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Chapter 12 Neural Tissue Lecture Presentation by Lee Ann Frederick University of Texas at Arlington © 2015 Pearson Education, Inc. An Introduction to the Nervous System • The Nervous System • Includes all neural tissue in the body • Neural tissue contains two kinds of cells 1. Neurons • Cells that send and receive signals 2. Neuroglia (glial cells) • Cells that support and protect neurons © 2015 Pearson Education, Inc. An Introduction to the Nervous System • Organs of the Nervous System • Brain and spinal cord • Sensory receptors of sense organs (eyes, ears, etc.) • Nerves connect nervous system with other systems © 2015 Pearson Education, Inc. 12-1 Divisions of the Nervous System • The Central Nervous System (CNS) • Consists of the spinal cord and brain • Contains neural tissue, connective tissues, and blood vessels • Functions of the CNS are to process and coordinate: • Sensory data from inside and outside body • Motor commands control activities of peripheral organs (e.g., skeletal muscles) • Higher functions of brain: intelligence, memory, learning, emotion © 2015 Pearson Education, Inc. 12-1 Divisions of the Nervous System • The Peripheral Nervous System (PNS) • Includes all neural tissue outside the CNS • Functions of the PNS • Deliver sensory information to the CNS • Carry motor commands to peripheral tissues and systems © 2015 Pearson Education, Inc. 12-1 Divisions of the Nervous System • The Peripheral Nervous System (PNS) • Nerves (also called peripheral nerves) • Bundles of axons with connective tissues and blood vessels • Carry sensory information and motor commands in PNS • Cranial nerves – connect to brain • Spinal nerves – attach to spinal cord © 2015 Pearson Education, Inc. 12-1 Divisions of the Nervous System • Functional Divisions of the PNS • Afferent division • Carries sensory information • From PNS sensory receptors to CNS • Efferent division • Carries motor commands • From CNS to PNS muscles and glands © 2015 Pearson Education, Inc. 12-1 Divisions of the Nervous System • Functional Divisions of the PNS • Receptors and effectors of afferent division • Receptors • Detect changes or respond to stimuli • Neurons and specialized cells • Complex sensory organs (e.g., eyes, ears) • Effectors • Respond to efferent signals • Cells and organs © 2015 Pearson Education, Inc. 12-1 Divisions of the Nervous System • Functional Divisions of the PNS • The efferent division • Somatic nervous system (SNS) • Controls voluntary and involuntary (reflexes) skeletal muscle contractions © 2015 Pearson Education, Inc. 12-1 Divisions of the Nervous System • Functional Divisions of the PNS • The efferent division • Autonomic nervous system (ANS) • Controls subconscious actions, contractions of smooth muscle and cardiac muscle, and glandular secretions • Sympathetic division has a stimulating effect • Parasympathetic division has a relaxing effect © 2015 Pearson Education, Inc. Figure 12-1 A Functional Overview of the Nervous System. Organization of the Nervous System Central Nervous System (CNS) (brain and spinal cord) Peripheral Nervous System (PNS) (neural tissue outside the CNS) Integrate, process, and coordinate sensory data and motor commands Sensory information within afferent division Motor commands within efferent division includes Somatic nervous system (SNS) Autonomic nervous system (ANS) Parasympathetic division Effectors Receptors Special sensory receptors monitor smell, taste, vision, balance, and hearing © 2015 Pearson Education, Inc. Visceral sensory receptors monitor internal organs Somatic sensory receptors monitor skeletal muscles, joints, and skin surface Skeletal muscle Sympathetic division • Smooth muscle • Cardiac muscle • Glands • Adipose tissue 12-2 Neurons • Neurons • The basic functional units of the nervous system • The structure of neurons • The multipolar neuron • Common in the CNS • Cell body (soma) • Short, branched dendrites • Long, single axon © 2015 Pearson Education, Inc. Figure 12-2a The Anatomy of a Multipolar Neuron. Dendrites Perikaryon Cell body Nucleus Telodendria Axon a This color-coded figure shows the four general regions of a neuron. © 2015 Pearson Education, Inc. Figure 12-2b The Anatomy of a Multipolar Neuron. Nissl bodies (RER and free ribosomes) Dendritic branches Mitochondrion Axon hillock Initial segment of axon Axolemma Axon Telodendria Direction of action potential Golgi apparatus Neurofilament Nucleolus Axon terminals Nucleus Dendrite See Figure 12–3 Presynaptic cell b An understanding of neuron function requires knowing its structural components. © 2015 Pearson Education, Inc. Postsynaptic cell 12-2 Neurons • The Structure of Neurons • The synapse • Area where a neuron communicates with another cell © 2015 Pearson Education, Inc. 12-2 Neurons • The Structure of Neurons • The synapse • Presynaptic cell • Neuron that sends message • Postsynaptic cell • Cell that receives message • The synaptic cleft • The small gap that separates the presynaptic membrane and the postsynaptic membrane © 2015 Pearson Education, Inc. 12-2 Neurons • The Synapse • The synaptic terminal • Is expanded area of axon of presynaptic neuron • Contains synaptic vesicles of neurotransmitters © 2015 Pearson Education, Inc. 12-2 Neurons • Neurotransmitters • Are chemical messengers • Are released at presynaptic membrane • Affect receptors of postsynaptic membrane • Are broken down by enzymes • Are reassembled at axon terminal © 2015 Pearson Education, Inc. 12-2 Neurons • Types of Synapses • Neuromuscular junction • Synapse between neuron and muscle • Neuroglandular junction • Synapse between neuron and gland © 2015 Pearson Education, Inc. Figure 12-3 The Structure of a Typical Synapse. Telodendrion Axon terminal Mitochondrion Synaptic vesicles Presynaptic membrane Postsynaptic Synaptic membrane cleft © 2015 Pearson Education, Inc. Figure 12-4 Structural Classifications of Neurons. Anaxonic neuron Bipolar neuron Dendritic branches Unipolar neuron Multipolar neuron Dendrites Dendrites Initial segment Cell body Axon Dendrite Cell body Cell body Axon Cell body Axon terminals Axon Axon Axon terminals a Anaxonic neurons have more than two processes, and they are all dendrites. © 2015 Pearson Education, Inc. b Bipolar neurons have two processes separated by the cell body. c Unipolar neurons have a single elongated process, with the cell body located off to the side. Axon terminals d Multipolar neurons have more than two processes; there is a single axon and multiple dendrites. 12-2 Neurons • Functions of Sensory Neurons • Monitor internal environment (visceral sensory neurons) • Monitor effects of external environment (somatic sensory neurons) • Structures of Sensory Neurons • Unipolar • Cell bodies grouped in sensory ganglia • Processes (afferent fibers) extend from sensory receptors to CNS © 2015 Pearson Education, Inc. 12-2 Neurons • Three Types of Sensory Receptors 1. Interoceptors • Monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive) • Internal senses (taste, deep pressure, pain) 2. Exteroceptors • External senses (touch, temperature, pressure) • Distance senses (sight, smell, hearing) 3. Proprioceptors • Monitor position and movement (skeletal muscles and joints) © 2015 Pearson Education, Inc. 12-2 Neurons • Motor Neurons • Carry instructions from CNS to peripheral effectors • Via efferent fibers (axons) © 2015 Pearson Education, Inc. 12-2 Neurons • Motor Neurons • Two major efferent systems 1. Somatic nervous system (SNS) • Includes all somatic motor neurons that innervate skeletal muscles 2. Autonomic (visceral) nervous system (ANS) • Visceral motor neurons innervate all other peripheral effectors • Smooth muscle, cardiac muscle, glands, adipose tissue © 2015 Pearson Education, Inc. 12-2 Neurons • Motor Neurons • Two groups of efferent axons • Signals from CNS motor neurons to visceral effectors pass synapses at autonomic ganglia dividing axons into: • Preganglionic fibers • Postganglionic fibers © 2015 Pearson Education, Inc. 12-2 Neurons • Interneurons • Most are located in brain, spinal cord, and autonomic ganglia • Between sensory and motor neurons • Are responsible for: • Distribution of sensory information • Coordination of motor activity • Are involved in higher functions • Memory, planning, learning © 2015 Pearson Education, Inc. 12-3 Neuroglia • Neuroglia • Half the volume of the nervous system • Many types of neuroglia in CNS and PNS © 2015 Pearson Education, Inc. 12-3 Neuroglia • Four Types of Neuroglia in the CNS 1. Ependymal cells • Cells with highly branched processes; contact neuroglia directly 2. Astrocytes • Large cell bodies with many processes 3. Oligodendrocytes • Smaller cell bodies with fewer processes 4. Microglia • Smallest and least numerous neuroglia with many fine-branched processes © 2015 Pearson Education, Inc. Figure 12-5 An Introduction to Neuroglia (Part 1 of 2). Neuroglia are found in Central Nervous System contains Ependymal cells Astrocytes Line ventricles (brain) and central canal (spinal cord); assist in producing, circulating, and monitoring cerebrospinal fluid Maintain blood–brain barrier; provide structural support; regulate ion, nutrient, and dissolved gas concentrations; absorb and recycle neurotransmitters; form scar tissue after injury © 2015 Pearson Education, Inc. Oligodendrocytes Myelinate CNS axons; provide structural framework Microglia Remove cell debris, wastes, and pathogens by phagocytosis 12-3 Neuroglia • Ependymal Cells • Form epithelium called ependyma • Line central canal of spinal cord and ventricles of brain • Secrete cerebrospinal fluid (CSF) • Have cilia or microvilli that circulate CSF • Monitor CSF • Contain stem cells for repair © 2015 Pearson Education, Inc. 12-3 Neuroglia • Astrocytes • Maintain blood–brain barrier (isolates CNS) • Create three-dimensional framework for CNS • Repair damaged neural tissue • Guide neuron development • Control interstitial environment © 2015 Pearson Education, Inc. 12-3 Neuroglia • Oligodendrocytes • Myelination • Increases speed of action potentials • Myelin insulates myelinated axons • Makes nerves appear white © 2015 Pearson Education, Inc. 12-3 Neuroglia • Oligodendrocytes • Nodes and internodes • Internodes – myelinated segments of axon • Nodes (also called nodes of Ranvier) • Gaps between internodes • Where axons may branch © 2015 Pearson Education, Inc. 12-3 Neuroglia • Myelination • White matter • Regions of CNS with many myelinated nerves • Gray matter • Unmyelinated areas of CNS © 2015 Pearson Education, Inc. 12-3 Neuroglia • Microglia • Migrate through neural tissue • Clean up cellular debris, waste products, and pathogens © 2015 Pearson Education, Inc. Figure 12-6b Neuroglia in the CNS (Part 1 of 2). Gray matter White matter CENTRAL CANAL Ependymal cells Gray matter Neurons Microglial cell © 2015 Pearson Education, Inc. Figure 12-6b Neuroglia in the CNS (Part 2 of 2). Gray matter White matter Myelinated axons Internode Myelin (cut) Axon White matter Oligodendrocyte Astrocyte Axolemma Node Unmyelinated axon Basement membrane Capillary b A diagrammatic view of neural tissue in the CNS, showing relationships between neuroglia and neurons © 2015 Pearson Education, Inc. 12-3 Neuroglia • Neuroglia of the Peripheral Nervous System • Ganglia • Masses of neuron cell bodies • Surrounded by neuroglia • Found in the PNS © 2015 Pearson Education, Inc. Figure 12-5 An Introduction to Neuroglia (Part 2 of 2). Neuroglia are found in Peripheral Nervous System contains © 2015 Pearson Education, Inc. Satellite cells Schwann cells Surround neuron cell bodies in ganglia; regulate O2, CO2, nutrient, and neurotransmitter levels around neurons in ganglia Surround all axons in PNS; responsible for myelination of peripheral axons; participate in repair process after injury 12-3 Neuroglia • Neuroglia of the Peripheral Nervous System • Satellite cells • Also called amphicytes • Surround ganglia • Regulate environment around neuron © 2015 Pearson Education, Inc. 12-3 Neuroglia • Neuroglia of the Peripheral Nervous System • Schwann cells • Also called neurilemma cells • Form myelin sheath (neurilemma) around peripheral axons • One Schwann cell sheaths one segment of axon • Many Schwann cells sheath entire axon © 2015 Pearson Education, Inc. Figure 12-7a Schwann Cells, Peripheral Axons, and Formation of the Myelin Sheath (Part 1 of 2). Axon hillock Nucleus Myelinated internode Initial segment (unmyelinated) Nodes Axon Axolemma Myelin covering internode a A myelinated axon, showing the organization of Schwann cells along the length of the axon. © 2015 Pearson Education, Inc. Dendrite Figure 12-7c Schwann Cells, Peripheral Axons, and Formation of the Myelin Sheath. 1 A Schwann cell first surrounds a portion of the axon within a groove of its cytoplasm. Schwann cell 2 The Schwann cell then begins to rotate around the axon. 3 As the Schwann cell rotates, myelin is wound around the axon in multiple layers, forming a tightly packed membrane Myelin Axon Schwann cell cytoplasm c Stages in the formation of a myelin sheath by a single Schwann cell along a portion of a single axon. © 2015 Pearson Education, Inc. 12-3 Neuroglia • Neurons and Neuroglia • Neurons perform: • All communication, information processing, and control functions of the nervous system • Neuroglia preserve: • Physical and biochemical structure of neural tissue • Neuroglia are essential to: • Survival and function of neurons © 2015 Pearson Education, Inc. 12-3 Neuroglia • Neural Responses to Injuries • Wallerian degeneration • Axon distal to injury degenerates • Schwann cells • Form path for new growth • Wrap new axon in myelin © 2015 Pearson Education, Inc. 12-3 Neuroglia • Nerve Regeneration in CNS • Limited by chemicals released by astrocytes that: • Block growth • Produce scar tissue © 2015 Pearson Education, Inc. Figure 12-8 Peripheral Nerve Regeneration after Injury (Part 1 of 4). 1 Fragmentation of axon and myelin occurs in distal stump. © 2015 Pearson Education, Inc. Axon Myelin Proximal stump Distal stump Figure 12-8 Peripheral Nerve Regeneration after Injury (Part 2 of 4). 2 Schwann cells form cord, grow into cut, and unite stumps. Macrophages engulf degenerating axon and myelin. Macrophage Cord of proliferating Schwann cells © 2015 Pearson Education, Inc. Figure 12-8 Peripheral Nerve Regeneration after Injury (Part 3 of 4). 3 Axon sends buds into network of Schwann cells and then starts growing along cord of Schwann cells. © 2015 Pearson Education, Inc. Figure 12-8 Peripheral Nerve Regeneration after Injury (Part 4 of 4). 4 Axon continues to grow into distal stump and is enclosed by Schwann cells. © 2015 Pearson Education, Inc. 12-4 Membrane Potential • Ion Movements and Electrical Signals • All plasma (cell) membranes produce electrical signals by ion movements • Membrane potential is particularly important to neurons © 2015 Pearson Education, Inc. 12-4 Membrane Potential • Five Main Membrane Processes in Neural Activities 1. Resting potential • The membrane potential of resting cell 2. Graded potential • Temporary, localized change in resting potential • Caused by stimulus © 2015 Pearson Education, Inc. 12-4 Membrane Potential • Five Main Membrane Processes in Neural Activities 3. Action potential • Is an electrical impulse • Produced by graded potential • Propagates along surface of axon to synapse © 2015 Pearson Education, Inc. 12-4 Membrane Potential • Five Main Membrane Processes in Neural Activities 4. Synaptic activity • Releases neurotransmitters at presynaptic membrane • Produces graded potentials in postsynaptic membrane 5. Information processing • Response (integration of stimuli) of postsynaptic cell © 2015 Pearson Education, Inc. 12-4 Membrane Potential • The Membrane Potential • Three important concepts 1. The extracellular fluid (ECF) and intracellular fluid (cytosol) differ greatly in ionic composition • Concentration gradient of ions (Na+, K+) 2. Cells have selectively permeable membranes 3. Membrane permeability varies by ion © 2015 Pearson Education, Inc. A&P FLIX Resting Membrane Potential © 2015 Pearson Education, Inc. 12-4 Membrane Potential • Passive Forces Acting across the Plasma Membrane • Chemical gradients • Concentration gradients (chemical gradient) of ions (Na+, K+) • Electrical gradients • Separate charges of positive and negative ions • Result in potential difference © 2015 Pearson Education, Inc. Figure 12-9 Resting Membrane Potential. © 2015 Pearson Education, Inc. Figure 12-9 Resting Membrane Potential (Part 1 of 2). Passive Chemical Gradients Active Na+/K+ Pumps The intracellular concentration of potassium ions (K+) is relatively high, so these ions tend to move out of the cell through potassium leak channels. Similarly, the extracellular concentration of sodium ions (Na+) is relatively high, so sodium ions move into the cell through sodium leak channels. Both of these movements are driven by a concentration gradient, or chemical gradient. Sodium–potassium (Na +/K+) exchange pumps maintain the concentration of sodium and potassium ions across the plasma membrane. Cl– + – + + + Sodium– potassium exchange pump K+ leak channel – + – + – KEY Sodium ion (Na+) + Potassium ion (K ) – Chloride ion (Cl ) + - © 2015 Pearson Education, Inc. – + + + + + Na+ leak channel – – – + ADP ATP 2 K+ + + – + 3 Na+ + + + + + + + + + – – Protein – + – + – + – – Protein – + Figure 12-9 Resting Membrane Potential (Part 2 of 2). Passive Electrical Gradients Potassium ions leave the cytosol more rapidly than sodium ions enter because the plasma membrane is much more permeable to potassium than to sodium. As a result, there are more positive charges outside the plasma membrane. Negatively charged protein molecules within the cytosol cannot cross the plasma membrane, so there are more negative charges on the cytosol side of the plasma membrane. This results in an electrical gradient across the plasma membrane. – + Resting Membrane Potential Whenever positive and negative ions are held apart, a potential difference arises. We measure the size of that potential difference in millivolts (mV). The resting membrane potential for most neurons is about –70 mV. The minus sign shows that the inner surface of the plasma membrane is negatively charged with respect to the exterior. –30 –70 0 EXTRACELLULAR FLUID +30 + mV – + + + + + + + – – – + Plasma membrane – + + + + – – – + – – Protein – – Protein + © 2015 Pearson Education, Inc. – – – + – CYTOSOL + KEY + + – Sodium ion (Na+) Potassium ion (K+) Chloride ion (Cl-) 12-4 Membrane Potential • Electrical Currents and Resistance • Electrical current • Movement of charges to eliminate potential difference • Resistance • The amount of current a membrane restricts © 2015 Pearson Education, Inc. 12-4 Membrane Potential • The Electrochemical Gradient • For a particular ion (Na+, K+) is: • The sum of chemical and electrical forces • Acting on the ion across a plasma membrane • A form of potential energy © 2015 Pearson Education, Inc. 12-4 Membrane Potential • Equilibrium Potential • The membrane potential at which there is no net movement of a particular ion across the cell membrane • Examples: • K+ = -90 mV • Na+ = +66 mV © 2015 Pearson Education, Inc. Figure 12-10a Electrochemical Gradients for Potassium and Sodium Ions. Potassium Ion Gradients a At normal resting membrane 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 chemical gradient Resting membrane potential –70 mV + Potassium electrical gradient + + + + + + – – – – + + + –+ + + + K + – – Cytosol + © 2015 Pearson Education, Inc. Net potassium electrochemical gradient + + Plasma membrane + + – – – + – + + – + Protein – – Figure 12-10b Electrochemical Gradients for Potassium and Sodium Ions. Potassium Ion Gradients b If the plasma membrane were freely permeable to potassium ions, the outflow of K+ would continue until the equilibrium potential (–90 mV) was reached. Note how similar it is to the resting membrane potential. Potassium chemical gradient Equilibrium potential –90 mV + + Potassium electrical gradient + + + + + + + Plasma membrane + + + + + + + + – – – – – + + +– + –+ + – + K – + + + + – Protein – – – Cytosol – – + © 2015 Pearson Education, Inc. Figure 12-10c Electrochemical Gradients for Potassium and Sodium Ions. Sodium Ion Gradients c At the normal resting membrane potential, chemical and electrical gradients combine to drive sodium ions (Na+) into the cell. Sodium chemical Sodium gradient electrical gradient Resting membrane potential –70 mV + + – – – + – © 2015 Pearson Education, Inc. + + + + + + + + + + + + + + + + + + + + + + + Plasma membrane – – – Net sodium electrochemical gradient – – – – Cytosol – + – Protein + – Figure 12-10d Electrochemical Gradients for Potassium and Sodium Ions. Sodium Ion Gradients d If the plasma membrane were freely permeable to sodium ions, the influx of Na+ would continue until the equilibrium potential (+66 mV) was reached. Note how different it is from the resting membrane potential. Sodium electrical gradient Sodium chemical gradient Equilibrium potential +66 mV – – + + + + © 2015 Pearson Education, Inc. + + + – + + + + + + Cytosol – + + – + + + + + + – – – – + + + + + Plasma membrane + + – + + + + + + + – + – Protein + – + – + 12-4 Membrane Potential • Active Forces across the Membrane • Sodium–potassium ATPase (exchange pump) • Is powered by ATP • Carries 3 Na+ out and 2 K+ in • Balances passive forces of diffusion • Maintains resting potential (-70 mV) © 2015 Pearson Education, Inc. 12-4 Membrane Potential • The Resting Potential • Because the plasma membrane is highly permeable to potassium ions: • The resting potential of approximately -70 mV is fairly close to -90 mV, the equilibrium potential for K + • The electrochemical gradient for sodium ions is very large, but the membrane’s permeability to these ions is very low • Na+ has only a small effect on the normal resting potential, making it just slightly less negative than the equilibrium potential for K+ © 2015 Pearson Education, Inc. 12-4 Membrane Potential • The Resting Potential • The sodium–potassium exchange pump ejects 3 Na+ ions for every 2 K+ ions that it brings into the cell • It serves to stabilize the resting potential when the ratio of Na+ entry to K+ loss through passive channels is 3:2 • At the normal resting potential, these passive and active mechanisms are in balance • The resting potential varies widely with the type of cell • A typical neuron has a resting potential of approximately -70 mV © 2015 Pearson Education, Inc. 12-4 Membrane Potential • Changes in the Membrane Potential • Membrane potential rises or falls • In response to temporary changes in membrane permeability • Resulting from opening or closing specific membrane channels © 2015 Pearson Education, Inc. 12-4 Membrane Potential • Sodium and Potassium Channels • Membrane permeability to Na+ and K+ determines membrane potential • They are either passive or active © 2015 Pearson Education, Inc. 12-4 Membrane Potential • Passive Channels (Leak Channels) • Are always open • Permeability changes with conditions • Active Channels (Gated Channels) • Open and close in response to stimuli • At resting potential, most gated channels are closed © 2015 Pearson Education, Inc. 12-4 Membrane Potential • Three States of Gated Channels 1. Closed, but capable of opening 2. Open (activated) 3. Closed, not capable of opening (inactivated) © 2015 Pearson Education, Inc. 12-4 Membrane Potential • Three Classes of Gated Channels 1. Chemically gated channels 2. Voltage-gated channels 3. Mechanically gated channels © 2015 Pearson Education, Inc. Figure 12-11a Gated Channels. a Chemically gated channel + Resting state Presence of ACh + ACh Binding site Gated channel Channel closed + + + + + Channel open A chemically gated (ligand-gated) Na+ channel that opens in response to the presence of ACh (ligand) at a binding site. © 2015 Pearson Education, Inc. Figure 12-11b Gated Channels. b Voltage-gated channel –70 mV Activation gate + + Channel closed –60 mV Channel open +30 mV Inactivation gate + + + + + + Channel inactivated A voltage-gated Na+ channel that responds to changes in the membrane potential. At its resting membrane potential of –70 mV, the channel is closed; at –60 mV, the channel opens; at +30 mV, the channel is inactivated. © 2015 Pearson Education, Inc. Figure 12-11c Gated Channels. c Mechanically gated channel + + + + + Channel closed + Applied + + + pressure + + Channel open + + + + Pressure removed + Channel closed A mechanically gated channel, which opens in response to distortion of the membrane. © 2015 Pearson Education, Inc. Figure 12-12 Graded Potentials (Part 1 of 3). Resting State Resting membrane with closed chemically gated sodium ion channels Initial segment + Extracellular + + Fluid –70 mV + + + + + + + + + + + + + + + + + + – © 2015 Pearson Education, Inc. – – – – – – – – – – – – Cytosol – Figure 12-12 Graded Potentials (Part 2 of 3). 1 Stimulation Membrane exposed to chemical that opens the sodium ion channels Stimulus applied here + + –65 mV + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + – – – – – + + + + + + + + + © 2015 Pearson Education, Inc. – – – – + + + + + + + + + + – – – – – – Figure 12-12 Graded Potentials (Part 3 of 3). 2 Graded Potential Spread of sodium ions inside plasma membrane produces a local current that depolarized adjacent portions of the plasma membrane + + + Local + + + + current + + + + + + + – – © 2015 Pearson Education, Inc. – – –60 mV + + + – – + + –65 mV + + + + – + + + + + + + + + + – – + + + + + – + + + + + + + + + + + + + + Local current + + + – – – – –70 mV + – – – – – 12-4 Membrane Potential • Graded Potentials • Repolarization • When the stimulus is removed, membrane potential returns to normal • Hyperpolarization • Increasing the negativity of the resting potential • Result of opening a potassium channel • Opposite effect of opening a sodium channel • Positive ions move out, not into cell © 2015 Pearson Education, Inc. STOP: NEXT CLASS……. • READ UP ON ACTION POTENTIAL © 2015 Pearson Education, Inc. 12-5 Action Potential • Initiating Action Potential • Initial stimulus • A graded depolarization of axon hillock large enough (10 to 15 mV) to change resting potential (-70 mV) to threshold level of voltage-gated sodium channels (-60 to -55 mV) © 2015 Pearson Education, Inc. 12-5 Action Potential • Initiating Action Potential • All-or-none principle • If a stimulus exceeds threshold amount • The action potential is the same • No matter how large the stimulus • Action potential is either triggered, or not © 2015 Pearson Education, Inc. Figure 12-14 Generation of an Action Potential (Part 3 of 9). RESTING POTENTIAL –70 mV + + – – – – + + + + + + + + + – + + The axolemma contains both voltagegated sodium channels and voltagegated potassium channels that are closed when the membrane is at the resting potential. KEY + + © 2015 Pearson Education, Inc. = Sodium ion = Potassium ion 12-5 Action Potential • Four Steps in the Generation of Action Potentials • Step 1: Depolarization to threshold • Step 2: Activation of Na+ channels • Step 3: Inactivation of Na+ channels and activation of K+ channels • Step 4: Return to normal permeability © 2015 Pearson Education, Inc. Figure 12-14 Generation of an Action Potential (Part 4 of 9). 1 Depolarization to Threshold –60 mV Local current + + + + + – + + + – + + + + + – + + The stimulus that initiates an action potential is a graded depolarization large enough to open voltage-gated sodium channels. The opening of the channels occurs at the membrane potential known as the threshold. KEY + + © 2015 Pearson Education, Inc. = Sodium ion = Potassium ion Figure 12-14 Generation of an Action Potential (Part 5 of 9). 2 Activation of Sodium Channels and Rapid Depolarization +10 mV + + + + – + + + – + + + + + + + + When the sodium channel activation gates open, the plasma membrane becomes much more permeable to Na+. Driven by the large electrochemical gradient, sodium ions rush into the cytoplasm, and rapid depolarization occurs. The inner membrane surface now contains more positive ions than negative ones, and the membrane potential has changed from –60 mV to a positive value. KEY © 2015 Pearson Education, Inc. + + = Sodium ion = Potassium ion Figure 12-14 Generation of an Action Potential (Part 6 of 9). 3 Inactivation of Sodium Channels and Activation of Potassium Channels +30 mV + + + + + + + + + + + + + + + + As the membrane potential approaches +30 mV, the inactivation gates of the voltagegated sodium channels close. This step is known as sodium channel inactivation, and it coincides with the opening of voltage-gated potassium channels. Positively charged potassium ions move out of the cytosol, shifting the membrane potential back toward resting levels. Repolarization now begins. KEY © 2015 Pearson Education, Inc. + + = Sodium ion = Potassium ion Figure 12-14 Generation of an Action Potential (Part 7 of 9). 4 Closing of Potassium Channels –90 mV + + + – – – + + + + + + – – + + + + – + The voltage-gated sodium channels remain inactivated until the membrane has repolarized to near threshold levels. At this time, they regain their normal status: closed but capable of opening. The voltage-gated potassium channels begin closing as the membrane reaches the normal resting potential (about –70 mV). Until all these potassium channels have closed, potassium ions continue to leave the cell. This produces a brief hyperpolarization. KEY © 2015 Pearson Education, Inc. + + = Sodium ion = Potassium ion 12-5 Action Potential • The Refractory Period • The time period: • From beginning of action potential • To return to resting state • During which membrane will not respond normally to additional stimuli © 2015 Pearson Education, Inc. A&P FLIX Generation of an Action Potential © 2015 Pearson Education, Inc. 12-5 Action Potential • Absolute Refractory Period • Sodium channels open or inactivated • No action potential possible • Relative Refractory Period • Membrane potential almost normal • Very large stimulus can initiate action potential © 2015 Pearson Education, Inc. 12-5 Action Potential • Powering the Sodium–Potassium Exchange Pump • To maintain concentration gradients of Na+ and K+ over time • Requires energy (1 ATP for each 2 K+/3 Na+ exchange) • Without ATP • Neurons stop functioning © 2015 Pearson Education, Inc. Figure 12-14 Generation of an Action Potential (Part 9 of 9). ABSOLUTE REFRACTORY PERIOD 3 +30 Membrane potential (mV) 0 D E P O L A R I Z AT I O N R E P O L A R I Z AT I O N Threshold 1 4 A graded depolarization brings an area of excitable membrane to threshold (–60 mV). During the relative refractory period, the membrane can respond only to a larger-thannormal stimulus. During the absolute refractory period, the membrane cannot respond to further stimulation. 0 1 Time (msec) © 2015 Pearson Education, Inc. Potassium channels close, and both sodium and potassium channels return to their normal states. Voltage-gated sodium channels open and sodium ions move into the cell. The membrane potential rises to +30 mV. –40 –70 Sodium channels close, voltage-gated potassium channels open, and potassium ions move out of the cell. Repolarization begins. 2 Resting potential –60 RELATIVE REFRACTORY PERIOD 2 12-5 Action Potential • Propagation of Action Potentials • Propagation • Moves action potentials generated in axon hillock • Along entire length of axon • Two methods of propagating action potentials 1. Continuous propagation (unmyelinated axons) 2. Saltatory propagation (myelinated axons) © 2015 Pearson Education, Inc. 12-5 Action Potential • Continuous Propagation • Of action potentials along an unmyelinated axon • Affects one segment of axon at a time • Steps in propagation • Step 1: Action potential in segment 1 • Depolarizes membrane to +30 mV • Local current • Step 2: Depolarizes second segment to threshold • Second segment develops action potential © 2015 Pearson Education, Inc. 12-5 Action Potential • Continuous Propagation • Steps in propagation • Step 3: First segment enters refractory period • Step 4: Local current depolarizes next segment • Cycle repeats • Action potential travels in one direction (1 m/sec) © 2015 Pearson Education, Inc. 12-5 Action Potential • Saltatory Propagation • Action potential along myelinated axon • Faster and uses less energy than continuous propagation • Myelin insulates axon, prevents continuous propagation • Local current “jumps” from node to node • Depolarization occurs only at nodes © 2015 Pearson Education, Inc. A&P FLIX Propagation of an Action Potential © 2015 Pearson Education, Inc. 12-6 Axon Diameter and Speed • Axon Diameter and Propagation Speed • Ion movement is related to cytoplasm concentration • Axon diameter affects action potential speed • The larger the diameter, the lower the resistance © 2015 Pearson Education, Inc. 12-6 Axon Diameter and Speed • Three Groups of Axons 1. Type A fibers 2. Type B fibers 3. Type C fibers • These groups are classified by: • Diameter • Myelination • Speed of action potentials © 2015 Pearson Education, Inc. 12-6 Axon Diameter and Speed • Type A Fibers • Myelinated • Large diameter • High speed (140 m/sec) • Carry rapid information to/from CNS • For example, position, balance, touch, and motor impulses © 2015 Pearson Education, Inc. 12-6 Axon Diameter and Speed • Type B Fibers • Myelinated • Medium diameter • Medium speed (18 m/sec) • Carry intermediate signals • For example, sensory information, peripheral effectors © 2015 Pearson Education, Inc. 12-6 Axon Diameter and Speed • Type C Fibers • Unmyelinated • Small diameter • Slow speed (1 m/sec) • Carry slower information • For example, involuntary muscle, gland controls © 2015 Pearson Education, Inc. 12-6 Axon Diameter and Speed • Information • “Information” travels within the nervous system • As propagated electrical signals (action potentials) • The most important information (vision, balance, motor commands) • Is carried by large-diameter, myelinated axons © 2015 Pearson Education, Inc. 12-7 Synapses • Synaptic Activity • Action potentials (nerve impulses) • Are transmitted from presynaptic neuron • To postsynaptic neuron (or other postsynaptic cell) • Across a synapse © 2015 Pearson Education, Inc. 12-7 Synapses • Two Types of Synapses 1. Electrical synapses • Direct physical contact between cells 2. Chemical synapses • Signal transmitted across a gap by chemical neurotransmitters © 2015 Pearson Education, Inc. 12-7 Synapses • Electrical Synapses • Are locked together at gap junctions (connexons) • Allow ions to pass between cells • Produce continuous local current and action potential propagation • Are found in areas of brain, eye, ciliary ganglia © 2015 Pearson Education, Inc. 12-7 Synapses • Chemical Synapses • Are found in most synapses between neurons and all synapses between neurons and other cells • Cells not in direct contact • Action potential may or may not be propagated to postsynaptic cell, depending on: • Amount of neurotransmitter released • Sensitivity of postsynaptic cell © 2015 Pearson Education, Inc. 12-7 Synapses • Two Classes of Neurotransmitters 1. Excitatory neurotransmitters • Cause depolarization of postsynaptic membranes • Promote action potentials 2. Inhibitory neurotransmitters • Cause hyperpolarization of postsynaptic membranes • Suppress action potentials © 2015 Pearson Education, Inc. 12-7 Synapses • The Effect of a Neurotransmitter • On a postsynaptic membrane • Depends on the receptor • Not on the neurotransmitter • For example, acetylcholine (ACh) • Usually promotes action potentials • But inhibits cardiac neuromuscular junctions © 2015 Pearson Education, Inc. 12-7 Synapses • Cholinergic Synapses • Any synapse that releases ACh at: 1. All neuromuscular junctions with skeletal muscle fibers 2. Many synapses in CNS 3. All neuron-to-neuron synapses in PNS 4. All neuromuscular and neuroglandular junctions of ANS parasympathetic division © 2015 Pearson Education, Inc. 12-7 Synapses • Events at a Cholinergic Synapse 1. Action potential arrives, depolarizes synaptic terminal 2. Calcium ions enter synaptic terminal, trigger exocytosis of ACh 3. ACh binds to receptors, depolarizes postsynaptic membrane 4. ACh removed by AChE • AChE breaks ACh into acetate and choline © 2015 Pearson Education, Inc. Figure 12-16 Events in the Functioning of a Cholinergic Synapse (Part 1 of 4). 1 An arriving action potential depolarizes the axon terminal of a presynaptic neuron. Mitochondrion 1 1 A Ch A Ch A Ch Synaptic vesicle A Ch Axon terminal Synaptic cleft Postsynaptic neuron © 2015 Pearson Education, Inc. ACh receptor (chemically gated Na+ channel) Figure 12-16 Events in the Functioning of a Cholinergic Synapse (Part 2 of 4). 2 Calcium ions (Ca2+) enter the cytosol of the axon terminal. This results in ACh release from the synaptic vesicles by exocytosis. Mitochondrion Voltage-gated Ca2+ channel A Ch 2 2 Ca2+ Ca2+ A Ch 2 A Ch © 2015 Pearson Education, Inc. A Ch 2 A Ch Figure 12-16 Events in the Functioning of a Cholinergic Synapse (Part 3 of 4). 3 ACh diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane. Sodium channels open, producing a graded depolarization. A Ch A Ch Na+ © 2015 Pearson Education, Inc. A Ch 3 Chemically gated Na+ channels 3 Initiation of a graded potential, or action potential, if threshold is reached at the initial segment Na+ Figure 12-16 Events in the Functioning of a Cholinergic Synapse (Part 4 of 4). 4 Depolarization ends as ACh is broken down into acetate and choline by AChE. The axon terminal reabsorbs choline from the synaptic cleft and uses it to resynthesize ACh. Mitochondrion Acetyl-CoA 4 CoA A Ch Acetylcholine Ch A Ch Axon terminal Choline Ch 4 Acetate A Acetylcholinesterase (AChE) Postsynaptic neuron © 2015 Pearson Education, Inc. A Ch Propagation of action potential (if generated) 12-7 Synapses • Synaptic Fatigue • Occurs when neurotransmitter cannot recycle fast enough to meet demands of intense stimuli • Synapse inactive until ACh is replenished © 2015 Pearson Education, Inc. 12-8 Neurotransmitters and Neuromodulators • Other Neurotransmitters • At least 50 neurotransmitters other than ACh, including: • Biogenic amines • Amino acids • Neuropeptides • Dissolved gases © 2015 Pearson Education, Inc. 12-8 Neurotransmitters and Neuromodulators • Important Neurotransmitters • Other than acetylcholine • Norepinephrine (NE) • Dopamine • Serotonin • Gamma aminobutyric acid (GABA) © 2015 Pearson Education, Inc. 12-8 Neurotransmitters and Neuromodulators • Norepinephrine (NE) • Released by adrenergic synapses • Excitatory and depolarizing effect • Widely distributed in brain and portions of ANS • Dopamine • A CNS neurotransmitter • May be excitatory or inhibitory • Involved in Parkinson’s disease and cocaine use © 2015 Pearson Education, Inc. 12-8 Neurotransmitters and Neuromodulators • Serotonin • A CNS neurotransmitter • Affects attention and emotional states • Gamma Aminobutyric Acid (GABA) • Inhibitory effect • Functions in CNS • Not well understood © 2015 Pearson Education, Inc. 12-8 Neurotransmitters and Neuromodulators • Many Drugs • Affect nervous system by stimulating receptors that respond to neurotransmitters • Can have complex effects on perception, motor control, and emotional states © 2015 Pearson Education, Inc. 12-8 Neurotransmitters and Neuromodulators • Neuromodulators • Other chemicals released by synaptic terminals • Similar in function to neurotransmitters • Characteristics of neuromodulators • Effects are long term, slow to appear • Responses involve multiple steps, intermediary compounds • Affect presynaptic membrane, postsynaptic membrane, or both • Released alone or with a neurotransmitter © 2015 Pearson Education, Inc. 12-8 Neurotransmitters and Neuromodulators • Neuropeptides • Neuromodulators that bind to receptors and activate enzymes • Opioids • Neuromodulators in the CNS • Bind to the same receptors as opium or morphine • Relieve pain © 2015 Pearson Education, Inc. 12-8 Neurotransmitters and Neuromodulators • Four Classes of Opioids 1. 2. 3. 4. Endorphins Enkephalins Endomorphins Dynorphins © 2015 Pearson Education, Inc. 12-8 Neurotransmitters and Neuromodulators • How Neurotransmitters and Neuromodulators Work • Direct effects on membrane channels • For example, ACh, glycine, aspartate • Indirect effects via G proteins • For example, E, NE, dopamine, histamine, GABA • Indirect effects via intracellular enzymes • For example, lipid-soluble gases (NO, CO) © 2015 Pearson Education, Inc. 12-8 Neurotransmitters and Neuromodulators • Direct Effects • Ionotropic effects • Open/close gated ion channels © 2015 Pearson Education, Inc. Figure 12-17a Mechanisms of Neurotransmitter Function. Examples: ACh, glutamate, aspartate a Direct effects ACh + Binding site Chemically gated channel © 2015 Pearson Education, Inc. + + + + + 12-8 Neurotransmitters and Neuromodulators • Indirect Effects – G Proteins • Work through second messengers • Enzyme complex that binds GTP • Link between neurotransmitter (first messenger) and second messenger • Activate enzyme adenylyl cyclase • Which produces second messenger cyclic-AMP (cAMP) © 2015 Pearson Education, Inc. Figure 12-17b Mechanisms of Neurotransmitter Function. b Indirect effects by G proteins Examples: E, NE, dopamine, histamine, GABA Neurotransmitter Receptor G protein (inactive) G protein (active) ATP Opens ion channels Adenylate cyclase cAMP Activates enzymes that change cell metabolism and activity © 2015 Pearson Education, Inc. 12-8 Neurotransmitters and Neuromodulators • Indirect Effects – Intracellular Receptors • Lipid-soluble gases (NO, CO) • Bind to enzymes in brain cells © 2015 Pearson Education, Inc. Figure 12-17c Mechanisms of Neurotransmitter Function. c Indirect effects by intracellular enzymes Examples: Nitric oxide, carbon monoxide Nitric oxide Opens ion channels Production of secondary messengers Changes in cell metabolism and activity © 2015 Pearson Education, Inc. Activation of enzymes 12-9 Information Processing • Postsynaptic Potentials • Graded potentials developed in a postsynaptic cell • In response to neurotransmitters • Two Types of Postsynaptic Potentials 1. Excitatory postsynaptic potential (EPSP) • Graded depolarization of postsynaptic membrane 2. Inhibitory postsynaptic potential (IPSP) • Graded hyperpolarization of postsynaptic membrane © 2015 Pearson Education, Inc. 12-9 Information Processing • Inhibition • A neuron that receives many IPSPs: • Is inhibited from producing an action potential • Because the stimulation needed to reach threshold is increased • Summation • To trigger an action potential: • One EPSP is not enough • EPSPs (and IPSPs) combine through summation 1. Temporal summation 2. Spatial summation © 2015 Pearson Education, Inc. 12-9 Information Processing • Temporal Summation • Multiple times • Rapid, repeated stimuli at one synapse • Spatial Summation • Multiple locations • Many stimuli, arrive at multiple synapses © 2015 Pearson Education, Inc. Figure 12-18a Temporal and Spatial Summation. 1 First stimulus arrives FIRST STIMULUS Initial segment 2 Second stimulus arrives and is added to the first stimulus 3 Action potential is generated SECOND STIMULUS Threshold reached a Temporal Summation. Temporal summation occurs on a membrane that receives two depolarizing stimuli from the same source in rapid succession. The effects of the second stimulus are added to those on the first. © 2015 Pearson Education, Inc. ACTION POTENTIAL PROPAGATION Figure 12-18b Temporal and Spatial Summation. 1 Two stimuli arrive simultaneously TWO SIMULTANEOUS STIMULI 2 Action potential is generated ACTION POTENTIAL PROPAGATION Threshold reached b Spatial Summation. Spatial summation occurs when sources of stimulation arrive simultaneously, but at different locations. Local currents spread the depolarizing effects, and areas of overlap experience the combined effects. © 2015 Pearson Education, Inc. 12-9 Information Processing • Facilitation • A neuron becomes facilitated • As EPSPs accumulate • Raising membrane potential closer to threshold • Until a small stimulus can trigger action potential © 2015 Pearson Education, Inc. 12-9 Information Processing • Summation of EPSPs and IPSPs • Neuromodulators and hormones • Can change membrane sensitivity to neurotransmitters • Shifting balance between EPSPs and IPSPs © 2015 Pearson Education, Inc. Figure 12-20a Presynaptic Inhibition and Presynaptic Facilitation. Action potential arrives GABA release Inactivation of calcium channels Ca2+ 2. Less calcium enters 1. Action potential arrives 3. Less neurotransmitter released a Presynaptic inhibition © 2015 Pearson Education, Inc. 4. Reduced effect on postsynaptic membrane Figure 12-20b Presynaptic Inhibition and Presynaptic Facilitation. Action potential arrives Serotonin release Activation of calcium channels Ca2+ Ca2+ 2. More calcium enters 1. Action potential arrives 3. More neurotransmitter released b Presynaptic facilitation © 2015 Pearson Education, Inc. 4. Increased effect on postsynaptic membrane