Internal Features and Pathways of the Spinal Cord

Dorsal horn of gray matter

contain cells that process incoming sensory information

ventral horn of gray matter

contain cell bodies of motor neurons innervating skeletal muscles

lateral horn of gray matter

contain cell bodies of autonomic motor neurons innervating smooth muscle

where is the lateral horn of gray matter located

thoracolumbar region

gray commissure

axons that cross between the left and right sides, surrounds central canal

where are the ascending and descending tracts located

in white matter

Ascending tracts

carry sensory information from the periphery to the brain

descending tracts

carry information from the brain to the periphery

dorsal column of white matter

made up of ascending axons responsible for fine touch, vibration, conscious proprioception, and 2-point discrimination

lateral column of white matter

made up of ascending axons for paint, temperature, crude touch, and non-conscious proprioception; also contains descending axons for voluntary movement of limb muscles

ventral column of white matter

contains descending axons for voluntary movements of axial muscles

where is the white matter most dense

cervical levels

where is grey matter most dense

cervical and lumbar enlargements

where do the ascending tracts send conscious sensations to

sensory cortex

where do ascending tracts send non-conscious sensations to

cerebellum

what are the two dorsal column tracts

fasciculus gracilis and fasciculus cuneatus

fasciculus gracilis

carries information from the lower limbs to the brain

fasciculus cuneatus

carries sensory information from the upper limb to the brain

which ascending tracts are responsible for conscious sensations

dorsal column (fasciculus gracilis and fasciculus cuneatus) and anterolateral tracts (spinothalamic)

which ascending tracts are responsible for non-conscious sensations

spinocerebellar

spinocerebellar tracts

relay information from joint and muscle receptors to the cerebellum, has dorsal and ventral tracts

which type of ascending tract uses the 3 neuron sequence

tracts that relay conscious sensations (dorsal columns and spinothalamic)

decussation

the process when of the 2nd order neuron crossing over to the other side

which neuron remains ipsilateral

1st order neurons

3 neuron sequence of the dorsal columns

sensory fibers enter spinal cord via the dorsal root → axons enter dorsal column and ascend to the lower medulla → 1st order neuron synapses onto 2nd order neuron → 2nd order neuron decussates → 2nd order neuron ascends thru the pons and midbrain to reach thalamus → 2nd order neuron synapses onto 3rd order neuron → 3rd order neuron travels to and terminates in the contralateral somatosensory cortex

3 neuron sequence of spinothalamic tracts

sensory fibers enter spinal cord via the dorsal root → axons synapse onto 2nd order neurons in the dorsal horn → 2nd order neuron decussates to reach the contralateral spinothalamic tract → neuron ascends through the brainstem to reach the thalamus → 2nd order neuron synapses onto 3rd order neuron in thalamus → 3rd order neuron travels to and terminates in the contralateral somatosensory cortex

which descending pathways are responsible for skilled voluntary movement

pyramidal tracts - lateral corticospinal and anterior corticospinal

lateral corticospinal tract

controls limb movements

anterior corticospinal tract

controls trunk movement

which descending pathways are responsible for postural control and head-eye coordination

extrapyramidal tracts - vestibulospinal, recticulospinal, tectospinal, rubrospinal

which side of the brain controls the voluntary movements for the right side of the body

left side of the brain

neuron pathway for the lateral corticospinal tract

neuron originates in motor cortex (precentral gyrus) → axons descned thru midbrain and pons to reach medulla → axons decussate where medulla meets the cervical cord → axons descend in contralateral spinal cord → axons terminate on lower motor neurons (alpha and gamma) in anterior horn

neuron pathway for the anterior corticospinal tract

neurons originate in motor cortex and premotor areas → axons descend thru midbrain and pons to reach medulla → axons reach the spinal cord and DO NOT decussate → axons descend in anterior white matter along ventral median fissure → once reaching target level, axons divide and terminate on alpha motor neurons bilaterally

typical motor examinations

muscle strength, muscle tone, spinal reflexes, gait

spinal reflex

involuntary, stereotyped response to a stimulus

minimum components of a spinal reflex

receptor, afferent neuron, synapse, efferent neuron, effector organ

monosynaptic spinal reflex

a single synapse between afferent and efferent neurons

polysynaptic spinal reflex

two or more synapses

deep tendon reflexes

type of monosynaptic reflex where muscle spindle activation evokes a motor response in the corresponding muscles

examples of deep tendon reflexes

biceps, brachioradialis, triceps, patellar tendon (most common), achilles tendon

reciprocal inhibition

collateral branch of afferent axon synapses on an inhibitory interneuron causing the antagonist muscle to relax; polysynaptic reflex

flexion withdrawal

afferent stimulus from pain receptors evokes muscle contraction to withdraw the limb; sensory neuron synapses on excitatory interneuron, which then synapses on motor neuron; polysynaptic reflex

upper motor neuron (UMN)

located in lateral and anterior corticospinal tracts; project from the cerebral cortex onto lower motor neurons in the anterior horn of the spinal cord

lower motor neuron (LMN)

project from anterior horn to skeletal muscles

mononeuropathy

LMN lesion that causes weakness and/or sensory impairment in the distribution of a single peripheral nerve (ex - carpal tunnel syndrome)

plexopathy

LMN lesion that causes weakness and/or sensory impairment in a distribution that corresponds to entire nerve plexus and impacts an entire limb

polyneuropathy

LMN lesion that causes weakness and/or sensory impairment in a 'glove’ or ‘stocking’ distribution of multiple peripheral nerves (ex - diabetic neuropathy)

UMN lesions above the pyramidal decussation

motor deficits are contralateral to the side of the lesion, ex - stroke and TBI

UMN lesion below pyramidal decussation

motor deficits are ipsilateral to the side of the lesion, ex - SCI

signs of an UMN

hyperreflexia, clonus, and babinski sign

dorsal column syndrome

bilateral loss of discriminative touch, proprioception, and vibration below the level of the lesion

central cord syndrome

bilateral loss of pain and temperature sensation at the level of the lesion, ex - syringomyelia

syringomyelia

central cord syndrome where a fluid filled cyst forms centrally in the cervical spine

spinal cord transection

complete loss of motor and sensory function below the level of the lesion

hemiplegic gait

leg is stiff and extended, foot in plantarflexed, hip hiking on the involved side during swing phase, looks like the leg swings outward in a circle; due to stroke or TBI

diplegic gait

both legs are stiff and extended, feet are plantarfelxed, thighs/knees cross each other with each step, scissoring gait; seen with cerebral palsy

ataxic gait

unsteady and uncoordinated, staggering side to side, wide base of support, feet/legs brought forward quickly, patient looks down to guide steps

steppage gait

excessive hip and knee flexion to lift foot off the floor (high-stepping), foot drop occurs during swing phase, foot brought down to the floor with a slap

parkinsonian gait

stopped posture, body held rigidly, short shuffling steps, episodes of freezing