Corticofugal Systems and the

Control of Movement

Alejandro F. Diaz,MDMaria Antonia Moral-Valencia, MD

Motor System Hierarchy

Lecture Outline

Corticospinal Tracts – Origin , Course and termination

Corticonuclear Tracts- Origin, Course and Termination

UMN versus LMN signs Cerebellar and Pallidal influences on

Motor output Pathologic states

Corticospinal Systems Neurons arise from layer V of the cerebral cortex Betz cells account for only 1-2% of the CST bundle Found mainly in 6 areas of the brain:

Primary motor cortex (MI) Area 4- 31% Premotor cortex Area 6 >60% Supplementary motor cortex Area 6 -29% Post central gyrus Areas 3,1,2 Superior parietal lobule- Areas 5, 7 40% Cingulate gyrus

Motor Homonculus Anterior paracentral

area- foot, leg, thigh Medial 2/3 precentral

gyrus-trunk, UE Lateral 1/3 precentral

gyrus-head, face and oral cavity

Disproportion of body part size in the homonculus reflects the density of neurons devoted to the control of a particular region.

Corticospinal Tract Course Motor cortex Corona radiata Posterior limb of the

Internal Capsule Middle third of the midbrain

crus cerebri Basilar pons 80-90% cross at the

medullospinal junction Crossed fibers end as the

lateral CST ( lateral funiculus)

Uncrossed- anterior CST (ant. Funiculus)

Termination of the CST Topographically organized Fibers from the frontal lobe synapse in lamina

VII-IX of the intermediate zone and ant. Horn Fibers from the parietal lobe synapse in lamina

IV-VI in the posterior horn Most fibers terminate in the SC enlargements.

55% in the cervical, 25% lumbosacral

Corticonuclear System Consists of cortical

neurons that influence the movements of striated muscles of the head innervated by: Cranial nerves V,VII and

XII Nucleus ambiguus CN IX

and X Accessory nucleus

Origins of the Corticonuclear Systems

Originates from the face and head area of the precentral gyrus- face motor cortex

CN III, IV and VI nuclei do not receive input from the face motor cortex. Eye movement is mediated by the frontal and parietal motor eye fields that connect with the midbrain and pontine reticular formation. Not included in the Corticonuclear system.

Corticonuclear fibers send equal numbers of fibers to the left and right motor nuclei in the brainstem with the following exemptions ( predominantly contralateral input): Control of the soft palate and uvula Tongue Lower half of the face

Corticonuclear tract Course Face Motor cortex Genu of the Internal

capsule At the brainstem they

arch superiorly into the: Trigeminal nuclei Facial nuclei Hypoglossal nucleus Nucleus Ambiguus

Other Corticofugal Systems

Corticorubral system Cortical projections to the red nucleus

from areas 4,6 and to a lesser extent 5 and 7

Primarily influences flexor musculature Supplements the function of the CST.

Other Corticofugal Systems

Corticoreticular system Pontine and medullary nuclei give rise to

the reticulospinal tracts receive cortical input from the premotor cortex and supplementary motor cortex.

Influences extensor muscles including the paravertebral extensors.

Other Corticofugal Systems Corticopontine

Axons from nearly all regions of the cerebral cortex contribute to the corticopontine projection.

Fontopontine, temporo-, parieto- and occipitopontine fibers synapse with the ipsilateral basilar pontine nuclei which send their axons to the cerebellum .

Serves as an important route of communication between th cerebral cortex and the cerebellum

Motor Related Cortical Areas Primary Motor Cortex

Functions mainly to EXECUTE movement Does not simply code for flexion and

extension of muscles but also the regulation of movement in terms of the amount of force required to make the movement.

Motor cortical neurons are informed of the result of their output through the long-latency reflex (sensory info from SI)

Supplementary Motor Cortex Occupies Brodmann Area 6 Contain a less precise homonculus Receives input from the parietal lobe Projects to the MI and reticular formation of

the Spinal Cord In contrast to single muscle movements of

the MI, these involve sequences or groups of muscles and orient the body or limbs in space.

ORGANIZING and PLANNING of movement

Premotor Cortex Occupies a portion of Area 6 Contains a less precise somatotopic

representation of the body musculature Receives input from the sensory areas of the

parietal cortex Projects to the MI and reticular formation of the

Spinal cord Involved in PREPARATION to move Organizes the postural movements required to

make a movement.

Posterior Parietal Cortex Comprised of Brodmann Areas 5 and 7 Carry out “background computations” for movements in

space. Project to the supplementary motor and premotor

cortices with few spinal and brainstem targets Area 5 receives projections from the somatosensory

areas and vestibular system (hand manipulation neurons)

Area 7 processes visual information related to the location of objects in space (eye-hand coordination neurons)

Cingulate Motor Cortex

Because of the proximity to the limbic lobe , these may be involved in movements that have an intense motivational or emotional component

Final Common Pathway

UMN versus LMN Signs LMN SIGNS

Flaccid paralysis Atrophy Fibrillations and

fasciculations Hypotonia Areflexia

UMN SIGNS Initially flaccid

paralysis which becomes spastic

Hypertonia Hyperreflexia Pathologic reflexes

e.x. Babinski sign

Cerebellar and Pallidal Influences

Cerebellar nuclei and Globus Pallidus project to the ventral anterior and ventral lateral and oral parts of the ventral posterolateral nuclei of the dorsal thalamus.(motor areas of thalamus)

Motor areas of the thalamus gives rise to thalamocortical projections . Pallidal origins project mainly to the supplementary motor cortex and cerebellar inputs project to the Primary motor cortex.

Signal transmission in the CST and CNT systems can be modified by outputs from the cerebellum,basal nuclei and thalamus

DIRECT PATHWAY

NET EFFECT: ↑ activity of the thalamus excitation of the cerebral cortex

INDIRECT PATHWAY

NET EFFECT: ↓ activity of thalamus ↓ activity of the cerebral cortex

What happens in Parkinson’s Disease?

Motor Loop

Hypokinetic Disturbances Akinesia- impairment in the initiation of movement

secondary to an impairment in the ability to plan or guide a movement

Bradykinesia- a reduction in the velocity of and amplitude of movement due to the imbalance of outflows to the thalamus of the direct and indirect pathways.

Both are characteristic of patients with Parkinson’s disease

Considered as lesions of the neostriatum causing the thalamus to not be disinhibited leading to a decrease flow of information through the thalamus to the cortex

Hyperkinetic Disturbances Take the form of dyskinesias Ballismus- uncontrolled flinging (ballistic) of

an extremity, most commonly seen in lesions of the contralateral subthalamic nucleus

Chorea- generalized irregular ( brisk) dance-like movements of the limbs

Athetosis- continuous writhing of the distal portions of the extremity

Caused by the disruption of the indirect pathway resulting in the loss of excitatory subthalamopallidal neurons

Cerebellar influences

Vestibulocerebellar module-(archicerebellum) influences posture, balance and equilibrium including orienting the eyes during movement. Deficits produce: ataxia, titubation, nystagmus and deficits in pursuit eye movements

Cerebellar influences

Spinocerebellar module- focused primarily on the control of axial musculature through the vermis and fastigial efferents and the limb throught the globose and emboliform nuclei.

Cerebellar influences

Pontocerebellar module- functions in the planning and control of precise dexterous movements of the hand, arm, forearm and in the timing of these movements.

Lesions in the cerebellar hemisphares result in motor deficits on the ipsilateral side of the body.

Disturbances Decomposition of movement- dyssynergia Hypotonia Ataxia Dysmetria (past-pointing) Kinetic or intention tremor Dysdiadochokinesia awkward rapid

alternating movements Rebound phenomenon or impaired check Dysarthria Nystagmus

Summary Motor system control is achieved by parallel

systems formed by somatotopically organized, descending cortical projections that link the various motor related areas of the cortex more with spinal motor circuits.

Such that each pathway contributes its own element or series of elements to movement control.