Learn first: The nervous system
Higher nervous activity is the activity of the cerebral cortex and the nearby subcortical structures; it provides the finest adaptation of the organism to a changing environment. Behaviour, learning, memory, speech and thinking are all based on it. The exam often asks you to distinguish conditioned and unconditioned reflexes, and covers types of inhibition, the signal systems and temperament types.
The reflex nature of brain activity was first argued by I. M. Sechenov ("Reflexes of the Brain", 1863). I. P. Pavlov created the theory of conditioned reflexes and studied higher nervous activity experimentally.
Unconditioned reflexes are inborn, inherited, species-specific (the same in all members of a species) and permanent; their reflex arcs are ready at birth; they are carried out mainly by the spinal cord and brainstem. Examples: sucking, swallowing, blinking, pulling a hand away from something hot, salivating when food enters the mouth.
Conditioned reflexes are acquired during life, not inherited, individual and changeable (they form and can fade); the cerebral cortex takes part in forming them. Examples: salivating at the sight of a lemon, entering the classroom at the bell, the skill of riding a bicycle.
Instincts are complex inborn forms of behaviour made up of chains of unconditioned reflexes: nest building in birds, web spinning in spiders, a mother animal caring for her young.
Pavlov experimented on dogs: the duct of a salivary gland was led to the outside (a fistula), so the amount of saliva could be measured precisely. Before feeding, a lamp was switched on or a bell rung. After several repetitions the dog began to salivate to the light or bell alone, without food.
Conditions for forming a conditioned reflex:
A conditioned reflex is based on a temporary connection formed in the cortex between two centres of excitation.
A firm conditioned reflex can serve as the base for a new one — a second-order conditioned reflex. A system of reflexes repeated in a fixed order is a dynamic stereotype: daily routine, habits, skills. It is hard to change.
A bell rang before every feeding. After 15 repetitions the dog began to salivate to the bell. Then for several days the bell rang but no food was given, and salivation stopped. Explain what happened.
The two basic processes of the nervous system are excitation and inhibition. They are interlinked: one does not exist without the other. Excitation can spread across the cortex (irradiation) and gather in one place (concentration). A. A. Ukhtomsky discovered the principle of the dominant: the centre of excitation that prevails at a given moment inhibits other reflexes.
Types of inhibition:
Inhibition is not passive rest but an active nervous process. It stops unnecessary reactions and makes behaviour flexible.
Memory is the receiving, storing and recalling of information. There is short-term and long-term memory; by content — motor, visual (image), emotional and verbal-logical memory. Repetition and reading with understanding strengthen long-term memory.
Sleep is a periodic state needed for the brain to recover. It consists of slow-wave and rapid (paradoxical, REM) phases that alternate several times a night; dreams occur mainly in the rapid phase. Lack of sleep impairs attention, memory and mood.
Types of higher nervous activity (after Pavlov) are based on the strength, balance and mobility of nervous processes and correspond to the ancient temperament types:
Alcohol, nicotine and drugs upset the balance of excitation and inhibition in the cortex and weaken memory and thinking.
According to Pavlov’s theory, information about the external and internal environment is received and analysed by analysers. Each analyser has three parts:
Damage to any part of an analyser abolishes the sensation: even with healthy eyes a person cannot see if the visual cortex is damaged.
Main areas of the cerebral cortex:
Each hemisphere controls the opposite side of the body: damage to the motor area of the left hemisphere impairs movement on the right side. In most people (especially right-handers) the speech centres lie in the left hemisphere: the motor speech centre (Broca’s area, in the frontal lobe) — for producing speech, and the auditory speech centre (Wernicke’s area, in the temporal lobe) — for understanding it. These centres are the physical basis of the second signal system.
Conditioned reflexes also rely on analysers: through differential inhibition a dog learns to salivate to a metronome beating 100 times a minute but not to one beating 60 times — the auditory analyser tells two similar sounds apart.
A patient has healthy eyes but cannot see; an examination shows a haemorrhage at the back of the brain. Explain.
A dog has a firm conditioned reflex to a bell. Now a lamp is switched on before the bell, and no food is given; after several repetitions the dog salivates to the lamp as well. What kind of reflex is this?
A pupil is easily excited and throws himself into tasks with enthusiasm, but quickly flares up and cannot restrain himself. What is his temperament?