Sensory Tracts

7/29/2019 Sensory Tracts

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Sensory tracts



Ascending tracts / sensory tracts:

4 groups:

Dorsal column medial leminiscal system / Dorsalcolumn tract / Dorsal white column.

Antero-lateral System: includes mainly spino-thalamictract (lateral & anterior).

Spino-cerebellar tracts: (Dorsal / posterior, Ventral /

anterior spino-cerebellar tracts) Miscellaneous group / tracts: (Spino-tectal, Spino-

reticular, Spino-olivary, Visceral Sensory pathways).



Comparison between DCMLS &

STT: DCMLS: Rapidly conducting fibers. (120

M/s) Sensations carried require accurate

localization. (spatial orientation of

nerve fibers in this tract) Involve perception of fine grades

of intensity. Crossing occurs in medulla. Fibers from upper parts of body

are added on lateral aspect of tract,

above the spinal cord origin, beforecrossing over. Receptors are Meissner’s &

Pacinian corpuscles. Sensations carried are: fine touch,

fine pressure, proprioception,

vibration, 2 point tactile discrimination.

STT: Slowly conducting fibers. (40 M/s) Do not require accurate

localization. Do not involve fine perception.

Crossing occurs below the level of medulla.

Fibers from upper parts of body are added on medial aspect of tract,after crossing over.

Receptors are free nerve endings

mainly. Sensations carried are: crude touch,

tickle & itch, temperature, pain &pressure sensation.



Dorsal column medial leminiscal

system / Dorsal column tract.

Grey matter of spinal cord is divided into 9laminae.

There are certain groups of neurons in grey matter of spinal cord:

Substantia gelatinosa &

Clark’s column. Important features of this tract have been

discussed in comparison.



C = cervical segment. S = Sacral segment. VPL = Ventral posterolateral nucleus. SI =Primary somatosensory cortex. VM = Ventromedial prefrontal cortex. MD = medial

dorsal thalamic nucleus. IL = Intralaminar nucleus. VPM = Ventral posteromedialnucleus. Main V = Main trigeminal nucleus. Spinal V = Spinal trigeminal nucleus.

http://en.wikipedia.org/wiki/Ventral_posterolateral_nucleus

http://en.wikipedia.org/wiki/Primary_somatosensory_cortex

http://en.wikipedia.org/wiki/Ventromedial_prefrontal_cortex

http://en.wikipedia.org/wiki/Medial_dorsal_thalamic_nucleus


http://en.wikipedia.org/wiki/Intralaminar_nucleus

http://en.wikipedia.org/wiki/Ventral_posteromedial_nucleus


http://en.wikipedia.org/wiki/Main_trigeminal_nucleus

http://en.wikipedia.org/wiki/Spinal_trigeminal_nucleus

http://en.wikipedia.org/wiki/Spinal_trigeminal_nucleus

http://en.wikipedia.org/wiki/Main_trigeminal_nucleus



http://en.wikipedia.org/wiki/Intralaminar_nucleus



http://en.wikipedia.org/wiki/Ventromedial_prefrontal_cortex

http://en.wikipedia.org/wiki/Primary_somatosensory_cortex

http://en.wikipedia.org/wiki/Ventral_posterolateral_nucleus



1st order nerve fibers:

Carrying impulses enter the spinal cord throughposterior nerve root. They are located in posterior rootganglion.

These pass to posterior white column on the same side without any synapse dorsal column medial leminiscalsystem.

The tract is formed in the lower part of spinal cord.

As tract ascends fibers from upper parts of body areadded on lateral aspect of tract.



At the level of T6, a septum appears dividesit into 2 parts medial = fasiculus gracilus(contains fibers from lower parts of body),

lateral = fasiculus cuneatus (contains fibers fromupper parts of body).

These enter Medulla & synapse into 2 nuclei:medially with nucleus gracilus & laterally withnucleus cuneatus, both located in lower part of medulla.



2nd order nerve fibers:

From nucleus gracilus & nucleus cuneatus in medulla,2nd order nerve fibers (internal arcuate fibers) arise decussate in medulla Sensory decussation.

After crossing over / decussation, these fibers form

medial leminiscus on each side. There in no crossing over in spinal cord but crossing

occurs in medulla. This leminiscus is joined by fibers carrying impulses of

same sensations from head region. These fibers come from 5th / Trigeminal nerve. Medial leminiscus synapse onto 3rd order neurons.



3rd order nerve fibers:

These are located in Ventro basal complex of thalamicnuclei. This complex consists of VPL (Ventral-posterolateral) & VPM (Ventral-posteromedial)nucleus.

In thalamus there is ventral posterior nucleus, whichhas 2 parts: lateral part & medial part. Lateral part iscalled VPL & Medial part is called VPM.

FIBERS FROM MEDIAL LEMINISCUS MAINLY

SYNAPSE IN VPL BUT FIBERS FROM HEAD & FACE AREA SYNAPSE

IN VPM.



From ventro-basal complex, 3rd order nervefibers arise, which pass through internal capsule

to terminate in S1 (in post central gyrus).

Some fibers from cuneate nucleus cerebellum(cuneo-cerebellar fibers / posterior externalarcuate fibers). These fibers supply sensory

(proprioceptive) information to cerebellum fromthe muscles & joints.



If disease of spinal cord sensory loss is on thesame side (no crossing in spinal cord).

If lesion of medulla sensory loss on oppositeside (due to crossing in medulla).



Antero-lateral system

Mainly Spino-thalamic tract.

Some include Spino-tectal.

Important features have been discussed in comparison.

Divided into: ventral / anterior & lateral spino-thalamic tracts.

Sensations carried by anterior spino-thalamic tract:

crude touch, tickle & itch, pressure & sexual sensations. Sensations carried by lateral spino-thalamic tract:

pain & temperature mainly.



Ventral spino-thalamic tract:

1st order nerve fibers

Carrying impulses enter spinal cord throughposterior nerve root & synapse into laminae V & VI in grey matter of spinal cord.



2nd order nerve fibers:

They arise from laminae V & VI cross overto opposite side through anterior commissure anterior white column of opposite side.

Tract is formed in lower part of spinal cord.

As tract ascends, fibers from upper part of body are added on medial aspect (after crossing over).

Tract enters medulla, where it joins the spinalleminiscus.



Spinal leminiscus:

It consists of 3 tracts:

Ventral spino-thalamic tract

Lateral spino-thalamic tract

Spino-tectal tract

Spinal leminiscus ascends synapse into ventrobasal complex of thalamic nuclei.

Fibers carrying impulses of same sensationsfrom head & face area join spinal leminiscus inbrain stem. These fibers come from the 5th /

Trigeminal nerve.



3rd order neurons / Ventro-basal

complex:

From 3rd order neurons fibers arise internal capsule terminate in S1 in postcentral gyrus.



Lesion of tract:

If lesion is in spinal cord opposite side iseffected. (crossing occurs in spinal cord).



Lateral spino-thalamic tract:

1st order nerve fibers

Carries pain & temperature sensations.

1st order nerve fibers carrying impulses arisefrom posterior root ganglion of spinal cord.

These synapse with 2nd order neurons in laminaeI, II, III. (including substantia gelatinosa).



2nd order nerve fibers

These arise from laminae I, II, III, which also includesubstantia gelatinosa.

They cross over obliquely to opposite side lateral

white column of opposite side of spinal cord. New fibers are added on medial side of tract from

upper part of body.

Tract ascends & enters medulla, where it joins spinal

leminiscus. Fibers carrying fast pain & temperature ventro-basal

complex of thalamic nuclei.



3rd order nerve fibers:

These arise from ventro-basal complex of thalamus.

They terminate in S1.

Fibers carrying slow pain impulses do not goto ventro-basal complex of thalamus, but go toother areas nuclei of reticular formation,

tectum of midbrain, peri-aqueductal grey matter fibers arise from here & go to intralaminar& midline nuclei of thalamus from herefibers also go to sensory cortex.



Pathway of fast pain /

Neo-spino-thalamic tract:

Fast pain is carried by A delta fibers.

Neuro-transmitter is Glutamate.

Main fibers go to ventro-basal complex of thalamus & then to sensory cortex.



Pathway of slow pain /

Paleo-spino-thalamic tract:

Slow pain impulses are carried by C fibers.

Neuro-transmitter is substance P.

Most fibers go to nuclei of reticular formation,tectum of midbrain, peri-aqueductal grey matter

fibers arise from here & go to intralaminar& midline nuclei of thalamus (instead of ventro-

basal complex) from here fibers also go to sensory cortex.



Thalamus as sub-cortical centre of

pain:

If lesion of sensory cortex pain sensation isleast effected.



Spino-cerebellar tract:

Consist of rapidly conducting nerve fibers & velocity of conduction = 120 M/ s.

2 Components:

Dorsal / Posterior

Ventral / Anterior



Dorsal spino-cerebellar tract:

1st order neurons:

Enter spinal cord through posterior nerve root

synapse with neurons in Clark’s column /nucleus dorsalis. Clark’s column is present in

grey matter of spinal cord in segments T1 – L3or L4.



2nd order neurons:

Arise at Clark’s column enter lateral white columnon same side tract ascends medulla inferiorcerebellar peduncle terminate into vermis &intermediate zone of cerebellum on same side.

Tract carries impulses from proprioceptors &cutaneous receptors help cerebellum to coordinatemovement of limbs & maintenance of posture.

So impulses from lower limb & trunk

cerebellumbut impulses from upper limb do not go tocerebellum



Ventral spino-cerebellar tract:

1st order neurons:

Enter spinal cord through posterior nerve root

synapse with neurons located near the Clark’scolumn in grey matter of spinal cord. These

neurons are present in all segments of spinalcord.



2nd order neurons:

Arise from here. Most fibers cross to oppositeside ascend in lateral white column as ventralspinothalamic tract tract enters medulla, pons

& midbrain oasses through superiorcerebellar peduncle fibers terminate into vermis & intermediate zone of cerebellum onboth sides.

Fibers of tract which cross in spinal cord cross back in cerebellum before termination.



Functions of spino-cerebellar tract:

Impulses from proprioceptors cerebellum coordination of movements & posturemaintenance.

Information to cerebellum about motorimpulses, which have reached the ventral hornmotor neurons along the cortico-spinal & rubro-

spinal tracts.



Miscellaneous ascending tracts:

Spino-tectal tract:

1st order neurons: Enter spinal cord through posterior nerve root

synapse with neurons in the grey matter of spinal cord.2nd order neurons: Arise from here cross over to opposite side

antero-lateral white column tract ascends medulla joins spinal leminiscus terminate into superiorcolliculus in tectum of midbrain.

Function: Afferent pathway for spino-visual reflexes control

movement of head & eyes towards source of stimulation.




Spino-reticular tract:

1st order neurons: Enter spinal cord through posterior nerve root

synapse with neurons in grey matter of spinal cord.2nd order neurons: Arise from here lateral white column on same side

(no crossing over) ascends as spino-reticular tract terminates in reticular formation of medulla, pons &midbrain.

Function: Important in control of level of consciousness /

alertness.




Spino-olivary tract:

1st order neurons: Enter the spinal cord through posterior nerve root

synapse with neurons in grey matter of spinal cord.2nd order neurons: Arise from here cross over to opposite side

lateral white column ascends medulla synapse with neurons in inferior olivary nucleus.

3rd order neurons: Arise here cross to opposite side inferior

peduncle terminate in cerebellum.Function: Carries proprioceptive impulses.




Visceral sensory pathway:

Sensory impulses from pain & stretch receptosin viscera of thorax & abdomen are carried by nerve fibers, which join spino-thalamic tract.

These impulses VPL of thalamus sensory cortex in posterior central gyrus.

Impulses about fullness of bladder & rectum just

before micturition & defecation go along dorsal column tract.

Sensory Tracts

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