The ability to react to external stimuli is a basic characteristic of living tissue. In single-celled organisms, all the stages of receiving, transmitting, processing, and responding to stimuli take place in a single cell. In more highly developed, mul-ticellular animals, the stimulus is transmitted by special conductive tissues—the nerves. Receiving, conducting, and processing stimuli enables living beings to orient themselves in their surroundings and to act and react independently.
Ganglia (singular: ganglion) are the groups and knots of nerve cells known as ganglion cells.They make up the nervous system with one center for processing stimuli, namely the brain and the spinal cord. The size of nerve cells varies from 4 to 120 um, excluding the processes located on the cells. Adult nerve cells have lost their ability to divide in favor of very pronounced differentiation; in other words, dead nerve cells are not replaced. There are around 14 billion nerve cells in the cerebral cortex, which are already preformed at birth.
Nerve cells (neurocytes) have various long extensions or processes. Processes that direct the stimulus away from the nerve cell are called neu-rites and can be very long (up to 1 m). Other processes that receive the stimulus from other nerve cells are very short and are called dendrites. A nerve cell with its processes is referred to as a neuron and is the smallest unit in the nervous system (Fig 7-49). The nerve fibers are the processes that emanate from the nerve cells.
Synapses are the contact points between the extensions of different nerve cells (Fig 7-50). The nerve fibers touch in the synapses but do not merge into each other. The transmission of nerve impulses by contact takes place by means of chemical transmitters: epinephrine (adrenaline) and acetylcholine.
Nerve cells have a strong tendency to form processes and synapses but also the ability to break down synapses. The nerve cells retain this ability for life; in other words, the connection of nerve cells among themselves remains variable, changeable, and expandable throughout life. In addition to the number of nerve cells in the brain, the number of synaptic connections between the nerve cells in particular accounts for the functional capacity of the brain.