierarchy

depends on sensory information• Sensory information generates a mental

image of the body and its relationship to the environment

• Tactical decisions are based on memory of sensory information from past movement

• Sensory feedback is used to maintain posture, muscle length, and tension, before and after each voluntary movement

Descending Spinal Tracts• How the brain communicates with motor

neurons of the spinal cord• Axons from the brain descend through the

spinal cord along 2 major pathways:• Lateral Column of the spinal cord

– Involved in voluntary movement of the distal muscles

– Under direct cortical control

• Ventromedial Column of the spinal cord– Involved in control of posture and locomotion– Controlled by the brainstem

Lateral Pathways

• Lateral Pathways include two tracts:• Corticospinal tract• Rubrospinal tract • Coordinated movements of joints,

speed, precision are all mediated by lateral pathway

• Rubrospinal tract can compensate for damage to the corticospinal tract

Corticospinal Tract• Most important component of the lateral pathway• Originates in the neocortex (mostly motor cortex)

thru the internal capsule thru the cerebral peduncle thru the pons to the base of the medulla– bulges on the surface of the medulla

• Cut cross section is ~ triangular pyramidal tract• At junction of medulla & spinal cord tract decusates

– fibers cross right motor cortex commands movements of the left side of the body & vice versa

• Axons terminate in the dorsolateral ventral horns– Where motor neurons & interneurons that control distal

muscles are located

Rubrospinal Tract • Smaller part of lateral pathway• Originates in red nucleus in midbrain

– decussation in the pons– joins the corticospinal tract in the lateral

column of the spinal cord

• In humans most of its function is supplanted by the corticospinal tract

• Corticospinal tract seems to be a later development than rubrospinal tract – in humans, rubrospinal tract may be less used

even than in monkeys

Ventromedial Pathways • Four descending tracts that originate in the

brainstem and terminate in spinal interneurons controlling proximal & axial muscles:– Vestibulospinal– Tectospinal– Pontine reticulospinal– Medullary reticulospinal

• These pathways use information about balance, body position, and the environment to reflexively maintain balance and body posture

Vestibulospinal & Tectospinal Tracts

• Vestibulospinal tract – vestibular nucleus in medulla

bilateral spinal neurons – compensation for head movement

• Tectospinal tract – superior colliculus in midbrain

contralateral spinal neurons – move body and head to orient toward

stimulus

Pontine & Medullary Reticulospinal Tracts

• Pontine Reticulospinal Tract – pontine reticular formation ipsilateral spinal

neurons – enhances antigravity spinal reflexes

• Medullary Reticulospinal Tract – reticular formation in medulla ipsilateral

spinal neurons – liberates antigravity muscles from reflex

control

• All four of these pathways can be activated from motor cortex

Principal Motor Domains

• The primary motor cortex (M1) lies along the precentral gyrus, and generates the signals that control the execution of movement.

• Secondary motor areas are involved in motor planning.

Planning & the Motor Cortex• Primary motor cortex

– Brodmann Area 4 – precentral gyrus – Somatotopic map like somatosensory cortex – Final output pathway from cortex to lower centers

• Premotor area (PMA) – lateral Brodmann Area 6 – Somatotopic map – Projects to reticulospinal neurons in proximal

motor units

• Supplementary motor area (SMA) – medial Brodmann Area 6 – Somatotopic map – Projects directly to distal motor units

Motor Homunculus

• A figurative representation of the body map encoded in primary motor cortex.

• Body parts with complex repertories of fine movement, like the hand, require more cortical space in M1, while body parts with relatively simpler movements, like the hip, require less cortical space.

Posterior Parietal & Prefrontal Cortex• Mental body image is constructed from

somatosensory, proprioceptive & visual inputs to posterior parietal cortex

• Posterior parietal areas 5 and 7 – Area 5 receives input from primary

somatosensory cortex – Area 7 receives input from higher order visual

cortices such as MT

• Prefrontal cortex – Area 8 – center for abstract thought, decision making,

etc.– Extensively interconnected with areas 5 & 7

Hierarchy

• Movements are planned in areas 5, 7 & 8

• Area 6 is the place where the action desired is converted into how the action will be carried out

• Area 4 becomes active when the movement is initiated

• The command to initiate comes from the subcortical basal ganglia

Initiating Movement• Input to area 6 arises in the ventral

lateral nucleus (VL) of the dorsal thalamus

• Input to the VL arises from the basal ganglion in the telencephalon

• The basal ganglia are targets of the frontal, prefrontal, & parietal cortex

• This forms a loop: information cycles from the cortex, to the basal ganglia and thalamus, and back to the cortex

• One function of this loop is selection & initiation of voluntary movements

The Basal Ganglia• Consists of:

– the caudate nucleus, putamen, globus pallidus, & subthalamus

– caudate + putamen = striatum– striatum is the target of cortical input to the basal

ganglion– Substantia nigra, a midbrain structure is reciprocally

connected and closely associated in function

• General connection pattern: – Neocortex striatum globus pallidus VLo

(thalamus) neocortex (SMA)

• The basal ganglia are a complex set of computing machines involved not only with motor systems, but also with memory and cognitive function

Motor Loop Thru the Basal Ganglia• Frontal cortex (area 8) • excites putamen • inhibits globus pallidus • disinhibits VLo • excites SMA • Inputs from substantia nigra to putamen

are also excitatory • Inputs from subthalamus to globus

pallidus are also excitatory • The signal to initiate may be when the

SMA neurons are dis-inhibited by the motor loop

Anatomy of The Cerebellum• Vermis along midline

– outputs to ventromedial pathways

• Hemispheres on either side of vermis – outputs to lateral pathways

• Cerebellar cortex is folded into folia and lobules

• Although only 10% of brain volume, contains 50% of brain neurons

• Deep cerebellar nuclei – relay centers for cerebellar output

Motor Loop Through the Cerebellum

• Layer V pyramidal cells in motor areas 4 & 6, somatosensory areas, and posterior parietal cortex massive projection to pontine nuclei (20 times larger than pyramidal tract) lateral cerebellum VL in thalamus Primary motor cortex

• This pathway is essential for smooth execution of voluntary multi-joint movements

• What is intended is compared with what has happened to produce proper sequence of outputs

• Motor learning involves programming the cerebellum

Initiating Movement• The basal ganglia and cerebellum are

large collections of nuclei that modify movement on a minute-to-minute basis.

• The motor cortex sends information to both, and both structures send information back to cortex via the thalamus.

• The output to the descending corticospinal tract comes from pyramidal cells in layer V – M1 (Betz cells)

Cerebellum & Basal Ganglia Dynamic

• Output of the cerebellum is excitatory

• Out put of the basal ganglia are inhibitory.

• The balance between these two systems allows for smooth, coordinated movement

• A disturbance in either system will show up as movement disorders.

Movement Disorders

• Parkinson’s Disease• Huntington's Disease • Ballism

Parkinson's Disease • Degeneration of dopaminergic neurons in

substantia nigra • Reduction in excitatory input to putamen

in motor loop • net reduction in excitation to SMA • Hypokinesia - paucity of movement • Bradykinesia - slowness of movement • Akinesia - difficulty in initiating voluntary

movements • Rigidity - increased muscle tone

Huntington’s Disease• Hereditary, progressive, lethal syndrome • Hyperkinesia - excess movement • Dyskinesia - abnormal movements • Chorea - uncontrollable, purposeless

movements, with flicking movements of limbs • Profound loss of neurons in caudate, putamen

and globus pallidus • Loss of inhibitory input from basal ganglia • Cortical degeneration also occurs dementia

Ballism

• Damage to subthalamus, usually from stroke

• Reduction in excitatory input to globus pallidus in motor loop - net increase in excitation to SMA

• Hyperkinesia - excess movement • Violent, flinging movements of the

extremities

Summary • External stimulus or internal thought

produces the desire for movement • Parietal, prefrontal, and PMA/SMA

become active, planning strategy for movement

• The basal ganglia motor loop triggers initiation

• SMA becomes more active • M1 is activated

Summary Continued

• The cerebellar motor loop sets the timing of the M1 output to accomplish the movement desired

• Cortical input to the reticular formation releases the antigravity muscles from reflex control via the ventromedial pathway

• Lateral pathways engage the motor units - movement occurs