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