Nervous Tissue
I. General Definitions
A. CNS vs PNS
§ CNS- brain and spinal chord
§ PNS- outlying nervous tissue
B. Gray vs White Matter (CNS)
§ Brain- gray outside, white inside
§ Spinal chord- gray inside, white outside
§ White matter- axons, vessesl, glia, no neurons
§ Gray matter- neurons, glia, neuronal processes, vessesl
C. Location of Neuron Cell bodies
§ CNS- neuron cell bodies only in gray matter (brain- located in nuclei, column, or layers); axons course in bundles called tracts
§ PNS- neuron cell bodies found in ganglia; axons course together in nerves
D. Meninges
§ Covering of CNS-brain, brain stem, spinal chord
§ Dura mater- (pachymeninges)-outer thickest layer
§ Arachnoid mater-(leptomeninges)-thin membrane with overlying flattened meningothelial cells (arachoid cap cells), avascular; sends think fibrous trabeculae down to attach to pia
§ Pia mater-(leptomeninges)- closest to surface of brain and spinal chord
§ Subarachnoid space between pia and arachoid; contains cerebrospinal fluid; blood vessels run on top of pia and within arachoidal tabeculae
II. Embryologic Origin of nervous tissue
A. Neurectodermàneural plateàneural grooveàneural tubeàCNS
B. Neural crest
§ Postganglionic neurons of ANS
§ Schwann cells
§ Satellite cells
§ Sensory nerves of cranial and spinal ganglia
§ Chromaffin ceels of adrenal medualla
§ Melanocytes of epidermis
§ Pia mater and arachnoid
III. Histologic feature of nervous tissue
A. Cell types
§ Soma- cell body
§ Axon-transmits impulses from soma
§ Dendrite-incoming processes
Classification of Neurons
§ Unipolar
Single axon, branching at terminal end
Not in adult humans, common in embryos and invert
§ Pseudounipolar
Anatomically- one cell process from soma
Developmentally- begin as bipolar neurons that grow together and fuse for appearance of one process
Functionally- one process carries impulses to cell; other carries it away
Adult pseudounipolar appears uniplar, but develop and func bipolar
Found in peripheral sensory ganglia (dorsal root ganglia); incoming signals do not route through body of neuron, pass directly from incoming to outgoing process
§ Bipolar
Single axon and dendrite arise at opposite poles of cell body
Found in sensory neurons such as retina, olfactory and auditory
§ Mulltipolar- Most Common
Multiple dendritic processes, one axon
Cell body found in brain, spinal cord, peripheral ganglia
Details of Multipolar Neuron
§ cell body (soma, perikaryon)
surfaces have receptor site for numerous nerve endings
cyotoplasm usual organelles
some may contain a lot of melaninàdark nuclei
with aging- lipofuscin granules can occupy half of cytoplasm
nissel substance- well developed poly ribosomes and RER- seen in motor neurons of sp.ch.
abundant neurofilaments- intermediate filaments of perikaryon cytoskeleton
anterograde- from cell body down axon (away)
microtubules play large role in movement
slow flow compenent (1-4.5 mm/day) is seen with cytoskeleta elements such as microtubules and microfilaments this is maximal rate of regeneration of nerve fiber after injury
neurotransmiteers, hormones, and protein for membrane renewal move much faster (40 mm/day)
§ nucleus
single large nucleus, centrally placed
movement to periphery if injured
prominent nucleolus
§ cell processes
dendrites
numerous short processes that branch with distance from perikaryon
provide receptor sites for up to 200,000 terminals from other neurons
cary impulses to soma
contain nissl bodies, mitochondria, neurofilaments, microtubules, NO GOLGI
axons
single axon
transmit impulses from perikaryon
unbranched until terminates
constant in diameter
arises from axon hillock- NO GOLGI, NO NISSEL SUBSTANCE
axolemma- axon plasma membrane; surrounded by protective layers provided by glial cells
axoplasm- axon cytoplasm-contain longitudinal eurotubules and neruofilaments, mitoch, SR, NEVER NISSEL OR GOLGI
somal-axonal communication is two way stret: destruc of certain amount of axon will reselt in retrograge changes at level of soma (movement of nucleus to periphery, chromatolysis-destruc of nissel sub, cell death)
polio and herpes travel up and down axons, resting in dormant periods in soma of affected neurons
synapses
contact points between axon and dendrite, perikaryons, or other axons
axon terminates in round membrane call bouton terminaux
actual synapse formed by thicken presynaptic and postsynaptic membranes with 20-20 nM cleft b/t
thickening due to dense layer of cytoplasmic protein
each bouton contains vesicles with neurotransmitters wich are released with arrival of AP, diffuse across synapse, act upon receptors on postsynaptic membrane
membrane depolarizes and another AP is generated
Neuroglia
§ Glial Cells of CNS
Macroglia
Astrocytes
Largest of neuroglia
Large oblong nuclei and long tentacle-like processes that give them stellate appearance
Cytoplasm contain abundant glycogen and eosino intermediate fil unique to this cell type: glial fibrillary acidic protein (GFAP)
Function- distal processes expand to form “end feet” which separtes pia from underlying nervous tissue, separate capillaries from surrounding tissue
Form interface between neural tissue of brain and spinal chord where they meet meninges and blood vessels
Function- physical support and electrochemical regulation of neurons
Proliferate and form scar tissue following injury and loss of neurons
Oligodendroglia
Smaller than astrocyte
Small round dense nuclei and clear cytoplasm
GFAP and glycogen not present
Fuction- responsible for myelination in CNS, one oligo can myelinate several axons
Microglia
Smaller than other glial cells
Small dense, elongated cellular and nuclear shapes
Appear only following injury and phagocytose debris
Part of mononuclear phagocyte system,, mesodermal origin
Primarysite of HIV infection within CNS
Ependymal Cells
Cuboidal to columnar cells with microvilli and cilia lining ventricles and central canal of spinal chord
Derived from innermost epithelial lining of neural tube and retain epithelial appearance and polarization
Function-line ventricles to form barrier between brain and cerebrospinal fulid
Within ventricular spaces, ependymal cells cover outgrowth of fibrovasuclar cores that differentiate into cerebrospinal fluid CSF- secreting cell of choronoid plexus
§ Glial cells of PNS
Satellite Cells
Closely cluster around and provide support for neuron (ganglion) cell bodies in peripheral autonomic or sensory ganglia
Neurocrest origin
Schwann Cells
Enshealthment of axon in PNS
Neurocrest origin
B. Nerve fibers
Unmylinated Fibers
§ CNS- no shealth or supporting cell cytoplasm to surround naked axons
§ PNS- small axons embedded and encircled by cytoplasm of schwann cells, singe cell may embrace a dozen axons
Mylinated Fibers
§ Axons become myelinated are larger than 1 mM in diameter
§ Myelin deposited just beyond axon hillock and continues to region of termination
§ CNS- myelin is laid down by oligodendroglia, each one being able to myelinate several axons simultaneously
§ PNS- cytoplasm of schwann cell envelops myelin, neurilemmal sheath, one schwanncell myelinates one axon
C. Process of myelination
§ Schwann cell completely wrapps itself repeatedly around an axon
§ Cytoplasm being squeezed out and adjoining plasma membranes come together and fuse
§ Myelin composed of fused pasmalemmae, rich in lipid
§ Myelin does not form continuous layer, interrupted at regular intervals (Nodes of Ranvier)
§ Distance between two nodes is internode (represents distance covered by single schwan cell
§ Electrical resistance lowest at node
§ Permit skipping or salutatory movement of electrical current down axon
§ Increase velocity of conduction of nerve impulse
§ CNS-oligodendrocytes responsible for formation of myelin
§ Single oligo can myelinate multiple axons
D. Nerves
§ Peripheral nerve is collection of nerve fibers grouped into bundles. Several tunics surround and divide peripheral nerve:
Epineurium
§ Dense fibroconnective tissue that surrounds fascicles of nerve fibers
§ Also contains some elastic (elastic recoil upon stretchin)
§ Arteries, veins, and lymphatics run longitudinally within epinurium
Perineurium
§ Surrounds each bunde of nerve fibers
§ Concentric layers of flattened epithelial cells separated by collagen
§ Inner perineurial cells interconnect with tight junctions to form blood nerve barrier to exclude macromolecultes
§ As caliber of nerve fascicle decreases, layers of perineurial cells decrease
§ Blood nerve barrier is absent in dorsal root ganglia and autonomic ganglia (permit toxicity to substances ie mercury)
Endoneurium
§ Compartment within perineurial sheath
§ All components of nerve fascile within perineurum are endoneeurium
§ Includes axons, schwann cells, fibroblasts, capillaries, ground substance, mast cells
Tags: Axon, axons, bipolar, brain, brain stem, CNS, cyotoplasm, Dendrite, dura matter, ganglia, gray matter, herpes, meninges, microtubules, Nervous Tissue, Neurocrest origin, neuroglia, Neuron Cell, Oligodendroglia, pachymeninges, PNS, ppolio, Pseudounipolar, schwann cells, Soma, somal-axonal communication, spinal chord, synapses, Unipolar, Unmylinated Fibers, white matter
