NERVOUS TISSUE

INTRODUCTION

The nervous tissue is composed of interconnecting network of specialised cells called neurons (nerve cells) supported by neuroglial cells. There are about 10 million neurons in human beings. The function of neurons is to receive stimuli and conduct them to a central site, the central nervous system (CNS), where they are analysed and integrated to produce a desired response in the effector organs.

ANATOMICAL CLASSIFICATION OF NERVOUS SYSTEM

Nervous system

Central nervous system (CNS)Peripheral nervous system (PNS)(nerves and ganglia outside CNS)

Brain Spinal cord Cerebrospinalnerves

Autonomic nervoussystem

Sympatheticnervous system

Parasympatheticnervous system

Classifi cation of nervous system.

CLASSIFICATION OF NEURONS

A. Morphological (based on the number of processes)

1. Unipolar neuron—has a single process (rare), e.g. mesencephalic nucleus of V cranial nerve.

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ABC of Human Histology

7

Flowchart 7.1

2. Bipolar neuron—has two processes (an axon and a dendrite; Fig. 7.1), e.g. spiral ganglion, bipolar cells in retina, etc.3. Multipolar neuron—has many processes (an axon and many dendrites; Fig. 7.2), e.g. autonomic ganglia motor neurons, etc.4. Pseudo-unipolar neuron—has a single process that divides into an axon (central process) and a dendrite (peripheral process; Fig. 7.3), e.g. cranial and spinal ganglia (sensory neurons).

1

Nervous system can be classified into two categories, central nervous system and peripheral nervous system (Flowchart 7.1).

Dendrite

Cell body

Axon

Dendrite

Cell body

Axon

Bipolar neuron. Multipolar neuron.

Peripheral processCentral process

Cell body

Nucleus

From receptorTo CNS

Pseudo-unipolar neuron.

B. Functional (based on the function performed)

1. Sensory neuron—receives stimuli from receptors and conducts impulses to CNS, e.g. sensory ganglia.

2. Motor neuron—conducts impulses from CNS to effector organs (muscles), e.g. ventral horn cells.

3. Interneuron—connects sensory and motor neurons and completes the functional circuit.

STRUCTURE OF A NEURON (MULTIPOLAR)

Cell body/Soma/Perikaryon (5–150 �m):

The cell bodies of all neurons are situated in the grey matter of the CNS and in the ganglia of PNS. �

1. Nucleus—is large, euchromatic, spherical and centrally located.

2. Nissl bodies or Nissl substance—are composed of large aggregations of rough endoplasmic reticulum

– are observed as basophilic clumps by light microscopy

– extend into dendrites but not into axon and axon hillock

– disintegrate as a result of injury to axon (chromatolysis).

3. Golgi complex—are found near the nucleus.

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Fig. 7.2Fig. 7.1

Fig. 7.3

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� The cell body of a neuron contains the nucleus and the following cytoplasmic organelles and inclusions (Figs 7.4 and 7.5):

Cell body (soma)

Dendrite

Nissl granules

Nucleus

Axon hillock

AxonMyelin sheath

Schwann cellnucleus

Node of Ranvier

Axon terminals

Boutonterminal

Structure of a multipolar neuron.

DendriteSpine (site of synaptic contact)

Golgi complexPigment granuleRough endoplasmicreticulum (Nissl body)

Mitochondria

Nucleolus

Nucleus

NeurofilamentsAxon hillock

Microtubule

Axon

Ultrastructure of a neuron.

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Fig. 7.4

Fig. 7.5

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4. Mitochondria—are numerous and rod shaped.

5. Neurofi laments (10 nm dia) and microtubules (25 nm dia)—form neuronal cytoskeleton provid ing structural support and intracellular transport.

6. Melanin pigments—dark brown granules.

7. Lipofuscin pigments—residual bodies not digested by lysosomes (increase with age).

Dendrites

Are highly branched, tapering processes of a neuron. So their diameter is not uniform. �

Are covered by thorny spines (gemmules) which are sites of synaptic contact. �

Receive stimuli from sensory cells and other neurons and transmit them towards the soma. So they can be regarded as �

major sites of information input into neuron.

Axon

Single, long, cylindrical process of a neuron. So its diameter is uniform. �

Does not branch profusely; but may give rise to collaterals. �

Arises from a cone-shaped portion of the cell body called � axon hillock, which is devoid of Nissl bodies, but contains bundles of microtubules.

The cytoplasm of the axon is called � axoplasm and the plasma membrane is called axolemma.

Terminates by dividing into many small branches, � axon terminals, ending in small swellings—terminal boutons.

Conducts impulses away from the cell body to the axon terminals from which impulses are transmitted to another neuron �

or another target cell.

Axons are commonly referred to as � nerve fi bres.

Are often surrounded by � myelin sheath, which is derived either from Schwann cells (PNS) or oligodendrocytes (CNS).

When an axon is cut, peripheral part degenerates. �

Regeneration of the axon is possible only when the cell body of the neuron is intact. �

Neurons do not regenerate in the event of cell body death, i.e. they do not multiply.

Myelinated and Unmyelinated Axons

In the PNS, all axons are enveloped by Schwann cells which provide both structural and metabolic support. �

Many axons with small diameter invaginate into one Schwann cell longitudinally and are simply surrounded by the �

cytoplasm of Schwann cells. They are called unmyelinated nerve fi bres.

There are gaps (areas of axon not covered by myelin) along the length of myelin sheath at regular intervals called � nodes of Ranvier.

In large myelinated axons, the nerve impulse jumps from node to node resulting in faster conduction (saltatory �

conduction).

The segment of myelin between two nodes of Ranvier is called � internode.

The myelin of one internode is formed by a single Schwann cell. �

Schmidt-Lantermanncleft

Schwann cellnucleus

Node of Ranvier Myelin sheath

Axon

A myelinated peripheral nerve fi bre.

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� Other axons, especially the ones with larger diameter, invaginate into the Schwann cell and are wrapped by concentric layers of the Schwann cell plasma membrane forming myelin sheath. These axons are called myelinated nerve fibres (Fig. 7.6).

Fig. 7.6

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The myelin sheath shows cone-shaped clefts called � Schmidt-Lantermann clefts. They are areas of remnants of cytoplasm of Schwann cells present within the myelin sheath.

Myelination

With further rotation cytoplasm between the concentric layers of plasma membrane is squeezed out and the opposing �

inner surfaces of the plasma membrane fuse with each other forming myelin sheath. Thus myelin sheath is actually com-posed of many layers of modifi ed cell membrane of Schwann cell.

Peripheral Nerve

Each peripheral nerve (spinal or cranial) is made of bundles (fascicles) of nerve fi bres (axons) which may be myelinated �

and/or unmyelinated.

The bundles are held together by connective tissue which provides structural support as well as nutritional support by �

carrying blood vessels to nerve fi bres.

Mesaxon

Myelin sheathAxon

Nucleus ofSchwann cell

Schwann cellmembraneMesaxon

Axon

Axon

Schwann cell

Nucleus

(A) (B) (C)

Stages of myelin formation in PNS.

Oligodendrocyte

AxonMyelin sheath

Myelin formation in CNS.

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� In the PNS, the myelin sheath of an individual axon is provided by many Schwann cells lying along the length of the axon (Fig. 7.7A).� Myelination begins with the invagination of the axon into the Schwann cell. The invaginated axon is suspended from the periphery of the cell by a fold of fused plasma membrane called mesaxon (Fig. 7.7B).� As myelination proceeds the Schwann cell and mesaxon rotates itself around the axon several times resulting in enveloping the axon in concentric layers of Schwann cell cytoplasm and plasma membrane alternately (Fig. 7.7C).

� In the CNS, the myelin sheath is formed by processes of oligodendrocytes (Fig. 7.8).

Fig. 7.7

Fig. 7.8

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– Epineurium: Dense connective tissue sheath surrounding the entire nerve.

– Perineurium: A sleeve of fl attened specialised epithelial cells surrounding the bundles of nerve fi bres.

– Endoneurium: Loose connective tissue composed of reticular fi bres supporting individual nerve fi bres.

Epineurium

Perineurium

Endoneurium

Bundle of nerve fibres

Blood vessel

Peripheral nerve (C.S.).

Peripheral nerve (C.S.).Presence of

(i) bundles of nerve fi bres; (ii) perineurium around each bundle; (iii) darkly stained axon and lightly

stained myelin.

Perineurium

Venule

Arteriole

Loose Connective Tissue

Bundle of Nerve Fibres

Peripheral nerve (C.S.)

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� The connective tissue framework is well appreciated in cross section of a nerve (Fig. 7.9; Box 7.1), where following struc- tures can be observed:

� In the case of optic nerve, it is surrounded by meninges of brain (Box 7.2).

Fig. 7.9

Box 7.1

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Optic Nerve.Presence of

(i) central vessels of retina;(ii) bundles of myelinated nerve fi bres; (iii) dura, arachnoid and pia maters

surrounding the nerve.

Dura Mater

Subdural Space

Arachnoid Mater

Subarachnoid Space

Pia Mater

Nerve Bundles

Central Artery of Retina

Central Vein of Retina

Optic nerve

GANGLIA

Differences between sensory and motor ganglia

Sensory ganglion (somatic), Motor ganglion (autonomic),e.g. spinal ganglion e.g. sympathetic ganglion

Epineurium(capsule)

Pseudo-unipolarneuronsNerve fibres

Satellite cells

Epineurium(capsule)

Multipolarneurons

Nerve fibres

Satellite cells

1. Pseudo-unipolar neurons 1. Multipolar neurons

Large, rounded and of varying size (in section) � Small, angular and of uniform size (in section) �

Nucleus centrally placed � Nucleus eccentrically placed �

Found in groups � Found scattered �

2. Well-defi ned satellite cells 2. Poorly defi ned satellite cells

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Box 7.2

Ganglia are oval bodies made of aggregation of cell bodies of neurons outside the CNS. They serve as relay centres in the neuronal pathway. They are usually covered by a dense connective tissue capsule known as epineurium. The cell bodies of the neurons are enveloped by a layer of cuboidal cells called satellite cells. Two types of ganglia can be distinguished on the basis of morphology and function; sensory and motor ganglia (Boxes 7.3 and 7.4). Their distinguishing features are enumerated in Table 7.1.

Table 7.1

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Spinal GanglionPresence of

(i) groups of rounded pseudo-unipolar neurons of varying size;

(ii) well defi ned satellite cells;(iii) centrally placed nuclei in the

perikaryon.

Capsule of the Ganglion(Epineurium)

Bundle of Nerve Fibres

Pseudo-unipolar Neurons

Satellite Cells

Fat Cells

Spinal ganglion

Sympathetic Ganglion.Presence of

(i) small scattered, angular multipolar neurons of uniform size;

(ii) poorly defi ned satellite cells; (iii) eccentrically placed nuclei in the

perikaryon.

Capsule of the Ganglion(Epineurium)

Multipolar Neuron

Bundle of Nerve Fibres

Satellite Cells

Sympathetic ganglion

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Box 7.3

Box 7.4

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NEUROGLIA (IN CNS)

Distinguishing features of astrocytes, oligodendrocytes and microglia

Astrocytes Oligodendrocytes Microglia

Protoplasmic Fibrous

Diagram

Nucleus

Granularcytoplasm

Bloodvessel

Vascularfoot

Cell size Large Large Medium Small elongated

Shape of nucleus Oval(lightly stained)

Oval(lightly stained)

Small spherical(darkly stained)

Small elongated(darkest)

Cytoplasmicprocesses

Many short thickprocesses

Many longslender processes

Few short beadedprocesses

Short thin processeswith spines

Cytoplasm Granular(organelles free)

Fibrillar(organelles free)

Organelles present Organelles present

Occurrence (Predominant in)

Grey matter White matter White matter Grey and white matters

Function Supporting and nutritive,repair, barrier to diffusion of toxic substance

Myelination Phagocytosis

Embryological origin

Neural ectoderm Neural ectoderm Mesoderm

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Neuroglia are highly branched cells that support the neurons by occupying the spaces between them, providing both structural and metabolic support. There are four principal types of neuroglia in the CNS; namely astrocytes, oligodendrocytes, microglia and ependymal cells. Of the four types, ependymal cells form a specialised simple low columnar epithelium which lines the ventricles of brain and central canal of spinal cord. The epithelium lacks a basement membrane. The characteristic features and functions of the other three neuroglial cells are presented in Table 7.2.

Table 7.2

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