Spinal Cord Ascending Pathways Part 1 Flashcards
Somatosensory System
- carries sensations of touch, pain, temperature, vibration, and proprioception to conscious centers (cerebrum) and/or subconscious centers (cerebellum)
- some carry information to adjacent levels and permit intersegmental coordination (fasciculus proprius)
- pathways: posterior (dorsal) column medial lemniscal pathway: proprioception, vibration sense, fine touch; anterolateral pathways-spinothalamic and other anterolateral pathways: pain, temperature, crude touch
Ascending Tracts
- tracts to conscious levels (posterior (dorsal) columns and spinothalamic tracts)
- sequence of three neurons:
- first neuron (the cell body lies in the dorsal root ganglion, synapses in the ipsilateral dorsal horn or medulla, receives impulses from Meissner’s & Pacinian, Merkels & Ruffini, spindles and GTOs)
- second neuron (cell body is in the dorsal horn or medulla, decussates and ascends to the thalamus)
- third neuron (cell body is in the thalamus, ascends to the primary sensory cortex of the parietal lobe)
Posterior (Dorsal) Column-Medial Lemniscal Pathway
- larger diameter, myelinated axons
- carry impulses of vibration, conscious proprioception and discriminative touch
- fasciculus gracilis (medial): fibers from the lower thoracic, lumbar and sacral roots
- fasciculus cuneatus (lateral): fibers from the upper thoracic and cervical roots
Posterior Dorsal Columns Neuron Sequence
-fibers (first neuron-pseudounipolar bifurcating axon) ascend uninterrupted ipsilaterally until they synapse in the medulla: fasciculus gracilis to nucleus gracilis; fasciculus cuneatus to nucleus cuneatus
-some axon collaterals synapse onto interneurons and motor neurons
-second neuron decussates in medulla and travels
to the thalamus via the medial lemniscus where it synapses with the third neuron in the ventral posterior lateral nucleus (VPL) of the thalamus
-third neuron then carries information to the somatosensory cortex via the posterior limb of the internal capsules in the thalamic somatosensory radiation
-after decussation legs anterior arms posterior
Anterolateral: Spinothalamic Tract
- smaller-diameter, unmyelinated axons
- carries pain and thermal sensations as well as non discriminative touch and pressure
- carries second order neurons
- first neuron synapses in Rexed laminae I-III (substantia gelatinosa)
- second neuron decussates obliquely in the gray matter in the spinal cord anterior commissure:
- pain (fast pain, sharp and pricking) and temperature
- touch and pressure
- after decussating - travels up the spinothalamic tract and the spinal lemniscus in the brainstem and synapses in the thalamus in the ventral posterior lateral nucleus (VPL)
- third neuron projects to the somatosensory cortex via the thalamic somatosensory radiations
Clinical Significance
-it takes 2-3 spinal segments for the decussating fibers to reach the opposite side, so a lateral cord lesions will affect contralateral pain and temperature a few segments below the level of the lesion
Anterolateral-Spinothalamic Tract
- temperature: lateral spinothalamic
- pain: lateral spinothalamic
- touch: anterior spinothalamic
Anterolateral-Spinoreticular Tract
- slow pain (dull and aching) from Rexed VI-VIII
- accompanies the spinothalamic tract
- half of spinoreticular fibers remain uncrossed
- two functions: arouses the cerebral cortex to induce or maintain the waking state, reports to somatosensory cortex the nature of the stimulus with resultant affective/emotional response (pleasurable or aversive)
Anterolateral-Spinomesencephalic Tract
- arise from spinal cord laminae I and V
- projects to the midbrain periaqueductal gray matter and superior colliculi
- periaqueductal gray: central pain modulation
What happens when the signal gets to the somatosensory cortex?
-primary somatosensory cortex
-secondary somatosensory area: information is sent to secondary somatosensory association cortex in the parietal operculum
-parietal association cortex: superior parietal lobule also processes somatosensory information
-motor cortex: extensive connections exist to/from the motor cortex
-damage to these areas leads to cortical sensory
loss
Spinocerebellar Tracts
- dorsal and ventral
- both carry information derived from the muscle spindles, golgi tendon organs, and tactile receptors to the cerebellum
- coordination and control of posture
- 2 neuron system
- cell bodies are located in the dorsal horn: dorsal: ascends ipsilaterally, ventral: decussates (some redecussate in the cerebellum)
Thalamus
-primary relay center of the brain
-pathways project onto the thalamus and then are projected to the cerebral cortex
-gray matter located deep within the cerebral white matter just above brainstem and behind basal ganglia
-shaped like eggs with posterior end angled out (forming an inverted V in horizontal sections) contains multiple nuclei
-each sensory pathway has a different nuclear
area where synapses occur
-cortical-thalamic connections are reciprocal
-non-sensory pathways also relay to thalamus: brainstem reticular formation, basal ganglia, cerebellum, limbic pathways
Paresthesia
- altered sensation: posterior column – medial lemniscal pathway lesions: tingling, numbness, tight, bandlike sensation around trunk or limbs
- anterolateral pathway lesions: sharp pain, burning pain, searing pain
Lesions and Resultant Paresthesia: Pareital Lobe, Thalamus, Cervical Spine, Nerve Roots, Peripheral Nerves
- parietal lobe and somatosensory cortex: contralateral numb tingling
- thalamus-severe contralateral pain (Dejerine-Roussy syndrome)
- cervical spine: electricity like sensation down back and into extremities with cervical flexion (Lhermitte’s sign)
- nerve roots: radicular pain-radiates down limb in a dermatomal distribution often with numbness and tingling provoked by stretching nerve root
- peripheral nerves: pain, numbness and tingling in sensory distribution of nerve
Sensory Loss: Patterns and Localization
- lesions can occur anywhere in pathway
- patterns of sensory loss and associated deficits are provided in the following slides
