The greatest achievement of evolution is the brain and the developed nervous system of organisms, with an increasingly complex information network based on chemical reactions. A nerve impulse running along the processes of neurons is the quintessence of complex human activity. An impulse arises in them, it moves along them, and it is the neurons that analyze them. The processes of the neuron are the main functional part of these specific cells of the nervous system, and they will be discussed.
Origin of neurons
The question of the origin of specialized cells is open today. There are at least three theories on this subject — Kleinenberg (Kleinenberg, 1872), the Gertwig brothers (Hertwig, 1878), and Zavarzin (Zavarzin, 1950). All of them boil down to the fact that neurons arose from primary sensitive ectodermal cells, and their precursors were globular proteins, united in bundles. Proteins that subsequently received the cell membrane were able to perceive irritation, generate and conduct excitation.
Modern ideas about the structure of the neuron and processes
A specialized cell of nerve tissue consists of:
- The soma or body of the neuron in which the organelles, neurofibrils and nucleus are located.
- Many short processes of a neuron called dendrites. Their function is the perception of arousal.
- One long process of a neuron - an axon, covered as a "sleeve" of the myelin sheath. The main function of the axon is to conduct excitation.
All neuron structures have different membrane structures and they are all completely different. Among the many neurons (in our brain there are about 25 billion of them) there are no absolute doubles both in appearance, in structure and, most importantly, in the specifics of functioning.
Short processes of neurons: structure and functions
The body of the neuron has many short and branched processes that are called the dendritic tree or dendritic region. All dendrites have many branches and points of contact with other neurons. This perceptual network improves the collection of information from the surrounding neuron environment. All dendrites have the following features:
- They are relatively short - up to 1 millimeter.
- They have no myelin sheath.
- These processes of the neuron are characterized by the presence of ribonucleotides, the endoplasmic reticulum and an extensive network of microtubules, which has its own uniqueness.
- They have specific processes - spines.
Spikes of Dendrites
These outgrowths of the dendritic membrane can be found in large numbers on their entire surface. These are additional points of contact (synapses) of the neuron, many times increasing the area of interneuronal contacts. In addition to expanding the perceptual surface, they play an important role in situations of sudden extreme effects (for example, with poisoning or ischemia). Their number in such cases sharply changes in the direction of increase or decrease and stimulates the body to increase or decrease the speed and number of metabolic processes.
Conductive process
The long process of a neuron is called an axon (ἀξον - axis, Greek), it is also called an axial cylinder. At the site of axon formation on the body of the neuron there is a mound, which plays an important role in the formation of a nerve impulse. This is where the action potential coming from all neuron dendrites is summed up. There are microtubules in the axon structure, but almost no organelles. The nutrition and growth of this process is completely dependent on the body of neurons. When the axon is damaged, their peripheral part dies, and the body and the rest remain viable. And sometimes a neuron can grow a new axon. The axon diameter is only a few micrometers, but the length can reach 1 meter. Such, for example, are the axons of neurons of the spinal cord that innervate the human limbs.
Axon myelination
The shell of the long processes of the neuron is formed by Schwann cells. These cells encircle the axon, and their tongue wraps around it. The cytoplasm of Schwann cells is almost completely lost and only the membrane of lipoproteins (myelin) remains. The purpose of the myelin sheath of the long processes of neuron bodies is to provide electrical isolation, which leads to an increase in the speed of the nerve impulse (from 2 m / s to 120 m / s.). The shell has gaps - Ranvier constrictions. In these places, an impulse, like a galvanic current, freely enters the medium and enters back. And it is in the constrictions of Ranvier that the appearance of the potential of action occurs. Thus, the impulse moves along the axon in jumps - from constriction to constriction. Myelin is white, it was this that served as the criterion for dividing the nerve substance into gray (neuron bodies) and white (pathways).
Axon bushes
At the end, the axon forks many times and forms a bush. At the end of each branch there is a synapse - the site of contact of the axon with another axon, dendrite, body of neurons or somatic cells. Such multiple branching allows multiple innervation and duplication of momentum transmission to be achieved.
Synapse - a place of transmission of a nerve impulse
Synapses are unique formations of neurons where the signal is transmitted through substances called neurotransmitters. The action potential (nerve impulse) reaches the end of the appendix - the axon thickening, which is called the presynaptic region. There are multiple vesicles with neurotransmitters (vesicles). Neurotransmitters are biologically active molecules designed to transmit a nerve impulse (for example, acetylcholine in muscle synapses). When the transmembrane current in the form of action potential reaches the synapse, it stimulates the operation of membrane pumps, and calcium ions enter the cell. They initiate rupture of the vesicles, the mediator enters the synaptic cleft and binds to the receptors of the postsynaptic membrane of the pulse successor. This interaction starts the work of sodium-potassium membrane pumps, and a new action potential arises, identical to the previous one.
Axon and target cell
In the process of embryogenesis and postembryogenesis of the body, neurons grow axons to those cells that should be innervated by them. And this growth is strictly directed. The mechanisms of neuronal growth have been discovered not so long ago, and they are often compared with the owner leading a dog on a leash. In our case, the owner is the body of the neuron, the leash is the axon, and the dog is the growth point of the axon with pseudopodia (pseudopods). The orientation and choice of axon growth direction depends on many factors. This mechanism is complex and in many respects has not yet been fully studied. But the fact remains that the axon reaches precisely its target cell, and the processes of the motor neuron that is responsible for the little finger grow exactly in the muscles of the little finger.
Axon Laws
When conducting a nerve impulse along axons, four main laws work:
- The law of anatomical and physiological integrity. Carrying out is possible only on intact processes of neurons. This rule also applies to damage due to changes in membrane permeability (under the influence of drugs or poisons).
- The law of isolation of excitation. One axon - conducting one excitation. Axons do not share nerve impulses with each other.
- The law of unilateral conduct. Axon conducts an impulse either centrifugally or centripetally.
- The law of no loss. This is a property of non-decrement - when conducting an impulse, it does not stop and does not change.
Types of neurons
Neurons are star-shaped, pyramidal, granular, basket-like - they can be like this in body shape. By the number of processes, neurons are: bipolar (one dendrite and one axon) and multipolar (one axon and many dendrites). According to the functional, neurons are sensory, plug-in and executive (motor and motor). Golgi type 1 and Golgi type 2 neurons are distinguished. This classification is based on the length of the process of the axon neuron. The first type is when the axon goes far beyond the area of the body (pyramidal neurons of the cerebral cortex). The second type - the axon is in the same area as the body (cerebellar neurons).