somatosensory evoked potential pathways and their recording during intraoperatively.

1. SSEP Pathways
Shehzad Hussain
17-18/05/2017

2. Contents:
1. Introduction
2. Spinal cord
3. Different sensory fibers
4. Neuronal exchanges at different level of
spinal cord
5. The afferent and efferent system

3. Introduction:
A somatosensory evoked potential (SSEP) is an evoked
potential caused by a physical stimulus (usually a small
electric pulse). Electrodes positioned over particular
areas of the body record responses of the SSEP, these are
then observed as a reading on an electroencephalogram
(EEG). A SSEP can most commonly involve stimulation of
the median nerve at the wrist, or the posterior tibial
nerve at the ankle. This investigation therefore tests the
pathway of the sensory nerves to the sensory areas of
the brain, even though the stimuli are non-physiological.

4. Spinal cord:
• The spinal cord begins at the foramen magnum
• The spinal cord is a soft bundle of nerves that extends from
the base of the brain to the lower back. It runs through the
spinal canal and is protected by the bones of the
spine (vertebrae). Messages between the brain and the nerve
roots travel up and down the spinal cord, making it possible
for the brain and body to communicate. The discs cushion the
vertebrae and provide flexibility to the spine and spinal cord.

5. Anatomy of the sensory system;
• There are three types of sensory fibers, each carrying
different sensation and each having different conduction
velocities; (i) pain and temperature sensation carried through
the slowest conduction fibers, (ii) touch and pressure senses
are conveyed though medium slow-conducting fibers, and (iii)
vibration and position senses travel through the fastest
conducting fibers. Sensory input originate from various type
of sensory receptors. Each receptor respond to a specific
modality of stimulus. The free nerve ending receive pain
sensations. Merkel’s disc and Meissner’s corpuscles react to
touch sensation. Pacinian corpuscles receive vibration senses.
Ruffini’s corpuscles respond when the skin is stretched.

6. Cont…
• After entering the spinal cord the sensory input travels
through different pathways depending on the type of
sensation. Pain/temperature or touch/pressure sensations
cross to the opposite site of the spinal cord after the synaptic
connection of the cells of the substantia gelatinosa (second-
order neurons). The pain/tem and touch/pressure input
ascends the lateral spinothalamic tract and touch and
pressure senses ascends through anterior spinothalamic
tract, respectively. At the brain stem, they ascends as spinal
lemniscus. The vibration and proprioceptive senses travel on
the same side after entering the spinal cord and ascends
through posterior column.

7. Cont…
• The posterior column consist of two fasciculi, fasciculus
gracilis and fasciculus cuneatus, which carry sensory inputs
from the lower and upper extremity respectively. The
fasciculus gracilis runs medially to the fasciculus cuneatus in
the spinal cord. The fibers of fasciculus gracilis and fasciculus
cuneatus end at the gracilis and the cuneatus nuclei,
respectively, at the lower part of medulla. The second-order
neurons that originate from the nucleus gracilis and nucleus
cuneatus cross to the opposite site of the spinal cord (sensory
decussation) and then they ascend as a single compact
bundle called the medial lemniscus. This runs through the
medial part of the medulla, pons and midbrain.

9. Cont…
• SSEP elicited by electrical stimulation are primarily mediated
through posterior column-medial lemniscus system not
through spinothalamic-spinal lemniscus system. Thus most of
SSEPs are normal in patient with pain-temperature senses
alone.
• At the pons, both the spinal and medial lemniscui ascends
together and end at the synaptic connection with the ventral-
posterior-lateral (VPL) nuclei of the thalamus. After synaptic
connection at the VPL nuclei, the third order fiber pass
through the posterior limb of the internal capsule and finally
end at the primary sensory cortex (area 3b) of the post
central gyrus of the cerebral cortex.

13. SSEP
• The SSEP consists of short latency (<30ms), medium latency
(30-100ms), and long latency (>100ms) components. Short
latency SSEP is generally resistant to change in level of
consciousness. Medium or long latency potentials vary
depending on the patients vigilance and various cognitive
functions. Although the amplitude of short latency SSEP is
smaller than the medium or long latency SSEP, the stable
nature of short latency SSEP make it more suitable for the
neurological diagnostic tests. Most clinical SSEP studies,
therefore, have focused on short latency SSEP, which consist
of near-field as well as far-field potentials.

14. Generation mechanism of far-field potential
(FFP)
• Although the concept of FFP first introduced in BAEP, its
generation mechanism has been explored more extensively in
SSEP. FFP are potentials recorded at a distance from the
neural generators. B/c FFPs have a wide field distribution,
conventional bipolar recording with short interelectrode
distance cancels out the FFP due to equipotentiality b/w the
two electrodes.

15. ACNS recommended montages:
Upper limb
 CPc-Cpi (N20)
 Cpi-Epc (P9, P11, P13/14,
N18)
 C5s-Epc (P9, P13)
 Epi-Epc
Lower limb
 CPi-FPz (P40)
 CPz-FPz (P40)
 FPz-C2S (P31, N34)
 T12S-Iliac crest (N23)
N9/P9= Distal portion of brachial
plexus
N10= erbs point
N11/P11= Root entry zone
N13= Cervical Cord
P13/14= high cervical cord
N18= brain stem and N20= cortex (3b)
N8= Popliteal fossa
N21= Root entry zone
N23= Conus medullaris
P31= brainstem (Medial lem)
N34= brainstem
P40= Cortex and N37= cortex (3b)