Somatosensory pathways Flashcards
What are the cutaneous sensory fibers? What are the muscular sensory fibers? Which ones are unmyelinated?
Muscular:
Group 1a - muscle spindles
Groub 1b - golgi tendon organ
Group II - secondary endings in muscle spindle
Group III and IV - thermoreceptors, nociceptors
Cutaneous:
Abeta - touch, pressure, vibration, proprioception
Adelta and C fibers - thermoreceptors, nociceptors, free nerve endings
The only unmyelinated fibers are C fibers and Group IV fibers
Sensory fibers from the DRG enter the dorsal spinal cord and branch into three paths. Where do each of the three paths go?
1 - courses ventrally through the grey matter and terminates in either the nucleus proprius if cutaneous afferent or clarke’s column if muscle afferent
2 - descends caudally and terminates in either the nucleus proprius if cutaneous afferent or clarke’s column if muscle afferent
3 - ascends in one of the dorsal columns (fasciculus gracilis if below T6, fasciculus cuneatus if T6 or above) to the medulla
NB: each of branch remains on the same side of the cord as which the dorsal roots enter. No crossing yet!
What is the pathway of sensory fibers of fine touch, pressure and vibration from the body? What is the pathway of sensory fibers of crude touch, pain and temperature from the body?
DC-ML = Sensory fibers from the body enter the spinal cord and ascend in either the gracile fasiculus if below T6 or the cuneate fasiculus if above T6 on the ipsilateral side -> synapses in the medulla in the gracilus nucleus and cuneate nucleus -> the 2nd order neuron arising from the medulla crosses the midline and ascends on the contralateral side -> ascends in the medial lemniscus -> terminates in the ventral posterior lateral nucleus in the thalamus. Thalamic fibers project to somatosensory cortex S1 and S2
Spinothalamic tract - pain and temperature fibers from the body enter the spinal cord synapse in the substantia gelatinosa of the dorsal horn -> the 2nd order neuron crosses decussates via the ventral white commissure and ascend in the contralateral spinothalamic tract -> synapses in the VPL (ventral posterior lateral) nucleus of the thalamus -> thalamic fibers project to S1 and S2
What is the pathway of sensory fibers of fine touch, pressure and vibration from the face? What is the pathway of sensory fibers of crude touch, pain and temperature from the face?
Touch, pressure and vibration sensory fibers from the face descend and synapse in the ipsilateral principal/primary trigeminal nucleus in the pons. 2nd order neurons decussate in the pons, and join the ascending medial lemniscal fibers on the opposite side. Fibers from the face terminate in the VPM (ventral posterior medial) nucleus of the thalamus. Thalamic fibers project to S1 and S2.
Pain and temperature sensory fibers from the face descend ipsilaterally into the medulla in the spinal tract of CN V and terminate in the spinal nucleus of CN V. Neurons in this nucleus then send axons across the midline to join the ascending spinothalamic tract and terminate in the VPM nucleus of the thalamus. Thalamic fibers project to S1 and S2.
What is the pathogenesis of Brown-Sequard syndrome? What are the clinical manifestations?
Brown-Sequard syndrome occurs when one half of the spinal cord is destroyed (from a tumor or gunshot wound).
Lesion to the:
corticospinal tract = below the lesion, ipsilateral spastic paralysis. At the level of the lesion, ipsilateral flaccid paralysis
dorsal column - medial lemniscal tract = ipsilateral loss of fine touch, vibration and proprioception
Spinothalamic tract = contralateral loss of pain and temperature
What is the ascending pathway of cutaneous pain?
Ascending - Adelta and C fibers enter the dorsal horn of the spinal cord and branch up and down a few segments (Lisshauer’s tract) and synapse in lamina I and V of the dorsal horn. The secondary fibers cross to the contralateral side and ascend in the spinothalamic tract.
Synapses of the secondary neurons:
1) secondary neurons synapse in the ventral posterior nucleus of the thalamus. Tertiary neurons from the VP nucleus go to the somatosensory cortex
2) secondary neurons synapse in the periaqueductal gray area (activate descending pathways)
3) secondary neurons synapse in the rostralventral medulla
4) secondary neurons synapse in the thalamic ventral medial (projects to -> insula) and medial dorsal (MD) nuclei (projects to -> cingulate cortex)
5) secondary neurons synapse in the ventromedial hypothalamus, amygdala and globus pallidus
What is the descending pathway for the control of pain?
Descending - The perception of pain in somatosensory cortex, amygdala, hypothalamus insula and limbic systems, causes these systems to send information to the periaqueductal gray. Axons from the PAG synapse on various structures that comprise the rostralventral medulla (RVM), including the locus ceruleus and dorsal raphe nuclei. Neurons from the LC and DRN release norepinephrine and serotonin onto local inhibitory interneurons in the spinal cord. The local inhibitory interneurons release enkephalins into lamina I and V of the dorsal horn, inhibiting the primary pain fibers (Adelta and C fibers) and secondary pain fibers (spinothalamic ascending neurons).
What are the three changes that lead to chronic neuropathic pain?
1) Upregulation of Na+ channels n peripheral axons -> increased membrane excitability
2) upregulation of glutamate receptors on secondary and tertiary neurons -> increased synaptic efficiency
3) decreased inhibition in local interneuron networks of the spinal cord
How does the thickness/diameter of the axon correlate with the speed of the electical signal?
The wider/thicker the axon, the faster the conduction velocity
