Motor Pathways: UMN and LMN

Clinical Neuroanatomy of Motor Disorders: Motor Pathways

• The main anatomical pathways, key regions, and functions related to voluntary movements and motor control are discussed

Objectives

• The main anatomical pathways responsible for voluntary movements are understood
• UMN (Upper Motor Neuron) and LMN (Lower Motor Neuron) roles are clear
• The anatomical regions associated with motor pathways are understood
• Motor dysfunction's anatomical basis can be explained
• Signs of LMN vs. UMN lesions are understood and can be localized
• A general understanding about movement control is developed

Motor Pathways

• The motor pathway consists of 2 sets of neurons: UMN and LMN
• The motor pathway is responsible for control of skeletal muscles for voluntary movements and control of balance and posture
• UMNs control the LMN function
• LMNs directly innervate the skeletal muscles and is the final common pathway

Lower Motor Neurons

• Cranial motor nerves innervate the skeletal muscles of the head and neck
• Spinal motor nerves innervate the trunk and limbs

Location of LMN in Spinal Cord

• The cell bodies of LMN for spinally innervated muscles are in the anterior horn
• Cell bodies of distal limb muscles are placed laterally
• Lower limb UMNs are located more peripherally
• Proximal limb and trunk muscles are placed medially

Arrangement of LMN of Cranial Nerves in Brainstem

• LMN cell bodies for muscles innervated by cranial nerves are in the motor nuclei of respective cranial nerves
• Midbrain cranial nerve nuclei contain the oculomotor and trochlear nuclei
• The pons contains the trigeminal motor nucleus and the superior oblique
• The medulla contains the hypoglossal nucleus and the nucleus ambiguus which are involved in swallowing and speech

Regulation of LMN

• LMNs are controlled by supraspinal centers with upper motor neurons
• Direct control includes:
  • Brainstem level for posture and muscle tone control
  • Cerebral cortex level for voluntary movements
• Indirect control includes:
  • Basal ganglia for planning, supervision, and regulation
  • Cerebellum for coordination

Upper Motor Neurons

• UMNs start from any supraspinal centers
• Multiple UMNs innervate the LMNs
• Primary motor cortex has Pyramidal pathways for voluntary movements
• Upper limb UMNs are located more centrally
• Brainstem Nuclei have Non-Pyramidal pathways
• These UMNs constitute the motor pathways

Motor Pathways – UMN – Non-Pyramidal / Extrapyramidal

• Non-Pyramidal pathways start from:
  • Red Nucleus which carries the Rubrospinal pathway
  • Vestibular Nucleus which carries the Vestibulospinal pathway
  • Reticular Nucleus which carries the Reticulospinal pathway, influences the lower motor system, important for posture, balance and extensor tone, and is located in the brainstem
  • Tectal Nucleus which carries the Tectospinal pathway and is for head and eye movement

Loss of Cortical Inhibition on Non-Pyramidal Pathways

• Decorticate posture: this is rigidity that results from lesions above the midbrain
• Both Rubrospinal and Vestibulospinal pathways are intact which increases tone and flexion, with increased flexion and extension in lower limbs
• Decerebrate posture: this is rigidity resulting from lesions in the lower midbrain or upper pons where Rubrospinal pathways are not intact which means legs are extended, toes are pointed away from the body and turned inward, arms are tensed, and rotated inward

Motor Pathways – UMN – Pyramidal

• Pyramidal pathways have voluntary movements
• Upper Motor Neurons terminate in the brainstem (cortical bulbar) or spinal cord (cortical spinal fibers)
• Structures include:
  • Primary Motor Cortex
  • Corticobulbar Pathway from cortical areas through the midbrain
  • Lateral Corticospinal Pwy
  • Anterior Corticospinal Pwy
  • Lower Motor Neurons

Lateral Cortico-Spinal Pathway (LCSP)

• LCSP innervates distal limb muscles and controls skilled/fine movements
• LCSP starts in the UL and LL region of the motor cortex and goes through the corona radiata -> posterior limb of the internal capsule -> passes the cerebral peduncle ->anterior pons --> medullary pyramid and decussates and runs into the lateral white column of the spinal cord before synapsing with a LMN
• UL muscle axons descends to the cervical region and LL muscles descend to the lumbosacral spine

Lateral Cortico-Spinal Pathway (LCSP) - Lesions

• Medial aspect of primary motor deficit cortical area lesions on the right will result in upper motor paralysis of the left -- right side of fibers controlling the left side
• A lesion above the posterior limb of the internal capsule causes damage to cortical spinal fibers and upper MN paralysis
• disrupt motor signals to lower motor neuron paralysis on left side causes motor weakness below

Anterior Cortico-Spinal Pathway (ACSP)

• The ACSP innervates axial/trunk muscles and proximal limb girdle muscles to help to maintain posture
• The pathway starts in the trunk region of M1 --> passes through corona radiata -> posterior limb of internal capsule -> cerebral peduncle --> anterior pons -> medullary pyramid --> before passing into the anterior white column without decussating
• Anterior corticospinal tract are uncrossed - each segment, cross and synapse to contralateral side
• Dual cortical innervation is received by the trunk and proximal limb muscles because at each spinal segment the ACSP neurons synapse with LMN supplying ipsilateral and contralateral trunk and proximal limb muscles

Anterior Cortico-Spinal Pathway (ACSP) - Lesions

• Lesions on the left mean motor weakness on the right, contralateral. This affects axial and proximal muscles and leads to complete paralysis of axial and proximal sides bilaterally

Location of Motor Pathways in Spinal Cord

• The ventral corticospinal tract is a descending tract located in the ventral white commissure
• The lateral corticospinal tract and rubrospinal tract are also descending tracts

Somatotopic Organization of Lateral Corticospinal Pathway in Spinal Cord

• Motor pathway neurons (UMN) for LL are located more peripherally, while Upper limb UMNS are located more centrally
• Central lesions -> Sacral sparing (less LL muscle involvement)
• Similar to main sensory pathways