The Somatic Sensory System
Functional Organization of the
System, which serves:
Sense of:
• Touch
• Limb position
• Pain
• Temperature
• Regulation of arousal
• Sensory regulation of movement
• Ascending SC Pathways, which serve the
trunk and limbs.
• [the trigeminal system, which transmits
somatic sensory info from the head, has a
similar organization, but will be discussed
later].
I. Functional Anatomy of two 1°
Somatic Sensory Pathways
A. Dorsal-column-Medial lemniscal system –
mediates touch and limb position.
B. Anterolateral system – mediates pain and
temp sense and to some degree, crude
touch.
Both systems decussate before reaching the
brain, although at a different anatomical
location.
So, 1 side is receiving info from the opposite
side of the body.
Medial Lemniscal
System without the
Cerebellum (Fig. 5-1a)
Dorsal ColumnMedial Lemniscal
System (Fig. 5-2a):
Anterolateral System
without cerebellum
(Fig. 5-5b)
Anterolateral
System (Fig. 5-2b):
4 Essential Differences
1. Different types of sensory receptors:
DC: mechanical receptors
AL: receptors sensitive to noxious or thermal
stimuli.
A pure lesion of the DC system  only crude
touch and position sense.
A pure lesion of the AL system  decr
sensation to pain.
4 Essential Differences
2. Distinct relay nuclei in SC and brainstem.
DC: dorsal column nuclei in the medulla
AL: dorsal horn of the SC (close to point of
entry).
3. Distinct levels of decussation: for both
systems, the 2nd-order neuron decussates
shortly after synapsing.
DC: in medulla
AC: in SC.
-important implications for lesions?
4 Essential Differences
4. Distinct pathways which synapse in different
diencephalon regions:
DC: travels in 1 path in dorsal column  DC n. 
VPL of thalamus.
AC: lateral and ventral columns of SC via 3 different
paths, which synapse in different brain regions:
i. Spinothalamic tract – (a) brings info re painful
stimuli to separate regions of CPL (along with
DC, serves ability to discriminate and localize
the stimulus). (b) also brings the info to
intralaminar n. (note diffuse projections) 
emotional aspects of pain.
4 Essential Differences
ii. Spinoreticular tract – synapsing on reticular
formation of pons and medulla, along with
intralaminar n. of the thalamus.
iii. Spinomesencephalic tract – terminates on
midbrain tectum and PAG
Integration of all
senses for orientation
feedback regulation of
pain transmission in SC.
II. Regional Anatomy: Following the course of these
systems all the way from the sensory receptors to the
brain
A. Somatic sensory receptor.
B. Entry and initial pathways of DR axons in the SC.
C. Pathway through the medulla and the somatic sensory
decussation.
D. Projections to the reticular formation (medulla + pons)
from the AL system + paths in midbrain.
E. Descending pain supression pathways from the
brainstem.
F. Thalamic n. and processing of somatic sensory info.
G. The 1° somatic sensory cortex and its organization.
H. 2° (higher-order) somatic sensory cortical areas.
II. Regional Anatomy: Somatic Sensory
Neuronal Path
II. Regional Anatomy:
A. Somatic Sensory Receptors
DRG neurons transmit sensory info  neural signals.
Located close to SC in intervertebral foramen:
Neurons are pseudounipolar.
Distal terminal = sensory receptor structure:
1. bare nerve endings: nociceptors, thermoreceptors sense noxious
or damaging stimuli
II. Regional Anatomy:
A. Somatic Sensory Receptors
2. Encapsulated receptors: mechanoreceptors
5 morphological types shown in Fig. 5-3 (previous slide).
Which receptor types serve DC and AL systems?
Note: mechanoceptors have the largest-diameter axons
and are the fastest-conducting, covered with thick
myelin sheath (what kind of cell provides this sheath?)
After DRG cell, axons enter SC at the dorsal root.
Dermatome: Area of skin innervated by axons in a single
dorsal root segment (Fig. 5-4, next slide).
II. Regional anatomy: B. Entry and initial
pathways of DR axons in the SC.
1. 2 systems are segregated even at the level
of entry.
Large-diameter axons serving touch and
position enter at dorsal column.
The smaller-diameter axons serve pain and
temperature sensation lateral to the DC:
Lissauer’s Tract
{This overlies the dorsal horn}
Once inside the SC, DRG axons branch into:
II. Regional anatomy: B. Entry and initial
pathways of DR axons in the SC.
a. Segmental branches  enter gray matter
and synapse immediately (i.e., reflex
circuits, interneurons).
b. Ascending branches  carry sensory info
upward (as we have noted).
c. Descending branches  synapse on
interneurons below level of entry (i.e.,
limb withdrawal reflexes).
II. Regional anatomy: B. Entry and initial
pathways of DR axons in the SC.
2. Within the SC, both gray and white matter
have a topographical organization.
a. Rexed’s laminae of gray matter and
separation of terminations.
DC: segmental branches of large-diameter
neurons: skin  L III-VI
muscle  L V, VI, VII, IX
AL: segmental branches of small-diameter
neurons  L I, II only.
Rexed’s Laminae (Fig. 5-5c)
II. Regional anatomy: B. Entry and initial pathways of DR
axons in the SC.
Somatotopy:
b. White matter topographically (somatotopically)
organized according to what part of body the sensory info
comes from.
Dorsal column:
lower body  medial portion of DC {gracile fascicle}
upper body  mostly lateral {cuneate fascicile}
This ensures that the local relationship in the periphery are
preserved in the CNS.
*Clinical e.g,: lower sc injury  atrophy and demyelination
of gracile fascicle (medial only). (fig. 5-8)
AL System: axons decussate in sc before ascending (ventral
commissure). (fig. 5-1b)
AL axons: lower body  lateral
upper body  medial (fig. 5-1b)
White Matter Somatotopy (Fig. 5-7)
Lower spinal cord lesion (Fig. 5-8)
II. Regional anatomy
C. Pathways through the Medulla and the Somatic Sensory
Decussation.
1. DC nuclei:
fine-tuning functions (not just processing info)
gracile fascicle  gracile nucleus (medial)
cuneati fascicle  cuneate nucleus (lateral)
decussation (interal arcuate fibers) contralateral ML (fig.
5-11)
Note: ML is now ventral to central canal, but still dorsal
to the pyramids.
2. AL fibers: still anterolateral (fig. 5-1b)
D. Projections to the Reticular Formation of the Medullas and
Pons from the AL system.
1. Spinoreticular tract (AL system)  reticular formation
(arousal, emotional aspects of pain)
Myelin stained section (Fig. 5-5a)
Pathways through Medulla (Fig. 5-11)
II. Regional anatomy
2. In midbrain, the AL system travels dorsal to the ML (fig. 5-11)
spinothalamic (to thalamus) + spinomesencephalic tracts  superior
colliculus + periaquaductal grey (PAG) (fig. 5-15)
E. Descending Pain Suppression Pathway from Brainstem.
PAG (in midbrain)  raphe n. (in medulla) – uses 5-HT projections
to dorsal horn (in SC).
5-HT suppresses pain transmission in dorsal horn by:
i. Inhibiting ascending AL system pain projection neurons.
ii. Exciting inhibitory (enkephalin-producing) interneurons in dorsal
horn.
F. Thalamic Nuclei and Somatic Sensory Information.
Thalamus is nodal point and relay n. for transmission of sensory info from
SC and brainstem  cerebral cortex.
DC-ML and AL systems  VP n. and posterior n. (VPL for
discrimination)
Pathways through Midbrain (Fig. 5-15)
Spinoreticular termination (Fig. 5-14A)
II. Regional anatomy
Spinothalamic  intralaminar n. (affective aspects of pain
sensation).
Lateral VP n. (VPL) is 1° input to  1° somatic sensory
cortex (postcentral gyrus).
VPM sends info from the face and perioral structures
(trigeminal system) (fig. 5-16 ).
Somatotopic Organization: functional units of neurons from
some part of the body and the same sensory modality
cluster together and project to specific location in
postcentral gyrus (cortical columns).
G. The 1° Somatic Sensory Cortex and its Organization.
posterior limb
VPL --------- 1° cortex
of internal capsule
II. Regional anatomy
1. Columnar Organization:
cortical columns:
– receive inputs from the same peripheral location.
- receive inputs from the same class of sensory receptor
VPL ---------------------- layer 4 (and deep layer 3)
thalamocortical axons
*{this is why Layer 4 is the most highly developed (thickest)
in sensory cortices}
2. Somatotopic Organization:
See homonculus.
Lower body: lateral portion of VPL to medial portion of 1°
cortex
Upper body: medial VPL to lateral 1° cortex
Cortical Somatotopy: Homunculus
(Fig. 5-16B)
Pathways through Pons (Fig. 5-14b)
Ventral Posterior nuclei of the Thalamus
(Fig. 5-17b)
II. Regional anatomy
Note: greater representation for body parts with richer
sensory innervation, such as the fingers.
*the representation is not static, however. Rather, it is based
on use.
(if a body’s sensory paths are damaged from a particular area,
its cortical representation atrophies (shrinks)).
3. Modality-specific organization.
1° somatic sensory cortex contains 4 Brodman’s areas:
1, 2, 3a, 3b. (fig. 5-19)
2, 3a: limb position, shape discrimination
1, 3b: touch perception
4. Efferent projections:
a. Corticocorticol (“ascending”)  higher-order sensory
cortices, motor cortex.
- process info, planning and controlling from layers 2, 3.
Brodman’s Areas (Fig. 5-19b)
II. Regional anatomy
b. Callosal
Through corpus callosum  to opposite side of brain
(coordinate sensory input from both sides of the body) –
also located in layers 2, 3
c. Descending
striatum
VPL of thalamus
brainstem (DC nuclei)
dorsal horn of SC
“gate-keeper” or filter function:
Regulating the quantity of somatic sensory info ascending
through the CNS.
H. Higher-order somatic sensory cortical areas.
Much info passes from 1° cortex to other areas of parietal
lobes, which process the sensory info. For perception and
higher brain functions.
e.g., Posterior parietal cortex – for perception of body image.
Clinical Correlation
Sensory deficits that follow SC injury permit the localization
of the trauma.
- Deficits at the level of and caudal to lesion.
- Brown-Sequard Syndrome: - SC hemisection
- As I present the “S” and “O” of this case, consider
importantly,
1. The modality affected
2. The laterality
3. The level (body regions affected)
-to help localize the lesion.
Clinical Correlation
S
A 30-yr old man presents with progressive numbness and
weakness in his left leg and hip and left trunk. He noted
no difficulties on the right side, except for observations
that he felt no pain when he accidently banged his right
foot against a door.
Medical history (-); basically healthy.
O
On physical exam, patient was found to have marked
diminishment in sensation of blunt pressure from
ambilicus downward on left side.
L: light touch was also diminished:
2-point discrimination nearly absent (normally ~ 3 cm)
Clinical Correlation
• Graphesthesia absent (draw letter “A” on leg)
• Vibration sense diminished on left leg  hip (tuning fork).
• Position sense: Patient showed a (+) Romberg sign (falling
to left when made to stand erect with eyes closed).
• All the preceding was intact on the right side.
• Temp, pain sensation were intact on left side, but
diminished on right side from lower hip downward
– inability to distinguish head of pins from sharp points.
- inability to discriminate test tubes filled with warm or
cold H2O.
A
• Test: MRI showed a tumor encroaching on the SC from the
left at the level of T10.
Clinical Correlation
• How do these symptoms relate to crossing-over of the 2
pathways?
• Why is the loss of pain, temp a couple of segments lower
than the lesion?*
P
• Treatment Plan:
Diagnose tumor (myoma) – not invasive (pressing on SC).
- progressive return of functions after surgical removal
(85% within 1 year)
*The axons of the AL system decussate over a distance of 1-2
spinal segments before ascending to the brainstem and
diencephalon.