Physiology of nervous processes

Synapses — the basis of cell communication

A synapse is a specialized contact zone. It can be located both between two nerve cells and between a neuron and another excitable structure. The main task of this formation is to guarantee the transmission of excitation in such a way that the initial informational significance of the signal is completely preserved.

In physiology, it is customary to classify synapses according to several key criteria:

• By localization: central (located within the brain and spinal cord) and peripheral contacts are distinguished.
• By development in ontogenesis: there are stable synapses that form unconditioned reflex arcs, as well as dynamic ones that gradually appear as the individual develops.
• By the mechanism of signal transmission: contacts are divided into chemical, electrical, and mixed.
• By the ultimate physiological effect: excitatory and inhibitory.

Considering exclusively chemical synapses, they are additionally divided according to the contact shape into terminal (having the appearance of a bulbous connection) and transitory (appearing as a varicosity of the axon). By the nature of the released neurotransmitter, they can be cholinergic, adrenergic, GABAergic, and so on.

Mechanisms of nervous inhibition

Inhibition is a crucial active process that ensures the coordination of body functions. Within the physiology of nervous processes, two basic mechanisms are distinguished.

Reciprocal (conjugate) inhibition This is a physiological mechanism in which the activation of one group of neurons predictably causes the suppression of activity in another group performing the opposite function. The process is carried out with the mandatory participation of intercalated inhibitory neurons (INs). The sequence of events is as follows:

1. A nerve impulse arrives and is conducted along the main excitatory pathway.
2. Contraction of the target innervated muscle occurs.
3. Simultaneously, through an axon collateral of the excitatory neuron, the intercalated inhibitory cell is activated.
4. This inhibitory neuron reliably blocks signal conduction in the alternative (antagonistic) motoneuron pathway.

Presynaptic inhibition This type of inhibition unfolds directly in the presynaptic terminal.

• Neurotransmitter: gamma-aminobutyric acid (GABA) is released into the synaptic cleft.
• Ionic shifts: under the action of the neurotransmitter, chloride ions (Cl⁻) exit the cell. This leads to the development of membrane depolarization of the presynaptic terminal.
• Result: the conduction of nerve impulses along presynaptic pathways to the postsynaptic cell is blocked. The main outcome of this cascade is the release of an insufficient amount of neurotransmitter into the cleft, which prevents the excitation of the next neuron in the chain.

Age-related features of nervous regulation

Nervous processes in early childhood have pronounced specifics. In a newborn child, subcortical structures and brainstem mechanisms unconditionally predominate in the regulation of functions. This explains the abundance of transient unconditioned reflexes, which normally fade away within the first months of life.

Higher nervous activity (HNA) of an infant is characterized by the fact that excitation processes significantly predominate over inhibition processes. In addition, a pronounced irradiation of nervous processes is typical for childhood — excitation easily spreads, encompassing vast zones.

Physiological adaptation of newborns

In addition to the features of the nervous system, early-age physiology includes a number of critical adaptation mechanisms that are triggered immediately after birth.

• Thermoregulation: the main method of heat production in an infant is non-shivering thermogenesis. It is carried out due to brown adipose tissue (textus adiposus fuscus), rather than through muscle shivering. At the same time, heat loss is relatively higher than heat production due to the large body surface area and thin layer of subcutaneous fat, which creates a high risk of hypothermia.
• Respiration: the very first breath is initiated by a complex of factors after the cessation of placental circulation. These include an increase in carbon dioxide levels (paCO₂), a drop in oxygen tension (paO₂), and a decrease in blood pH, as well as temperature and tactile stimuli. Alveolar stability upon expiration is supported by surfactant, whose system matures by the 34th–36th week of gestation.
• Blood circulation: the fetus has three shunts functioning (ductus venosus, foramen ovale, ductus arteriosus) aimed at bypassing the non-functioning lungs. After birth, pulmonary vascular resistance drops, placental circulation stops, which changes the pressure gradient. The foramen ovale closes functionally immediately, and the ductus arteriosus closes within the first hours or days.
• Digestion and constants: the digestive system of an infant is adapted to milk nutrition — there is high lactase activity with low amylase activity (up to 3–4 months). Breast milk itself changes over time: from colostrum to transitional and mature milk. It is important to remember that all physiological constants (heart rate, respiratory rate, blood pressure, temperature) predictably change with age, so the assessment of the condition is always carried out taking into account the age norm.

Class and exam questions

What else to study on this topic

• Detailed classification of chemical synapses by neurotransmitter type
• Ionic mechanisms of depolarization during chloride ion efflux in the presynaptic terminal
• Stages of surfactant system maturation in embryogenesis
• Dynamics of changes in breast milk composition (colostrum, transitional, mature)
• Age norms of physiological constants (heart rate, respiratory rate, blood pressure, temperature)

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

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