THE OVERALL SENSITIVITY AND ITS DISORDERS

Sensitivity is the ability of a living organism to perceive irritations from the environment or from its own tissues and organs and to respond by differentiated forms of reactions.

The organism continuously undergoes a variety of irritations: mechanical, panic, temperature, etc. All external agents primarily affect skin coverings. These irritations are perceived by a large number of nerve fibres, which are distal portions of the dendritic cells of the spinal units. For the most part they are the so-called free nerve endings, there are so many of them that they form whole plexus. The end of some fibres enters the epithelial structure in the shape of the flask, disk, or bulbs. These ends apparatus and dendrites are receptors. The free ends of the fibres belonging to special receptors perceive irritation and transform it into nerve impulses.

If skin is irritated the intervertebral ganglion cell (first neuron) or homologous to it cranial nerve ganglion sends perceived and processed pulses not only in effector neuron to form segmental reflex. At the same time it sends information to the second sense neuron in the spinal cord and brainstem formations; the third neuron (in the optic thalamus) transmits impulses to the cerebral cortex. A huge complex of cortical nerve cells and nerve impulse enters mind here. Then a feeling occurs. The classical idea of ​​the formation of sensations was formed in this way as a result of body irritation. All perceptions of external and internal environments in physiology are usually combined in the concept of "reception." However not everything what is perceived by nerve receptors is sensed, it is not a part of the consciousness. The concept of reception is broader than the concept of sensitivity, for example, the signals from the musculoskeletal system in the cerebellum. They regulate muscle tone and are involved in the coordination of movements, but impulses do not cause the emergence of feelings.

The reception brain functions have always been an object for philosophical debates on cognition because of its subjective sensation and an important role in the established relationship of the organism with environment.

The doctrine of analyzers was established to replace the idealism and agnosticism which prevailed in the physiology of the senses in the last century [Pavlov, IP, 1936]. This doctrine became the basis of the foundation of the natural science of the material conception of nature and mechanisms of sensation, the processes of learning and behaviour.

Analyzers are the functional units of structures of the peripheral and central nervous system fulfilling the function of perception and analysis of information of phenomena occurring in the outer and internal environment.

Analyzers are divided into 2 groups. External (exteroceptive) analyzers analyze information about phenomena occurring in the environment or within the body. They are visual, olfactory, auditory, tactile and other analyzers

Internal (interoceptive) analyzers analyze information about changes in the internal environment of the body, such as information on the state of the gastrointestinal tract, cardiovascular system, lungs and other internal organs. One of the major internal analyzers is motor analyzer, informing brain about the state of the muscle-joint system. The muscular system is not only an executive motor apparatus, but also an organ of proprioceptive sensitivity. In 1863 Sechenov shows that "dark muscular sense" plays an important role in the mechanisms of regulation of movements.

Vestibular analyzer has an intermediate position between the external and internal analyzers. The receptor is located within the body (the semicircular canals), but it is excited by external factors (acceleration and deceleration of the rotational and linear movements).

Each analyzer consists of a peripheral (receptor) part, wires and cortical parts.

Peripheral part of the analyzer is represented by specialized receptors that transform certain forms of energy (light, sound, heat) into nerve impulses. Due to the specialization of receptors (Pic. 21) there is the primary analysis of external irritation.

The meaning ​​of these signals is differentiated in the brain. The reason is that the signals of the receptor are encoded. In addition to pulse-code communication specific functional electrotonic connection with various areas of the brain are extremely important.

The conduction part of analyzers is represented not only by the neurons of thalamic nuclei and their projections to the appropriate areas of the cerebral cortex, but also by such entities as the reticular formation, the structure of the limbic system, cerebellum. It is established that the afferent signal which comes even by a single fibre, is sent to multiple neurons in specific, community and non-specific thalamic nuclei, which in turn switch each pulse to an even greater number of cortical neurons.

Cortical analyzer has a certain location, for example visual - mainly in the occipital region, the auditory - in the temporal lobe, motional - in the parietal cortex of the brain. The boundaries of these analyzer zones are not exact. In the cortical areas of the analyzer there are neurons that respond only to a certain sensory irritation. This is a specific projection neurons (the core of the cortical end of the analyzer). The next are non-specific nerve cells that respond to different sensory irritation, i.e. they have a multi-sensory convergence. Such neurons are especially numerous in the associative cortex. Due to the convergence of excitation of cortical neurons interaction between multiple analyzers is possible.

On the base of the analysis signals coming to the brain from external and internal receptors there the afferent synthesis of information comes with the following formation of the program behaviour and the apparatus of evaluation - the results of the acceptor. [PK Anokhin, 1955].

The activity of the analyzer is not limited by only the analysis of internal and external information, but it includes the reverse effect on the higher parts of the receptor and the conductor part of the analyzer. Receptor sensitivity (a perceived part) and functional state of transfer relay (conductor of the analyzer) are defined by a downward influences of the cerebral cortex allowing the organism actively select the most appropriate at this time sensory information from many irritations. This is expressed in the form of scrutinizing, squinting, listening and physiologically explained by a decrease of limen to visual or auditory irritation.

Receptors depending on its location: they are conditionally divided into exteroceptor (pain, temperature, tactile receptors), proprioceptors (located in muscles, tendons, ligaments, joints, receptors that give information about the position of the limbs and body in space, the degree of muscle contraction) interretseptory (located in the internal organs and the baro- and chemo- receptors).

In clinical practice different receptors are analyzed when appropriate irritations are applied; arising feelings are referred to as the overall sensitivity.

Feelings from irritation of exteroreceptors are called superficial (exteroceptive) sensitivity. The following forms of exteroceptive sensitivity are distinguished – a pain, heat, cold and tactile sensitivity.

The feeling of the posture of body and limbs in space (muscle-joint sense), a feeling of pressure and body weight, vibration, kinesthetic sensitivity, two-dimensional, spatial sense are referred to deep sensitivity (bathesthesia).

There are also complex types of sensitivity conditioned by combined activity of different types of receptors and cortical analyzers (for example a feeling of locating, recognition of objects by touch - stereognosis).

Interoceptive sensitivity is a sensation that occurs during irritation of internal organs, blood vessel walls. In large extent they are related to the field of autonomic innervation. The impulses from the internal organs are almost realized under normal circumstances. However when there is a feeling of irritation a feeling of discomfort, heaviness, pain of varying intensity occurs. This kind of feeling is not strictly localized, in other cases they are at different degrees are localized and connected with a specific organ.

In addition to the overall sensitivity there is a special sensitivity that occurs in response to irritation of the special senses from outside. This sensitivity includes vision, audition, smell, taste, besides vision, audition and sense of smell also called distant, i.e. irritable at a distance, in contrast to the contact in which the irritation is in direct contact with the skin or mucous membranes. Receptors of tactile sensation (sense of touch) and partly pain and temperature receptors are referred to contact receptors.

Conductors of pain temperature sensation(Pic. 22). A motion of conductors of pain and temperature sensitivity (conductors A) differs from that one by deep sensitivity (conductors B).

The first neuron of conductors of pain and temperature sensitivity like other tracts of overall sensitivity are presented by spinal ganglion nerve cell with its T-shaped dividing dendracson (appendix in which the beginning of the dendrite and the axon are close together and there is an impression of the merger). Peripheral appendix of the cells in the spinal nerve, plexus and peripheral nerve trunk goes to the corresponding dermatome (dermatome is a zone of innervation of skin from one spinal ganglion to the corresponding segment of the spinal cord). Dendrites which receive cold irritations contain receptors as in the form of encapsulated nerve sensitive endings (Krause's bulbs) and thermal fibre - in the form of non-encapsulated nerve endings (Ruffini's end). The axons of the ganglion cells form spinal nerve and dorsal root. Entering the substance of the spinal cord this fibre passes through a marginal zone (Lissauer zone), then - gelatinous substance and it forms the base of the posterior horn of the synapse with the second neuron of sensitive way. The second neuron cells form its own nucleus (a column of nerve cells that run along the spinal cord). Even before the formation of the synapses the axon of the neuron of the spinal ganglion gives collateral branch to the arc of appropriate segmental reflex. Then axon of the second neuron passes through the front commissure on the opposite side to a funicle side, but fibre goes horizontal not perfectly but in oblique and upward way. The transition is performed by 1-2 segments above.

Date: 2015-12-11; view: 1989