METHODS TO STUDY CORTICAL CONNECTIONS
AND FUNCTIONS
BY CUTTING OR
DESTRUCTION OF NERVE CELL
1. If the nerve cell body is destroyed, degenerative changes occur throughout the axon arising from it. By using Marchi staining technique, course of the nerve fiber could be traced. If any part of motor area is destroyed, the degeneration of the fibers in the pyramidal tracts can be traced. If arm fibers are involved, the degeneration occurs up to lower cervical and upper thoracic level.
2. If an axon is cut, the nerve
cell body (from which the axon arises) undergoes chromatolysis. If any fiber in pyramidal tract is cut, the chromatolysis
occurs in nerve cell body situated in motor cortex.
Thus, this method is used
to study connections and localization in motor cortex. It is also used for the
study of connections of different parts of cerebral cortex.
BY RECORDING
ELECTRICAL ACTIVITY – EVOKED POTENTIAL
When an impulse passes through
a nerve, its route and the termination can be determined by recording the electrical
potentials using microelectrodes at different points
along the course of the nerve fiber. This method is used to trace certain pathways
from or to the cortex, particularly auditory pathway and pyramidal tract.
Evoked Potential
Evoked potential is the
electrical potential or electrical response in a neuron or group of neurons in
the brain produced by an external stimulus. It is also called evoked
cortical potential. When any receptor of skin or a sense organ (eye or ear) is
stimulated, the impulses pass through the afferents and reach cerebral cortex.
By using scalp electrodes, the potentials developed in cortical areas can be
recorded. This method is used to determine the functions of various cortical
areas. It is also used to map out the cortical representation of body (localization)
for sensory function. Evoked potential is recorded by placing the exploring electrode
on the surface of the head over the primary cortical area of the particular
sensation. Indifferent electrode is placed on a distant area of head. In human
beings small disk like electrodes are placed on different areas of head by
using a tape or washable paste. Electrode cap, which is placed over the head
can also be used. Analysis and interpretation of the potential is done by
computer. Evoked potential is characterized by two types of response.
1. Primary evoked
potential
When the stimulus is applied
to the receptor or sense organ, the primary evoked potential appears after
a latent period of 5 to 10
milliseconds. It includes a positive wave followed by a small negative wave.
Primary evoked potential
is highly localized and appears specifically on the cortical
surface where the
particular sensory pathway
terminates.
2. Diffuse secondary
evoked potential
Finally another larger and
prolonged positive wave called secondary evoked potential is recorded. It is
not
localized. It appears on
the diffused areas of cortex. Thus, the evoked potential includes P1-N1-P2
sequence,
i.e. first positive wave – first negative wave – second positive wave.
Diagnostic Uses of
Evoked Potential
An evoked potential test
determines the functional status of a nervous pathway. It also measures the
time
taken by the nerves to respond
to stimulation. Intensity of response is also measured. Nerves from different areas
of the body may be tested. However, three types of evoked potentials are
commonly used in diagnosis.
1. Visual evoked potential, which is recorded when the visual
receptors are stimulated by looking at a
test pattern
2. Auditory
evoked potential that is recorded when auditory receptors are stimulated by
listening to a
test sound
3. Somatosensory evoked potential, which is recorded when the
somatic nerves of the limbs are stimulated by electrical stimulus.
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