Cytology, General Histology and Embryology - V. K. Napkhanyuk 2002

Tissues
Nervous tissue
Nerve fibers. Nerve endings. Synapses

Nerve fibers (neurofibra) are the processes of Nerve Cells covered by sheaths.

In different parts of The Nervous system, the sheaths of nerve fibers differ significantly in their Structure; therefore, according to their structural features, nerve fibers are divided into two groups: unmyelinated and myelinated.

Cytology/practical/64.html">Unmyelinated nerve fibers (neurofibra amyelinata) are fibers with a diameter of 1-4 µm that are predominantly part of the Autonomic nervous system.

Unmyelinated nerve fibers consist of a neurolemma, an axon (axis cylinder), and a basement membrane.

The neurolemma (neurolemma) is the sheath of a nerve fiber formed by neurolemmatocytes (Schwann cells) that lie closely together to form strands. They are directly adjacent to the axon and feature oval-shaped nuclei located at certain distances from one another.

The axon (axis cylinder) is a process of a nerve Cell. By invaginating the Sheath of the neurolemmatocytes, the axon penetrates deeply into this strand, forming a sleeve-like structure. The neurolemmatocyte sheaths tightly wrap around the axons and, closing above them, form deep folds. The apposed areas of the neurolemmatocyte sheath at the folds form a double membrane known as the mesaxon, which seemingly suspends the axon.

Nerve fibers of Internal Organs within the neurolemmatocyte strands contain not just one, but several axons (10-20). These unmyelinated fibers are referred to as cable-type fibers.

Externally, the nerve fiber is covered by a basement membrane.

The speed of Nerve Impulse propagation in an unmyelinated nerve fiber is 1-2 m/s.

Myelinated nerve fibers (neurofibra myelinata) are fibers consisting of an axon, a myelin sheath, a neurolemma, and a basement membrane; they have a diameter of 1-20 µm and are located in the central and peripheral nervous systems.

The sheath of a mature nerve fiber consists of two layers: an inner, thicker layer (the myelin layer) and an outer, thin layer comprising the Cytoplasm and nuclei of neurolemmatocytes (the neurolemma).

The myelin layer (stratum myelini) contains Lipids, which is why it stains intensely dark brown when treated with osmium tetroxide. In this case, the entire fiber appears as a uniform cylinder with light lines—myelin clefts (incisurae myelinicae)—located at regular intervals from one another. At certain intervals (ranging from a few micrometers to a few millimeters), segments of the fiber lacking the myelin layer occur, known as nodes of Ranvier (nodus neurofibrae). These nodes correspond to the boundaries between adjacent neurolemmatocytes. The segment of the fiber located between adjacent nodes is called the internodal segment, and its sheath consists of a single glial cell.

During The Development of a myelinated fiber, the axon penetrates the neurolemmatocyte, invaginating its sheath to form a deep fold and thereby creating the mesaxon. As development continues, the mesaxon elongates, wrapping concentrically around the axon to form the myelin layer around it. Under Electron Microscopy, each turn of the mesaxon appears as a light layer about 8-12 nm wide, corresponding to the lipid layers of the two leaflets of the neurolemmatocyte Plasmalemma.

The neurolemma (neurolemma) is a thin, light-appearing sheath when treated with osmium tetroxide, located external to the myelin layer and formed by the cytoplasmic parts of neurolemmatocytes and their nuclei.

The basement membrane (membrana basalis) covers the myelinated fiber externally and is connected to dense bundles of Collagen fibrils that are longitudinally oriented and uninterrupted at the node of Ranvier.

The axon is a process of a nerve cell. It consists of neuroplasm containing longitudinally oriented neurofilaments and neurotubules. The neuroplasm of the axon contains Mitochondria, which are more numerous in the immediate vicinity of the nodes of Ranvier and particularly abundant in the terminal apparatuses of the fiber.

Externally, the axon is covered by a membrane called the axolemma. The axolemma is an extension of the neurocyte cell membrane that ensures the conduction of the nerve impulse. The speed of nerve impulse propagation along a thick myelinated fiber ranges from 5 to 120 m/s.

Slides for study

Slide 35. Myelinated nerve fibers (Fig. 111).

Low magnification. Locate a myelinated fiber at this magnification.

High magnification. Clearly visible is the pale-stained axon, along which lies the dark myelin layer with nodes of Ranvier and myelin clefts appearing as oblique, narrow, light slits. The neurolemma is clearly noticeable in the region of the node of Ranvier. Make a drawing of the slide.

Label the following on the drawing: 1) axon; 2) neurolemma: a — myelin; b — node of Ranvier; c — neurolemma cleft.

Nerve endings (terminationes nervorum) are the terminal apparatuses of nerve fibers.

According to their functional purpose, they are divided into three groups:

— effector nerve endings (effectors);

— sensitive nerve endings (receptors, or afferent endings);

— interneuronal synapses (terminal apparatuses).

Effector nerve endings are terminal apparatuses of axons and effector Cells of the somatic or autonomic nervous system.

Effector nerve endings are of two types: motor and secretory.

Motor nerve endings are terminal apparatuses of axons of motor cells of the somatic or autonomic nervous system, through which nerve impulses are transmitted to the Tissues of effector organs.

Motor endings in striated Muscles are called neuromuscular junctions (terminatio neuromuscularis). These are axon terminals of cells from the motor nuclei of the anterior horns of the Spinal Cord or the motor nuclei of the Brain. These endings consist of terminal Branches of the axon cylinder of a nerve fiber and a specialized region of the Muscle fiber. Upon approaching the muscle fiber, the myelinated nerve fiber loses its myelin sheath and enters the muscle fiber, drawing its plasmalemma inward. Meanwhile, Connective Tissue elements merge into the outer layer of the muscle fiber sheath.

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Fig. 111. Myelinated nerve fibers. Osmium tetroxide staining: objective — x 40, eyepiece — x 15:

1 — axon cylinder; 2 — neurolemma; 3 — myelin; 4 — node of Ranvier; 5 — cleft of Lanterman

The plasmalemmas of the axonal terminal branches and the muscle fiber are separated by a synaptic cleft about 50 nm wide. In addition, the muscle fiber membrane itself forms numerous folds that shape the secondary synaptic clefts of the effector ending.

The plasmalemma of the axon terminal serves as the presynaptic membrane, and the plasmalemma of the muscle fiber serves as the postsynaptic membrane. The space between them, about 50 nm wide, is the synaptic cleft.

Secretory nerve endings are terminal thickenings or dilations of a fiber containing synaptic vesicles filled predominantly with acetylcholine.

Sensitive nerve endings (receptors) are terminal apparatuses of dendrites of sensory neurocytes. Based on their functional characteristics, they are divided into two groups: exteroceptors and interoceptors (Fig. 112).

Fig. 112. Classification of receptors by functional characteristics

Depending on structural features, sensory nerve endings are divided into several groups (Fig. 113).

Free nerve endings (terminatio nervi libera) consist solely of the terminal branches of the axon cylinder.

Non-free nerve endings contain all the components of a nerve fiber, namely, the branches of the axon cylinder and glial cells.

Furthermore, non-free nerve endings may be covered by a connective tissue capsule, in which case they are called encapsulated (corpusculum nervosum capsulatum). Non-free nerve endings lacking a connective tissue capsule are termed unencapsulated (corpusculum nervosum noncapsulatum).

Interneuronal synapses are specialized contacts between nerve cells that transmit impulses in a single direction (Fig. 114).

According to morphological criteria, synapses are divided into three groups:

— axosomatic (terminal branches of the axon of the first neuron terminate on The Cell body of the second);

— axodendritic (terminal branches of the axon of the first neuron form synaptic connections with the dendrite of the second);

— axoaxonic (terminals of the axon of one neuron terminate on the axon of another).

Fig. 113. Classification of receptors according to morphological features

Based on their morphofunctional characteristics, synapses are divided into two groups: chemical and electrical.

Chemical (vesicular) synapses (synapsis vesicularis) are characterized by the presence of presynaptic vesicles in the presynaptic terminal, filled with a neurotransmitter—a substance involved in the transmission of excitation to the postsynaptic terminal.

Fig. 114. Structure of synapses:

A — diagram of synapse cytotopography; B — diagram of synapse structure (a — inhibitory type; b — excitatory type; c — electrical); C — diagram of synaptic vesicle structure (a — cholinergic (clear); b — adrenergic (dense-cored); c — purinergic; d — peptidergic) (after L. D. Markina); 1 — axosomatic synapse; 2 — axodendritic; 3 — axoaxonic; 4 — dendrites; 5 — dendritic spine; 6 — axon; 7 — synaptic vesicles; 8 — presynaptic membrane; 9 — postsynaptic membrane; 10 — synaptic cleft; 11 — postsynaptic density

Such Neurotransmitters are noradrenaline in adrenergic synapses and acetylcholine in cholinergic ones.

A chemical synapse comprises three parts: the presynaptic part, the postsynaptic part, and the synaptic cleft.

The presynaptic part (pars presynaptica) contains presynaptic vesicles (vesiculae presynapticae). In cholinergic synapses, they are clear (vesiculae lucidae), whereas in adrenergic synapses, they have a dense core and are termed dense-cored vesicles (vesiculae densae).

The presynaptic part terminates in the presynaptic membrane (membrana presynaptica), the inner surface of which contains the presynaptic density (densitas presynaptica).

The postsynaptic part (pars postsynaptica) contains the postsynaptic density (densitas postsynaptica) and occasionally submembranous cisterns of the smooth Endoplasmic reticulum. The postsynaptic part includes the postsynaptic membrane (membrana postsynaptica).

The synaptic cleft (fissura synaptica) is approximately 20 nm wide and is located between the pre- and postsynaptic membranes.

Slides for Study

Slide 36. Encapsulated nerve ending. Vater — Pacini corpuscle (Fig. 115).

Low magnification. Locate the encapsulated nerve ending, which consists of inner and outer bulbs, is large and round in shape, and has a spherical capsule. A pale-stained inner bulb is visible in the center of the corpuscle. Make a drawing of the slide.

Label the following on the drawing: 1) terminal portions of the gland; 2) Longitudinal section of the lamellar corpuscle: a) lamellae of the outer bulb; b) inner bulb; 3) Cross section of the lamellar corpuscle; 4) nerve fibers approaching the lamellar corpuscle.

Slide 37. Peripheral nerve. Cross section of the sciatic nerve (Fig. 116).

Low magnification. Myelinated fibers appear as clear circles with a dark central dot. Connective tissue resembles pink strands with the purple nuclei of connective tissue cells. It forms Three types of layers: endoneurium — within a bundle of nerve fibers, perineurium — around a bundle of nerve fibers, and epineurium — surrounding the entire nerve.

Fig. 115. Encapsulated nerve ending. Vater — Pacini corpuscle. Hematoxylin and eosin staining. x 120:

1 — terminal portions of the Pancreas; 2 — longitudinal section of the lamellar corpuscle (a — lamellae of the outer bulb; b — inner bulb); 3 — cross section of the lamellar corpuscle; 4 — nerve fibers approaching the lamellar corpuscle

Make a drawing of the slide. Label the following on the drawing: 1) myelinated nerve fibers; 2) endoneurium; 3) perineurium; 4) epineurium; 5) Blood Vessels; 6) adipocytes.

Electron micrographs (Figs. 117–122) will assist you in studying this topic.

Fig. 116. Peripheral nerve. Cross section of the sciatic nerve. Hematoxylin and eosin staining. x 120:

1 — myelinated nerve fibers; 2 — endoneurium; 3 — perineurium; 4 — epineurium; 5 — blood vessels

Fig. 117. Cross-section of a myelinated nerve fiber from a frog sciatic nerve. Electron micrograph, ×65,000:

1 — lemocyte (Schwann cell) cytoplasm; 2 — lemocyte cell membrane; 3 — mesaxon; 4 — mesaxon turns; 5 — axolemma; 6 — axoplasm; 7 — mitochondrion (after V. L. Borovyagin)

Fig. 118. Node of Ranvier in a myelinated sciatic nerve fiber. Electron micrograph, ×7,000:

1 — axon (axis cylinder); 2 — axolemma; 3 — endoplasmic reticulum in the axoplasm; 4 — mitochondria in the axoplasm; 5 — lemocyte (Schwann cell) mitochondria; 6 — finger-like projections of two lemocytes at their contact sites (from Rodin's atlas)

Fig. 119. STRUCTURE OF THE mesaxon at the neurolemma cleft (Schmidt–Lantermann cleft). Longitudinal section of the lateral part of a myelinated sciatic nerve fiber. Electron micrograph, ×65,000:

1 — axolemma; 2 — lemocyte cytoplasm located between two layers of its cell membrane within the neurolemma cleft; 3 — loosening of the mesaxon in the area of the neurolemma cleft (from Rodin's atlas)

Fig. 120. Unmyelinated nerve fiber of the cable type. Electron micrograph, ×24,000:

1 — lemocyte (Schwann cell) Nucleus; 2 — unmyelinated nerve axons; 3 — basement membrane; 4 — nucleus of a perineurial CONNECTIVE TISSUE CELL; 5 — cytoplasm of a connective tissue cell; 6 — collagen fibrils (after Z. M. Getling)

Fig. 121. Lamellar (Vater–Pacini) corpuscle. Electron micrograph (a and b to scale):

1 — axon; 2 — mitochondria; 3 — cleft of the inner bulb; 4 — processes of lamellar cells of the inner bulb; 5 — pinocytotic vesicles (after V. L. Cherepnov)

Fig. 122. Unmyelinated nerve. Cross-section. Electron micrograph, ×17,000:

1 — axon of an unmyelinated nerve fiber; 2 — lemocyte (Schwann cell) nucleus; 3 — mesaxon; 4 — cross-sections of endoneurial collagen protofibrils (after Elfvin)

Review Questions

1. Nerve fibers: classification and Functions.

2. Structure of unmyelinated nerve fibers. Mechanisms of nerve impulse propagation.

3. Structure of myelinated nerve fibers. Mechanisms of nerve impulse propagation.

4. Morphology of nerve fiber myelination.

5. Stages of nerve fiber regeneration following injury.

6. Nerve endings. Classification.

7. Sensory nerve endings. Functions. Classification and Structure.

8. Structural Features of Vater–Pacini corpuscles, Meissner corpuscles, Merkel Touch menisci, and neuromuscular spindles.

9. Effector nerve endings. Functions and structure of the motor end-plate.

10. Synapses. Functions. Classification and structure.

11. Principles of Structural Organization of chemical and electrical synapses.

12. Structure of a peripheral nerve.

Situational problems

1. One slide shows a terminal branching of an axon accompanied by glial cells, while the other shows branching of the axon alone. Which morphological types do the First and Second nerve endings belong to?

2. Two micropreparations are presented. One of them shows a nerve ending surrounded by a connective tissue capsule. The other lacks a capsule, and the axon branches are accompanied by neurolemmocytes (Schwann cells). Which morphological types do these nerve endings belong to?

3. An electron micrograph shows two nerve processes. One contains small clear vesicles (30–50 nm in diameter), while the other contains both small clear vesicles and larger ones (50–90 nm in diameter) with a central dense core. The formation of which types of synapses do the first and second nerve processes take part in?

Sample examination questions

1. Structure of nerve fibers. Classification. Functional purpose.

2. Nerve endings. Classification. Morphofunctional characteristics.

3. Synapses. Classification. Structure.

4. Structure of a peripheral nerve.