Biology 1001 Ch42&43 Lecture Notes PDF

Biology 1001 Brooklyn College, CUNY E. Wiech Chapter 42 Chapter 43 Please note, you are prohibited from sharing, posting, or distributing these PowerPoint slides...

Biology 1001 Brooklyn College, CUNY E. Wiech Chapter 42 Chapter 43 Please note, you are prohibited from sharing, posting, or distributing these PowerPoint slides (the entire file or selected slides). © 2020 McGraw-Hill Education. All rights reserved.  Chapter 42 Cells of the Nervous System Key Concepts: Cellular Components of Nervous Systems Electrical Properties of Neurons and the Resting Membrane Potential Generation and Transmission of Electrical Signals Along Neurons Neurons Communicate Electrically or Chemically at Synapses 2  Chapter 43 Neuroscience II: Evolu*on, Structure, and Func*on of the Nervous System Key Concepts: The Evolu*on and Development of Nervous Systems 3  The organization of the nervous system: Central nervous system (CNS) – ______________which in vertebrates brain and nerve cord extends from the brain through the vertebral column and is called the spinal cord. Peripheral nervous system (PNS) – all neurons that are ______________ outside of the CNS In certain invertebrates with a simple nervous system, the distinction is less clear. 4  Representa)ve nervous systems All animals have a nervous system except for sponges. All animals except sponges have neurons Number of neurons varies widely as a function of size and behavioral complexity. 5  Representa)ve nervous systems ________: Nerve Net –Simplest nervous system –Cnidarians (jellyfish, hydras, anemones) –Neurons connect to each other in a network. –AcFvaFon of neurons in one area leads to acFvaFon of all or most neurons: ExcitaFon spreads in all direcFons at once Usually sFmulates contracFle cells 6  Representa)ve nervous systems Echinoderms –Slightly more complex –Nerve ring around mouth connected to larger radial nerves extending to arms –Mouth and arms operate independently 7  Nervous systems: Cephaliza)on Trend toward cephaliza*on –ConcentraFon of sense organs at anterior end –__________________that Increasingly complex brain is composed of more than one anatomical and funcFonal region 8  Representa)ve nervous systems Platyhelminthes (flatworms) – First animal phylum to evolve a brain with defined regions with many synapFc connecFons. – ___________: Nerve Ladder Two nerve cords extend length of animal connected by transverse nerves 9  Representa)ve nervous systems Annelids – _________ Ganglia (collecFon of neuron cell bodies with limited processing ability, limited synapses, and few to no subdivisions like those found in a brain) and nerves in each segment – Coordinate local sensory and motor acFviFes Simple mollusks –Similar to annelids –Pair of anterior ganglia –Paired nerve cords 10  Representa)ve nervous systems Insects – Drosophila – Brain has several subdivisions with separate funcFons Advanced mollusks –Well-developed brain with ________ subdivisions –Coordinates complex visual sensing and motor behaviors 11 Representa)ve nervous systems Chordates – Brain connected to dorsal spinal cord [Central nervous system (CNS)] – Peripheral nervous system (PNS) – composed of nerves that route informaFon into and out of Central nervous system 12  Representa)ve nervous systems Invertebrate nervous systems vary widely in complexity: a) The simplest systems are nerve nets – cnidarians. b) A flatworm has collections of neurons called ganglia at its head end, with a nerve ladder running the length of the body. c) Earthworm and arthropod nervous systems are more centralized, with ventral nerve cords connected to an anterior brain. 13  Cellular Components of Nervous Systems Neurons – ________________cells Electrically excitable – Cells that send and receive electrical and chemical signals to and from other neurons or other cells throughout the body. 14  Neuron structure: _________________ Cell body or soma q Contains nucleus and organelles Dendrites q Extensions of plasma membrane, single or branching q _______________signals Receiving incoming Axons q Extension of plasma membrane q Typically single q _______________other Send signals to cells q Axon hillock near cell body; the axon hillock is important in the generaFon of the electrical signals that travel along an axon ________convey q Axon terminals electrical or chemical message to other cells such as other neurons or muscle cells at synapse 15 Within an animal's body, many axons tend to run in parallel bundles to form nerves, which are covered by a protective layer of connective tissue. Nerves enter and leave the Central nervous system and transmit signals between the Peripheral nervous system and the Central nervous system. Along the way, the axon terminals communicate with particular cells of the body. 16  Glia - cells that surround the neurons. In vertebrates, specialized glial cells wrap around the axons at regular intervals to form an insulating layer called a _____ myelin The sheath is periodically interrupted by noninsulated gaps called Nodes ______of Ranvier Node of Ranvier a) In the human brain and spinal cord, the myelin- producing glial cells are Myelin sheath called ____________ aligodendrocytes Glial cell b) the glial cells that form myelin on axons that A single glial cell wraps itself around an axon to form a segment of the myelin sheath. travel outside the brain and spinal cord are called _________________ Schwann Cells A myelinated neuron a: © James Cavallini/BSIP/Phototake Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Signal direction Dendrites _____ Dendrites receive electrical and chemical messages from other Cell body neurons. ____processes Cell body incoming Axon hillock signals and generates outgoing signals. Signal direction ____ Axon sends outgoing signals to axon terminals. ____make Axon terminals contact with Axon terminals nearby cells and transmit signals to them. A myelinated neuron a: © James Cavallini/BSIP/Phototake Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Three main types of neurons: n _____________neurons Sensory (Afferent neurons) q Detect information from the outside world or internal body conditions q transmit to CNS q have a long, single axon that branches into a peripheral process and a central process, with the cell body in between n __________neurons Motor q Send signals away from CNS (efferent neurons) to elicit response (movement) q have long axons, but do not branch into two main processes. n ______________ Interneurons or association neurons q Form interconnections between other neurons in the CNS q have many dendrites, and their axons are typically short and highly branched. This arrangement allows interneurons to form complex connecFons with many other cells. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (a) Sensory neuron Cell body Axon Dendrites (b) Motor neuron Dendrites (c) Interneuron To CNS Cell body Axon Axon Dendrites Cell body Peripheral Central process process 19 Direction of signal Plasma membrane is not very permeable to cations and anions. ->Separates charge by keeping different ions largely inside or outside cell. Membrane potential: _________ Difference in charge inside and outside the cell. Distribution of charges across the neuronal plasma membrane - Extracellular + - - + - environment - + - + + + + + - - - - - - + + - + - + - - - Neuron + - - + Positive charges outside the - + + - + - - - + - + neuron and negative charges + - + - inside attract each other and + - + line up along the membrane + - - + - + surfaces. + - - - + - + + - + + - - + - + + - - - - - - + - + - Cytosol + - + - + + + - -+ + + - - + - - - - - + - + + - - - + + - + + + + + Resting membrane potential: i. When neurons are not sending signals. ii. _______resting -70 mV potential inside cell. iii. ________ Interior more negative than __________. exterior  Three factors that influence the resting potential Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Na+/K+ Extracellular ____________establishes -ATPase environment and maintains the concentration gradients for Na+ and K+. Upgated__________________ k+ and Na+ channels At rest, the membrane is more Plasma + + allow passive permeable to K+ than to Na+. membrane 3 Na+ Na+/K+-ATPase movements Na+ tends to diffuse into cells Na+ of these ions. and K+ diffuses out of cells + + when the cell is at rest. + + + + The greater number of leak Na+ channel channels for K+, therefore, + means that excess positive + charges exit the cell. – K+ Leak channels 2 K+ – K+ channels – – + – – Anion negatively –– + __________molecu + charged – – les such as – – proteins are more abundant – inside the cell. – Cytosol 22  Only neurons and muscle cells are excitable – capacity to generate electrical signals Ion diffusion Ion Extracellular environment through channel + + + + + + + ++ - - + + ++ + - + - For a cell to communicate using Voltage change - + + + ++ + - electrical signals, it must be able to - - - - - - - - change its membrane potential very + - - - - ++ rapidly. This is accomplished by - - - + + + + - - - - - gated ion channels: - Closed channel Open channel Cytosol (a) Voltage-gated ion channel a) ____________ion Voltage-gated channels open and close in response to changes in voltage across the Extracellular Neurotransmitter Ion diffusion environment binding site through channel membrane. + + Neurotransmitter + + + + + Neurotransmitter + b) ____________ion Ligand-gated channels, binding also known as chemically gated + + + ++ ion channels, open or close - - + - - - - - - when ligands, such as - - neurotransmitters, bind to them. - - - - - - - - Closed channel Open channel Cytosol (b) Ligand-gated ion channel Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The opening and closing of ligand-gated and voltage-gated ion channels are responsible for two types of changes in a neuron's membrane potential: graded potentials and action potentials. Two types of changes: 1. _______________ Graded potentials q Depolarization or hyperpolarization q Varies depending on strength of stimulus q Occur locally on dendrites or cell body q Spreads a short distance and dies out q Act as triggers for action potential  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. _______ – cell membrane less polarized, less Depolarization _________ – cell membrane more Hyperpolarization negative relative to surrounding solution polarized, more negative qGated channels open allowing Na+ to qK+ moves out of the cell making flow in and membrane potential becomes the cell membrane less positive more positive (less negative) (more negative) Graded potential: Graded potential: Depolarization Hyperpolarization 0 Resting Membrane potential (mV) potential Strong stimulus Resting potential –50 Depolarization Weak stimulus –70 Strong Weak stimulus stimulus Hyperpolarization 110 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 Time (msec)  The opening and closing of ligand-gated and voltage-gated ion channels are responsible for two types of changes in a neuron's membrane potential: graded potentials and action potentials. Two types of changes: 2. ________________ Action Potentials q Carry electrical signal along an axon q Always large amplitude depolarization q All-or-none fashion– cannot be graded q Actively propagated – regenerates itself as it travels 26  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. -30 3 Action potential 0 Membrane potential (mV) 2 -50 1 Resting Threshold 4 5 -70 potential potential 0 1 2 3 Time (msec) Voltage-gated Voltage-gated Na+ channels K+ channels 1 Resting membrane potential: Closed Closed The membrane is at the resting potential, occasionally generating graded potentials. Cytosol  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 3 -30 Action potential 0 Membrane potential (mV) 2 -50 1 Resting Threshold 4 5 potential -70 potential 0 1 2 3 Time (msec) Voltage-gated Voltage-gated Na+ channels K+ channels 2 Depolarization to threshold: Open Closed An action potential is triggered when the threshold potential of about −55 to −50 mV is reached. Voltage-gated Na+ channels open. Na+ diffuses Na+ into the cell and depolarizes the membrane.  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. -30 3 Action potential 0 Membrane potential (mV) 2 -50 1 Resting Threshold 4 5 -70 potential potential 0 1 2 3 Time (msec) Voltage-gated Voltage-gated Na+ channels K+ channels 3 Peak of action potential: At about +30 mV, voltage- Inactivated Open gated Na channels are K+ inactivated, and voltage-gated K+ channels open. K+ exits the cell and repolarizes the membrane. At this time, the membrane is in its absolute refractory period. Inactivation gate  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. -30 3 Action potential 0 Membrane potential (mV) 2 -50 1 Resting Threshold 4 5 -70 potential potential 0 1 2 3 Time (msec) Voltage-gated Voltage-gated Na+ channels K+ channels 4 Repolarization: Voltage-gated Na+ channels Closed Open change from inactivated to closed. Voltage-gated K+ channels remain open, causing a hyperpolarization of the membrane. The membrane is now in its relative refractory period.  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. -30 3 Action potential 0 Membrane potential (mV) 2 -50 1 Resting Threshold 4 5 -70 potential potential 0 1 2 3 Time (msec) Voltage-gated Voltage-gated Na+ channels K+ channels 5 Restoration of resting Closed Closed membrane potential: Voltage-gated K+ channels close, and the resting 26.3 mh potential of the membrane is restored by the Na+ /K+-ATPase pump.  Voltage-gated K+ channels also open at threshold potential, but 1 msec later than Na+ channels K+ leave cell and membrane becomes negative again So many K+ leave that membrane hyperpolarizes Voltage-gated K+ channels close and resting membrane potential is restored Evolution of K+ channels with a _________________________than slightly slower opening time Na+ channels was a key event that led to the formation of nervous systems If both opened at the same time, they would negate each other’s effects 32  Absolute refractory period qWhile inactivation gates of Na+ channels are closed, cell is unresponsive to another stimulus qPlaces limits on the frequency of action potentials qAlso ensures action potential does not move backward toward cell body 33  Conduction Graded potentials reach threshold potential at the _______________ axon hillock Triggers opening of voltage-gated ____ Na+ channels just beyond hillock region Depolarizes area farther along axon Sequential opening of _______ Na+ channels conducts a wave of depolarization from axon hillock to axon terminal Inactivation of gated _______ Na+ channels prevents backward movement toward cell body. 34  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Axon hillock Na+ channels along the axon Axon terminals 1 Unstimulated neuron; Na+ channels are closed. KEY Closed Na+ Open Na+ Inactivated Na+ channels channels channels 35  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Action potential Na+ channel open + + 2 Threshold potential is reached at axon hillock. Na+ channels open, and an action potential is generated. KEY Closed Na+ Open Na+ Inactivated Na+ channels channels channels 36  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Inactivation gate closed + + 3 Entry of Na+ depolarizes membrane and opens channels toward the axon terminal, generating new action potentials. Previously opened Na+ channels are inactivated. KEY Closed Na+ Open Na+ Inactivated Na+ channels channels channels 37  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. + + 4 Continued entry of Na+ depolarizes the membrane and opens channels farther down the axon. Previously opened Na+ channels are inactivated. Previously inactivated Na+ channels switch to a closed state when K+ channels (not shown) open and restore the resting potential. KEY Closed Na+ Open Na+ Inactivated Na+ channels channels channels 38  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. + + 5 Process continues and action potential moves down axon. KEY Closed Na+ Open Na+ Inactivated Na+ channels channels channels 26 mh-review 39  Neurons Communicate Electrically or Chemically at Synapses Synapses Junction where nerve terminal meets a neuron, muscle cell, or gland Presynaptic cell (_________), synaptic sends signal cleft and postsynaptic cell (__________) receives signal Two types qElectrical – electric charge freely flows through gap junctions from cell to cell qChemical – neurotransmitter acts as signal from presynaptic to postsynaptic cell 40  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Axon terminal of presynaptic cell Synaptic cleft Presynaptic Neuron1 Postsynaptic Postsynaptic cell membrane Neuron 2 Axon Neuron 3 terminal Presynaptic Synapses Postsynaptic Presynaptic Synapses Postsynaptic 41  Chemical synapse In a presynaptic cell, an action potential opens 1 voltage-gated ___ Ca2 channels.Ca2+ + __enters the cytosol. Presynaptic cell Presynaptic nerve cell contains vesicles of neurotransmitter. Action potential Ca2+ 2 Intracellular ___binds Ca2+ to vesicles and causes them to channel Vesicle fuse with the presynaptic cell membrane, releasing Ca2+ binds to vesicle neurotransmitter into the synaptic cleft via exocytosis. Ca2+ Exocytosis of neurotransmitter Reuptake of neurotransmitter Neurotransmitter Exocytosis releases neurotransmitter into synaptic cleft Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Chemical synapse Presynaptic cell Action potential Ca2+ channel Vesicle Ca2+ binds Neurotransmitter to vesicle Ca2+ 3 molecules diffuse across Exocytosis of neurotransmitter Synaptic cleft the synaptic cleft and bind to receptors in the post synaptic cell membrane Neurotransmitter receptors Postsynaptic cell Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Chemical synapse Presynaptic cell Action potential Ca2+ channel Vesicle Ca2+ binds to vesicle In this example, the receptor Ca2+ Exocytosis of Synaptic cleft 4 is a ligand-gated ion channel neurotransmitter that opens in response to neurotransmitters and allows the movement ofcautions ____ Reuptake of neurotransmitter Neurotransmitter into the postsynaptic cell. This depolarizes the membrane. cations Receptors with bound neurotransmitter are open Postsynaptic neurotransmitter Postsynaptic Degrading enzymes receptor cell Some neurotransmitter molecules are New gr aded po 5 taken back up into the presynaptic cell or te are broken down by degrading enzymes. generate ntial d 26.4 mh Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Neurons Communicate Electrically or Chemically at Synapses Neurotransmitters qMore than 100 different ones in animals qCategorized by size or structure Excitatory and inhibitory neurotransmitters qLike brake and accelerator on a car All nervous systems operate with combined excitatory and inhibitory neurotransmitters 45  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites of postsynaptic neuron Presynaptic neurons E2 E1 I1 Cell body of I2 postsynaptic neuron Axon hillock of postsynaptic KEY neuron Synapses that are excitatory (E) Synapses that are inhibitory (I) (a) A single neuron receiving many inputs 46  Postsynaptic receptors In some cases, same neurotransmitter can have excitatory or inhibitory effects Response of postsynaptic cell depends on receptor type Ionotropic receptors – ligand-gated ion channels open in response to neurotransmitter Metabotropic receptors – G-protein coupled receptors (GPCRs) initiate changes in postsynaptic cell 47  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Neurotransmitter Neurotransmitter Extracellular environment binding site Pore (open) Pore (closed) Subunit Cytosol (a) Ionotropic receptor 48  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Neurotransmitter Extracellular environment Neurotransmitter binding site Cytosol G-protein binding site (b) Metabotropic receptor Trimeric G protein 49  Five classes of neurotransmitters 1. Acetylcholine q One of most widespread neurotransmitters q Released at neuromuscular junctions q Excitatory in brain and skeletal muscles but inhibitory in cardiac muscles 2. Biogenic amines q Widespread physiological effects and psychoactive q Abnormally high or low levels associated with a variety of mental illnesses (schizophrenia, depression) 50 3. Amino acids q Glutamate most widespread excitatory neurotransmitter q GABA (gamma aminobutyric acid) most common inhibitory neurotransmitter 4. Neuropeptides q Often called neuromodulators – can alter response of postsynaptic neuron to other neurotransmitters q example: Opiate peptides 51 5. Gaseous neurotransmitters q Nitric oxide, NO q Carbon monoxide, CO q Not sequestered into vesicles q Produced locally as required q Short-acting – influence other cells by diffusion q Drugs for male sexual dysfunction enhance erections by increasing or mimicking action of NO on smooth muscle q Function of CO uncertain 52  Generation and Transmission of Electrical Signals Along Neurons Action potentials are the electrical events that carry a signal along an axon. In contrast to a graded potential, an action potential is always a large depolarization; all action potentials in a given neuron have very similar amplitudes, which is the degree to which an action potential changes the membrane potential away from its resting state. Once an action potential has been triggered, it occurs in an all-or-none fashion. In other words, it cannot be graded. Unlike a graded potential, an action potential is actively propagated along the axon, regenerating itself as it travels. Action potentials travel rapidly down the axon to the axon terminals, where they initiate a response at the junction with the next cell. 53  Use your understanding of how action potentials are generated and propagated 1.to answer All cellsthe following maintain questions. an ionic (and therefore electrical) potential difference across their membranes. In most cells, this potential difference is between 250 and 2100 mV. 1.That Allis, cells the maintain an ionic inside of the (and cells is moretherefore negativeelectrical) potential than the outside bydifference across their 50 to 100 mV. membranes. Although In most all cells in thecells, body this potential maintain differencedifference this potential is between across and 2100 mV. 250their That is, theonly membranes, inside of the certain cells cells (forisexample, more negative neurons)thanarethe outside capable ofby 50 to 100 mV. generating Although action all cells in the body maintain this potential difference across their potentials. membranes, only certain cells (for example, neurons) are capable of generating a. action How is potentials. this potential difference across the cell membrane generated? Sodium-potassium pumps in cell membranes actively pump Na1 ions out of cells a.andHow K1 is this ions potential into cells. Indifference across general, for everythe cellNa three membrane 1 generated? ions pumped out of a cell, two K1 ions are pumped Sodium-potassium into the pumps cell.membranes in cell As a result,actively the inside of the pump 1 Nacell contains ions out of cells fewer and positively K1 ions into charged cells.ions than thefor In general, outside; everythat is,Na three the1inside of the cell ions pumped outisof a cell, two K1 ions are pumped into the cell. As a result, the inside of the cell contains 22 fewer positively charged ions than the outside; that is, the inside of the cell Activity is 48.2 Copyright © 2017 Pearson Education, Inc. 322 Activity 48.2 Copyright © 2017 Pearson Education, Inc. RF.indd 322 54 6_PRF.indd 322 1. All cells maintain an ionic (and therefore electrical) potential difference across their membranes. In most cells, this potential difference is between 250 and 2100 mV. That is, the inside of the cells is more negative than the outside by 50 to 100 mV. Although all cells in the body maintain this potential difference across their membranes, only certain cells (for example, neurons) are capable of generating more action negative than the outside of the cell. This difference in ion distribution can potentials. be measured as a voltage or potential difference across the cell membrane. For the is a. How squid axon, thedifference this potential potentialacross difference the cellismembrane 270 millivolts (mV). generated? Sodium-potassium pumps in cell membranes actively pump Na1 ions out of cells b. and What characteristics K1 ions into cells. Inofgeneral, membranes allow for every threecells Na1toions concentrate pumped outorofexclude a cell, ions? 1 two K ions are pumped Cell membranes into Cell the cell.permeable. membrances are selectively As a result,Ions are selectivellythe inside such of permable as the Nacell 1 contains and K1 cannot fewer positively charged ions than the outside; that is, the inside of the cell is diffuse across the phospholipid bilayer. Ions must either be actively transported or diffuse through specific ion channels in the membranes. The ratio of Na1 to 22 1 Activity 48.2 K ion channels can also affect the relative Copyright concentrations of the © 2017 Pearson ions Inc. Education, in cells. c. What is it about neurons (nerve cells) that make their properties different from those of other cells? In other words, what enables nerve cells to produce action RF.indd 322 potentials? All cells generate a potential difference across their membranes. However, only nerve cells have voltage-gated ion channels that open and close in response 55 to  the squid axon, the potential difference is 270 millivolts (mV). 1. All cells maintain an ionic (and therefore electrical) potential difference across their b. What characteristics membranes. In most cells,ofthis membranes potential allow cellsistobetween difference concentrate 250 orandexclude ions? 2100 mV. 1 1 ThatCell membranes is, the inside of are the selectively cells is morepermeable. Ionsthe negative than such as Naby outside and 50 toK100cannot mV. diffuseall Although across cells the phospholipid in the body maintainbilayer. Ions must this potential either beacross difference actively transported their membranes, or diffuseonly certain through cells (for specific ion example, channels neurons) are capableThe in the membranes. of generating ratio of Na1 to action K1potentials. ion channels can also affect the relative concentrations of the ions in cells. a. How c. Whatisisthis potential it about difference neurons (nerveacross cells) the thatcell makemembrane generated? their properties different from Sodium-potassium those of other cells? pumps in cell In other membranes words, actively what enables pump nerve Na1toions cells out ofaction produce cells and K1 ions into cells. In general, for every three Na1 ions pumped out of a cell, potentials? 1 two All cellsions K are pumped generate into the a potential cell. As across difference a result, the membranes. their inside of the However, cell containsonly fewer positively nerveNeurons cells have charged ionsionthan voltage-gated have voltage- gated ionthe outside; channels channels thatopen that that open and is, the close andinside in closeofinthe cell is to response response to changes in the potential difference across the membrane. changes in the potential difference across the membrane. Postsynaptic neurons 22 also contain chemically gated ion channels that open and close in response Activity 48.2 to Copyright © 2017 Pearson Education, Inc. neurotransmitters released from presynaptic neuron terminals. d. How is an action potential started and propagated? Refer to Figure 48.11 of Campbell Biology, 11th edition, and keep in mind that all RF.indd 322 of the following responses are recorded at a single site on the axon. r

-PDBM
EFQPMBSJ[BUJPOT
JO
UIF
OFVSPO
NFNCSBOF
PQFO
TPNF
PG
UIF
/B1 gates and 56 1