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Zoology 142 Nervous Tissue – Ch 12 Dr. Bob Moeng Nervous Tissue Homeostatic Control • Nervous system and endocrine system are basis of homeostasis – Nervous system responds rapidly (in msec) – Endocrine system responds slowly • Three major components of control – Sensing internal or external levels – Interpreting and integrating – Reacting (motor) via muscle or glandular (effectors) stimulation General Organization of NS • Central vs. peripheral – Central: brain & spinal cord – Peripheral: cranial and spinal nerves • Sensory (afferent) vs. motor (efferent) • Somatic, autonomic & enteric – Somatic (voluntary): sensory neurons from cutaneous and special sense receptors and motor neurons to skeletal muscle – Autonomic (involuntary): sensory neurons from visceral organs and motor neurons to smooth & cardiac muscle and glands – Enteric (involuntary): sensory neurons from chemical & stretch receptors in GI tract, enteric plexus, motor neurons to smooth muscle and secretory functions • Autonomic connections to CNS • Sympathetic vs. parasympathetic – Sympathetic: regulates energy expenditure – Parasympathetic: regulates energy restoration and conservation Organization of NS (graphic) Histology of Nervous Tissue • Major cell types – Neurons (conducting cells) and neuroglia (support cells) • Neuroglia – Glia - glue; but much more – 5-50X the number of neurons – Can mitotically divide Neuroglia • Types – Astrocytes: manage interstitial environment - metabolism of neurotransmitters, balance of K+ ions, help form blood-brain barrier – Microglia: protect CNS from foreign and damaged materials through phagocytosis (derived from mesodermal cells that also give rise to monocytes & macrophages) 1 Zoology 142 Nervous Tissue – Ch 12 Dr. Bob Moeng – Ependymal cells: epithelial cells that line ventricles of brain & central canal of spinal cord, produce CSF and promote its circulation (many are ciliated) – Oligodendrocytes: most common glial cell in CNS, form myelin (w/o neurolemma) – Neurolemmocytes (Schwaan cells): form neurolemma and myelin sheath around neurons in PNS – Satellite cells: support cell bodies in PNS ganglia Types of Neuroglia (graphic) Myelin • Composed of lipid & protein concentrically laid membrane (up to 100 layers) • Enhances rate of conduction of excitation and “insulates” • Myelination different between CNS and PNS • Presence of neurolemma increases probability of neuron regrowth • Myelination increases during early childhood increasing rapidity and coordination of response • Multiple sclerosis – Myelin produced by oligodendrocytes deteriorates – Auto-immune response – Slows conduction and short-circuits excitation Formation of Myelin (graphic) Gray and White Matter • In CNS, clear difference between myelinated and unmyelinated tissue • Gray - areas w/o myelin including unmyelinated regions of myelinated neurons, unmyelinated neurons and neuroglia • Brain: gray matter on outside of cerebrum and cerebellum and inner nuclei (groupings of cell bodies and dendrites) • Spinal cord: H-shaped gray matter inside Gray vs. White (graphic) Neuronal Structure • Size – Varying length: <1mm in CNS to >1.5 m in PNS – Varying diameter: 5-135m which along with myelin affect conduction rates (1280mph) • Major parts – Dendrites (afferent) usually not myelinated, include rough ER (Nissl bodies), mitochondria, and other cell organelles – Cell body (or soma) include nucleus, Golgi, mitochondria, Nissl bodies, and other cell organelles • Clustered cell bodies in PNS form ganglia – Axon (efferent) plus axon hillock, initial segment, axon terminal • Nerve: in PNS, both sensory and motor neurons arranged in bundles surrounded by connective tissue • Tract: in CNS, bundled neurons w/o connective tissue 2 Zoology 142 Nervous Tissue – Ch 12 Dr. Bob Moeng – Synapse including synaptic end bulbs or varicosities with synaptic vesicles Microanatomyof Neuron (graphic) Tissue Cultured Neurons (graphic) Relational Structure • Multipolar - multiple dendrites and one axon – Typical of brain and spinal cord • Bipolar - one dendrite and one axon – Typical of many sensory neurons including retina, inner ear and olfactory • Unipolar - dendrite leads directly to axon – Typical of sensory neurons for touch and pain Sample Shapes (graphic) Axonal Transport • Slow axonal transport – Similar to cytoplasmic movement in other cells but usually only toward axon – 1-5 mm per day • Fast axonal transport – Organelle or molecular movement by protein motor, bi-directional – 200-400 mm per day – Important for long axonal lengths – Various viruses and toxins transported this way • Herpes & rabies virus, toxin from tetanus bacteria Physiology of Excitable Cells • Membrane potential: voltage difference between inside and outside of cell due to ionic concentration differences (e.g. Na+, K+, Cl- and others) – Resting potential - little current (ionic) flow – Graded potential - varying current flow over a short distance – Action potential - predictable current flow over a long distance • Current flow due to combined ion movement through channels and pumps Ion Channels • Leakage (non-gated) channels (some always open) • Gated channels (either opened or closed) – Voltage gated - respond directly to changes in membrane potential – Ligand (chemically) gated - respond to neurotransmitters, hormones, H+ and Ca2+ • Direct or G protein/2nd messenger mediated – Mechanically gated - respond to vibration, pressure or stretch Ion Channels (graphic) Resting Potential • Polarization due to separation of charge across cell membrane – Ranges from -40mV to -90mv in neurons (typical ~-70 mV) – Due to concentration gradients of ions across membrane and relative permeability of the ions through the membrane 3 Zoology 142 Nervous Tissue – Ch 12 Dr. Bob Moeng • Na+ and C- outside, K+ inside • Permeability of K+ 50-100 > than Na+ (leakage channels) – K+ equilibrium potential (-90 mV) has greatest influence over resting potential • Membrane permeability greater for K+ than Na+ or Cl– Na/K electrogenic pump moves ions in 3:2 ratio – Anions (Cl-) have little effect Ions Across Membrane (graphic) Graded Potentials • Voltage change due to ion flow through chemically (ligand) or mechanically gated channels • Amount of voltage change (graded) dependent on # of gates open at one time and how long – Change is localized (not conducted) – Change may be depolarization or hyperpolarization • Usually limited to dendrites and cell body of neurons, and many sensory cells • Synapse - postsynaptic potential, Sensory receptor - receptor potential, Sensory neuron - generator potential Graded Potentials (graphic) Action Potential • An all or none voltage change that starts when the neuron is depolarized to a threshold level by graded potential(s) • Carried (or conducted) over long distances • Amplitude, period (about 1 msec) and conducting rate are dependent on nature of neuron • Primary carrier of information in nervous system either alone, in sequence or spatially Phases of AP (graphic) AP Depolarization • Caused by rapid opening of voltage-gated Na+ channels to where polarization is reversed • Channels open at a threshold level (neuron specific) - ~-55mv • Initially Na+ is driven by both concentration & electrical gradients • Voltage-gated Na+ channels have an activation & inactivation gate • Activation gates opened by super-threshold voltage • Inactivation gates closed after specific period of time (a few 10,000th of a sec) • Number of Na+ ions that move are small relative to total concentration outside cell thus little change in gradient (important for next AP) • Electrogenic pump return the Na+ ions to exterior • Local anesthetics frequently prevent opening of Na+ channels (e.g. Novocaine or Lidocaine), also tetrodotoxin AP Gate Action (graphic) AP Repolarization 4 Zoology 142 Nervous Tissue – Ch 12 • • • Dr. Bob Moeng Voltage-gated K+ channels also open at threshold, but slowly K+ ion movement more evident (repolarization) as Na+ channels close Voltage current is reversed bringing neuron back to resting potential (usually some hyperpolarization prior to all K+ channels closing) Refractory Period • Absolute refractory period - neuron cannot be re-stimulated (Na+ channels are inactivated with both gates closed) • Relative refractory period - neuron can be stimulated by suprathreshold level (K+ channels are still open) • Refractory period determines rate of AP generation (frequency) – large diameter cells - ARP about 0.4 msec (2500 APs per sec) – small diameter cells - ARP about 4 msec (250 APs per sec) AP Conduction • Or propagation • In unmyelinated neuron, current flow in one membrane region affects voltage in adjacent region - continuous conduction – If Na+ channels are in resting state, they are activated – If Na+ channels are in inactive state, there is no effect – Thus APs are conducted in one direction • In myelinated neuron, current flow is through nodes of Ranvier only - saltatory conduction – Na+ and K+ channels open in nodal regions only – Thus AP jumps from node to node increase rate of conductance – Fewer membrane channels open per unit length of neuron decreasing the required work of the electrogenic pump AP Conduction (graphic) Conduction Rates • Conduction rate is dependent on neuron diameter as well as presence of myelin • A fibers - diameter of 5-20 m, short absolute refractory period, with conduction rates 12-130 m/sec – Typical of sensory and motor neurons needed for quick response • B fibers - diameter of 2-3 m, longer absolute refractory period, with conduction rates 15 m/sec – Typical of visceral sensory and ANS motor neurons to autonomic ganglia • C fibers - diameter of 0.5-1.5 m, longest absolute refractory period, with conduction rates 0.5-2 m/sec – Typically unmylenated neurons, many peripheral neurons associated with pain, and post-ganglionic ANS motor neurons Synapse • Functional junction between neurons and between neuron and effector 5 Zoology 142 Nervous Tissue – Ch 12 • Dr. Bob Moeng Gap junctions are a form of electrical synapse (e.g. intercalated discs of heart muscle, also CNS) - two way flow & fast • Chemical synapses (e.g. NMJ) - one way flow & slower – Review components – Synaptic delay about 0.5 msec Synaptic Action • AP arrives at presynaptic region of axon • Voltage-gated Na+ and Ca2+ channels open • Ca2+ moves inward • Ca2+ initiates exocytosis of synaptic vesicles • Neurotransmitter diffuses across synaptic cleft (20-50nm) and bind to receptors • Neurotransmitter receptors on postsynaptic membrane open ligand- gated ion channels • Na+, Ca2+ inflow - excitatory • K+ outflow or Cl- inflow - inhibitory Synaptic Action (graphic) Postsynaptic Potentials • Excitatory or inhibitory potentials depend on the neurotransmitter, receptor, and channels opened (EPSP or IPSP) • They are graded potentials (not propagated, varying amplitude, no refractory period) • Excitatory - opening of cation channels allowing flow of Na+, Ca2+, and K+ – Since Na+ furthest away from its equilibrium potential, more Na+ flow • Inhibitory - opening of Cl- or K+ channels Excitatory Synapse (graphic) Removal of Neurotransmitter • Diffusion • Enzymatic breakdown – e.g. acetycholinesterase • Recycling uptake – By presynaptic neuron or neuroglia – Involves neurotransmitter transporters in membrane – Cocaine blocks transporters of dopamine (brain NT) causing heightened stimulation – Prozac (anti-depressant) inhibits uptake of serotonin Postsynaptic Summation • CNS neurons may have 1000s of synapses • Spatial summation • Temporal summation • Summed effect on postsynaptic cell – Excitatory PSP 6 Zoology 142 Nervous Tissue – Ch 12 Dr. Bob Moeng – Action potential (sufficient EPSP at initial segment) – Inhibitory PSP Examples of Summation (graphic) Neurotransmitters • 100 or so possible substances • Modulation – Rate of synthesis – Blocked or enhanced release • Botulinum toxin inhibits release of ACh – Inhibited or enhanced removal – Blocked or enhanced active site • Curare blocks ACh receptors • Agonist vs. Antagonist – Therapeutic/drug related Acetylcholine • PNS neurons and some CNS neurons • Excitatory in NMJ • Inhibitory at heart via G protein/2nd messenger (parasympathetic vagus) Amino Acids • Glutamate and aspartate – Excitatory in CNS (some w/Ca+ gates) – Glutamate ~half of brain synapses and removed by transporters • Gamma aminobutyric acid (GABA) and glycine – Inhibitory via Cl - channels – GABA most prevalent inhibitory transmitter in brain – Valium (diazepam) is an agonist for GABA – In spinal cord, both about equally found Biogenic Amines • Catecholamines including nor-epi, epi and dopamine – Derived from tyrosine – Can be either excitatory or inhibitory – Removed by transporters to pre-syn. membrane – Rigidity of skeletal muscle in Parkinson’s disease due to relatively low dopamine production (inhibitory in brain) • Serotonin (5-hydroxytryptamine) Adenosine Derivatives • Adenosine, ATP, ADP, AMP • Excitatory in CNS & PNS • Frequently co-released with other transmitters Nitric Oxide Gas 7 Zoology 142 Nervous Tissue – Ch 12 • • • • Dr. Bob Moeng Derived from arginine Not packaged, but enzymatically formed as released Short-lived (10 sec) Lipid soluble (diffuses through membrane) - no vesicles, no receptor channels; 2nd messenger (GMP) in post-synaptic cell • Produced by endothelial cells of vessels to relax smooth muscle (↓BP); Viagra ↑ NO effect • Transmitter in brain and ANS Neuropeptides • CNS & PNS • Excitatory & inhibitory • Manufactured in cell body • First found when looking for effects of opiates • Include enkephalins, endorphins, dynorphins • Bodies natural painkillers? Effects of acupuncture? • Substance P – for communication of pain (PNS to CNS) – Inhibited by enkephalins • Others: Angiotensinogen II, CCK, releasing and inhibiting hormones from hypothalamus Neural Circuits • Simple series - no integration • Diverging – Increased number of terminal neurons; distribution to multiple regions • Converging – Increases likelihood of excitation or inhibition of terminal neuron or effector • Reverberating – Causes repetitive stimulation in terminal neuron – Inhibition required to stop – May be important for breathing, muscle coordination, short-term memory • Parallel after-discharge – Causes extra EPSP or IPSP in terminal neuron Integrating Circuits (graphic) Regeneration • Plasticity - new dendrites, synapses • In PNS, dendrite or axon w/o neurolemma damage may repair – Chromatolysis (breakdown of Nissl bodies), Wallerian (distal) degeneration, mitotic division of neurolemmocytes and formation of regeneration tube • In CNS, neither unmylenated or mylenated (by oligodendrocytes) neurons regenerate – Astrocytes fill space 8 Zoology 142 Nervous Tissue – Ch 12 – Research - stem cells & epidermal growth factor Regeneration (graphic) Epilepsy • READ 9 Dr. Bob Moeng