Spread and Convergence of Excitation
Two basic processes determine how excitation is distributed within neural tissue: signal divergence and signal concentration onto specific cells.
Irradiation is the process by which an impulse spreads 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 primary result of this phenomenon is a significant expansion of the zone of excitation.
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 frequently localized in the cerebral cortex, reticular formation, and non-specific thalamic nuclei. Depending on the number of modalities (sensory inputs), neurons are classified as bisensory, trisensory, or polysensory.
Four main types of convergence are distinguished:
- Multisensory: convergence of signals from various sensory organs.
- Multibiological: combination of excitations from centers of different biological states (pain, thirst, hunger).
- Sensory-biological: a combination of the previous types (e.g., 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 within neural circuits allows for the modification of an incoming stimulus by increasing its frequency or duration.
Multiplication converts a single input impulse into an entire series of output discharges. This occurs because the signal diverges along parallel chains of interneurons. These chains have varying lengths and, consequently, varying numbers of synapses. Due to synaptic delays, impulses traverse the pathways in different amounts of time and reach the final neuron not simultaneously, but in strict succession.
Prolongation (after-discharge) ensures that excitation persists 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 ceases only upon the depletion of neurotransmitter stores in synapses (synaptic fatigue) or upon the arrival of an external inhibitory signal.
Principles of Coordination of Reflex Activity
Nervous centers constantly interact with one another, requiring strict coordination. An important role here is played by the phenomenon of overlap of synaptic fields, where the receptive fields of two different reflex reactions partially overlap.
Due to the presence of shared nerve cells (overlap neurons), the mechanism of occlusion arises. The essence of this phenomenon is that upon simultaneous stimulation of two inputs, the total response is less than the simple arithmetic sum of the reactions when they are stimulated separately. A portion of the neurons activated by the first stimulus can no longer provide an additional response to the second input.
Another fundamental law was formulated by Charles Sherrington: the principle of the final common path. Spinal motor neurons serve as a single output for the execution of multiple reflexes. Sherrington illustrated this with two comparisons:
- The 'Funnel': a broad input from a vast number of afferent pathways narrows down to a narrow output (motor neurons).
- The 'Paycheck': effector cells react to an incoming impulse with action, completely regardless of the source from which that impulse originally arrived.