nervous tissue
DESCRIPTION
nervous tissue anatomyTRANSCRIPT
Permissible Noise Exposure
NERVOUS SYSTEM
most COMPLEX system in the HUMAN BODY
formed by a network of >100 MILLION nerve cells (neurons) assisted by many more glial cells
CELLS OF NERVOUS SYSTEM
NEURONS
Functional unit of structure of nervous tissue
For receptive, integrative and motor functions of the nervous system
5 150 um in diameter
NEUROGLIAL CELLS
For supporting and protecting neurons
Do not receive or transmit impulses
PARTS OF A NEURON
1. CELL BODY / PERIKARYON / SOMA
- Central portion which contains the nucleus & perinuclear cytoplasm
in CNS:
- generally polygonal with concave surfaces between many cell processes
in DRG (sensory ganglion of PNS):
- have a round cell body from which only 1 process exits
NUCLEUS
Large, spherical to ovoid, centrally located
Prominent nucleolus
Contains finely dispersed chromatin (may appear vesicular)
Less obvious in smaller neurons (greater conc. of chromatin)
Sex chromatin of females prominent
CYTOPLASM
With abundant RER with many cisternae in parallel arrays
NISSL BODIES
stacked RER cisternae & polyribosomes seen as clumps of basophilic material
represent sites of protein synthesis
SER
- Abundant, extends into the axons and dendrites
forms a broad, flat, fenestrated hypolemmal cisterna
sequester Ca and contain CHONs and provide a pathway for their distribution throughout the cell
GOLGI COMPLEX
Located only in the cell body
Consists of multiple parallel arrays of smooth cisternae arranged around the periphery of the nucleus
Responsible for packaging of neurotransitter substances
MITOCHONDRIA
Found in soma, dendrites & axon
Most abundant in axon terminals
More slender
Constantly moving along microtubules in the cytoplasm
CENTRIOLE
Characteristic of preneuronal multiplying cells during embryologic development
only occasionally encountered in adult neurons
believed to be vestigial structures (because neurons do not undergo cell division)
INCLUSIONS
A. Melanin coarse dark-brown/black granules
location:
- certain regions of the CNS (substantia nigra & locus ceruleus, dorsal motor nucleus of the vagus & spinal cord)
- sympathetic ganglia of the PNS
- thought to accumulate as by-product of the synthesis of neurotransmitters dihydrophenylalanine or methyldopa
B. Lipofuscin golden-brown granules
- irregular in shape
- remnants of lysosomal enzymatic activity
- increase with advancing age, may even crowd the organelles & nucleus to one side possibly affecting cellular function
Purkinje cells of cerebellar cortex
Iron containing pigments
C. Lipid droplet result of faulty metabolism or normal energy reserves
D. Secretory granules
- observed in neurosecretory cells
- many contain signaling molecules
CYTOSKELETAL COMPONENTS
microtubules
- 20 28 nm in diameter
- essential role in transport of vesicles & organelles that move along their surface w/in the cell body & along the length of the axon
neurofilaments
- intermediate, 10 nm in diameter
- abundant in perikaryons & cell processes
microfilaments
- 6 nm in diameter
- composed of 2 strands of polymerized G-actin arranged in a helix
neurofibrils
- up to 2 um in diameter
- possibly represent clumped bundles of neurofilaments
2. DENDRITE
- cell body projections
-with abundant mitochondria
- receives stimuli from sensory cells, axons and other neurons
- impulse received are transmitted towards soma
Dendrite branching pattern permits a neuron to receive & integrate multiple impulses
Some have SPINES (permit dendrites to form synapses with other neurons)
Sometimes contain vesicles & transmit impulses to other dendrites
3. AXON (axis cylinder)
Varying diameter
Usually very long processes (may be up to 100cm in length)
1 neuron: 1 axon
Conducts impulses away from the soma to other neurons, muscles or glands
Axolemma - cell membrane
Axoplasm axon cytoplasm
Axon Hillock where axon arises, absent RER
Collateral branches
Axon Terminal
Nerve Fiber - axon + certain sheaths of ectodermal origin
SYNAPSE region where impulses can be transmitted between cells
Functions:
Impulse conduction
Axonal transport
crucial to trophic relationships (within axons & between neurons & muscles and glands)
Interruptions lead to atrophy of target cells
Anterograde transport from cell body to axon terminal; MAP : kinesin
Retrograde transport from axon terminal to the cell body; MAP : dynein
Clinical Correlate
Retrograde Axonal Transport
pathway followed by toxins ( e.g., tetanus toxin) and neurotropic viruses (e.g., herpes simplex and rabies) to penetrate and invade the CNS
Axon : MYELIN SHEATH
MYELIN
Fatlike substance covering axons
concentric layers of mixed lipids alternating with thin layers of the protein neurokeratin
associated only with axons
* Unmyelinated Axons
* Myelinated Axon
Produced by Oligodendrocytes (CNS), Schwann cells (PNS)
Structure of MYELIN SHEATH
Nodes of Ranvier sites of discontinuity between successive Schwann cells along the axon
Internodal segments consists of a singular Schwann cell & its concentric lamellae of myelin around the axon, delineated by successive nodes of Ranvier
Incisure of Schmidt-Lantermann aligned sites of local separation of the myelin lamellae by residues of cytoplasm trapped in the spiral
Functions of Myelin Sheats
A. Increases the speed of conduction from 1 m/s in slender unmyelinated axons to 120 m/s in heavily myelinated axons of large caliber
B. Serves as a high-resistance low-capacitance insulator
C. Role in nutrition of the axon
D. Protective role assuring continuing conductivityMECHANISM OF MYELINATION
- Schwann cell (or oligodendrocytes) concentrically wraps its membrane around the axon to form the myelin sheath
- wrapping may continue for more than 50 turns
- cytoplasm is squeezed back into the body of the Schwann cell bringing the cytoplasmic surfaces of the membranes in contact with each other forming the major dense line that spirals through the myelin sheath
PERIPHERAL NERVE SHEATHSEPINEURIUM
outermost sheath
envelops the nerve & sends extensions into it to surround the separate nerve fascicles w/in it
thick & strong investment composed of dense irregular connective tissuePERINEURIUM
covers each bundle of nerve fiber (fascicle)
more dense; consists of a few to several layers of flattened fibroblast-like cells bounded both internally & externally by a basal lamina
barrier to passage of particulate tracers, dye molecules/toxins into the endoneurium, thus protecting the perineural compartment
ENDONEURIUM
surround individual nerve fibers (axons)
delicate, loose connective tissue consisting of small fibrils of collagen, fibroblasts, fixed macrophages, capillaries, perivascular mast cells, & EC fluid
Classification of NEURONS
ACCORDING TO MORPHOLOGY:
BIPOLAR
posses 2 processes emanating from the soma, a single dendrite and a single axon
found in the vestibular & cochlear ganglia & in the olfactory epithelium of the nasal cavity
PSEUDOUNIPOLAR
when a single process, morphologically an axon, leaves the body and soon bifurcates
BRANCHES:
A. peripheral proceeds to its destination in the body
B. central enters the CNS
present in the dorsal root ganglia & the ganglia of some cranial nerves
MULTIPOLAR
most common
possess various arrangement of multiple dendrites emanating from the soma and a single axon
most are motor neurons
some are named according to morphology (e.g., Pyramidal cells) or after the scientist who 1st described them (e.g., Purkinje cells)
UNIPOLAR
posses only 1 PROCESS emanating from the cell body
exists in early embryonic life
ACCORDING TO FUNCTION:
A. SENSORY (AFFERENT) NEURONS
Receives & transmits impulses to the CNS for processing
B. MOTOR (EFFERENT) NEURONS
Originates in the CNS & transmits impulses to effector organs throughout the body
C. INTERNEURONS
located completely in the CNS
function as interconnectors or integrators that established networks of neuronal circuits between sensory & motor neurons and other interneurons
Synapse
Site of transmission of nerve impulses
Point of contact of a neuron & another cell
Allows neurons to communicate with each other or with effector cells (muscle & gland)
Types of SYNAPSES
A. ELECTRICAL
Uncommon
Few places in the brain stem, retina & cerebral cortex
Transmission is much more rapid
Transmit impulse through gap junctions that cross the pre- & postsynaptic membranes
Ions pass freely through these gap junctions
B. CHEMICAL
impulse transmission occur mostly through the release of neurotransmitters at axon terminal
Components:
A. Presynaptic membrane
B. Synaptic cleft
small gap between that separates the pre- & post-synaptic membranes
12-20 nm
may contain polysaccharides & some fine intersynaptic filaments
Postsynaptic membrane
Types of CHEMICAL SYNAPSE
AXONDENDRITIC axon synapses with a dendrite
AXOSOMATIC axon synapses with a cell body
AXOAXONIC axon synapses with another axon
DENDRODENDRITIC
SOMATODENDRITIC
SOMATOSOMATIC
SOMATOAXONIC
DENDROAXONIC
AXOAXODENDRITIC Presynaptic neuron neuron that transmits the impulse
Postsynaptic cell cell that receives the impulse (neuron, muscle or gl.)
Bouton expanded portion of the process that is involved in the formation of a synapse
Synaptic vesicles contain chemical neurotransmitters, fill the bouton
NEUROGLIAL CELLS
1. Astrocytes
largest & most numerous
star-shaped & have numerous, branching processes
involved in metabolic processes
form scar tissue in damaged areas
Have bundles of intermediate filaments made of glial fibrillary acid protein that reinforce their structure
Bind neurons to capillaries & to the pia mater
Types of Astrocytes
A. Protoplasmic
many short branching processes
abundant cytoplasm & bigger & paler-staining nucleus
found mainly within the gray matterB. Fibrous
with few long processes mostly unbranched
closely associated with the pia mater & blood vessels
located chiefly in the white matter
possess euchromatic cytoplasm containing only a few organelles, free ribosomes, & glycogen
2. Oligodendrocytes
smaller, fewer & shorter processes
scanty cytoplasm & smaller ovoid/spherical nucleus
located in the white matter where they form the myelin sheath
3. Microglia
Dense elongated nuclei
small cell with short processes
cytoplasm scanty & contains many lysosomes
phagocytic in nature
Represent the mononuclear phagocytic system in nervous tissue & derived from precursor cells in the bone marrow
Involved with inflammation & repair in the adult CNS
produce & release neutral proteases & oxidative radicals
4. Ependyma
Low columnar to cuboidal epithelial cells that line the cavities of the CNS
posses short cytoplasmic processes, free surface possesses microvilli
cytoplasm contains abundant mitochondria and bundles of intermediate filaments
some are ciliated, a feature that facilitates the movement of CSF
5. Schwann cells
flattened cells whose cytoplasm contains a flattened nucleus, small golgi apparatus and few mitochondria
form both myelinated & unmyelinated coverings over axons of the PNS
Origin and Principal Functions of Neuroglial Cells
Glial Cell TypeOriginLocationMain Functions
OligodendrocyteNeural ectodermCNSMyelin production,
Electric insulation
Schwann CellNeural ectodermPeripheral NervesMyelin production,
Electric insulation
AstrocyteNeural ectodermCNSStructural support,
Repair processes, BBB,
Metabolic exchanges
Ependymal CellNeural ectodermCNSLining cavities of central nervous system
MicrogliaMesoderm CNSMacrophagic
activity
NERVE REGENERATION: CNS
Connective tissue sheaths are absent in the CNS
Injured cells are phagocytosed by special macrophages (microglia)
Spaced liberated by phagocytosis is occupied by proliferation of glial cells form cell mass (GLIAL SCAR)
Glial cell mass hinder the process of repair thus damage to the CNS is permanentNERVE REGENERATION: PERIPHERAL NERVE FIBER
Neuron attempts to repair the damage, regenerate the process, and restore function
Axon reactions localized in 3 regions:
1. site of damage (local changes)
2. distal to the site of damage (anterograde changes)
3. proximal to the site of damage (retrograde changes)
Some changes occur simultaneously, others weeks or months apart
LOCAL REACTION
Involves repair & removal of debris by neuroglial cells
ANTEROGRADE REACTION
Portion of the axon distal to an injury undergoes degeneration and is phagocytosed
RETROGRADE REACTION & REGENERATION
Proximal portion of the injured axon undergoes degeneration followed by sprouting of a new axon whose growth is directed by Schwann cells
CNS
GRAY MATTER
- contains neuronal cell bodies, dendrites & the initial unmyelinated portions of axons & glial cells
- Region where synapses occur
- Prevalent at surface of cerebrum & cerebellum
- Nuclei aggregates of neuronal cell bodies forming islands of gray matter embedded in the white matter
CNS
WHITE MATTER
contains the myelinated axons & the myelin producing oligodendrocytes
it does not contain neuronal cell bodies
the myelin sheath imparts the white color
Parts of a NEURON
NUCLEUS
spherical/ovoid with unusually large, euchromatic (pale staining) nucleus with prominent nucleolus Nerve ProcessesDENDRITES
Provide the surface for receiving signals from other neurons
Relatively thick but taper gradually along their length
Fairly short & confined to the immediate vicinity of the soma
Bifurcate, at acute angles, into primary, secondary, tertiary & higher orders of branches
Found abundant in nerve cells (considerably increase the receptive area of the cell)AXONCELL TYPES
Nerve cells or neurons - show numerous long processes
a. Glial cells
- have short processes
- support & protect neurons
- participate in neural activity, neural nutrition, & defense processes of the CNSb. PERIKARYON OR CELL BODY
1. ORGANELLES
SER
Abundant
extends into the axons & dendrites
forms a broad, flat, fenestrated hypolemmal cisterna
sequester Ca & contain CHONs & provide a pathway for their distribution throughout the cell
transport vesicles & synaptic vesicles may bud off from it
Nissl Bodies
clumps of intensely chromatophilic material
consist of cisternae of granular ER in ordered parallel array
ribosomes are arranged in rows, loops & spirals on the outer surface of the cisternae
Axon Hillock
area from which the axon arises ER is absent
C. AXONS
Arise from the cell body at the axon hillock
A cylindrical process that varies in length & diameter according to the type of neuron
Constant diameter & do not branch profusely
All axons originate from a short pyramid-shaped region, the Axon hillock, that usually arises from the perikaryon
When severed, its peripheral parts degenerate and die
MOTOR COMPONENT
SOMATIC SYSTEM
Impulses transitted directly via a single neuron to skeletal muscles
AUTONOMIC SYSTEM
Impulses transmitted to an autonomic ganglion via 1 neuron
second neuron (from autonomic ganglia) transmits impulses to smooth muscles, cardiac muscles or glandsQUIZ!!
1. identify picture shown: fibrous astocyte (answer)
2. identify: perineurium
3. what are microglia for? Phagocytic (answer)
4. identify types of synapse: axoaxoneme (answer)
5. identify type: pseudounipolar (answer)
6. give 2 functions of myelin: protective and conduction of impulse
7. microtubule for anterograde transport: Kinesin (answer)
8. draw a neuron include the three main structure:
9. axon, dendrite and
10. soma