Preparation of the biological specimen
To conduct I.M. Sechenov's classic experiment, a specially prepared specimen is used. The initial stage is 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 the thalamic structures.
An important condition for the correct execution of the method is the removal of the cerebral hemispheres located above the line of the cut. The site of the created section is necessarily dried to prepare the nervous tissue for the subsequent application of a chemical stimulus. Such thorough preparation allows isolating the influence of overlying regions and focusing exclusively on the interaction of diencephalic centers with the spinal cord.
Course of the classic experiment
The essence of the method lies in comparing the speed of an unconditioned motor response before and after active stimulation of brain structures.
- The baseline reflex latency is determined as a control parameter. For this, the animal's hind paw is immersed in a weak hydrochloric acid solution (concentration 0.25% HCl), and the time until the limb is withdrawn is recorded. This response represents a standard acid protective withdrawal reflex.
- The main phase of the experiment begins: a chemical stimulant — a crystal of table salt (NaCl) — is placed on the dried section of the optic thalami. The salt provides intense and strong stimulation of the exposed nerve centers.
- Against the background of the ongoing action of this stimulus, the paw is re-immersed in the acid solution, and the latency of the acid withdrawal reflex is measured again.
Results and physiological mechanism
The main result of the experiment is a reliable increase in reflex latency. Under conditions of brain stimulation, the motor withdrawal response slows down significantly, and with a sufficiently strong effect, it may be completely absent (suppressed).
The physiological mechanism underlying this phenomenon is described by the following sequence of events in the central nervous system:
- The application of the NaCl crystal provokes a powerful focus of excitation in the nerve centers of the optic thalami.
- The generated excitation begins to spread along descending pathways, heading towards the spinal motor centers.
- Upon reaching the segments of the spinal cord, the descending signal activates specialized cells — inhibitory interneurons.
- The activated interneurons exert a powerful inhibitory influence on motor neurons, thereby functionally blocking the reflex arc of withdrawal.
Evolution of I.M. Sechenov's views
A thorough analysis of the obtained results forced 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 are localized in the optic thalami, the sole task of which is the direct suppression of spinal cord activity.
- The final conclusion turned out to be more profound. The researcher established that central inhibition is a direct consequence of a complex interaction of two or more excitation flows converging simultaneously on the neurons of the spinal cord.
Within the framework of this experiment, two flows interact: descending excitation (coming from the salt-stimulated structures of the optic thalami) and afferent excitation (originating from skin pain receptors of the paw upon contact with the aggressive acid solution). It is precisely the collision of these powerful flows that leads to the formation of the inhibitory effect.
Briefly: answers to topical questions
What are the main differences between Sechenov inhibition and Vvedensky's pessimal inhibition?
The main differences lie in the conditions of occurrence and the described mechanism.
| Sign | Sechenov inhibition | Vvedensky's pessimal inhibition |
|---|---|---|
| Condition of occurrence | In the experiment: strong stimulation of the optic thalami with an NaCl crystal on the frog's brain section. According to the final conclusion, the interaction of descending excitation from brain stimulation by salt and afferent excitation from paw pain receptors under the action of acid. | Occurs if the frequency of the stimulus exceeds the lability measure of the excitable tissue; for stable depolarization inhibition, a high frequency of excitation and an excess/accumulation of the mediator are indicated. |
| Mechanism | Descending excitation spreads to spinal motor centers, activating inhibitory interneurons; they inhibit motor neurons and block the reflex arc of withdrawal. | Stable depolarization due to sustained activation of Na+ and K+ channels; prolongation of the absolute refractory period. |
| Manifestation | Increase in the latency of the acid withdrawal response: the reaction slows down or is suppressed. | When the lability measure of the muscle is exceeded, instead of the expected increase in contraction amplitude, its decrease is noted. |
| Mediator aspect | The specific mediator is not specified in the description of Sechenov's experiment; inhibitory interneurons are indicated. | An excess of the mediator/accumulation of the excitatory mediator is described as a condition for stable depolarization inhibition. |
Which other brain structures, besides the thalamus, are capable of exerting a descending inhibitory influence on the spinal cord?
According to the cited sources, besides the optic thalami, inhibitory influences on spinal structures are described for the following structures.
- Reticular formation: exerts either activating or inhibiting influence on spinal motor neurons.
- Medial (vermal) zone of the cerebellum via related structures: its projections are indicated to the fastigial nucleus, vestibular nucleus of the pons, and reticular formation; impulses travel via the vestibulospinal tract, and among the effects on the spinal cord, inhibition of flexor motor neurons is indicated.
- Vasomotor center of the medulla oblongata in baroreflex regulation: upon blood pressure elevation, increased impulsation from baroreceptors leads to pessimal inhibition of the center's neurons; the center stops sending impulses to spinal sympathetic neurons, and vascular tone decreases. Inhibition of the nervous sympathetic system is also described in the depressor reflex.
The secret of Sechenov's experiment: SALT on the THALAMI inhibits ACID on the PAW.
Class and exam questions
Why is a 0.25% hydrochloric acid solution used in Sechenov's experiment?
The acid solution is necessary for applying a dosed painful stimulus, which elicits a standard acid protective defense reflex, allowing its baseline latency to be measured.
At what level is it necessary to make a transverse cut of the frog's brain?
The section is made in the region of the optic thalami (thalamus), while the cerebral hemispheres located above the line of the cut are completely removed.
Why does the paw withdrawal reflex latency increase?
Because descending excitation from the optic thalami activates inhibitory interneurons in the spinal cord, which block the motor neurons of the reflex arc.
What final conclusion regarding the nature of inhibition did I.M. Sechenov arrive at?
He concluded that inhibition does not require specific centers, but is the result of a complex interaction of descending and afferent excitation flows on spinal neurons.
What else to study on this topic
- The role of spinal inhibitory interneurons in blocking reflex arcs
- Mechanisms of propagation of descending excitation from the thalamus
- Interaction of afferent and efferent excitation flows in the central nervous system
- Principles of formation of protective defense reflexes
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