Nervous System – Histology

21Jan08

Nervous tissue

2 large components:
Central
Peripheral

CNS

Brain
Spinal cord

PNS

Nerves
Small aggregates of neuron – neuroganglia

Nervous System

sensory motor (from external/internal environment)
integrated component for processing n analyzing
integrated components (from CNS to muscles n glands)

Nervous System – 2 systems

somatic system – provide voluntary control of skeletal muscle.
autonomic system – involuntary control of smooth, cardiac muscles n glands.

Nervous tissue include 2 cell classes:

Nerve cells/ neurons – can generate, conduct, switch n receive nerve impulses.
Glial cells/ neuroganglia – support, nurture n protect neurons.

Development:

-developed from ectoderm  neural plate  neural tube (brain n spinal cord)  neural crest <migrate to form ganglia>
-2 cell types
- Neuroblast : give rise to all kinds of neurons
- Spongioblast : most of neuroganglia

NEURONS
-Protoplasm

– neuroplasm <cytoplasm>
- nucleus <karyon>

-3 parts:

– Cell body : trophic centre, have nucleus n cytoplasm
- Dendrites :receive stimuli from environment
- Axons : conduct nerve impulses thru another cells

- Function

– receive info from dendrites  cell body  axons

- Classification according to no. of processes:

– multipolar
- bipolar
- pseudounipolar : has a single process which divides into 2 branches  axons n dendrites

- Classification according to the distance btw cell body – axon:

– long axon neuron (Golgi’s first type) : make up distance connection. Eg: skeletal muscles
- short axon neuron (Golgi’s 2nd type) : provide internal connection in nervous centre.

- Classification according to their functions:

– motorneurons : control muscle fibers or effecteral organs eg: endocrine.
- sensoryneurons : receive sensory stimuli.

INTERNEURONS

– 99%
- establish relationship with other neurons forming complete functional circuit.

Neuron nuclei

– large, spherical, pale-stained
- chromatin finely dispersed, reflecting the intense synthetic activity of this cell.
- in females : nucleus is a sex chromatin/Barr body

- dense, condense n inactivate during interphase x-chromosome

Neuron body

– body/ soma or perikaryon
- trophic centre n recepting function.
- Neuroplasm

- organelles
- highly developed GER organized into aggregates of parallel cysterna

- synthesizes structural proteins n proteins for transport.

- many free ribosomes

- NISSL’S BODY – blue-stained GER : basophilic clumps or granules
- chromatolysis – Nissl’s bodies dissolve n disappear.
- Golgi apparatus

- located only in perikaryon around the nucleus.
- consist of 6 multiple parallel arrays of smooth cysterna.

- Mitochondria : abundant especially in axon terminals.
- neurofilaments (10nm) n microtubules (24nm).
- neurofibrils : aggregation of neurofilaments
- Inclusion of perikaryon:

- trophic : represented by glycogen n lipid.
- pigmented:

- melanin (in substantia nigra, locus coeruleus, etc) : darkbrown/black granules
- lipofuscent : light brown/ yellow lipid (in adult n senile age, increase with age)

DENDRITES

– arborizated (branched) : can receive n integrate a number of axon terminals.
- 200 000 axon terminals make contact with dendrites of Purkinje neurons in cerebellum.
- diameter decrease from proximal to distal end.
- no Golgi complex.
- Nissl’s body n mitochondria in proximal end only.
- numerous neurofilaments n microtubules.
- dendrite surfaces sometimes is covered by short branches called spines : special sites for contact with different terminals.

AXONS

– Only 1 axon
- axon hillock : beginning of axon after perikaryon.-
- no GER n ribosomes
- neurofilaments n microtubules are arranged in parallel fascicles.
- plasma membrane : axolemma n contains axoplasm.
- have constant diameter.
- sometimes axons give rise to a recurring (recurrent) collateral axonal branch toward perikaryon area.
- poor in organelles, few mitochondria n quite developed neurofibrils.

NEUROGLIA

– smallest cells of nervous tissue
- Function : metabolic, support, n protection.
- 2 types :

- macroglia

- astrocytes
- oligodendrocytes
- ependimocytes

- microglia

ASTROCYTES

– largest
- long processes
- part of processes form pedicles at the end which attach to the wall of blood capillaries almost completely n form the spherical glial sheath
- pedicles : end of processes
- upon the neuron surface, is called astrocyte feet.
- processes of the astrocytes are also present at the periphery of the brain n spinal cord forming a layer under pia mater : separates conn. Tissue of pia mater from the nerve cells.
- 2 types:

- protoplasmic : in grey matter of brain n spinal cord

- processes are shorter n thicker

- fibrous : in white matter

- long smooth processes

OLIGODENDROCYTES

– smaller than astrocytes
- lesser processes n shorter
- can be found both in grey n white matter.
- Function:

- take part in metabolism of neurons making unmyelinic n myelinic species.
- form protective barrier around neurons.
- live symbiotically with neurons : cant survive without ODCs

- ODCs are analogous for the lemmacytes n satellite’s cells of peripheral nerves n ganglia.

EPENDIMOCYTES

– looks like an epithelial layer.
- line the central canal of spinal cord n ventricles of the brain which are filled with cerebrospinal fluid.
- during development , basal processes of ependimocytes extend btw neural elements as far as the pia mater.
- take part in elaboration of cerebrospinal fluid.
- serve as a barrier btw fluid n neural elements.
- at last they form supporting framework of the system.

-
MICROGLIA

– small cells, highly-densed elongated bodies n short processes with numerous small branches.
- condensed, elongated nucleus.
- during embryogenesis, microglia cells are derived from mesenchyme (different from other Nervous Tissue)
- in adults, microglia are derived from monocytes of the bone marrow blood n they are phagocytes.

NERVE FIBERS

– Long, neural processes enveloped by special sheath.
- 2 types :

- unmyelinated

- surrounded by unmyelinated sheath.
- Every sheath is formed by lemmacytes n axon lies in the fold of its plasmalemma.
- A single lemmacytes can form folds of several axons.

- Myelinated

- surrounded by both myelinated sheath n lemmacytes.
- have a smaller diameter of axons than unmyelinated one. The diameter corresponds to the thickness of the myelin sheath. If the 1st is larger, the 2nd is thicker.
- the development of the myelin during histogenesis represent the multiple wrapping of lemmacyte’s plasma membrane around the axon.
- in this case the nucleus n most of cytoplasm of lemmacytes become displaced on the outside.
- electron micrograph shows the lamellar texture of the myelin. The myelin sheath doesn’t uniformly cover the axon.
- the myelin is absent btw adjacent lemmacytes.
- such areas are known as Node of Ranvier.
- internode distance varies in dimension from 0.08-1μm.
- along internode under the LM, myelin shows cone-shaped cleft : cleft of Schmidt-Lantermann
- they r actually helical tunnel from the outside of the sheath to the inside. They appear as sparsely dispersed lamella separated by cytoplasm.

NERVE ENDINGS:

– sensory /receptor n efferent endings.
- receptor : formed by termination of dendrites.
- efferent : divided into motor endings in muscles n glands n interneuronal synaptic ending. Formed by termination of axons.

SYNAPSES

– most of axon terminals are ended upon the surface of neuron..
- such specialized junctions btw neurons are called SYNAPSES
- they are used for the switch of nerve impulses.
- there r various types of synapses based on their sites.
- they could be :

o axo-dendrited synapse (btw axon n dendrite)
o axo-somatic synapse (btw axon n neuron body)
o axo-axonit synapse (btw axons)
o dendro-dendritic synapse (btw dendrites)

- majority : axo-dendritic n axo-somatic
- 3 parts of synapse

o pre-synaptic part
o post-synaptic part
o synaptic cleft (btw them)

- 2 types of synapse

o Chemical
o Electrical

- pre-synaptic part of chemical synapse contains synaptic vesicles.
- pre-synaptic part contains pre-synaptic membrane n some quantity of mitochondria.
- the post-synaptic part contains post-synaptic membrane n underlined area of neurocytoplasm.
- synaptic vesicles : small, membrane-bounded, 40-60nm in d.
- they contain neurotransmitter substance mediator <eg: acetylcholine, nor epinephrine, etc>  discharge into the synaptic gap.
- neurotransmitter provoke an excitation n inhibition of post-synaptic neuron.
- electrical synapses appear as gap junction n don’t contain synaptic vesicles.
- switch of impulses occurs by passage of ions from neuron to neuron w/o transmitters.

-
REGENERATION & DEGENERATION

– numerous kinds of agents can provoke regeneration, degeneration n cell death.
- neurons are unable to divide but glial cells can be divided n fill in the area left by that neuron.
- neurons are able to regenerate lost or injured processes in case the perikaryon hasn’t been destroyed.
- in such cases, intracellular regeneration occurs in perikaryon.

PERIPHERAL NERVOUS SYSTEM
- consist of

o sensory ganglia
o autonomic ganglia
o nervous plexuses
o nerves
o nerve terminals

NERVES
- myelinated or unmyelinated fibers
- 4 types of nerve fibers:

o somatic efferent (SE)
o visceral efferent (VE)
o somatic afferent (SA)
o visceral afferent (VA)

- SE fibers

o myelinated axons
o multipolar neurons
o ventral horn of spinal cord

- VE fibers

o Unmyelinated
o arise from sympathetic ganglion cells.
o join the nerve trunk by way of grey communicating ramus.

- SA n VA fibers

o pseudounipolar neurons
o located in sensory ganglia
o maybe myelinated (major) n unmyelinated (minor)

- cranial nerves n spinal nerves consist of

o efferent
o afferent
o mixed nerves

STROMA
- Consist of connective tissue
- cover nerve trunk
- 3 envelops/layers:

o epineurium : external, densed connective tissue
o perineurium : 2nd layer, cover bundle or nerve fibers, has epithelioid fibroblast layer which is joined by tight junction.
o endoneurium : surround individual nerve fibers, continuous with perineurium, fine connective tissue fibers, contain fibroblast n histiocytes (inactive macrophage), capillaries n blood vessels

GANGLIA
- capsulated aggregations of nerve cell bodies outside the CNS.
- 2 types:

o cranio-spinal ganglia

 sensory ganglia
 associated with cranial nerves n dorsal root of spinal nerves.
 1st : cranial ganglia
 last : dorsal root/ spinal ganglia
 consist of pseudounipolar neurons : transmit info from sensory receptors to CNS.
 sensory fibers/ processes:

• central – axons, form dorsal spinal root.
• peripheral – form mixed nerves (motor n sensory)

o autonomic gangli

 associated with autonomic nerves.
 multipolar neurons
 stellate glial cells surround the nerve cell bodies.
 ganglion neurons

• long axon cells/neurons – form post ganglionic fibers for effector organ.
• associative neurons – conn. btw autonomic neurons.

 contain connective tissue components (densed) – located btw neurons, nerve fibers, n ganglia.

AUTONOMIC NERVOUS SYSTEM

- Control smooth muscle, gland secretion n modulation of heart rhythms.
- involuntary control of effector organs.
- 3 types

o Sympathetic
o Parasympathetic
o anteric (related to digestive system)

- ANS is composed of :

o connection of nerve cells located in the CNS..
o nerve fibers which leave the CNS thru cranial or spinal nerve.
o nerve ganglia situated in the part of these fibers n post ganglionic nerve fibers with their termination.

SYMPATHETIC N PARASYMPATHETIC DIVISION

– 1st Neurons in the CNS are called preganglionic neurons n their axons form the preganglionic nerve fibres.

SYMPATHETIC DIVISION :

– Pre-ganglionic neuron bodies are located in the thoracic n 1st two lumbar segments.
- the axons are sent via the ventral roots to the vertebral n paravertebral ganglia.
- the ganglia in the vertebral n paravertebral sympathetic chain contain the cell bodies of the post synaptic effector neurons of the sympathetic division.
- the post synaptic axons leave the ganglion to the effector organs.
- Norepinephrine : neurotransmitter of post ganglionic synapses

PARASYMPATHETIC DIVISION

– cell bodies are in certain cranial nerves : 3,7,8,9 n also thru the 2nd, 3rd, n 4th sacral spinal segments.
- axons of preganglionic neurons leave the branch of spinal cord n enter parasympathetic ganglia (ciliary, otic, submandibular glands)
- head: parasympathetic ganglia are located in some distances from their effector organs. So post ganglionic fibers are distributed by cranial nerves but visceral ganglia in the remainder of the body are located close to the effector organs n often within their wall.
- the post ganglionic fibers are short.
- Acetylcholine  neurotransmitter for both pre-ganglionic n post-ganglionic synapses.

-
ANTERIC DIVISION

– Consist of nervous plexuses – located within the wall of alimentary tract.
- is connected with other organs.
- mostly, organs are innervated by the sympathetic (stimulator) n parasympathetic (inhibitor) fibers.

CENTRAL NERVOUS SYSTEM

– Consist OF brain (cerebrum n cerebellum) n spinal cord : contain grey n white matter.
- grey matter :

o contain neuronal cell bodies (axons n dendrites)
o contain neuroglia
o neurophil : the meshwork of axonal dendritic n glial processes w/o neuron bodies

- white matter :

o composed of myelinated n unmyelinated fibers n neuroglia : form conductive tract.

- grey matter neuronal organization

o stratified type : cerebral n cerebellar cortex
o nuclear type : represent a compact group of neuron cell bodies. Eg: subcorticals, brainstem, n SC
o reticular type : thin irregular arrangement of neuron bodies.

MENINGES

– 3 mater

o dura mater – outermost, densed connective tissue envelop.
o arachnoid mater – trabecular connective tissue: form a loose network (subarachnoid space) filled with CSF
o pia mater – Innermost, loose connective tissue : contains blood vessels, continuous with the perivascular connective tissue sheath of the blood vessels.

SPINAL CORD
- TRACTS

o Dorsal
o Lateral
o Ventral

Cerebrum
- composed of

o cortex
o subcortex
o brain stem

- cerebrocortex

o grey matter
o have gyrus, separated by sulcus
o the underlined white matter forms an inner core of medulla.

CEREBRAL CORTEX LAYERS:

– molecular cell layers

o small neurons, afferent fibers n neuroglia.

- outer granular layer

o granule neurons, afferent fibers n neuroglia (more compact than 1)

- outer pyramidal layer

o medium n small pyramidal neurons, granule neurons, afferent fibers n neuroglia.

- internal granular layer

o narrow band
o granule neurons, afferent fibers, n neuroglia

- ganglionic layer

o large n medium pyramidal neurons, afferent fibers n neuroglia
o largest pyramidal neurons.

- multiform layer

o various shapes of afferent fibers n neuroglia.

CEREBELLUM

– cortex n inner coat : medulla
- medulla : contain cerebellar nuclei surrounded by tracts of white matter.
- 3 layers:

o 1. outer/ molecular layer

 fused, small neurons.
 upper located stellate cells (star shaped)
 deeply disposed basket cells.

o 2. ganglionic layer / Purkinje’s cell layer.

 Large, pear-shaped bodies of neurons : purkinje’s cell – surrounded by axon terminals of basket cells.
 purkinje’s cells : located in a single layer, axons are myelinated, dendrites are densely branched n disposed in molecular layer.

o 3. granular layer

 numerous, closely packed, very small neurons (granules)

*Afferent fibers n glial cells are located in all layers.

SPECIAL SENSE
- 2 types

o receptor/ sensory endings (dendrites)
o effector endings (axons)

- sensory receptors

o extraceptors
o proprioceptors
o intraceptors

- EXTRACEPTORS
o Near body surface
o stimulus from external environment (touch, temperature, pain, pressure)
o general somatic afferent pathway.
o Specialized (light, sound, smell, taste.)

- PROPRIOCEPTORS
o in joint capsules, tendons, ligaments, n in muscles.
o general somatic afferent receptors.
o awareness of the body in space n movements.
o receptors of vestibular mechanism : inner ear.

- INTRACEPTORS
o within organs of the body
o general visceral afferent.

SPECIALIZED PERIPHERAL RECEPTORS
- 3 TYPES
o Mechanoreceptors

 mechanical stimuli (touch, vibrations, stretch, pressure)

o Thermoreceptors

 temperature difference (2° above 0)
 warmth n cold receptors

o Nociceptors

 sensitive to pain
 caused by mechanical stress, extreme temperature n cytokines (bio-active substances: bradykinin, serotonin, histamine)



5 Responses to “Nervous System – Histology”

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