Macroscopic Recording: Evoked Potentials and MEG
To assess the functional state of the brain's afferent systems, the recording of evoked potentials (EPs) is used. This method captures electrical potentials generated in response to brief stimulation of peripheral receptors or central nervous system (CNS) structures.
Evoked potentials represent stereotyped complex waveforms whose morphology reflects the functional state of neural structures. A critical characteristic of EPs is a strictly defined latency period (the time elapsed between stimulus delivery and the appearance of the response). These waveforms are clearly distinguishable from background electroencephalographic rhythms.
The EP method is utilized for functional localization, specifically for mapping sensory projection zones. The procedure involves:
- Repeatedly stimulating sensory systems at various levels (receptors, nerve trunks, relay nuclei).
- Shifting electrodes across the surface of the brain to record electrical activity.
- Identifying the territory where the responses reach maximum amplitude.
As a result, researchers can map the precise boundaries of the cortical area where the pathways of a specific sensory modality terminate.
Another essential functional method is magnetoencephalography (MEG). It is used to identify active brain regions during cognitive tasks and to analyze pathological disruptions. Compared to standard electroencephalography (EEG), MEG offers superior spatial resolution (up to 5 mm), while providing ultra-high temporal resolution (around 1 ms) that significantly surpasses fMRI and PET.
Investigation of Single Neuron Activity
To record the spike activity of individual neurons, microelectrode techniques are employed. Subjects include cultured neural tissue, invertebrate ganglia (e.g., snails), and CNS cells of anesthetized vertebrates.
The highest recording quality is achieved using glass microelectrodes. The most technically challenging yet information-rich approach involves recording action potentials in freely behaving subjects.
A contactless counterpart to this is optical recording of impulse activity. It tracks the propagation of excitation along nerve fibers without physical microelectrodes. The core principles are:
- Special dyes that bind to the cell membrane are introduced into the tissue.
- As the membrane potential changes, the membrane alters its light absorption or exhibits fluorescence.
- The modulated light flux is transmitted via fiber optics, amplified, and converted into an electrical signal.
Study of Molecular Mechanisms and Chemosensitivity
One of the most refined neurophysiological techniques is microiontophoresis. It enables the simultaneous recording of electrical impulses from a single neuron while delivering metered quantities of biologically active substances (BAS) to it. Technically, this is achieved using a multibarrel glass microelectrode. Substances are delivered from individual barrels directly into the vicinity of the cell or into the structure itself.
This opens the door to the pharmacological analysis of neuronal activity. The method allows researchers to study the chemosensitivity of nerve cells to various agents:
- Classical neurotransmitters (e.g., acetylcholine, catecholamines);
- Peptide neurotransmitters;
- Protein synthesis blockers and other pharmacological agents.
Analysis of Neuronal Activity During Behavioral Acts
Modern instrumental methods make it possible to correlate cellular activity with the stages of goal-directed behavior. A classic example is recording from a visual cortex neuron in a cat during the acquisition of a food-acquisition conditioned reflex.
A neuronogram (spike activity) is recorded, and cumulative histograms are constructed (with the vertical axis representing the impulse count and a bin width of 50 ms).
The resulting graphs demonstrate the visual cell's response not only to an adequate stimulus (light) but also to a conditioned stimulus of another modality (sound). This clearly shows the dynamics of neuronal activity at different stages of behavior:
- At the onset of the conditioned signal (light or sound).
- During the animal's pedal press (instrumental response).
- At the moment food reinforcement appears in the feeder (milk).
- At the start of food consumption (lapping).