16.4 The Sensory and Motor Exams

The spinal cord is a vital hub for sensory and motor function, connecting the brain to the rest of the body. Its complex organization of gray and white matter enables the transmission of signals that control movement and process sensations like touch, pain, and temperature.

Injuries to the spinal cord can have profound effects on sensory and motor function, depending on the location and severity of the damage. Understanding the differences between upper and lower motor neuron diseases is crucial for diagnosing and treating neurological disorders affecting movement and muscle control.

Spinal Cord and Sensory/Motor Exams

Organization of spinal cord regions

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• Spinal cord organized into gray matter and white matter
  • Gray matter contains cell bodies of neurons
    • Dorsal (posterior) horn receives sensory input from peripheral nerves (pain, temperature, touch)
    • Ventral (anterior) horn contains motor neuron cell bodies that innervate skeletal muscles
  • White matter contains ascending and descending tracts of myelinated axons
    • Dorsal columns carry ascending sensory information related to proprioception and fine touch (cuneate and gracile fasciculi)
    • Lateral corticospinal tract carries descending motor information from the primary motor cortex to control voluntary movements
• Sensory and motor pathways organized somatotopically within the spinal cord
  • Each spinal cord level corresponds to a specific dermatome for sensory innervation (C5 dermatome covers the lateral upper arm) and myotome for motor innervation (C5 myotome includes the deltoid muscle)

Effects of spinal cord injuries

• Spinal cord injuries result in sensory and/or motor deficits depending on the level and severity of the injury
• Complete transection of the spinal cord
  • Results in loss of sensation and voluntary movement below the level of injury (paraplegia or tetraplegia)
  • Spinal shock is a temporary loss of reflexes below the level of injury due to the sudden disruption of descending input
• Incomplete spinal cord injuries
  • Anterior cord syndrome damages the anterior portion of the spinal cord
    • Loss of motor function and pain/temperature sensation below the level of injury due to damage to the anterior spinothalamic tract
    • Preservation of proprioception and vibration sense mediated by the dorsal columns
  • Brown-Séquard syndrome is a hemisection of the spinal cord
    • Ipsilateral loss of motor function, proprioception, and vibration sense below the level of injury due to damage to the lateral corticospinal tract and dorsal columns
    • Contralateral loss of pain and temperature sensation below the level of injury due to damage to the anterior spinothalamic tract

Upper vs lower motor neuron diseases

• Upper motor neuron (UMN) diseases affect motor neurons in the brain or spinal cord
  • Examples include stroke, multiple sclerosis, and amyotrophic lateral sclerosis (ALS)
  • Clinical features of UMN lesions
    • Spasticity is increased muscle tone and resistance to passive movement due to loss of inhibitory input from the brain
    • Hyperreflexia refers to exaggerated deep tendon reflexes (brisk patellar reflex) due to loss of descending inhibition
    • Positive Babinski sign is an extensor plantar response indicating a lesion in the corticospinal tract
    • Weakness and loss of fine motor control due to impaired voluntary motor activation
• Lower motor neuron (LMN) diseases affect motor neurons in the spinal cord or peripheral nerves
  • Examples include poliomyelitis, spinal muscular atrophy, and ALS (which affects both UMNs and LMNs)
  • Clinical features of LMN lesions
    • Flaccid paralysis is decreased muscle tone and weakness due to loss of motor neuron innervation
    • Hyporeflexia or areflexia refers to diminished or absent deep tendon reflexes (absent Achilles reflex) due to interruption of the spinal reflex arc
    • Muscle atrophy is the loss of muscle mass due to denervation and disuse
    • Fasciculations are spontaneous muscle twitches visible under the skin due to the spontaneous firing of damaged motor neurons

Significance of neurological reflexes

• Deep tendon reflexes assess the integrity of the spinal reflex arc
  • Examples include the biceps (C5-C6), triceps (C7-C8), patellar (L2-L4), and Achilles (S1-S2) reflexes
  • Grading ranges from 0 (absent) to 4+ (hyperreflexia) with 2+ considered normal
• Babinski reflex assesses the integrity of the corticospinal tract
  • Elicited by stimulating the lateral aspect of the sole of the foot with a blunt object
  • Positive Babinski sign is an extensor plantar response (upgoing big toe) indicating an UMN lesion
• Hoffmann's reflex assesses the integrity of the corticospinal tract in the upper extremities
  • Elicited by flicking the distal phalanx of the middle or ring finger
  • Positive Hoffmann's sign is a flexion of the thumb and index finger indicating an UMN lesion in the cervical spinal cord or brain

Sensory and Motor Integration

• Sensory receptors in the skin, muscles, and joints detect various stimuli and convert them into electrical signals
• Peripheral nerves transmit sensory information from receptors to the spinal cord and motor commands from the spinal cord to muscles
• Spinal reflexes, such as the stretch reflex, allow for rapid, automatic responses to stimuli without involving higher brain centers
• Proprioception provides information about body position and movement, crucial for coordinating motor actions
• The neurological examination assesses sensory and motor function to identify potential nervous system disorders