The somatosensory system is the part of the sensory system concerned with the conscious perception of touch, pressure, pain, temperature, position, movement, and vibration, which arise from the muscles, joints, skin, and fascia. The somatosensory system is a 3-neuron system that relays sensations detected in the periphery and conveys them via pathways through the spinal cord, brainstem, and thalamic relay nuclei to the sensory cortex in the parietal lobe Impulses are carried from receptors via sensory afferents to the dorsal root ganglia, where the cell bodies of the first-order neurons are located. Here the fibers split into 2 functional groups: a lateral group (or anterolateral system) and a medial group (or dorsal column-medial lemniscal system). The lateral group carries mainly unmyelinated fibers that subserve pain and temperature sensations, whereas the medial group carries mainly myelinated fibers that convey proprioceptive impulses Their axons then travel through the spinal cord either in an ipsilateral or a contralateral fashion. Note that second-order neuron cell bodies are located in different anatomical areas depending on the sensation they carry. Broadly, the spinal cord contains the second-order neurons for the fibers carrying pain, touch, and temperature sensations. The lateral group of fibers enters the spinal cord, then ascend to terminate on the substantia gelatinosa and the nucleus proprius, where the second-order neurons are housed Fibers then ascend via the brainstem to the thalamus in the spinothalamic tracts (or STT). The medulla contains the second-order neurons for fibers carrying touch, position, and vibratory sensations. The fibers are then either conveyed to the thalamus (where the third-order neurons are located) The medial group also sends its fibers into the posterior spinal cord; however, upon reaching it, most fibers ascend to the dorsal column nuclei in the medulla and synapse there These tracts synapse on a second-order neuron in the nucleus gracilis and cuneatus, which are located in the medulla. Their axons then decussate form a bundle known as the medial lemniscus. Fibers of the posterior columns and medial lemniscus are concerned primarily with position sense and fine discriminative touch These fibers travel to the midbrain on their way to the thalamus. Once in the thalamus, they synapse on third-order neurons in the ventral posterior lateral (VPL) nucleus. The third-order neurons then project to the primary somatosensory cortex, which is located in the postcentral gyrus (also known as Brodmann areas 1, 2, and 3) of the parietal lobe Primary somatosensory cortex subserves general and proprioceptive sensations and serves to integrate sensory information Somesthetic cortex is organized in a sensory homunculus Body areas particularly important to the sensory system (for example the face, lips, and hand) are given larger representation than other areas Sensory receptorsperipheral nerve dorsal

1. Anatomy of Somatosensory
pathways and technical
aspects
FAIZAN ABDULLAH
30/10/23

2. • The somatosensory system is the part of the sensory system
concerned with the conscious perception of touch, pressure, pain,
temperature, position, movement, and vibration, which arise from
the muscles, joints, skin, and fascia.
• The somatosensory system is a 3-neuron system that relays
sensations detected in the periphery and conveys them via pathways
through the spinal cord, brainstem, and thalamic relay nuclei to the
sensory cortex in the parietal lobe

3. Sensory receptors in skin

4. • Impulses are carried from receptors via sensory
afferents to the dorsal root ganglia, where the cell
bodies of the first-order neurons are located.
• Here the fibers split into 2 functional groups: a lateral
group (or anterolateral system) and a medial group (or
dorsal column-medial lemniscal system).
• The lateral group carries mainly unmyelinated fibers
that subserve pain and temperature sensations,
whereas the medial group carries mainly myelinated
fibers that convey proprioceptive impulses
• Their axons then travel through the spinal cord either
in an ipsilateral or a contralateral fashion. Note that
second-order neuron cell bodies are located in
different anatomical areas depending on the
sensation they carry.
• Broadly, the spinal cord contains the second-order
neurons for the fibers carrying pain, touch, and
temperature sensations.

5. • The lateral group of fibers enters the
spinal cord, then ascend to terminate on
the substantia gelatinosa and the nucleus
proprius, where the second-order
neurons are housed
• Fibers then ascend via the brainstem to
the thalamus in the spinothalamic tracts
(or STT).
• The medulla contains the second-order
neurons for fibers carrying touch,
position, and vibratory sensations. The
fibers are then either conveyed to the
thalamus (where the third-order neurons
are located)

6. • The medial group also sends its fibers into the
posterior spinal cord; however, upon reaching it,
most fibers ascend to the dorsal column nuclei
in the medulla and synapse there
• These tracts synapse on a second-order neuron
in the nucleus gracilis and cuneatus, which are
located in the medulla.
• Their axons then decussate form a bundle
known as the medial lemniscus.
• Fibers of the posterior columns and medial
lemniscus are concerned primarily with position
sense and fine discriminative touch
• These fibers travel to the midbrain on their way
to the thalamus. Once in the thalamus, they
synapse on third-order neurons in the ventral
posterior lateral (VPL) nucleus.

7. • The third-order neurons then project to the
primary somatosensory cortex, which is located
in the postcentral gyrus (also known as
Brodmann areas 1, 2, and 3) of the parietal lobe
• Primary somatosensory cortex subserves
general and proprioceptive sensations and
serves to integrate sensory information
• Somesthetic cortex is organized in a sensory
homunculus
• Body areas particularly important to the sensory
system (for example the face, lips, and hand) are
given larger representation than other areas

9. Dorsal column pathway
• Sensory receptorsperipheral nerve dorsal root
ganglionposterior nerve rootsSpinal cord (substantia
gelatinosa/post.column)  neucleus gracialis and cutaneous
medial lemniscus thalamus  somatosensory cortex

10. EP
• Ep are potentials produced in response to stimulation of nervous
system ;evoked’ by sensory, electrical, magnetic stimulation

11. • SEP is the response to electrical stimulation of PN
• Any nerve testing is possible. Commonly used are
• Median ulnar peroneal,tibial
• Wave larger with mixed nerve stimulation

12. Indicatins
• Organic lesions in Sens pathway
• Multiple sclerosis
• Friedreichs ataxia: large fibers involvement vibration and
proprioception.
• Spinocerebellar degeneration: absent cervical and cortical
• Huntingtons chorea: showed reduced amp of potentials
• Coma/ Brain Death: in brain death N18 and N20- should be absent
with cervical present.

13. • Short latency: <30ms more stable, amp less, resistant to change in
conscious level.
• Medium latency:30-100ms
• Long latency:>100ms
• Mixed nerves have better response that can be seen by muscle
twitch.

15. Method/ recording
For upper limbs
• Median nerve
• Stimulation at wrist
• Recording electrodes are ERBS
point, cervical, and cortical
electrodes.
For lower limbs
• post tibial nerve
• Stimulation at ankle
• Recording electrodes are pop
fossa, lumber, cortical
electrodes.

17. Method/ recording
• Stimulus electrodes applied to the skin overlying the nerve.
• electrode with anode distal and cathode proximal 2-3 cms prox to
wrist crease.
• Placed where smallest stimulus needed.
• Constant current pulses given.
• Constant current is preferable to constant voltage stimulation.
• Stimulate to supramaximal
• Supramax is lower in short lat sep .

18. • Duration of stim= 0.1-0.3 ms
• Intensity of current=10-20mA
• Stim rate= 3-4Hz or sometimes upto 10H.
• Higher stim rates can attenuate the amplitudes of longer latency waves (so lower limb
sep may be affected)
• LFF=5-30 Hz
• HFF=1-3KHz OR
• Wide open filter=1-3000Hz
• Average summations=1000 for short lat sep
(In case signal-to-noise ratio is low, more averaging is required.)

19. Far field potentials.
• FFPs are potentials recorded at a
distance from the neural generators.
• Wide field distribution
• FFPs are recorded only when the
distance between the two electrodes
is sufficiently great such that an
amplitude difference exists between
the two electrodes
• Bipolar recording with short
interelectrode distance cancels out
FFPs due to equipotential.
• Conversely NFP’s are recorded by
short distance electrodes.
• Can be both positive and negative.

20. FFP CONT…
• why the FFP is generated as the nerve impulse passes
through a certain and fi xed anatomical site.
1). Earlier studies of BAEP proposed that the FFPs of waves II
through V were generated when the impulse passed
through each synaptic site of the brainstem auditory
pathway.
However, in the P9 model of SSEP, there is no synaptic
connection at this shoulder area (the first synaptic
connection in the ascending sensory pathway is at the
dorsal root ganglion, close to the entrance to the spinal
cord). Therefore, the synaptic theory does not explain the
generation of FFP.
2). The major change occurring at the shoulder area is the
change of volume surrounding the nerve, that is, the
traveling impulse suddenly enters from the small (arm) to
large (body) volume area
a sudden change of volume conductive media
surrounding the nerve generates the FFPs
3). The other mechanism is the change of the
direction/orientation of the nerve impulse. This was
proposed by the fi nding that changing the arm position
altered waveforms and latency of the P9 FFP

21. Physiological Factors that affect SSEP
• Age
• Body size
• Temperature
• Sleep

22. Age
• Latency is dependent upon the peripheral-central conduction velocity and height
(for lower extremities) or arm length (for upper extremity)
• Periph CV adult values by age 3.
• Spinal CV adult values by age 5.
• Stable from teens to 50yrs.
• Normal median nerve SSEP latencies are observed by age 6 to 8 yr; normal
posterior tibial nerve latencies are observed by age 5 to 7 yr (normal values at
age 5)
• the latency changes are secondary to the combined effects of maturation of
peripheral and central somatosensory pathways, and the increase in these
pathways along with body growth.
• myelination of the peripheral fi bersprogresses faster than that of the central
pathways, the interpeak latency (IPL) between cervical N13 and cortical N20
(central conduction time) progressively decreases until about 6 to 7 years of age,
whereas the IPL between Erb’s potential and cervical N13 remains constant

23. • Body size
• Arm length and height are linearly Correlated for absolute latencies not for IPL.
• if IPLs are used then effect of differences in adults can be ignored.
Temperature and sleep
Decrease temperature increases latency - 1’c - 1.15ms
• Sleep has little effect on short-latency far-fi eld potentials but signifi cantly alters medium- (40–
100 ms) and long-latency (>100 ms) components.
• The medium-latency components, for example, N60 of median nerve SSEP shows latency
prolongation and amplitude reduction
• N20 latency prolongs slightly in deep sleep. However, the latency prolongation in stage I or II
• sleep is minimal; thus, it can be ignored
• Detailed analysis of N20 has shown multiple fast components (400–600 Hz) superimposed over
the N20 waveform.
• These fast-frequency components (commonly referred to as high-frequency oscillation or HFO)
are best recognized in wakefulness and disappear in non-REM sleep but reappear in REM ssleep

26. Short lat-UE SSEP
• Median mostly used.
• Ulnar nerve stimulation has smaller amp and longer latency.
• Both ffp and nfp are recorded.
• FFPs recorded by non cephalic reference usually contralateral erbs
point.
• Some FFPs, for example P9, have higher amplitude when the
reference electrode is further away from the head, the use of very
long interelectrode distances encounters more technical problems
such as an increase in stimulus artifact, sample rejection, and muscle
or movement artifacts.

27. • P9 and N 9 FFP
• Distal brachial plexus / proximal nerve
• obTained from Cpi – contra-noncephalic ref (Epc)
• C5s - contra-noncephalic ref (Epc)
• Anterior neck more positive than posterior neck
• So in derivation of posterior - anterior neck we get N9
• N10 - Erbs point

28. • P11 N11
• Smaller, inconsistent
• Presynaptic - at dorsal root entry zone
• Cpi – contra-noncephalic ref (Epc)
• Small notch on rising phase of N13
• N13 (cervical)
• Cervical cord, dorsal horn interneuron
• C2s/ C5s - contra-noncephalic ref (Epc)
• Independent of dorsal column medial leminiscal system
• P13/14
• Cpi – contra-noncephalic ref (Epc)
• Just before rising of N18
• High cervical cord / brainstel medial lemniscus

29. • N18 (subcortical)
• Wide field, although less than p9
• Can be recroded over ipsi hemisphere and bifrontal regions
• Ear reference ok
• Low frequency potential – enhanced by ___ LFF
• Genertor – Branstem
• N20 P20 P22 (cortical)
• NFP
• Post central contraletral electrode
• Cortex area 3b
• If recorded from frontal electrode – P20
• Parietal N20 and frontal P20 - horizontally oriented dipole directed
tangentially from parietal to frontal
• P 22 -supplementary motor cortex

31. • For FFP – Active electrode is ipsilateral to stimulation
• For cortical potentials – Active is contralateral to stimulation site
• Alternatively, we can use Fc and CPc both ref to ipsi ear.
• Fc register p13,14, N18, P20
• CPc registers p13, 14, N20
• Assist in peak identification
• Phase reveral of n20 p20

34. Montages

35. • Rationale for the Above Montage
• Both CPc and CPi electrodes contain P9, P11,
P13/P14, and N18 FFPs
• Only CPc includes the near-fi eld potential of
N20 (first cortical potential).
• Channel 1, CPc-CPi, thus cancels all FFPs and
registers only N20.
• Channel 2 records all FFPs (P9,P11, P13/P14,
and N18) only.
• Channel 3 registers P9 FFP andN13 (cervical
potential), and
• channel 4 records Erb’s potential(N10).

37. Rationale
• Channel 3 registers N9, N11, and N13
potentials. Of these, N9 is derived from
P9 FFP.
• P13/P14 FFPs can be identifi ed by fi
nding at both channels 1 and 2.
• These 2 channels also help to distinguish
N18 FFP and N20 cortical potential by
their latency difference between channels
1 and 2

38. Lower limb SEP – tibial at ankle

39. rationale
• Chaneel 1 and 2 cancel out FFP p31 and N 34 and record NFP P40.
• Channel 3 records FFP
• T12 records local spinal potential N23
• Either CPz or Cpi may have higher amplitude or longer latency.
• Inter individual anatomical contribution contribute to this.

41. Channel 1 and 2 record P40, however only channel 1 records p31 hence
differentiation from P40 (37)
integritiy of peripheral nerve can be seen with channel 4.
if chanel 4 is absent, it can be peripheral neuropathy, inadequate stimulus or
improper electrode placement.

45. Spinal response..
• Recorded from T12 spine to L 4 spine
• Ref is iliac crest.
• A horizontal line connecting 2 iliac crest is L4 level
• A horizontal line connecting floating ribs/ last rib is T12.
• T12 is N23 (conus, dorsal horn interneurons)
• L4 is 21 (cauda equine)
• Corrsponds to Cervical N13
• Can be absent in older/ obese persons

46. Scalp potentials
• Using a non cephalic ref 3 positive FFP are noted
• P17- distal sacral plexus…. Similar to P9 of UL
• P24 – unknown origin
• P31 – most easily recordable… lower brainstem - similar to P14 of UL
Useful for spinal conduction time (N23 to P31)
• N34 ipsilateral, widespread bifrontal and ipsipateral central parietal field.
Similar to N18 of UL.
May not be identified in some.
• N37/ P40 – cortical potential, n midline or ipsilateral, NFP, similar to N20

49. Obligate potentials
Upper Lower
N10 N8
N13
P13/14
N18
N20 P40/37

51. > Absent N13 or prolonged Low cervical cord
N11 – N13 (P13 onwards potentials can be normal)
> Normal N18 and absent N20 VPL thalamus or cortex (sensory)
High cervical cord

53. Medium and long latency SEP
• P26, N32, P40, N60
• At contralateral hemisphere
• Depend upon conscious level/ vigilance, attention
• Affected by thalamic lesion (other than VPL/ and diffuse cortical
lesions)
• Can still be recorded even if erbs and short latency SEP are absent
due to neuropathy

54. Localize

55. Median SEP normal