Physiological correlations of the brain activity. Performing of localization by electrophysiological methods: electrodes, electroencephalography (EEG) and evoked potentials, microelectrodes. Spatial and temporal resolution.

Neurons are cells specialized for the integration and propagation of electrical events. It is through such electrical activity that neurons communicate with each other as well as with muscles and other end organs. Therefore, an understanding of basic electrophysiology is fundamental to appreciating the function and dysfunctions of neurons, neural systems, and the brain.

Neurons are typically composed of a soma, or cell body, a dendritic tree and an axon. The majority of vertebrate neurons receives input on the cell body and dendritic tree, and transmits output via the axon, although there is great heterogeneity throughout the nervous system, as well as throughout the animal kingdom, in the size, shape and function of neurons. For invertebrate neurons, the information flow is less well defined.

Membrane potential (left) and the synaptic connection (right)

Neurons communicate via chemical and electrical synapses, in a process known as synaptic transmission. The fundamental process underlying synaptic transmission is the action potential, a propagating electrical signal that is generated by exploiting the electrically excitable membrane of the neuron.

Structure of action potential (AP)

Studying the electrical activity of brain neurons in vivo is the current challenge for neuro-electrophysiologists. The task of visualizing the anatomy of the living brain is shifted towards a much more complex task: to visualize the function of the living brain without disrupting it. Brain electrophysiology is classic tool for studying how the brain functions. Recording the electrical activity of the whole brain or some of its parts using single- or multiunit spike activity, EEG (electro-encephalography), EP (evoked potentials), intracellular recording is very common now.

The neuron's ability to fire is due to the presence of an electrical gradient across the neuron's membrane. The intracellular charge of the neuron is typically -70 mV in its resting state compared to the surrounding extracellular matrix. Upon activation, sodium channels open in the membrane allowing ions to flow rapidly down their electro-chemical gradient toward a new steady state. This new state--depolarization--is reached when the intracellular charge reaches +50 mV with respect to the outside. This is an activated state, which does not last long because the ion channels quickly change their configuration again and the cell membrane returns to its chore of pumping positively charged ions outside the cell-repolarization.

The entire membrane does not depolarize at once. Rather, depolarization starts in one area and spreads by diffusion to adjoining regions. The entire process of depolarization and repolarization takes between 0.5 and 1.5 milliseconds. The abrupt and transient change in the neuron's electrical charge is known as the action potential, and it is the key physical process that underlies the neuron's ability to transmit information. An action potential travels down the axon to its end and causes the release of a neurotransmitter into the synapse. As the neurotransmitter binds to the post-synaptic membrane, ion channels start to open, causing a small depolarization of the post-synaptic cell. This small depolarization is known as the excitatory post-synaptic potential (EPSP). It is not until the EPSP reaches a critical value that the post-synaptic neuron opens its remaining sodium channels, producing the next action potential. There are some neurotransmitters which, instead of depolarizing, hyperpolarize the post-synaptic cell by causing ion channels to close. As a result, the post-synaptic neuron becomes more negative. Neurons operating with such neurotransmitters have an inhibitory effect on the post-synaptic neuron.

Recording the electrical activity from individual neurons in vivo presents numerous challenges. Foremost among them is the fact that recording must be made within 100 micrometers of the neuron. Moreover, the probe used to obtain the recording must not damage the cell before recording from it. The cell body diameter, which ranges from 10 to 40 micrometers, makes intracellular recording even more challenging.

The only electrical event an extracellular electrode will detect is the fully on and off action potential. It is unable to detect sub-threshold changes in neuronal membrane voltages. Intracellular recordings that can do this are much more difficult to achieve than extracellular ones. It is chiefly a daunting mechanical task: an ultra fine glass microelectrode must be inserted into a cell of 10 to 40 micrometers in diameter; the electrode must not irreversibly damage the cell; and the electrode must maintain these constraints for the duration of the recording period despite the constant pulsations of the extracellular matrix. For these reasons, intracellular recordings have been carried out usually in vitro but not in vivo.

Extracellular microelectrode and the neuron

Electroencephalography (EEG) is the neurophysiologic measurement of the electrical activity of the brain by recording from electrodes placed on the scalp or, in special cases, subdurally or in the cerebral cortex. The resulting traces are known as an electroencephalogram (EEG) and represent an electrical signal (postsynaptic potentials) from a large number of neurons; these are sometimes called brainwaves. Electrical currents are not measured, but rather voltage differences between different parts of the brain.

EEGs are frequently used in experimentation because the process is non-invasive to the research subject. The subject does not need to make a decision or behavioral action in order to log data, and it can detect covert responses to stimuli, such as reading. The EEG is capable of detecting changes in electrical activity in the brain on a millisecond-level. It is one of the few techniques available that has such high temporal resolution.

EEG recording: electrodes placed on the head surface (top, left), screenshot of the EEG-analyzer (right) and the map of the EEG activity generated after recording session (bottom, left)