Instrumental methods for studying brain function

Macroscopic recording: Evoked potentials and MEG

To determine the state of the brain's afferent systems, the recording of evoked potentials (EPs) is used. This method records electrical potentials that arise during the short-term stimulation of peripheral receptors or CNS structures.

Evoked potentials are stereotyped oscillation complexes whose waveform reflects the functional state of neural structures. A critical characteristic of EPs is the presence of a strictly defined latency period (the time from stimulus delivery to the appearance of the response). These oscillations are clearly distinct from changes in normal background rhythmicity.

The EP method is used for functional localization, specifically for locating analyzer projection areas. To do this:

1. Sensory systems are repeatedly stimulated at various levels (receptors, nerve trunks, relay nuclei).
2. Electrodes are moved across the brain surface to record activity.
3. The area where responses are maximally expressed is determined.

As a result, the researcher identifies the precise contours of the cortical area where the pathways of a specific analyzer terminate.

Another important functional method is magnetoencephalography (MEG). It is used to search for brain areas active during problem-solving and to analyze abnormalities in pathology. Compared to EEG, the method has higher spatial resolution accuracy (up to 5 mm), and compared to fMRI and PET, it boasts ultra-high signal acquisition speed (about 1 ms).

Investigation of single neuron activity

To record the spike activity of individual neurons, the microelectrode technique is used. The subjects are nerve cells in tissue culture, invertebrate ganglia (e.g., of the edible snail), and CNS cells of anesthetized vertebrate animals.

The highest quality of recording is achieved using glass microelectrodes. The most complex, yet simultaneously most informative option, is recording impulse activity under conditions of the subject's free behavior.

There is also a non-contact counterpart — optical recording of impulse activity. It tracks the propagation of excitation along fibers without microelectrodes. The essence of the method:

• Special dyes that bind to the cell membrane are injected into the tissue.
• When the membrane potential changes, the membrane alters its light absorption or causes cell fluorescence.
• The modulated light flux passes through fiber optics, is amplified, and is converted into an electrical signal.

Study of molecular mechanisms and chemosensitivity

One of the subtlest neurophysiological techniques is microiontophoresis. It allows for the simultaneous recording of electrical impulses from a single neuron and the metered delivery of biologically active substances (BAS) to it. Technically, this is implemented using a multi-channel glass electrode. Substances are delivered from its separate barrels directly to the area of the cell's location or inside the structure itself.

This opens the way to the pharmacological analysis of neuronal activity. The method makes it possible to study the chemosensitivity of nerve cells to various agents:

• Classic neurotransmitters (acetylcholine, catecholamines);
• Peptide neurotransmitters;
• Protein synthesis blockers and other substances.

Analysis of neuronal activity in a behavioral act

Modern instrumental methods make it possible to correlate cell activity with the stages of goal-directed behavior. As an example, the visual cortex neuron of a cat is examined during the elaboration of a food-acquisition conditioned reflex.

A neuronogram (spike activity) is recorded, and summary histograms are constructed (where the vertical axis represents the number of impulses, and the bin width is 50 ms).

On the graphs, one can track the response of a visual cell not only to an adequate stimulus (light) but also to a conditioned stimulus of a different modality (sound). At the same time, the dynamics of neuronal activity at different stages of behavior are clearly visible:

1. At the moment the conditioned signal (light or sound) is turned on.
2. During the animal's press of the pedal (instrumental response).
3. At the moment reinforcement appears in the feeder (milk).
4. At the onset of food consumption (lapping).

Class and exam questions

What else to study on this topic

• Features of slow delta-wave and alpha-rhythm generation
• Localization of cortical association areas and subcortical nuclei
• Structure and functions of the mitochondrial membrane and synaptic cleft
• Detailed mechanisms of cellular chemosensitivity to peptide neurotransmitters

Other topics in the "General Physiology of the Central Nervous System" section

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