Mechanisms of neuronal interaction

Spread and convergence of excitation

Two basic processes determine how excitation is distributed within neural tissue: the divergence of signals and their concentration on specific cells.

Irradiation is the process of the spread of an impulse from a single source to multiple other elements. Arriving via afferent fibers, the signal diverges through the axon collaterals of the first neuron to a large number of other cells. The main result of this phenomenon is a significant expansion of the excitation zone.

The opposite process is convergence, which is the convergence of impulses from multiple fibers onto a single neuron. In this case, the neuron responds with a specific discharge pattern. Such cells are often localized in the cerebral cortex, reticular formation, and non-specific thalamic nuclei. Depending on the number of modalities (sensory inputs), neurons are divided into bisensory, trisensory, and polysensory.

There are four main types of convergence:

• Multisensory: convergence of signals from various sense organs.
• Multibiological: combination of excitations from centers of different biological states (pain, thirst, hunger).
• Sensory-biological: combination of the previous types (for example, signals from the retina and the hypothalamic hunger center converge on a single cell).
• Axonal-sensory-biological: the neuron receives inputs from three sources simultaneously (from collaterals of pyramidal neurons, receptor zones, and biological centers).

Multiplication and after-discharge of impulses

Signal processing in neural circuits allows for the modification of the incoming stimulus by increasing its frequency or duration.

The mechanism of multiplication turns a single input impulse into a whole series of output discharges. This occurs because the signal diverges along parallel chains of interneurons. These chains have different lengths and, consequently, different numbers of synapses. Due to the presence of synaptic delays, impulses travel along the pathways in different amounts of time and reach the final neuron not simultaneously, but strictly sequentially.

Prolongation (after-discharge) ensures the persistence of excitation even after the stimulus has ceased to act. This is based on the phenomenon of reverberation — the circulation of impulses in closed neural circuits (often called «neural traps»). A copy of the main signal travels via a collateral to an interneuron, which returns the excitation back to the original cell, creating a closed loop. Reverberation stops only when neurotransmitter stores in the synapses are depleted (synaptic fatigue) or when an external inhibitory signal arrives.

Principles of coordination of reflex activity

Nerve centers constantly interact with each other, which requires strict coordination. An important role here is played by the phenomenon of overlap of synaptic fields, where the receptive fields of two different reflex responses partially overlap.

Due to the presence of common nerve cells (overlap neurons), the mechanism of occlusion (blockade) arises. The essence of the phenomenon is that during simultaneous stimulation of two inputs, the total response turns out to be less than the simple arithmetic sum of the responses when they are stimulated separately. Some of the neurons activated by the first stimulus can no longer produce an additional response to the second input.

Another fundamental law was formulated by Charles Sherrington — the principle of the final common path. Spinal motoneurons serve as a single output for the realization of multiple reflexes. Sherrington illustrated this with two comparisons:

1. «Funnel»: a broad input from a vast number of afferent pathways narrows down to a narrow output (motoneurons).
2. «Bearer check»: effector cells respond to an incoming impulse with an action, completely independently of the source from which this impulse originally arrived.

Class and exam questions

What else to study on this topic

• Features of synaptic delays in various types of neural circuits
• The phenomenon of facilitation during the interaction of synaptic fields
• Mechanisms of external inhibition interrupting reverberation
• The role of synaptic fatigue and neurotransmitter depletion
• Functional significance of non-specific thalamic nuclei in convergence

Other topics in the "General Physiology of the Central Nervous System" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.