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Termination

These two tracts terminate in the medulla oblongata. The fibers of fasciculus gracilis terminate in the nucleus gracilis and the fibers of fasciculus cuneatus terminate in the nucleus cuneatus. The cells of these medullary nuclei form the second order neurons.

The axons of the second order neurons form the internal arcuate fibers. The internal arcuate fibers from both the sides cross the midline forming sensory decussation and then ascend through pons and midbrain as medial lemniscus. The fibers of medial lemniscus terminate in ventral posterolateral nucleus of thalamus. From here, fibers of the third order neurons relay to cerebral cortex.

Functions

The tracts of the posterior white funiculus convey impulses of following sensations:

1. Fine, i.e. epicretic tactile sensation.

2. Tactile localization: It is the ability to locate the area of skin where the tactile stimulus is applied.

3. Tactile discrimination: It is the ability to recognize the two stimuli, which are applied over the skin simultaneously.

4. Sensation of vibration: This is the ability to perceive the vibrations (from a vibrating tuning fork placed over bony prominence) conducted to deep tissues through skin.

5. Conscious kinesthetic sensation: It is the sensation (awareness) of various muscular activities in different parts of the body.

6. Stereognosis: It is the ability to recognize the known objects by touch with closed eyes.

The lesion in the fibers of these tracts or lesion in the posterior white column leads to the following symptoms. The symptoms appear on the same side below the lesion.

1. Loss of fine tactile sensation. However, crude touch sensation is normal.

2. Loss of tactile localization.

3. Loss of two point discrimination.

4. Loss of sensation of vibration.

5. Astereognosis: It is the inability to recognize known objects with closed eyes.

6. Lack of ability to differentiate the weight of different objects.

7. Loss of proprioception: There is inability to appreciate the position and movement of different parts of the body. Because of the loss of proprioception, the voluntary movements become incoordinated, slow and clumsy. This condition is known as sensory ataxia or posterior column ataxia.

Ascending tracts of spinal cord

Situation

Tract

Origin

Course

Termination

Function

Anterior white funiculus

1.Anterior spinothalamic

tract

Chief

sensory cells

Crossing in

spinal cord,

forms

spinal lemniscus

Ventral

posterolateral

nucleus

of thalamus

Crude

touch sensation

Lateral

white funiculus

2. Lateral

spinothalamic

tract

Substantia gelatinosa

Crossing in

spinal cord,

forms spinal

lemniscus

Ventral

posterolateral

nucleus

of thalamus

Pain and

temperature

sensations

3. Ventral

spino-cerebellar

tract

Marginal

cells

Crossing in

spinal cord

Anterior

lobe

of cerebellum

Subconscious

kinesthetic

sensations

4. Dorsal

spino-cerebellar

tract

Clarke's

column cells

Uncrossed

fibers

Anterior

lobe of

cerebellum

Subconscious

kinesthetic

sensations

Spinotectal tract

Chief

sensory cells

Crossing in

spinal cord

Superior

colliculus

Concerned

with spinovisual

reflex

6.Fasciculus dorsolateralis

Posterior nerve

root ganglion

Component

of lateral

spinothalamic tract

Substantia gelatinosa

Pain and

temperature

sensations

7. Spinoreticular

tract

Intermedio-

lateral cells

Crossed and

uncrossed

fibers

Reticular

formation

of brainstem

Consciousness

and awareness

8.Spino-olivary

tract

Not specific

Uncrossed

fibers

Olivary nucleus

Proprioception

9.Spino-vesti-

bular tract

Not specific

Crossed and

uncrossed

fibers

Lateral vestibular

nucleus

Proprioception

Posterior white funiculus

10.Fasciculus

gracilis

Posterior nerve

root ganglia

Uncrossed fibers.

No synapse

in spinal cord

Nucleus gracilis

in medulla

Tactile sensation.

Tactile localization.

Tactile

discrimination.

Vibratory sensation.

Conscious kinesthetic

sensation. Stereognosis.

11.Fasciculus cuneatus

Posterior nerve

root ganglia

Uncrossed fibers.

No synapse

in spinal cord

Nucleus cuneatus

in medulla

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