Motor Pathways: Pyramidal vs Extrapyramidal

Overview of Motor Pathways

• This lecture focuses on major motor pathways
• Emphasis is put on identifying key structures and tracts
• Also differentiating the functions of the pyramidal versus extrapyramidal pathways.

Objectives

• Compare the corticonuclear tract to the corticospinal, reticulospinal, and vestibulospinal tracts
• Understand the general functions and fiber courses of the corticotectal, corticorubral, corticoreticular, and vestibularspinal tracts
• Learn the general path of the corticonuclear tract
• Compare deficits after upper motor neuron (UMN) lesions with lower motor neuron (LMN) lesions involving the corticonuclear tract

Cranial Nerve Motor Nuclei and Motor Innervation

• Cranial nerves III, IV, V, VI, VII, IX, X, XI, and XII provide motor innervation to head and neck muscles
• The trigeminal, facial, glossopharyngeal, vagus, accessory, and hypoglossal nerve motor nuclei receive corticonuclear (UMN) axon terminals

Corticonuclear Tract Function

• The corticonuclear tract transmits motor commands to control movement of the jaw, face, palate, tongue, pharynx, larynx, and neck via UMN axons

Specific Nuclei and UMN Projections

• Trigeminal motor nucleus (CN V) receives bilateral UMN projections, except for LMNs innervating the lateral pterygoid muscle, which receive contralateral projections
• Facial nucleus (CN VII) receives bilateral UMN projections in its upper half and contralateral projections in its lower half
• Nucleus ambiguus (serving CN’s IX and X) receives bilateral UMN projections
• Accessory nucleus (CN XI) receives ipsilateral corticonuclear projections
• Hypoglossal nucleus (CN XII) receives bilateral UMN projections, except for the LMNs innervating the genioglossus muscle, which receive contralateral projections

Motor Eye Field Input to Specific Nuclei

• Oculomotor, trochlear, and abducens motor nuclei receive motor messages from the frontal and parietal motor eye fields
• These messages travel via a different group of axons, stopping in the reticular formation and projecting to cranial nerves III, IV, and VI

Corticonuclear Fibers and UMNs

• Corticonuclear fibers are axons of upper motoneurons (UMNs)
• Fibers descend alongside corticospinal fibers to the brainstem

Corticonuclear Fiber Termination

• At the pontine level, corticonuclear fibers exit the tract to terminate bilaterally on interneurons near the V motor nucleus
• These interneurons then project to the V motor nucleus.

Lateral Pterygoid Muscle Innervation

• LMNs innervating the lateral pterygoid muscle receive contralateral corticonuclear projections

Trigeminal Motor Nucleus Function

• The V motor nucleus innervates muscles of mastication which involves moving the jaw up and down, and side to side during chewing and speaking

Corticonuclear Fiber Termination in the Facial Nucleus

• Motor cortex sends ~equal ipsilateral and contralateral UMN projections to the superior half of the facial nucleus
• The superior half contains LMNs that innervate the muscles of the upper half of the face

Corticonuclear Fiber Innervation of the Inferior Half of the Facial Nucleus

• The motor cortex sends predominantly contralateral UMN projections to the inferior half of the facial nucleus
• The inferior half contains LMNs that innervate the muscles of the lower half of the face

Facial Nucleus Lesions Manifestation

• Upper half of the face is spared because it receives bilateral corticonuclear (UMN) projections, allowing for "backup" projections from the opposite side
• The lower half of the face shows weakness/paralysis contralateral to the lesion because it predominantly receives contralateral corticonuclear projections with few "backup" UMN fibers

Vulnerability of Lower Facial Muscles

• The muscles of the lower half of the face are more susceptible to weakness/paralysis following an UMN lesion

Motor Cortex Lesions Affect on Face and Limbs

• A lesion in the primary motor cortex impacting face and upper limb control may cause deficits on the same side of the body
• The primary motor cortex area for the face is near the areas for the hand and tongue

Corticonuclear Fiber Proximity

• Fibers travel close to corticospinal fibers in the internal capsule, basis pedunculi (crus cerebri) of the midbrain, basilar pons, and pyramid of the medulla
• A lesion damaging corticonuclear fibers may also damage axons of the corticospinal tract

UMN Lesions

• UMN lesions involving both corticonuclear and corticospinal tracts can lead to symptoms in muscles of the upper and/or lower limbs on same side as facial weakness

Corticonuclear Projections and Nucleus Ambiguus

• Nucleus ambiguus receives bilateral corticonuclear projections
• Contains cell bodies of LMNs that run in branches of the glossopharyngeal and vagus nerves
• These nerves innervate muscles of the soft palate, larynx, and pharynx

Corticonuclear Projections and Accessory Nerve

• Corticonuclear projections to the accessory nucleus are ipsilateral
• The UMNs synapse with LMNs in the accessory nucleus of the cervical spinal cord, projecting to sternocleidomastoid and trapezius muscles

Corticonuclear Projections and Hypoglossal Nucleus

• Hypoglossal nucleus receives bilateral corticonuclear projections
• Motor cortex projects bilaterally to neurons that innervate most tongue muscles, except those for the genioglossus

Genioglossus Muscle Control

• Neurons of the hypoglossal nucleus innervating the genioglossus muscle mainly receive contralateral corticonuclear input

XII Nerve Lesions Impact

• If fibers destined for the XII nucleus are damaged, there are no deficits in most tongue muscles

Genioglossus Muscle Weakness

• Because the genioglossus muscle receives contralateral UMN projections, damage results in weakness and deviation to the weak side upon protrusion

Motor Cortex and Eye Movements

• The motor cortex influences eye movements via the reticular formation
• This involves the frontal and parietal motor eye fields projecting to the midbrain reticular formation and PPRF

Oculomotor Nerve Function

• Motor nuclei of oculomotor (III), trochlear (IV) and abducent (VI) nerves produce conjugate eye movements contralateral to the origin of cortical input.

Blood Supply to the Corticonuclear Tract

• Precentral gyrus (lateral surface, head area) is supplied by middle cerebral artery
• Genu and posterior limb of the internal capsule supplied by lenticulostriate arteries
• Midbrain supplied by posterior cerebral artery and basilar artery
• Pons supplied by pontine arteries from the basilar artery
• Rostral medulla supplied by anterior spinal artery and vertebral artery
• Caudal medulla supplied by anterior spinal artery

Corticospinal Tract

• Lesions affecting corticospinal tract often involve corticonuclear tract due to their close proximity
• Corticonuclear tract descends in genu of internal capsule and shifts posteriorly into anterior posterior limb

Tract Position

• Corticospinal tract descends in anterior half of posterior limb of internal capsule
• The corticonuclear tract descends medial to the corticospinal tract in the basis pedunculi of the midbrain

Fiber Association

• Corticonuclear tract fibers mingle with corticospinal tract fibers in the basis pontis and pyramid in the medulla

Corticotectal Tract Origin

• The Origin is from Visual association cortex, of occipital lobe

Corticotectal Tract Termination

• Termination is in the following nuclei of the midbrain
• Oculomotor accessory nuclei, which project via the MLF to nuclei that control extraocular muscles for reflex eye movements
• Superior colliculus which is a reflex center of the visual system

Tectospinal Tract Origin

• Origin of the tectospinal tract is the superior colliculus

Tectospinal Tract Termination

• Termination of the tectospinal tract occurs at the cervical and upper thoracic spinal cord levels
• Coordinates movements of the head with the eyes in response to visual, auditory, vestibular, or cutaneous stimuli

Auditory Example

• Loud noise leads to reflex turning of the head and upper thorax
• Process involves ears, auditory nuclei in brainstem, inferior/superior colliculi, tectospinal tract, and neck/upper thoracic spinal cord levels

Cutaneous Example

• Feeling something crawling on your hand leads can trigger the tectospinal tract.

Corticorubral Tract Origin

• The sensoritmotor cortex gives rise to fibers that form this tract

Corticorubral Tract Termination

• This tract terminates in the red nucleus of the midbrain

Rubrospinal Tract

• Red nucleus gives rise to the rubrospinal tract
• It crosses in the midbrain
• It descends to the upper spinal cord

Rubrospinal Tract Function

• Controls movements of the hand and digits
• Facilitates activation of motor neurons to distal flexors
• Inhibits activation of motor neurons to distal extensors
• Supplements the function of the corticospinal tract
• More important in other primates

Rubrospinal Tract Makeup

• The rubrospinal tract consists of UMN's
• A lesion will result in UMN signs.

Corticoreticular Fibers

• Motor cortex gives rise to corticoreticular fibers that descend to the brainstem reticular formation, located in the pons and medulla

Medial Reticulospinal Tract

• Pontine nuceli give rise to the medial (pontine) reticulospinal tract
• It descends to terminate at spinal chord levels
• Synapses occurs with interneurons and upper motor neurons

Medial Reticulospinal Tract Function

• Tract has excitatory influence on motoneurons that innervate paravertebral and limb extensors
• Inhibibits flexors
• Used when standing

Lateral Reticulospinal Tract

• A medullary nucleus gives rise to this tract
• Tract descends to terminate at all spinal chord levels
• Synapses occurs with interneurons

Lateral Reticulospinal Tract Function

• Tract has inhibitory influence on motoneurons that innvervate the paravetebral and limb extensors
• Excitatory for flexors
• Used when lying down

General Reticulospinal Tract Info

• Consists of UMNs
• A leasion will result in UMN sign
• Controlled by corticoreticular fibers arising in reticular formation
• Descends to terminate at spinal chord levels
• Gamma motor nuerons innervate antigravity muscles
• Plays a role in modulation of muscle tone and maintanence of posture from a spinal origin

Medial Vestibulospinal Tract

• Originates from Medial Vestibular Nucleus
• Terminates at cervical and Upper Thoracic spinal cord levels on interneurons, or with alpha motor neurons and innervate neck muscles
• Coordinates head and eye movements

Lateral Vestibulospinal Tract

• Originates from Lateral Vestibular Nucleus
• Terminates at all spinal chord levels on interneurons, or with alpha motor neurons.
• Excitatory to antigravity muscles
• Functions in maintainence of posture and balance, adjust muscle tone, and to regain posture

Overview of major motor pathways, focusing on identifying key structures and tracts. Differentiating the functions of the the pyramidal versus extrapyramidal pathways. Also comparing upper motor neuron (UMN) lesions with lower motor neuron (LMN) lesions.