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 we will talk about them.
Origin of neurons
The question of the origin of specialized cells is still open today. There are at least three theories on this subject - Kleinenberg (Kleinenberg, 1872), brothers Hertwig (Hertwig, 1878) and Zavarzin (Zavarzin, 1950). All of them boil down to the fact that neurons arose from primary sensitive ectodermal cells, and their predecessors were globular proteins that united into bundles. Proteins that subsequently received cellularmembrane, turned out to be capable of perceiving irritation, generating and conducting excitation.
Modern ideas about the structure of the neuron and processes
A specialized cell of the nervous tissue consists of:
- Soma or body of a neuron, which contains organelles, neurofibrils and a nucleus.
- Many short processes of a neuron called dendrites. Their function is to perceive arousal.
- One long process of a neuron - an axon, covered like a "clutch" with a myelin sheath. The main function of the axon is to conduct excitation.
All structures of a neuron have a different structure of membranes and they are all completely different. Among the many neurons (there are about 25 billion of them in our brain) there are no absolute twins both in appearance and in structure and, most importantly, in the specifics of functioning.
Short processes of neurons: structure and functions
The body of a neuron has many short and branched processes, which are called a dendritic tree or a dendritic region. All dendrites have many branches and points of contact with other neurons. This network of perception increases the level of information gathering from the environment surrounding the neuron. All dendrites have the following features:
- They are relatively short - up to 1 millimeter.
- They don't have a myelin sheath.
- These neuron processes are characterized by the presence of ribonucleotides, the endoplasmic reticulum and an extensive network of microtubules, which has its ownuniqueness.
- They have specific processes - spines.
Dendrite spines
These outgrowths of the dendritic membrane can be found on their entire surface in large numbers. These are additional contact points (synapses) of the neuron, which greatly increase the area of interneuronal contacts. In addition to expanding the receptive surface, they play an important role in situations of sudden extreme effects (for example, in case of poisoning or ischemia). Their number in such cases changes dramatically in the direction of increase or decrease and stimulates the body to increase or decrease the rate and number of metabolic processes.
Conducting 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 a neuron, there is a mound that plays an important role in the formation of a nerve impulse. It is here that the action potential received from all the dendrites of the neuron is summed up. The structure of the axon contains microtubules, 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, while the body and the remaining part remain viable. And sometimes a neuron can grow a new axon. The diameter of the axon is only a few micrometers, but the length can reach 1 meter. Such, for example, are the axons of spinal cord neurons that innervate human limbs.
Axon myelination
The shell of the long processes of the neuron is formed by Schwann cells. These cells wrap around sections of the axon, and their uvula wraps around it. The cytoplasm of Schwann cells is almost completely lost and only a membrane of lipoproteins (myelin) remains. The purpose of the myelin sheath of the long processes of the neuron bodies is to provide electrical insulation, which leads to an increase in the speed of the nerve impulse (from 2 m/s to 120 m/s). The shell has ruptures - constrictions of Ranvier. In these places, the impulse, like a current of a galvanic nature, freely enters the medium and enters back. And it is in the constrictions of Ranvier that the action potential occurs. Thus, the impulse moves along the axon in jumps - from constriction to constriction. Myelin is white, this is what served as a criterion for dividing the nerve substance into gray (neuronal bodies) and white (conducting pathways).
Axon bushes
At its end, the axon branches many times and forms a bush. At the end of each branch there is a synapse - the point of contact of an axon with another axon, dendrite, body of neurons or somatic cells. This multiple branching allows for multiple innervation and duplication of impulse transmission.
The synapse is the site of nerve impulse transmission
Synapses are unique formations of neurons, where the signal is transmitted through substances called mediators. The action potential (nerve impulse) reaches the end of the process - the axon thickening, which is called the presynaptic region. There are multiple vesicles with mediators (vesicles). Neurotransmitters are biologically active molecules designed to transmit a nerve impulse (for example, acetylcholine in muscle synapses). When a transmembrane current in the form of an action potential reaches the synapse, it stimulates the membrane pumps, and calcium ions enter the cell. They initiate the rupture of vesicles, the mediator enters the synaptic cleft and binds to the receptors of the postsynaptic membrane of the impulse receiver. This interaction triggers the sodium-potassium pumps of the membrane, and a new action potential, identical to the previous one, arises.
Axon and target cell
In the process of embryogenesis and post-embryogenesis 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 to an owner leading a dog on a leash. In our case, the host is the body of the neuron, the leash is the axon, and the dog is the growth point of the axon with pseudopodia (pseudopodia). The orientation and direction of axon growth depends on many factors. This mechanism is complex and largely not yet fully understood. But the fact remains - the axon reaches exactly its target cell, and the processes of the motor neuron, which is responsible for the little finger, will grow into 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. Conduction is possible only along intact processes of neurons. This rule also applies to damage resulting from changes in membrane permeability (under the influence of drugs or poisons).
- The law of excitation isolation. One axon - conduction of one excitation. Axons do not share nerve impulses with each other.
- The law of unilateral holding. The axon conducts impulse either centrifugally or centripetally.
- The law of no loss. This is the property of non-decrement - when conducting an impulse, it does not stop and does not change.
Varieties of neurons
Neurons are stellate, pyramidal, granular, basket-shaped - they can be like that in the shape of the body. By the number of processes, neurons are: bipolar (one dendrite and axon each) and multipolar (one axon and many dendrites). By functionality, neurons are sensory, plug-in and executive (motor and motor). Neurons of Golgi type 1 and Golgi type 2 are distinguished. This classification is based on the length of the axon neuron process. The first type is when the axon extends far beyond the location of the body (pyramidal neurons of the cerebral cortex). The second type - the axon is located in the same zone as the body (cerebellar neurons).
