1) The document describes the ascending sensory tracts that carry sensations from the body to the brain. It focuses on the dorsal column-medial lemniscal system and the anterolateral system. 2) The dorsal column tract carries fine touch, vibration, and proprioception. It ascends ipsilaterally in the spinal cord and crosses over in the medulla before terminating in the thalamus. 3) The anterolateral system includes the ventral and lateral spinothalamic tracts. The ventral tract carries crude touch and the lateral tract carries pain and temperature. They both cross over in the spinal cord and ascend to the thalamus.

1. Ascending Tracts/ Sensory
Tracts
Made by
Mehreen roohi

2. • Four groups:
• 1) Dorsal column, medial leminiscal
system or Dorsal column tract.
2) Antero-lateral system (includes mainly spinothalamic
tract)
3) Spino-cerebellar tract
4) Miscellaneous group/tracts

3. Dorsal column tract
• 1) Fasciculus Gracilis:
• Carries sensation from lower half of
• body.
• 2) Fasciculus cuneatus:
• Carries sensation from upper half of body.
• This tract has certain special features consists of rapidly
conducting myelinated fibers.
• The velocity of conduction is 120 m/sec.

4. Sensation carried by Dorsal column
tract
• Sensations carried by this tract involve
accurate localization (exact point of
touch is localized).
• Fine touch, two-point tactile discrimination,
pressure, vibration and Proprioception
from muscles and joints i.e; position
sense.

5. • Grey matter is divided into nine laminae.
• There are certain groups of neurons in grey matter.
• 1) Substantia gelatinosa
• 2) Clarke’s column
• First order nerve fibers carrying impulses enter spinal
cord through posterior nerve root, where these nerve
fibers pass to posterior white column on the same side,
without any synapse.

6. • 1st
order neuron → posterior root
ganglion
• They ascend as dorsal column medial
lemniscal system.
• The tract is formed in lower part of spinal
cord. As tract ascends, fibers from upper
part of body are added on lateral of tract.

7. • At the level of 6th
thoracic segment, a septum appears, in this tract
which divides it into two parts.
• The medial is called Fasciculus Gracilis and lateral is called
Cuneatus.
• Fasciculus Gracilis contains fibers from lower part of body and
Fasciculus Cuneatus from upper part of body.
• These enter Medulla Oblongata, where these synapse into two
nuclei; nucleus gracilis (medial one) and nucleus cuneatus
(lateral one) located in lower part of medulla oblongata.
• 2nd
order neuron → nucleus gracilis and nucleus cuneatus in
medulla oblongata.

8. • From these two nuclei, second order nerve fibers
arise, called internal arcuate fibers.
• These fibers decussate in medulla to form sensory
decussation.
• After crossing over, these fibers form medial lemniscus
on each side.
• This lemniscus is joined by fibers carrying impulses of
same sensation from head region.
•
• These fibers come from 5th
nerve (trigeminal nerve).

11. • 3rd
order neuron→ ventrobasal complex of thalamus
(VPL nucleus)
• Medial lemniscus synapse onto the 3rd
order neurons
located in ventro-basal complex of thalamic nuclei.
• In Thalamus, there is ventral posterior nucleus, it has
two parts:
• Lateral part called VPL and medial part called VPM.
• Ventro-basal complex consists of Ventral Posterolateral
nucleus and Ventral Postero-medial nucleus (VPL &
VPM).

12. • Fibers of medial lemniscus mainly synapse in VPL.
• But fibers from Head & face area synapse in VPM.
• From ventro-basal complex, 3rd
order nerve fibers arise, which
pass through internal capsule to terminate in somatic sensory area
1, located in post-central gyrus.
• Some fibers from Cuneate nucleus go to Cerebellum & these fibers
are called Cuneo-cerebellar/ Post-external arcuate fibers.
• These fibers provide Sensory Proprioceptive information to
cerebellum from the muscles and joints.

15. • Key points:
• First order neurons are located in
Post.root ganglion.
• There is no crossing over in spinal cord.
• Crossing occurs in medulla.

16. • If there is disease in spinal cord, sensory loss is on the
same side but if there is lesion of medulla, there is loss
on opposite side.
• Lesion effects:
• Below medulla:
• Loss of ipsilateral sensation.
• Above medulla:
• Loss of contralateral sensation.

17. Somatic sensory cortex
• Areas of fine movement occupy more space in
sensory cortex.
• Behind central sulcus is posterior central gyrus
SI or somesthetic Area I (Broadman area 3,1,2).
• Opposite half of body represented upside down
i.e; from up → down feet, legs, thighs, trunk,
thorax, upper limb, hand, neck, head and face.

18. • Sensory area I (SI):
• It is located in the post-central gyrus, also extends to the
medial surface of the hemisphere into Para-central
lobule.
• Its Broadmann area is 3, 1, 2.
• This area consists of granular type of cortex.
• In the sensory cortex, neurons are arranged in the
vertical column and each of these column is specific for
a specific stimulus and is specific for a particular part of
the body.

19. • In sensory area I, there is contralateral
representation of the various parts of the body.
• The parts of the body on the Rt. Side is
represented on the left side.
• The body is representing upside down. When
the representation parts of the body are joined
together, a figure of the body is formed, which is
called Sensory Homenculus.

23. • At the lower end of the lateral surface of hemisphere
there are pharynx, tongue, face, hand, hip, lower limb
and genitals are on the medial surface.
• Area of the representation of different parts of the body
is not according to the anatomical size but according to
the functional importance.
• Parts of body which contains greater number of sensory
receptors are represented by a larger area e.g; tongue,
lips, fingers, thumb, hand are occupied by much larger
area.

25. Sensory cortex functions

30. Somatic Sensory Area II
• It is located in the superior wall of the sylvian fissure or lateral
fissure.
• SII is behind SI, in the lower part above lateral sulcus.
• Body is represented from front → backwards, face, arm and legs.
• Here body parts are incompletely represented.
• Face is anterior, while legs are posterior.
• This area receives pain impulses. Physiologically, this area is not so
important.

34. Somesthetic Association Area (SIII)
• It is located behind somatic sensory area I, in
the posterior parietal lobule.
• S III:
• Sensory association area is present in parietal
cortex, behind SI and above SII.
• It receives information from SI, SII and
ventrobasal complex of thalamus.
• Its Broadmann area: 5 &7.

35. Somesthetic Association Area (SIII)
• This area receives impulses from the somatic sensory
area, visual area (17, 18, 19) and Auditory area (41),
where these senses are analyzed and interpreted.
• Than these information go to motor cortex and order is
given for response.
• This area is also involved in stereognosis.
• So when we touch something with hand, it goes to this
area and than with the help of stored information (stored
past memory is associated with stereognosis), we can
identify the object.

36. Somesthetic Association Area (SIII)
• Effects of lesion or section of dosal
column tract:
• If lesion is at the spinal cord:
• The sensation are lost at the same side.
• Below the level of lesion:
• There will be Astereognosis.

39. Somesthetic Association Area (SIII)
• Functions of this area:
• 1) It receives impulses of all the somatic
sensations from the ventro-basal complex of the
thalamus (consisting mainly of VPL & VPM).
• Fine touch, tickle, itch, temperature,
proprioception, vibration & pain sensation (all
sensory tracts) go to sensory area.

40. • 2) It is concerned with perception of the grades of
intensity of stimulation.
• 3) It is involved in stereognosis (ability to identify shape,
structure of the object with closed eyes.
• Most effected sensations are fine touch and
proprioception and than temperature and least is pain
sensation.
• When there is recovery from the lesion:
• First to recover is pain sensation than temperature and
than finally fine touch and the proprioception.

41. ii) Antero-lateral system
• Spinothalamic tract is divided into two:
• 1) Ventral Spinothalamic tract
• 2) Lateral Spinothalamic tract

42. ii) Antero-lateral system
• Mainly Spinothalamic tract:
• Spinothalamic tract has certain features:
• 1) Fibers of tract are less rapidly conducting.
• Velocity of conduction is upto 40m/sec.
• 2) Sensation carried by this tract don’t require accurate
• localization.
• 3) Sensation carried don’t involve perception of fine
• grades of intensity.

44. Ventral Spinothalamic tract
• Sensation carried are crude touch, tickle, itch, pressure and sexual
sensation.
• First order nerve fibers carrying the impulses enter spinal cord through
Post.nerve root and synapse into laminae 5 & 6 in grey matter of spinal
cord.
• From here 2nd
order nerve fibers arise, which cross over in the anterior
commissure and reach the anterior white column of the opposite side and
form the anterior spinothalamic tract.
• Tract is formed in lower part of spinal cord. As tract ascends, fibers of upper
part of body are added on medial aspect (after crossing over).
• Fibers from lower parts of the body are situated laterally and from upper
regions more medially.

46. • Tract enters medulla oblongata, where it joins spinal
lemniscus.
• Spinal lemniscus consists of three tracts:
• 1) Ventral spinothalamic tract.
• 2) Lateral spinothalamic tract.
• 3) Spinotectal tract.
• The tract continues to ascend through the posterior part
of the pons.

47. • Spinal lemniscus ascends to synapse into
ventro-basal complex of thalamic nuclei.
• Fibers carrying impulses of same sensation from
head and face area, join spinal lemniscus in
brain stem.
• These fibers come from the 5th
nerve.
• From 3rd
order neurons, fibers arise, which pass
to internal capsule, to terminate in somatic
sensory area 1 in post.central gyrus.

48. • If lesion is in spinal cord:
• Opposite side is effected.
• Destruction of this tract produces little tactile
disturbances as touch is also carried in
posterior white columns.
• Bilateral destruction of these columns results
in a complete loss of itching, tickling and sexual
sensations showing that these sensations are
carried only by these tracts.

50. Lateral Spinothalamic tract
• Carries pain and temperature sensations.
• 1st
order nerve fibers carrying the impulses,
synapse with 2nd
order neurons, located in
laminae 1,2, 3.
• These also include substantia gelatinosa.
• From here, 2nd
order nerve fibers arise, which
cross over obliquely to opposite side to enter
lateral white column of opposite side.

51. • New fibers are added on medial aspect of tract from
upper part of body.
• Tract enters medulla, where it joins spinal lemniscus.
• Temperature fibers end mainly in the ventrobasal
complex.
• Fibers carrying fast pain end in ventro-basal complex of
thalamic nuclei and intralaminar nuclei.
• From these nuclei, 3rd
order nerve fibers arise, which
go to the sensory area 1, in postcentral gyrus.

52. • Fibers carrying slow pain impulses do not go to
ventro-basal complex of thalamic nuclei.
• Most of these fibers go to other areas and these
are:
• Nuclei of reticular formation
• Tectum of mid-brain and peri-aqueductal
grey matter.
•

54. • From these areas, fibers arise, which go to the intralaminar and midline nuclei of
thalamus.
• From these nuclei, fibers also go to sensory cortex.
• Fibers which carry slow pain don’t go to ventro-basal complex.
• Pathway of fast pain is called Neospino- thalamic tract.
• Pathway of slow pain impulses is called Paleo-spinothalamic
• tract.
• In its passage in the spinal cord, the fibers carrying pain and temperature sensations
are split; those carrying pain signals are present anterolaterally, while those carrying
temperature sensation are present posteriorly.

55. • Unilateral section of this tract produces a complete loss
of pain and thermal sense (analgesia and
thermoanesthesia) on the opposite side of the body.
• The anesthesia involves the superficial and deep
portions of the body wall, but not the viscera which
appear to be represented bilaterally.
• After a certain period, there may be some return of pain
and thermal sensations.

56. Neo-spinothalamic tract
• A-delta fibers carrying fast pain (6-30m/s),
synapse with neurons in laminae 1(lamina
marginalis).
• At this synapse, neurotransmitter is glutamate.
• Fibers cross to opposite side and ascends.
• Main fibers go to ventro-basal complex and than
to sensory cortex.

58. Paleospinothalamic pathway for
slow-chronic pain
• Slow pain impulses are carried by C-fibers→ enter spinal cord → synapse
with laminae 2, 3 (substantia gelatinosa).
• At this synapse, neurotransmitter is substance P.
• Fibers cross over to opposite side obliquely→ most fibers go to:
•
• 1) reticular nuclei of the medulla, pons and mesencephalon.
• 2) The tectal area of the mesencephalon deep to superior and inferior
• colliculi.
•
• 3) Periaqueductal gray region surrounding the aqueduct of Sylvius.
• 4) Midline and intralaminar nuclei (instead of Ventro-basal complex).