Marieb Human Anatomy & Physiology Twelfth Edition Chapter 11 PDF

Marieb Human Anatomy & Physiology Twelfth Edition Chapter 11 Fundamentals of the Nervous System and Nervous Tissue...

Marieb Human Anatomy & Physiology Twelfth Edition Chapter 11 Fundamentals of the Nervous System and Nervous Tissue PowerPoint® Lecture Slides prepared by Ashley Spring, Ph.D., Eastern Florida State College Copyright © 2025 Pearson Education, Inc. All Rights Reserved Why This Matters Understanding neurotransmitter function will help you be aware of how drugs affect a patient’s nervous system Copyright © 2025 Pearson Education, Inc. All Rights Reserved Video: Why This Matters (Career Connection) Click here to view ADA compliant video: Why This Matters (Career Connection) https://mediaplayer.pearsoncmg.com/assets/secs_wtm_ch_11_dea_v3 Copyright © 2025 Pearson Education, Inc. All Rights Reserved 11.1 The Nervous System Receives, Integrates, and Responds to Information Nervous system is master controlling and communicating system of body Cells communicate via electrical and chemical signals – Rapid and specific – Usually cause almost immediate responses Copyright © 2025 Pearson Education, Inc. All Rights Reserved Functions of Nervous System (1 of 5) Nervous system has three overlapping functions 1. Sensory input ▪ Information gathered by sensory receptors about internal and external changes 2. Integration ▪ Processing and interpretation of sensory input 3. Motor output ▪ Activation of effectors (muscles and glands) produces a response Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Nervous System’s Functions Figure 11.1 The nervous system’s functions. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Functions of Nervous System (2 of 5) Nervous system is divided into two principal parts: – Central nervous system (CNS) ▪ Brain and spinal cord of dorsal body cavity ▪ Integration and control center – Interprets sensory input and dictates motor output – Peripheral nervous system (PNS) ▪ The portion of nervous system outside CNS ▪ Consists mainly of nerves that extend from brain and spinal cord – Spinal nerves to and from spinal cord – Cranial nerves to and from brain – Walls of gastrointestinal tract also contain neurons called the enteric nervous system Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Nervous System Figure 11.2 The nervous system. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Functions of Nervous System (3 of 5) Peripheral nervous system (PNS) has two functional divisions – Sensory (afferent) division ▪ Somatic sensory fibers: convey impulses from skin, skeletal muscles, and joints to CNS ▪ Visceral sensory fibers: convey impulses from visceral organs to CNS – Motor (efferent) division ▪ Transmits impulses from CNS to effectors – Muscles and glands ▪ Two divisions – Somatic nervous system – Autonomic nervous system Copyright © 2025 Pearson Education, Inc. All Rights Reserved Functions of Nervous System (4 of 5) Somatic nervous system, or voluntary nervous system – Somatic motor nerve fibers conduct impulses from CNS to skeletal muscle – Conscious control of skeletal muscles Copyright © 2025 Pearson Education, Inc. All Rights Reserved Functions of Nervous System (5 of 5) Autonomic nervous system (ANS), or involuntary nervous system – Consists of visceral motor nerve fibers – Regulates smooth muscle, cardiac muscle, and glands – Two functional subdivisions work in opposition of each other ▪ Sympathetic division ▪ Parasympathetic division Copyright © 2025 Pearson Education, Inc. All Rights Reserved Organization of the Nervous System Figure 11.3 Organization of the nervous system. Copyright © 2025 Pearson Education, Inc. All Rights Reserved 11.2 Neuroglia Nervous tissue consists of two principal cell types – Neuroglia (glial cells): small cells that surround and wrap delicate neurons – Neurons (nerve cells): excitable cells that transmit electrical signals Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuroglia in the CNS (1 of 6) Four main neuroglia support CNS neurons – Astrocytes – Microglial cells – Ependymal cells – Oligodendrocytes Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuroglia in the CNS (2 of 6) Astrocytes – Most abundant, versatile, and highly branched of glial cells – Cling to neurons, synaptic endings, and capillaries Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuroglia in the CNS (3 of 6) Astrocytes (cont ) inued – Functions include: ▪ Support and brace neurons ▪ Play role in exchanges between capillaries and neurons ▪ Guide migration of young neurons ▪ Control chemical environment around neurons – Clean up leaked potassium ions – Recapture and recycle neurotransmitters ▪ Influence neuronal functioning Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuroglia: Astrocytes Figure 11.4a Neuroglia. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuroglia in the CNS (4 of 6) Microglial cells, or microglia – Small, ovoid cells with thorny processes that touch and monitor neurons – Migrate toward injured neurons – Can transform to phagocytize microorganisms and neuronal debris Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuroglia: Microglial Cells Figure 11.4b Neuroglia. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuroglia in the CNS (5 of 6) Ependymal cells – Range in shape from squamous to columnar – May be ciliated ▪ Cilia beat to circulate CSF (cerebral spinal fluid) – Line the central cavities of the brain and spinal column – Form permeable barrier between cerebrospinal fluid (CSF) in cavities and tissue fluid bathing CNS cells Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuroglia: Ependymal Cells Figure 11.4c Neuroglia. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuroglia in the CNS (6 of 6) Oligodendrocytes – Branched cells with fewer processes than astrocytes – Processes wrap CNS nerve fibers forming insulating myelin sheath Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuroglia: Oligodendrocytes Figure 11.4d Neuroglia. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuroglia in the PNS Two major neuroglia seen in PNS Satellite cells – Surround neuron cell bodies in PNS – Function similar to astrocytes of CNS Schwann cells, or neurolemmocytes – Surround all peripheral nerve fibers and form myelin sheaths around the thicker nerve fibers ▪ Similar function as oligodendrocytes – Vital to regeneration of damaged peripheral nerve fibers Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuroglia: Satellite Cells and Schwann Cells Figure 11.4e Neuroglia. Copyright © 2025 Pearson Education, Inc. All Rights Reserved 11.3 Neurons Neurons, or nerve cells, are structural units of nervous system Large, highly specialized cells that conduct impulses Special characteristics – Extreme longevity (lasts a person’s lifetime) – Amitotic (lose their ability to divide) with few exceptions – High metabolic rate and require continuous supply of oxygen and glucose All have a cell body and one or more slender processes Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuron Cell Body Cell body, soma, contains spherical nucleus with nucleolus, surrounded by cytoplasm – In most neurons, plasma membrane is part of receptive region that receives information from other neurons – Biosynthetic center and metabolic center of neuron ▪ Protein- and membrane-making machinery – Free ribosomes, Golgi apparatus – Rough endoplasmic reticulum also called chromatophilic substance or Nissl bodies ▪ Cytoskeletal elements – Maintain shape with microtubules and neurofibrils (bundles of intermediate filaments, or neurofilaments) ▪ Pigment inclusions Copyright © 2025 Pearson Education, Inc. All Rights Reserved Structure of a Motor Neuron (1 of 4) Figure 11.5a Structure of a motor neuron. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuron Cell Bodies in the CNS v s. PNS er su Most neuron cell bodies are located in CNS – Nuclei: clusters of neuron cell bodies in CNS – Ganglia: clusters of neuron cell bodies in PNS Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuron Processes Processes extend from cell body of all neurons – CNS contains both neuron cell bodies and their processes – PNS contains chiefly of neuron processes (most cell bodies in CNS) Two types of processes – Dendrites – Axon Copyright © 2025 Pearson Education, Inc. All Rights Reserved Structure of a Motor Neuron (2 of 4) Figure 11.5a Structure of a motor neuron. Copyright © 2025 Pearson Education, Inc. All Rights Reserved IP Anatomy Review Animation: Nervous Nervous 1 I Click here to view ADA compliant Animation: IP Anatomy Review Animation: Nervous I https://mediaplayer.pearsoncmg.com/assets/secs-ipweb-nervous1-anat-rev Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuron Processes: Dendrites Dendrites – Motor neurons can contain hundreds of these short, tapering, diffusely branched processes ▪ Contain same organelles as in cell body – Receptive (input) regions of neuron – Convey incoming messages toward cell body as graded potentials (short-distance signals) – In many brain areas, finer dendrites are highly specialized to collect information ▪ Contain dendritic spines, appendages with bulbous or spiky ends Copyright © 2025 Pearson Education, Inc. All Rights Reserved Structure of a Motor Neuron (3 of 4) Figure 11.5b Structure of a motor neuron. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuron Processes: Axon (1 of 3) The Axon: Structure – Each neuron has only one axon ▪ Starts at cone-shaped area called axon hillock ▪ Initial segment of the axon starts uniform diameter of remainder of axon – Axon length varies ▪ In some neurons, axons are short or absent ▪ In others, axon comprises almost entire length of neuron ▪ Longest cells in body are motor neurons from lumbar region to toes ▪ Long axons are called nerve fibers – Some axons branch ▪ Axons have occasional branches called axon collaterals ▪ Axons branch profusely at their end (terminus) into 10,000 or more terminal branches, or terminal arborizations – Knob-like distal ends are called axon terminals Bundles of axons in CNS are called tracts Bundles of axons in PNS are called nerves Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuron Processes: Axon (2 of 3) The Axon: Functional Characteristics – Axon is the conducting region of neuron ▪ Generates nerve impulses and transmits them away from cell body along axon plasma membrane axolemma to axon terminals – Axon terminals are the secretory region of neuron ▪ Release neurotransmitters (signaling chemicals) into the extracellular space ▪ Neurotransmitters excite or inhibit neurons, muscles, or glands – Each neuron receives and sends signals with different neurons simultaneously – Axons rely on cell bodies for membrane components and mRNA – Axons quickly decay if cut or damaged Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuron Processes: Axon (3 of 3) Axonal Transport – Axons have efficient internal transport mechanisms ▪ Molecules and organelles are moved along axons by motor proteins and cytoskeletal elements – Movement occurs in both directions ▪ Anterograde movement: away from cell body – Examples: mitochondria, cytoskeletal elements, membrane components, enzymes ▪ Retrograde movement: toward cell body – Examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins Copyright © 2025 Pearson Education, Inc. All Rights Reserved Clinical—Homeostatic Imbalance 11.1 Certain viruses and bacterial toxins damage neural tissues by using retrograde axonal transport – Example: polio, rabies, and herpes simplex viruses, and tetanus toxin Research is under way to investigate using retrograde transport to treat genetic diseases – Viruses containing “corrected” genes or microRNA to suppress defective genes can enter cell through retrograde transport Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuron Processes: Myelin Sheath (1 of 4) Myelin sheath – White, fatty substance that coats many axons, particularly long or large axons – Function of myelin ▪ Protect and electrically insulate axon ▪ Increase speed of nerve impulse transmission – Myelinated axons: segmented sheath surrounds most long or large-diameter axons – Nonmyelinated axons: do not contain sheath ▪ Conduct impulses more slowly Note, myelin sheaths are only associated with axons; dendrites are never myelinated Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuron Processes: Myelin Sheath (2 of 4) Myelination in the PNS – Formed by Schwann cells ▪ Wrap around axon in jelly roll fashion ▪ One cell forms one segment of myelin sheath – Outer collar of perinuclear cytoplasm (formerly called neurilemma): peripheral bulge containing nucleus and most of cytoplasm – Plasma membranes have less protein ▪ No channels or carriers, so good electrical insulators ▪ Interlocking proteins bind adjacent myelin membranes Copyright © 2025 Pearson Education, Inc. All Rights Reserved Myelination of an Axon (Nerve Fiber) (1 of 2) Figure 11.6a Myelination of an axon (nerve fiber). Copyright © 2025 Pearson Education, Inc. All Rights Reserved Myelination of an Axon (Nerve Fiber) (2 of 2) Figure 11.6b Myelination of an axon (nerve fiber). Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuron Processes: Myelin Sheath (3 of 4) Myelination in the PNS (cont ) inued – Myelin sheath gaps, or nodes of Ranvier ▪ Gaps between adjacent Schwann cells ▪ Sites where axon collaterals can emerge – Nonmyelinated fibers ▪ Thin fibers not wrapped in myelin; surrounded by Schwann cells but no coiling; one cell may surround 15 different axons Copyright © 2025 Pearson Education, Inc. All Rights Reserved Structure of a Motor Neuron (4 of 4) Figure 11.5a Structure of a motor neuron. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuron Processes: Myelin Sheath (4 of 4) Myelination in the CNS – Formed by processes of oligodendrocytes, not whole cells – Each cell can wrap up to 60 axons at once – Myelin sheath gap is present – No outer collar of perinuclear cytoplasm – Thinnest fibers are unmyelinated, but covered by long extensions of adjacent neuroglia Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neuroglia Figure 11.4d Neuroglia. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Classification of Neurons (1 of 2) Structural classification – Three types grouped by number of processes extending from cell body 1. Multipolar neurons: many processes (1 axon and many dendrites) – 99% of neurons; major neuron type in CNS 2. Bipolar neurons: two processes (1 axon and 1 fused dendrite) – Rare neuron type; found in retina, ear, and olfactory mucosa 3. Unipolar neurons : one process (T-shaped process exits cell body and divides into 2 branches of axon) – Peripheral process: branch associated with sensory receptor – Central process: branch enters CNS – Also called pseudounipolar neurons – Found mainly in PNS ganglia where function as sensory neurons Copyright © 2025 Pearson Education, Inc. All Rights Reserved Table 11.2-1 Comparison of Structural Classes of Neurons Neuron type Structural Class: Neuron Type According to the Number of Processes Extending from the Cell Body Multipolar Bipolar Unipolar (pseudounipolar) Many processes extend from the cell body. Two processes extend from the cell One process extends from the cell body and forms All are dendrites except for a single axon. body. One is a fused dendrite, the central and peripheral processes, which together other is an axon. comprise an axon. A m ult ipolar ne ur o nhasma nysh or tproce sse st h at e xtendfr o m thecel bo dylabe ledDend r it e sandalon gproce sslabe ledAxon. A bipolar neu ro nhastwoproce sse st h at e xtendfrom thecel bo dy,oneisafuse dden drit ean dt h eother isana xon. A pseudoun ipolar ne uronha sonep r ocesstha t exte ndsfrom thecel bo dyan df o r msce ntr a l an dperipheralpr o cesses,whichtog ethercompriseana xon. Thea xonh asrece ptiveend ings. Relationship of Anatomy to the Three Functional Regions Multipolar Bipolar Unipolar (pseudounipolar) A bipolar neu ro nhasatr igg er zoneinther e cep t ivereg ion, wh ichr eceivesstimulus.Manyb ipolar n euronsd onotgen erateactionpo te ntials. Int h osetha t do , theloca t iono f thetr igge r zo neisnotunive r sa l. ) A m ult ipolar ne ur o nhasareceptiver e gion(r eceivesstimulus), atrigger zone(co ndu ctingr eg iont h at g ene ra te sactionpo te ntial) , an dasecre to r yregion(axo nt e r minalsreleaseneu r otransm it e r s). A pseudoun ipolar ne uronha sat riggerzon ein thecond uctingr e gionthatgen eratesana ctionpoten t ial. (Many bipolar neurons do not generate action potentials. In those that do, the location of the trigger zone is not universal.) Copyright © 2025 Pearson Education, Inc. All Rights Reserved Table 11.2-2 Comparison of Structural Classes of Neurons Relative Abundance and Location in Human Body Multipolar Bipolar Unipolar (pseudounipolar) Most abundant in body. Major neuron type Rare. Found in some special sensory Found mainly in the P NS. Common only in dorsal in the CNS. organs (olfactory mucosa, eye, ear). root ganglia and sensory ganglia of cranial nerves (described in Chapter 13 ). Structural Variations Multipolar Bipolar Unipolar (pseudounipolar) A diagram shows the Purkinje cell of the cerebellum and the pyramidal cell which are the structural variations of the multipolar neuron. The Purkinje cell of the cerebellum has a single axon and multiple A d i a g ra m sh ows the o l fa ctory ce l l an d reti na l ce l l whi ch are the b i po la rn eu ron s. Ea ch ce l l ha s a ce l l bo dy , a s in gl e a xon , a nd fu sed de nd rites. A diagr am sh owsthedo r sa l rootgan glioncell wh ichis au nipolar n eurontha t ha sacell b ody,ace ntr alproce ss( a xon ) ,andae pr iph eral p r ocess(axon) tha t ha sr e cep t ivee nding s. branches of dendrites extending from the cell body. The pyramidal cell has a conical-shaped cell body, an axon, and multiple dendrites. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Classification of Neurons (2 of 2) Functional classification of neurons – Three types of neurons grouped by direction nerve impulse travels relative to CNS 1. Sensory (afferent) neurons – Transmit impulses from sensory receptors toward CNS – Almost all are unipolar neurons – Cell bodies located in ganglia in PNS 2. Motor (efferent) neurons – Carry impulses from CNS to effectors – Multipolar neurons – Most cell bodies located in CNS (except some autonomic neurons) 3. Interneurons, or association neurons – Lie between motor and sensory neurons in neural pathways – Shuttle signals through CNS pathways, most are entirely within CNS – 99% of body’s neurons are interneurons Copyright © 2025 Pearson Education, Inc. All Rights Reserved Table 11.2-3 Comparison of Structural Classes of Neurons Neuron type Functional Class: Neuron Type According to Direction of Impulse Conduction Multipolar Bipolar Unipolar (pseudounipolar) Most multipolar neurons are interneurons Essentially all bipolar neurons are Most unipolar neurons are sensory neurons that that conduct impulses within the CNS, sensory neurons that are located in conduct impulses along afferent pathways to the CNS integrating sensory input or motor output. some special sense organs. For for interpretation. (These sensory neurons are called May be one of a chain of CNS neurons, or example, bipolar cells of the retina are primary or first-order sensory neurons.) a single neuron connecting sensory and involved in transmitting visual inputs motor neurons. from the eye to the brain (via an Some multipolar neurons are motor intermediate chain of neurons). neurons that conduct impulses along the efferent pathways from the CNS to an effector (muscle/gland). An ilust r at ionsh owsaninterne ur o nin theg re ym a t terof thespinalcordandamo to r neu ro nin thespinalcordasane xamp le fothemult ipo lar ne ur o n. T hemotor n euronco ndu ctsim p ulsetot h eske let a l muscle. An ilust r at ionsh owsabipo lar ne ur o nof retinaofeyetha t tr an smit sthevisualim pu lsef romt h eeyetothebrain. An ilust r at ionsh owsase nso r yn euronasa nexa mpleof ap seu dou nipolar n euroncond uctinganimpulsefrom theskint othespinalcord Copyright © 2025 Pearson Education, Inc. All Rights Reserved 11.4 Membrane Potentials Like all cells, neurons have a resting membrane potential Unlike most other cells, neurons can rapidly change resting membrane potential Neurons are highly excitable Copyright © 2025 Pearson Education, Inc. All Rights Reserved Basic Principles of Electricity (1 of 5) Opposite charges are attracted to each other Energy is required to keep opposite charges separated across a membrane Energy is liberated when the charges move toward one another When opposite charges are separated, the system has potential energy Copyright © 2025 Pearson Education, Inc. All Rights Reserved Basic Principles of Electricity (2 of 5) Definitions – Voltage: a measure of potential energy generated by separated charge ▪ Measured between two points in volts (V) or millivolts (mV) ▪ Called potential difference or potential – Charge difference across plasma membrane results in potential ▪ Greater charge difference between points = higher voltage – Current: flow of electrical charge (ions) between two points ▪ Can be used to do work ▪ Flow is dependent on voltage and resistance – Resistance: hindrance to charge flow ▪ Insulator: substance with high electrical resistance ▪ Conductor: substance with low electrical resistance Copyright © 2025 Pearson Education, Inc. All Rights Reserved Basic Principles of Electricity (3 of 5) Definitions (cont.) inued – Ohm’s law: gives relationship of voltage, current, resistance Current (I) = voltage (V) / resistance (R) – Current is directly proportional to voltage ▪ Greater the voltage (potential difference), greater the current ▪ No net current flow between points with same potential – Current is inversely proportional to resistance ▪ The greater the resistance, the smaller the current Copyright © 2025 Pearson Education, Inc. All Rights Reserved Basic Principles of Electricity (4 of 5) Role of membrane ion channels – Plasma membranes have large proteins that acts as ion channels ▪ K + ion channel allows only K + to pass through – Two main types of ion channels ▪ Leakage (nongated) channels, which are always open ▪ Gated channels, in which part of the protein changes shape to open/close the channel – Chemically gated channels, or ligand-gated channels Open only with binding of a specific chemical (ex: neurotransmitter) – Voltage-gated channels Open and close in response to changes in membrane potential – Mechanically gated channels Open and close in response to physical deformation of receptors, as in sensory receptors Copyright © 2025 Pearson Education, Inc. All Rights Reserved Operation of Gated Channels Figure 11.7 Operation of gated channels. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Basic Principles of Electricity (5 of 5) When gated channels are open, ions diffuse quickly across the membrane – Direction into or out of the cells is determined by the electrochemical gradient ▪ Concentration gradient: ions move from area of higher concentration to area of lower concentration ▪ Electrical gradient: ions move toward area of opposite electrical change ▪ The strongest gradient drives the net flow of ions – Ion flow creates an electrical current, and voltage changes across membrane ▪ Expressed by rearranged Ohm’s law equation: V = IR Copyright © 2025 Pearson Education, Inc. All Rights Reserved Generating the Resting Membrane Potential (1 of 3) A voltmeter can measure potential (charge) difference across membrane of resting cell Resting membrane potential of a resting neuron is approximately −70 mV – The cytoplasmic side of membrane is negatively charged relative to the outside – The actual voltage difference varies from −40 mV to − 90 mV – The membrane is said to be polarized Potential is generated by: – Differences in ionic composition of ICF and ECF – Differences in plasma membrane permeability Copyright © 2025 Pearson Education, Inc. All Rights Reserved Measuring Membrane Potential in Neurons Figure 11.8 Measuring membrane potential in neurons. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Generating the Resting Membrane Potential (2 of 3) Differences in ionic composition – Cell cytosol has lower concentration of Na + than ECF ▪ Balanced chiefly by chloride ions (Cl− ) – Cell cytosol has higher concentration of K + than ECF ▪ Balanced by negatively charged proteins – K + plays most important role in membrane potential Copyright © 2025 Pearson Education, Inc. All Rights Reserved A&P Flix: Resting Membrane Potential Click here to view ADA compliant Animation: A&P Flix: Resting Membrane Potential https://mediaplayer.pearsoncmg.com/assets/apf-resting-membrane-potential Copyright © 2025 Pearson Education, Inc. All Rights Reserved Resting Membrane Potential (1 of 2) Generating a resting membrane potential depends on (1) differences in K + and Na + concentrations inside and outside cells, and (2) differences in permeability of the plasma membrane to these ions. Focus Figure 11.1-1 Resting Membrane Potential. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Generating the Resting Membrane Potential (3 of 3) Differences in plasma membrane permeability – Impermeable to large anionic protein – Slightly permeable to Na + (through leakage channels) ▪ Sodium diffuses into cell down concentration gradient – 25 times more permeable to K + than sodium (more leakage channels) ▪ Potassium diffuses out of cell down concentration gradient − – Quite permeable to Cl – More potassium diffuses out than sodium diffuses in ▪ As a result, the inside of the cell is more negative ▪ Establishes resting membrane potential – Sodium-potassium pump (Na + − K + ATPase) stabilizes resting membrane potential ▪ Maintains concentration gradients for Na + and K + ▪ Three Na + are pumped out of cell while two K + are pumped back in Copyright © 2025 Pearson Education, Inc. All Rights Reserved Resting Membrane Potential (2 of 2) Focus Figure 11.1-2 Resting Membrane Potential. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Changing the Resting Membrane Potential (1 of 2) Membrane potential changes when: – Concentrations of ions across membrane change – Membrane permeability to ions changes Changes produce two types of signals – Graded potentials ▪ Incoming signals operating over short distances – Action potentials ▪ Long-distance signals of axons Changes in membrane potential are used as signals to receive, integrate, and send information Copyright © 2025 Pearson Education, Inc. All Rights Reserved Changing the Resting Membrane Potential (2 of 2) Terms describing membrane potential changes relative to resting membrane potential – Depolarization: decrease in membrane potential (moves toward zero and above) ▪ Inside of membrane becomes less negative than resting membrane potential ▪ Probability of producing impulse increases – Hyperpolarization: increase in membrane potential (away from zero) ▪ Inside of membrane becomes more negative than resting membrane potential ▪ Probability of producing impulse decreases Copyright © 2025 Pearson Education, Inc. All Rights Reserved Depolarization and Hyperpolarization of the Membrane Figure 11.9 Depolarization and hyperpolarization of the membrane. Copyright © 2025 Pearson Education, Inc. All Rights Reserved 11.5 Graded Potentials Graded potentials are short-lived, localized changes in membrane potential – The stronger the stimulus, the more voltage changes and the farther current flows Triggered by stimulus that opens gated ion channels – Results in depolarization or sometimes hyperpolarization Named according to location and function – Receptor potential (generator potential): graded potentials in receptors of sensory neurons – Postsynaptic potential: neuron graded potential – End-plate potential: special type of graded potential that occurs in muscle cell, instead of postsynaptic neuron, triggering action potential for muscle contraction Copyright © 2025 Pearson Education, Inc. All Rights Reserved Graded Potentials Once gated ion channel opens, depolarization spreads from one area of membrane to next Current flows but dissipates quickly and decays (declines) Graded potentials acts as signals only over short distances, such as dendrites and cell body Graded potentials are essential in initiating action potentials Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Spread and Decay of a Graded Potential (1 of 2) Figure 11.10a,b The spread and decay of a graded potential. Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Spread and Decay of a Graded Potential (2 of 2) Figure 11.10c The spread and decay of a graded potential. Copyright © 2025 Pearson Education, Inc. All Rights Reserved 11.6 Action Potentials An action potential (AP) is a brief reversal of membrane potential with a change in voltage of ~ 100 mV – From −70 mV to + 30 mV Neurons send signals over long distances by generating and propagating (transmitting) APs Occur only in excitable membranes – Neurons and muscle cells APs do not decay over distance as graded potentials do – In neurons, also called a nerve impulse ▪ Typically, only generated in axons of neurons – Involves opening of specific voltage-gated channels Copyright © 2025 Pearson Education, Inc. All Rights Reserved Action Potential (1 of 7) Focus Figure 11.2 Action Potential. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Generating an Action Potential (1 of 5) Four main steps 1. Resting state: All voltage-gated Na + and K + channels are closed ▪ Only leakage channels for Na + and K + are open – Maintains the resting membrane potential ▪ Each Na + channel has two voltage-sensitive gates – Activation gates: closed at rest; open with depolarization, allowing Na + to enter cell – Inactivation gates: open at rest; block channel once it is open to prevent more Na + from entering cell ▪ Each K + channel has one voltage-sensitive gate – Closed at rest – Opens slowly with depolarization Copyright © 2025 Pearson Education, Inc. All Rights Reserved Action Potential (2 of 7) Focus Figure 11.2 Action Potential. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Generating an Action Potential (2 of 5) Four main steps (cont.) inued 2. Depolarization: voltage-gated Na + channels open ▪ Depolarizing local currents open voltage-gated Na + channels, and Na + rushes into cell ▪ Na + activation and inactivation gates open ▪ Na + influx causes more depolarization, which opens more Na + channels – As a result, ICF becomes less negative ▪ At threshold ( −55 to − 50 mV ), positive feedback causes opening of all Na + channels – Results in large action potential spike – Membrane polarity jumps to +30 m V Copyright © 2025 Pearson Education, Inc. All Rights Reserved Action Potential (3 of 7) Focus Figure 11.2 Action Potential. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Generating an Action Potential (3 of 5) Four main steps (cont ) inued 3. Repolarization: Na + channels are inactivating, and voltage-gated K+ channels open ▪ Na + channel inactivation gates close – Membrane permeability to Na + declines to resting state – AP spike stops rising ▪ Voltage-gated K + channels open – K + exits cell down its electrochemical gradient ▪ Repolarization: membrane returns to resting membrane potential Copyright © 2025 Pearson Education, Inc. All Rights Reserved Action Potential (4 of 7) Focus Figure 11.2 Action Potential. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Generating an Action Potential (4 of 5) Four main steps (cont.) inued 4. Hyperpolarization: Some K + channels remain open, and Na + channels reset ▪ Some K + channels remain open, allowing excessive K + efflux – Inside of membrane becomes more negative than in resting state ▪ This causes hyperpolarization of the membrane (slight dip below resting voltage) ▪ Na + channels also begin to reset Copyright © 2025 Pearson Education, Inc. All Rights Reserved Action Potential (5 of 7) Focus Figure 11.2 Action Potential. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Action Potential (6 of 7) Focus Figure 11.2 Action Potential. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Action Potential (7 of 7) Focus Figure 11.2 Action Potential. Copyright © 2025 Pearson Education, Inc. All Rights Reserved A&P Flix: Generation of an Action Potential Click here to view ADA compliant Animation: A&P Flix: Generation of an Action Potential https://mediaplayer.pearsoncmg.com/assets/apf-generation-action-potential Copyright © 2025 Pearson Education, Inc. All Rights Reserved Generating an Action Potential (5 of 5) Repolarization resets electrical conditions, not ionic conditions After repolarization, Na +  K + pumps (thousands of them in an axon) restore ionic conditions Copyright © 2025 Pearson Education, Inc. All Rights Reserved Threshold and the All-or-None Phenomenon Not all depolarization events produce APs For an axon to “fire,” depolarization must reach threshold voltage to trigger AP At threshold: – Membrane is depolarized by 15 to 20 mV – Na + permeability increases – Na + influx exceeds K+ efflux – The positive feedback cycle begins All-or-none phenomenon: AP either happens completely, or does not happen at all Copyright © 2025 Pearson Education, Inc. All Rights Reserved Propagation of an Action Potential Propagation allows AP to be transmitted from origin down entire axon length toward terminals Na + influx through voltage gates in one membrane area cause local currents that cause opening of Na + voltage gates in adjacent membrane areas – Leads to depolarization of that area, which in turn causes depolarization in next area Once initiated, an AP is self-propagating – In nonmyelinated axons, each successive segment of membrane depolarizes, then repolarizes – Propagation in myelinated axons differs Since Na + channels closer to the AP origin are still inactivated, no new AP is generated there – AP occurs only in a forward direction Copyright © 2025 Pearson Education, Inc. All Rights Reserved Propagation of an Action Potential (AP) Figure 11.11 Propagation of an action potential (AP). Copyright © 2025 Pearson Education, Inc. All Rights Reserved A&P Flix: Propagation of an Action Potential Click here to view ADA compliant Animation: A&P Flix: Propagation of an Action Potential https://mediaplayer.pearsoncmg.com/assets/apf-propagation-action-potential Copyright © 2025 Pearson Education, Inc. All Rights Reserved Coding for Stimulus Intensity All action potentials are alike and are independent of stimulus intensity CNS tells difference between a weak stimulus and a strong one by frequency of impulses – Frequency is number of impulses (APs) received per second – Higher frequencies mean stronger stimulus Copyright © 2025 Pearson Education, Inc. All Rights Reserved Relationship between Stimulus Strength and Action Potential Frequency Figure 11.12 Relationship between stimulus strength and action potential frequency. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Refractory Periods Refractory period: time in which neuron cannot trigger another AP – Voltage-gated Na + channels are open, so neuron cannot respond to another stimulus Absolute refractory period – Time from opening of Na + channels until resetting of the channels – Ensures that each AP is an all-or-none event – Enforces one-way transmission of APs Relative refractory period – Follows absolute refractory period ▪ Most Na + channels returned to their resting state, some K + channels still open ▪ Repolarization is occurring – Threshold for AP generation is elevated – Only exceptionally strong stimulus can stimulate an AP Copyright © 2025 Pearson Education, Inc. All Rights Reserved Absolute and Relative Refractory Periods in an AP Figure 11.13 Absolute and relative refractory periods in an AP. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Conduction Velocity (1 of 3) APs occur only in axons, not other cell areas AP conduction velocities in axons vary widely Rate of AP propagation depends on two factors: 1. Axon diameter ▪ Larger-diameter fibers have less resistance to local current flow, so have faster impulse conduction 2. Degree of myelination ▪ Two types of conduction depending on presence or absence of myelin Copyright © 2025 Pearson Education, Inc. All Rights Reserved Conduction Velocity (2 of 3) APs can propagate in two ways: – Nonmyelinated axons: slower conduction, called continuous conduction, that occurs in nonmyelinated axons – Myelinated axons: faster conduction, called saltatory conduction, occurs only in myelinated axons and is about 30 times faster ▪ Myelin sheaths insulate and prevent leakage of charge ▪ Voltage-gated Na + channels are located at myelin sheath gaps ▪ APs generated only at gaps ▪ Electrical signal appears to jump rapidly from gap to gap Copyright © 2025 Pearson Education, Inc. All Rights Reserved Action Potential Propagation in Nonmyelinated and Myelinated Axons (1 of 3) Figure 11.14a Action potential propagation in nonmyelinated and myelinated axons. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Action Potential Propagation in Nonmyelinated and Myelinated Axons (2 of 3) Figure 11.14b Action potential propagation in nonmyelinated and myelinated axons. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Action Potential Propagation in Nonmyelinated and Myelinated Axons (3 of 3) Figure 11.14c Action potential propagation in nonmyelinated and myelinated axons. Copyright © 2025 Pearson Education, Inc. All Rights Reser