Motor System, Motor Cortex, and Basal Ganglia

Kinds of movement categories

reflexive
rhythmic
voluntary

medial activating system

reticulospinal tract
vestibulospinal tract
tectospinal tract

descends in ventral column and terminates in ventromedial area of spinal gray matter for postural control

lateral activating system

rubrospinal tract

descends in contralateral dorsolateral column and terminates in ventrolateral area of gray matter

corticospinal tract function

Fine movement of limbs and trunk movement

corticobulbar tract function

control of muscles of facial muscles and mastication

upper motor neuron lesion

hyperreflexia w/ hypertonia (excessive muscle tone)

lower motor neuron lesion

hyporeflexia (loss of or decreased amplitude of reflexes) w/ hypotonia

corticobulbar tract pathway

cortex --> internal capsule --> brainstem to synapse bilaterally with lower motor neurons that go to head, neck, and face

reflexive movements

controlled at spinal cord and brainstem and has its own sensory stimulus with its stereotypical response

ex. blinking, stretch/withdrawal/pupillary reflexes

rhythmic movements

semiautomatic movements unless something changes in environment

ex. respiration, walking, chewing, swallowing

voluntary movements

intentional, goal directed movement

reticulospinal tract

originates in pontine RF and terminates in ventromedial area of spinal gray

controls postural muscles and limb extensors

vestibulospinal tract

originates in vestibular nuclei and terminates in ventromedial area of spinal gray

medial division: head/neck position, coordinate head/eye movements

lateral division: excitatory to motorneurons of proximal limb extensors and paravertebral muscles

tectospinal tract

originates in superior colliculus (in the tectum) and terminates in ventromedial area of spinal gray

coordinates head and eye movements

rubrospinal tract

originates in red nucleus and crosses over in the midbrain; terminates in the ventrolateral area of spinal gray matter

- proximal limb flexors
- modulates sensitivity of flexor responses
- fine, goal-directed limb function

lateral corticospinal tract pathway

UMN originates at cortex --> crosses at caudal medulla --> continues down spinal cord to synapse w/ LMN that exit at designated level

ventral corticospinal tract pathway

UMN originates at cortex --> down spinal cord at same level --> cross over to ventral gray horn at designated level --> synapses w/ LMN and it exits

upper motor neurons

neurons that stay in CNS that synapse with lower motor neurons

greater branching for proximal muscles (larger coordinated movements)
least branching for distal muscles (fine movements)

lower motor neurons

neurons that leave the CNS and become peripheral nerves, as they innervate the rest of the body

main cortical motor areas

-primary motor cortex
-lateral premotor cortex
-supplementary motor area

important in eliciting voluntary movement

Homunculus organization

lower extremity is medial, upper extremity and face is lateral

Betz Cells

aka pyramidal cells or upper motor neurons; in layer 5 of cortex (second layer up from white matter) and release glutamate

Betz Cells Pathways

axons descend to brainstem and spinal tracts to become corticospinal tracts

lateral corticospinal tract function

provide innervation to LMNs that activate distal muscles for voluntary movement

ventral corticospinal tract function

provide innervation to LMNs that control proximal muscles for posture and balance movements

can have bilateral actions

lateral premotor cortex function

movements in response to sensory/external cues

primary motor cortex function

all movements but is the only one activated for simple movements

supplementary motor area

provide motor plan based on desired movement and direct input to proximal muscles (postural control)

activated alongside primary motor cortex for complex movements

Lateral premotor and supplementary motor area lesion

impaired ability to execute complex, purposeful movements

lesion to only lateral premotor cortex

impaired ability to respond correctly to visual/verbal cues

basal ganglia nuclei

striatum (caudate+putamen)
subthalamic nucleus
globus pallidus external segment (GPe)
substantia nigra pars compacta (SNpc)
globus pallidus internal segment (GPi)
substantia nigra pars reticulata (SNpr)

inputs to basal ganglia

primary motor cortex and secondary motor areas
frontal and prefrontal areas
somatosensory cortex
posterior parietal areas
brainstem
thalamus

outputs to basal ganglia

thalamus to cortex
*pedunculopontine nucleus to reticulospinal tract
midbrain locomotor region to reticulospinal tract

influence of basal ganglia - motor loop

motor, premotor, somatosensory cortex (cortical input) --> putamen --> lateral globus pallidus internal segment --> ventral lateral and anterior nuclei --> back to cortex

Influence of basal ganglia - prefrontal loop

dorsolateral prefrontal cortex --> anterior caudate --> globus pallidus internal segment + substantia nigra pars reticulata --> mediodorsal and ventral anterior nuclei --> back to cortex

influence of basal ganglia - limbic loop

amygdala, hippocampus, orbitofrontal, anterior cingulate, temporal cortex --> ventral striatum --> ventral pallidum --> mediodorsal nucleus --> back to cortex

functions of basal ganglia

-automatic control of movement
- internally generated movements
- motor learning
- sequential movement
- movement optimization
- focused selection and inhibition of activity

Basal Ganglia Neurologic Disorders

hypokinetic disorders (not enough movement)
hyperkinetic disorders (uncontrolled, involuntary movement)

direct pathway

- cortex releases glutamate, exciting striatum
- striatum inhibits GP interna and SNpr
- SNpc (dopamine) enables the inhibition of GP interna and SNpr from striatum
- GP interna and SNpr inhibit thalamus
- thalamus excites cortex

indirect pathway

- cortex releases glutamate, exciting striatum
- striatum inhibits GP externa
- SNpc (dopamine) inhibits the inhibition of GP externa, which in turn facilitates movement
- GP externa inhibits STN
- STN excites GP interna and SNpr
- GP interna and SNpr inhibit thalamus
- thalamus excites cortex