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

- 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

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

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

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

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