Histology of nerve system

Nerve Tissue & Nervous System

Assoc. Prof Dr. Karim Al-JashamyIMS/MSU 2010

Nerve Tissue & the Nervous System:

The human nervous system, by far the most complex system in the

human body, is formed by a network of more than 100 million nerve

cells (neurons), assisted by many more glial cells.

Each neuron has, on average, at least 1000 interconnections with other

neurons, forming a very complex system for communication.

Neurons are grouped as circuits. Like electronic circuits, neural circuits

are highly specific combinations of elements that make up systems of

various sizes and complexities.

Nerve tissue is distributed throughout the body as an integrated

communications network.

Anatomically, the nervous system is divided into the central nervous

system, consisting of the brain and the spinal cord, and the peripheral

nervous system, composed of nerve fibers and small aggregates of

nerve cells called nerve ganglia

Structurally, nerve tissue consists of two cell types: nerve

cells, or neurons,

Usually show numerous long processes, and several types of

glial cells which have short processes, support and protect

neurons, and participate in neural activity, neural nutrition,

and the defense processes of the central nervous system.

Neurons react promptly to stimuli with a modification of

electrical potential that may be restricted to the place that

received the stimulus or may be spread (propagated)

throughout the neuron by the plasma membrane. This

propagation, called the action potential, or nerve impulse,

is capable of traveling long distances; it transmits information

to other neurons, muscles, and glands.

Central Nervous System (CNS)

The CNS consists of the brain (encephalon), which is

enclosed in the skull, and the spinal cord, which is contained

within the vertebral canal.

Nervous tissue of the CNS does not contain connective

tissue other than that in the three meninges (dura mater,

arachnoid membrane and pia mater) and in the walls of large

blood vessels.

Collagenous fibers or fibrocytes/blasts are consequently not

observed, which is quite unlike other tissues. Because of the

absence of connective tissue, fresh CNS tissue has a very

soft, somewhat jelly-like consistency.

The major classes of cells that make up the nervous tissue

are nerve cells, neurones, and supporting cells, glia.

The Central Nervous System

The central nervous system consists of the cerebrum, cerebellum, and spinal cord. It has almost no connective tissue and is therefore a relatively soft, gel-like organ.

When sectioned, the cerebrum, cerebellum, and spinal cord show regions that are white (white matter) and that are gray (gray matter).

The differential distribution of myelin in the central nervous system is responsible for these differences: The main component of white matter is myelinated axons and the myelin-producing oligodendrocytes.

White matter does not contain neuronal cell bodies.

Meninges

The delicate, innermost, mesh-like layer of the meninges. Thepia mater closely envelops theentire surface of the brain,running down into the fissuresof the cortex.

It joins with the ependymawhich lines the ventricles toform choroid plexuses thatproduce cerebrospinal fluid. Inthe spinal cord, the pia materattaches to the dura mater bythe denticular ligamentsthrough the arachnoid

membrane.

Meninges

Gray and White Matter

Microscopically, the CNS contains 2 neural elements: Neuron cell bodies (clusters are

known as nuclei) Nerve fibers (axons) in bundles

called tracts.

Viewed macroscopically, CNS tissues can be distinguished by color: Gray matter consists of somata,

dendrites, and unmyelinatedaxons.

White matter consists primarily of myelinated axons.

Gray matter contains neuronal cell bodies, dendrites, and the

initial unmyelinated portions of axons and glial cells.

Gray matter is prevalent at the surface of the cerebrum and

cerebellum, forming the cerebral and cerebellar cortex

whereas white matter is present in more central regions.

Aggregates of neuronal cell bodies forming islands of gray

matter embedded in the white matter are called nuclei

In the cerebral cortex, the gray matter has six layers of cells

with different forms and sizes. Neurons of some regions of

the cerebral cortex register afferent (sensory) impulses; in

other regions, efferent (motor) neurons generate motor

impulses that control voluntary movements.

Cells of the cerebral cortex are related to the integration of

sensory information and the initiation of voluntary motor

responses

Cerebral Cortex

Allows for sensation, voluntary movement, self-awareness, communication, recognition, and more.

Gray matter!

40% of brain mass, but only 2-3 mm thick.

Each cerebral hemisphere is concerned with the sensory and motor functions of the opposite side (contralateralside) of the body.

CEREBELLAR CORTEX

Impregnated with silver nitrate1 - molecular layer 2 - ganglionic layer (Purkinje cell layer) 3 - granular cell layer 4 - Purkinje neurons (cells) 5 - fold of the cortex 6 - white matter

CEREBELLAR CORTEX

Stained with H&E

1 - molecular layer

2 - ganglionic layer

(Purkinje cell layer)

3 - granular cell layer

4 - Purkinje neurons (cells)

Silver-stained section of cerebral cortex showing many pyramid-shaped neurons with their processes and a few glialcells.

The cerebellum (H&E) does not reveal

the unusually large dendritic of the

Purkinje cell,

Section of the

cerebellum with

distinct Purkinje

cells. One

Purkinje cell

shows part of

its rich dendritic

arborization.

The gray matter of the spinal

cord showing several motor

neurons with their basophilic

bodies (Nissl bodies). Nucleoli

are seen in some nuclei. The

neurons are surrounded by a

mesh of neuronal and glial

processes. PT stain. Medium

magnification

Cerebellum• Lies inferior to the cerebrum and

occupies the posterior cranial fossa.

• 2nd largest region of the brain.• 10% of the brain by volume, but it

contains 50% of its neurons

• Has 2 primary functions:

1. Adjusting the postural muscles of the body

• Coordinates rapid, automatic adjustments, that maintain balance and

equilibrium

2. Programming and fine-tuning movements controlled at the

subconscious and conscious levels

• Refines learned movement patterns by regulating activity of both the

pyramidal and extrapyarmidal motor pathways of the cerebral cortex

• Compares motor commands with sensory info from muscles and joints

and performs any adjustments to make the movement smooth

Cerebellum

Cerebellar cortex contains huge, highly branched Purkinje cells whose extensive dendrites can receive up to 200,000 synapses.

Internally, the white matter forms a branching array that in a sectional view resembles a tree – for this reason, it’s called the arbor vitae

In cross sections of the spinal cord, white matter is peripheral and gray matter is central, assuming the shape of an H.

In the horizontal bar of this H is an opening, the central canal, which is a remnant of the lumen of the embryonic neural tube.

It is lined with ependymal cells. The gray matter of the legs of the H forms the anterior horns. These contain motor neurons whose axons make up the ventral roots of the spinal nerves.

Gray matter also forms the posterior horns (the arms of the H), which receive sensory fibers from neurons in the spinal ganglia (dorsal roots).

Spinal cord neurons are large and multipolar,

especially in the anterior horns, where large motor

neurons are found

Spinal Cord

Cross section of the spinal

cord in the transition between

gray matter (below) and white

matter (above).

The gray matter contains

neuronal bodies and abundant

cell processes, whereas the

white matter consists mainly of

nerve fibers whose myelin

sheath was dissolved by the

histological procedure. PT

stain. Medium magnification.

Section of spinal cord gray

matter. The meshwork of

cell neuron and glial

processes appears

distinctly.

The small nuclei are from

glia cells. Note that these

cells are more numerous

than neurons. H&E stain.

Medium magnification.

Ganglion cells will typically beseveral times larger than othercells in the ganglia

The perikaryon is very large andsurrounds a large and lightnucleus. Only the cellsimmediately surrounding theganglion cells as one flattenedlayer are satellite cells.

Ganglion cells are of course incontact with other parts of thenervous system and with theperipheral tissues which theyinnervate. Consequently, nervefibers will be visible close to orwithin the ganglion.

Peripheral Nerve longitudinal H&E stained sections it is

possible to identify the axon running in its myelin sheath, nodes of Ranvierand Schwann cell nuclei.

Components of the connective tissue elements, which accompany the nerve, should be visible and identifiable in both longitudinal and transverse sections.

transversely cut preparations give a good picture of the axon in the middle of a ring-like structure (sometimes fussy), which represents the remains of the myelin sheath.

Due to their small size and the lack of a myelin sheath, type C fibres are very difficult to detect in either osmium or H&E stains.