Download - Nervous System
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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 o stacked RER cisternae &
polyribosomes seen as clumps of basophilic material
o 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)
INCLUSIONSA. 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
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- many contain signaling molecules
CYTOSKELETAL COMPONENTSmicrotubules - 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
processesmicrofilaments - 6 nm in diameter- composed of 2 strands of polymerized
G-actin arranged in a helixneurofibrils- 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 SHEATHMYELIN • 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 SheatsA. 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 axonD. Protective role assuring continuing
conductivity
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MECHANISM 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 SHEATHS
EPINEURIUM 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 tissue
PERINEURIUM 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 bodyB. 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
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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 dendriteAXOSOMATIC – axon synapses with a cell bodyAXOAXONIC – axon synapses with another axonDENDRODENDRITICSOMATODENDRITICSOMATOSOMATICSOMATOAXONICDENDROAXONICAXOAXODENDRITIC
• 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 CELLS1. Astrocytes
o largest & most numerouso star-shaped & have numerous,
branching processeso involved in metabolic processes
o form scar tissue in damaged areaso Have bundles of intermediate filaments
made of glial fibrillary acid protein that reinforce their structure
o Bind neurons to capillaries & to the pia mater
Types of Astrocytes
A. Protoplasmico many short branching
processeso abundant cytoplasm & bigger &
paler-staining nucleuso found mainly within the gray
matter
B. Fibrous o with few long processes mostly
unbranchedo closely associated with the pia
mater & blood vesselso located chiefly in the white matter o possess euchromatic cytoplasm
containing only a few organelles, free ribosomes, & glycogen
2. Oligodendrocytes o smaller, fewer & shorter processeso scanty cytoplasm & smaller
ovoid/spherical nucleuso located in the white matter where they
form the myelin sheath
3. Microgliao Dense elongated nucleio small cell with short processeso cytoplasm scanty & contains many
lysosomeso phagocytic in natureo Represent the mononuclear phagocytic
system in nervous tissue & derived from precursor cells in the bone marrow
o Involved with inflammation & repair in the adult CNS
o produce & release neutral proteases & oxidative radicals
4. Ependymao Low columnar to cuboidal epithelial cells
that line the cavities of the CNSo posses short cytoplasmic processes,
free surface possesses microvillio cytoplasm contains abundant
mitochondria and bundles of intermediate filaments
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o some are ciliated, a feature that facilitates the movement of CSF
5. Schwann cellso flattened cells whose cytoplasm
contains a flattened nucleus, small golgi apparatus and few mitochondria
o form both myelinated & unmyelinated coverings over axons of the PNS
Origin and Principal Functions of Neuroglial Cells
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 permanent
NERVE 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 cellsANTEROGRADE 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
CNSGRAY 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 Processes
DENDRITES 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)
Glial Cell Type Origin Location Main Functions
Oligodendrocyte Neural ectodermCNS Myelin production,Electric insulation
Schwann Cell Neural ectodermPeripheral Nerves
Myelin production,Electric insulation
Astrocyte Neural ectodermCNS Structural support,Repair processes, BBB,Metabolic exchanges
Ependymal Cell Neural ectodermCNS Lining cavities of central nervous system
Microglia Mesoderm CNS Macrophagic activity
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AXON
CELL TYPESNerve 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 CNS
b. 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 glands