ASCENDING TRACTS

ASCENDING TRACTSASCENDING TRACTSPOSTERIOR COLUMNPOSTERIOR COLUMN

DR. NILESH N. KATE M.D

ASSOCIATE PROFESSOR

Department Of Physiology

ESIC Medical College and hospital, Gulbarga.

OBJECTIVESAt the end of the class, you

should know1.1. Tracts of spinal cord & brain stemTracts of spinal cord & brain stem

2.2. Posterior columnPosterior column

3.3. Origin Origin

4.4. DistributionDistribution

5.5. CourseCourse

6.6. CrossingCrossing

7.7. Termination Termination

8.8. Applied Applied

Dorsal

Ventral

Dorsal

Lateral

Ventral

Horns

Spinal Cord

Spinal cord

Dorsal horn - sensory inputLateral horn - autonomic N.S.Ventral horn - motor output ~

Ascending Pathways

Three-neuron pathways:Three-neuron pathways:Primary sensory neurons:Primary sensory neurons:

From external receptorsFrom external receptorsTravel through dorsal roots of spinal cordTravel through dorsal roots of spinal cord

Secondary neurons:Secondary neurons:Make up tracts in spinal cord and Make up tracts in spinal cord and

brainstembrainstem

Tertiary neurons:Tertiary neurons:From thalamus to primary sensory cortexFrom thalamus to primary sensory cortexTravel through internal capsuleTravel through internal capsule

Ascending Pathways

For conscious perception:For conscious perception:Spinothalamic systemSpinothalamic systemMedial Lemniscal systemMedial Lemniscal system

For unconscious perception:For unconscious perception:SpinocerebellarSpinocerebellarSpino-olivarySpino-olivarySpinotectalSpinotectalSpinoreticular Spinoreticular

Medial Lemniscus System

Also called posterior column system.Also called posterior column system.Carries sensations for two-point Carries sensations for two-point

sensation (fine touch), pressure, and sensation (fine touch), pressure, and vibration.vibration.


Primary fibers ascend entire length Primary fibers ascend entire length of spinal cord and synapse with of spinal cord and synapse with secondary neurons in medulla:secondary neurons in medulla:

Fasciculus gracilisFasciculus gracilis

Fasciculus cuneatusFasciculus cuneatus

Medial Lemniscus

RedRed:: GracilisGracilis

BlueBlue cuneatuscuneatus


Fibers of fasciculus gracilis synapse Fibers of fasciculus gracilis synapse in nucleus gracilis:in nucleus gracilis:

Convey sensations from below midthoracic Convey sensations from below midthoracic level.level.

Fibers of fasciculus cuneatus synapse Fibers of fasciculus cuneatus synapse in nucleus cuneatus:in nucleus cuneatus:

Convey sensations from above midthoracic Convey sensations from above midthoracic level.level.

Also conveys proprioceptive sensation from Also conveys proprioceptive sensation from arms to arms to cerebellum.cerebellum.


Secondary fibers decussate.Secondary fibers decussate.Secondary fibers ascend to synapse Secondary fibers ascend to synapse

in VPL of thalamus.in VPL of thalamus.Tertiary fibers ascend through Tertiary fibers ascend through

internal capsule to primary sensory internal capsule to primary sensory cortex.cortex.

Medial Lemniscus

Medial Lemniscus

Somatosensory Pathways

Crossover at spinal cordPain and TemperatureTickle and ItchPoorly localised touch

Crossover in medullaDiscriminative touchShape, size texture, weightVibrationProprioception

Criteria for a sensory pathway

All sensation has to All sensation has to pass through pass through thalamus thalamus

3 Order Neuronal 3 Order Neuronal pathway is essentialpathway is essential

Body represented in Body represented in the contralateral the contralateral cerebral hemisphere cerebral hemisphere

POSTERIOR COLUMN

Spino thalamic (lateral & anterior)

Description of a tract Synonyms Synonyms OriginOriginDistribution Distribution Course Course CrossingCrossingTermination Termination Function Function Applied Applied

Ascending Spinal TractsAscending Spinal Tracts

Convey sensory Convey sensory information from information from cutaneous cutaneous receptors, receptors, proprioceptors and proprioceptors and visceral receptors to visceral receptors to cerebral cortex.cerebral cortex.

Sensory fiber tract Sensory fiber tract decussation may decussation may occur in medulla or occur in medulla or spinal cord.spinal cord.

Ascending tracts Major Major

1)1)posterior posterior column column

2)2)Anterolateral Anterolateral system(spinosystem(spinothalamic)thalamic)

3)3)Spino Spino cerebellar cerebellar tracttract

Sensations carried by Dorsal columns:

Light touch (tactile sensation,2. Tactile discrimination 3. Tactile localization)4. Conscious proprioception –5. Joint position and movement.6. Vibration sensation7. Stereognosis.8. Pressure sensation with fine

gradations.

Functions of posterior column

Somatosensory Cortex

Area on somatosensory cortex related to degree of innervation

c. Dorsal Column c. Dorsal Column PathwayPathway

..The axons from The axons from these first-order these first-order neurons pass neurons pass upward in the upward in the fasciculus gracilis fasciculus gracilis or fasciculus or fasciculus cuneatus.cuneatus.

c. Dorsal Column Pathwayc. Dorsal Column Pathway

The axons from the second-The axons from the second-order neurons (nucleus order neurons (nucleus gracilis or nucleus cuneatus) gracilis or nucleus cuneatus) cross to the opposite side of cross to the opposite side of the medulla and ascend to the medulla and ascend to the thalamus through the the thalamus through the medial lemniscus of the medial lemniscus of the medulla, pons, and midbrain.medulla, pons, and midbrain.

Ascending PathwaysAscending Pathways

Figure 13.33a

Tracts of the Spinal CordTracts of the Spinal Cord

Figure 13.32

The Spinal CordThe Spinal Cord

Ventral

Dorsal

3 GROUP OF CELLS IN THE POSTERIOR HORN

1.Substantia gelatinosa of rolando(sgr)

2.Chief sensory nucleus

3.Clarke’s column

Posterior ColumnPosterior Column

Spinothalamic Pathways

Somatosensory cortex of Postcentral GyrusSomatosensory cortex of Postcentral Gyrus

Relative sizes of Relative sizes of cortical areascortical areas proportional to number proportional to number

of sensory receptors of sensory receptors proportional to the proportional to the

sensitivity of each part sensitivity of each part of the bodyof the body

Can be modified with Can be modified with learninglearning learn to read Braille & learn to read Braille &

will have larger area will have larger area representing fingertipsrepresenting fingertips

Sensory HomunculiSensory Homunculi

Sensory HomunculiSensory Homunculi

Somatic Sensory Somatic Sensory PathwaysPathways

First-order neuron conduct impulses to First-order neuron conduct impulses to brainstem or spinal cordbrainstem or spinal cordeither spinal or cranial nerves either spinal or cranial nerves

Second-order neurons conducts impulses from Second-order neurons conducts impulses from spinal cord or brainstem to thalamus--cross spinal cord or brainstem to thalamus--cross over to opposite side before reaching thalamusover to opposite side before reaching thalamus

Third-order neuron conducts impulses from Third-order neuron conducts impulses from thalamus to primary somatosensory cortex thalamus to primary somatosensory cortex (postcentral gyrus of parietal lobe)(postcentral gyrus of parietal lobe)



• Proprioception, vibration, discriminative touch, weight discrimination & stereognosis

• Signals travel up spinal cord in posterior column

• Fibers cross-over in medulla to become the medial lemniscus pathway ending in thalamus

• Thalamic fibers reach cortex

Two-Point TouchTwo-Point Touchdiscrimination discrimination

Minimum distance at Minimum distance at which 2 points of touch which 2 points of touch can be perceived as can be perceived as separate.separate. Measures of distance Measures of distance

between receptive fields.between receptive fields. Indication of tactile Indication of tactile

acuity.acuity. If distance between 2 If distance between 2

points is less than points is less than minimum distance, only 1 minimum distance, only 1 point will be felt.point will be felt.

Receptive FieldsReceptive Fields

Receptive fields in the thalamusthalamus have centre-surround organization.

Cortical receptive Cortical receptive fields (left)fields (left) are smaller in the fingers and larger on the hand and forearm.

STEREOGNOSISSTEREOGNOSIS

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ASCENDING TRACTS