Nervous tissue

1. Neurons.

2. Neuroglia.

Neuron

A neuron consists of a cell body which gives off a variable number of processes. The cell body is also called the soma or perikaryon. The shape of the cell body is dependent on the number of processes arising from it. The most common type of neuron gives off several processes (one axon and several dendrites) and the cell body is, therefore, multipolar. Some neurons have only one axon and one dendrite and are bipolar. Another type of neuron has a single process (axon), this neuron is unipolar. Some neurons have a single process which after a very short course divides into two (axon and dendrite), those neurons are pseudounipolar.

Like a typical cell neuron consists of a mass of cytoplasm surrounded by a cell membrane. The cytoplasm contains a large central nucleus (usually with prominent nucleolus), numerous mitochondria, lysosomes and a Golgi complex. Centrioles are also present.

The cytoplasm of a neuron has some distinctive charactertistics not seen in other cells:

1. The cytoplasm shows the presence of a granular material that stains intensely with basic dyes; this material is the Nissl substance (also called Nissl bodies or granules). These bodies are composed of rough endoplasmic reticulum. The presence of abundant granular endoplasmic reticulum is an indication of the high level of protein synthesis in neurons. The proteins are needed for maintenance and repair, and for production of neurotransmitters and enzymes.

2. The cytoplasm is permeated by a network of fibrils (neurofibrils). The neurofibrils consist of microfilaments and microtubules.

The processes arising from the cell body of a neuron are called neurites. These are of two kinds. Most neurons give off a number of short branching processes called dendrites and one longer process called an axon.

The dendrites are characterized by the fact that they terminate near the cell body. They are irregular in thickness, and Nissl granules extend into them. They bear numerous small spines which are of variable shape.

The axon may extend for a considerable distance away from the cell body. The longest axons may be as much as a metre long. Each axon has a uniform diameter, and is devoid of Nissl substance.

In a dendrite, the nerve impulse travels towards the cell body whereas in an axon the impulse travels away from the cell body.

Myelinated fibre

During its formation each axon comes to be associated with certain cells that provide a sheath for it. The cells providing this sheath for axons lying outside the central nervous system are called Schwann cells. Axons lying within the central nervous system are provided a similar covering by a kind of neuroglial cell called an oligodendrocyte. An axon lying near a Schwann cell invaginates into the cytoplasm of the Schwann cell. In this process the axon comes to be suspended by a fold of the cell membrane of the

Schwann cell: this fold is called the mesaxon. In some situations the mesaxon becomes greatly elongated and comes to be spirally wound around the axon, which is thus surrounded by several layers of cell membrane. These layers of the mesaxon, along with the lipids, form the myelin sheath. Outside the myelin sheath a thin layer of Shwann cell cytoplasm persists to form an additional sheath which is called the neurilemma.

Axons having a myelin sheath are called myelinated axons. The presence of a myelin sheath increases the velocity of conduction It also reduces the energy expended in the process of conduction.

An axon is related to a large number of Schwann cells over its length. Each Schwann cell provides the myelin sheath for a short segment of the axon. At the junction of any two such segments there is a short gap in the myelin sheath. These gaps are called the nodes of Ranvier.

Nonmyelinated fibre

There are some axons that are devoid of myelin sheaths. These unmyelinated axons invaginate into the cytoplasm of Schwann cells, but the mesaxon does not spiral around them. Several such axons may invaginate into the cytoplasm of a single Schwann cell.

Neurons vary considerably in the size and shape of their cell bodies and in the length an manner of branching of their processes. The cell body varies in diameter from about five micrometers, in the smallest meurons, to as much as one hundred and twenty micrometers in the largest ones. The shape of the cell body is dependent on the number of processes arising from it. The most common type of neuron gives off several processes and the cell body is,therefore, multipolar. Some neurons have only one axon and one dendrite and are bipolar.

Another type of neuron has a single process (which is highly convoluted). After a very short course this process divides into two. One of the divisions represents the axon; the other is functionally a dendrite, but its structure is indistinguishable from that of an axon. This neuron is pseudounipolar.

Neuroglia

In addition to neurons, the nervous system contains several types of supporting cells. These are:

1) Neuroglial cells, found in the parenchyma of the brain and spinal cord.

2) Ependymal cells, lining the ventricular system.

3) Schwann cells, forming sheaths for axons of peripheral nerves. They are also called lemmocytes or peripheral glia.

4) Capsular cells (also called satellite cells or capsular gliocytes) that surround neurons in peripheral ganglia.

5) Various types of supporting cells found in relation to motor and sensory terminals of nerve fibres.

Neuroglial cells may be divided into two major categories: macroglia (or large glial cells) and microglia (or small glial cells)

1. Macroglia (or large glial cells).

a. Astrocytes, which may be subdivided into fibrous and protoplasmic astrocytes. These are small star-shaped cells that give off a number of processes. The processes are often flattened into leaf-like laminae that may partly surround neurons and separate them from other neurons. Astrocytes play a role in maintenance of the blood brain barrier.

b. Oligodendrocytes. These cells have rounded or pear shaped bodies with relatively few processes. These cells provide myelin sheaths to nerve fibres that lie within the brain and spinal cord.

2. Microglia (or small glial cells). These are the smallest neuroglial cells. The cell body is flattened. The processes are short. These cells are frequently seen in relation to capillaries. They become active after damage of nervous tissue by trauma or disease and act as phagocytes.

Questions:

1. What is nervous tissue made up of?

2. Types of neurons.

3. Structural features of neuron.

4. Types of nerve fibres.

5. Structural features of the myelinated nerve fibre.

6. Structural features of the nonmyelinated nerve fibre.

7. Classification of the neuroglia.

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THE EYE

The eyeball consists of three layers:

1. sclera;

2. choroid;

3. retina.

Sclera

The sclera is composed of dense fibrous tissue and maintains the shape and size of the eye. The flat bundles of collagen fibres of the sclera lie mainly parallel to the surface, but they intersect in all directions, with fine networks of elastic fibres between the bundles, some ground substance, and a few flattened, branching fibroblasts.

Choroid

Choroid is a spongy, brown membrane that contains extensive venous plexuses.

Retina

The retina is the innermost layer of the eyeball. It in its turn consists of ten layers. In cross section, from external to internal, the layers of the retina are as follows:

1. Pigment epithelium

2. Layer of rods and cones

3. External limiting membrane

4. Outer nuclear layer

5. Outer plexiform layer

6. Inner nuclear layer

7. Inner plexiform layer

8. Ganglion cell layer

9. Optic nerve fibre layer

10. Internal limiting membrane

CORNEA

The cornea is made up of five layers.

1. The outermost layer is of non-keratinized stratified squamous epithelium (corneal epithelium). The cells of the deepest layer of the epithelium are columnar; in the middle layers they are polygonal; and in the superficial layers they are flattened.

2. The corneal epithelium rests on the anterior limiting lamina (also called Bowman’s membrane) which is made up of fine collagen fibrils embedded in matrix.

3. Most of the thickness of the cornea is formed by the substantia propria (or corneal stroma). The substantia propria is made up of collagen fibres embedded in a ground substance containing sulphated glycosaminoglycans.

4. Deep to the substantia propria there is a thin homogeneous layer called the posterior limiting lamina (or Descemet’s membrane). It is true basement membrane.

5. The posterior surface of the cornea is lined by a single layer of flattened cells that constitute the endothelium of the anterior chamber. This layer is in contact with the aqueous humour of the anterior chamber.

The cornea has no blood vessels or lymphatics. It receives nutrition from vessels around its periphery. The cornea has a rich nerve supply.

Questions:

1. What layers does the eyeball consist of?

2. Structural features of the sclera.

3. Structural features of the choroid.

4. Structural features of the retina.

5. Structural features of the cornea.

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THE EAR

The ear is made up three main parts called the external ear, the middle ear, and the internal ear.

THE EXTERNAL EAR

The auricle. The auricle consists of a thin plate of elastic cortilage covered on both sides by true skin. The skin is closely adherent to the cartilage. Hair follicles, sebaceous glands, and sweat glands are present in the skin.

The external acoustic meatus. The wall of the external acoustic meatus is made up partly of elastic cartilage (in its outer part) and partly of bone (in its inner part). The meatus is lined by skin. The skin lining the bony part is thin and is firmly adherent to the underlying bone. In the cartilaginous part the skin is thick and contains hair, sebaceous glands, and ceruminous glands. The ceruminous glands secrete the wax of the ear. They are modified sweat glands lined by a columnar, cuboidal or squamous epithelium.

THE MIDDLE EAR

The tympanic membrane. The tympanic membrane has three layers. The middle layer is made up of fibrous tissue, which is lined on the outside by skin (continuous with that of the external acoustic meatus), and on the inside by mucous membrane of the tympanic cavity.

The fibrous layer contains collagen fibres and some elastic fibres. The fibres are arranged in two layers. In the outer layer they are placed radially, while in the inner layer they run circularly.

The mucous membrane is lined by an epithelium which may be cuboidal or squamous.

The tympanic cavity. The walls of the tympanic cavity are formed by bone which is lined by mucous membrane. The mucous membrane also covers the ossicles (malleus, incus, stapes). The lining epithelium varies from region to region. Typically it is cuboidal or squamous. At places it may be ciliated columnar. The ossicles of the middle ear consist of compact bone, but do not have marrow cavities.

The auditory tube. The wall of the auditory tube is partly bony (lateral part) and partly cartilaginous (medial part, nearer the nasopharynx). The bone or cartilage is covered by mucous membrane which is lined by ciliated columnar epithelium. Near the pharyngeal end of the tube the epithelium becomes pseudostratified columnar. Goblet cells and tubulo-alveolar mucous glands are also present.

INTERNAL EAR

The internal ear is in the form of a complex system of cavities lying within the petrous temporal bone. It has a central part called the vestibule. Continuous with the front of the vestibule there is a spiral shaped cavity called the cochlea. Posteriorly, the vestibule is continuous with three semicircular canals.

Because of the complex shape of these intercommunicating cavities the internal ear is also called the labyrinth. The space bounded by bone is bony labyrinth. Its wall is made up of bone that is more dense than the surrounding bone. Its inner surface is lined by periosteum. Lying within the bony labyrinth there is a system of ducts which constitute the membranous labyrinth. The space within the membranous labyrinth is filled by a fluid called the endolymph. The space between the membranous labyrinth and the bony labyrinth is filled by another fluid called the perilymp.

The bony labyrinth consists of a central part called the vestibule. The vestibule is continuous anteriorly with the cochlea; and posteriorly with three semicircular canals. .

The cochlear part of the bony labyrinth is divisible into two parts. One part, the scala vestibuli opens into the vestibule; while the second part called the scala tympani opens into the middle ear through an aperture called the fenestra cochlea.

Within each semicircular canal the membranous labyrinth is represented by a semicircular duct. The part of the membranous labyrinth (bounded by vestibular membrane and basilar membrane) present in the cochlea is called the duct of the cochlea. The part of membranous labyrinth that lies within the vestibule is in the form of two distinct membranous sacs called the saccule and the utricle.

The bony cochlea. The cochlea has a striking resemblance to a snail shell. It is basically a tube that is coiled on itself for two and three-fourth turns. The “turns” rest on a solid core of bone called the modiolus.

Ampullary crest. One ampullary crest is present in the ampulated end of each of the three semicircular duct. Each crest is an elongated ridge projecting into the ampulla, and reaching almost up to the opposite wall of the ampulla. A long axis of the crest lies at right angles to that of the semicircular duct. The crest is lined by a columnar epithelium in which two kinds of cells are present. These are hair cells which are specialized mechanoreceptors, and supporting (or sustentacular) cells.

Spiral organ of Corti. The spiral organ of Corti is so called because (like other structures in the cochlea) it extends in a spiral manner through the turns of the cochlea. In sections it is seen to be placed on the basilar membrane and to be made up of epithelial cells that are arranged in a complicated manner. The cells are divisible into the true receptor cells or hair cells, and supporting elements which are given different names depending on their location. The cells of the spiral organ are covered from above by a gelatinous mass called the membrana tectoria.

Questions:

1. Three main parts of the ear.

2. The main parts of the internal ear.

3. The main parts of the bony labyrinth

4. Structural features of the bony cochlea.

5. Structural features of the ampullary crest.

6. Structural features of the spiral organ of Corti.

Cytotrophoblast

Syncytiotrophoblast