histology of nervous system
TRANSCRIPT
NERVOUS TISSUEDr. Ashfaqur Rahman
MBBS, M.Phil (Anatomy)Associate Professor and Head, Anatomy
Monno Medical College
INTRODUCTION
NEUROANATOMY: THE STUDY OF THE NERVOUS SYSTEM.
2 body systems control all the functions of the body-1. Nervous system- which works very rapidly2. Endocrine system- which works more or less slowly.
Nervous system is also called ‘master system of the body’.
Functions of the nervous system:
1. Reception of sensory stimuli2. Integration of sensory information3. Coordination and control of motor activities4. Assimilation and storage of experiences5. Programming of basic instincts
Sensory stimuli Correlation Effectors
Memory
Classification of nervous systemAnatomically:Central nervous system (CNS)-Brain and spinal cordFunctions:1. Integration, processing and
coordination of sensory data2. Giving appropriate motor commands3. The seat for higher functionsPeripheral nervous system (PNS)-peripheral nerves and associated gangliaFunctions:4. Provides sensory information to CNS5. Carries motor commands of CNS to
the effectors
Functionally/physiologically:1. Somatic nervous system (SNS)2. Autonomic nervous system (ANS)
CELLULAR ORGANIZATIONThe tissue forming the nervous system is called nervous tissue, which is one of the 4 basic tissues of the body. Essentially nervous tissue is a cellular type of tissue, composed of 2 types of cells without ICM.
Composition:1. Neurons/nerve cells-Excitable, non-dividing cell2. Neuroglia/glial cells-Non-excitable, dividable cell
Neuroglia are 10 times more in number than neurons. Blood vessels are also found within nervous tissue which provide nutrition to the neurons and neuroglia.
Organization of grey and white matter- 1. Primitive organization-inner grey matter, outer white
matter- e.g. Brain stem and spinal cord.2. Modified organization-outer grey matter, inner white
matter- e.g. Cerebrum and cerebellum.
Types of the nervous tissue:
Grey matter- The nervous tissue which appears Grey. It is grey due to rich blood supply and absence of myelin. It has rich blood supply because it contains cell bodies of the neuron.Composition: 1. Cell bodies of neurons2. Dendrites and proximal/initial (non-myelinated) portion of
axons, sometimes whole axons (also non-myelinated) 3. Supporting cells-neuroglia (Protoplasmic astrocyte,
perineuronal oligodendrocyte, microglia)4. Blood vessels (abundant)
White matter- The nervous tissue which appears white. It is white due to presence of myelin and less blood supply. It has less blood supply because it does not contain cell bodies of the neuron.Composition: 1. Distal myelinated portion of axons2. Supporting cells-neuroglia (fibrous astrocyte,
intrafascicular oligodendrocyte, microglia)3. Blood vessels (few) (supplying the neuroglia present in
the white matter)
Neuron-Structural & functional unit of nervous systemNeuron doctrine: by Ramon y Cajal- 6 tenets in 1 sentenceNeuron is the anatomical, genetic, functional, polarized, pathologic and regenerative unit of nervous system.
2 parts of neuron-1. Cell body/soma/perikaryon2. Process/neurite
2 types of neurite-3. Axon4. DendriteAxon (& dendrite) is called nerve fiber.
Collection of neuron (cell body) inside CNS is called NUCLEUS.Collection of neuron (cell body) outside CNS is called GANGLION. Collection of nerve fiber inside CNS (specially within spinal cord) is called TRACT.Collection of nerve fiber outside CNS is called NERVE (PERIPHERAL NERVE).
Classification of neurons:According to polarity (number of neurites)/morphological classification:
• Unipolar (pseudounipolar) neurons- Functionally sensory neurons. E.g. Neurons of DRG, neurons of sensory cranial ganglia, neurons of mesencephalic nucleus of trigeminal nerve.
• Bipolar neurons- Functionally sensory neurons. E.g. Bipolar neurons of retina, olfactory cells, neurons of vestibular (Scarpa’s ganglion) and cochlear (spiral) ganglion.
• Multipolar neurons- Functionally motor neurons. E.g. Anterior horn cells of spinal cord, pyramidal cells of cerebral cortex, Purkinje cells of cerebellar cortex, interneurons, neurons of autonomic ganglia. Most of the neurons of CNS are multipolar type.
Classification of neurons:
According to function:
Motor/effector neurons- E.g. Anterior horn cells of spinal cord, pyramidal cells of cerebral cortex, Purkinje cells of cerebellar cortex, neurons of autonomic ganglia.• UMN • LMN• Preganglionic• Post ganglionic
Sensory/receptor neurons- E.g. Neurons of DRG, neurons of sensory cranial ganglia, neurons of mesencephalic nucleus of trigeminal nerve, bipolar neurons of retina, olfactory cells, neurons of vestibular (Scarpa’s ganglion) and cochlear (spiral) ganglion.• Primary• Secondary• Tertiary
Interneurons/internuncial/intercalated/central/associational/connector neurons. >99.9% neurons are of this type. E.g. Column cells of spinal cord.
Classification of neurons:
According to length of neurites (axons)/according to size of neuron:
Golgi type I neurons- E.g. Anterior horn cells of spinal cord, pyramidal cells of cerebral cortex, Purkinje cells of cerebellar cortex. They act as relay neurons.
Golgi type II neurons (microneurons)- E.g. Granule cells and stellate cells of cerebellar cortex, interneurons. They act as associational/interneurons.
Classification of neurons:
Other types:1. Amacrine neurons2. Microneurons (7µM)3. Paraneurons (apolar neurons)
Locations of neurons (cell bodies):1. Within gray matter of CNS2. Within white matter of CNS as nuclei3. Within peripheral ganglia4. Within olfactory epithelium, retina
Development of neurons:1. CNS neurons- from neural tube2. Neurons of peripheral ganglia- from neural crest and
ectodermal placodes
Structure of a neuron:Cell body • Nissle body
(substance/granules)/tigroid or chromodial substance
• Neurofibrils• Centrosome• Axon hillockDendrites• Dendritic spine/gemmulesAxon• Parts
• Preaxon/initial segment• Axon proper• Axon terminale (terminal part)
• Telodendria• Terminal boutons/synaptic
knob• Axolemma• AxoplasmEndoneuriumPerineuriumEpineurium
Difference between axon and dendrite
Neurolemma/neurolemma sheath/sheath of SchwannSchimidt-Lantermann incisura/fissure and segment
Nerve fibers
Axons (and dendrites) of neurons are called nerve fibers.
Classification:According to presence of myelin sheath:1. Myelinated nerve fibers2. Unmyelinated nerve fibers- unmyelinated peripheral
nerves are known as fibers of Remak
According to functions:1. Motor (efferent) nerve fibers2. Sensory (afferent) nerve fibers
Periphery
Nerve fibers
Fiber type
Sub type
CV m/s
Diameter m
Function Myelin
A 70-120 12-20 Motor (extra-fugal fibers of skeletal muscle)
+
40-70 5-12 Sensory (touch, pressure, vibration) +
10-50 3-6 Motor (intra-fugal fibers of skeletal muscle)
+
6-30 2-5 Sensory (temperature, pain-sharp & localized)
+
B 3-15 <3 Motor (preganglionic autonomic fiber) +
C 0.5-2.0 0.4-1.2 Motor (postganglionic autonomic fiber)Sensory (temperature, pain-diffuse & deep)
_
Combined classification:
Receptors (SENSORY)
Biological transducers responsible for reception of any sensory stimulation and conversion of this stimulation into nerve impulse to be carried by nerve fibers are called receptors.
Receptors are sensory nerve endings specialized for reception of stimuli and transforming them in the form of nerve impulses (ref. Vishram Singh).
Classification:
• According to location- Exteroceptors (cutaneous receptors)-from superficial
structures like skin, superficial fascia Interoceptors-from viscera and blood vessels Proprioceptors-from muscle and joint
• Functional classification- Mechanoreceptors Thermoreceptors Nociceptors Chemoreceptors Electromagnatic receptors
• Anatomical classification-
Uncapsulated receptors
Free nerve ending Epidermis, cornea, gut, dermis, fascia, tendon, ligaments, joint capsules, bone, periosteum, perichondrium, dental pulp, tympanic membrane, muscle
Pain, temperature, touch (crude), pressure
Tactile discs of Merkel/Merkel’s disc
Thick (hairless) skin-epidermis Touch and pressure
Hair follicle receptor/Peritrichial plexus
Thin (hairy) skin-dermis Touch
Encapsulated receptors
Meissner’s tactile corpuscle
Dermis of thick (hairless) skin, external genitalia, nipple
Touch (2 point TD)
Pacinian corpuscle Dermis, S/C tissue, joint capsules, ligaments, peritoneum, pleura, external genitalia, nipple
Vibration and pressure
Ruffini’s corpuscle Dermis of thin (hairy) skin Stretch
End bulbs of Golgi-Mazzoni
Genital skin
End bulbs of Krause Mucocutaneous junctions, conjunctiva Temperature (cold)
Neuromuscular spindles Skeletal muscle Stretch-muscle length
Golgi tendon organ Tendons Compression-muscle length
Cutaneous receptorsSensory modalities Type of receptors
Pain, touch and temperature
Free nerve endings
Merkel’s discs
Peritrichial plexus
Pressure and vibration Meissner’s corpuscles
Pacinian corpuscles
Deep pressure Ruffini’s corpuscles
Free nerve endings
Ruffini’s corpuscles
Pacinian corpuscles
Golgi tendon organ
Joint receptors
Neuroglia/Glial cells
• The non-excitable, cellular supporting element of NS is called neuroglia.
• They represent connective tissue of nervous system. They are 5-10 times more numerous than neurons.
• Neuroglia (except microglia), like neurons, develop from neuroectoderm.
• But the basic difference is that neuroglia are capable of division by mitosis whereas neurons are not.
Macroglia Microglia
Astrocyte Oligodendrocyte/oligodendroglia
Ependymal cell
Perineuronal/satellite
Intrafascicular
Resting microglia Gitter cell
Choroidal Epithelial cell
TanycyteEpendymocyteProtoplasmicFibrous
A. Neuroglia of CNS (central neuroglia):
Classification:
B. Neuroglia of PNS (peripheral neuroglia):(some authors regard supporting cells of PNS are not
neuroglia. They regard supporting cells of CNS as neuroglia)-
• Schwann cell/neurolemmal cell/neurolemmocyte• Capsular cell/satellite cell/perineuronal cell/ganglionic
gliocyte/amphicyte
• Others: Pituicyte, Bergman glial cell, Muller cell
Astrocyte• Most numerous glial cell• Star shaped (stellate) body with numerous
radiating process
Astrocyte• Cell body and process contain intermediate
filament, named GFAP (gliofibrils)• Source of development- Neuroectoderm (neural
tube)
• Perivascular feet, forms BBB• Outer glial limiting membrane, forms pia-glial
membrane• Inner glial limiting membrane, forms ependyma-
glial membrane
• Fibrous astrocyte- found in white matter, each process is long, slender, smooth, not much branched. Number of filament numerous.
• Protoplasmic astrocyte- found in gray matter, each process is shorter, thicker, more branched. Number of filament fewer.
Functions of astrocytes• Structural support-for nerve cell bodies and fibers• Act as a scaffolding for migration of young nerve
cell during IUL
• Act as electrical insulator by covering synapse• Form barrier for spread of neurotransmitter• Absorb neurotransmitters and limit actions• Take up excess K+ from ECF• Form BBB• Store glycogen and help in nutrition of nerve cells• Phagocytosis• Replacement gliosis• Secrete trophic substances for neurons
Blood brain barrier
The composite barrier that exist between blood in the vessel and neurons of brain is called blood brain barrier.
Composition:1. Nonfenestrated capillary endothelium and tight junctions
between them.2. Continuous basement membrane of capillary
endothelium.3. Perivascular foot process (and cell bodies) of astrocytes.
Functions:1. Protection of brain from circulating harmful
substances of blood.2. Selective transport of substances from blood by
specialized transport system.
Blood brain barrier
The barrier is absent in:Median eminence of hypothalamusPosterior pituitaryPineal body Area postremaOVLTIntercolumnar tubercleSubfornical organChoroid plexus
Oligodendrocyte (oligodendroglia)• Small cell body with few delicate process (hence
oligodendro)• No intermediate and other filaments• Location:
1. gray matter around nerve cell bodies as satellite cell 2. white matter in rows along myelinated nerve fiber
• Source of development- Neuroectoderm (neural tube)
Functions of oligodendrocyte• Intrafascicular/perifascicular oligodendrocyte- Formation
and maintenance of myelin sheath (medullary sheath) around nerve fibers in CNS
• Perineuronal satellite oligodendrocyte- Surround nerve cell bodies and influence biochemical environment of neuron.
Microglia• Smallest of all neuroglia• They are part of MPS• Small cell body with wavy branching process that give off
numerous spine-like projections• Possess amoeboid movement when activated.• Location: gray and white matter• Source of development- macrophage (mesenchyme).
They migrate to CNS during fetal life.
Functions of microglia• Normally they are inactive- resting microglia• Enlarge and proliferate in disease condition- Gitter cell-
help in phagocytosis, act as scavenger cells of CNS, act as APC
Ependymal cell• Source of development- Neuroectoderm (neural
tube).• Have no basal lamina.
1. Ependymocyte- cuboidal or low columnar shaped cell, with cilia and microvilli, line ventricles of brain and central canal of spinal cord.
2. Tanycyte- line floor of the 3rd ventricle overlying median eminence of hypothalamus, possess long basal processes.
3. Choroidal epithelial cell- cover surfaces of choroid plexus. Form blood-CSF barrier.
Functions of ependymal cell1. Ependymocyte- Help in circulation of CSF by ciliary
beat. Absorb CSF by microvilli.2. Tanycyte- Transport chemical substances from CSF
to hypophyseal portal system and play a part in control of hormone production by anterior pituitary.
3. Choroidal epithelial cell- Production and secretion CSF from choroid plexus.
Blood CSF barrier
The composite barrier that exist between blood in choroid capillary and CSF within ventricles of brain is called blood CSF barrier. It is a weaker barrier than blood brain barrier.
Composition:1. Fenestrated capillary endothelium.2. Continuous basement membrane of capillary endothelium.3. Pale cell layer.4. Continuous basement membrane of choroidal epithelial
cell.5. Choroidal epithelial cell and tight junctions (performing
actual barrier function) between them.
Functions:1. Protection of brain from circulating harmful
substances of blood.2. Selective transport of substances from blood by
specialized transport system
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Schwann Cells (Neurolemmocytes)
Derived from the neural crest.
Found only in the PNS
Responsible for myelination in PNS
One Schwann cell forms myelin around a segment of one axon, in contrast to the ability of oligodendrocytes to branch and sheath parts of more than one axon.
Satellite Cells of Ganglia/Ganglionic Gliocytes
Derived from the neural crest.
Form a covering layer over the large neuronal cell bodies in PNS ganglia.
The satellite cells exert a trophic or supportive role, but the molecular basis of their support is poorly understood.
Synapse Specialized areas of contacts between two or more
neurons. Now-a days neuromuscular junctions are also called synapse.
Classification of synapse-1. Chemical synapse2. Electrical synapse
Chemical synapseSynapse where transmission of impulse occurs through chemical substances called neurotransmitters.Structure of chemical
synapse-• Presynaptic neuron• Presynaptic
(synaptic/terminal) knob (bouton terminale/end feet)
• Presynaptic (transmitter) vesicle (synaptosome)
• Presynaptic membrane, presynaptic density, release site, voltage gated Ca ++ channel
• Postsynaptic membrane, postsynaptic density (synaptic or subsynaptic web)
• Synaptic cleft (neuropil area) 20nm
• Synaptic delay (0.5msec)• Enzymes• Neurotransmitters-
excitatory/inhibitory• Receptors-
excitatory/inhibitory• Parts of a receptor protein-
• binding component• ionophore component• 2 types-
1. ion channel- cation channel/anion channel
2. 2nd messenger activator • EPSP/IPSP
Types-
Structurally and functionally-
1. Asymmetrical/type 1/excitatory synapse
2. Symmetrical/type 2/inhibitory synapse
According to synapsing structures-
1. Axodendritic- 80-95%
2. Axosomatic- 5-20%3. Axoaxonic4. Dendrodendritic5. Somatodendritic6. Somatosomatic
1. Small molecule, rapidly acting transmitters Class I
Acetylcholine- Both excitatory and inhibitory
Class II: The Amines Norepinephrine- Both excitatory and
inhibitory Epinephrine- Both excitatory and
inhibitory Dopamine- Both excitatory and
inhibitory Histamine Serotonin- Inhibitory
Class III: Amino acids GABA- Inhibitory Glycine- Inhibitory Glutamate- Excitatory Aspartate- Excitatory
Class IV NO
2. Large molecule, slowly acting transmitters (Neuropeptides)
• Principal neurotransmittersAcetylcholine
(nicotinic)GlutamateGABA
• NeuromodulatorsAcetylcholine
(muscarinic)SerotoninHistamineAdenosine Neuropeptides
Neurotransmitters/NeuromediatorsChemical substances secreted by presynaptic neuron at synaptic cleft of chemical synapse which transmit nerve impulse to the post synaptic neuron. Classification-
Structure of autonomic ganglion
Connective tissue capsule
Cell bodies of post ganglionic neurons
Interneurons – SIF cells
Satellite/capsular cells
Nerve fibers-Pre- and postganglionic fibers
and fibers traversing the ganglion without synapse
GanglionCollection of (cell bodies of) neuron outside CNS is
called ganglion.
Structure:• Ovoid in shape • Contain collection of cell bodies of neurons-
(pseudo)unipolar or bipolar in case of sensory ganglion, multipolar in case of motor ganglion.
• Each cell body is covered by a layer of capsular/satellite cells.
• Each ganglion is covered by a connective tissue capsule.
Classification
Sensory ganglion (Somatic ganglion)• Cranial ganglion
• Trigeminal ganglion of C5• Geniculate ganglion of C7• Vestibular and spiral ganglion of C8• Superior and inferior ganglion of C9• Superior and inferior ganglion of C10
• Spinal ganglion• Dorsal root ganglion
Motor ganglion (Autonomic ganglion)Sympathetic ganglion• Lateral (paravertebral) ganglia- Ganglionated
sympathetic trunk• Subsidiary ganglia
• Collateral (prevertebral) ganglia• Superior mesenteric ganglion• Inferior mesenteric ganglion• Aorticorenal ganglion (renal)• Superior hypogastric ganglion
• Terminal ganglia- Adrenal medulla
Motor ganglion (Autonomic ganglion)Parasympathetic ganglion• Collateral ganglia (cranial parasympathetic
ganglia)• Ciliary ganglion• Pterygopalatine ganglion• Submandibular ganglion• Otic ganglion
• Terminal ganglia- close to or within wall of viscera
• Ganglia of cardiac plexus• Ganglia of pulmonary plexus• Ganglia of myenteric plexus• Ganglia of submucosal plexus• Inferior hypogastric ganglion
Myelination
CNS-By OligodendrocyteOf spinal cord-at 4th month of IUL, sensory fibers firstOf brain-at 6th month of IUL, motor fibers first
PNS-By Schwann cell
Structure of a myelinated nerve fiberOf CNS-
AxonMyelin sheath
Of PNS-AxonMyelin sheathNeurilemmaBasement membraneEndoneurium