The brain

Our brain is the most complicated mechanism which has ever been constructed.

The weight of the human brain is from 1 to 2 kg. It has a volume of about 3,21 l and consists of about 12 billion cells. They are the most delicate of all the cells of the human body. Each cell is connected to the other directly or indirectly by nerve fibers.

The brain is the centre of a wide system of communication. A constant flow of stimuli comes into the brain through the spinal cord from our eyes, ears and other sense organs for pain, temperature, smell and other feeling. We can eat, hear, see and do many other things when all the received stimuli are summarized and analyzed in the brain and then it sends orders through the nerve fibers in the spinal cord to different parts of the body.

There are those areas of the brain which control vision, hearing, physical movements and even emotions. For example, the motor cortex controls body movements, but the hypothalamus controls function of blood pressure.

At the moment of tiredness of the cortical nervous cells the process of inhibition begins to act. When the process of inhibition extends over a great number of cells, spreads widely over the cortex and even over the subcortical areas of the brain one falls asleep. During a sound quite sleep the whole activity of the human organism changes, the body metabolism decreases, and the respiratory and pulse rates become slower, the body temperature drops. Though the stimuli continue to come into the brain, the inhibited cortical cells don’t react to them.

Prolonged sleeplessness increases the amount of sugar in blood and white blood cells count, decreases the level of vitamins B1 and B6, and the amount of iron in the blood.

Reflexes

All the visceral and somatic reflexes including the protective, feeding and others are formed by various internal and external stimuli. These stimuli produce reactions not depending on surrounding conditions. The great Russian physiologist Pavlov called them unconditioned reflexes.

In human beings the nervous system has the additional ability to form cortical associations which increase the range of reactions, i.e. the process in which an ineffective reflex stimulus forms the same reactions as the stimulus with which it has become associated. These individually obtained reflexes are conditioned, because they are developed only in connection with some other reflexes. They begin to develop already in infancy. During human development their number is much increased through training and education.

In higher animals it is in the cortex that conditioned reflexes are formed.