Conduction velocity and types of nerve fibers
The velocity at which a nerve impulse (excitation) propagates along neuron processes strictly depends on the anatomical structure and diameter of the fiber itself. Based on these parameters, three main categories of conductors are distinguished:
- Type A (thick myelinated fibers): The fastest pathways of the nervous system. Their diameter ranges from 3 to 20 µm, and the excitation transmission velocity reaches a record 120 m/s.
- Type B (thin myelinated fibers): Structures with more modest parameters. With a diameter of about 3 µm, they are capable of conducting a nerve signal at velocities up to 15 m/s.
- Type C (unmyelinated fibers): The slowest conductors in the body. Lacking a dense myelin insulation, they transmit impulses at a velocity not exceeding 2 m/s.
Architecture of the myelin fiber in the CNS
In the brain and spinal cord, oligodendrocytes—specialized macroglial cells—are responsible for creating reliable electrical insulation. Their unique feature is that a single such element can, through its processes, myelinate several neighboring axons simultaneously (unlike the peripheral nervous system, where this task is performed individually by Schwann cells).
Structurally, the myelin fiber of the central nervous system consists of the following elements:
- Axon — the central axis cylinder of the nerve cell along which the electrical signal directly travels.
- Myelin sheath — powerful electrical insulation representing a dense spiral of multiple turns of the oligodendrocyte plasma membrane around the axon.
- Node of Ranvier — a narrow unmyelinated segment of the axon located strictly at the boundary of myelinated territories.
- Inner mesaxon — the starting point of wrapping. It is a doubled membrane fold (plasma membrane duplicity) that lies directly adjacent to the axon.
- Outer mesaxon — the final plasma membrane duplicity located on the outer surface of the myelin sheath (the site where wrapping ends).
- Cytoplasmic pockets — small isolated areas (islands) of the creator cell's cytoplasm preserved between the insulation turns within the sheath.
Ultrastructure of the synapse
Electron microscopy data permanently disproved the old theory of syncytium (a continuous network of neurons). It was reliably proven that each neuron is enclosed within its own closed plasma membrane. Communication between cells is carried out exclusively via synapses using specific chemical messenger substances—neurotransmitters.
Morphologically, a classical synapse is divided into three key zones:
- Presynaptic part: the terminal expansion of the axon of the transmitting neuron. This is a kind of warehouse where a supply of neurotransmitter is stored in special synaptic vesicles. Mitochondria are also present here in large quantities, providing energy-demanding processes.
- Synaptic cleft: the physical space completely separating the contacting cells. Neurotransmitter molecules must cross this distance for successful impulse transfer.
- Postsynaptic part: the receptive membrane of the target cell that reads the incoming chemical information.
Classification of synapses and transmission initiation
On electron micrographs in the synaptic region, special osmiophilic thickenings on the membranes are clearly revealed. Based on their prominence, synapses are divided into two classes (this hypothesis regarding the relationship between morphological structure and function was fully confirmed by immunocytochemical studies using antibodies against neurotransmitters and their biosynthetic enzymes):
- Asymmetric (Gray's type I synapses): the receiving (postsynaptic) membrane is predominantly thickened. Functionally, these are excitatory contacts.
- Symmetric (Gray's type II synapses): both contacting membranes are thickened approximately equally. They perform an inhibitory function.
The initial stage of synaptic transmission represents a strict sequence of physiological events:
- An excitatory impulse (action potential) arrives at the synaptic terminal of the axon.
- Rapid depolarization of the presynaptic membrane occurs.
- In response to the change in electrical charge in the membrane, voltage-gated calcium channels open.
- Calcium ions rush en masse into the terminal swelling.
- Calcium interacts with various intracellular proteins, ultimately leading to the fusion of synaptic vesicles with the presynaptic membrane and preparation for neurotransmitter release.
Briefly: answers to topical questions
Which cells make up macroglia and microglia?
Macroglia and microglia comprise various types of glial cells of the central and peripheral nervous system.
- Macroglia — includes astroglia (stellate cells), ependymal glia, and oligodendroglia. Varieties of oligodendroglia in the peripheral nervous system also include mantle gliocytes (satellite cells, ganglion gliocytes) and neurolemmocytes (Schwann cells).
- Microglia — represented by glial macrophages that develop from promonocytes. According to their functional state, they are subdivided into amoeboid, resting (branched), and reactive microglia.
What functions do astrocytes perform in the central nervous system?
Astrocytes perform numerous functions in the central nervous system, which are realized through their processes.
- Supporting — form a supporting network throughout the entire volume of the brain, and the processes of fibrous astrocytes form a framework (septa) in the white matter.
- Barrier — participate in the formation of the blood-brain barrier by forming glial limiting membranes (astrocytic end-feet) on the surface of blood vessels.
- Transport and trophic — ensure the transport of substances between capillaries and neurons.
- Regulatory — secrete neurocyte growth factors, regulating brain development and regeneration.
- Metabolic — participate in neurotransmitter metabolism.
What are the structure and functions of ependymal glia?
Ependymal glia is a tissue lining the central canal of the spinal cord and the brain ventricles.
Structure of the ependyma:
- Mostly a single-layer sheet of cylindrical cells (ependymocytes), though it may be multi-layered in certain areas.
- Differs from epithelium by the absence of a basement membrane and keratin filaments.
- The apical surface of the cells bears microvilli and motile cilia (kinocilia), while long processes may extend from the basal surface (such cells are called tanycytes).
- Cells are connected by interdigitations and gap junctions (nexuses).
Functions of the ependyma:
- Production and transport of CSF — secretion of cerebrospinal fluid and facilitation of its flow via cilia.
- Barrier — participation in the formation of the blood-cerebrospinal fluid barrier.
Asymmetric synapse — Active (excitatory, type I). Symmetric — Quiet (inhibitory, type II).
Class and exam questions
How does myelination differ in the CNS and PNS?
In the central nervous system, myelin is formed by oligodendrocyte processes, with a single cell capable of enveloping several axons at once. In the peripheral system, Schwann cells perform this task.
What is a node of Ranvier?
This is a narrow segment of the axon devoid of a myelin sheath. It is located at the boundary of myelinated territories and plays a crucial role in the conduction of the nerve impulse.
How do calcium ions participate in synaptic transmission?
Upon membrane depolarization, calcium channels open. Calcium ions enter the axon terminal swelling and bind to proteins, causing synaptic vesicles to fuse with the membrane and release the neurotransmitter.
What else to study on this topic
- The role of specific neurotransmitters in the synaptic cleft
- Immunocytochemical studies of biosynthesis enzymes
- Mechanisms of presynaptic membrane depolarization
- Subsequent stages of synaptic transmission following vesicle fusion
- Differences between Schwann cells and CNS macroglia
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