ASCENDING TRACTS

1. ASCENDING TRACTSASCENDING TRACTS
POSTERIOR COLUMNPOSTERIOR COLUMN
DR. NILESH N. KATE
M.D
ASSOCIATE PROFESSOR
Department Of Physiology
ESIC Medical College and
hospital, Gulbarga.

2. OBJECTIVES
At the end of the class, you should
know
1.1.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

3. Dorsal
Ventral
Dorsal
Lateral
Ventral
Horns
Spinal Cord

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

5. Ascending Pathways
Three-neuron pathways:Three-neuron pathways:
Primary sensory neurons:Primary sensory neurons:
From external receptorsFrom external receptors
Travel 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 cortex
Travel through internal capsuleTravel through internal capsule

6. Ascending Pathways
For conscious perception:For conscious perception:
Spinothalamic systemSpinothalamic system
Medial Lemniscal systemMedial Lemniscal system
For unconscious perception:For unconscious perception:
SpinocerebellarSpinocerebellar
Spino-olivarySpino-olivary
SpinotectalSpinotectal
Spinoreticular Spinoreticular

7. 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.

8. Medial Lemniscus System
Primary fibers ascend entire length of Primary fibers ascend entire length of
spinal cord and synapse with spinal cord and synapse with
secondary neurons in medulla:secondary neurons in medulla:
Fasciculus gracilisFasciculus gracilis
Fasciculus cuneatusFasciculus cuneatus

9. Medial Lemniscus
RedRed::
GracilisGracilis
BlueBlue
cuneatuscuneatus

10. Medial Lemniscus System
Fibers of fasciculus gracilis synapse in Fibers of fasciculus gracilis synapse in
nucleus gracilis: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.

11. Medial Lemniscus System
Secondary fibers decussate.Secondary fibers decussate.
Secondary fibers ascend to synapse in Secondary fibers ascend to synapse in
VPL of thalamus.VPL of thalamus.
Tertiary fibers ascend through Tertiary fibers ascend through
internal capsule to primary sensory internal capsule to primary sensory
cortex.cortex.

12. Medial Lemniscus

13. Medial Lemniscus

14. Somatosensory Pathways
Crossover at spinal cord
Pain and Temperature
Tickle and Itch
Poorly localised touch
Crossover in medulla
Discriminative touch
Shape, size texture, weight
Vibration
Proprioception

15. 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

16. POSTERIOR COLUMN
Spino
thalamic
(lateral &
anterior)

17. Description of a tract
Synonyms Synonyms
OriginOrigin
Distribution Distribution
Course Course
CrossingCrossing
Termination Termination
Function Function
Applied Applied

18. 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.

19. Ascending tracts
Major Major
1)1)posterior posterior
column column
2)2)Anterolateral Anterolateral
system(spinothsystem(spinoth
alamic)alamic)
3)3)Spino Spino
cerebellar tractcerebellar tract

20. 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 sensation
7. Stereognosis.
8. Pressure sensation with fine
gradations.
Functions of posterior column

21. Somatosensory Cortex
Area on somatosensory cortex related to degree of innervation

23. 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.

24. 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 the medulla and ascend to the
thalamus through the medial thalamus through the medial
lemniscus of the medulla, lemniscus of the medulla,
pons, and midbrain.pons, and midbrain.

25. Ascending PathwaysAscending Pathways
Figure 13.33a

26. Tracts of the Spinal CordTracts of the Spinal Cord
Figure 13.32

27. 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

28. Posterior ColumnPosterior Column

29. Spinothalamic Pathways

30. 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

31. Sensory HomunculiSensory Homunculi

32. Sensory HomunculiSensory Homunculi

33. Somatic Sensory Somatic Sensory
PathwaysPathways
First-order neuron conduct impulses to First-order neuron conduct impulses to
brainstem or spinal cordbrainstem or spinal cord
either spinal or cranial nerves either spinal or cranial nerves
Second-order neurons conducts impulses Second-order neurons conducts impulses
from spinal cord or brainstem to thalamus--from spinal cord or brainstem to thalamus--
cross over to opposite side before reaching cross over to opposite side before reaching
thalamusthalamus
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)

34. Posterior ColumnPosterior Column

35. Posterior ColumnPosterior 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

36. Two-Point TouchTwo-Point Touch
discrimination 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.

37. 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.

38. STEREOGNOSISSTEREOGNOSIS

39. Thank you…Thank you…
Today’s
PHYSIOLOGY
is
tomorrow’s
MEDICINE.

40. Thank
you.