Chapter 12: Nervous Tissue
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Chapter Outline and Objectives
OVERVIEW OF THE NERVOUS SYSTEM
Identify the structures that make up the nervous system.
Classify the organs of the nervous system into central and peripheral divisions and their subdivisions.
List and explain the three basic functions of the nervous system and indicate the direction of afferent and efferent information flow.
HISTOLOGY OF NERVOUS TISSUE
List the three parts of a neuron.
Describe the structures located in a neuron cell body and their functions.
List the names given to collections of cell bodies in the CNS and PNS.
Describe the function of the dendrites.
Describe the direction of information flow through an axon.
List the names given to collections of axons in the CNS and PNS and how they are bundled together.
Identify neurons on the basis of their structural and functional classifications.
Contrast the general functions of neuroglia and neurons.
Describe the relative number of neuroglia compared to neurons.
List the four types of neuroglia in the CNS and describe the function for each.
List the two types of neuroglia in the PNS and describe the function for each.
Identify the cells that produce myelin, describe how the sheath is formed, and discuss its function.
Define the neurolemma and discuss its function.
Describe the Nodes of Ranvier and tell why they are important for axon signal transmission.
Discuss multiple sclerosis in terms of anatomical changes and causes.
Describe the difference between gray and white matter, and give examples of each.
ELECTRICAL SIGNALS IN NEURONS
Distinguish between action potential and graded potentials.
Identify the basic types of ion channels and the stimuli that operate gated ion channels.
Describe the ions, channels, and integral-protein pumps that contribute to generation of a resting membrane potential.
Discuss the All or none principal in regards to neurons.
Discuss the features of the graded potential including areas where generated, size, properties, and type.
Describe the effect the sum of the excitatory and inhibitory stimuli has on the neuron.
List the sequence of events involved in generation of a nerve impulse.
Define and give a value for the threshold voltage.
Describe the events involved in depolarization of the nerve cell membrane and tell which charges are located where.
Describe the repolarization of the nerve cell membrane and tell which charges are located where.
Define refractory period and describe why it occurs.
Discuss how the sodium ion flow in one area of an axon leads to initiation of an action potential in an adjacent region of the axon membrane.
Compare and contrast continuous and saltatory conduction.
Outline the factors that alter the rate of action potential propagation along an axon.
SIGNAL TRANSMISSION AT SYNAPSES
Describe the structure of a chemical synapse.
Go through the sequence of events that allow an action potential on an axon to be transmitted into a graded potential on a postsynaptic membrane.
Indicate the voltage changes associated with EPSPs and IPSPs, and how these potentials are related to various ion channels.
Distinguish between spatial and temporal summation.
Note that there must be a mechanism to diminish neurotransmitter concentrations in the synaptic cleft to be able to turn the stimulus off.
Describe and give examples and functions of the various neurotransmitter classes.
Be able to identify the group to which a specific neurotransmitter belongs.
Describe the effect of various drugs and disorders on normal neurotransmitter function.
CIRCUITS IN THE NERVOUS SYSTEM
Describe the various types of neuronal circuits in the nervous system.
Chapter Lecture Notes
Homeostasis The Nervous System is the body's most rapid means of maintaining homeostasis (maintenance of constant internal environment) Central nervous system (CNS) (Fig 12.1) Brain - 100 billion neurons (each synapse with 1,000 -10,000 other neurons) Spinal Cord Peripheral nervous system (PNS) - communication between CNS and rest of body (Fig 12.10) Structural Divisions Cranial nerves (12 pairs) Spinal nerves (31 pairs) Functional Divisions (uses both cranial and spinal nerves) Somatic nervous system Controls skeletal muscle Voluntary Autonomic nervous system Sympathetic division (responds to short term stress) Parasympathetic division (returns body to normal functions following stress) Controls smooth muscle, cardiac muscle and glands Involuntary
Enteric Nervous System
Controls smooth muscle and glands of the digestive system
Functional Classification of Nervous System Sensory (Afferent) = Input - senses changes in external and internal environment & transmits changes via sensory neurons/afferent neurons to CNS Integrative (Processing) - Interprets changes (solely in CNS) Motor (Efferent) = Output - Responds to changes in form of muscular contraction/gland secretion via motor neurons/efferent neurons Neurons Neurons – specialize in conducting action potential (nerve impulse) Amitotic but high rate of metabolism that requires abundant supply of O 2 and glucose Parts of a Neuron (Fig 12.2 & Table 12.3) Clustered into ganglia in PNS Clustered into nuclei in brain Clustered into horns in spinal cord Contains nucleus Contains Nissl bodies - rough ER - site of protein synthesis Contains neurofibrils - cytoskeleton that extends into axons and dendrites and used to transport neurotransmitters, nutrients, etc NOTE: herpes, rabies, and polio viruses and toxin from Clostridium tetani travels along neurofibrils of axons to cell bodies where they can multiply and cause damage Dendrites Extensions that receive information along with the cell body in motor neurons and interneurons or generate input in sensory neurons (once extension becomes myelinated, it is then called an axon) (Fig 12.4) Axons Conduct action potentials toward the axon terminal Distal end of axons swell into synaptic end bulbs that contain neurotransmitters in synaptic vesicles Bundles of neuron axons in CNS = tracts (axons bundled with neuroglia) Bundles of neuron axons in PNS = nerves (axons bundled with endoneurium/perineurium/epineurium) Frequently myelinated in both CNS and PNS Structural Classification of Neurons Structural classification: classification of neurons according to the number of process from the cell body (Fig 12.3, 12.4 & 12.5) Sensory or afferent in function Begins as a bipolar neuron in embryo but fuses into single extension Bipolar neuron - 2 extensions from cell body Examples: rods and cones (shapes of dendrites) of retina, olfactory neurons, inner ear neurons Multipolar neuron - many extensions from cell body Most of CNS (interneurons) and all motor neurons Functional Classification of Neurons Functional classification: classification according to the direction which impulses are conducted relative to the CNS (Fig 12.10) Sensory (afferent) neuron - strictly PNS - transmit impulses toward CNS from receptors Includes both unipolar and bipolar neurons In unipolar neurons, cell bodies are just outside the spinal cord in a structure called the posterior (dorsal) root ganglia Motor (efferent) neuron - transmits impulses away from CNS to muscles/glands Cell bodies are in spinal cord All are multipolar All are multipolar Make up 90% of total neurons Neuroglia = Glial cells Neuroglia - support, connect, and protect the impulse conducting cells of the nervous system (neurons) in both CNS and PNS Cancer of NS - (gliomas) involves neuroglia and not neurons because neuroglia have retained mitotic ability but neurons have not retained mitotic ability beyond infancy Neuroglia outnumber neurons by 5 - 50 X Neuroglia from CNS (Fig 12.6) Astrocytes – star shaped Twine around neurons to form supporting network Attach neurons to blood vessels Create blood-brain barrier Produce "scar tissue" if there is damage to CNS Microglia - derived from monocytes Become phagocytic and remove injured brain or cord tissue Ependymal cells - epithelial cells that line ventricles of brain and central canal of cord Ciliated to assist in circulation of CSF Produce myelin sheath in CNS Neuroglia from PNS (Fig 12.7) Schwann cells - produce myelin sheath in PNS Satellite cells - support cell bodies in PNS Myelin Sheath Myelin sheath produced around an axon by the following two neuroglia cells Oligodendrocytes CNS Can myelinate up to 15 different neurons (axons) Schwann cells PNS Can have up to 500 Schwann cells along the longest neurons (myelinate only one axon) Myelin sheath = multilayered lipid and protein covering surrounding axons in PNS and CNS (actually multilayers of cell membrane from Schwann cell or extension from oligodendrocyte) Myelin sheath electrically insulates the axon and increases speed of nerve impulse conduction (action potential) Schwann cell wraps like a jelly roll so that up to 100 layers of the cell rolls around the axon The outer part of the cell contains the nucleus and the Schwann cell membrane (Fig 12.8) The Schwann cell membrane is called the neurolemma Evidence has shown that the neurolemma aids in repair and regeneration of axons in the PNS (absent in CNS) (Fig 12.29) Guillain-Barre Syndrome – demyelination of axons in the PNS by macrophages macrophages destroy Schwann cells which can regenerate person suffers from acute paralysis but most patients recover completely Oligodendrocytes have "octopus-like extensions" that wrap several different axons and therefore do not have neurolemma (may be one reason why CNS neurons don't regenerate) (Fig 12.6) Multiple Sclerosis - autoimmune disorder in which Killer T-cells destroy oligodendrocytes that are replaced by plaques (scleroses) from astrocytes Interferes with impulse transmission MS is also known as a demyelination disorder of the CNS Nodes of Ranvier - gaps in myelinated neuron where myelin absent (Fig 12.2 & 12.7) Nodes of Ranvier are produced by both Schwann cells as well as oligodendrocytes, so nodes of Ranvier are present in both CNS & PNS White matter - cell processes (axons) with myelin (Fig 12.9) Gray matter - parts of neuron, especially cell bodies and dendrites, that lack myelin Always located in protected areas of CNS Ganglia would also be gray because cell bodies are not myelinated Neurophysiology Action potential - An electrical signal that propagates along the membrane of a neuron or muscle fiber Neurophysiology = Excitability - ability to respond to a stimulus (stimulus – any condition capable of altering the cell’s membrane potential) and convert it into an action potential Nerve conduction of action potentials involves an electrochemical mechanism Ion Channels Proteins in the cell membrane Don’t require ATP - movement of ions is by channel-mediated facilitative diffusion (Fig 12.11 & Table 12.1) Nongated Leakage channels - randomly open Cell membranes of muscle/neurons have more K + leakage channels than Na + leakage channels Chemical (ligand) gated - open and close in response to chemicals like neurotransmitters, hormones, ions (dendrites and cell bodies) Mechanically gated - open and close in response to mechanical vibration or pressure such as sound waves or pressure of touch/stretch (dendrites of sensory neurons) Voltage gated - open and close in response to voltage (axons only) Require ATP - movement of ions is by active transport Na +K + Pump (Na +K + ATPase) - movement of three Na + ions out of the cell and two K + ions into the cell by active transport which requires ATP Resting Membrane Potential (RMP) Nonconducting neuron has a RMP of -70mV (Fig 12.12) Reason for resting membrane potential (Fig 12.13) The inside of the membrane has non-diffusible anions (-) (phosphate and protein anions) K + ions are more numerous on the inside than outside – Remember Na + and Cl - ions more numerous outside Small amounts of K + move to the outside through leakage (nongated) channels with anions following (cannot diffuse through the membrane and get stuck at the membrane) Note: there are more K + leakage channels than Na + leakage channels The inside of the cell has a more negative charge than the outside which is positive; overall the inside of the membrane is -70mV Maintain resting membrane potential with Na +K + Pump Membrane is said to be polarized because of the difference in charge across the membrane = resting membrane potential K + is inside, Na + is outside, Inside = (-) All or None Principal All or None Principle - Neuron transmits action potentials according to all or none principle If the stimulus is strong enough to generate an action potential, the impulse is conducted down the neuron at a constant and maximum strength for the existing conditions Stimulus must raise membrane potential to less negative than -55mV (Threshold potential) (Fig 12.19) Graded Potentials Graded potentials – local potentials (Table 12.2) Affected at site of stimulation and effect decreases with distance Spreads passively The stronger the stimulus, the greater the change in potential and the larger area affected (Fig 12.16) The potential change could be either negative or positive (Fig 12.14 & 12.15) Excitatory stimulus - Increases Na + into cell making membrane hypopolarized Partially depolarizes and makes membrane less negative Causes depolarization (but not to -55 mV) Single excitatory stimulus usually does not initiate nerve impulse but membrane is closer to the threshold and more likely to reach threshold with next excitatory stimulus Inhibitory stimulus - Increases K + outward or increases Cl - inward Makes the membrane hyperpolarized (as low as -90mV) Generation of action potential is now more difficult Must add up all the excitatory and inhibitory stimuli (summation) that are influencing the neuron to determine if an action potential will be sent (Fig 12.17 & 12.26) Action Potentials Action Potential (AP) = rapid change in membrane potential (polarity) that can spread down the length of the axon The membrane will depolarize and then repolarize Only muscle and neurons can produce an AP In neurons, an AP lasts about 1 ms or less Propagation of APs down axons = nerve impulses Steps in generating an Action Potential (Fig 12.18 - 12.20) 1. Depolarization When a stimulus is applied, if the sum of stimuli is excitatory (mechanical gated or chemical gated ion channels open and cause a net flow of Na + into the cell) and depolarization occurs to threshold potential (threshold = -55mV) At -55 mV, voltage gated Na + channels open and Na + rushes in (Na + inflow), making the inside of the cell positive This is the depolarization (Na + inflow) phase = normal polarized state is reversed Inside = (+) K + is inside, Na + is inside, Inside = (+) Repolarization is due to K + ions flowing outward (K + outflow) through voltage gated K + channels Voltage gated Na + channels inactivate and close Voltage-gated K + channels open in response to positive membrane and remain open until membrane potential returns to a negative value Ion distribution is reverse of that at resting Inside = (-) K+ is outside, Na+ is inside, Inside = (-) Refractory Period - period of time during which an excitable cell cannot generate another action potential Voltage gated Na + channels cannot reopen until they become reactivated Because ion distribution has not returned to resting, sufficient potential has not built up on either side of the membrane to generate a new action potential The refractory period begins at depolarization and continues until the resting membrane ion distribution is restored The refractory period can be short (0.4 ms in skeletal muscle) because only a few Na + rush in and only a few K + move out with each nerve impulse 3. Restoration of Resting Membrane Potential Leakage channels allow ions to flow into and out of the cell The Na +K + pump also operates in restoring the resting ion distribution by pumping Na + out of the cell and K + into the cell K + is inside, Na + is outside, Inside = (-) 4. Propagation of Action Potentials (Fig 12.21) Each action potential acts as a stimulus for development of another action potential in an adjacent segment of membrane The Na + inflow during the depolarization phase of an action potential diffuses to an adjacent membrane segment Increase in Na + concentration raises the membrane potential of that membrane segment to the threshold potential, generating a new action potential Action potentials do not travel but are regenerated in sequence along an axon like tipping dominos Refractory period prevents action potential from going backwards Action potentials continue to be regenerated in sequence until the potential reaches the end of the axon Saltatory Conduction Saltatory conduction - Action potentials are only generated at the Nodes of Ranvier (Fig 12.21) Action potential will skip from node to node Ionic movement is inhibited beneath myelin sheath Conserves energy because Na-K Pump is not needed as extensively because only Nodes of Ranvier are depolarized and repolarized Conducts up to 50x faster than unmyelinated neuron Speed of Impulse Conduction Speed of impulse conduction (propagation) determined by: Presence of myelin sheath - the further the nodes are apart, the faster the transmission Diameter of fiber - the greater the diameter the greater density of voltage gated Na + channels; the greater the diameter, the faster the transmission Temperature - the greater the temperature the faster the transmission Localized cooling can block impulse conduction; therefore pain can be reduced by application of ice Types of nerve fibers based on transmission speed A fibers - myelinated and large diameter; fastest conduction; in areas where split second responses can mean survival; speeds up to 280 mph B fibers - myelinated and smaller diameter; speeds up to 32 mph C fibers - unmyelinated and smaller diameter; speeds up to 4 mph Note: B and C fibers are going to and from viscera Synapse Synapse - connection between axon terminal and another neuron, muscle (neuromuscular junction), or gland (neuroglandular junction) Electrical synapse: ionic current spreads directly from one cell to another through gap junctions (found in cardiac and smooth muscle) Chemical synapses: neurotransmitter is secreted from one cell and a second cell responds to it
Flow of information is in one direction
Structure of chemical synapse (Fig 12.22) Synaptic end bulb of first neuron (presynaptic neuron) = presynaptic membrane Presynaptic electrical signal is converted to chemical signal Presynaptic neuron releases neurotransmitter Synaptic cleft: 20-50 nm gap between neuron and next structure Impulse cannot jump cleft, therefore, will need chemical transmission in form of neurotransmitter Cell membrane of second neuron (postsynaptic neuron) = postsynaptic membrane Postsynaptic neuron has receptors for neurotransmitter Postsynaptic neuron receives chemical signal (neurotransmitter) and in turn may generate an electrical signal (action potential) Exocytosis of neurotransmitter (Fig 12.23) When nerve impulse (action potential) arrives at synaptic end bulb, the depolarization phase opens voltage gated Ca 2+ channels Extracellular Ca 2+ flows inward Increase in Ca 2+ inside the neuron, triggers exocytosis of synaptic vesicles Neurotransmitter enters synaptic cleft Neurotransmitter can either be excitatory or inhibitory inhibitory neurotransmitters prevents chaos in nervous system Neurotransmitters have to be inactivated or transported back into the presynaptic neuron neurotransmitters must be removed from cleft Neurotransmitters interact with receptor sites of chemically gated ion channels on the postsynaptic membrane to produce: EPSP – excitatory postsynaptic potential - a type of graded potential (Fig 12.24)
Typically results from the opening of chemically gated Na + channels.
IPSP - inhibitory postsynaptic potential - a type of graded potential
Typically results from the opening of chemically gated K + channels or Cl - channels.
Summation of EPSP & IPSP = inhibition or excitation Spatial (multiple synapse stimulation) (Fig 12.25) Temporal (time) Integration of EPSP and IPSP is at axon hillock (trigger zone) Whether a neurotransmitter is excitatory or inhibitory is determined by the postsynaptic membrane receptor Must have a mechanism to remove neurotransmitters from synaptic cleft to be able to turn signal off Neurotransmitters At least 75 neurotransmitters (Fig 12.27) Acetylcholine (ACh) - main neurotransmitter of PNS (not common in CNS) Excitatory for skeletal muscle Inhibitory for cardiac muscle Important in brain for memory consolidation (destroyed in Alzheimer’s) Adenosine – excitatory in PNS and CNS Caffeine acts as a competitive inhibitor at adenosine receptors in the brain Catecholamines Affect mood 6-C ringed structure with 2 hydroxyl groups and an attached amine They are degraded by catechol-O-methyltransferase and monoamine oxidase (MAO) Dopamine (DA) Excitatory for emotional response but inhibitory in motor functions Low levels are associated with Parkinson's Excess DA associated with schizophrenia DA seems to be the neurotransmitter involved in addiction to heroin, methamphetamines, cocaine, marijuana, alcohol, nicotine, caffeine Cocaine prevents DA reuptake Norepinephrine (NE) Found in brain and secreted by sympathetic nervous system Affects mood Low levels are associated with depression Methamphetamines (speed) - prevents NE reuptake Epinephrine = adrenaline Secreted by the adrenal gland Enhances sympathetic nervous system response Produced from amino acid, tryptophan High amts in milk and turkey Serotonin is secreted in parts of the brain and spinal cord Affects mood Induces sleep Aids in memory Prozac, Paxil, Zoloft, Luvox, Celexa and Lexapro inhibit its reuptake by the presynaptic membrane LSD blocks the activity of serotonin Ecstasy inhibits its reuptake by the presynaptic terminal and induces the presynaptic neuron to release even more serotonin Ecstasy also induces the release of norepinephrine and dopamine Amino acids and Amino acid like compounds Glycine Common inhibitory neurotransmitter in spinal cord (1/2 of inhibitory synapses in cord use glycine) Tetanus toxin inhibits glycine, causing "lockjaw“ Strychnine blocks glycine receptors, causing the diaphragm to continuously contract which leads to suffocation GABA (Gamma aminobutyric acid) Common inhibitory neurotransmitter in brain (as many as 1/3 of brain synapses use GABA) Prevents chaos in nervous system GABA reduces anxiety Valium, Xanax, alcohol and barbituates enhance the action of GABA Treatment for epilepsy is drug that increases GABA Glutamate (Glutamic acid) Common excitatory in CNS Increase in glutamate after stroke may lead to death of neurons because of oxygen deprivation to glutamate transporters that work by active transport (requires oxygen for ATP synthesis) Asparagine (Aspartic acid) Common excitatory in CNS Peptides - series of covalently linked amino acids (Table 12.4) Substance P - neurotransmitter in pain pathways (mediates our perception of pain) Enkephalins and endorphins - endogenous morphine-like substances Both are structurally similar to morphine and bind to morphine receptors Modulate pain by inhibiting release of substance P Runner’s high Natural child birth Certain disorders such as Parkinson's disease, Alzheimer's disease, depression, anxiety, schizophrenia involves problems relating to neurotransmitters Circuits Circuits (Fig 12.28) Typical neuron receives input from 1,000 to 10,000 synapses Each presynaptic neuron may branch and synapse with up to 25,000 or more different postsynaptic neurons Convergence - single postsynaptic neuron controlled by converging signals coming from 2 or more presynaptic neurons
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