Surgical methods for studying CNS connections
This group of methods is strictly experimental in nature. The main essence of surgical intervention lies in the isolation of certain neural structures. This is achieved by the physical interruption of nerve impulse flows—both afferent (incoming) and efferent (outgoing).
The general significance of such operations is that surgical isolation makes it possible to sever the connections between the brain, spinal cord, and peripheral organs. As a result, researchers can analyze the completely autonomous functioning of isolated neural structures.
Main types of surgical interventions on the cerebral cortex:
- 'Undercutting' surgery: used to study the functioning of neurons in a small area of the cortex under conditions where their direct connections with underlying neural structures are disrupted.
- 'Trimming' surgery: makes it possible to investigate the functioning of neurons in a specific cortical area that is completely isolated from neighboring cortical regions.
Electrical irritation (stimulation)
Electrical stimulation is a method of strictly localized action on selected brain structures. This effect is carried out using special implanted or surface electrodes.
A number of strict physical and technical requirements are imposed on electrodes:
- Size: the contact area must be sufficiently large. This is critically important to avoid thermal damage (burns) to neural tissue, as well as to encompass the necessary tissue volume with the electrical field to achieve a pronounced physiological effect.
- Materials: chemically inert and conductive metals—nichrome, tungsten, or gold—are used for manufacturing.
- Insulation: the shaft (body) of the electrode must be coated with a special varnish. Only the active tip remains uninsulated, with an area of approximately 0.1 square millimeters.
Stimulation parameters: in the classic variant, strictly rectangular pulses are used, with the duration of each such pulse being about 1 millisecond.
An important advantage of the method is convenient localization control. The exact same electrodes used to deliver current can be used for the backward recording of electrical brain signals. This allows researchers to accurately confirm that the electrode tip has hit the desired brain structure based on characteristic physiological activity.
Magnetic brain stimulation
Unlike surgery and electrical stimulation with implanted electrodes, magnetic stimulation is a completely non-invasive method. Its main task is to assess the functional state of central motor pathways.
The theoretical basis of the method relies on the physical properties of the field: a powerful magnetic field is able to penetrate dense cranial bones unhindered. Upon reaching the neural 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 diseases of the nervous system.
- Traumatic spinal cord injuries.
Chemical irritation
Chemical irritation is a method of targeted administration of active substances for the local stimulation of strictly defined brain zones.
The localization of drug administration may vary depending on the objectives:
- Directly inside the brain ventricular system.
- Straight into the thickness of the brain tissue (parenchyma).
Technique: special cannulae are used to deliver substances. They are implanted in advance and securely attached to the cranial bones, which allows for repeated and precise chemical interventions on selected nerve centers.
Briefly: answers to topical questions
Which specific chemical substances (neurotransmitters) are used for the chemical stimulation of neurons?
The cited sources indicate acetylcholine and noradrenaline as substances applied microiontophoretically to the neuron.
Their action:
- acetylcholine: when applied microiontophoretically to a neuron, it causes the elimination of discharge upon pressing the pedal (phase I) and a decrease in impulse activity during reinforcement (phases II–III);
- noradrenaline: when applied microiontophoretically, it causes a general decrease in the baseline activity of the neuron at all stages of food-procuring behavior — before and after pressing, and during food intake.
During microiontophoresis, substances are delivered in metered doses through a multi-channel glass electrode into the neuron's location zone or inside the studied structure, with simultaneous recording of the impulse activity of an individual neuron.
What does the microiontophoresis method for studying the CNS entail?
The microiontophoresis method consists of simultaneously recording the impulse activity of an individual neuron and delivering metered amounts of various biologically active substances to this neuron.
Technically, the method is performed using a multi-channel glass electrode: substances are delivered from separate channels of the electrode into the zone of the neuron's location or inside the studied structure. The source refers to microiontophoresis as one of the most delicate and effective neurophysiological techniques for studying the characteristics of CNS cells.
CHAM: four methods of action: Surgery (cutting), Electricity (delivering shocks), Magnet (exciting with a field), Chemistry (dripping substances).
Class and exam questions
What is the difference between cortical 'undercutting' and 'trimming' operations?
During 'undercutting', the connections of a cortical area with underlying brain structures are disrupted. During 'trimming', a cortical area is isolated exclusively from neighboring, surrounding zones.
Why should the tip of the stimulation electrode not be too small?
If the electrode area is too small, the high current density will cause thermal damage (a burn) to the tissue. In addition, a larger size is needed to encompass a sufficient volume of neurons.
In which diseases is magnetic stimulation indicated?
The method is used to assess the pyramidal tract in ischemic stroke, spinal cord injuries, and degenerative diseases of the central nervous system.
What else to study on this topic
- Peculiarities of neuronal function in the isolated cortex
- Parameters of rectangular pulses in electrical stimulation
- Technique of implanting cannulae into cranial bones
- Mechanism of interneuron and pyramidal cell activation by a magnetic field
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