The nerve cells that form the nervous tissue are of two types: neurocytes (neurons) and gliocytes (glial cells). Neurons perceive irritation, generate an action potential, conduct and transmit nerve impulses, establish contacts with each other, and gliocytes provide conditions for the optimal functioning of neurons, i.e. isolate them, protect them, take part in the exchange of mediators and the allocation of neurocyte growth factor.
According to current information, the human brain contains 25 billion neurons, two-thirds of them are located in the cortex, and the number of glial cells is about 10 times higher.
Neuron
Nerve cells contain neurons, which are the main structural and functional element of the nervous system. A neuron is a process cell 4-130 microns in size, consisting of a body and processes, which are of two types: axon and dendrites. A process of a nerve cell - an axon - is otherwise called neuritis. The length of the processes reaches 1.5 m. There is only one axon in the cell, long, weakly branching; an impulse goes through it from the cell body. Dendrites are usually numerous, highly branched, short. Through them, the impulse enters the body of the neuron. Dynamic polarization is inherent in neurons; they conduct a nerve impulse in only one direction - from the dendrite to the axon. That is, a neuron in its structure resembles a funnel. The cell body basically performs the function of trophism in relation to the processes. The shape of the body can be different - from pyramidal to round.
Types of Neurons
According to the number of processes, nerve cells are divided into several main types.
- unipolar - have a single process, only an axon. These cells exist only in embryos as an intermediate stage in the development of neurocytes;
- bipolar - contain axon and dendrite. Similar nerve cells in humans are in the retina and in the inner ear;
- multipolar - have 2 or more processes, axon and dendrites. This is the most common type of neurons in the body, they are both in the central part of the nervous system, and in the peripheral;
- pseudo-unipolar cells - the only common process comes out of the cell body, including the axon and dendrite, later it is divided into two independent ones. These bipolar neurons are located in the nodes of the cranial and spinal.
Nerve cell structure
The cell is covered by neurilemma, which, in addition to the barrier, receptor and metabolic functions, performs a specific function of conducting a nerve impulse.
Nerve cells have a cytoplasm, which includes common organelles (mitochondria, endoplasmic reticulum, cell center, Golgi complex, lysosomes) and special-purpose organelles, the so-called neurofibrils. The nucleus of nerve cells is bright, round, contains 1 or 2 nucleoli.
Cell types by purpose
In accordance with the functional purpose, nerve cells are classified into sensitive, motor and insertion.
Sensitive neurons are cells whose body is located in the ganglia of the peripheral system. The dendrites of these cells end with sensitive endings, while the axon is sent to the brain stem or spinal cord.
Insertion nerve cells are responsible for the transmission of neuron excitation.
Motor or secretory cells are called depending on the structure (muscle fiber or iron), where their axon ends.
There are also auxiliary nerve cells, the so-called gliocytes, which isolate neurons from each other.
Ependymocytes are similar to epithelial tissues and line the cavities of the spinal cord and brain. Their function is basic and distinguishing.
Astrocytes are multi-process small stellate cells. According to the structure of the processes, astrocytes are protoplasmic and fibrous.
Nerve fibers are formed from processes of nerve cells and lemocytes. Outside, the nerve fiber is covered by a thin sheath of loose fibrous connective tissue called the basal lamina.