Motor Pathways: Upper and Lower Neurons

Overview

• The lecture discusses major motor pathways
• Focus is on identifying structures, tracts and differentiating pyramidal and extrapyramidal pathway functions

Objectives

• Contrast premotor cortical areas with the primary motor cortex in movement production
• Contrast corticospinal, corticonuclear, reticulospinal, and vestibulospinal tracts
• Describe the corticospinal tract's path from the motor cortex to the muscle
• Contrast deficits from upper motor neuron (UMN) lesions vs. spinal lower motor neuron (LMN) lesions

Contralateral Relationships

• Each cerebral hemisphere connects to the opposite side of the body via motor tracts
• Ipsilateral refers to the same side of the body

Major Components of the Motor System

• Motor Cortex (Cerebrum): Initiates motor actions by integrating somatosensory info, memory, learning, and decision-making processes
• Basal Ganglia (Cerebrum): Initiates and accelerates voluntary movements and suppresses extraneous movements
• Cerebellum: Coordinates motor actions with proprioceptive feedback and vestibular information and controls force, velocity, and timing of muscle action

Motor Control

• Movement is controlled by the cerebral cortex, thalamus, basal ganglia, cerebellum, brainstem, and spinal cord
• Complex, voluntary, motor tasks are planned and executed by the cerebral cortex
• Primary and premotor cortical areas influence movement directly via projections to the brainstem and spinal cord

Major Motor Pathways/Tracts

Pyramidal Tract (Activation Pathway)

• Corticospinal tract: Controls motor function of the torso and limbs
• Corticobulbar tract: Controls motor function of the head via cranial nerves

Extrapyramidal Tracts (Modulation Pathways)

• Rubrospinal tract: Has cerebellar influence on ventral horn motor neurons and affects force and duration
• Reticulospinal tract: controls sensitivity of muscle spindles
• Vestibulospinal tract: regulates posture and balance and is excitatory to extensor (anti-gravity) muscle groups

Cortical Areas Involved in Motor Activity

Supplementary Motor Area (SMA) and Premotor Cortex (PMC)

• Located in the frontal lobe (Brodmann's area 6)
• Programs the design and sequence of complex movements involving muscle groups
• Transmits intended movement "program" to the primary motor cortex for execution
• fMRI shows increased neural activity when movement is being planned
• Controls axial (trunk) and proximal limb (girdle) musculature to orient the trunk and limbs toward the intended movement direction

Frontal Eye Field (FEF)

• Located in the frontal lobe (Brodmann's area 8)
• Projects to brain stem centers that control ocular movements
• Used to coordinate eye movements
• Plays a role in visual tracking

Posterior Parietal Cortex

• Brodmann’s area 7
• Associated with visual guidance of movement
• Evaluates the position of the body and limbs, and forms a movement plan to accomplish tasks

Primary Motor Cortex (M-I)

• Located in the precentral gyrus (Brodmann’s area 4) of the frontal lobe
• Axons descend to terminate in the brainstem and spinal cord
• Functions in the execution of distinct, well-defined, voluntary movement
• fMRI shows this area lights up when intended movement is in progress
• Controls movement of the opposite side of the body

Primary Somatosensory (Somesthetic) Cortex (S-I)

• Located in the postcentral gyrus (Brodmann’s areas 3, 1, 2) of the parietal lobe
• Fibers descend to terminate in the brainstem and spinal cord
• Influences movement by modulating ("filtering") sensory input from visceral and somatic structures

Related Structures/Pathways

• Precentral gyrus is the location of the primary motor cortex
• Internal capsule is the confluence of motor fibers within the cerebrum
• Decussation of pyramids is the site where corticospinal motor fibers cross in the medulla

Motor Homunculus

• Each motor cortex displays a distorted representation of the contralateral side of the body
• The body's distortion reflects the precision of control that is needed

Layer V of the Cerebral Cortex

• The internal pyramidal layer is very prominent in the motor cortex
• Contains cell bodies of upper motor neurons whose axons form descending motor pathways

Upper Motor Neurons (UMNs)

• Cell bodies are in the motor cortex or brainstem
• Influence lower motor neurons (LMNs) in the brainstem or spinal cord

Lower Motor Neurons (LMNs)

• LMNs control body movement and reside in the ventral horn of the spinal cord
• Axons run in peripheral nerves that innervate skeletal muscle

Corticospinal Tract

• Is a long, descending tract involved in the motor control of the opposite side of the body
• Organized somatotopically throughout its entire path
• The motor homunculus reflects disproportionate innervation

Origin of Corticospinal Tract

• Cortex of the frontal and parietal lobes

Descent in the Brain

• Descends through the corona radiata, posterior limb of the internal capsule, basis pedunculi, pons, and medulla
• In the medulla, corticospinal tract fibers assemble to descend in the pyramid

Decussation

• The corticospinal tract splits into two bundles of axons when it reaches the caudal medulla
  • 85-90% of axons decussate in the pyramidal decussation to the opposite side
  • the remaining 10-15% descend on the same side of origin

Descent in Spinal Cord

• Crossed axons descend in the lateral funiculus as the lateral corticospinal tract
• These axons synapse at all spinal cord levels, but mostly in the cervical and LS levels (for limb control)
• This tract controls the distal muscles of the upper limb and involved in skilled movement of the fingers
• Uncrossed axons descend as the anterior corticospinal tract
• These axons cross near their level of termination
• This tract controls axial muscles (neck, shoulder, trunk)

Upper Motor Neuron (UMN) Lesion Signs

• Involves UMNs from the motor cortex to their termination in the brainstem or spinal cord

Spastic Paralysis

• Initially, UMN lesions cause temporary muscle weakness or flaccid paralysis
• Hypotonia and hyporeflexia develop
• Function of proximal limb muscles and crude movements of the affected limbs may return over time

UMNs Impact

• Neighboring descending motor tracts may intermingle as the corticospinal tract descends. Damage here results in UMN lesion signs
• UMNs transmit messages from the motor cortex, cerebellum, and basal ganglia to the LMNs
• UMN damage interrupts inhibitory and excitatory influences to LMNs
• Gamma motor neurons become overactive due to diminished inhibitory influence, leading to hypertonia
• Stretching of muscles leads to contraction of extensors, and extension of the hand

Severe Hypertonicity/Spasticity

• Affects distal limb muscles and fine movements most
• Is caused by exaggeration of the stretch reflex and causes resistance to passive motion

Note

• There's no resistance to passive stretch if a muscle is hypotonic

Hyperreflexia, Muscle Atrophy, and Babinski Sign

• UMN lesions cause hyperreflexia due to overactive gamma motor neurons
• Mild muscle atrophy and Babinski sign (extensor plantar response) may develop
• Babinski sign in adults indicates damage to the lateral corticospinal tract

Understanding Spasticity, Hypertonia, and Hyperreflexia

• Not well understood, but thought to occur because Gamma motor neurons receive messages from UMNs for muscle tone
• UMN lesion diminishes inhibitory influences leading to overactive gamma motor neurons and causes hypertonia
• Spinal cord stimulation of inhibitory interneurons may be diminished causing hypertonia and spasticity
• Agonist muscles are impacted because UMNs stimulate inhibitory interneurons which inhibit LMNs to the antagonist muscles

Lower Motor Neuron (LMN) Lesion Signs

• Involves the cell bodies of LMNs in the ventral horn of the spinal cord and/or their axons
• Signs include decreased muscle strength and tone (hypotonia) because the motor neurons are damaged and the signals cannot get through
• There is a weakening/absence of reflexes (hyporeflexia or areflexia)
• The reflex arc is interrupted because there are damaged motor neurons
• Can also lead to flaccid paralysis, severe muscle atrophy, fibrillations and fasciculations

The Corticospinal Tract's Blood Supply

• Precentral gyrus
  • Medial surface (hip, leg, and foot area) is supplied by the anterior cerebral artery
  • Lateral surface (trunk, upper limb, head area) is supplied by the middle cerebral artery
• Posterior limb of the internal capsule is supplied by lenticulostriate arteries
• Midbrain is supplied by the posterior cerebral artery and basilar artery
• Pons is supplied by pontine arteries of the basilar artery
• Rostral medulla is supplied by the anterior spinal artery and vertebral artery
• Caudal medulla & Cervical spinal cord supplied by the anterior spinal artery

Explore major motor pathways: structures, tracts, and functions. Differentiate between pyramidal and extrapyramidal pathways. Contrast upper motor neuron (UMN) and lower motor neuron (LMN) lesion deficits.