Ascending Sensory System D.Nimer D.Rania Gabr D.Safaa D.Elsherbiny

Objectives Define the meaning of a tract. Distinguish between the different types of tracts. Locate the position of each tract. Describe the sensory pathway. Describe gracile and cuneate tracts and pathways for conscious proprioception, touch, pressure and vibration from the limbs and trunk. Describe lateral spinothalamic tract and pathways for pain and temperature from the limbs and trunk. Describe ventral spinothalamic tract and pathways for simple touch from the limbs and trunk. Describe dorsal and ventral spinocerebellar tracts and pathways for unconscious proprioception from the limbs and trunk. Describe Reticulothalamic and spinothalamic tracts for dull aching pain

TRACTS Bundles or fasciculi of fibers that occupy more or less definite positions in the white matter. They have the same Origin, Termination and carry the same Function. They serve to join the brain to the spinal cord. They are classified into: 1-Ascending (sensory or afferent). 2-Descending (motor or efferent).

Ascending Tracts Carry impulses from pain, thermal, tactile, muscle and joint receptors to the brain. Some of this information eventually reaches a conscious level (the cerebral cortex), While some is destined for subconscious centers (e.g. the cerebellum). Ascending sensory pathways between the peripheral receptors and the cerebral cortex, are organized in a three neuronal chain: - First order neurone - Second order neurone - Third order neurone

Ascending sensory pathway 2nd order neuron (S.c or brain stem) Receptors 1st order neuron (DRG) 2nd order neuron (S.c or brain stem) 3rd order neuron (PVNT)

Ascending tracts of dorsal funiculus

Tracts of dorsal funiculus Gracile & Cuneate 1. Gracile tract: medial, carry sensations from below T6 segment. 2. Cuneate tract: lateral, carry sensations from above T6 segment.

Function of G&C Carry sensation from same side of body 1. Conscious proprioception (sense of position and sense of movement) 2. Discriminative touch Localization Stereognosis 2 points-discrimination 3. Sense of vibration

Pathway of G&C 1st order neuron: DRG Second-order neurons : G&C nuclei Peripheral fibers: ends in the receptors Central fibers: enter the spinal cord Long ascending fibers travel upward in the posterior column of the same side as fasciculus gracilis and fasciculus cuneatus Fibers of Fasciculus Gracilis enter via the sacral, lumbar and lower thoracic 6 levels; (lower limbs). Fibers of Fasciculus Cuneatus enter via the upper 6 thoracic and cervical levels; (upper limbs). ….ascend in the posterior white column of the same side …..end in the gracile and cuneate nuclei of medulla oblongata of the same side. Second-order neurons : G&C nuclei gracile and cuneate nuclei of medulla oblongata of the same side………. DECUSSATE in the medulla as the internal arcuate fibers (sensory decussation) ……..ascend in (opposite side) as Medial Lemniscus……… Ventral postero- lateral nucleus of thalamus (VPLNT) Third-order neurone: VPLNT (VPLNT) …….. internal capsule ………postcentral gyrus of cerebral cortex area 3, 1 and 2 ) : Somatosensory cortex.

[ nucleus G & C ] in medulla fasciculus cuneatus fasciculus gracilis cervical segments upper 6 thoracic segments lower 6 thoracic segments lumbar segments sacral segments G C fasciculus gracilis

Lesion of G&C Loss of proprioception discriminative touch, sense of vibration In dorsal funiculus: on same side of body below level of lesion. In medial lemniscus: on opposite side of body (extremely rare to have a lesion of the spinal cord to be localized as to affect one sensory tract only )

Ascending tracts of ventral funiculus VSPT

Tracts of ventral funiculus Lesion: not detected clinically Ventral spinothalmic tract Function: carries simple touch and pressure sensation from opposite side of the body 1st order neuron: Dorsal root ganglia Peripheral fibers: ends in the receptors Central fibers: enters the spinal cord ……end in main sensory nucleus of many segments of SC 2nd order neuron: SGR from main sensory nucleus ….cross to the opposite side in the anterior white commissure…..ascend in contralateral white column as ventral spinothalamic tract ….form spinal lemniscus with LSTT …… terminate in PVLNT 3rd order neuron: PVLNT PVLNT …… internal capsule ….. ends in area 3,1,2 Lesion: not detected clinically

Ascending tracts of lateral funiculus DSPCT LSPT VSPCT S.T S.O

Tracts in the lateral funiculus 1. Dorsal spinocerebellar tract: unconscious proprioception from trunk & lower limb 2. Ventral spinocerebellar tract: unconscious proprioception from lower limb 3. Cuneocerebellar tract: unconscious proprioception from upper limb gg 4. Spinotectal tarct: spinovisual reflexes 5. lissauer's tract

6. Lateral Spinothalamic Tract. Function: carry pain and temperature from opposite side of the body Receptors: free nerve endings 1st order neuron: Dorsal root ganglia Peripheral fibers: ends in the receptors Central fibers: enters the spinal cord …..ascend 2 segments in the lateral funiculus……end in substantia gelatinosa (SGR) of posterior gray column of SC 2nd order neuron: SGR from SGR ….cross to the opposite side in the anterior white commissure…..ascend in contralateral white column as lateral spinothalamic tract ….form spinal lemniscus with VSTT …… terminate in PVLNT 3rd order neuron: PVLNT PVLNT …… internal capsule ….. ends in area 3,1,2

2nd

Lesion of LSTT: loss of pain and temperature sensations on opposite side of body 2 segments below level of lesion.