Chapter 3. Nervous system

According to modern concepts, CNS activity is based on processes, associated, primarily, with the work of nerve cells. Neuronal theory successfully explains a wide range of phenomena occurring in the brain and spinal cord. a neuron is a structural and functional unit of the nervous system. The number of neurons in the brain and spinal cord is about 10 billion. Neurons consist of a body, dendrites originating from it with a large number of synapses and one axon, along which impulses run from the body to the periphery. Several collaterals that also terminates with synapses, may originate from the axon. Neurons can be afferent, associative, and efferent. Afferent neurons provide conduction of information from the sensory organs to the CNS. Associative neurons are entirely located in the CNS and provide communication between afferent and efferent neurons. Efferent neurons conduct impulses from the CNS to the muscles or glands. The following principles of the neural organization were formulated (R. Cajal).

• A neuron with processes forms a single morphological entity.
• Neurons are genetically unified, originating from the same neuroblasts.
• A neuron is functionally unified.
• Excitation along the neuron propagates in one direction - from the dendrites to the axon.
• Trophic unity is inherent for a neuron. If the bodies of neurons are removed, axon death is observed. Ligation of the axon also causes the death of the cell.
• The neuron as a unit is involved in pathological reactions. Destructive changes cover the entire neuron.

Neuronal theory

The aforementioned principles formed the basis of the neuronal theory of CNS organization. According to this theory, the structure and functions of CNS are determined by a variety of interconnected neurons with discrete properties. Along with this, the idea that CNS has a solid structure via the type of syncytium, inherent for, for example, in neuropil present in the nervous system of lower invertebrates (coelenterates) and the brain of amphibians (axolotl), does not lose its significance. Heterogeneous neurons in their interactions allow the human CNS to participate in millions of different reactions. On the bodies of single neurons, hundreds to thousands of synapses are located, which function with the help of various mediators and are associated with specific postsynaptic chemical reactions.

Electrical and chemical theories explain the processes of neuronal excitation.

Electrical theory of neuronal excitation

Is based on ideas about the morphofunctional heterogeneity of neurons. Most synapses are located on the dendrites of neurons, where they occupy 75% of the membrane’s area, and only 2% of synapses - in the brain cortex - on the bodies of neurons. Synapses on dendrites are called integrator synapses, and on the body of the cell closer to the axon origin - detonators. It was believed, that the membrane of dendrites is electrically undisturbed. Today, it has been proven that certain areas possess potential-controlled conductivity due to the presence of calcium channels, whose activation leads to calcium action potentials. The so-called “light axon hillock,” or “light spot,” where there are no synapses, is located near the place of the axon’s origin. This area of the neuron is positively charged in relation to other areas of the neuron membrane. During excitation of the synapses located on the dendrites and body of neurons, the potential difference between the “axon hillock” and other part of the neuron increases. When a certain value is reached, this generates axonal spike activity. The more rapidly the potential difference between the body of the neuron and “axon hillock” increases, the more frequent the axon output impulse is.

Chemical theory of neuronal excitation

Chemical theory connects the processes of neuronal excitation to the specifics of neurotransmitters, oligopeptides, and other biologically active substances released by the synapses of neurons. Excitatory transmitters (norepinephrine, acetylcholine, glutamate, etc.) depolarize the postsynaptic membrane and generate an excitatory postsynaptic potential. Inhibitory transmitters (e.g., GABA) generate inhibitory postsynaptic potentials on the postsynaptic membrane. The process of neuronal excitation depends on the specifics of postsynaptic chemical reactions that propagate to the genetic apparatus of neuronal nuclei.

Transmitter functions of oligopeptides and amino acids

Oligopeptides-opioids, substance P, angiotensin II, vasopressin, oxytocin, as well as cytokines modulate transmitter action in the synapses of the CNS. Amino acids - glutamic acid, aspartic acid, and GABA - perform a specific transmitter function. Specific glutamate receptors (NMDA receptors with affinity to N-methyl-D-aspartate) were detected in some neurons of the CNS. GABA is the most common inhibitory transmitter in the CNS. Another inhibitory transmitter, whose point of application is the neurons of the medulla oblongata and spinal cord, is glycine.

Functions of neurons

• Receptor. The ability to perceive information is ensured via receptors of postsynaptic membranes, which are part of neuron-to-neuron synapses, and whose number can reach several thousand.
• Neuronal memory allows both genetic and acquired information to be stored.
• Integrative. Basic neuronal activity consists of the intracellular processing of information, which can be used at any given moment of time.
• Synthesizing. The processes of synthesis of biologically active substances occur in the body of a neuron. These include neurotransmitters and neuromodulators. The former ensures the transmission of a signal to the cell, while the latter can affect the efficiency of transmission, largely determining its nature. Today the processes of synthesiare are known to occur also in the cortical zone of the neuronal axon.
• Transport. The synthesized substances are transported inside the axon of the nerve cell to the synaptic terminals. There are two types of axonal transports: fast, with a speed of up to 20 cm/day, and slow - about 10 mm/day. Transport is performed through the action of special fibrillar proteins.
• Generator. The external neuronal reaction consists of the action potential generation, which occurs in the initial segment of the neuron. At present, it has also been shown that electrical potential generation is possible at the sites of collaterals originating from the main axon.
• Encoding. A neuron possesses the property of encoding, i.e., transmission of information in the conditional form of a code. The pattern of action potential sequence is the means of neuron participation in a particular experimental or behavioral situation.
• Conduction. A neuron possesses the property of a conductor, and action potential from the site of the initial segment propagates along the axonal membrane to the working synapses.
• Secretory. It is manifested in the release of transmitters and some peptides in the axonal terminal.
• Trophic. It should be noted that the growth of axons and synaptic connection function are affected by so-called trophic factors and, in particular, the nerve growth factor (NGF), which is produced by target cells, captured by neurons and retrogradely transported to the soma, where it affects the enzymatic processes, transmitter production, and axonal growth.