medicine cns

1. The anatomy of
the somatosensory system
Dr. Tin Moe Nwe
Associate Professor
FMHS, UNIMAS
2.11.2020

2. Ascending tracts
• Represents functional
pathways that convey sensory
information from soma or
viscera to higher levels.
• Usually consist of a chain of
three neurons: first, second
and third order neurons.
• Usually decussate/cross
before reaching third order
neurons.
• Destination is primary
somatosensory cortex.
Neuron I
Neuron II
Neuron III

3. General sensory pathway
• The first order neuron – in
the dorsal root ganglion (DRG)
• The second order neuron – in
the brain stem or spinal cord.
Then axon of the second order
neuron crosses the midline.
• The third order neuron – in
the thalamus
• Then axon of the third order
neuron projects to the primary
somatosensory cortex.

4. Dorsal column-
medial lemniscus pathway
• Carries sensory
information for two point
discrimination, joint position
(conscious proprioception),
form recognition, vibratory
and pressure sensation from
trunk and limb.
• Receives input from Pacini
and Meissner corpuscles,
muscle spindles and Golgi
tendon organs.

5. Dorsal column-medial lemniscus pathway
• The first order neuron –
in the dorsal root ganglion
• Gives rise to the
fasciculus gracilis from the
lower extremities and the
fasciculus cuneatus from
the upper extremities
• The second order
neuron – in the gracile
and cuneate nuclei of the
medulla.

6. • Axons of the second order
neuron gives rise internal
arcuate fibres that decussate in
the midline and form a bundle,
the medial lemniscus.
•The third order neuron – in
the VPL nucleus of thalamus
• Then axons of the third order
project via the posterior limb
of internal capsule to the
primary somatosensory cortex.
Dorsal column-medial lemniscus pathway

7. • Lesion of the dorsal column
results in a loss of joint and
position sensation and two
point discrimination, vibratory
and pressure sensation. There is
also loss of the ability to identify
an object by active touch of the
hands without visual input is
called astereognosis.
• If the lesion is in the spinal
cord all the sensory deficits are
found below the level of lesion
on the ipsilateral/same side.
Lesion
loss of joint position
sensation and two pint
discrimination, vibratory
and pressure sensation
Clinical correlations

8. • Romberg sign
Ask the patient to place his/her feet together with eyes
closed. If there is deterioration of posture, this is a positive
Romberg sign, suggesting that lesion is in the dorsal column.
• With the eyes open, interruption of proprioceptive input
of dorsal column can be compensated by cerebellum.
• if the patient has balance problems and tends to sway with
eyes open, this is indicative of cerebellar lesion.
Clinical correlations

9. Spinothalamic tracts
• Ventral spinothalamic tract
carries light touch sensation.
Receives input from free
nerve endings and Merkle
tactile disk.
• Lateral spinothalamic tract
carries pain and
temperature sensation.
Receives input from free
nerve endings and thermal
receptors.

10. • The first order neuron –
in the dorsal root
ganglion. Project axons via
the dorsoateral tract of
Lissaurer to the second
order neuron.
• The second order
neuron – in dorsal horn.
Gives rise to the axons
that decussate in the
ventral white
commissure.
dorsoateral tract
of Lissaurer
Spinothalamic tracts

11. • Axons of the second order
neuron form ventral
spinothalamic tract ascend in
ventral white column and
those of the lateral
spinothalamic tract ascend in
lateral white column.
•The third order neuron – in
the VPL nucleus of thalamus
• Then axons of the third order
project via the posterior limb
of internal capsule to the
primary somatosensory cortex.
Spinothalamic tracts

12. • Because the lateral spinothalamic
tracts crosses as soon as it enters
the spinal cord, any unilateral lesion
in the spinal cord or brain stem will
result in contralateral(opposite
side) loss of pain and temperature
sensation.
• This is an extremely useful clinical
sign it means that if a patient
present with analgesia (loss of pain
and temperature sensation) on one
side of the trunk or limbs, the
location of the lesion must be on
the contralateral side.
Loss of pain and
Temperature sensation
Lesion
Clinical correlations

13. • Dorsal and ventral
spinocerebellar tracts carry
input from the lower
extremities and lower
trunk.
• Cuneocerebellar carries
input from the upper
extremities and upper
trunk.
• They carries unconscious
proprioception.
Spinocerebellar
pathways

14. • Lesions that affect only the spinocerebellar tracts
are uncommon.
• In Friedreich ataxia, which is an autosomal recessive
trait, the spinocerebellar tract, dorsal columns,
corticospinal tracts and cerebellum are involved.
• Ataxi of gait is the most common initial symptoms
of this disease.
Clinical correlations

15. • Dorsal column- Ipsilateral loss
of position and vibrator senses
below the level of lesion
• Spinal thalamic tract (lat)–
contralateral loss of pain and
temperature one to two
segments below the level of
lesion (some fibres ascend and
descend a few segment)
• Lateral corticospinal tract –
Ipsilateral UMN type of lesion
below the level of lesion
Hemisection of the spinal cord
Lesion
Loss of pain
and Temperature
sensation
loss of joint position
sensation and two pint
discrimination, vibratory
and pressure sensation

16. Ascending
trigeminothalamic tracts
• Carries GSA information from face.
• Ventral and dorsal trigeminothalamic
tracts.
• Neuron I – Trigeminal ganglion
• Neuron II
Ventral – spinal trigeminal nucleus
Dorsal – principal sensory nucleus
• Neuron III – VPM nucleus of thalamus
• Destination – primary somatosensory
cortex (face area)
• Lesion – Loss of sensation on face