General functions of the nervous system
The nervous system (systema nervosum) acts as a universal coordinator. It not only ensures the coordinated work of absolutely all organs, but also allows the body to adequately respond to various stimuli.
Globally, its functionality comes down to three consecutive stages:
- Perception of stimuli coming from outside or inside.
- Analysis, synthesis, and storage of the received information.
- Transmission of generated signals to the final effector organs (muscles or glands).
Neuron: structure and impulse direction
The basis of nervous tissue is the neuron, which performs its work with the mandatory support of neuroglia.
Each such cell has a strictly defined structure:
- Cell body (corpus) — the central part of the cell.
- Dendrites (dendritae) — short, actively branching processes.
- Axon (axon, or neuritum) — a long process.
Neurons line up into long chains along which the nerve impulse travels. It is important to remember that the signal always moves in one direction: first it is perceived by the dendrite, then it passes to the neuron body, and from there is directed via the axon to the next cell or tissue.
Interneuronal contacts: synapses
For an impulse to pass from one neuron to another, a special contact is needed — the synapse (synapsis). The signal here is transmitted not directly, but with the help of specific chemical substances called neurotransmitters.
Synaptic transmission is characterized by several important features:
- The transmission process is always accompanied by a small temporal delay.
- A single neuron is capable of forming an incredible number of connections — the count can reach thousands of synapses.
- It is the processes of formation and subsequent destruction of synaptic contacts that underlie the subtle mechanisms of our memory.
Classification of neurons and the reflex arc
Depending on the task performed, neurons are divided into three groups:
- Afferent (sensory) — neurones afferentes.
- Interneurons (association) — neurones associativi.
- Efferent (motor) — neurones efferentes.
These cells form reflex arcs. The simplest reflex arc consists of three links: the signal goes from the afferent neuron to the interneuron, and is then transmitted to the efferent neuron. In more complex reflex arcs, there are many interneurons that combine into extensive neuronal complexes.
Nerves and types of nerve fibers
What we call nerves are essentially bundles of neuronal processes. The fibers in their composition are divided into two main types.
Afferent fibers:
- Formed by dendrites of sensory neurons.
- End with specialized receptive structures — receptors.
- The bodies of these neurons lie outside the brain: in spinal ganglia or cranial nerve ganglia.
Efferent fibers:
- Formed by neuronal axons.
- The bodies of these cells are located directly in the gray matter of the spinal cord and brain.
- Such fibers are called motor or centrifugal. They carry the command and end on effectors (glands or muscle tissue).
For the most part, peripheral nerves are mixed, meaning they contain both afferent and efferent fibers.
Division of the nervous system
Anatomically and functionally, the nervous system is conventionally divided into several divisions.
According to the anatomical and functional principle, the following are distinguished:
- Central nervous system (systema nervosum centrale): includes the brain and spinal cord.
- Peripheral nervous system (systema nervosum periphericum): unites cranial and spinal nerves, as well as nerve nodes and plexuses.
Based on functional features, the system is divided into somatic (systema nervosum somaticum) and vegetative (autonomic — systema nervosum autonomicum). In turn, the autonomic nervous system breaks down into two parts: sympathetic and parasympathetic, which have opposite effects on organs.
Briefly: answers to topical questions
How are neurons classified by the number of processes?
According to the number of processes, four main morphological types of neurons are distinguished. The classification includes the following types of nerve cells:
- Unipolar — the first morphological type.
- Bipolar — the second type of neurons.
- Pseudounipolar — considered a variety of bipolar neurons.
- Multipolar — the fourth morphological type.
Based on this classification of neurons, the corresponding pathways of the nervous system are formed.
What types of cells form neuroglia?
Neuroglia is classified based on localization and embryonic origin into glia of the central and peripheral nervous systems. CNS glia includes:
- Microglia — cells of mesenchymal or monocytic origin.
- Macroglia — cells of neural origin, which include astroglia, ependymal glia, and oligodendroglia.
PNS glia is considered a variety of oligodendroglia and includes:
- Mantle gliocytes — satellite cells or ganglionic gliocytes.
- Neurolemmocytes — Schwann cells.
Oligodendrocytes are further subdivided into satellite cells surrounding neuronal cell bodies and nerve fiber oligodendrocytes that form myelin.
Which specific chemical mediators transmit signals across synapses?
Signal transmission in synapses is carried out by primary neurotransmitters and their accompanying co-mediators. In cholinergic synapses, the following are released:
- Acetylcholine — primary mediator.
- Vasoactive intestinal polypeptide — co-mediator.
In adrenergic synapses, the following operate:
- Norepinephrine — primary mediator of nerve endings.
- Epinephrine — released by adrenal medulla cells.
- Dopamine — catecholamine and co-mediator.
- ATP — adenosine triphosphate.
- Gonadotropin — co-mediator.
- Neuropeptide Y — neuropeptide.
- Somatostatin — co-transmitter.
Many interneuronal synapses contain multiple mediators that can be released into the synaptic cleft together or separately.
What are the specific functional differences between the sympathetic and parasympathetic divisions?
The functional differences between the sympathetic and parasympathetic divisions lie in their opposing effects on internal organs based on the principle of the unity and struggle of opposites.
| System | Effect on the GI tract | Effects on the cardiovascular system |
|---|---|---|
| Sympathetic | Inhibits peristalsis and secretion | Increased heart rate, vasoconstriction |
| Parasympathetic | Enhances peristalsis and secretion | Bradycardia, relaxation of vascular wall musculature, blood pressure reduction |
Only the balanced activity of both systems ensures normal functioning of internal organs.
The rule "DTA" will help you remember the direction of an impulse in a neuron: Dendrite → Soma → Axon. The signal always travels strictly in this order.
Class and exam questions
What is the difference between dendrites and an axon?
Dendrites are typically short, highly branched processes that conduct impulses toward the neuron soma. The axon is a long process through which the impulse travels away from the cell body.
What are afferent nerve fibers formed by?
According to the classical description, afferent fibers are formed by the dendrites of sensory neurons and terminate in peripheral receptors.
Where are the cell bodies of efferent (motor) neurons located?
Their cell bodies (perikaryons) are located within the central nervous system — in the gray matter of the brain and spinal cord.
What else to study on this topic
- Morphological classification of neurons (unipolar, bipolar, multipolar)
- Role and types of neuroglial cells in nervous tissue
- Types of chemical mediators in the synaptic cleft
- Differences between the sympathetic and parasympathetic divisions of the autonomic nervous system
- Mechanisms of formation of complex multi-neuronal reflex arcs
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