The Motor System

Basal Ganglia

Initiation and execution of motor activity

Automatic stereotyped movement of a postural and reflex nature

The "Striatal Motor System"

Cerebellum

Maintenance of posture, balance, and muscle tone

Coordination of voluntary motor activity

Lower Motor Neurons

Axons leave the CNS, extend through PNS to skeletal muscles. Cell bodies in anterior horns of spinal cord and in cranial nerve nuclei of brainstem

Damage = flaccid paralysis

Upper Motor Neurons

Motor neurons in the central nervous system that control the lower motor neurons in the peripheral nervous system

Damage = partial paralysis with hyperflexia

Pyramidal UMN

Projections from the cortex

Corticospinal

Descending tract whose fibers conduct motor impulses to skeletal muscles

Synapse on LMN of the spinal cord

Lateral and ventral

Damage = contralateral hemiparesis of trunk and extremities

Corticobulbar

Direct control of movements in head and neck

Synapse on brainstem nuclei

Damage = contralateral weakness of lower face

Extrapyramidal UMN

Projections from the brainstem nuclei

Rubrospinal

From the red nucleus to the spinal cord

Vestibulospinal

From the vestibular nuclei to the spinal cord

Reticulospinal

From the reticular nuclei to the spinal cord

Tectospinal

From the midbrain tectum to the spinal cord

Lateral Corticospinal Tract

A set of axons from the primary motor cortex, surrounding areas, and midbrain area that is primarily responsible for controlling the peripheral muscles

85% cross in the caudal medulla

Ventral Corticospinal Tract

The system of axons that originates in the motor cortex and terminates in the ipsilateral ventral gray matter of the spinal cord; controls movements of the upper legs and trunk

Continues uncrossed

Pyramidal System

The motor system that includes neurons within the cerebral cortex and their axons, which form the pyramidal tract.

Originates from neurons in the primary motor cortex, premotor cortex, supplementary motor cortex, and somatosensory cortex

Internal Capsule

A large collection of axons that connects the telencephalon with the diencephalon

Pyramidal tracts, among other things, pass through here

Corona Radiata

All the fibers connecting cortical and subcortical regions also pass through here

Motor Homunculus

Broad areas of primary motor cortex devoted to controlling movements of different body regions

Descending UMN of the Extrapyramidal System

Fibers from various regions of the brainstem descend and synapse on LMN

More diffuse and multisynaptic than corticospinal tracts

Involved in non-skilled, voluntary movements

Influenced by cerebral cortex via descending fibers synapsing in the brainstem

Rubrospinal Tract

From red nucleus

Crosses in the midbrain

Pontine Reticulospinal Tract

A tract originating in the pontine reticular formation and terminating in the spinal cord, involved in the control of movement.

Medullary Reticulospinal Tract

A tract originating in the medullary reticular formation and terminating in the spinal cord; involved in the control of movement.

Lateral Vestibulospinal Tract

Axons arising in the lateral vestibular nucleus that project ipsilaterally to facilitate lower motor neurons to extensor muscles and simultaneously inhibit lower motor neurons to flexor muscles via interneurons.

Medial Vestibulospinal Tract

Axons arising in the medial vestibular nucleus that project bilaterally to the cervical and thoracic spinal cord. Affects the activity of lower motor neurons that control the neck and upper back muscles.

Components of the Basal Ganglia

Caudate nucleus

Putamen

Globus pallidus

Subthalamic nucleus (in diencephalon)

Substantia nigra (in midbrain)

Thalamus

Lenticular Nucleus

Consists of the putamen and globus pallidus

Striatum

Consists of the caudate nucleus and putamen

Substantia Nigra

An area of the midbrain that is involved in motor control and contains a large concentration of dopamine-producing neurons

Largest nucleus in the midbrain

Substantia Nigra Degeneration

Parkinson's disease

Main Circuit of the Basal Ganglia

Cerebral cortex -->

Striatum -->

Globus pallidus -->

Thalamus -->

Cerebral cortex

Subthalamus

Subthalamus: origin of neuroblasts that form the globus pallidus

A stroke in this region results in hemiballism

Corticostriate Fibers

Originate all 4 lobes

Terminate in striatum (caudate, putamen)

Basal Ganglia Ipsilateral Connections

Cerebral cortex

Thalamus

Basal Ganglia Contralateral Connections

Side of the body

Hypertonia

Increase in muscle tone

Hypotonia

Decrease in muscle tone

Dyskinesias

Movement disorders caused by lesions in the basal nuclei

Resting Tremor

Rhythmic oscillations of the hand or head

Chorea

Sudden, rapid, jerky, purposeless movement involving limbs, trunk, or face

Athetosis

Bizarre, slow, twisting, writhing movement, resembling a snake or worm

Dystonia

A condition of abnormal muscle tone that causes slow, sustained contractions of the head and trunk

Ballism

A movement disorder caused by damage to the subthalamus, characterized by violent, flinging movements of the extremities.

Parkinson's Disease

A slow, progressive neurological disease caused by degeneration of dopaminergic neurons in the substantia nigra - results in depletion of dopamine in the striatum

Symptoms of Parkinson's

Resting tremors (pill rolling)

Rigidity

Akinesia

Bradykinesia

Impaired postural reflexes

Masked face (I can move, but would prefer not to)

Parkinsonism

Parkinson's disease-like extrapyramidal symptoms that are adverse effects associated with particular drugs or brain injuries

Progressive Supranuclear Palsy

Associated with Parkinson's and characterized by supranuclear ophthalmoplegia

Primarily downward gaze paresis, followed by paresis of other eye muscles

Huntington's Chorea

Autosomal dominant movement disorder, onset in mid-thirties

Localized to a single gene defect on chromosome 4

Symptoms: chorea, dementia, slowness of eye saccades, and inability to perform saccade without head movement

Wilson's Disease (Hepatolenticular Degeneration)

Autosomal recessive movement disorder localized on chromosome 13; caused by a defect in copper metabolism

Results in lesions in the liver and lentiform nucleus (putamen)

Symptoms: psychiatric problems, personality disorders, dementia tremor, rigidity, choreiform and athetotic movements

Kayser-Fleischer Ring

Gray/green or red/gold pigmented ring on the outer margin of the cornea

Used as a diagnostic sign of Wilson's disease

Hemiballism

An involuntary and violent movement of a large body part

Caused by a stroke in the subthalamus

Symptoms: violent, flinging movements of the contralateral arm and/or leg

Cerebral Palsy

A loss or deficiency of motor control with involuntary spasms caused by permanent brain damage present at birth due to oxygen deficiency

Symptoms: dystonia and athetosis

Flocculonodular Lobe

A region of the cerebellum; involved in control of postural reflexes

Associated with the thalamus

Flocculus

Small but dense lobe involved in eye movements and balance

Nodule

Medial protrusion seen on the midsagittal view of the cerebellum; part of the vermis

Tonsil of Cerebellum

Two elevated masses on inferior surface of hemispheral portion just nearby foramen magnum

Can herniate into the foramen magnum, exerting pressure on the medulla

Cerebrocerebellum

Planning and initiation of voluntary activity

Vestibulocerebellum

Maintenance of balance, control of eye movements

Vermal Zone

Composed of flocculonodular lobe

Controls eye movement and axial musculature related to balance and posture

Connects two hemispheres of cerebellum

Spinocerebellum

Enhances muscle tone and coordinates skilled, voluntary movements

Fastigial, globose and emboliform nuclei

Input to the Cerebellar Cortex

Climbing fibers and mossy fibers

Output from the Cerebellar Cortex

Transmitted via GABAergic Purkinje cell fibers

Only output from cortex

Final Output from the Cerebellum

From the deep cerebellar nuclei

Path Through the Cerebellum

Fibers from spinal cord and brainstem enter and synapse in both cerebellar deep nuclei and cerebellar cortex

Purkinje cells in the cortex send axons to the deep nuclei, modulating the output of the nuclei

Deep nuclei send fibers out of the cerebellum to the brainstem and thalamus

Cerebrocerebellum Circuitry

Input to contralateral cerebral cortex indirectly via the pons

Output to the contralateral red nucleus which projects to the thalamus (VL/VA) and contralateral thalamus relays info to the cortex

Lateral nuclei = dentate nuclei

Double crossing: the cerebellum talks to the initiating cerebral hemisphere

Cerebellopontine Angle

The junction at the base of the brain where the cerebellum, medulla, and pons communicate

Vestibulocerebellum Circuitry

Input from the vestibular nuclei and nerve

Output to the vestibular nuclei, giving rise to the vestibulospinal tract and to the reticular nuclei, giving rise to the reticulospinal tract

Spinocerebellum Circuitry

Input from the cerebral cortex (program) and sensory inputs (environmental cues)

Output to the UMNs of the brainstem (red, reticular, vestibular) which communicate with LMNs through extrapyramidal pathways

Somatotopic Map of the Cerebellum

The neuronal regions associated with regulation of the distal musculature are shown in green, and the regions associated with the axial musculature are shown in red

Superior Cerebellar Peduncles

Connects cerebellum to midbrain

Contains contralateral tracts that output to the red nucleus and thalamus

Middle Cerebellar Peduncles

Connect pons to cerebellum

Contain contralateral tracts that receive input from the pons - pontocerebellar

Inferior Cerebellar Peduncles

Connect medulla oblongata to cerebellum

Contains an ipsilateral tract that receives input from the spinal cord - dorsal spinocerebellar

Contains both ipsilateral and contralateral tracts connecting to the reticular and vestibular systems - vestibulospinal and reticulospinal

Contains a contralateral tract connecting to the inferior olivary nucleus - olivocerebellar

Blood Supply of the Cerebellum

Vertebral-basilar system branches: superior cerebellar, AICA, and PICA

Cerebellar Dysfunction

Ipsilateral defects where the somatotopic organization is reflected

Lesions damaging the deep cerebellar nuclei produce more severe symptoms than lesions only damaging the cerebellar cortex

Signs and symptoms include hypotonia, disequilibrium, and dyssynergy

Dyssynergy

Lack of coordinated action of the muscle groups

Cerebellar Ataxia

Staggering, wide-based gait; difficulty with turns; uncoordinated movement with positive Romberg sign

Intention Tremor

Involuntary trembling when an individual attempts a voluntary movement

Dysdiadochokinesia

Inability to perform rapid alternating movements

Dsyarthria

Difficulty speaking due to disruption of muscle control

Nystagmus

Involuntary, jerking movements of the eyes

Phenytoin Intoxication

Caused by use of the anti-epileptic drug, phenytoin

Symptoms: ataxia, nystagmus, gait disturbances, and dysarthria

Cerebellar Atrophy

Caused by certain specific inherited disorders

Symptoms: gait ataxia, dysarthria, intention tremor

Tonsillar Herniation

Caused by the cerebellar tonsils herniating through the foramen magnum, resulting in blockage of CSF flow

Symptoms: related to pressure on the cerebellum and medulla, involvement of CNs IX, X, XI, and XII

Arnold-Chiari Phenomenon

Displacement of hindbrain downward through foramen

magnum to obstruct CSF flow

Associated with craniovertebral anomalies, such as spina bifada