THE HIGHER FUNCTIONS OF THE NERVOUS SYSTEM Lecture

THE HIGHER FUNCTIONS OF THE NERVOUS SYSTEM Lecture 24

All the reflexes taking place in the organism are divided into two principal groups: Unconditioned reflexes 1. Congenital, hereditary are predetermined Conditioned reflexes During individual development on the basis of life experience 2. Specific Individual 3. Constant enough (exception Not constant, may be developed, reinforced - some unconditioned reflexes of or extinguished newborns) 4. Evoked in response to adequate stimuli applied definite receptive fields 5. Function of the lower divisions of the CNS, the subcortical nuclei, brain stem, spinal cord Can be developed under the action of the stimuli applied to different receptive fields Function of the cortex and the nearest subcortical formations

CLASSIFICATION OF UNCONDITIONED REFLEXES Nutritional reflexes swallowing, chewing, sucking, salivation, secretion of gastric and pancreatic juice, etc. Defensive reflexes reactions eliminating injurious and painful stimuli. Sexual and parental reflexes associated with the sex behaviour and with the feeding and rearing of progeny. Stato - kinetic and locomotor reflexes reactions of maintaining a definite posture and of moving the body in space. Homeostatic reflexes of thermoregulation, respiration, cardiac activity, vascular reflexes, etc. Orientation reflexes reflex response to novelty: "What is it It? ” (“ That such? ” ) expressed at in turns of a head an eye aside of stimulus, extinguishes comparatively quick with repeated application of the same stimulus. It does not depend on a modality of stimulus.

RULES OF CONDITIONED REFLEX FORMATION • 1. A conditioned reflex can be produced only with a combination of the neutral (future conditioned) stimulus with the unconditioned stimulation. • 2. The beginning of a conditioned signal should precede the unconditioned stimulation. • 3. The intensity of the unconditioned should be more than the intensity of the conditioned signal. • 4. These combinations should be repeated many times. • 5. There particularly should be a normal active state of the CNS and the cerebral hemispheres. • 6. The absence of pathological processes in the organism is an indispensable condition for developing of conditioned reflexes. • 7. There should be absence of any stimuli arousing orientation reflexes

INHIBITION IN HIGHER NERVOUS ACTIVITY Prof. Irina V. Mukhina 2012

Inhibition of conditioned reflexes Unconditioned inhibition: 1. Extinguished inhibitor; 2. Constant inhibitor; 3. Excessive or protective inhibition Conditioned (internal) inhibition

Unconditioned inhibition: • Conditioned reflexes are easily inhibited by the action of various external stimuli. If, for example, the beginning of a conditioned nutritional reflex in a dog is suddenly preceded by a strange sound, conditioned reflex will be reduced or completely extinguished. • The explanation is that any new stimulus arouses an orientation reflex in the dog. This orientation reflex inhibits the conditioned reaction. The inhibited influence of new stimuli is extinguished with the repeat of them. This variety of external inhibition is the so-called "extinguished inhibitor".

Unconditioned inhibition: • A similar effect can be produced by stimulation of the visceral organs. For example, an inflammatory process in any organ or pain will inhibit conditioned nutritional reflexes. This variety of external inhibition is the so-called "constant inhibitor". • All these cases of inhibition have a common feature: they are due to stimuli arousing a new reflex reaction. Pavlov called this inhibition external.

Unconditioned inhibition: • The other variety of external inhibition is excessive or protective inhibition. A conditioned reflex can be inhibited by an excessive increase in the strength of the conditioned stimulus. The inhibited influence of these stimuli is not extinguished with the repeat of them. • For example, nutritional reflex on tone "mi" in a dog has been produced. The increase in force of tone up to the certain limit conducted to increase of value of a conditioned reflex. However the further increase in force of tone is higher than this limit resulted in reduction of effect. Reduction of force of stimulus resulted in the increase of a conditioned reflex again.

Conditioned (internal) inhibition • Conditioned inhibition are divided on: • 1. extinctive inhibition, • 2. differentiated inhibition, • 3. conditioned inhibition, • 4. delayed inhibition.

1. EXTINCTIVE inhibition • A conditioned reflex can exist as long as the conditioned signal is accompanied with and reinforced by an unconditioned stimulus. • But, if a conditioned signal is used alone is not reinforced by an unconditioned stimulus, the stable conditioned reflex earlier established will gradually weaken after several applications and finally be extinguished. • An extinguished conditioned reflex can recover spontaneously if the conditioned stimulus is not applied for some time. The phenomenon is known as disinhibition.

2. DIFFERNTIATED inhibition • If a conditioned reflex to some stimulus is established, similar stimuli are capable of eliciting a conditioned reaction. The phenomenon is known as generalization of the conditioned reflex. • If no special measures are taken to prevent it, a conditioned reflex will long retain its generalized character. To achieve accurate specificity the stimulus has to be constantly reinforced by an unconditioned stimulus, and signal close to it have to be used without reinforcement. • Then the reflex to them will gradually be extinguished, while the reflex to a reinforced signal will be retained. Pavlov called this process differentiation of stimuli. It is based on the development of conditioned inhibition in the cortex.

3. CONDITIONED inhibition • The so-called conditioned inhibitor can be developed if signal A is constantly reinforced by unconditioned stimulation, and the combination of signals B and A is never used with unconditioned stimulation. • Initially the combination of stimuli B+A elicits the same conditioned reflex as is produced by application of signal A alone. Later, the combination B+A loses its positive signal significance. Conditioned stimulus A, however, when used alone, retains its capacity to evoke a conditioned reflex. Stimulus B, which inhibits the conditioned reflex to signal A, was called a conditioned inhibitor. • For example, the person loves a cake, but he will refuse it if on a cake the fat green fly (signals B) sits.

4. DELAYED inhibition • If an unconditioned stimulus (for example, food) is used for a short time (1 to 5 seconds) after the beginning of a conditioned agent, the conditioned-reflex reaction (for instance, salivation) occurs immediately after the beginning of signal stimulation. • But if the reinforcing stimulus is gradually delayed to 2 or 3 minutes after the beginning of the conditioned stimulation, the conditioned-reflex reaction will more and more behind. • The mechanism of internal inhibition also accounts for the delay of the conditioned reflex. The reason for its development is that the action of the conditioned stimulus is not reinforced by the unconditioned reflex during the first minutes. During that time, therefore, the stimulus has an inhibitory significance and only later plays the role of a positive signal. • For example, if a reinforcement of food to make in 3 minutes after the beginning of sounding of a bell at development at(in) a dog the conditioned nutritional reflex on a bell salivation will begin too in 3 minutes.

Analysis and synthesis • Analysis and synthesis of stimulation are major functions of the cerebral cortex. • Analysis of stimuli consists of discrimination between different stimuli, and differentiation of the various influences on the organism. It begins right in the receptor apparatus, but the process is the most developed in the cortex. • The form of analysis specific to the cortex consists of the differentiation of the stimuli according to their importance as signals, which is secured by internal inhibition. • Synthesis of stimuli consists of the association, generalization, and unification of excitation arising in different areas of the cortex owing to the interaction of different neurons and groups of neurons. It is expressed in the formation of the temporary connections on which every conditioned reflex is formed.

Dynamic stereotypy • The complex forms of cortical synthesis are distinctly expressed in phenomena designated as dynamic stereotypy. • If different conditioned stimuli eliciting conditioned reflex are applied day after day in a strictly defined sequence, a definite stereotype of cortical reactions to the whole system of stimulation will be developed. • The cortex reacts to a signal after a definite pattern, in accordance with the existing dynamic stereotype. A conditioned signal is perceived not as an isolated stimulus but as an element in a definite system of signals, associated with both the preceding and the subsequent stimuli. • For example, the person gets used to wake up during certain time, in strict sequence to carry out a toilet, to follow in the same way to a place of work or study, to perform familiar (habitual) work. Thus, daily, weekly, annual stereotypes of behaviour are formed at him. At change of conditions of an environment these stereotypes can be reconstructed, therefore them name dynamic stereotypes.

TYPES OF THE HIGHER NERVOUS ACTIVITY • Conditioned-reflex activity depends on the individual qualities of the CNS. • The aggregate of these qualities, which largely determine the character of higher nervous activity, is preconditioned by the hereditary characteristics of the individual and previous life experience, and is called the type of nervous system or type of higher nervous activity.

Types is based on a rating of three basic features of processes of excitation and inhibition: 1. The force (intensity) of the excitation and inhibition processes, 2. Their reciprocal equilibrium (steadiness), in other words, the ratio of their intensities, and finally 3. Their mobility, i. e. the rate at which excitation is replaced by inhibition, and vice versa.

Principal types of higher nervous activity Intensity 1. Strong Mobility Highly mobile nervous processes – "impetuous" type 2. Strong Equilibrium Unbalanced, predominance of excitation over inhibition Well - balanced 3. Strong Well - balanced Low mobility of nervous processes - "calm" or inert type 4. Weak - Highly mobile nervous processes - "lively" or active type Extremely weak development of both excitation and inhibition, quick fatigability leading to loss of work capacity The 4 types of higher nervous activity met in animals and man coincide with the 4 temperaments in man described by Hippocrates. The strong, unbalanced type with predominance of excitation over inhibition coincides with the choleric temperament. The strong, well balanced, active type coincides with the sanquine temperament. The strong, well balanced, inert type coincides with the phlegmatic temperament. The weak type coincides with the melancholic temperament.

RETICULAR FORMATION OF BRAIN STEM. ELECTRICAL PHENOMENA IN THE CEREBRAL CORTEX. SLEEP

Reticular formation • In the central part of the brain stem there is an anatomical formation consisting of diffuse aggregations of cells of various types and sizes, which have numerous fibers passing in different directions. • Because the nerve tissue in this region resembles a net under the microscope, it is called reticular formation. • Reticular formation is of the highest importance for regulating the excitability and tone of all divisions of the CNS

• Through the descending reticulospinal tracts it is capable of exerting both an activating and an inhibitory influence on the reflex activity of the spinal cord. • Through the ascending tracts it produces an activating influence on the cerebral cortex.

• Impulses from the reticular formation and the nonspecific nuclei of the thalamus maintain the cortex in a wakeful state. Under its influence reflex reactions become stronger and more accurate. • Magoun and Moruzzi (1949) found that stimulation of the reticular formation caused changes in the electrical activity of the cortex characteristics of wakening and of the natural waking state. • With destruction of the reticular formation an animal induces sleep. It indicates that the normal activity of the cortex depends largely on the tonic and activating influences of the reticular formation of the brain stem.

• The activity of the reticular formation, through which it can exert its ascending and descending influences, is sustained by the arrival of impulses through the collaterals of various afferent pathways. As a result, the most varied stimulation of the receptors affects its condition. • In addition, its neurons are extremely sensitive to various chemical agents, hormones and certain metabolites. The reticular formation also receives impulses from the effecter centers of the cerebral hemispheres and the cerebellum. • Both ascending afferent and descending efferent impulses interact in the region of the reticular formation. Circulation of impulses through its closed, circular neuronal chains is also possible.

ELECTRICAL PHENOMENA IN THE CEREBRAL CORTEX • The electroencephalogram (EEG) is a rhythmic electrical activity recorded from the surface of the skull. • In humans, the EEG is recorded from a grid of standard leads. • The EEG recording is due to large synaptic potentials by whole groups of mainly pyramidal cells. The EEG pattern is desynchronized by sensory inputs through the thalamus. • During neurosurgery the electrical activity is recorded from the surface of the cortex as an electrocorticogram(ECo. G).

• The amplitude of the potentials from the scalp in man varies from 5 or 10 to 200 or 300 microvolts (µV) and the frequency from 0, 5 to above 70 cycles per second (cps) or Hz. • Five basic types of EEG - alpha, beta, gamma, delta, and theta are distinguished, according to the frequency, amplitude, and physiological characteristics of their electrical waves.

Rhythm Frequency (Hz) Amplitude (µV) gamma above 30 below 15 beta 14 - 30 15 - 20 alpha 8 -13 45 - 70 theta 4 -7 100 - 300 delta 1 -3 300 - 500

Thank for you attention!