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Brian Nichols, B.S., CNIM, REPT, RPSGT

Upper SSEPs• Wrist to Cortex

Lower SSEPs• Ankle to Cortex

BAERs

• External Ear to Upper Brainstem

VEPs• Eyes to Occipital lobe

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Upper SSEPs• Erbs Point, Cervical Potential, P14, N18, N20, P24

Lower SSEPs• Pop, LP, P31, N34, P37, N45

BAERs

• Waves I, II, III, IV, V

VEPs• N75, P100, N145

Polarity convention

Types of potentials and how to record

them

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Start with electrical

stimulation

• Median Nerve

Cathode 2 cm proximal

to the wrist crease

• Ulnar Nerve

Cathode 2 cm proximal

to the wrist crease

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The electrical impulse begins at the

cathode and moves up the nerve

toward the body.

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Impulse travels up the median

Nerve deep to the flexor muscles

of the forearm.

Impulse travels medial to

the Biceps Brachii and approaches

the Brachial Plexus.

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Impulse travels through the

Brachial Plexus.

Continues up the nerve toward the

spinal cord.

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Impulse travels up the Dorsal Nerve Root

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Impulse travels up the Dorsal nerve root

Impulse travels from the Dorsal root into

Fasciculus Cuneatus of the spinal cord.

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Impulse travels up the Fasciculus Cuneatus

of the Spinal cord.

Impulse travels up the Fasciculus Cuneatus

of the Spinal cord.

At C1-C2 the spinal cord becomes the

medulla. The lower medulla contains

the decussation of the pyramids.

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Decussation of the Pyramids-

Lower Medulla

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Decussation of the Pyramids-

Lower Medulla

Crossed over

Pre decussation is the spinal cord.

Post decussation is the

Medial Lemniscus

Pons

Impulse travels along the

Medial Lemniscus

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Midbrain

The Medial Lemniscus continues

carrying the impulse upward

through the Midbrain.

Thalamus

The impulse arrives at the Thalamus

where the Medial Lemniscus

terminates.

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Thalamocortical Tracts

Projections from the thalamus

Radiate outward to the

Somatosensory strip of the cortex.

Sensory Cortex

• Also called the Somatosensory

strip

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Sensory Homunculus

End of Upper SSEP

pathway

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Recording begins at stimulation

0 msec

Traveling along the median nerve

4 msec

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The impulse has reached the brachial

Plexus. As the impulse travels, the first

recording electrode at the erbs point

begins to record the propagating

Potential.

8 msec

Peripheral response recorded from the

Skin at the Supraclavicular Fossa

10 msec Erbs Point: Brachial Plexus

Potential

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The impulse has traveled through the Brachial

Plexus and converged via the dorsal nerve root

Into the spinal cord and an electrode at C5

records the potential. The N13 waveform

reflects postsynaptic activity in the cervical

spinal cord.

13 msec Cervical Potential: N13

Reflects activity in the Caudal Medial Lemniscus

as the impulse moves through the brainstem.

Cephalic and Noncephalic recording

electrodes are used.

14 msec Subcortical Response: P14

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The impulse continues its route along the

Medial Lemniscus and approaches the

Thalamus.

The Medial Lemniscus terminates at

the thalamus.

18 msec Subcortical Response: N18

The impulse arrives at the cortical

somatosensory region where it is processed.

The processing of this impulse by a large group

of cells, or their post synaptic activity, is what

gives us this potential.

20 msec Cortical Response: N20

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20 msec Cortical Response: N20

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Start with electrical stimulation

• Tibial Nerve

Cathode is posterior to the medial malleolus

• Peroneal Nerve

Cathode 2 cm proximal to the ankle crease

• Tibial nerve stimulation provides better waveforms

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Electrical stimulation creates an impulse that moves up the Posterior Tibial

Nerve.

The impulse continues

proximally. Note the

Post. Tib. Nerve at this

region runs deep

to the

Gastrocnemius.

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The impulse arrives

at the Popliteal

Fossa. The Post Tib

and Peroneal nerves

Soon merge to form

the Sciatic nerve.

The impulse is now

Moving up the sciatic

nerve toward the

spinal column.

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The sciatic nerve moves up the leg,

eventually separates and merges

with other nerves to form the

LumboSacral plexus.

The LumboSacral Plexus

innervates L4-S3

Arriving at the Sacral Plexus

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The nerves that are encased in the vertebral

column are considered the Cauda Equina

At T12, the

cauda equina

becomes the

Spinal Cord

at a region

known as the

Conus

Medullaris.

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The impulse has traveled up

The spinal cord and arrives

At the lower brainstem where

The crossover takes place.

The impulse is now

contralateral to the side of

stimulation and is traveling

up the Medial Lemniscus.

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Traveling the Medial Lemniscus.

Continuing along the Medial Lemniscus.

To the Thalamus.

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Passing through the thalamus and

traveling via thalamocortical tracts to

the sensory cortex.

Impulse is processed in the

somatosensory cortex.

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The Sensory Homunculus.

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Recording begins at stimulation

0 msec

Impulse travels Posterior Tibial nerve

6 msec

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10 msec

Peripheral response recorded at the popliteal fossa

15 msec

Impulse passes popliteal fossa and moves up sciatic

nerve toward the next recording electrode at T12.

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17 msec

Impulse moves up Sciatic Nerve, through the

lumbosacral plexus and Cauda Equina toward

the next recording

electrode at T12.

20 msec

Impulse arrives at the Conus Medullaris where the

Cauda equina synapses with the spinal cord. The

Post synaptic activity creates a potential that is

recorded by an

electrode placed at the

T12 region.

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25 msec

Impulse travels up the spinal cord.

31 msec

Impulse reaches the lower brainstem, crosses over

and begins its route up the medial lemniscus

Cephalic and

noncephalic

Recording Electrodes

are used.

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32 msec

Impulse moves up through the medial lemniscus.

34 msec

Impulse reaches the thalamus where the medial

lemniscus terminates.

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35 msec

Impulse passes thalamus and via thalamocortical tracts

it is moving toward the somatosensory strip.

37 msec

Impulse has reached the somatosensory strip. Cephalic

electrodes are used to record the response.

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Start with audible tone

clicks

• Ear inserts

• Headphones

Soundwave enters ear canal

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Soundwave arrives at the tympanic

Membrane, also called the eardrum.

The eardrum vibrates causing

Movement in the ossicles (bones of

Middle ear). The ossicles send the

vibrations into the Cochlea where tiny

hair cells convert the physical

movement into an electrical sensory

impulse.

Wave I: Distal Auditory nerve

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Wave II: Proximal Auditory nerve

Wave III: Superior Olivary Complex

At the junction of the Pons

and Medulla Oblongata.

This is the beginning of the

Lateral Lemniscus.

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Wave IV: Lateral Lemniscus

The Lateral Lemniscus

travels upward through the

pons and terminates at the

Inferior Colliculus

Wave V: Inferior Colliculus

Located in the midbrain.

Acts as a relay station to

send the sensory impulse

to the correct area in the

brain.

End of the Lateral

Lemniscus.

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Start with

checkerboard pattern

reversal.

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Waveforms

•N75

•P100

•N145

•N100

The waveforms N75 and

P100 are generated in

the primary visual cortex.

Note the Left retina halves of each eye are processed in the left lobe.

And vice versa. The right retina halves of each eye are processed in the

right lobe.

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Polarity Convention: How the waveforms are displayed are based on the

polarity of the impulse and what amplifier input is recording that impulse.

• Waveforms with an N are

negative. i.e. N20, N18

• Waveforms with a P are

positive. i.e. P14, P31

• Peripheral and spinal

potentials are negative.

• In BAER’s, Wave 1 is negative

and waves II through V are

positive.

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Keep in mind, both amplifier inputs (Active(-) and Reference(-)) record data.

The polarity convention applies to each of the 2 inputs separately. Then the

data from each input is combined and we see the final waveform.

Using the properties of polarity convention, we’ll

look at 2 different types of potentials, and how they

are displayed

1. Propagating

2. Stationary

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A moving potential that travels along a nerve.

•Examples: Erb’s point, Pop Fossa

•Occurs on peripheral nerves.

•Polarity is negative

•Active electrode records at a volley or where the

nerve becomes superficial.

•Reference electrode

records from an area

where there is little

contamination from

muscle and away

from potential. Also

called a “quiet” area.

Propagating Potential

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A potential generated at a specific point after the impulse

stops at a relay station along a nerve route.

•Examples: Cervical and lumbar potentials

•As the nerve cells are transmitting the impulse across

a synapse, or connection between 2 nerve cells, a

potential is created

•Polarity is negative

•Active electrode records where the potential is most

superficial.

•Reference electrode records from an area where

there is little contamination from muscle and away

from the potential. Also called a “quiet” area.

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Near field- Potential lies close to the

surface of the brain. • Potential is higher amplitude because the field

Is generated near the recording electrode.

Far field- Potential lies deep within the

brain

• Potential is lower amplitude because the field

Is generated far from recording electrode.

•Example of a potential being recorded at CP3.

•The negative potential is

recorded by the active (-)

input thus creating an upward

deflection at CP3. The

reference (+) CP4 is

in a quiet area and does

not record the potential.

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•Example of subcortical potential being recorded at CP3.

•Requires cephalic to non-cephalic channel.

•The negative potential is

recorded by the active(-)

input thus creating an upward

deflection at CP3. The

Reference(+) A2 is in a quiet

area off the head and does

not record the potential.

•Why cephalic to cephalic montage does not work to obtain

far field responses.

• Example of CP3-CP4

montage

This is why you can record

a cortical potential and not

have it contaminated with a

Subcortical potential.

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References:

Spehlman, R., 1985, Evoked Potential Primer: Visual, Auditory, and

Somatosensory Evoked Potentials in Clinical Diagnosis

Chiappa K., 1983, Evoked Potentials in Clinical Medicine

American Clinical Neurophysiology Society Guidelines

Thank You.