ASCENDING TRACTS

ASCENDING ASCENDING TRACTSTRACTS

POSTERIOR POSTERIOR COLUMNCOLUMN

DR. NILESH N. KATE M.DASSOCIATE PROFESSOR

Department Of Physiology

OBJECTIVESAt the end of the class, you should know1. Tracts of spinal cord & brain stem

2. Posterior column

3. Origin

4. Distribution

5. Course

6. Crossing

7. Termination

8. Applied

Dorsal

Ventral

Dorsal

Lateral

Ventral

Horns

Spinal Cord

Spinal cord Dorsal horn - sensory input Lateral horn - autonomic N.S. Ventral horn - motor output ~

Ascending Pathways Three-neuron pathways:

Primary sensory neurons:From external receptorsTravel through dorsal roots of spinal cordSecondary neurons:Make up tracts in spinal cord and brainstemTertiary neurons:From thalamus to primary sensory cortexTravel through internal capsule

Ascending Pathways For conscious perception:

Spinothalamic systemMedial Lemniscal system

For unconscious perception:SpinocerebellarSpino-olivarySpinotectalSpinoreticular

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

sensation (fine touch), pressure, and vibration.

Medial Lemniscus System Primary fibers ascend entire length of

spinal cord and synapse with secondary neurons in medulla:

Fasciculus gracilisFasciculus cuneatus

Medial Lemniscus Red:

Gracilis Blue

cuneatus

Medial Lemniscus System Fibers of fasciculus gracilis synapse in

nucleus gracilis:Convey sensations from below midthoracic level.

Fibers of fasciculus cuneatus synapse in nucleus cuneatus:Convey sensations from above midthoracic level.Also conveys proprioceptive sensation from arms to cerebellum.

Medial Lemniscus System Secondary fibers decussate. Secondary fibers ascend to synapse in

VPL of thalamus. Tertiary fibers ascend through

internal capsule to primary sensory 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 pass through thalamus

3 Order Neuronal pathway is essential

Body represented in the contralateral cerebral hemisphere

POSTERIOR COLUMN

Spino thalamic (lateral & anterior)

Description of a tract Synonyms OriginDistribution Course CrossingTermination Function Applied

Ascending Spinal Tracts

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

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

Ascending tracts Major 1)posterior

column 2)Anterolateral

system(spinothalamic)

3)Spino cerebellar tract

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 Pathway

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

c. Dorsal Column Pathway

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

Ascending Pathways

Figure 13.33a

Tracts of the Spinal Cord

Figure 13.32

The 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 Column

Spinothalamic Pathways

Somatosensory cortex of Postcentral Gyrus

Relative sizes of cortical areas proportional to number

of sensory receptors proportional to the

sensitivity of each part of the body

Can be modified with learning learn to read Braille &

will have larger area representing fingertips

Sensory Homunculi

Sensory Homunculi

Somatic Sensory Somatic Sensory PathwaysPathways

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

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

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

Posterior Column

Posterior Column

• 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 Touchdiscrimination Minimum distance at

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

between receptive fields. Indication of tactile

acuity. If distance between 2 points

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

Receptive FieldsReceptive 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.

STEREOGNOSIS

Thank you…

Today’s PHYSIOLOG

Y is

tomorrow’s MEDICINE.

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OBJECTIVES.

Monday, May 1, 2023

Thank You