Surgical Methods for Studying CNS Pathways
This group of methods is strictly experimental. The main purpose of surgical intervention is the isolation of specific neural structures. This is achieved by physically interrupting neural impulse streams—both afferent (incoming) and efferent (outgoing).
The general significance of such operations is that surgical isolation separates the brain from the spinal cord and peripheral organs. As a result, researchers can analyze the completely autonomous function of isolated neural structures.
Main types of surgical interventions on the cerebral cortex:
- Undercutting: used to study the function of neurons in a small cortical area while disrupting their direct connections with subcortical neural structures.
- Isolation (ostro-cut) procedure: allows researchers to investigate the functioning of a specific cortical area that is completely isolated from adjacent cortical regions.
Electrical Stimulation
Electrical stimulation is a method of highly localized action on selected brain structures, performed using special implanted or surface electrodes.
Electrodes must meet several strict physical and technical requirements:
- Size: The contact area must be sufficiently large. This is critical to avoid thermal injury (burns) to neural tissue and to encompass the necessary volume of tissue with the electrical field to produce a prominent physiological effect.
- Materials: Chemically inert and conductive metals are used, such as nichrome, tungsten, or gold.
- Insulation: The shaft (body) of the electrode must be coated with special varnish. Only the active tip is left uninsulated, with an area of approximately 0.1 square millimeters.
Stimulation parameters: Classically, strictly rectangular pulses are used, with a duration of about 1 millisecond for each pulse.
An important advantage of the method is convenient localization control. The same electrodes used to deliver current can be used to record electrical brain signals back. This allows researchers to accurately confirm that the electrode tip has reached the target brain structure based on characteristic physiological activity.
Transcranial Magnetic Stimulation
Unlike surgery and invasive electrical stimulation, magnetic stimulation is a completely non-invasive method. Its primary task is to assess the functional state of central motor pathways.
The theoretical basis of the method relies on physical field properties: a powerful magnetic field can penetrate dense cranial bones without resistance. Reaching the brain tissue, it selectively excites motor cortex neurons, primarily interneurons and large pyramidal cells.
In clinical practice, magnetic stimulation is used to accurately assess the state of the pyramidal tract in the following pathologies:
- Ischemic stroke.
- Degenerative nervous system diseases.
- Traumatic spinal cord injuries.
Chemical Stimulation
Chemical stimulation is a method of targeted administration of active substances for the local stimulation of strictly defined brain zones.
The site of administration can vary depending on the objectives:
- Directly into the brain ventricular system.
- Directly into the brain tissue parenchyma.
Technique: Special cannulae are used to deliver substances. They are implanted in advance and securely anchored to the cranial bones, allowing repeated and precise chemical interventions on selected neural centers.