Preparation of the Biological Specimen
To perform I.M. Sechenov's classic experiment, a specially prepared specimen is required. The initial stage involves the surgical exposure of the frog's brain. After gaining access to the neural structures, the researcher makes a careful transverse cut at a strictly defined level—in the region of the optic thalami, which anatomically correspond to thalamic structures.
An essential condition for the correct execution of the technique is the removal of the cerebral hemispheres located above the transection line. The cut surface is thoroughly dried to prepare the nervous tissue for the subsequent application of a chemical stimulant. Such meticulous preparation isolates the influence of higher centers and focuses exclusively on the interaction between diencephalic centers and the spinal cord.
Course of the Classic Experiment
The essence of the technique is to compare the speed of an unconditioned motor response before and after active stimulation of brain structures.
- As a baseline measurement, the initial reflex time is determined. For this, the animal's hind paw is immersed in a weak solution of hydrochloric acid (concentration of 0.25% HCl) and the time until withdrawal of the limb is recorded. This reaction represents a standard acid-induced protective defense reflex.
- The main phase of the experiment begins: a chemical stimulant—a crystal of table salt (NaCl)—is placed on the dried cut surface of the thalamus. The salt provides intense and strong stimulation of the exposed neural centers.
- Against the background of the ongoing action of this stimulant, the paw is re-immersed in the acid solution, and the latency of the acid defense reflex is measured again.
Results and Physiological Mechanism
The primary result of the experiment is a reliable increase in reflex time. Under conditions of brain stimulation, the motor withdrawal reaction slows down significantly and, with sufficiently strong stimulation, may be completely absent (suppressed).
The physiological mechanism of this phenomenon involves the following sequence of events in the central nervous system:
- The application of the NaCl crystal triggers a powerful focus of excitation in the neural centers of the thalamus.
- The generated excitation begins to propagate along descending pathways toward spinal motor centers.
- Upon reaching the spinal cord segments, the descending signal activates specialized cells—inhibitory interneurons.
- The activated interneurons exert a powerful inhibitory effect on motor neurons, thereby functionally blocking the withdrawal reflex arc.
Evolution of I.M. Sechenov's Views
A thorough analysis of the results compelled the scientist to revise and refine his initial hypotheses regarding the nature of the phenomenon he discovered. I.M. Sechenov's views underwent an important evolution:
- The initial hypothesis was based on the assumption that specific "delaying" (or inhibitory) centers were localized in the thalamus, whose sole task was the direct suppression of spinal cord activity.
- The final conclusion proved to be more profound. The researcher established that central inhibition is a direct consequence of the complex interaction between two or more streams of excitation that converge simultaneously on spinal cord neurons.
Within this experiment, two streams interact: descending excitation (arriving from the salt-stimulated thalamic structures) and afferent excitation (arriving from skin pain receptors of the paw upon contact with the aggressive acid solution). It is the collision of these powerful streams that leads to the formation of the inhibitory effect.