Ascending Tracts of the Spinal cord

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

• The first-order neurone or (primary afferent neurone) :

• enters the spinal cord through the dorsal root of a spinal nerve and its cell body lies in the dorsal root ganglion.

• The main fibers remain on the ipsilateral side of the cord and terminate by syapses with the second neurone either in the spinal grey matter or in the medulla oblongata of the brain stem.

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• The second order neurone • Has its cell bodies in the cord or

medulla oblongata. • Its axon cross over (decussates)

to the opposite side of the CNS and ascend to the thalamus, where it terminate upon the third neurone.

• The third-order neurone Has its cell bodies in the thalamus.

• Its axons pass to the somatosensory cortex of the parietal lobe of the cerebral hemisphere.

SECOND ORDER NEURON

• cross the mid line • in front of central canal

VPL

1st

2nd

Ascending Tracts

• Dorsal column tracts• Spinothalamic tracts( Ventral AND Lateral)• Posterior spinocerebellar• Anterior spinocerebellar• Cuneocerebellar• Spinotectal• Spinoreticular• Spino-olivary

Functions of Ascending Tracts• Lateral spinothalamic• Anterior spinothalamic• Posterior column

• Anterior and posterior spinocerebellar and cuneocerebellar tracts

• Spinotectal

• Spinoreticular

• Painful and thermal sensations• Crude touch and pressure• Descriminative fine touch and

joint and muscle information and vibration sense

• Muscle and joint unconscious information

• Pain, thermal and tactile information to superior colliculus

• Muscle and joint information to reticular information

Ascending Or Sensory Tracts

1- Dorsal column tracts: (Gracile & Cuneate)

Function: Transmit a. Proprioceptive (deep)

sensations (sense of movement, position, vibration).

b. Fine touch sensations (tactile localization, tactile discrimination & stereognosis).

These senses reach a Conscious level (cerebral cortex).

2- Spinothalmic tracts:Function: Transmit impulses concerned with specific sensory

modalities: pain, temperature, touch, that reach a Conscious

level (cerebral cortex).

3- Spinocerebellar tracts: Function: Carries unconscious information from muscles,

joints, skin, subcutaneous tissues Transmit impulses from tactile and stretch receptors

to Subconscious centers (cerebellum) for muscle tone and coordinations.

1. Dorsal column TractsFasciculus gracilis and fasciculus cuneatus

• Function: carry discriminative(fine) touch, proprioception: vibratory sense and conscious muscle joint sense

• Inputs from pacinian corpuscles, Messiner’s corpuscles, joint receptors, muscle spindles and Golgi tendon organs )

Axon of 1st order neuron enter the spinal cord through the dorsal roots of the spinal nerves.• passes directly to the posterior white column

of the same side ( without synapsing )

• 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).

• Synapse on the 2nd order neuron in the nucleus gracilis and cuneatus of medulla oblongata of the same side.

lower 6 thoracic segments

lumbar segments

sacral segments

cervical segments

upper 6 thoracic segments

fasciculus gracilis

fasciculus cuneatus

[ nucleus G & C ]

in medulla

GC

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• Axons of second-order neurones :

• decussate in the medulla as the internal arcuate fibers(sensory decussation).

• Then it ascend in (opposite side) through brain stem as the Medial Lemniscus.

• The medial lemniscus ASCENDS through medulla oblongata, pons, and midbrain terminates in the ventral posterior (VP) nucleus of the thalamus.

• Third-order neurone: • (thalamocortical, or sensory radiations- run in

the internal capsule,corona radiata to reach the postcentral gyrus of cerebral cortex area 3, 1 and 2 ) : somatosensory cortex.

Clinical application

destruction of fasciculus gracilis and cuneatus

• loss of muscle joint sense, position sense, vibration sense and tactile discrimination

• on the same side • below the level of the lesion

(extremely rare to have a lesion of the spinal cord to be localized as to affect one sensory tract only)

Prof. Makarem 21

• They carry pain and thermal (temperature) sensations and also non-discriminative (crude) touch and pressure. – The Lateral spinothalamic tracts carry pain and temperature

sensations. – The ventral spinothalamic tracts carry crude touch and pressure.

• But fibers carrying these modalities are probably intermingled, at least to some extent.

Spinothalamic tracts lie lateral and ventral to the ventral horn of the spinal

grey matter .

2 -Spinothalamic tracts

A-Lateral Spinothalamic Tract (pain & temperature) Function:• Carries pain & Temperature to

thalamus and sensory area of the cerebral cortex.

Neurones: 3 Neurones• Neurone I: Small cells in the dorsal

root ganglia.• Neurone II: Cells of substantia

gelatinosa of Rolandi in the posterior horn.

• Neurone III: Cells of (VPL) nucleus of the thalamus.

• The spinothalamic tract contains second-order neurones, the cell bodies of which lie in the contralateral dorsal horn.

Pain and thermal impulses ( input from free nerve endings,

thermal receptors )• transmitted to spinal cord in

delta A and C fibres • central process enters the

spinal cord through posterior nerve root,

• the central process of 1st order neuron synapse with cell body of 2nd order neuron in substantia gelatinosa of posterior gray column of the spinal cord

• the axon of 2nd order neuron cross to the opposite side in the anterior white commissure and ascend in contralateral white column as lateral spinothalamic tract

• end by synapsing with 3rd order neuron in the ventral posterolateral nucleus (VPL )of thalamus

• axon of the 3rd order neuron passes through the posterior limb of internal capsule and corona radiata to reach the postcentral gyrus of cerebral cortex ( area 3, 1 and 2 )

Clinical application destruction of LSTT• loss of

– pain and thermal sensation – on the contralateral side– below the level of the lesion

patient will not respond to pinprick recognize hot and cold

Lesion:•Syringomyelia, (widening of the central canal) leads to Loss of pain & temperature below the level of the lesion.

B- Ventral Spinothalamic (Crude touch & Pressure)

Function:• Carries crude touch &

pressure to thalamus and sensory cortex.

• input from free nerve endings, Merkel’s tactile disks

Neurones: 3 Neurones• Neurone I: Medium sized cells in the dorsal root ganglia.• Neurone II: Cells of main sensory nucleus or (nucleus proprius). cross

to the opposite side (decussate)– ascend in the contralateral ventral column as ASTT– Ends in the VPL nucleus of the thalamus.

• Neurone III: Cells of VPL nucleus of thalamus project to cerebral cortex

( area 3, 1 and 2 ) Effect of lesion: Loss of crude touch sensation below the level of the

lesion. on the contralateral side of the body

• Axons carrying pain and temperature decussate within one segment of their origin,

• while those carrying touch and pressure may ascend for several segments before crossing.

• It is highly organised somatotopically; consequently, the origin of sensory stimuli can be accurately localised.

• It is thought to be the route via which sharp, pricking pain (sometimes called 'fast' pain) is conducted

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• Ascending pathways that carry impulses to a subconscious level are represented by the spinocerebellar tracts.

• Fibres of spinocerebellar tracts form dorsal and ventral tracts that are located at the dorsolateral & ventrolateral surfaces of the cord, respectively.

3 -Spinocerebellar tracts

• Both tracts carry information derived from muscle spindles, Golgi tendon organs and tactile receptors to the cerebellum for the control of posture & coordination of movement.

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• The spinocerebellar system consists of a sequence of only two neurones.

• Neurone I:Large cells of dorsal root ganglia.

• Neurone II: cells of the nucleus dorsalis Clark's nucleus.

• give rise to axons ascending to the cerebellum of the same side

• Both spinocerebellar tracts contain second-order neurones.

• The tract neurons terminate directly in the cerebellar cortex. MOSTLY VERMIS

Dorsal (Direct) Spinocerebellar tract• Fibers of the dorsal

spinocerebellar tract originate from the cells of Clarke's column at the base of the posterior horn.

• The axons ascend ipsilaterally to enter the cerebellum through the inferior cerebellar peduncle.

Ventral (Indirect) Spinocerebellar Tract

• Fibers of the ventral spinocerebellar tract decussate, ascend on the contralateral side of the cord and enter the cerebellum via the Superior cerebellar peduncle.

• Some axons then recross within the cerebellar white matter.

• The spinoreticulothalamic system represents an additional route by which sensory impulses ascend to higher centers.

• Second-order neurons arising from the dorsal horn ascend in the ventrolateral region of the cord

• They terminate in the brain stem reticular formation, particularly within the medulla.

• Reticulothalamic fibres then ascend to the intralaminar thalamic nuclei, which in turn activate the cerebral cortex.

Spinoreticulothalamic

• The spinoreticulothalamic system is thought to be the route via which dull, aching pain (sometimes called 'slow' pain) is transmitted to a conscious level.

• Activation of spinothalamic and spinoreticular fibers, which may ultimately be perceived as unpleasant or painful, can be modulated by descending pathways from the brain.

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