HUMAN ANATOMY – VOLUME 1

Fig. 23. Sequenced stages (A, B, C) of forming of myelinated nerve fiber (acc. to V.G.Eliseev and others).

1 — contact of axolemma and Schwann (Neurolemnal) sheath; 2 — intercellular gap (fissure); 3 — axolemma and Schwann (neurolemnal) sheath; 4 — cytoplasma of neurolemmocyte; 5 — mesaxon.

cells, is continuous and crosses over the nodes of Ranvier without interrupting. These nodes are areas of penetration for sodium ions during nerve impulse depolarization /electric current/. Depolarization in nodes of Ranvier allows for fast conduction of nerve impulses along myelinated nerve fibers. Impulses are conducted along these fibers in saltatory jumps from one node of Ranvier to the next. In unmyelinated nerve fibers depolarization takes place in all parts, which is why impulses are conducted slower. Thus, the speed of conduction along unmyelinated fibers is 1–2 m/s and along myelinated ones is 5–120 m/s.

Classification of neurons. Depending on the number of processes there are u n i p o l a r (have one process) and b i p o l a r (two processes) neurons (Fig.25). M u l t i p o l a r neurons have many processes. There are also pseu- do-unipolar neurons, which pertain to bipolar neurons. These neurons have

Fig. 24. Structure of myelinated nerve fiber and neurofiber node of Ranvier (acc. to V.G. Eliseev and others).

1 — mesaxon; 2 — axial cylinder; 3 — myelin incisure; 4 — neurofiber node; 5 — cytoplasma of neurolemmocyte; 6 — nucleus of neurolemmocyte; 7 — neurolemma; 8 — endoneurium.

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rections and a long axon.

The large pyramidal neurons of the cerebral cortex have a triangular shape and a large number of short dendrites. Their axon extends from the base of the pyramidal cell.

Both dendrites and axons end with nerve endings. Dendrite nerve endings are sensitive, while the axon ending is effectorial.

According to their function, nerve cells are divided into sensory, effectorial and associative cells.

Sensory (receptor) neurons are able to perceive various stimulations with their endings. Impulses generated in the nerve endings (receptors) of these cells are conducted to the cerebellum. Because of this sensory neurons are also called afferent. Effectorial nerve cells are also called efferent. Associated (interneurons, relay neurons) conduct impulses from afferent to efferent neurons. There are also neurons, which can produce secretion, these are called neurosecretory. Their secretion (neurosecretion) is excreted in the form of granules and transported by blood. Neurosecretion is a way of interaction of the nervous and cardiovascular (humoral) systems.

Depending on their location receptors are divided into exteroceptors, interoceptors and proprioceptors. Exteroceptors perceive influences from the environment. They are situated in the external teguments of the body— the skin, mucosae and sensory organs. Interoceptors sense stimuli, which are produced by changes in chemical composition of the internal environment (chemoreceptors) and pressure inside tissues and organs (baroreceptors, mechanoreceptors). Proprioceptors perceive stimuli in muscles, tendons, ligaments, fasciae and joint capsules.

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Fig. 26. Structure of various receptors (acc. to A.Ham and D.Kormack).

A — free nerve ending: 1 — nerve ending; 2 — border between dermis and epidermis. B — free (Merckel’s) ending: 1 — modified epidermal cell (Merckel’s cell); 2 — basal membrane; 3 — end disk of afferent fiber; 4 — myeline; 5 — neurolemmocyte. C — Pacinian corpuscle: 1 — subcapsulr space; 2 — capsule; 3 — myelin; 4 — neurolemmocyte; 5 — external bulbus; 6 — basal membrane; 7 — internal bulbus; 8 — terminal process of afferent fiber. D — Meissner’s lamillar corpuscle: 1 — flatterned neurolemmocytes; 2 — capsule; 3 — basal membrane; 4 — spiral terminals of afferent fiber; 5 — neurolemmocytes; 6 — myelin. E — Ruffini’s tactile corpuscle: 1 — bundles of collagen fibers within body’s nucleus; 2 — terminal branches of afferent fiber; 3 — capsule. F — terminal

variosity (Krause’s corpuscle): 1 — terminal branches of afferent fiber; 2 — capsule.

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Based on their function there are thermoreceptors, which perceive changes in temperature, and mechanoreceptors, which sense mechanical influences (touch, pressure on the skin). There are also nociceptors, which are able to perceive pain stimuli.

Nerve ending can also be divided into free (naked) nerve endings and encapsulated ending, which are covered by a tunic (capsule) made up of neuroglia cells or connective tissue fibers (Fig. 26).

F r e e n e r v e e n d i n g s are found in the skin, where they branch out between epitheliocytes. Such endings are also found in mucosae and cornea of the eye. Terminal free nerve endings can perceive pain, heat and coldness.

E n c a p s u l a t e d n e r v e e n d i n g s are covered by a connective tissue capsule. This group of endings includes tactile (Meissner’s) corpuscles, lamellated (Pacinian) corpuscles, bulbous (Golgi-Mazzoni) corpuscles and corpuscles of Ruffini. All these nerve endings are mechanoreceptors. This group also includes bulbous corpuscles of Krause, which are thought to be thermoreceptors.

L a m e l l a t e d (Va t e r - P a c i n i a n) c o r p u s c l e s are the largest among encapsulated endings. They reach 3 – 4 mm in length and 2 mm in thickness. They are found in connective tissue of internal organs, dermis and subcutaneous tissue. On the outside the corpuscle is covered with a connective tissue capsule, which has a lamellated structure and is

rich in capillaries.

Ta c t i l e (M e i s s n e r ’s) c o r p u s c l e s are small (50–160 m long and about 60 m wide). They are especially concentrated in the papillary dermal layer of the skin on fingers, lips, edges of eyelids and external genital organs. Their capsule is formed by several layers of epithelioid cells. Meissner’s corpuscles are mechanoreceptors, which perceive touch and pressure applied to the skin.

C o r p u s c l e s o f R u f f i n i are spindle-shaped receptors, found in the skin on fingers, hands and feet, in joint capsules and walls of blood vessels. The corpuscles are covered with thin capsules, which are formed by perineurial cells. Corpuscles of Ruffini are mechanoreceptors; they are thought to perceive heat and are proprioceptors.

K r a u s e ’ s c o r p u s c l e s (bulbous corpuscles) are spherically shaped and are found in skin, eye conjunctiva and mucosa of the mouth. The corpuscles have a thick connective tissue capsule. Krause’s corpuscles are cold receptors and, possibly, mechanoreceptors.

The papillary dermis in the skin of glans penis and clitoris contains many of the so-called genital corpuscles, which are similar to bulbous corpuscles. These are mechanoreceptors.

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Proprioceptors perceive muscle contractions, stretching of tendons, joint capsules and muscle strength, necessary for movement or keeping body parts in a certain position. This group includes neuromuscular spindles and tendon organs.

Tendon organs are located in places of transition between muscles and tendons. They take form of bundles of tendon (collagen) fibers, are connected with muscle fibers

Fig. 27. Neuro-muscular terminal. and surrounded by a connective tissue capsule.

Neuromuscular spindles are large, 3–5 mm long and 0.5 mm thick, and are covered with a connective tissue capsule. Such a capsule contains up to 10–12 short thin striated muscle fibers of various structures.

Muscles also contain effectorial neuromuscular endings, which are found on each muscle fiber (Fig. 27). These are thickened endings covered by Schwann cells and their basement membrane, which transcends into the basement membrane of the muscle fibers. The axolemma of each nerve ending touches the sarcolemma of one muscle fiber, as if pressing it inwards. A gap between the nerve ending and the muscle fiber is filled with an amorphous substance, which, as in a synapse, contains acetylcholinesterase.

Effectoral nerve endings of smooth muscles form dilations, which contain mitochondria and synaptic vesicles, containing noradrenaline and dopamine. The majority of nerve endings are in contact with the basement membrane of myocytes, while some endings perforate it. In contacts between nerve fibers and smooth muscle cells the axolemme is separated from the cytolemme by a 10 nm gap.

Neurons perceive, conduct and transmit electrical signals (nerve impulses) to other nerve cells or working organs (muscles, glands, etc.).

Nerve impulses are transmitted between neurons through specialized intercellular contacts called synapses. Synapses can be divided into axosomatic, axodendritic and axoaxonic. Axosomatic synapses are formed between the nerve ending of one neuron and the cell body of another. Axodendritic synapses are contacts between axons of one cell and dendrites of an-

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other. Axoaxonic contacts are synapses between axons of two neighboring cells. Formation of these contacts creates chains of neurons. Nerve impulses are transmitted along neuron chains with the aid of bioactive substances called neurotransmitters. These neurotransmitters include noradrenalin, acetycholine, monoamines (adrenalin, serotonin, etc.), as well as some neuropeptides (enkephalins, neurotensin, somatostatin, etc.).

Each synapse has presynaptic and postsynaptic parts, which are separated from each other by a 20–30 nm gap. When a nerve impulse reaches the presynaptic part it causes Ca2+ channels to open. The increase in Ca2+ concentration results in the release of neurotransmitters, which are stored in synaptic vesicles, into the synapse. The neurotransmitter binds to receptors on the postsynaptic membrane. This causes a postsynaptic potential to generate in the form of a nerve impulse. The magnitude of the postsynaptic potential is directly proportional to the quantity of excreted neurotransmitter.

Also part of the nervous

system is neuroglia cells, which

Fig. 28. Neuroglia (acc. to V.G.Eliseev carry out supporting, trophic, and others).

defensive, isolating and secretory functions. Neuroglia is divided into glia of the nervous sys-

tem (ependymocytes, astrocytes, oligodendrocytes and microglia) and glia of the peripheral nervous system (neurolemmocytes) (Fig. 28).

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E p e n d y m o c y t e s line on the inside the ventricles of the brain and the cerebrospinal canal. These cells have a cuboidal or prismatic shape; they lie in a single layer and have microvilli. Their basal surface is in contact with blood capillaries. Ependymocytes participate in the formation of cerebrospinal fluid and carry out functions of support and demarcation.

A s t r o c y t e s are the main connective element of the central nervous system. There are protoplasmic and fibrous astrocytes.

Fibrous astrocytes are multiprocessed cells, which are predominant in the white matter of the central nervous system. Their processes are located between nerve fibers. Some of the processes reach blood capillaries. Protoplasmic astrocytes have a stellar shape and long cytoplasmic protrusions stretching in all directions from the cell body. These protrusions provide support for neuron processes and create a reticulum in which neuron cell bodies lie. Astrocyte processes, which reach the brain surface, connect with each other to form a continuous boundary membrane. This glial membrane creates specific microsurroundings for neurons.

O l i g o d e n d r o c y t e s are small (6–8 mm) ovoid cell with processes and a large nucleus. They are situated around neurons and their processes. Oligocytes, which form sheaths of neuron processes in the peripheral nervous system, are called neurolemmocytes, or Schwann cells.

M i c r o g l i a (O r t e g ’s c e l l s) is made up of small cells of an undefined shape. There are numerous processes of various sizes and shrublike appearance stretching from cell bodies. Microglia cells are mobile and have a phagocytic ability.

Questions for revision and examination

1.Describe the structure and functions of a nerve cell and nerve fibers /myelinated and unmyelinated/.

2.Give the classification of nerve endings, describe their morphological and functional characteristics.

3.What are synapses, what features are used to classify them?

4.Describe the structure of a synapse and the mechanism of nerve impulse transmission though it.

5.Give the classification of neuroglia, describe its structure and the functions of its different components.

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MAIN STAGES OF DEVELOPMENT IN ONTOGENESIS

Each human being possesses individual traits of outer appearance and internal organ structure, which are determined by hereditary factors and influences of the environment.

Individual development of a human organism lasts throughout all periods of life — from conception until death. Ontogenesis of a person is divided into the intrauterine (prenatal) and the «after birth» (postnatal) periods. During the intrauterine period, which lasts from conception until birth, the embryo develops inside the mothers womb. In the course of the first 8 weeks of embryogenesis the main processes of organ and body part formation take place. This period is called the embryonic period, and the future human organism — the embryo. Starting on the 9th week, when main external features have begun to designate, the organism enters the fetal period of development and begins to be called a fetus.

After conception, which usually takes place in a uterine tube, fused sex cells (ovum and spermatozoid) form a unicellular ‘germ’ called a zygote, which possesses all the attributes of both sex cells. From this moment begins the development of a new /daughter/ organism.

During its first week of development the zygote divides into daughter cells (cleavage stage). In the first 3–4 days the zygote is simultaneously dividing and moving along the uterine tube towards the uterus. As a result of its divisions the zygote transforms into a multicellular vesicle (blastula) with a cavity inside (Fig.29). The walls of this vesicle are made up partially of larger and partially of smaller cells. The smaller cells form the outside layer of the blastula wall and is called the trophoblast. Later on the trophoblast cells form the outer layer of the extraembryonic membranes. The larger cells (blastomeres) form a cell mass called an embryoblast, which is situated to the inside of the trophoblast. This inner cells mass /embryoblast/ later develops into the embryo. Between the trophoblast and the embryoblast accumulates a small amount of fluid.

By the end of the 1st week of development (6–7th day of pregnancy) the embryo implants into the mucosa of the uterus. For this to happen, cells of the trophoblast secrete an enzyme, which loosens the surface layer of the mucosa, which at this time is already prepared for the implantation. At the moment of ovulation (excretion of the ovum from the ovary) the mucosa inside the uterus is thickened (up to 8 mm) and has uterine

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Fig. 29. Cleavage of a zygote and formation of germinal layers (acc. to R. Kristic, modified, 1984).

A — insemination: 1 — spermatozoid; 2-oocyte; Â,C – cleavage of zygote; D — moruloblast: 1 — embryoblast; 2— trophoblast; E — blastocyst: 1 — embryoblast; 2 — trophoblast; 3 — amniotic cavity. F — blastocyst: 1 — embryoblast; 2 — amniotic cavity; 3 — blastocele; 4 — embryonic entoderm; 5 — amniotic epithelium. G, K, L: 1 — ectoderm; 2 — entoderm; 3 — mesoderm.

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glands and vessels proliferating in it. The trophoblast forms numerous protrusions called villi, which increase its contact surface area with the mucosa. The trophoblast transforms into an extraembryonic nourishing membrane called the chorion. At first there are chorionic villi all around the blastula, but later the villi remain only on the side in contact with the uterine wall. Out of the chorion and the adjoining mucosa a new organ called the placenta («baby’s place») is formed. The placenta is the organ, which connects the mother’s organism with the embryo, providing it with nourishment.

The second week of embryonic development is a stage when cells of the embryoblast divide into two layers (discs), which form into two vesicles (Fig. 30).

The outside cell layer, which adjoins the trophoblast, forms the ectoblastic vesicle, which becomes filled with amniotic fluid. The inside layer

Fig. 30. Positioning of a human embryo and embryonic tunics during different stages of development.

A — embryo weeks 2—3; B — embryo weeks 4: 1 — amniotic cavity; 2 — embryo body; 3 — yolk sac; 4 — trophoblast; C — embryo weeks 6; D — embryo months 4—5: 1 — body; 2 — amnion; 3 — yolk sac; 4 — chorion; 5 — umbilical cord.

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