ASCENDING TRACTS

1. ASCENDINGASCENDING
TRACTSTRACTS
POSTERIORPOSTERIOR
COLUMNCOLUMN
DR. NILESH N. KATE M.D
ASSOCIATE PROFESSOR
Department Of Physiology

2. OBJECTIVES
At the end of the class, you should
know
1. Tracts of spinal cord & brain stem
2. Posterior column
3. Origin
4. Distribution
5. Course
6. Crossing

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:
Primary sensory neurons:
From external receptors
Travel through dorsal roots of spinal
cord
Secondary neurons:
Make up tracts in spinal cord and
brainstem
Tertiary neurons:
From thalamus to primary sensory
cortex

6. Ascending Pathways
 For conscious perception:
Spinothalamic system
Medial Lemniscal system
 For unconscious perception:
Spinocerebellar
Spino-olivary
Spinotectal
Spinoreticular

7. Medial Lemniscus System
 Also called posterior column system.
 Carries sensations for two-point
sensation (fine touch), pressure, and
vibration.

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

9. Medial Lemniscus
 Red:
 Gracilis
 Blue
 cuneatus

10. 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.

11. 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.

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
pass through thalamus
 3 Order Neuronal
pathway is essential
 Body represented in
the contralateral
cerebral hemisphere

16. POSTERIOR COLUMN
Spino thalamic
(lateral &
anterior)

17. Description of a tract
Synonyms
Origin
Distribution
Course
Crossing
Termination
Function
Applied

18. 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.

19. Ascending tracts
 Major
1)posterior
column
2)Anterolateral
system(spinoth
alamic)
3)Spino
cerebellar 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
Pathway
.The axons from
these first-order
neurons pass upward
in the fasciculus
gracilis or fasciculus
cuneatus.

24. 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.

25. Ascending Pathways
Figure 13.33a

26. Tracts of the Spinal Cord
Figure 13.32

27. 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

28. Posterior Column

29. Spinothalamic Pathways

30. 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

31. Sensory Homunculi

32. Sensory Homunculi

33. Somatic SensorySomatic 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)

34. Posterior Column

35. 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

36. Two-Point Touch
discrimination
 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.

37. Receptive Fields
Receptive fields in
the thalamusthalamus have
centre-surround
organization.
Cortical receptiveCortical receptive
fields (left)fields (left) are
smaller in the fingers
and larger on the
hand and forearm.

38. STEREOGNOSIS

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

40. Thank
you.

41. OBJECTIVES.
Wednesday, June 8, 2016

42. Thank
You