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 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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Dorsal (posterior) horn receives sensory input from (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
carry ascending sensory information related to and fine touch (cuneate and gracile fasciculi)
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 for sensory innervation (C5 dermatome covers the lateral upper arm) and 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 ( or )
is a temporary loss of reflexes below the level of injury due to the sudden disruption of descending input
Incomplete spinal cord injuries
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
Preservation of and vibration sense mediated by the dorsal columns
is a of the spinal cord
Ipsilateral loss of motor function, proprioception, and vibration sense below the level of injury due to damage to the 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
(UMN) diseases affect motor neurons in the brain or spinal cord
Examples include stroke, multiple sclerosis, and (ALS)
Clinical features of UMN lesions
is increased muscle tone and resistance to passive movement due to loss of inhibitory input from the brain
refers to exaggerated deep tendon reflexes (brisk ) due to loss of descending inhibition
Positive 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 , , and ALS (which affects both UMNs and LMNs)
Clinical features of LMN lesions
is decreased muscle tone and weakness due to loss of motor neuron innervation
or refers to diminished or absent deep tendon reflexes (absent ) due to interruption of the spinal reflex arc
Muscle atrophy is the loss of muscle mass due to denervation and disuse
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 is an extensor plantar response (upgoing big toe) indicating an UMN lesion
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
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
, 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 assesses sensory and motor function to identify potential nervous system disorders