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11 Neural Tissue PowerPoint® Lecture Presentations prepared by Alexander G. Cheroske Mesa Community College at Red Mountain © 2011 Pearson Education, Inc. Section 1: Nervous System Components • Learning Outcomes • 11.1 Sketch and label the structure of a typical neuron, and describe the functions of each component. • 11.2 Classify and describe neurons on the basis of their structure and function. • 11.3 Describe the locations and functions of neuroglia in the CNS. • 11.4 Describe the locations and functions of Schwann cells and satellite cells. © 2011 Pearson Education, Inc. The major components and functions of the nervous system Central Nervous System The central nervous system (CNS) consists of the brain and spinal cord and is responsible for integrating, processing, and coordinating sensory data and motor commands. Information processing includes the integration and distribution of information in the CNS. Peripheral Nervous System The motor division of the PNS carries motor commands from the CNS to peripheral tissues and systems. The peripheral nervous system (PNS) includes all the neural tissue outside the CNS. includes The sensory division of the PNS brings information to the CNS from receptors in peripheral tissues and organs. Somatic sensory receptors provide position, touch, pressure, pain, and temperature sensations. The somatic nervous system (SNS) controls skeletal muscle contractions. Special sensory receptors provide sensations of smell, taste, vision, balance, and hearing. Visceral sensory receptors monitor internal organs. Receptors are sensory structures that detect changes in the internal or external environment. Skeletal muscle The autonomic nervous system (ANS) provides automatic regulation of smooth muscle, cardiac muscle, glands, and adipose tissue. • Smooth muscle • Cardiac muscle • Glands • Adipose tissue Effectors are target organs whose activities change in response to neural commands. Figure 11 Section 1 © 2011 Pearson Education, Inc. Module 11.1: Neurons • Neuron components • Dendrites • Highly branched, bearing spines 0.5–1 µm long (dendritic spines) • CNS neurons receive most information here • Neuron receives stimuli from environment or other neurons at dendrites • Cell body • Contains nucleus • Organelles contained within perikaryon (peri, around + karyon, nucleus) • Cytoskeleton comprised of neurofilaments and neurofibrils (extend into dendrites and axon) © 2011 Pearson Education, Inc. Module 11.1: Neurons • Neuron components (continued) • Axon • Carries information toward other cells • Transport of materials (enzymes and lysosomes) using neurotubules (= axoplasmic transport) • Occurs in both directions • Back toward cell body = retrograde flow • Components • Axon hillock (base or initial segment) • Axolemma (plasma membrane of axon) • Axoplasm (cytoplasm of axon with organelles, structural components, and transported materials) • Collateral branches (communicate with other cells) © 2011 Pearson Education, Inc. Module 11.1: Neurons • Neuron components (continued) • Telodendria (telo-, end + dendron, tree) • Axonal extensions at end of axon trunk • Terminate at synaptic terminals • Where neuron communicates with other cells Animation: Neurophysiology: Neuron Structure © 2011 Pearson Education, Inc. A diagrammatic view of a representative neuron Dendrites Axon Dendritic spines of dendrites Axon hillock Axolemma Axoplasm Nissl bodies (clusters of RER and free ribosomes) Mitochondrion Nucleus Nucleolus Telodendria Cell Body Perikaryon Neurofilament Synaptic terminals Figure 11.1 © 2011 Pearson Education, Inc. 1 Module 11.1: Neurons • Synapse • Specialized site of communication between neuron and another cell • Components • Presynaptic cell (before synaptic cleft) • Usually a neuron • May have synaptic knob • Has synaptic vesicles that contain neurotransmitters (chemical messengers synthesized in cell body) • Presynaptic membrane (where neurotransmitters are released) © 2011 Pearson Education, Inc. Module 11.1: Neurons • Synapse (continued) • Components (continued) • Postsynaptic cell (after synaptic cleft) • Can be a neuron or other type of cell • Postsynaptic membrane (bears receptors for neurotransmitters • Synaptic cleft (narrow space between cells) © 2011 Pearson Education, Inc. A representative synapse Telodendrion of presynaptic cell Mitochondrion Synaptic knob Endoplasmic reticulum Presynaptic membrane Synaptic vesicles Synaptic cleft Cytoplasm of postsynaptic cell Postsynaptic membrane Figure 11.1 © 2011 Pearson Education, Inc. 2 The type of synapses Synapses with another neuron Neuron 1 Synapses with another neuron Collateral branch Dendrites Neuron 2 Axolemma Neuromuscular junctions Neuron Collateral branch Neuromuscular junctions Telodendria Skeletal muscle fibers Synaptic terminals Neuroglandular synapses Neuron Neuroglandular synapses Gland cells Figure 11.1 © 2011 Pearson Education, Inc. 3 Module 11.1: Neurons • Most CNS neurons lack centrioles and cannot divide • Neurons lost to injury or disease are seldom replaced • Some neural stem cells exist but mostly inactive • Exceptions: • Olfactory epithelium (smell) • Retina of eye • Hippocampus (area of brain for memory storage) © 2011 Pearson Education, Inc. Module 11.1 Review a. Name the structural components of a typical neuron. b. Describe a synapse. c. Why is a CNS neuron not usually replaced after it is injured? © 2011 Pearson Education, Inc. Module 11.2: Classification of neurons • Four major anatomical classes of neurons 1. Anaxonic neurons • All cell processes look alike (dendrites vs. axons) • Located in brain and special sense organs • Functions are poorly understood 2. Bipolar neurons • Two distinct processes 1. One with branching dendritic processes 2. One axon • Rare, but occur in special sense organs • Small (30 µm in length) © 2011 Pearson Education, Inc. Module 11.2: Classification of neurons • Four major anatomical classes of neurons (continued) 3. Unipolar neurons • Dendrites and axon are continuous (fused) • Cell body lies off to one side • Initial segment where dendrites converge • Most sensory neurons of peripheral nervous system • May extend 1 meter or more 4. Multipolar neurons • Two or more dendrites and one axon • Most common neurons in CNS • Can be as long as unipolar (voluntary motor neurons) © 2011 Pearson Education, Inc. The four major anatomical classes of neurons Dendrites Dendritic process An anaxonic neuron A bipolar neuron Cell body Axon Synaptic terminals Dendrites Initial segment Dendrites Axon A unipolar neuron A multipolar neuron Axon Axon Synaptic terminals © 2011 Pearson Education, Inc. Synaptic terminals Figure 11.2 1 – 4 Module 11.2: Classification of neurons • Three major functional classes 1. Sensory neurons (~10 million in body) 2. Interneurons (~20 billion in body) 3. Motor neurons (~500,000 in body) © 2011 Pearson Education, Inc. Module 11.2: Classification of neurons • Functional relationships of neurons • Sensory receptors (relay stimuli to sensory neurons) • Interoceptors (intero-, inside) • • Proprioceptors • • Monitor sensations inside body from various systems Monitor body position and movement of joints and muscles Exteroceptors (extero, outside) • Monitor sensations from external environment © 2011 Pearson Education, Inc. Module 11.2: Classification of neurons • Functional relationships of neurons (continued) • Afferent nerve fibers (axons from receptor to CNS) • Sensory ganglia • • Contain cell bodies of unipolar sensory neurons • Somatic sensory neurons (outside world) • Visceral sensory neurons (internal conditions) Central Nervous System • Interneurons • Usually between sensory and motor neurons • Also responsible for higher functions (memory, etc.) © 2011 Pearson Education, Inc. Module 11.2: Classification of neurons • Functional relationships of neurons (continued) • Central Nervous System and Peripheral Nervous System • Motor neurons (originate in CNS and transmit impulses to effectors through PNS) • Somatic motor neurons (skeletal muscles) • Visceral motor neurons (smooth and cardiac muscle, glands, and adipose tissue) • © 2011 Pearson Education, Inc. Synapse with 2nd set of neurons at autonomic ganglia Module 11.2 Review a. Classify neurons according to their structure. b. Classify neurons according to their function. c. Are unipolar neurons in a tissue sample of the PNS more likely to function as sensory neurons or motor neurons? © 2011 Pearson Education, Inc. Module 11.3: CNS neuroglia • Neuroglia (or glial cells) • Cells that support and protect neurons • Are abundant and diverse • ~Half the volume of nervous system © 2011 Pearson Education, Inc. Module 11.3: CNS neuroglia • CNS neuroglia 1. Ependymal cells • Form epithelia (ependyma) lining fluid-filled passageway in brain and spinal cord • Fluid = cerebrospinal fluid (CSF) • Also surrounds brain and spinal cord • 2. Assist in producing, circulating, and monitoring CSF Microglia • Embryologically related to monocytes and macrophages • Migrate into CNS • Persist as mobile phagocytic cells • Remove cellular debris, waste products, and pathogens © 2011 Pearson Education, Inc. Module 11.3: CNS neuroglia • CNS neuroglia 3. Astrocytes • Maintain blood–brain barrer • Isolates CNS from chemicals and hormones in the blood • Provide structural support • Regulate ion, nutrient, and dissolved gas concentrations in interstitial fluid • Absorb and recycle neurotransmitters • Form scar tissue after CNS injury © 2011 Pearson Education, Inc. Module 11.3: CNS neuroglia • CNS neuroglia 4. Oligodendrocytes (oligo-, few) • Provide CNS framework by stabilizing axons • Produce myelin • • Coats axons and increases speed of neural impulse transmission Cell process wraps axon with layers of myelin and plasma membrane creating myelin sheath • One oligodendrocyte wraps axonal segments of many neurons • Myelin sheath is incomplete • Myelin-wrapped areas = internodes • Gaps between internodes = nodes © 2011 Pearson Education, Inc. Module 11.3: CNS neuroglia • CNS neuroglia 4. Oligodendrocytes (continued) • • Axons that have myelin sheath = myelinated • Appear white due to lipid content • Constitute white matter of the CNS Axons that lack myelin sheath = unmyelinated • Contribute to gray matter of the CNS • © 2011 Pearson Education, Inc. Along with neuron cell bodies and dendrites Astrocytes Oligodendrocyte Myelin sheath in internode Section of spinal cord Capillary Ependymal cell Unmyelinated axon Microglial cell Neurons Myelinated axons Myelin (cut) Gray matter Nodes White matter Figure 11.3 © 2011 Pearson Education, Inc. Module 11.3 Review a. Identify the neuroglia of the central nervous system. b. Which glial cell protects the CNS from chemicals and hormones circulating in the blood? c. Which type of neuroglia would occur in increased numbers in the brain tissue of a person with a CNS infection? © 2011 Pearson Education, Inc. Module 11.4: PNS neuroglia • PNS neuroglia • Schwann cells • Form sheath around peripheral axons • Outer surface of Schwann cell is called neurilemma • Cover both myelinated and unmyelinated axons • A single cell myelinates an axon • A single cell can wrap many unmyelinated neurons • Satellite cells • Surround neuron cell bodies in ganglia • Regulate intercellular environment (much like astrocytes) © 2011 Pearson Education, Inc. Nucleus Axon hillock Internode (myelinated) Cell body Initial segment (unmyelinated) Dendrite Node Schwann cell Neurilemma Myelin covering internode Axon Axolemma A Schwann cell © 2011 Pearson Education, Inc. Figure 11.4 1 Schwann cell nucleus Axon Neurilemma Myelin covering internode The steps in the myelination of an axon in the PNS © 2011 Pearson Education, Inc. Figure 11.4 2 A single Schwann cell forming the internode of many unmyelinated axons Satellite cells Schwann cell #1 Schwann cell Schwann cell nucleus Schwann cell #2 Neurilemma Axons Schwann cell #3 nucleus Axons Figure 11.4 © 2011 Pearson Education, Inc. 3 Module 11.4: PNS neuroglia • Repair of damaged nerves in PNS 1. Axon and myelin degenerate distal to injury 2. Schwann cells proliferate along original axon path • • Macrophages move in and remove cellular debris 3. Axon grows along original path created by Schwann cells 4. Schwann cells wrap around elongating axon • If axon makes normal synaptic contacts, normal function may be regained • If axon stops growing or wanders off, normal function may not return Repair that does not restore full function = Wallerian degeneration © 2011 Pearson Education, Inc. The process of repair of damaged PNS nerves, or Wallerian degeneration Site of injury Step 1: Distal to the injury site, the axon and myelin degenerate and fragment. Step 2: The Schwann cells do not degenerate; instead, they proliferate along the path of the original axon. Over this period, macrophages move into the area and remove the degenerating debris distal to the injury site. Axon Myelin Proximal stump Distal stump Macrophage Cord of proliferating Schwann cells Step 3: As the neuron recovers, its axon grows into the site of injury and then distally, along the path created by the Schwann cells. Step 4: As the axon elongates, the Schwann cells wrap around it. If the axon reestablishes its normal synaptic contacts, normal function may be regained. However, if it stops growing or wanders off in some new direction, normal function will not return. Figure 11.4 © 2011 Pearson Education, Inc. 4 Module 11.4: PNS neuroglia • Only limited repair can occur in CNS due to: 1. Many more axons involved 2. Astrocytes produce scar tissue that can prevent axon growth 3. Astrocytes release chemicals that block axon regrowth © 2011 Pearson Education, Inc. Module 11.4 Review a. Identify the neuroglia of the peripheral nervous system. b. Describe the neurilemma. c. In which part of the nervous system does Wallerian degeneration occur? © 2011 Pearson Education, Inc. Section 2: Neurophysiology • Learning Outcomes • 11.5 Explain how the resting potential is created and maintained. • 11.6 Describe the functions of gated channels with respect to the permeability of the plasma membrane. • 11.7 Describe graded potentials. • 11.8 Describe the events involved in the generation and propagation of an action potential. • 11.9 Describe continuous propagation and saltatory propagation, and discuss the factors that affect the speed with which action potentials are propagated. © 2011 Pearson Education, Inc. Section 2: Neurophysiology • Learning Outcomes • 11.10 Describe the general structure of synapses in the CNS and PNS, and discuss the events that occur at a chemical synapse. • 11.11 Discuss the significance of postsynaptic potentials, including the roles of excitatory postsynaptic potentials and inhibitory postsynaptic potentials. • 11.12 Discuss the interactions that make the processing of information in neural tissue possible. © 2011 Pearson Education, Inc. Section 2: Neurophysiology • Neurophysiology • Transmembrane potential • An unequal distribution of charge across a cell membrane • Inside membrane is slightly negative • Due to slight excess of negatively charged ions and proteins • Outside membrane is slightly positive • Due to slight excess of positively charged ions • Results from differences in membrane permeability to various ions and active transport • Is characteristic of all cells Animation: Transmembrane Potentials © 2011 Pearson Education, Inc. The unequal distribution of charges inside and outside the plasma membrane, which produces a transmembrane potential Extracellular fluid Plasma membrane Protein Cytosol Protein Protein Figure 11 Section 2 1 © 2011 Pearson Education, Inc. Section 2: Neurophysiology • Neural activity and transmembrane potential • Changes in transmembrane potential can cause muscle contraction, gland secretion, or transfer of information • Resting potential • Transmembrane potential of a cell at rest • All neural activities begin with a change from resting potential • Graded potential • Temporary, localized change in resting potential due to typical stimulus • Decreases with distance from stimulus © 2011 Pearson Education, Inc. Section 2: Neurophysiology • Neural activity and transmembrane potential (continued) • Action potential • Electrical event involving one location of axonal membrane • Can be triggered by sufficiently large graded potential • Is propagated along axon surface toward synaptic terminals • Synaptic activity • Typically involves release of neurotransmitters (like ACh) by presynaptic cell • Compounds bind to receptors on postsynaptic cell, changing its permeability producing a graded potential © 2011 Pearson Education, Inc. Section 2: Neurophysiology • Neural activity and transmembrane potential (continued) • Information processing • Integration of stimuli at individual cell level • Response of postsynaptic cell to stimulated receptors and other stimuli © 2011 Pearson Education, Inc. An overview of the role of the transmembrane potential in neural activity Graded potential Resting stimulus potential produces may produce Action potential triggers Information processing Presynaptic neuron Postsynaptic cell Figure 11 Section 2 2 © 2011 Pearson Education, Inc. Module 11.5: Resting potential • Extracellular fluid (ECF) has high concentrations of Na+ and Cl– • Cytosol has high concentrations of K+ and negatively charged proteins (Pr–) • These proteins cannot cross plasma membrane • Neuron resting potential is usually near 0.07 volts (V) or –70 millivolts (mV) (slightly negative inside) • Charged ions cannot freely cross plasma membrane • Can move across membrane only through membrane channels or active transport mechanisms © 2011 Pearson Education, Inc. Module 11.5: Resting potential • Leak channels • Always open • Size, shape, and structure determine which ions will pass • Potassium ions diffuse out of cell through K+ leak channels • Sodium ions diffuse into cell through Na+ leak channels • Primarily causes the transmembrane potential Animation: Neurophysiology: Ion Movement © 2011 Pearson Education, Inc. Plasma membrane Passive leak channels, which are primarily responsible for the transmembrane potential Figure 11.5 © 2011 Pearson Education, Inc. 1 Module 11.5: Resting potential • Active transport • Sodium–potassium exchange pump • Ejects 3 Na+ for 2 K+ recovered from ECF • Maintains stable resting potential Animation: Neurophysiology: Sodium Potassium Exchange Pump © 2011 Pearson Education, Inc. Potassium ions can diffuse out of the cell through potassium leak channels. The sodium–potassium exchange pump ejects 3 Na+ for every 2 K+ recovered from the extracellular fluid. At a transmembrane potential of –70 mV, the rate of Na+ entry versus K+ loss is 3:2, and the exchange pump maintains a stable resting potential. Sodium ions can diffuse into the cell through sodium leak channels. EXTRACELLULAR FLUID The unit of measurement of potential difference is the volt (V), and the transmembrane potential of a neuron is usually near 0.07 V. Such a value is usually expressed as –70 mV (or –70 millivolts—thousandths of a volt) with the minus sign indicating that the interior is negatively charged. Sodium– potassium exchange pump Plasma membrane CYTOSOL Protein The cytosol contains an abundance of negatively charged proteins, whereas the extracellular fluid contains relatively few. These proteins cannot cross the plasma membrane. Protein Protein An overview of the events responsible for the normal resting potential of a neuron Figure 11.5 © 2011 Pearson Education, Inc. 2 Module 11.5: Resting potential • Electrochemical gradients • Chemical gradient • Concentration gradient for an ion across plasma membrane • Electrical gradient • Attraction between opposite charges or repulsion between like charges (+/+ or –/–) • Equilibrium potential • When electrical and chemical gradients are equal and opposite, resulting in no net movement across membrane • In most cells, the gradients for Na+ and K+ are most important © 2011 Pearson Education, Inc. Module 11.5: Resting potential • Potassium ion gradients • At normal resting potential, the electrical and chemical gradients are in opposition, but not equal • The net electrochemical gradient for K+ is out of the cell • If the plasma membrane were freely permeable to potassium ions, K+ would continue to leave the cell until an equilibrium potential of –90 mV © 2011 Pearson Education, Inc. The dynamics of potassium ion gradients Potassium Ion Gradients At normal resting potential, an electrical gradient opposes the chemical gradient for potassium ions (K+). The net electrochemical gradient tends to force potassium ions out of the cell. Potassium Potassium chemical electrical gradient gradient Net potassium electrochemical gradient Resting potential If the plasma membrane were freely permeable to potassium ions, the outflow of K+ would continue until the equilibrium potential (–90 mV) was reached. Potassium chemical gradient Potassium electrical gradient Equilibrium potential Plasma membrane Cytosol Protein Plasma membrane Cytosol Protein Figure 11.5 © 2011 Pearson Education, Inc. 3 Module 11.5: Resting potential • Sodium ion gradients • At normal resting potential, both the chemical and electrical gradients cause Na+ to move into the cell • If the plasma membrane were freely permeable to sodium, Na+ would continue to enter the cell until an equilibrium potential of +66 mV was reached A&P Flix: Resting Membrane Potential © 2011 Pearson Education, Inc. The dynamics of sodium ion gradients Sodium Ion Gradients At the normal resting potential, chemical and electrical gradients combine to drive sodium ions (Na+) into the cell. Sodium chemical gradient Sodium electrical gradient If the plasma membrane were freely permeable to sodium ions, the influx of Na+ would continue until the equilibrium potential (+66 mV) was reached. Net sodium electrochemical gradient Sodium chemical gradient Sodium electrical gradient Equilibrium potential Resting potential Plasma membrane Cytosol Protein Plasma membrane Cytosol Protein Figure 11.5 © 2011 Pearson Education, Inc. 3 Module 11.5 Review a. Define resting potential. b. What effect would decreasing the concentration of extracellular potassium ions have on the transmembrane potential of a neuron? c. What happens at the sodium–potassium exchange pump? © 2011 Pearson Education, Inc. Module 11.6: Gated channels • Gated channels • Resting potential remains stable until the cell is disturbed or stimulated • Changes in transmembrane potential primarily occur due to gated channels opening or closing in response to stimuli • Three different gated channel classes 1. Chemically gated channels 2. Voltage-gated channels 3. Mechanically gated channels © 2011 Pearson Education, Inc. Figure 11.6 © 2011 Pearson Education, Inc. 1 Module 11.6: Gated channels • Chemically gated channels • Open when they bind specific chemicals • • Example: neuromuscular junction receptors that bind ACh Most abundant on dendrites and cell body of neurons • Where most synaptic communication occurs © 2011 Pearson Education, Inc. The function of chemically gated channels Extracellular fluid ACh Binding site ACh Plasma membrane Cytosol Resting state Gated channel (closed) Arrival of ACh Gated channel opens Figure 11.6 © 2011 Pearson Education, Inc. 1 Module 11.6: Gated channels • Voltage-gated channels • Characteristic of excitable membranes (capable of generating and conducting an action potential) • Open or close in response to changes in transmembrane potential • Most important for neurons • Voltage-gated potassium channels • Voltage-gated calcium channels • Voltage-gated sodium channels • Have two gates that function independently • Activation gates (open on stimulation) • Inactivation gates (close to stop sodium entry) © 2011 Pearson Education, Inc. The function of voltage-gated channels Activation gate Channel closed Inactivation gate Channel open Channel inactivated Figure 11.6 © 2011 Pearson Education, Inc. 2 Module 11.6: Gated channels • Mechanically gated channels • Open in response to physical distortion of membrane • Important in sensory receptors • Examples: touch, pressure, vibration © 2011 Pearson Education, Inc. The function of mechanically gated channels Applied pressure Channel closed Channel open Pressure removed Channel closed Figure 11.6 © 2011 Pearson Education, Inc. 3 Module 11.6 Review a. Define gated channels. b. Identify the three types of gated channels, and state the conditions under which each operates. c. What effect would a chemical that blocks voltagegated sodium channels in a neuron’s plasma membrane have on its transmembrane potential? © 2011 Pearson Education, Inc. Module 11.7: Graded potentials • Graded potentials • Also known as local potentials • Changes in transmembrane potential that cannot spread far from stimulation site • Example: effects of chemically gated sodium channels © 2011 Pearson Education, Inc. Module 11.7: Graded potentials • Graded potentials produced by chemically gated Na+ channels • At resting potential, chemically gated sodium channels are closed • Binding of chemical, opens channels allowing sodium influx • • Positively charged ions entering the cell cause depolarization (shift from resting potential to more positive) Intracellular Na+ spread out, attracted to negative charges lining membrane (= local current) • Extracellular Na+ moves to replace © 2011 Pearson Education, Inc. Initial segment The events in the propagation of a graded potential Extracellular Fluid Cytoplasm A neuron plasma membrane at normal resting potential A chemical stimulus opens the chemically gated sodium channels, producing a depolarization. Local current Local current Movement of positive charges causes a local current. © 2011 Pearson Education, Inc. Figure 11.7 1 – 3 Module 11.7: Graded potentials • • Degree of depolarization decreases with distance from stimulation site • Ions enter at one location • Spread occurs in all directions Change in transmembrane potential proportional to stimulus intensity • Greater stimulus = more open channels = more ion flow © 2011 Pearson Education, Inc. The effect of distance from the stimulation site on the degree of depolarization Transmembrane potential Figure 11.7 © 2011 Pearson Education, Inc. 4 Module 11.7: Graded potentials • Graded potentials produced by chemically gated Na+ channels (continued) • With removal of chemical stimulus, membrane returns to resting potential • Na+ pumped out of cell • = Repolarizition © 2011 Pearson Education, Inc. Module 11.7: Graded potentials • Effects of chemically gated potassium channels • Some chemicals open K+ channels • Potassium ions leave cytoplasm • Results in more negative transmembrane potential • = Hyperpolarization © 2011 Pearson Education, Inc. A chemical stimulus opens chemically gated sodium ion channels. Removal of the chemical stimulus leads to repolarization. A different chemical stimulus opens chemically gated potassium channels, causing hyperpolarization. Chemical stimulus removed Repolarization Resting potential Transmembrane potential (mV) Depolarization Hyperpolarization Return to resting potential Time The changes in transmembrane potential over time when different chemical stimuli are applied to the axon hillock Figure 11.7 © 2011 Pearson Education, Inc. 5 Figure 11.7 © 2011 Pearson Education, Inc. 6 Module 11.7 Review a. Define graded potential. b. Describe depolarization, repolarization, and hyperpolarization. c. What factors account for the local currents associated with graded potentials? © 2011 Pearson Education, Inc. Module 11.8: Action potential generation • Action potential generation • Information transfer in neurons • Reception of information as graded potentials on dendrites and cell bodies • At synaptic terminals, graded potentials cause release of neurotransmitters • Distance between cell body and synaptic terminals can be large • Graded potentials only travel short distances • Action potentials can travel longer distances • Are propagated changes in transmembrane potential that affect entire excitable membrane Animation: Neurophysiology: Action Potential © 2011 Pearson Education, Inc. Module 11.8: Action potential generation • Channel types and transmembrane potential • Leak channels are responsible mainly for resting potential • Chemically gated channels often produce graded potentials • Voltage-gated channels produce action potentials © 2011 Pearson Education, Inc. Module 11.8: Action potential generation • Prior to action potential generation • Transmembrane potentials are at resting levels • Sodium channels are closed but capable of opening • Activation gate closed • Inactivation gate open • Potassium channels are closed but capable of opening • Single gate closed A&P Flix: Generation of an Action Potential © 2011 Pearson Education, Inc. Module 11.8: Action potential generation • Steps of action potential generation and propagation 1. Depolarization to threshold • Graded depolarization large enough to open voltage-gated sodium channels • = Threshold • Approximate transmembrane potential of –60 mV Activation of Na+ channels and rapid depolarization 2. • Sodium ions rush into cell through open channels • Causes rapid depolarization • From –60 mV to a positive value © 2011 Pearson Education, Inc. Module 11.8: Action potential generation • Steps of action potential generation and propagation (continued) Inactivation of Na+ channels and activation of K+ channels 3. • At ~+30 mV, sodium inactivation gates close • • = Sodium channel inactivation Voltage-gated potassium channels open • Potassium ions leave the cell • 4. Begins repolarization Potassium channels close • As membrane reaches resting potential (–70 mV) • K+ ions continue to leave cell until all channels are closed • Produces brief hyperpolarization © 2011 Pearson Education, Inc. A graded depolarization brings an area of excitable membrane to threshold (–60 mV). DEPOLARIZATION Transmembrane potential (mV) The changes in the transmembrane potential at one location during the generation of an action potential REPOLARIZATION Voltage-gated sodium channels open and sodium ions move into the cell. The transmembrane potential rises to +30 mV. Sodium channels close, voltage-gated potassium channels open, and potassium ions move out of the cell. Repolarization begins. Potassium channels close, and both sodium and potassium channels return to their normal states. Threshold Resting potential ABSOLUTE REFRACTORY PERIOD During the absolute refractory period, the membrane cannot respond to further stimulation. RELATIVE REFRACTORY PERIOD During the relative refractory period, the membrane can respond only to a larger-than-normal stimulus. Time (msec) Figure 11.8 © 2011 Pearson Education, Inc. 2 Module 11.8: Action potential generation • Graded and action potential analogy: gun firing • Graded potential • Pulling trigger of gun • • Enough pressure will cause gun to fire Action potential • Firing of gun • Enough pressure on trigger will cause gun to fire same way every time • Stimulus triggers action potential or not at all • = All-or-none principle © 2011 Pearson Education, Inc. Module 11.8 Review a. Define action potential. b. List the events involved in the generation and propagation of an action potential. c. Compare the absolute refractory period with the relative refractory period. © 2011 Pearson Education, Inc. Module 11.9: Action potential propagation • Action potential propagation • A generated action potential does not itself move along the axon • Once generated at the initial segment, the action potential is regenerated at each adjacent axonal segment • = Propagation (not conduction) • Two types of action potential propagation 1. Continuous propagation 2. Saltatory propagation Animation: Neurophysiology: Continuous and Saltatory Propagation © 2011 Pearson Education, Inc. Module 11.9: Action potential propagation • Continuous propagation • Occurs along unmyelinated axons • Appears to move in a series of tiny steps • Each step takes ~1 msec • = Propagation speed of ~1 m/s © 2011 Pearson Education, Inc. Module 11.9: Action potential propagation • Steps of continuous propagation 1. Action potential develops at initial segment • Transmembrane potential = +30 mV 2. Entering sodium spreads away from voltage-gated channels to depolarize adjacent segment to threshold 3. Action potential occurs in adjacent segment while initial segment begins repolarizing 4. Sodium enters new segment, spreads, and causes depolarization of next adjacent axonal segment • Action potential can only move forward because last axonal segment is in absolute refractory period © 2011 Pearson Education, Inc. Module 11.9: Action potential propagation Animation: Neurophysiology: Positive Potential A&P Flix: Propagation of an Action Potential © 2011 Pearson Education, Inc. Initial segment Axon hillock The events that occur in continuous propagation Step 1: As an action potential develops at the initial segment 1 , the transmembrane potential at this site depolarizes to +30 mV. Extracellular fluid Action potential Cell membrane Step 2: As the sodium ions entering at 1 spread away from the open voltage-gated channels, a graded depolarization quickly brings the membrane in segment 2 to threshold. Cytosol Graded depolarization Step 3: An action potential now occurs in segment 2 while segment 1 begins repolarization. Repolarization (refractory) Step 4: As the sodium ions entering at Segment 2 spread laterally, a graded depolarization quickly brings the membrane in Segment 3 to threshold. The action potential can only move forward, not backward, because the membrane at segment 1 is in the absolute refractory period of repolarization. Figure 11.9 © 2011 Pearson Education, Inc. 1 Module 11.9: Action potential propagation • Saltatory propagation (saltere, leaping) • Occurs in myelinated axons • Only exposed nodes can respond to depolarizing stimulus • Internodes covered with myelin prevent ion flow across membrane • • Prevents continuous propagation Much faster than continuous propagation • Speed varies with axon diameter • Larger axon = faster current © 2011 Pearson Education, Inc. Module 11.9: Action potential propagation • Steps of saltatory propagation 1. Action potential occurs at initial segment 2. Local current produces graded depolarization to threshold at next node 3. Action potential develops at node 4. Local current flow produces graded depolarization to threshold at next node © 2011 Pearson Education, Inc. The events that occur in saltatory propagation Step 1: Extracellular fluid An action potential has occurred at the initial segment 1 . Myelinated internode Myelinated internode Myelinated internode Cytosol Cell membrane Step 2: A local current produces a graded depolarization that brings the axolemma at the next node to threshold. Local current Step 3: An action potential develops at node 2 . Repolarization (refractory) Step 4: A local current produces a graded depolarization that brings the 3 axolemma at node to threshold. Local current Figure 11.9 © 2011 Pearson Education, Inc. 2 Module 11.9 Review a. Define continuous propagation and saltatory propagation. b. What is the relationship between myelin and the propagation speed of action potentials? © 2011 Pearson Education, Inc. Module 11.10: Synaptic events • Synaptic events • Transmission of a message or “nerve impulse” within a neuron • = Action potential generation and propagation • Transfer of a message between cells (from a neuron to another neuron or effector cell) • = Must be relayed across a synapse • Types of synapses 1. Chemical synapses 2. Electrical synapses Animation: Neurophysiology: Synapse © 2011 Pearson Education, Inc. Module 11.10: Synaptic events • Chemical synapses • Rely on neurotransmitter release • • Those that release acetylcholine (ACh) are cholinergic synapses Most abundant synapse type • Most of those between neurons • All synapses between neurons and other cells © 2011 Pearson Education, Inc. Module 11.10: Synaptic events • Events at a cholinergic synapse 1. Depolarization of synaptic knob by arriving action potential 2. Opening of voltage-gated calcium channels • Influx of Ca2+ causes exocytosis of ACh from synaptic vesicles • Ca2+ quickly removed to end release of ACh © 2011 Pearson Education, Inc. Module 11.10: Synaptic events • Events at a cholinergic synapse (continued) 3. ACh diffuses across synaptic cleft and binds to chemically gated Na+ channels • Na+ diffuses into postsynaptic cell and depolarizes membrane • More ACh bound = larger depolarization 4. Acetylcholinesterase (AChE, an enzyme) breaks down ACh • Makes effects on postsynaptic cell temporary • Occurs within 20 msec © 2011 Pearson Education, Inc. The events that occur at a cholinergic synapse Events Occurring at Synapse Mitochondrion 1 An arriving action potential depolarizes the synaptic knob. Acetyl-CoA 2 Calcium ions enter the cytoplasm, and after a brief delay, ACh is released through the exocytosis of synaptic vesicles. CoA Acetylcholine Synaptic vesicle 4 Depolarization ends as ACh is broken down into acetate and choline by AChE. SYNAPTIC KNOB Choline Acetate Acetylcholinesterase (AChE) POSTSYNAPTIC MEMBRANE 3 ACh binds to sodium channel receptors on the postsynaptic membrane, producing a graded depolarization. SYNAPTIC CLEFT 5 The synaptic knob reabsorbs choline from the synaptic cleft and uses it to synthesize new molecules of ACh. ACh receptor Figure 11.10 2 © 2011 Pearson Education, Inc. Module 11.10: Synaptic events • Chemical synapse physiology • Synaptic fatigue • After extended stimulation, the recycling of neurotransmitter unable to keep up with demand • Synapse weakens until neurotransmitter can be replenished • Synaptic delay • Release and binding of neurotransmitters takes ~0.2–0.5 msec • With many neurons, cumulative delay may be significant • Rapid reflexes involve few synapses © 2011 Pearson Education, Inc. Module 11.10: Synaptic events • Electrical synapses • Presynaptic and postsynaptic membranes are locked together by gap junctions • Changes in transmembrane potential are transferred directly between cells through local current flow • Occur in CNS and PNS but extremely rare • • Some areas of brain, eye, ciliary ganglia of PNS Less adaptable and complex compared to chemical synapses • Example: changes in chemical environment or multiple neurotransmitter affecting postsynaptic cell response © 2011 Pearson Education, Inc. The structure of an electrical synapse Presynaptic neuron Gap junctions connecting presynaptic and postsynaptic neurons Postsynaptic neuron Figure 11.10 3 © 2011 Pearson Education, Inc. Module 11.10 Review a. Describe the parts of a chemical synapse. b. Contrast an electrical synapse with a chemical synapse. c. What is synaptic fatigue, and how does the synapse recover? © 2011 Pearson Education, Inc. Module 11.11: Information processing within a neuron • Information processing within a neuron • Postsynaptic potentials • Graded potentials in postsynaptic cell in response to a neurotransmitter • Two types 1. Excitatory postsynaptic potential (EPSP) • Graded depolarization caused by neurotransmitter arrival • Shifts transmembrane potential closer to threshold (= facilitated) • © 2011 Pearson Education, Inc. More facilitation, the less additional stimulus needed to reach threshold Module 11.11: Information processing within a neuron • Postsynaptic potentials (continued) • Two types (continued) 2. Inhibitory postsynaptic potential (IPSP) • Graded hyperpolarization • • Example: opening of chemically gated K+ channels Shifts transmembrane potential farther from threshold (= inhibited) • © 2011 Pearson Education, Inc. More inhibition, larger-than-usual stimulus needed to reach threshold Postsynaptic potentials, graded potentials that develop in the postsynaptic membrane in response to a neurostransmitter An excitatory postsynaptic potential, or EPSP, a graded depolarization An inhibitory postsynaptic potential, or IPSP, a graded hyperpolarization Time 2: Hyperpolarizing stimulus applied EPSP Stimulus removed Time 3: Hyperpolarizing stimulus applied Resting potential Resting potential IPSP Time 1: Depolarizing stimulus applied Summation: the integration of the effects of graded potentials on a segment of the plasma membrane EPSP IPSP Time 3: Depolarizing stimulus applied Stimulus removed Stimuli removed Time Figure 11.11 1 © 2011 Pearson Education, Inc. Module 11.11: Information processing within a neuron • Integration of information at postsynaptic cell • Single postsynaptic cell may receive information from thousands of synapses • • • Some excitatory, some inhibitory Net effect at axon hillock determines cell response • Axon hillock is closest to initial segment where action potential is generated • Threshold at axon hillock is lowest of cell body Is the simplest information processing in the nervous system • Allows neurons to respond to changes in oxygen, nutrients, or abnormal chemicals © 2011 Pearson Education, Inc. Axon hillock Initial segment Glial cell processes Dendrite Synaptic knobs The axon hillock, the site at which a single neuron integrates the excitatory and inhibitory stimuli it receives across thousands of synapses Figure 11.11 2 © 2011 Pearson Education, Inc. Module 11.11: Information processing within a neuron • Summation • Integration of graded potential effects on plasma membrane segment • May be combining opposite stimulations (EPSP + IPSP) or similar stimulations (EPSP + EPSP or IPSP + IPSP) • Individual EPSP or IPSP has small effect on transmembrane potential (~0.5 mV) • Summation of EPSPs can lead to action potential • Threshold commonly ~10 mV © 2011 Pearson Education, Inc. Module 11.11: Information processing within a neuron • Two types of summation 1. Temporal summation (tempus, time) • A single synapse stimulated repeatedly • Example: before effects of one EPSP can dissipate, another arrives • = More ACh release = more postsynaptic cell depolarization • © 2011 Pearson Education, Inc. Possibly to threshold at initial segment Temporal summation, in which a single synapse is active repeatedly Temporal Summation FIRST STIMULUS Initial segment SECOND STIMULUS ACTION POTENTIAL PROPAGATION Threshold reached Figure 11.11 3 © 2011 Pearson Education, Inc. Module 11.11: Information processing within a neuron • Two types of summation (continued) 2. Spatial summation • Involves multiple synapses activated simultaneously • Example: EPSPs at multiple sites allowing Na+ channels to open • • May lead to action potential at initial segment Degree of depolarization dependent on 1. Number of synapses are active at a particular moment 2. Distance from initial segment © 2011 Pearson Education, Inc. Spatial summation, in which multiple synapses are active simultaneously Spatial Summation TWO SIMULTANEOUS STIMULI ACTION POTENTIAL PROPAGATION Threshold reached Figure 11.11 3 © 2011 Pearson Education, Inc. Module 11.11 Review a. Define excitatory postsynaptic potential (EPSP) and inhibitory postsynaptic potential (IPSP). b. Compare temporal summation with spatial summation. c. If a single EPSP depolarizes the initial segment from a resting potential of –70 mV to –65 mV, and threshold is at –60 mV, will an action potential be generated? © 2011 Pearson Education, Inc. Module 11.12: Higher-level information processing • Higher-level information processing • Involves regulatory neurons • Facilitate or inhibit presynaptic neurons by: • Affecting cell body membrane • Altering sensitivity of synaptic knobs © 2011 Pearson Education, Inc. The positions of regulatory neurons, which facilitate or inhibit the activities of presynaptic neurons Regulatory neurons Presynaptic neuron Postsynaptic neuron Figure 11.12 1 © 2011 Pearson Education, Inc. Module 11.12: Higher-level information processing • Involves different neurotransmitters • More than 100 exist and work in different ways • May have direct or indirect effects on ion channels • Indirect effects usually involve G proteins • Trigger formation or release of second messengers to alter postsynaptic cell activity © 2011 Pearson Education, Inc. Figure 11.12 2 © 2011 Pearson Education, Inc. Module 11.12: Higher-level information processing • In nervous system, complex information is translated to action potentials • Solely on frequency of action potentials • Example: muscle contraction changes in response to increasing action potential frequency © 2011 Pearson Education, Inc. How the rate of action potentials arriving at a neuromuscular junction determines the nature of the resulting muscle contraction Time KEY Muscle tension Maximum tension (in tetanus) Arrival = of action potential Incomplete tetanus Tetanus Muscle tension Twitch contractions Frequency of action potentials (per second) © 2011 Pearson Education, Inc. Figure 11.12 3 Figure 11.12 4 © 2011 Pearson Education, Inc. Module 11.12 Review a. Describe the role of regulatory neurons. b. What determines the frequency of action potential generation? c. The greater the degree of sustained depolarization at the axon hillock, the __________ (higher or lower) the frequency of generation of action potentials. © 2011 Pearson Education, Inc.