Histology                                                                                                         Stanley Shostak
BioSc 1450                                                                                                      Spring 05

Lecture 6. Nervous tissues, Spinal Cord and Peripheral Nervous Systam (PNS)

neurons (nerve cells) and neuroglia (glial cells) with all their processes and external laminae plus connective tissue sheaths

Staining methods: intermediate filaments (neurofilaments), lipids (for myelin sheath) and chemical & histochemical methods (for neruotransmitters) and ELISA: immunochemical- fluorescent methods (glial fibrillary acidic protein); metal impregnation (thick sections; spreads); Sudan black; formalin-induced fluorescence (for sympathetic noradrenergic nerve endings)

Neurons: specialized cells: sensory receptors, conducting pathways, sites of integration and analysis; lg cell body; perikaryon - nucleus and surrounding cytoplasm; terminally differentiated

nucleus: chromatin dispersed; nucleolus conspicuous

chromatophilic substance (nissl substance [bodies])

neurofilaments: intermediate filaments (provide structural support?)

microtubules: axonal transport of neurotransmitter substances, enzymes, membrane and cellular constituents

golgi: diffuse

multivesicular bodies: transport to organelles

 Processes: Note: may regenerate if neuron cell body is intact dendrites: drawn out extensions of cell; highly branched, tapering, either end in specialized sensory receptors (primary sensory neurons) or form synapses with neighboring neurons; receive stimuli; information input; generally convey impulse toward nerve cell body (afferent); contains rER; sER prominent;

axon (commonly: nerve fibers): differentiated (specialized) extension of cell; single; arises from axon hillock; cylindrical process (may be one meter long); terminates on other neurons or effector organs through branches ending in terminal boutons; generally convey impulse away from nerve cell body (efferent); has no rER (Nissl bodies) beyond hillock except in motor end plate with striated muscle; sER prominent; extremely slender and elongate mitochondria

 Basic neuron types: arrangement of axon and dendrites with respect to cell body. multipolar neuron: most common; numerous dendrites project from cell body; intermediate, integratory and motor neurons

bipolar neuron: single dendrite arises opposite origin of axon; receptor neurons for senses of smell, sight and balance

pseudo-unipolar neuron: primary sensory neurons; single dendrite and axon arise from common stem formed by fusion;


Anatomical division of nervous system:

CNS devoid of collagen except in vicinity of blood vessels and meninges; contains no lymphocytes

blood-brain barrier: CNS capillaries impermeable to certain plasma constituents especially larger molecules; absent in choroid plexus, pituitary and pineal glands and vomiting center of hypothalamus capillary endothelium: junctions btwn endothelial cells are sealed; little or no pinocytosis in endothelium; luminal surface membranes contain enzymes which destroy neurotoxic metabolites (neuroactive humoral substanes)

astrocyte foot processes: maintain barrier

 White matter (myelin): tracts of nerve fibers; substantial numbers myelinated

Gray (grey) matter: almost all neuron cell bodies (perikarya) and axons and neuropil: feltwork of axons and dendrites surrounding neurons and neuroglia; esosinophilic; generally devoid of myelin

non-myelinated nerve fibers: note: in PNS when neurolemmocyte only investment: small diameter axons (autonomic nervous system and small pain fibres)
myelinated nerve fibers: increased velocity of action potential;
Myelin sheath formation begins in the CNS of the human embryo at about 4 months gestational age with the formation of most sheaths at least commenced by about the age of one year. From this time, successive layers continue to be laid down with final myelin sheath thickness being achieved by the time of physical maturity.

Choroid plexus: arises from wall of four ventricles of brain; produces cerebrospinal fluid (CSF); consists of mass of capillaries projecting into ventricle; lined by modified ependymal cells separated from underlying capillaries and suporting tissue by basement membrane; long bulbous microvilli project from luminal surface ; continuous tight junctions (zonula occludens) form blood-CSF barrier

PNS

Spinal ganglia: on posterior nerve roots as they pass through intervertebral foramina primary sensory neurons: pseudo-unipolar; fascicle of nerve passes to center of ganglion; ganglion cells peripheral;

capsule cells = satellite cells (neural crest origin) surround each neuron; surronded by CT sheath continuous with perineurium-epineurium of peripheral nerve

 Sympathetic ganglia: multipolar ganglion cells widely spaced, separated by numerous axons and dendrites;; ganglion cells nuclei eccentric; peripheral cytoplasm contains variable quantity lipofuscin granules (brown) in residual bodies;

supporting cells = satellite cells small in number, irregularly placed.

 Parasympathetic ganglia: within or near effector; cell bodies may form well-organized ganglia but more commonly a few cell bodies are clumped with scattering supporting cells (between muscle layers) neurons: large nuclei with dispersed chromatin and prominent nucleoli; extensive basocphilic cytoplasm

afferent and efferent nerve fibers

 CT: endoneurium (loose, vascular ct surrounds neurolemmocytes) ; perineurium (dense ct surrounds fascicles); epineurium (loose ct [condensed peripherally] around peripheral nerves with more than one fascicle)

peripheral nerves: one or more bundles = fascicles of nerve fibers;

fascicle: surrounded by perineurium

each fiber contained in neurolemmocyte; surrounded by endoneurium

 neurolemmocyte (Schwann cell): invest all axons in PNS; variable number of axons per neurolemmocyte external lamina: neurolemmocytes (i.e., individual cells) have external lamina but neurons do not

Non-myelinated Nerve Fibers:

when neurolemmocyte only investment: small diameter axons (autonomic nervous system and small pain fibres)

neurolemmocytes: extend short distance along axons; abutt each other end to end.

mesaxon: zone of apposition of neurolemmocyte cell membrane

channel within neurolemmocyte: may be occupied by more than one axon

 Myelinated Nerve Fibers: increased velocity of action potential ensheathing neurolemmocyte forms myelin sheath; large diameter fibers Synapses and neuromuscular junctions = motor end plates; unidirectional but may be stimulatory or inhitibory; site of intercommunication btwn adjacent neurons main types of synapses axosodendritic

axosomatic

axoaxonic

 anatomy of synapse terminal bouton: not myelinated; contain mitochondria etc. synaptic vesicle: about 50 nm in diameter; derived from sER dense-cored vesicles: autonomic synapses presynaptic membrane: release contents of presynaptic vesicles upon arrival of action potential synaptic cleft (20 to 30 nm): uniform thickness; neurotransmitter diffuse across; contain hydrolytic and oxidative enzymes which inactivate released neurotransmitters;

postsynaptic membrane

receptors

postsynaptic web

 neurotransmitters: chemical transmitter substances initiate action potential in adjacent neuron or effector organ CNS: many types

PNS: 2 types

acetylcholine: neurotransmitter of somatic neuromuscular junctions; neurotransmitter of all preganglionic fibers in autonomic nervous system; main neurotransmitter of postganglionic parasympathetic neurons

noradrenaline (norepinephrine): main neurotransmitter of postganglionic sympathetic neurons (except sweat glands)

 motor end plate: contains concentraton of mitochondria and rER sole plate: recess in effector (muscle) cell surface; covered by extension of last neurolemmocyte (= Schwann cell); external lamina of neurolemmocyte merges with that of muscle fiber; endoneureum merges with endomycium

secondary synaptic clefts: deep folds in postsynaptic membrane; contain acetylacolinesterase

postsynaptic membrane: concentration of receptors for acetylcholine
 

 excitability: change in membrane permeability in response to stimuli > reversal of ionic gradient across membrane and plasma membrane is depolarized

action potential = wave of depolarization; depolarization spread along plasma membrane; faster in larger axons; incrased velocity with myelination

repolarization: membrane reestablishes resting potential

saltatory conduction: enhances conduction velocity; action potential travels by jumping from node to node

pathway: arrangement of integrating neurons conducting action potential from receptor to effector organ

effector organ: of voluntary nervous pathways: skeletal muscle of involuntary pathways: smooth muscle, cardiac muscle, muscle-like epithelial cells (myoepithelial cells) within some exocrine glands

ganglia: aggregations of primary sensory neurons and terminal effector neurons of autonomic nervous system at peripheral sites

   Neuroglia: non-neural cells of CNS; neurolemmocytes of PNS; ependyma lining ventricles and canal of CNS astrocytes: star-shaped with heavy metal impregnation; most numerous glial cells in gray matter; highly branched packing cells; form mass surrounding nerve cells processes; rounded, nuclei closely enmeshed in neuropil; mediate metabolic exchange btwn neurons and blood; regulate composition of intercellular environment in CNS.
  Glial fibrillary acidic protein (GFAP): unique intermediate filament; demonstrated by immunoperoxidase method
fibrous astrocytes: astrocytes of white matter with relatively straight cytoplasmic processes
protoplasmic astrocytes: astrocytes of gray matter with numerous short highly branched cytoplasmic processes
  glial limitans: relatively impermeable; foot processes invest basement membrane CNS and innermost layer of meninges (pia mater)

  perivascular feet: terminate upon basement membranes of capillaries; cover most of surface of capillary in CNS
 

 oligodendrocytes: invest axons in myelin; form multiple myelin internodes; contribute to ensheathment of as many as 50 individual axons; small rounded condensed nuclei; cytoplasm unstained by H&E; tend to be aggregated around neuron cell bodies; most numerous glial cell in white matter; analogous to satellite cells in ganglia; analogous to neurolemmocytes in PNS nerves light oligodendrocytes: capable of cell division; highly active in myelin sheath formation (predominant in fetus and neonate)

dark oligodendrocytes: main form in mature CNS

medium oligodendrocytes: immature form involved in myelination and maturation; capacity for remyelination after demyelinating disease such as multiple sclerosis

 ependymal cells: cuboidal or low columnar; luminal junctional complexes; no basement membrane; bases taper and break up into fine branches that ramify into underlying layer with astrocytes; luminal: variable number of cilia; microvilli also present; lining epithelium of ventricles and spinal canal
 microglia: monocyte-macrophage cells; invade CNS during fetal period; small irregular nuclei; relatively little cytoplasm forms fine, highly-branched processes; transform into large amoeboid phagocytic cells.

Myelination: formation myelin sheath: formed by oligodendrocytes in CNS and neurolemmocytes in PNS

mesaxon: zone of apposition of neurolemmocyte (oligodendrocyte) cell membrane that extends into jellyroll;

major dense lines: inner leaflets of plasma membrane fuse with each other; expands to form membrane loop containing cytoplasm as it approaches neurofibral node

intraperiod lines: represent closely applied outer membrane leaflets; multiple layers of membranes constitute myelin sheath; peripheral neurolemmocyte cytoplasm: main bulk of neurolemmocyte cytoplasm encircles myelin sheath; periaxonal neurolemmocyte cytoplasm: thin layer persists immediately surrounding axon

internode: single segment of myelin produced by each neurolemmocyte; internodal length function of diameter of axon; may be up to 1.5 mm in largest fibers

neurofibral node = node of Ranvier: interval at which axon not covered by myelin sheath; between neurolemmocytes; external lamina (basal lamina) continues across node

myelin fissure = Schmidt Lantermann cleft (incisures); narrow channels of uncompacted cytoplasm;


Functional division:

Somatic nervous system: voluntary functions

Autonomic nervous system: controls involuntary functions

Sensory receptors
 

functional classification:
exteroceptors: stimuli from outside: touch, light pressure, deep pressure, cutaneous pain, temperaure, smell, taste, sight and hearing

proprioceptors: located within skeletal muscle; conscious and unconscious information about orientation, skeletal position, tension and movement; vestibular apparatus of ear, tendon organs and neuromucsular spindles

interoceptors: viscera; chemoreceptors of blood, vascular (pressure) baroreceptors, receptors for state of distension of hollow viscera (gastrointestinal tract and urinary bladder; visceral pain, hunger, thirst, wellbeing and malaise
 

 morphological classification simple free nerve endings: branched or unbranched nerve endings of affarent fibers; small diameters; slow rates of conduction; devoid of myelin even where arising from myelinated nerves (e.g., nerves servicing tactile epitheliocytes); responsible for cutaneous pain and temperature;
tactile epitheliocyte = Merkel's cell; sensation of touch compound tactile body: corpuscle of touch; Meissner's corpuscle; reception of light discriminatory touch; oval shape; located in dermal papillae immediately beneath epidermis; delicate collagenous capsule surrounding mass of plump oval cells arranged transversely (specialized neurolemmocytes); non-myelinated branches of large myelinated sensory fibers ramify throughoug cell mass in helical manner lamellated corpuscle (Vater) Pacinian corpuscles; large encapsulated sensory receptors responsive to pressure or coarse touch, vibration and tension; in deeper layers of skin ligaments and joint capsules, some serous membranes, mesenteries, some viscera and erogenous areas; 1 to 4 mm in length; appearance of an onion; delicate capsule enclosing concentric lamellae of flattened (neurolemmocytes) cells separated by interstitial fluid space and delicate collage fibers; core contains single, unbranced non-myelinated nerve fibre with several club-like terminals; becomes myelinated upon leaving corpuscle
Ruffini corpuscles: robust spindle-shaped structures in soles of feet
Krause end bulbs: delicate receptors in lining of oropharynx and conunctiva of eye neuromuscular spindle: stretch receptor organs within skeletal muscle; regulate muscle tone via spinal stretch reflex; encapsulated, lymph filled fusiform up to 6 mm long by 1 mm in diameter; parallel to muscle fibers; embedded in endomysium or perimysium; 2 to 10 modified skeletal muscle fibers; evokes motor response from large, alpha motor neurons intrafusal fibers smaller than skeletal muscle fibers (= encapsulated extrafusal fibers); nuclei concentrated in central, nonstriated area
nuclear bag: central area dilated
nuclear chain nuclei arranged in row annulo-spiral endings: branched, non-myelinated endings of large, myelinated sensory fibers wrapped around central area
flower-spray endings: smaller, myelinated sensory fibers located on striated portions of intrafusal fibers; from Gamma motor fiber; increase sensitivity by causing contraction of striated portions of infrafusal fibers organs of special sense: eye, ear and receptors for senses of smell and taste

last revised: 01-19-05