Shahd’s Chapter 11 Notes Clean PDF

- Chapter 11 Fundamentals of the Nervous System and Nervous Tissue 391 You are in a crowded restaurant and hear the crash of 2. Inte grat ion. The nervous system processes a nd interprets f...

- Chapter 11 Fundamentals of the Nervous System and Nervous Tissue 391 You are in a crowded restaurant and hear the crash of 2. Inte grat ion. The nervous system processes a nd interprets falling plates. You immediately look toward the sound. Ayesha sensory in put and decides what shoul d be done at each smiles at the text message "lol" she's j ust received, knowing moment-a process called integra tion. her friend e njoyed the j oke. You are doz ing but you awaken 3. Moto r ou t put. T he nervous sys tem act ivates effector instantly when your infant son cries softly. organs-the muscles and glands-to cause a response, What do these three events have in common? They are all called motor outpu t. everyday examples of the function ing of your nervous system, Here's another example: You are driv ing a nd see a red light which has your body cells humming with activity nearly all the ahead (sensory input). Your nervous system integrates this time. in formation (red light means "stop"), and your foot hits the T he n er vous sys te1n is the master co ntrolling and com- brake (motor output). mun icating sys tem of the body. Every thought , actio n, and We have o ne highly integrated nervous system. For conve- emotion reflects its activity. Its cells commun icate by electrical nience, it is + divided into cord Spinal t\vo principal parts, central Dorsal body periph- andcavity Brain = and chemical signals, which are rapid and specific, and usually eral (Figure 11.2). cause almost irnmediate responses. The central nervous system (CNS) consists of the brain We begi n this chapter with a brief overview of the functions and spinal cord, which occupy the dorsal body cavity. The CNS and organization of the nervous system. Then we focus on the is the integrating and control center of the nervous system. It functional anatorny of nervous tissue, especially the nerve cells, interprets sensory input and dictates motor output based o n or neurons, wh ich are the key to neural cornmunication. reflexes, current conditions, and past experience. The peripher a l ner vous system (PNS) is the part of the nervous system outside the CNS. The PNS consists main ly I I The nervous system receives, of nerves (bundles of axons) that ex tend from the brain and spinal cord, a nd ganglia (collections of neuro n cell bodies). spinal nerves integrates, and responds to Spinal nerves carry impulses to and from the spinal cord, and information d to and from spinal cord cranial nerves carry impulses to a nd from the bra in. These Learning Outcomes cranial nerves.... List the basic f unct ions of the d tonervous and syste fromm. the brain II.... Explain t he st ructura l a nd functional d ivisions of the Central nervous Peripheral nervous ne rvous system. system (CNS) system (PNS) The nervous system has three overlapping fu nctions, illustrated Brain - ---- by the example of a thirsty person seeing and then lifti ng a glass of water (Figure 11.1): 1. Se nsory input. The nervous system uses its millions of sen- sory receptors to monitor changes occurring both inside and Spinal nerves outside the body. The gathered information is called sens ory input. Ganglia Sensory input ,, ,.. Integration ~ --~- M_o_to_r_o_ u t_p_ u~t _.~ Figure 11.2 The ne rvous syste m. The brain and spinal cord (tan) make up the central nervous system. The peripheral nervous Figure 11.1 The nervous syste m's functions. system (dark gold) mostly consists of pairs of cranial nerves, spinal nerves, and associated gangl ia.  392 UNIT 3 Regulation and Integration of the Body Central nervous system (C NS) Peripheral nervous system (PNS) Brain and spinal cord Cranial nerves and spinal nerves Integrative and control centers I Communication lines between the CNS and the rest of the body ti TO CNS I FROM CNS to effector Somatic sensory Fb% Sensory (afferent) division ' Motor (efferent) d ivis ion organs from Somatic and visceral sensory Motor nerve fibers convey impulses nerve fibers skin skeletal muscles joints. Conducts impulses from the CNS Conducts impulses from. to effectors (muscles and glands) Visceral ✓ receptors to the CNS I - - sensory fibers ,l. ,&. transmit impulses from the visceral organs peripheral nerves serve as commun icat ion lines that link all Somatic nervous Autonomic nervous system system (ANS) Involuntary parts of the body to the CNS. The PNS has two functional subdivisions, as Figure 11.3 Somatic (voluntary) Visceral (involuntary) motor nerve fibers motor nerve fibers shows. The sensory, or afferen t, division (af'er-ent; "carrying Conducts impulses Conducts impulses toward") consists of nerve fibers (axons) that convey impulses from the CNS to from the CNS to to the central nervous system from sensory receptors located skeletal muscles cardiac muscle, smooth muscle, throughout the body. voluntary and glands Somatic senso,yfibers convey impulses frorn the skin, skeletal I muscles, and j oints (so,na = body).J..1 Visceral sensory fibers transm it impulses from the visceral Sympathetic division Pa rasympathetic organs (organs within the ventral body cavi ty) Mobilizes body systems division The sensory division keeps the CNS constan tly informed of during activity Conserves energy events going on both inside and outside the body. Promotes house- fightorflightrestanddigestastrocytes.microgh.at The 1n otor, or effer en t , divis ion (ef'er-ent; "carryi ng keeping functions away") of the PNS transmits impulses from the CNS to effector Structure during rest.. Function organs, which are the muscles and glands. These impulses acti- vate muscles to contract and glands to secrete. In other words, Figure 11.3 Organization of the nervous system. they effect (bring about) a motor response. The motor division also has two main parts: The somatic ner vous system is composed of somatic motor The nervous systern consists mostly of nervous tissue, which nerve fibers that conduct impulses from the CNS to skeletal is highl y cellular. For example, Jess than 20% of the CNS is muscles. It is often referred to as the volunta r y n ervous sys- extracellular space, v,hich means that t he cells are densely tem because it allows us to consciously control our skeletal packed and tightly intertwined. Although it is very complex, 4 CNS [ muscles. nervous tissue is made up of just two principal types of cells: The au ton omic n ervous system (ANS) consists of visceral Supporting cells called neuroglia (or glial cells), small cells g epyndymal cellsmotor , nerve fibers thatcells regulate / the activity of srnooth mus- that surround and wrap the more delicate neurons [ cles, cardiac muscle, and glands. Autonomic means "a law Neurons, nerve cells that are excitable (respond to stimuli by oligodendrocytes unto itself," and because we generally cannot control such zpNs activities as the pumping of our heart or the movement of changing their membrane potential) and transmit electrical signals food through our digest ive tract, the ANS is also called the Look at Figure 4. 13 ( p. 140) to refresh your memory about involuntary ner vous system. - cells satellite these two kinds of cells before we explore them further in the As we will describe in Chapter 14, the ANS has two func- next two modules. tional subdivisions, the syn1pathetic division and the parasym- Schwann cells p a thetic division. lypically these divisions work in opposition ' to each other-whatever one stimulates, the other inhibits. Neuroglia support and maintain Check Your Understanding neurons 1. What is mea nt by "integration," cells and does it primarily occur in Howthe can be distinguished Learning Outcome ? supporting CNS or the PNS?.... List t he types of neurog lia a nd cite t heir functions. 2. fflQQN Wh ich subdivision of the PNS is involved in (a) relaying nuclei By their the feeling smaller of a "full stomach " after a meal, and (b) contracting size the muscles to lift your arm, and (c) increasing your heart rate? darker staining For answers, see Answers Appendix.  III Chapter 11 Fundamentals of the Nervous System and Nervous Tissue 393 Neurons associate closely with much smaller cells called neu- roglia (nu-rog' le-ah; "nerve glue"), or glial cells (gle'al). There......,.c_ apillary --i4} are six types of neuroglia-four in the CNS and t\vo in the PNS (Fi gure 11.4). Once considered merely the "glue" or scaffold- ing that supports the neurons, neuroglia are now known to have many other important and unique functions. Neuroglia in the CNS Neurogl ia in the CNS outnumber neurons and include aslro- cytes, ,nicroglial cells, ependy111al cells, and o/igodendrocyles (Figure l l.4a-d). Like neurons, most neuroglia have branch- ing processes (extensions) and a central cell body. They can be (a) Astrocytes are the most abundant CNS neuroglia. CNS distinguished, however, by their much smaller size and their darker-staining nuclei. Astrocytes Nearougia most abundant Shaped like delicate branching sea anemones, as t rocytes I (as' tro-sHz; "star cells") are the most abundant and versatile and capillaries ~ -Neuron cling to neurons glial cells. Their numerous radiating processes cling to neurons , ~=.=:;:-- - --:;-,r - Microglial functions: and their synaptic endings, and cover nearby capillaries. They cell support and brace the neurons and anchor lines them to their nutrient 1) support neurons anchors , them to nutrient supply supply lines (Figure l l.4a). 2) role in exchanges between capt and neurons. Astrocytes play a role in making exchanges between cap- 3) Determineillaries capandpermeability. neurons, helping determine capillary permeability. (b) Microglial cells are defensive cells in the CNS. They guide the migration ofneurons young neurons and formation of of migration 4) guide synapses (junctions)young between neurons. Astrocytes also control the chemical environment aroundneurons neurons, where their most Fluid-filled cavity 5) formation of synapses between important job is "mopping up" leaked potassium ions and Ict) leaked Further- 6) control recapturi ng andenvironment chemical ( mopping recycling released neurotransmitters. up more, astrocytes have been show n to respond to nearby nerve Ependymal 7) recapturing released neurotransmitters cells impulsesand recycling and released neurotransmitters. Connected by gap junctions, astrocytes each signalother each other with " Ca Brain or connected by gap junctions astrocytes signal with slow-paced intracellular calcium pulses (calcium waves), , spinal cord tissue and by releasing extracellular chemical messengers. They also waves by releasing extracellular messengers influence neuronal functioning and therefore participate in in- (c) Ependymal cells line cerebrospinal fluid- filled CNS cavities. 8) Participate info formationinprocessing processing in the brain. in the brain Axons Oligodendrocytes~ :;;=;>,;;;;;;;;;;; M icrog lial Cells Microglial cells (mi-kro' gle-al) are small and ovoid with rela- tively long "thorny" processes (Figure l l.4b). Their processes touch nearby neurons, monitoring their heal th , and whe n they sense that certain neuro ns are injured or in other trouble, the microglial cells migrate toward them. Where invading micro- Myelin sheath Myelin sheath gap organisms or dead neurons are present, the microglial cells (d) Oligodendrocytes have processes that form myelin transform into a special type of macrophage that phagocytizes sheath s around CNS nerve fibers. the microorganisms or neuronal debris. This protective role is important because cells of the immune system have limited access to the CNS. Ependymal Cells E pendymal cells (e-pen'di-mul; "wrapping garment") range...--- in shape from squamous to columnar, and many are ciliated (Figure l l.4c). They line the central cavities of the brain and the spinal cord, where they form a fairly permeable barrier (e) Satellite cells and Schwann cells (which form myelin bet\veen the cerebrospinal fluid that fills those cavities and the sheaths) surround neurons in the PNS. tissue fluid bathing the cells of the CNS. The beating of their Fi gure 11.4 Neurogl ia. (a-d) The four types of neuroglia of the cilia circulates the CSF. CNS. (e) Neuroglia of the PNS. fhotg-pf.gg 394 UNIT 3 Regulation and Integration of the Body cilia helps to circulate the cerebrospinal fluid that cushions the ability to divide. We pay a high price for this feature because brain and spinal cord. neurons cannot be replaced if destroyed. There are excep- Produce CNS myelin tions to this rule. For example, olfactory epithel ium a nd Oligodend rocytes some hippocampal regions of the brain contain stem cells - Though they also branch, the oligode ndrocytes (ol"T-go- that can produce new neurons throughout life. den 'dro-sTts) have fewer processes (oligo = few; de11dr = Neurons have an exceptio nally high metabolic rate and branch) than astrocytes. Oligodendrocytes line up along the requ ire continuous and abu ndant supplies of oxygen and thicker nerve fibers in the CNS and wrap their processes tightly glucose. They cannot survive for more than a few minutes around the fibers, producing an insulating covering called a without oxygen. 1nyeli11 sheath (Figure I l.4d). Although neurons vary in structure, they all have a cell body and one or more slender processes (Figure 11.5). Neuroglia in the PNS The two kinds of PNS neurogl ia-satellite cells and Schwa11n Neuron Cell Body cells--differ mainly in location. The neuron cell body consists of a spherical nucleus (with a Satellite cells surround neuro n cell bodies located in the conspicuous nucleolus) surrounded by cytoplasm. Also called peripheral nervous system (Figure I l.4e), and are thought to the perikaryon (peri = around, kary = nucleus) or soma , the have many of the same functions in the PNS as astrocytes do in cell body ranges in diameter from 5 to 140 µm. the CNS. Their name comes from a fancied resemblance to the In most neurons, the plasma membrane of the cell body acts moons (satellites) around a planet. as part of the receptive region that receives information from Schwa nn cells (also called neurole1111nocytes) surround all other neurons. nerve fibers in the PNS a nd form myel in sheaths arou nd the The cell body is the major biosy11thetic ce11ter and ,neta- thicker nerve fibers (Figures J J.4e and I I.Sa). In this way, they bolic center of a neuron. In addition to abu ndant mitochon- are functionally similar to oligodendrocytes. (We describe the dria, it contains many structures you are already familiar with, formation of rnyelin sheaths later in this chapter.) Schwann cells including: ~ Sub are vital to regeneration of damaged peripheral nerve fibers. Protein- and 111e1nbrane-11iaking 111achinery. Neuro n cell Check Your Understanding bodies (not axons) have the organelles needed to synthesize 3. Which type of neuroglia controls the extracellular f luid proteins-rough endoplasmic reticulum (ER), free ribosomes, environment around neuron cell bodies in the CNS? ER and Golgi apparatus. The rough ER, also called the chroma- Rough In the : tophilic substance (chromatophilic = color lovi ng) or Niss/ PNS? chroma tophive substance 4. Which two types of neurogl ia form insulating coverings called bodies (nis'I), stains darkly with basic dyes. I myelin sheaths? Cytoskeletal ele1ne11ts. Microtubules and neurofibrils, which _ _ _ _ _ _ _ _.....,_ For answers, see Answers Appendix. are bundles of intermediate filaments (neurofila,nents), mai ntain cell shape and integrity. They form a network throughout the cell body and its processes. Neurons are the structural units :¥a÷÷÷wn 11.3 Pig111e11t i11clusio11s. Pigments sometimes found inside neu - of the nervous system ¥ ron cell bodies include black melan in, a red iron-containing pigment, a nd a golden-brown pigment called lipofusci11 Learning Outcomes (lip"o-fu'sin). Lipofuscin, a harmless by-product of lysoso- ÷÷÷ Define neuron, describe its important structural components, and relate each to a functional role. mal activity, is sometimes called the "aging pigment" be- cause it accurnulates in neurons of elderly ind ividuals. Differentiate between (1) a nucleus and a ganglion, and Most neuron cell bodies are located in the CNS, where they (2) a nerve a nd a tract. Explain t he importance of the myelin sheath and describe are protected by the bones of the skull and vertebral column. how it is formed in the central a nd peripheral nervous Clusters of cell bodies in the CNS are called nuclei, \vhereas systems. those that lie along the nerves in the PNS are called ganglia Classify neurons by structure and by function. (gang' gle-ah; ganglio11 = "knot on a string," "swelli ng"). glial z Neurons, also called nerve cells, are the structural units of the Neuron Processes nervous system. There are bill ions of these (typically) large, supporting Arml ike processes extend from the cell body of all neurons. cells highly specialized cells that conduct messages in the form of The brain and spinal cord (CNS) contai n both neuron cell bod- nerve impulses from o ne part of the body to another. Besides ies and their processes. The PNS consists chiefly of neuron pro- their exci tability, they have three other special characteristics: cesses (whose cell bodies are in the CNS). Neurons have extre1ne longevity. Given good nutrition, they The two types of neuron processes, dendrites a nd axons can function optimally for a lifetime. (ak' sonz), differ in the structure a nd function of their plasma Neurons are a111itotic. As neuro ns assume the ir roles as mernbranes. The convention is to describe these processes using communicating links of the nervous system, they Jose their  Chapte r 11 Fundamentals of the Nervous System and Nervous Tissue 395 Dendrites Cell body (receptive (biosynthetic center regions) and receptive region) ' Neuron-- cell body --..... Nucleus ~t:!!e~- - Dendritic spine Nerveare wherepulses d where is the axon..... generate - ~ - - Initial ~ Axon (b) Nucleolus segment (impulse-generating of axon and conducting -''s zone) Chromatophilic - -~ E ' , + ,.s Inside.s positive s"' s"' ' J,.:' '' ', - 0 0 0. ( 0 - - ( > > Inside --...!! c negative more ④ --..; c c f?..&. -50 Depolarization..&... c -50 / Resting potential I E.. J:> :::; -70 "--Resting1 potenti~. J:> E :::; -70 I i:; :::....!__ Hyper- more -0 - 100 - 100 oolarization 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 Time (ms) Time (ms) (a) Depolarization: The membrane potential (b) Hyperpolarization: The membrane potential moves toward O mV, the inside becoming less increases, the inside becoming more negative. negative (more positive). Figure 11.9 Depolarization and hyperpolarization of t he membrane. The resting membrane potential is approximately - 70 mV (inside negative) in neurons. change in resting potential from -70 mV to -65 ,nv is a depo- cause current flows that decrease in magnitude with distance. larization (Figure 11.9a). Depolarization also includes events Graded potentials are called "graded" because their magni tude in which the membrane potential reverses and moves above varies directly with stimulus strength. The stronger the stimu- zero to become positive. lus, the more the vol tage changes and the farther the current Hyp erpolarization is an increase in membrane potential: flows. The inside of the membrane becomes more negative (moves Graded potentials are triggered by some change (a stimu- farther from zero) than the resti ng potential. For example, lus) in the neuron 's environment that opens gated ion channels. a ch ange from -70 mV to -75 mV is hyperpolarization Graded potentials are given d ifferent names, depend ing on (Figure I J.9b). As we will describe shortly, depolarization in- where they occur and the functions they perform. creases the probability of producing nerve impulses, whereas A receptor potential or a generator potential is produced hyperpolarization reduces this probability. when a sensory receptor is excited by its stimulus (e.g., light, pressure, chemicals). We will consider these two types of Check Your Understanding graded potentials in Chapter 13. 10. For an open channel, what factors determine in wh ich A postsynaptic potential is produced when the stimulus is a direction ions will move th rough that chan nel? neurotransmitter released by another neuron. Here, the neu- 11. For which cation are there the la rgest number of leakage chan nels in the plasma membrane? rotransmitter is released into a fluid-filled gap called a syn- apse and influences the neuron beyond the synapse. We will 12. jij34 )tiii Your patient, Ms. You ng, has fail ing kidneys that cause an increase in her extracellular fluid K+ concentration. discuss postsynaptic potentials in Module 11.8. What would this do to t he resting membrane pote ntial of her Fluids inside and outside cells are fairly good conductors, neurons and muscle cells? Explain. and current, carried by ions, flows through these fluids when- - - - - - - - - - - For answers, see Answers Appendix. ever voltage changes. Suppose a stimulus depolarizes a small area of a neuron 's plasma membrane (Figure 11.10a). Current (ions) flows on both sides of the membrane between the depo- I 11.s Graded potentials are brief, larized (active) membrane area and the adjacent polarized (rest- ing) areas. Positive ions migrate toward more negative areas short-distance signals within a neuron (the direction of cation movement is the direction of current Learning Outcome flow), and negative ions s imultaneously move toward more.... Describe graded potentials and name several examples. positive areas (Figure I I. I Ob). For our patch of plasma membrane, pos itive ions (mostly G raded poten tials are short-lived, localized changes in mem- K+) inside the cell move a\vay from the depolarized area and brane potential, usually in dendrites or the cell body. They can accumulate on the neighboring membrane areas, where they be e ither depolarizations or hyperpolarizations. These changes neutralize negative ions. Meanwhile, positive ions (rnostly Na +)  Chapter 11 Fundamentals of the Nervous System and Nervous Tissue 405 Consequently, the current d ies out within a few mill imeters of its origin and is said to be decre,nental (Figure 11. 1Oc). Because the current d issipates quickly and decays (declines) with increa~ing distance from the site of in itial depolarization, graded potentials can act as signals only over very short dis- Plasma tances such as in the dendrites and cell body. Nonetheless, they membrane are essential in initiating action potentials, the long-distance + ++ ++++ + - +++++++ signals of axons. (a) Depolarization: A small patch of the membrane (red area) depolarizes. Check Your Understanding 13. What determines the size of a graded potential? 14. Recall (from Chapter 9) that, at a neuromuscular junction, the AP in the muscle fiber is also triggered by a graded potential. Name that graded potential.. - - - - - - - - - -. For answers, see Answers Appendix. + + + + + + + - - - + + + + + + Action potentials are brief, (b) Depolarization spreads: Opposite charges attract each other. long-distance signals within a neuron This creates local currents (black arrows) that depolarize.. adjacent membrane areas, spreading the wave of depolanzat,on. Learning Outcomes Ill> Compare and contrast graded potentials a nd action 11.6 Action potentials potentials. Ill> Explain how action potentials are generated and Active area propagated alo ng neurons. (site of initial depolarization) Ill> Define a bsolute and relative refractory periods. r1-, Ill> Define saltatory conduction and explain how it differs from conti nuous conduction. The principal way neurons send signals over long distances is by generating and propagating (transmitting) act ion potentials. Only cells with excitable ,nembranes-neuro ns and muscle cells-can generate action potentials. : \ _ Resting potential An action potential (AP) is a brief reversal of membrane potential with a total ampl itude (change in voltage) of about JOO mV (from - 70 mV to +30 mV). Depolarization is fol- Distance (a few mm) lowed by repolarization and often a short period of hyperpolari- (c) Membrane potential decays w ith distance: Because current is zation. T he whole event is over in a fev, milliseconds. Unlike lost through the "leaky" plasma membrane, the voltage declines with graded potentials, action potentials do not decay with distance. distance from the stimulus (the voltage is decremental). In a neuron, an AP is also called a nerve im p ulse, and is Consequently, graded potentials are short-distance signals. typically generated only in axons. A neuron generates a nerve figure 11.10 The spread and decay of a g raded potential. impulse only when adequately stimulated. The stirnulus changes the permeability of the neuron's membrane by openi ng specific voltage-gated channels on the axon. These vol tage-gated chan- nels are generally found only on axons, where they are critical on the outside of the membrane move toward the depolarized for AP formation-no voltage-gated channels means no AP. region, which is momentarily Jess pos itive. As these positive Vol tage-gated c ha nne ls open and close in response to ions move, their "places" on the membrane become occupied changes in the me,nbrane potential. T hey are in itially activated by negative ions (such as CJ- and HC0 3 - ), sort of like ionic by local currents (graded potentials) that spread toward the musical c hairs. In this way, at regions next to the depolar- axon along the dendritic and cell body membranes. ized region, the inside becomes Jess negative and the outside In many neurons, the transition from local graded potential becomes Jess positive. The depolarization spreads as the neigh- - - to Jong-distance action potential takes place at the initial seg- bori ng membrane patch is, in turn, depolarized. 111ent of the axon. In sensory neurons, the action pote ntial is As just explained, the flow of current to adjacent membrane generated by the peripheral (axonal) process just proximal to areas changes the membrane potential there as well. However, the receptor region (see Table I 1.2). However, for simplic ity, the plasma membrane is permeable like a leaky garden hose, we will just use the term axon in our discuss io n. We'll look and rnost of the charge is quickly Jost through leakage channels. first at the generation of a n action potential and the n at its propagation. (Text conlinues on p. 408.). 2· Action Potential The act ion potential (AP) is a brief change in membrane potential in a patch of membrane that is depolarized by local currents. Watch a 3-D animation of this process: MasteringA&P" > St udy Area > Animations & Videos > A&P Flix The big picture What does this graph show? During the course of an action potential (below), voltage changes over time at a given point within the axon. inside becomes + inside becomes - @ Depolarization is @ Repolarization is caused by Na flowing caused by K+ flowing into the cell. out of t he cell. / ) / / / +30 © Hyperpolarizat ion is caused by K+ continuing to leave the cell. eienmoveinside 2 Threshold -70 Q) Resting state. No ions move through voltage-gated channels. 0 1 2 3 4 Time (ms) The key players Voltage-gated Na+ channels have two gates and Voltage-gated K+ channels have alternate between three different states. one gate and two states. Outside Outside cell Na Na Na cell I O Activation Inactivation l Inside gate gate Inside cell cell Closed at the Opened by lnactivated-{;hannels Closed at the Opened by resting state, so no depolarization, automatically blocked by resting state, so no depolarization, after Na enters the cell allowing Na to inactivation gates soon K exns the cell a delay, allowing K through them enter the cell after they open through them to exn the cell open inactivation closed inact. gate gate gate 406 Act - gale act. closed open  stale 1) Resting channels open The events only leakage are Each step corresponds to ↳ Maintains R P - one part of the AP graph. each Na channel has 2 voltage sensitive gates : 1)Activation gates closed :. at rest Na open at depolarization 4) Hyperpolarization Sodium Potassium Allow Nat in - channel channel " 2) Inactivation some Kt channels open allowing gates open :. at rest eflux -0 inside blocks channel exces-s-iue.tt = move cell to block Nat state More -0 than resting Activation = causes hyperpolarization Inactivation gates of the membrane gate reset Nat channels © Rest ing state: All gated Na and K channels ↳ open inacli. gates are closed. ↳ close act. gates Na \.__ ©__ Na -0 ® \_..._ ___ ) -. - \) K ⑦ © Hyperpolarization: Some K+ channels remain @ Depolarization: Na channels open, allowing open, and Na channels reset. Na entry. 2) Depolarization Nat channels open sodium + Depolarization opens Not enters 3) Repolarization gated channels , and and inactiv gates inactivating - activ Nat channels are Nat. - channels open Kt open more depot - Nat influx causes Inac. gates close causing more Nat channels membrane perm to Nat declines to open. to resting stale - so KFCinside) becomes less ↳ AP spike stops rising negative @Repolar ization: Na channels are inactivating. At threshold 1- 55mV to -50mV ) Kt channels OPEN K+ channels open, allowing K+to exit. feedback causes opening of it's inside becomes -0 + ↳ Kt exits cell down conc - grad → au Nat channels spike 407 = large Ap = Membrane returns to resting ↳ +30mV 408 UNIT 3 Regulation a nd Integration of the Body Generating an Action Potential of the resting neuron, an event called repolarization. Both the abrupt decline in Na+ permeability and the increased Focus on an Action Potential ( Focus Figure 11.2) on pp. 406- permeability to K+ contribute to repolarization. 407 describes how an action potential is generated. Let's start with an axon in the resting (polarized) state. © Hyperpola rization: Some K+ cha nne ls re ma in ope n, a nd Na + cha nnels reset. The period of increased K+ perme- Q) Resting sta te: All vo ltage-gated Na+ a nd K+ cha nne ls a re ability typically lasts longer than needed to restore the rest- closed. Only the leakage channels are open, maintaining ing state. As a result of the excessive K+ efflux before the resting me,nbrane potential. Each Na+ channel has two potassium channels close, a hyperpolarization is seen on gates: a voltage-sensitive activation gate that is closed at the AP curve as a slight dip following the spike. At the end rest and responds to depolarization by opening, and an of this phase, the Na+ channels have reset to their original inactivation gate that blocks the channel once it is open. position by changing shape to reopen their inactivation gates Thus, depolarization opens and then inactivates sodium and close their activation gates. channels. Both gates must be open for Na+ to enter, but the clos- Repolarization restores rest →ing electrical conditions. While conditions electrical ing of either gate effectively closes the channel. In contrast, you mightRepolarization resets think that the ionic conditions change as a result of each voltage-gated potassitun channel has a single voltage- movement of massive numbers of Na + and K+ ions during an action O Complete sensitive gate that is closed in the resting state and opens Nat / Kt pumps → resets - ionic a nconditions slowly in response to depolarization. potential, th is is not the case. Only interactive tutorial: ( small amounts of sodium and potas-maints - conc gradients MasteringA&P" > Study @ Depola rizat ion: Vo lta ge-g ated Na + channe ls ope n. As sium cross the membrane. (The Na+ Area > Interactive local currents depolarize the axon membrane, the voltage- and Kt back in influx required to reach thresholdNatPhysiology escorts out gated sodium channels open and Na+ rushes into the cell. produces only a 0.0 I 2o/o change in This influx of positive charge depolarizes that local patch intracellular Na+ concentration.) These s,nall ionic changes are of membrane further, opening ,nore Na + channels so the quickly corrected because an axon membrane has thousands of cell interior becomes progressively Jess negative. Na+-K+ pumps. When depolarization reaches a critical level called thr eshold (often between -55 and - 50 mV), depo- Threshold and the All-or-None larization becomes self-generating, urged on by positive feedback. As more Na + enters, the membrane depolar- Phenomenon izes further and opens still more channels until all Na+ Not all local depolarization events produce APs. The depolari- channels are open. At this point, Na+ permeability is about zation must reach threshold values if an axon is to "fire." What I000 times greater than in a resting neuron. As a result, determines the threshold point? the membrane potential becomes Jess and Jess negative and One explanation is that threshold is the membrane poten- then overshoots to about + 30 mV as Na + rushes in along tial at \vhich the outward current created by K+ movement is its electroche,nical gradient. This rapid depolarization and exactly equal to the inward current created by Na+ movement. polarity reversal produces the sharp upward spike of the T hreshold is typically reached when the membrane has been [ action potential (Focus Figure 11.2). depolarized by 15 to 20 mV from the resting value. This depo- Earlier, we stated that membrane potential depends on larization status represents an unstable equ ilibrium state. If one membrane permeability, but here we say that membrane more Na+ enters, further depolarization occurs, opening more permeability depends on membrane potential. Can both Na+ channels and allowing more Na+ to e nter. If, on the other statements be true? Yes, because these l\vo relationships hand, one more K+ leaves, the membrane potential is driven establish a positive feedback cycle: Increasing Na+ perme- away from threshold, Na+ channels close, and K+ continues to abili ty due to increased channel openings leads to greater ⑦ feedback diffuse oul\vard unt il the potential returns to its cycle resting value. depolarization, wh ich increases Na+ penneability, and so Triggers Recall that local depolarizations are graded potentials and on. T his explosive positive feedback cycle is responsible their magnitude increases when stimuli become more intense. - Brief weak stimuli (subthreshold stilnuli) produce subthreshold for the rising (depolarizing) phase of an action potential-it puts the "action" in the action potential. depolarizations that are not translated into nerve impulses. On @ Re pola rization: Na + cha nne ls are in activating, a nd the other hand, stronger threshold sti111u/i produce depolarizing voltage-gated K+ cha nnels open. The explosively rising currents that push the membrane potential toward and beyond phase of the action potential persists for only about I ms. the threshold voltage. As a result, Na + permeability rises to It is self-limiting because the inactivat ion gates of the Na+ such an extent that enteri ng sodium ions "swamp" (exceed) the channels begin to close at this point. As a result, the mem- outward movement of K+, establishing the positive feedback brane permeability to Na+ declines to resting levels, and cycle and generating an AP. the net influx of Na+ stops completely. Consequently, the T he critical factor here is the total amount of current that AP spike stops rising. flows through the membrane during a stimulus (electrical As Na+ entry declines, the slow voltage-gated K+ chan- charge x time). Stro ng stimul i depolarize the membrane to nels open and K+ rushes out of the cell, following its elec- threshold qu ickly. Weaker stimuli must be appl ied for longer trochemical gradient. This restores the internal negativity  KARAI Chapte r 11 Fundamentals of the Nervous System and Nervous Tissue 409 periods to provide the crucial amou nt of current flow. Very depolarize adjacent membrane areas in the forward direction AP (away from the origin of the nerve impulse), which ope ns HOW is weak sti muli do not trigger an AP because the local current flows they produce are so slight that they dissipate long before an vol tage-gated channels and triggers an action potential there threshold is reached. (Figu re 11.11). This process repeats down the length of the An A P is a n a ll-or-none p h eno,n en on: It either happens completely or doesn't happen at all. We can compare the gen- eration of an AP to lighting a match under a small dry t\vig. The generated ? axon so that an AP is self-propagating and continues along the axon at a constant velocity-something like falling dorninos. Following depolarization, each segrnent of axon membrane of Nations c ha nges occurring where the tw ig is heated are a nalogous to repolarizes, restoring the resting membra ne potential the influx in that the change in membrane permeability that initially allows more - By changes also set up local cur- region. Because these electrical ↳ Na+ to enter the cell. When that part of the twig becomes hot rents, the repolarization wave chases the depolarizat ion wave e nough (when enough Na+ enters the cell), it reaches the flash down the length of the axon. Kt eflux point (threshold) and the flame consumes the entire l\vig, even must exceed The propagation process we have just described occurs on if you blow out the match. Similarly, the AP is generated a nd nonmyeli nated axons. On p. 4 11, we will describe propagation propagated whether or not the stimulus continues. But if you along myelinated axons. blow out the match before the twig reaches the threshold tem- Although the phrase conduction of a nerve irnpulse is com- perature, ig nition will not take place. Likewise, if the number monly used, nerve impulses are not really conducted in the of Na+ ions entering the cell is too low to achieve threshold, no same way that an insulated wire conducts current. In fact, neu- AP will occur. rons are fairly poor conductors, and local current flows decline with distance because the charges leak through the membrane Propagation of an Action Potential (Figure 11. 10). The expression propagation of a nerve irnpulse If it is to serve as the neuron's signal ing device, an AP must is more accurate, because the AP is regenerated anew by the be p r opagated along the axon 's voltage-gated channels at each membrane patch, and every sub- entire length. As we have seen, Q Watch a 3-D sequent AP is ide ntical to the o ne that was generated init ially. voltage gated No the AP is generated bychannels the influx =animation no of this process: propagation Without voltage-gated channels, propagation can not occur (see of N a + th roug h a g iven area MasteringA&P > Study Figure l l. l 4a). 0 Area > Animations & of the membrane. T his in fl ux Videos > A&P Fl ix establishes loca l c urre nts tha t >E - "iv + 30 Voltage at2 ms "' 8. Vottage Voltage., at Oms at 4 ms i I "- ,:::, - 70 - I :. Recording electrode + ++ +++ ++ D + +++ + + + ++ +++ -( ~ +++++++++++++ + + + + + + - W,;+ + + + + + + + + + + + + + + + + - W,;+ (a) Time = O ms. Action potential has (b) Time = 2 ms. Action potential (c) Time = 4 ms. Action potential not yet reached the recording peak reaches the recording peak has passed the recording electrode. electrode. electrode. Membrane at the recording electrode is still D Resting potential hyperpolarized. Im Peak of action potential D Hyperpolarization Figure 11.11 Pro pagation of an action potential (AP). Recordings at three successive times as an AP propagates along an axon (from left to right). The arrows show the direction of local current flow generated by the movement of positive ions. This current brings the resting membrane at the leading edge of the AP to threshold, propagating the AP forward.  stimulus is Ht If enough 410 UNIT 3 Regulation and Integration of the Body Yoo e ¥ strong f. another AP , AP may Coding for Stimulus Intensity Ab~olute refractory period ✗ xx J here _ f Rel~tive refractory Lperiod in another generate Once generated,all all APs APare 's independent of stimulusofstrength, 1~-.. , _ - ~ - - - - ' - - - - - ~1 Once generated are independent and all APs are alike. So how can the CNS determine whether Depolarization stimulus strength a particular stimulus is intense or weak? For example, how can. (Na+ enters) + 30 you tell the difference between a quiet hum and a loudoften roar? generate >E - nerve impulses more The answerstimuli is really quite simple: Strong stimuli gener- Stronger ate nerve impulses more often in a given time #interval of than do impulses Stimulus intensity is coded for by the v,eak stimuli. Stimulus intensity is coded for by the number Kt leaves not increase in frequency of Ap is, byby or the of impulses per second-that the frequency of action →mJa+ \--....,;...- Repolarization strength cnmplitvdex potentials-rather ) than by increases in the strength (amplitude) (K+ leaves) of the individual APs (Figure 11.12). 9 frequency9 stimuli = enters ( Hyperpolarization [: Action :-\ Nat / potentials \ MAP frequency Stimulus ✓ gates 0 1 2 3 4 5 Time (ms) reset Kt still Figure 11.13 Absolute and relat ive refractory periods in leaves an AP. = hypetpol. more -., 9 Stinks The relative refractory period follows the absolute refrac-.. Stimulus "' Threshold 1.2 ::, "' tory period. During the relative refractory period, most Na+ E := - - - - - _J _- _r,,_~~ - -- - ·- 0 0 channels have returned to their resting state, some K+ chan- tii > nels are still open, and repolarization is occurring. The axon's Time (ms) -jl,, threshold for AP generation is substantially elevated, so a stim- ulus that would normally generate an AP is no longer sufficient. A subthreshold The stronger the stimulus, An exceptionally strong stimulus can reopen the Na+ channels stimulus does not the more frequently APs that have already returned to their resting state and generate generate an AP. are generated. another AP. Strong stimuli trigger more frequent APs by intrud- ing into the relative refractory period. Figure 11.12 Relationship between stimulus st rength and It might help you to remember the difference between the action potential frequency. absolute and relative refractory periods if you know that the word "refractory" means stubborn or unmanageable. Think of a dog that is refractory (disobedient). If he is absolutely in refrac- in slower tory, he won' t come no matter how loudly you call. But if he