Motor Systems: Spinal Cord & Muscle Anatomy

• Motor systems involve the structures in the nervous system responsible for controlling movement
• Dana McPhee PT, MS, NCS and Megan Slayton, PT, DPT, NCS are credited for the presentation
• It was created for the SFCC PTA/OTA Program, Winter 2023

Spinal Cord Anatomy Review

• The dorsal horns of the spinal cord contain sensory information.
• The ventral horns contain motor neurons.
• Ascending pathways relay sensory information to the brain.
• Descending pathways transmit motor commands from the brain to the spinal cord.

Muscle Anatomy

• Skeletal muscle is excitable, contractile, extensible, and elastic
• Muscle fiber contains myofibrils.
• Myofibrils contain sarcomeres delineated by Z lines.
• Sarcomeres which are composed of actin and myosin myofilaments
• Muscle contraction occurs through the sliding of actin relative to myosin.

Muscle Contraction Steps

• A nerve impulse reaches the neuromuscular junction, causing the release of acetylcholine (ACh).
• ACh diffuses across the synaptic cleft, opening Na+ channels in the muscle membrane.
• Na+ entry depolarizes the muscle cell.
• T-tubules transmit impulses to the sarcoplasmic reticulum, releasing Ca2+ ions.
• Ca2+ binds to troponin, exposing actin binding sites.
• Myosin binds to actin, forming cross-bridges.
• ATP is used to facilitate cross-bridge movement and subsequent actin strand pulling.
• Another ATP molecule is used for the recovery stroke which is responsible for the bend, attach and pull.
• When the action potential ceases, Ca2+ is pumped back into the sarcoplasmic reticulum, and tropomyosin covers the binding sites, preventing further myosin binding.

Sliding Filament Model

• Active sites on actin are exposed, allowing myosin heads to bind
• Myosin heads bend, pulling thin filaments
• Each head releases, binds to the next active site, and pulls again
• Action results in entire myofibril shortening

Upper vs. Lower Motor Neurons

• Upper motor neurons (UMNs) located in the cerebral cortex or brainstem, transmit motor impulses to the lower motor neurons. UMNs use glutamate
• Lower motor neurons (LMNs) located in the brainstem and spinal cord, transmit impulses from the UMNs to the muscles. LMNs use acetylcholine
• UMN damage results in Spastic paralysis, Increased DTR's, Pathological reflex present, Fasciculations absent
• LMN damage results in Flaccid paralysis, Severe atrophy, Absent DTR's, Absent pathological reflex, Fasciculations present

Motor Tract Lesions

• Signs of motor tract lesions include paresis/paralysis, abnormal reflexes, myoplasticity, abnormal muscle tone, loss of fractionated movements, and abnormal muscle synergies
• Hemiplegia is weakness on one side of the body
• Paraplegia affects the body below the arms
• Tetraplegia affects all four limbs
• Paresis in MT lesions from inadequate MN facilitation
• Paralysis occurs in muscles below a complete spinal cord lesion

Abnormal Cutaneous Reflexes

• Babinski's sign is extension of the great toe with fanning of other toes
• Test by stroking the lateral sole firmly
• Muscle stretch hyperreflexia, clonus, and clasp-knife response are common abnormal reflexes in chronic SCI

Clonus

• Clonus is involuntary, repeating, rhythmic muscle contractions
• Unsustained clonus fades after a few beats
• Sustained clonus is pathologic in origin, activates oscillating neural networks due to a MT control lack in the spinal cord

Clasp-Knife Response

• Clasp-knife response occurs when a paretic muscle is slowly stretched and resistance drops notably
• A change in resistance akin to a pocketknife opening resistance
• Type II afferents elicit the clasp-knife response

Muscle Tone Assessment

• Muscle tone is assessed via PROM

• Resistance to passive stretch in a resting muscle

• Potential findings include:

  • Hypotonia
  • Hypertonia
  • Spasticity
  • Rigidity
  • Modified Ashworth Scale

Abnormal Muscle Tone

• Resistance to stretch in resting muscle
• Muscle tone categorized on a continuum
• Resistance ranges from flaccid through normal to hypertonic to rigidity
• Velocity-dependent hypertonia is referred to as spasticity

Rigidity

• Resistance to passive movement
• Remains constant regardless of speed
• Velocity-dependent hypertonia
• Decerebrate rigidity: Rigid extension of upper limbs
• Decorticate rigidity: Flexed upper limbs, extended neck/lower limbs, plantarflexion

Disorders of Motor Neurons

• Lower motor neuron disorders include trauma, metabolic disorders (alcohol, diabetic neuropathy), myasthenia gravis, polio, Guillain-Barré, brachial plexus injury, carpal tunnel syndrome, multiple sclerosis, coma.
• Upper motor neuron disorders include amyotrophic lateral sclerosis, multiple sclerosis, cerebral vascular accident, cerebral palsy, tumors, trauma, infections, hypoxia.

Lower Motor Neurons

• The only neurons to convey signals to muscle spindle fibers
• Cell bodies in ventral horn
• Axons leave SC at ventral nerve root
• Neurons projecting to a single muscle form a motor pool

Motor Neurons

• Motor neurons in the medial ventral horn innervate muscles of the neck & back (axial & proximal)
• Motor neurons in the lateral ventral horn innervate distal muscles
• Motor neurons in the anterior ventral horn innervate extensor muscle groups
• Motor neurons in the posterior ventral horn innervate flexor muscle groups

Alpha and Gamma Motor Neurons

• MNs have large cell bodies and large myelinated axons
• Innervate extrafusal skeletal muscle, responsible for muscle contraction
• Medium sized. myelinated axons
• Project to intrafusal fibers in the muscle spindle
• Gamma motor neurons innervate what???
• Maintain the sensitivity of the muscle spindle throughout the normal range of muscle length
• Responsible for Proprioception
• Action results in Isometric mm contraction

Muscle Spindle vs. Golgi Tendon Organ

• Muscle spindles are small spindle-shaped sensory organs sensing length changes and lengthening rate
• Golgi tendon organs are sensory organs in the muscle-tendon unit sensing muscle tension changes
• During contraction, muscle spindles contract
• Golgi tendon organs however do not contract
• Muscle spindle protective function results in the Stretch reflex and reciprocal inhibition
• Golgi tendon organs leads to Autogenic inhibition
• Differentiated muscle fibers are held in a connective tissue sac
• Golgi tendon organs are braided collagen strands which are encapsulated

Golgi Tendon Organ

• GTO's Autogenic inhibition results in inhibition within same muscle stimulated
• Reciprocal Inhibition creates inhibition within functional opposing muscle

Reflex Characteristics

• Reflexes are rapid, preprogrammed, involuntary responses.
• Reflexes require a stimulus, and are always the same.
• Reflexes are a survival mechanism

Reflex Arc

• The neural pathway is responsible for generating the response
• Stimulus activates receptor
• Nerve signal propagated via sensory neuron to SC
• Signal processed in integration center by interneurons
• Signal propagated by motor neuron to effector
• Effector responds

Monosynaptic vs. Polysynaptic Reflexes

• Monosynaptic reflexes have direct communication between sensory & motor neurons (e.g., stretch reflex)
• Polysynaptic reflexes have interneurons facilitating sensory-motor communication (e.g., withdrawal reflex)

Involuntary Muscle Contractions

• Muscle cramps
• Fasciculations
• Myoclonus
• Fibrillations
• Tremors
• Movements related to BG dysfunction

Motor Neuron Lesions

• Interrupting MNs signal reduces/prevents muscle contraction
• Damage to MNs caused by trauma, demyelinating diseases (GBS), infection, or chronic neuropathy (DM)

Diagnostic Studies

• Nerve conduction studies
  • Useful as a diagnostic tool
  • Can differentiate between dysfunction at different sites
    • the nerve
    • neuromuscular junction
    • the muscle
• Electromyography (EMG):
  • Useful as a diagnostic tool
  • Can differentiate denervated muscle from myopathy

Motor Tracts

• Arise in cerebral cortex or brainstem
• Axons travel in descending tracts to synapse with MNs/interneurons in brainstem/spinal cord
• Cerebellum/BG are key in adjusting motor tract activity
• Classified according to the location of synapse

Descending Tracts

• They control movement, muscle tone, spinal reflexes, spinal autonomic functions, and sensory transmission modulation.
• They are divided into pyramidal (corticospinal) and extrapyramidal tracts (rubrospinal, vestibulospinal, reticulospinal, tectospinal).
• Each descending tract contains two interconnecting neurons
  • First-order motor neurons originate in the cerebral cortex/brainstem and travel down the spinal cord, synapsing in the anterior gray horn.
  • Second-order neurons travel from the spinal cord to skeletal muscles.
• There are two main pathways for cortex/brainstem to control movement: medial and lateral.
  • Medial pathways control axial and proximal muscles, while lateral pathways control distal extremity muscles

Reticulospinal Tract

• Begins in the reticular formation
• Facilitates bilateral motor neurons innervating postural & gross limb movement muscles
• Coordinates movement of proximal muscles during walking
• Involved in anticipatory postural adjustments & reaching
• Creates a pathway for neck reflexes to respond to visual/auditory input

Vestibulospinal Tract

• Lateral vestibulospinal tract
  • Lateral vestibular nuclei respond to gravity from vestibular apparatus I- psilateral motor neurons facilitate extensors, inhibiting ipsilateral motor neurons to flexors
  • Excites extensor muscles of trunk and proximal anti-gravity muscles
  • Maintains person's COG within BOS
• Medial vestibulospinal tract
  • Medial vestibular nuclei get info about head movement positions from the vestibular apparatus
  • Projects bilaterally to cervical & thoracic spinal cord
  • Impacts Motor neurons to control neck/upper back muscles

Rubrospinal Tract

• Non-pyramidal route
• Motor cortex & cerebellum which influence spinal motor actiivity
• Originates in the Red nucleus and ends at levels of the spinal cord
• Controls tone of limb flexor muscles
• Has an important role in a number of body reflexes

Tectospinal Tract

• Aka colliculospinal tract
• Originates in superior colliculus, crosses, descends in brainstem
• Mediates reflex postural movements of head/neck, in response to visual/auditory stimuli directing eyes/head towards objects in visual field

Anterior Corticospinal Tract

• Originated in the cerebral cortex, fiber tracts do not cross to the pyramidal dessucation
• Contralateral with spinal cord, terminate in that side of the Spinal cord
• Controls axial and proximal muscles of the neck and trunk

Lateral Corticospinal Tract

• Originates in cerebral cortex: premotor, primary motor, and supplementary motor areas
• Fibers cross in pyramidal decussation
• Synapses onto neurons to control muscles of extremities