Chapter 12 Lecture Outline 1 See separate PowerPoint

Chapter 12 Lecture Outline 1 See separate PowerPoint

Chapter 12 Lecture Outline 1 See separate PowerPoint slides for all figures and tables preinserted into PowerPoint without notes. Copyright McGraw-Hill Education. Permission required for reproduction or display. 1 Introduction The nervous system is very complex Nervous system is the foundation of our conscious experience, personality, and behavior Neurobiology combines the behavioral and life sciences 12-2 Overview of the Nervous System Expected Learning Outcomes Describe the overall function of the nervous system.

Describe its major anatomical and functional subdivisions. 12-3 Overview of the Nervous System Endocrine and Nervous systems maintain internal coordination or maintaining stable internal conditions by detecting and responding to stimuli Endocrine system: communicates by means of chemical messengers (hormones) secreted into to the blood Nervous system: employs electrical and chemical means to send messages from cell to cell 12-4 Overview of the Nervous System Nervous system carries out its task in three basic steps Sense organs receive information about changes in the body and external environment, and transmit

coded messages to the brain and spinal cord (CNS: central nervous system) CNS processes this information, relates it to past experiences, and determines appropriate response CNS issues commands to muscles and gland cells to carry out such a response 12-5 Overview of the Nervous System Two major subdivisions of nervous system Central nervous system (CNS) Brain and spinal cord enclosed by cranium and vertebral column Peripheral nervous system (PNS) All the nervous system except the brain and spinal cord; composed of nerves and ganglia Nervea bundle of nerve fibers (axons) wrapped in fibrous connective tissue, and organs of nervous system - Nerve is a group of peripheral axons Gangliona knot-like swelling in a group of nerve where neuron nerve-cell bodies are concentrated, located in PNS

12-6 Overview of the Nervous System Peripheral nervous system contains sensory and motor divisions each with somatic and visceral subdivisions Sensory is also known as (afferent) division: carries signals from receptors to CNS Somatic sensory division: carries signals from receptors in the skin, muscles, bones, and joints Visceral sensory division: carries signals from the viscera (heart, lungs, stomach, and urinary bladder) 12-7 Overview of the Nervous System Motor is also known as (efferent) division carries signals from CNS to effectors (glands and muscles that carry out the bodys response) Somatic motor division: carries signals to skeletal muscles Output produces muscular contraction as well as somatic

reflexesinvoluntary muscle contractions Visceral motor division (autonomic nervous system) carries signals to glands, cardiac and smooth muscle of the large intestine Its involuntary responses are visceral reflexes 12-8 Overview of the Nervous System Visceral motor division is also known as (autonomic nervous system) Sympathetic division (feed and breed) Tends to arouse, prepare body for action Accelerating heart beat and respiration, while inhibiting digestive and urinary systems Parasympathetic division (fight and flight) Tends to have calming effect Slows heart rate and breathing Stimulates digestive and urinary systems

12-9 Subdivisions of the Nervous System Figure 12.1 12-10 Subdivisions of the Nervous System Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Central nervous system Brain Peripheral nervous system Spinal cord Visceral sensory

division Figure 12.2 Sensory division Somatic sensory division Motor division Visceral motor division Sympathetic division Somatic

motor division Parasympathetic division 12-11 Properties of Neurons Expected Learning Outcomes Describe three functional properties found in all neurons. Define the three most basic functional categories of neurons. Identify the parts of a neuron. Explain how neurons transport materials between the cell body and tips of the axon. 12-12 Universal Properties of Neurons Excitability (irritability)

Respond to environmental changes called stimuli Conductivity Respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations Secretion When an electrical signal reaches the end of nerve fiber, the cell secretes a chemical neurotransmitter that influences the next cell 12-13 Functional Classes of Neurons Sensory (afferent) neurons Detect stimuli and transmit information about them toward the CNS Interneurons (association neurons) Interneurons lie entirely within gray matter of CNS connecting motor and sensory pathways (about 90% of all neurons)

Receive signals from many neurons and carry out integrative functions (make decisions on responses) Motor (efferent) neuron Send signals out to muscles and gland cells (the effectors) of the nervous system 12-14 Classes of Neurons Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Peripheral nervous system Central nervous system 1 Sensory (afferent) neurons conduct signals from receptors to the CNS. 3 Motor (efferent) neurons conduct

signals from the CNS to effectors such as muscles and glands. 2 Interneurons (association neurons) are confined to the CNS. Figure 12.3 12-15 Structure of a Neuron Somacontrol center of neuron Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Most metabolic and regulatory functions in a neuron happen Also called neurosoma or cell body Has a single, centrally located nucleus

with large nucleolus Cytoplasm contains mitochondria, lysosomes, Golgi complex, inclusions, extensive rough ER and cytoskeleton Inclusions: glycogen, lipid droplets, melanin, and lipofuscin pigment (produced when lysosomes digest old organelles) Cytoskeleton has dense mesh of microtubules and neurofibrils (bundles of actin filaments) that compartmentalizes rough ER into dark-staining Nissl bodies No centrioles, no mitosis Dendrites Soma Nucleus Nucleolus Trigger zone:

Axon hillock Initial segment Axon collateral Axon Direction of signal transmission Internodes Node of Ranvier (a) Myelin sheath Schwann cell Terminal arborization Synaptic knobs

Figure 12.4a 12-16 Structure of a Neuron Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites Dendritesbranches that come off the soma, usually highly branched Primary site for receiving signals from other neurons The more dendrites the neuron has, the more information it can receive Provide precise pathways for the reception and processing of information Soma

Nucleus Nucleolus Trigger zone: Axon hillock Initial segment Axon collateral Axon Direction of signal transmission Internodes Node of Ranvier Myelin sheath Schwann cell Terminal arborization

Synaptic knobs (a) Figure 12.4a 12-17 Structure of a Neuron Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Axon (refers to nerve fiber) originates from a mound on the soma called the axon hillock, carry nerve impulses toward cell body Axon is cylindrical, relatively unbranched for most of its length Axon collateralsbranches of axon Branch extensively on distal end Specialized for rapid conduction of signals to distant points

Axoplasm: cytoplasm of axon Axolemma: plasma membrane of axon Only one axon per neuron (some neurons have none) Dendrites Soma Nucleus Nucleolus Trigger zone: Axon hillock Initial segment Axon collateral Axon Direction of signal transmission Internodes

Node of Ranvier (a) Myelin sheath Schwann cell Terminal arborization Synaptic knobs Figure 12.4a 12-18 Structure of a Neuron Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Myelin sheath is white fatty material surrounds the axon and

increases the speed of the nerve impulse Distal end of axon has terminal arborization: extensive complex of fine branches Synaptic knob (terminal button)little swelling that forms a junction (synapse) with the next cell, is a tiny bulge at the end of the presynaptic neurons axon Contains synaptic vesicles full of neurotransmitter are chemicals that allow neurons to communicate with one another. Dendrites Soma Nucleus Nucleolus Trigger zone:

Axon hillock Initial segment Axon collateral Axon Direction of signal transmission Internodes Node of Ranvier Myelin sheath Schwann cell Terminal arborization Synaptic knobs (a)

Figure 12.4a 12-19 Structure of a Neuron Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Multipolar neuron One axon and multiple dendrites Most common type of most neurons in CNS Dendrites Axon Bipolar neuron Multipolar neurons One axon and one dendrite Olfactory cells, retina, inner ear

Dendrites Unipolar neuron Single process leading away from soma Sensory cells from skin and organs to spinal cord Anaxonic neuron Many dendrites but no axon Retina, brain, and adrenal gland Axon Bipolar neurons Dendrites Axon Unipolar neuron Dendrites

Anaxonic neuron Figure 12.5 12-20 Axonal Transport Many proteins made in soma must be transported to axon and axon terminal To repair axolemma, serve as gated ion channels, enzymes or neurotransmitters Axonal transporttwo-way passage of proteins, organelles, and other material along an axon Anterograde transport: movement down the axon away from soma Retrograde transport: movement up the axon toward the soma Microtubules guide materials along axon Motor proteins (kinesin and dynein) carry materials on their backs while they crawl along microtubules Kinesinmotor proteins in anterograde transport

Dyneinmotor proteins in retrograde transport 12-21 Axonal Transport Fast axonal transportrate of 20 to 400 mm/day Fast anterograde transport Organelles, enzymes, synaptic vesicles, and small molecules Fast retrograde transport For recycled materials and pathogensrabies, herpes simplex, tetanus, polio viruses Delay between infection and symptoms is time needed for transport up the axon Slow axonal transport0.5 to 10 mm/day Always anterograde Moves enzymes, cytoskeletal components, and new axoplasm down the axon during repair and regeneration of damaged axons Damaged nerve fibers regenerate at a speed governed by slow axonal transport 12-22

Supportive Cells (Neuroglia) Expected Learning Outcomes Name the six types of cells that aid neurons and state their respective functions. Describe the myelin sheath that is found around certain nerve fibers and explain its importance. Describe the relationship of unmyelinated nerve fibers to their supportive cells. Explain how damaged nerve fibers regenerate. 12-23 Supportive Cells (Neuroglia) About 1 trillion neurons in the nervous system Neuroglia outnumber neurons by at least 10 to 1 Neuroglia or glial cells - known as supporting cells Protect neurons and help them function Bind neurons together and form framework for nervous tissue In fetus, guide migrating neurons to their destination

If mature neuron is not in synaptic contact with another neuron, it is covered by glial cells Prevents neurons from touching each other Gives precision to conduction pathways 12-24 Types of Neuroglia Four types of glia occur in CNS: oligodendrocytes, ependymal cells, microglia, and astrocytes Oligodendrocytes Form myelin sheaths in the spinal cord of CNS that speed signal conduction Arm-like processes wrap around nerve fibers Ependymal cells Line internal cavities of the brain; secrete and circulate cerebrospinal fluid (CSF) Cuboidal epithelium with cilia on apical surface 12-25

Types of Neuroglia Four types of glia occur in CNS: Microglia Wander through CNS looking for debris and damage Develop from white blood cells (monocytes) and become concentrated in areas of damage Glial cell turns into a microbe eating cell in inflammed brain tissue 12-26 Types of Neuroglia Astrocytes - Most abundant glial cell in CNS, covering brain surface and most nonsynaptic regions of neurons in the gray matter - Diverse functions: Form supportive framework Have extensions (perivascular feet) that contact blood capillaries and stimulate them to form a seal called the blood brain barrier Convert glucose to lactate and supply this to neurons Secrete nerve growth factors

Communicate electrically with neurons Regulate chemical composition of tissue fluid by absorbing excess neurotransmitters and ions Astrocytosis or sclerosiswhen neuron is damaged, astrocytes form hardened scar tissue and fill in space 12-27 Neuroglial Cells of CNS Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Capillary Neurons Astrocyte Oligodendrocyte Perivascular feet Myelinated axon

Ependymal cell Myelin (cut) Cerebrospinal fluid Microglia Figure 12.6 12-28 Types of Neuroglia Two types occur only in PNS Schwann cells Glial cell is found wrapped around or envelope nerve fibers in PNS Wind repeatedly around a nerve fiber Produce a myelin sheath similar to the ones produced by oligodendrocytes in CNS Assist in regeneration of damaged fibers Satellite cells

Surround the neurosomas in ganglia of the PNS Provide electrical insulation around the soma Regulate the chemical environment of the neurons 12-29 Myelin Myelin sheathinsulation around a nerve fiber, composed primarily of lipids Formed by oligodendrocytes in CNS and Schwann cells in PNS Consists of the plasma membrane of glial cells 20% protein and 80% lipid Myelinationproduction of the myelin sheath Begins at week 14 of fetal development Proceeds rapidly during infancy Completed in late adolescence

Dietary fat is important to CNS development 12-30 Myelin In PNS, Schwann cell spirals repeatedly around a single nerve fiber Lays down as many as one hundred layers of membrane No cytoplasm between the membranes Neurilemma: thick, outermost coil of myelin sheath Contains nucleus and most of its cytoplasm External to neurilemma is basal lamina and a thin layer of fibrous connective tissueendoneurium 12-31 Myelin Sheath in PNS Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Axoplasm Axolemma Figure 12.4c

(c) Schwann cell nucleus Neurilemma Myelin sheath Nodes of Ranvier and internodes 12-32 Myelination in PNS Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Schwann cell Axon Basal lamina

Endoneurium Nucleus (a) Neurilemma Myelin sheath Figure 12.7a 12-33 Myelin In CNSan oligodendrocyte myelinates several nerve fibers in its immediate vicinity Glia cell produces and makes myelin for cells in the brain Anchored to multiple nerve fibers Cannot migrate around any one of them like Schwann cells Must push newer layers of myelin under the older

ones; so myelination spirals inward toward nerve fiber Nerve fibers in CNS have no neurilemma or endoneurium 12-34 Myelination in CNS Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Oligodendrocyte Myelin Nerve fiber Figure 12.7b (b) 12-35 Myelin Many Schwann cells or oligodendrocytes are needed to cover one nerve fiber

Myelin sheath is segmented Nodes of Ranvier: gap between segments, indentations between adjacent Schwann cells Internodes: myelin-covered segments from one gap to the next Initial segment: short section of nerve fiber between the axon hillock and the first glial cell Trigger zone: the axon hillock and the initial segment Play an important role in initiating a nerve signal 12-36 Glial Cells and Brain Tumors Tumorsmasses of rapidly dividing cells Mature neurons have little or no capacity for mitosis and seldom form tumors Brain tumors arise from: Meninges (protective membranes of CNS) Metastasis from nonneuronal tumors in other organs Often glial cells that are mitotically active throughout life Gliomas grow rapidly and are highly malignant

Bloodbrain barrier decreases effectiveness of chemotherapy Treatment consists of radiation or surgery 12-37 Diseases of the Myelin Sheath Degenerative disorders of the myelin sheath Multiple sclerosis Oligodendrocytes and myelin sheaths in the CNS deteriorate (loss of myelin) Myelin replaced by hardened scar tissue Nerve conduction disrupted (double vision, tremors, numbness, speech defects) Onset between 20 and 40 and fatal from 25 to 30 years after diagnosis Cause may be autoimmune triggered by virus 12-38 Diseases of the Myelin Sheath (continued) Degenerative disorders of the myelin sheath

TaySachs disease: a hereditary disorder of infants of Eastern European Jewish ancestry Abnormal accumulation of glycolipid called GM2 in the myelin sheath Normally decomposed by lysosomal enzyme Enzyme missing in individuals homozygous for TaySachs allele Accumulation of ganglioside (GM2) disrupts conduction of nerve signals Blindness, loss of coordination, and dementia Fatal before age 4 12-39 Unmyelinated Nerve Fibers Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Unmyelinated nerve fibers Schwann cell Basal lamina

Figure 12.7c Figure 12.8 Many CNS and PNS fibers are unmyelinated In PNS, Schwann cells hold 1 to 12 small nerve fibers in surface grooves Membrane folds once around each fiber 12-40 Conduction Speed of Nerve Fibers Speed at which a nerve signal travels along surface of nerve fiber depends on two factors Large Diameter of fiber Larger fibers have more surface area and conduct signals or nerve impulse more rapidly (fastest) Presence or absence of myelin

Myelin further speeds signal conduction 12-41 Conduction Speed of Nerve Fibers Conduction speed Small, unmyelinated fibers: 0.5 to 2.0 m/s Small, myelinated fibers: 3 to 15.0 m/s Large, myelinated fibers: up to 120 m/s Slow signals sent to the gastrointestinal tract where speed is less of an issue Fast signals sent to skeletal muscles where speed improves balance and coordinated body movement 12-42 Regeneration of Nerve Fibers

Regeneration of damaged peripheral nerve fiber can occur if: Its soma is intact At least some neurilemma remains Steps of regeneration: Fiber distal to the injury cannot survive and degenerates Macrophages clean up tissue debris at point of injury and beyond Soma swells, ER breaks up, and nucleus moves off center Due to loss of nerve growth factors from neurons target cell Axon stump sprouts multiple growth processes as severed distal end continues to degenerate Schwann cells, basal lamina and neurilemma form a regeneration tube Enables neuron to regrow to original destination and reestablish synaptic contact 12-43 Regeneration of Nerve Fibers Once contact is reestablished with original

target, the soma shrinks and returns to its original appearance Nucleus returns to normal shape Atrophied muscle fibers regrow But regeneration is not fast, perfect, or always possible Slow regrowth means process may take 2 years Some nerve fibers connect with the wrong muscle fibers; some die Regeneration of damaged nerve fibers in the CNS cannot occur at all 12-44 Regeneration of Nerve Fiber Figure 12.9 12-45 Nerve Growth Factor Nerve growth factor (NGF) protein secreted by a gland,

muscle, or glial cells and picked up by the axon terminals of neurons Prevents apoptosis (programmed cell death) in growing neurons Enables growing neurons to make contact with their targets Isolated by Rita LeviMontalcini in 1950s Won Nobel prize in 1986 with Stanley Cohen Use of growth factors is now a vibrant field of research Figure 12.10 12-46 Electrophysiology of Neurons Expected Learning Outcomes Explain why a cell has an electrical charge difference

(voltage) across its membrane. Explain how stimulation of a neuron causes a local electrical response in its membrane. Explain how local responses generate a nerve signal. Explain how the nerve signal is conducted down an axon. 12-47 Electrophysiology of Neurons Galen (Roman physician) thought brain pumped a vapor called psychic pneuma through hollow nerves and into muscles to make them contract Ren Descartes in the 17th century supported Galens theory Luigi Galvani discovered the role of electricity in muscle contraction in the 18th century Camillo Golgi developed an important method for staining neurons with silver in the 19th century Santiago Ramn y Cajal (1852-1934) used stains to trace neural pathways He showed that pathways were made of distinct neurons (not continuous tubes)

He demonstrated how separate neurons were connected by synapses 12-48 Electrophysiology of Neurons Cajals theory brought up two key questions: How does a neuron generate an electrical signal? How does it transmit a meaningful message to the next cell? 12-49 Electrical Potentials and Currents Electrophysiologystudy of cellular mechanisms for producing electrical potentials and currents Basis for neural communication and muscle contraction Electrical potentiala difference in concentration of charged particles between one point and another Living cells are polarized and have a resting membrane potential Cells have more negative particles on inside of membrane than outside Neurons have about 70 mV resting membrane potential

Electrical currenta flow of charged particles from one point to another In the body, currents are movements of ions, such as Na+ or K+, through channels in the plasma membrane Gated channels are opened or closed by various stimuli Enables cell to turn electrical currents on and off 12-50 The Resting Membrane Potential Resting membrane potential (RMP) exists because of unequal electrolyte distribution between extracellular fluid (ECF) and intracellular fluid (ICF) RMP results from the combined effect of three factors Ions diffuse down their concentration gradient through the membrane Plasma membrane is selectively permeable and allows some ions to pass easier than others Electrical attraction of cations and anions to each other

12-51 The Resting Membrane Potential Potassium (K+) ion has greatest influence on resting membrane potential (RMP) Plasma membrane is more permeable to K+ than any other ion Leaks out until electrical charge of cytoplasmic anions attracts it back in and equilibrium is reached (no more net movement of K+) K+ is about 40 times as concentrated in the ICF as in the ECF Cytoplasmic anions cannot escape due to size or charge (phosphates, sulfates, small organic acids, proteins, ATP, and RNA) 12-52 The Resting Membrane Potential Membrane is not very permeable to sodium (Na+) but RMP is slightly influenced by it Na+ is about 12 times as concentrated in the ECF as in

the ICF Some Na+ leaks into the cell, diffusing down its concentration and electrical gradients This Na+ leakage makes RMP slightly less negative than it would be if RMP were determined solely by K+ 12-53 The Resting Membrane Potential Na+/K+ pump moves 3 Na+ out for every 2 K+ it brings in Works continuously to compensate for Na+ and K+ leakage, and requires great deal of ATP (1 ATP per exchange) 70% of the energy requirement of the nervous system Necessitates glucose and oxygen be supplied to nerve tissue (energy needed to create the resting potential) The exchange of 3 positive charges for only 2 positive charges contributes about 3 mV to the cells resting membrane potential of 70 mV 12-54

The Resting Membrane Potential Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ECF Na+ 145 m Eq/L K+ Na+ channel 4 m Eq/L K+ channel Na+ 12 m Eq/L K+ 150 m Eq/L ICF Na+ concentrated outside of cell (ECF) K+ concentrated inside cell (ICF)

Large anions that cannot escape cell Figure 12.11 12-55 Local Potentials Local potentialschanges in membrane potential of a neuron occurring at and nearby the part of the cell that is stimulated Dendrites most local potentials form in neuron Different neurons can be stimulated by chemicals, light, heat, or mechanical disturbance 12-56 Local Potentials A chemical stimulant binds to a receptor on

the neuron Fully opens Na+ gates, while a neuron membrane is depolirizing and allows Na+ to enter cell Entry of a positive ion makes the cell less negative; this is a depolarization: a change in membrane potential toward zero mV Na+ entry results in a current that travels toward the cells trigger zone; this short-range change in voltage is called a local potential 12-57 Local Potentials Properties of local potentials (unlike action potentials) Graded: vary in magnitude with stimulus strength Stronger stimuli open more Na+ gates Decremental: get weaker the farther they spread from the point of stimulation Voltage shift caused by Na+ inflow diminishes with distance

Reversible: if stimulation ceases, the cell quickly returns to its normal resting potential Either excitatory or inhibitory local potential causes: some neurotransmitters make the plasma membrane potential more negativehyperpolarize itso it becomes less likely to produce an action potential 12-58 Local Potentials Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites Soma Trigger zone Axon Current ECF Ligand Receptor

Plasma membrane of dendrite Na+ ICF Figure 12.12 12-59 Action Potentials Action potentialdramatic change in membrane polarity produced by voltage-gated ion channels, also known as electrical signal or nerve impulse and most associated with neuron. Only occurs where there is a high enough density of voltage-regulated gates Soma (50 to 75 gates per m2 ); cannot generate an action potential Trigger zone (350 to 500 gates per m2 ); where action potential is generated

If excitatory local potential reaches trigger zone and is still strong enough, it can open these gates and generate an action potential 12-60 Action Potentials Action potential is a rapid up-and-down shift in the membrane voltage involving a sequence of steps: Arrival of current at axon hillock depolarizes membrane Depolarization must reach threshold: critical voltage (about -55 mV) required to open voltage-regulated gates Voltage-gated Na+ channels open, Na+ enters and depolarizes cell, which opens more channels resulting in a rapid positive feedback cycle as voltage rises 12-61 Action Potentials (Steps in action potential shift in membrane voltage, Continued) As membrane potential rises above 0 mV, Na+ channels are inactivated and close; voltage peaks at about +35 mV

Slow K+ channels open and outflow of K+ repolarizes the cell K+ channels remain open for a time so that membrane is briefly hyperpolarized (more negative than RMP) RMP is restored as Na+ leaks in and extracellular K+ is removed by astrocytes 12-62 Action Potentials Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Only a thin layer of the cytoplasm next to the cell membrane is affected 3 Depolarization Repolarization

Action potential Threshold 2 55 Local potential Action potential is often called a spike, as it happens so fast 5 0 mV In reality, very few ions are involved

4 +35 1 7 70 Resting membrane potential (a) 6 Hyperpolarization Time Figure 12.13a 12-63

Action Potentials Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. K+ Na + K+ channel Na+ channel 35 0 0 mV mV

35 70 2 Na+ channels open, Na+ enters cell, K+ channels beginning to open Resting membrane potential Depolarization begins 35 35 0 0

mV 3 Na+ channels closed, K+ channels fully open, K+ leaves cell 70 4 Na channels closed, K+ channels closing + 70 Depolarization ends, repolarization begins mV 1 Na+ and K+ channels closed Figure 12.14

70 Repolarization complete 12-64 Action Potentials Characteristics of action potential (unlike local potential) Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 +35 Follows an all-or-none law 5 0 Depolarization

mV If threshold is reached, neuron fires at its maximum voltage 3 Irreversible: once started, goes to completion and cannot be stopped Action potential Threshold If threshold is not reached, 55 it does not fire Nondecremental: do not get weaker with distance

Repolarization 2 Local potential 1 7 70 Resting membrane potential (a) 6 Hyperpolarization Time Figure 12.13a

12-65 Action Potential vs. Local Potential Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 +35 3 +35 Spike 5 0 0 Repolarization

Action potential Threshold mV mV Depolarization 2 55 Local potential 1 7 Hyperpolarization

70 Resting membrane potential 6 Hyperpolarization 70 0 Time (a) (b) 10 20 ms 30

40 50 Figure 12.13a,b 12-66 Action Potential vs. Local Potential Table 12.2 12-67

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