METHODS TO STUDY CORTICAL CONNECTIONS AND FUNCTIONS

 

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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