Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Higher Nervous Activity
Key Concepts

Higher Nervous Activity. This refers to The activity of the Cerebral Cortex AND subcortical structures that ensures the optimal adaptation of The Human Body to the environment; it can also be defined as the sum of unconditioned and conditioned Reflexes.

It is well known that the Central Nervous system (CNS) plays the primary regulatory role in the body. The activity of the CNS is reflexive in nature. A reflex is the body's response to a stimulus. The hypothesis that the central nervous system operates on a reflex principle was first put forward by the distinguished French scientist René Descartes. In 1863, the Russian physiologist I. M. Sechenov published his work «Reflexes of the Brain», in which he substantiated The connection between consciousness, thinking, and reflex activity. This same scientist is credited with discovering the phenomena of inhibition in the CNS.

The fundamental principles formulated by I. M. Sechenov were later confirmed in the works of Academician I. P. Pavlov. He conducted the majority of his scientific research within the walls of the Military Medical Academy. Pavlov's discovery of conditioned reflexes and his Development of concepts concerning the types, physiology, and pathology of higher nervous activity laid the foundation for further research in this field. He divided all reflex Reactions of the Organism into unconditioned and conditioned.

Unconditioned reflexes. These reflexes are innate, meaning they are inherited and persist throughout life. They have evolved over the course of evolution as adaptive responses.

Each unconditioned reflex has its own reflex arcs, and their central components have a strictly defined localization within the CNS.

Unconditioned reflexes are classified into the following types:

1) vitally essential (nutritive, protective);

2) social, arising from interaction with other individuals (orienting, sexual);

3) self-development reflexes (exploratory reflexes), aimed at acquiring new knowledge about the surrounding world and mastering new skills, such as the «what is it?» reflex.

Let us consider a few Examples of unconditioned reflexes. Salivation and an increase in gastric juice secretion upon The entry of food into the Oral Cavity are referred to as unconditioned salivary and secretory reflexes, respectively (see Ch. 7 «Anatomy and PHYSIOLOGY OF THE Digestive System»). The sucking reflex is also a type of feeding reflex: when the Skin around an infant's Lips is tactually stimulated, the baby begins to make sucking movements. Withdrawing a hand upon contact with a hot object and blinking in response to corneal irritation are examples of protective (defensive) reflexes. When a novel stimulus is presented, a response known as the «what is it?» reflex occurs, which is characterized by the shifting of attention to the new stimulus and its investigation.

Instincts represent the most complex form of unconditioned reflexes; they are species-specific, stereotyped patterns of behavior that essentially consist of a chain of unconditioned reflexes. In this process, the completion of one action triggers the initiation of the next, and so on. Examples of instincts include bird nest-building, among others.

Conditioned reflexes. These are reflexes acquired by the organism based on life experience. They are not inherited and are strictly individual, meaning they are specific to each given subject. Conditioned reflexes are necessary for the organism to adapt to changing environmental conditions. They are variable: as living conditions change, new reflexes emerge while old ones that are no longer needed fade away.

The formation of conditioned reflexes requires the mandatory participation of the cerebral cortex. They can only be formed on The basis of unconditioned reflexes. As already mentioned, salivation and increased gastric juice secretion in response to food (an unconditioned, adequate stimulus) contacting oral cavity receptors constitute an unconditioned reflex. If an indifferent signal, such as a light or a specific set of sounds, is presented to an experimental animal, no increase in salivations or secretions naturally occurs. These stimuli have no prior connection to food intake and are therefore indifferent. However, if one of these signals is repeatedly presented prior to each feeding, after a certain time, salivation and secretion will occur immediately upon its presentation, even before food is consumed. Such signals become conditioned stimuli. During the formation of conditioned reflexes, a temporary connection is established between the analyzer centers and the unconditioned reflex centers. Consequently, once the cortical center of the analyzer is excited by a specific stimulus, the unconditioned reflex center is also activated.

Thus, the establishment of a conditioned reflex requires the following conditions to be met:

✵ the unconditioned stimulus must be stronger than the conditioned one and biologically more significant;

✵ the presentation of the conditioned stimulus must precede that of the unconditioned stimulus;

✵ repeated pairings of the conditioned and unconditioned stimuli are required;

✵ an appropriate Setting must be provided, free from distracting extraneous stimuli.

Consequently, a conditioned reflex is an adaptive activity of the organism carried out in the higher Divisions of the central nervous system through the formation of temporary connections between the cortical centers of analyzers and unconditioned reflex centers. Conditioned reflexes serve as the foundation for the organism's acquired individual experience. They vary in complexity and biological significance. Conditioned reflexes form the basis for The Development of behavior in humans and higher animals. A comparative description of unconditioned and conditioned reflexes is presented in Table 16.1.

Based on their complexity, conditioned reflexes are divided into first-order, second-order, and higher-order reflexes. Examples of first-order reflexes are the salivary and secretory reflexes we have already discussed. If another stimulus (such as a sound) is added to the initial conditioned stimulus (light), after some time, salivation and gastric juice secretion will be observed following the presentation of the sound alone. This represents a second-order reflex, and so on.

Once formed and consolidated, a conditioned reflex can transform into a skill—an automatic action. For example, learning to write, play a musical instrument, or drive a car initially requires a high level of concentration and a vast amount of time. Once the skill is acquired, however, the person no longer thinks about how to write a particular letter or which pedal to press to stop the car. All these actions are performed automatically.

Class="center">Table 16.1 Comparative Characteristics of Reflexes

The structural basis of higher nervous activity. This represents a network of anatomically and functionally interconnected structures of the central nervous system. The presence of A large number of pre-existing morphological reflex arcs accounts for the existence of numerous unconditioned reflexes from the moment of birth. Unconditioned reflexes are integrated at the level of the Spinal Cord and brainstem. Cranial and Spinal Nerves also play a vital role. It should be noted that unconditioned reflex activity is coordinated by subcortical and cortical structures.

The structural basis of human mental activity is the brain. The Hypothalamus and the limbic system are among the key structures responsible for emotions and motivations. The thalamus Functions as a «filter» for all sensory information. The reticular formation is responsible for activating the cerebral cortex and regulating the wake-Sleep cycle.

Undoubtedly, the cerebral cortex plays the dominant role in mental activity. The projection and associative centers located here serve as the structural basis for various mental functions and regulate the activity of lower CNS divisions. The Basal Ganglia, which are part of the Telencephalon, are responsible for Muscle tone and the coordination of automatic movements.

Inhibition in The Nervous System. Conditioned reflexes may weaken or even disappear altogether. These processes are rooted in inhibition, which can be defined as a set of processes in the central nervous system that cause conditioned reflexes to fade. Inhibition is divided into two types: external and internal. Let us consider conditioned salivary and digestive reflexes as an example.

External inhibition is unconditional and innate. If an intense signal (light, sound, sharp odor) occurs after the presentation of a conditioned stimulus, it elicits an unconditioned orienting reflex (the "what-is-it?" reflex) in the test animal, and salivation and secretion do not occur. With repeated exposure to such situations, the established conditioned reflex may weaken or vanish entirely. This fact is consistent with the dominance principle formulated by Academician A. A. Ukhtomsky, according to which the currently dominant focus of excitation suppresses all others and determines The Nature of the body's response.

There are several types of internal inhibition: extinction, differentiation, delayed inhibition, and conditioned inhibition. If an animal with an established reflex to light is repeatedly presented with the conditioned stimulus over a long period without being reinforced by the unconditioned stimulus (food), salivation and secretion in response to the light will eventually cease. This is known as extinction, a type of internal inhibition of the conditioned reflex, during which the temporary connections between the analyzer centers and unconditioned reflexes weaken or disappear altogether. Differentiation inhibition develops when stimuli similar in parameters to the conditioned stimulus are left unreinforced. For example, if an animal has developed a salivary reflex to a specific sound signal, presenting a different sound signal—one not significantly different from the first—without food reinforcement will cause the animal to stop responding to the initial conditioned stimulus. Delayed inhibition occurs with a gradual increase in the interval between the conditioned stimulus and food reinforcement. Conditioned inhibition is developed through the alternating presentation of reinforced and unreinforced conditioned stimuli, where the latter is preceded by an auxiliary stimulus. After some time, this auxiliary stimulus causes the cessation of salivation and secretion in response to the conditioned stimulus.