Introduction Development Anatomy Characteristics Histology Pathology

The human CNS contains about 100 billion neurons.

It also contains 10-50 times this number of glial cells.

The neurons, the basic building blocks of the nervous system have evolved from primitive neuroeffector cells.

NEURON: A Specialized cell transmitting nerve impulses

Otherwise called as nerve cell.

Differ from other cells- Neuron has branches called axons and dendrites. Neuron does not have centrosome so it cannot

undergo division

NERVE:A whitish fibre or bundle of fibres in the body

that transmits impulses of sensation to brain or spinal cord,& impulses from these to muscles & organs.

DEPENDING UPON THE NUMBER OF POLES: UNIPOLAR NEURONS- have only one pole from

where both the dendrites and axons arise. Present only in embryonic stage in human

beings.

BIPOLAR NEURONS – two poles, one for axons and the other for dendrites.

MULTIPLE NEURONS – have many poles. One of the poles gives rise to axon and all the other poles give rise to dendrites.

DEPENDING UPON THE FUNCTION : MOTOR NEURON – also known as efferent nerve

cells. Carry the motor impulses from CNS to the

peripheral effector organs like muscles, glands, blood vessels.

The motor neurons have long axons and short dendrites.

SENSORY NEURONS – also called as afferent nerve cells. Carry the impulses from periphery to the CNS. They have short axons and long dendrites.

DEPENDING UPON THE LENGTH OF THE AXON :

GOLGI TYPE 1 NEURONS – have long axons.

The cell body is in CNS and their axons reach the remote peripheral organs.

GOLGI TYPE 2 NEURONS – have short axons. Present in the cerebral cortex and spinal cord.

In the early embryonic disc,3rd week of I.U life,a layer of ectodermal cells called neuroectodermal cells form a plate called the NEURAL PLATE or the medullary lamina.medullary lamina.

A median groove called the neural groove appears in this plate as its lateral edge raises as 2 folds called NEURAL FOLDSNEURAL FOLDS..

The neural folds starts uniting in the middle and the union proceeds in both cranial and caudal direction connecting the neural groove into a NEURAL TUBE.

CNS at 3rd week of IU

Neural plate

Neural folds

Neural Tube

Neuro Epithelial Cells

As the neural tube is being formed, cells at the junction of the neural fold and the skin ectoderm gets detached to lie as clusters on the dorsolateral side of the neural tube. The cluster of cells are called Neural crest or Neural crest or Neuro epithelial cells. Derivatives of the Neuro epithelial cells. Derivatives of the neural crest –neural crest –

1. Sheath cells of schwann- neurolemmal cells of the peripheral nerves.

2.2. Dorsal root ganglia of the spinal nerves, sensory ganglia of the cranial nerves

3. sympathetic and parasympathetic ganglia.

-The caudal part of the neural tube remains relatively narrow and gives rise to the spinal cord.

The neural tube in the beginning is lined by a single layer of pseudostratified epithelial cells which proliferate and migrate outward to form a second layer called mantle layer.mantle layer.

The cells which migrate from the mantle layer are of two types :

1.1. Neuroblasts Neuroblasts which differentiate into nerve cells or neurons

2.2. Spongioblasts Spongioblasts which differentiate into neuroglial cells, astrocytes and oligodendrocytes.

3. Microglia are mesodermal in origin and are mesodermal in origin and migrate into the neural tube along with migrate into the neural tube along with the blood vesselsthe blood vessels

The neuron is made up of three parts : 1. NERVE CELL BODY : irregular in shape,

constituted by a mass of cytoplasm called neuroplasm covered by a cell membrane.

• NUCLEUS : present in the central part of the nerve cell body.

• Has one or two nucleoli.• No centrosome.

NISSIL BODIES : small basophilic granules. Present throughout the soma except in axon

hillock.

Are called TIGROID SUBSTANCES since these bodies are responsible for tigroid or spotted appearance of soma after suitable staining.

Are membranous organelles containing ribosomes - concerned with protein synthesis in the neurons.

During injury of neuron, these bodies fragment disappear by a process called chromatolysis.

NEUROFIBRILS : thread like structure present in the soma and the processes. Consist of neurofilament and microtubules.

MITOCHONDRIA : form the powerhouse of the cell where ATP is produced.

GOLGI APPARATUS : concerned with the package of proteins into the granules.

Nerve cell body also contain lipofuscin pigment and melanin.

2. DENDRITE : are the branched process of the neuron.

Have nissl bodies and neurofibrils. Conductive in nature and transmit

impulse towards the nerve cell body.

3. AXON : is the longer process of the nerve cell.

Arises from axon hillock of the nerve cell body and devoid of NISSIL GRANULES

The distal ends of the terminal branches of the axons are often enlarged – TERMINALS OR BUTTONS terminaux.

Some axons near their termination show a series of swelling resembling a string of beads and are called as VARICOSITIES.

Structure of axon : the axons are arranged in different bundles called FASCICULI

The whole nerve is covered by tubular sheath which is formed by areolar membrane. This sheath is called as EPINEURIUM.

Each fasciculus is covered is by perineurium and each nerve fibre is covered by endoneurium.

The axon has a long central core of cytoplasm called AXOPLASM, covered by the tubular sheath like membrane called AXOLEMMA

Synapse is the junction between the two neurons.

The presynaptic membrane has two important structures :

• Mitochondria which help in the synthesis of chemical neurotransmitter.

• Synaptic vesicles which store neurotransmitter substance.

The membrane of the postsynaptic neuron is called postsynaptic membrane – contains receptor proteins.

The small space b/w pre and postsynaptic membrane is called synaptic cleft(20-30nm) – contains cholinesterase

SynapseSynaptic

Cleft

Conduct electrical impulses along the plasma membrane

Produce nerve impulse Produce action potential Longevity: can live and function for a life

time Do not divide: fetal neurons lose their

ability to undergo mitosis High metabolic rate: require abundant

oxygen and glucose

The Nerve is typically most WAVY in section although not elastic, because it must accommodate with increased length when the tissue in which it resides is strecthed.

Wavy nucleus is seen

Mass of Support cells in the Central Nervous System (CNS) are grouped together as neuroglia

Neuroglia literally means “nerve glue”. The cells are nonexcitable hence called the

nonneural cells ,glial cells. Play an important role in the reaction of nerve

during infection Provide both mechanical and metabolic support The function of neuroglia is to support, insulate,

and protect the delicate neurons of the brain

Astrocytes Star-shaped cells Half of all brain tissue Brace neurons; they provide Mechanical support as

well as mediating the exchange of metabolites between Neurons and the vascular system,thus regulating the composition of intercellular environment of CNS.

They also form a part of Blood-Brain barrier Play an important role in repair of CNS tissue after

damage.

Astrocytes are identified at molecular level by IHC staining for a Protein called Glial Fibrillary Acidic Protein(GFAP).

All Astrocytes contain bundles of intermediate filaments . The intermediate filaments are formed of GFAP, which is characteristic of Astrocytes.

Many of astrocytes process end in terminal expansions adjacent to the non-synaptic regions of neurons. Other processes of the same astrocyte terminate upon the basement membrane of the capillaries ,these Perivascular feet cover most of the surface of capillary basement membrane & form part of Blood-Brain barrier.

Microglia Spiderlike phagocytes (white blood cells)

derived from monocytes Dispose of debris like dead brains cells and

bacteria

Ependymal cells Lines the cavities of the brain and spinal cord Circulate cerebrospinal fluid by beating their

cilia Cerebrospinal fluid, fills the space that the

brain does not take up and forms a protective cushion around the brain and spinal chord

Oligodendrocytes Wrap around nerve cells in the brain and spinal

chord Produce myelin sheaths Myelin is a fatty, insulation covering the nerve

cells; allows for the electrical signal to transmit faster (like wire coating).

A single oligodendrocyte can contribute to the myelination of upto 50 axons from the same or the different fibre tracts . Conversly,any one axon will reqire the service of numerous different oligodendrocyte because of limited length of myelin segments (internodes) produced by oligodendrocytes.

Schwann cells Form myelin sheath in the

peripheral nervous system. A nerve is basically a bundle of

axons (long projections) from various neurons.  Around these axons (note the dark centers in the middle of the circular structures on the high power image on the right) are glial cells called Schwann cells.  These make up the white myelin which extends around the each axon. 

Satellite cells Protects neuron cell bodies which is where

the nucleus of the cell if found

TANCYTES TANCYTES : long basal processes with end feet on capillaries.

line the floor and the third ventricle. Transport the substances from CSF to

hypophyseal portal system.

Thoroidal epithelial cellsThoroidal epithelial cells : secrete CSF.

Multiple Sclerosis Rabies Herpes simplex & herpes zoster Poliomyelitis

MS affects the ability of nerve cells in the brain and spinal cord to communicate with each other.

In MS, the body's own immune system attacks and damages the myelin

Neuroprexia : : it transient blocks and the paralysis is incomplete, recovery is rapid and complete and no microscopic evidence of degeneration.

Pressure is the most important cause.Pressure is the most important cause.

Axonotmesis : : term applied to the nerve lesion in which the axons are damaged but the surrounding connective tissues sheaths remain more or less intact.

Crush injuries, traction and compression Crush injuries, traction and compression are the most common causesare the most common causes

Neurotmesis : : term applied to the lesion of complete section of the nerve trunk.

Sensory recovery is indicated by the appearance of tingling sensation in the areas of cutaneous distribution called Tinnel’s sign.Tinnel’s sign.

Light, touch and tactile sensation are the Light, touch and tactile sensation are the last sensation to return and are often last sensation to return and are often incomplete.incomplete.

1. K. SMITH - NEURONS PHYSIOLOGY2. ARTHUN BUTT - NEUROGLIA3. N.A. GREGSON - CNS MYELIN4. Wheater's Functional Histology5. The Neuron , by Richard H.Hall(1998)6. TORTORA - TEXTBOOK OF PHYSIO AND

ANATOMY7. HAM’S HISTOLOGY