Clinical Neuroanatomy: Motor Pathways

• The lecture covers the clinical neuroanatomy of motor disorders, focusing on motor pathways.

Objectives

• The main goal is to understand anatomical pathways responsible for movement.
• Clarify the roles of upper motor neurons (UMN) and lower motor neurons (LMN).
• Understand key anatomical locations and somatotopic organization of motor pathways.
• Explain the anatomical basis for motor dysfunction.
• Differentiate between signs of LMN vs UMN lesions and their localization.
• Develop a general understanding about movement control.

Role of Motor Pathways

• Responsible for control of skeletal muscles.
• Responsible for posture, balance, and voluntary movement.

Upper Motor Neurons (UMN) and Lower Motor Neurons (LMN)

• Motor pathways consist of 2 sets of neurons: LMN and UMN.
• UMNs control the function of LMNs.
• LMNs directly innervate skeletal muscles, forming the final common pathway.

Lower Motor Neurons

• Skeletal muscles of the head and neck are innervated via cranial motor nerves.
• The trunk & limbs are innervated by spinal motor nerves.

Location of LMN in the Spinal Cord

• Cell bodies of LMN for spinal nerves are located in the anterior horn of the spinal cord.
• LMNs for distal limb muscles are located laterally within the anterior horn.
• Proximal limb and trunk muscle LMNs are located medially.

Arrangement of LMN of Cranial Nerves in the Brainstem

• Cell bodies of LMN for muscles innervated by cranial nerves are in the motor nuclei.
• Oculomotor nucleus (CN III) mainly serves eye muscles.
• The Trochlear nucleus (CN IV) serves the extraocular muscle.
• The Trigeminal motor nucleus (CN V) serves the muscles of mastication.
• The Abducens nucleus (CN VI) serves the lateral rectus muscle.
• The Facial motor nucleus (CN VII) serves the facial muscles.
• Nucleus Ambiguous (CN IX, X) involved in pharynx/larynx control.
• The Hypoglossal nucleus (CN XII) controls the tongue.

Regulation of LMNs

• LMNs are controlled by supraspinal centers: brainstem and cerebral cortex.
• Brainstem controls posture and muscle tone.
• Cerebral cortex controls voluntary movements via signals from the precentral gyrus.
• Indirect Control via basal ganglia, planning, supervision, and regulation
• Indirect control via cerebellum, coordination
• Supraspinal centers control LMNs via UMNs.

Upper Motor Neurons (UMN) Origin

• UMNs originate from any supraspinal center.
• Multiple UMNs innervate LMNs.
• The UMNs constitute motor pathways.
• The UMNs that start from the primary motor cortex constitute the pyramidal tracts used for voluntary movements.
• Some UMNs that start from brainstem nuclei are part of the non-pyramidal tracts.

Motor Pathways - Non-Pyramidal/Extrapyramidal UMNs

• Non-pyramidal pathways originate from:
  • Red nucleus, giving rise to the rubrospinal tract.
  • Vestibular nuclei, giving rise to the vestibulospinal tract.
  • Reticular nuclei, giving rise to the reticulospinal tract.
  • Tectal nucleus, giving rise to the tectospinal tract
• The tectospinal tract controls head and eye movements, originating from the superior colliculus.
• The reticular formation influences lower motor neurons, regulates posture, balance, and extensor tone.

Loss of Cortical Inhibition and Rigidity

• Decorticate posture results from lesions above the midbrain; rigidity in upper limbs
• Decerebrate posture results from lesions in the lower midbrain/upper pons.
• In decerebrate posture, legs are extended, toes point away from body, arms are tensed and rotated inward.

Motor Pathways - Pyramidal System (UMN)

• Pyramidal pathways facilitate voluntary movements.
• Upper motor neurons originate in the cerebral cortex.
• Fibers terminate in brainstem as cortical bulbar, and in spinal cord as cortical spinal fibers.
• Pyramidal pathways include the corticobulbar tracts; originate from the cortical areas (head and neck).
• Corticospinal tracts supply the upper and lower limbs.
• Lateral corticospinal tracts cross midline and supply upper/lower limbs.
• Anterior corticospinal tracts run in the anterior coniculus of the spinal cord.

Lateral Corticospinal Pathway (LCSP)

• LCSP innervates the distal limb muscles for skilled/fine movements.
• Starts in the upper/lower limb region of M1.
• It passes through the corona radiata, posterior limb of internal capsule, cerebral peduncle, anterior pons, and medullary pyramid.
• Axons decussate at the lower end of pyramid.
• Controls the upper and lower limbs to the cervical and lumbosacral regions and synapse with LMN.

Lateral Corticospinal Pathway (LCSP) Lesions

• A medial aspect of primary motor cortical lesion on the right causes upper motor paralysis on the left.
• Lesions above the posterior limb of internal capsule cause damage to descending cortical spinal fibers, and upper motor neuron paralysis.
• A lesion on anterior cortical spinal tract on one side may cause motor weakness on one side.
• Disrupting motor signals to lower motor neurons results in paralysis and motor weakness.

Anterior Corticospinal Pathway (ACSP)

• The ACSP innervates axial/trunk muscles and proximal limb girdle muscles to maintain posture.
• Starts in the trunk region of M1.
• Passes through corona radiata, posterior limb of internal capsule, and cerebral peduncle to the pons.
• From the medulla, axons pass into the anterior white column without decussating.
• Neurons synapse with LMN in the ipsilateral & contralateral trunk & proximal limb muscles.
• The trunk and proximal limb muscles receive dual cortical innervation.

Termination of Anterior and Lateral CST

• The Lateral Corticospinal Tract (CST) runs lateral coniculus.
• Synapse with LMN to distal muscles, and distal limb muscles
• The Anterior Corticospinal tract crosses and synapse to contralateral side

Location of Motor Pathways in the Spinal Cord

• Descending tracts: Ventral Reticulospinal, Lateral Reticulospinal, Rubrospinal, and Corticospinal

Somatotopic Organization of Lateral Corticospinal Pathway in the Spinal Cord

• Motor pathway neurons for the leg are located more peripherally, and upper limb neurons located more centrally.
• Central lesions result in sacral sparing, with relatively less lower limb muscle involvement.