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Chapter 12 Neural Tissue An Introduction to the Nervous System • Learning Outcomes o 12-1 Describe the anatomical and functional divisions of the nervous system. o 12-2 Sketch and label the structure of a typical neuron, describe the functions of each component, and classify neurons on the basis of their structure and function. o 12-3 Describe the locations and functions of the various types of neuroglia. An Introduction to the Nervous System • Learning Outcomes o 12-4 Explain how the resting membrane potential is created and maintained. o 12-5 Describe the events involved in the generation and propagation of an action potential. o 12-6 Discuss the factors that affect the speed with which action potentials are propagated. An Introduction to the Nervous System • Learning Outcomes o 12-7 Describe the structure of a synapse, and explain the mechanism involved in synaptic activity. o 12-8 Describe the major types of neurotransmitters and neuromodulators, and discuss their effects on postsynaptic membranes. o 12-9 Discuss the interactions that enable information processing to occur in neural tissue. An Introduction to the Nervous System • The Nervous System o Includes all neural tissue in the body o Neural tissue contains two kinds of cells 1. Neurons o Cells that send and receive signals 2. Neuroglia (glial cells) o Cells that support and protect neurons © 2015 Pearson Education, Inc. An Introduction to the Nervous System • Organs of the Nervous System o Brain and spinal cord o Sensory receptors of sense organs (eyes, ears, etc.) o Nerves connect nervous system with other systems 12-1 Divisions of the Nervous System • Anatomical Divisions of the Nervous System o Central nervous system (CNS) o Peripheral nervous system (PNS) 12-1 Divisions of the Nervous System • The Central Nervous System (CNS) o Consists of the spinal cord and brain o Contains neural tissue, connective tissues, and blood vessels o 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 12-1 Divisions of the Nervous System • The Peripheral Nervous System (PNS) o Includes all neural tissue outside the CNS o Functions of the PNS Deliver sensory information to the CNS Carry motor commands to peripheral tissues and systems 12-1 Divisions of the Nervous System • The Peripheral Nervous System (PNS) o Nerves (also called peripheral nerves) Bundles of axons with connective tissues and blood vessels Carry sensory information and motor commands in PNS o Cranial nerves – connect to brain o Spinal nerves – attach to spinal cord 12-1 Divisions of the Nervous System • Functional Divisions of the PNS o Afferent division Carries sensory information From PNS sensory receptors to CNS o Efferent division © 2015 Pearson Education, Inc. Carries motor commands From CNS to PNS muscles and glands 12-1 Divisions of the Nervous System • Functional Divisions of the PNS o Receptors and effectors of afferent division Receptors o Detect changes or respond to stimuli o Neurons and specialized cells o Complex sensory organs (e.g., eyes, ears) Effectors o Respond to efferent signals o Cells and organs 12-1 Divisions of the Nervous System • Functional Divisions of the PNS o The efferent division Somatic nervous system (SNS) o Controls voluntary and involuntary (reflexes) skeletal muscle contractions 12-1 Divisions of the Nervous System • Functional Divisions of the PNS o The efferent division Autonomic nervous system (ANS) o Controls subconscious actions, contractions of smooth muscle and cardiac muscle, and glandular secretions o Sympathetic division has a stimulating effect o Parasympathetic division has a relaxing effect 12-2 Neurons • Neurons o The basic functional units of the nervous system o The structure of neurons The multipolar neuron o Common in the CNS Cell body (soma) Short, branched dendrites Long, single axon 12-2 Neurons • The Cell Body © 2015 Pearson Education, Inc. o o o o Large nucleus and nucleolus Perikaryon (cytoplasm) Mitochondria (produce energy) RER and ribosomes (produce neurotransmitters) 12-2 Neurons • The Cell Body o Cytoskeleton Neurofilaments and neurotubules in place of microfilaments and microtubules Neurofibrils: bundles of neurofilaments that provide support for dendrites and axon o Nissl bodies Dense areas of RER and ribosomes Make neural tissue appear gray (gray matter) 12-2 Neurons • Dendrites o Highly branched o Dendritic spines Many fine processes Receive information from other neurons 80–90 percent of neuron surface area 12-2 Neurons • The axon o Is long o Carries electrical signal (action potential) to target o Axon structure is critical to function o 12-2 Neurons • Structures of the Axon o Axoplasm Cytoplasm of axon Contains neurofibrils, neurotubules, enzymes, organelles o Axolemma Specialized cell membrane Covers the axoplasm 12-2 Neurons • Structures of the Axon © 2015 Pearson Education, Inc. o Axon hillock Thick section of cell body Attaches to initial segment o Initial segment Attaches to axon hillock 12-2 Neurons • Structures of the Axon o Collaterals Branches of a single axon o Telodendria Fine extensions of distal axon o Axon terminals Tips of telodendria 12-2 Neurons • The Structure of Neurons o The synapse Area where a neuron communicates with another cell 12-2 Neurons • The Structure of Neurons o The synapse Presynaptic cell o Neuron that sends message Postsynaptic cell o Cell that receives message The synaptic cleft o The small gap that separates the presynaptic membrane and the postsynaptic membrane 12-2 Neurons • The Synapse o The synaptic terminal Is expanded area of axon of presynaptic neuron Contains synaptic vesicles of neurotransmitters 12-2 Neurons • Neurotransmitters o Are chemical messengers o Are released at presynaptic membrane o Affect receptors of postsynaptic membrane © 2015 Pearson Education, Inc. o Are broken down by enzymes o Are reassembled at axon terminal 12-2 Neurons • Recycling Neurotransmitters o Axoplasmic transport Neurotubules within the axon Transport raw materials Between cell body and axon terminal Powered by mitochondria, kinesin, and dynein 12-2 Neurons • Types of Synapses o Neuromuscular junction Synapse between neuron and muscle o Neuroglandular junction Synapse between neuron and gland 12-2 Neurons • Structural Classification of Neurons o Anaxonic neurons Found in brain and sense organs o Bipolar neurons Found in special sensory organs (sight, smell, hearing) o Unipolar neurons Found in sensory neurons of PNS o Multipolar neurons Common in the CNS Include all skeletal muscle motor neurons 12-2 Neurons • • Anaxonic Neurons o Small o All cell processes look alike Bipolar Neurons o Are small o One dendrite, one axon 12-2 Neurons • Unipolar Neurons o Also called pseudounipolar neurons o Have very long axons © 2015 Pearson Education, Inc. • o Fused dendrites and axon o Cell body to one side Multipolar Neurons o Have very long axons o Multiple dendrites, one axon 12-2 Neurons • Three Functional Classifications of Neurons 1. Sensory neurons Afferent neurons of PNS 2. Motor neurons Efferent neurons of PNS 3. Interneurons Association neurons 12-2 Neurons • • Functions of Sensory Neurons o Monitor internal environment (visceral sensory neurons) o Monitor effects of external environment (somatic sensory neurons) Structures of Sensory Neurons o Unipolar o Cell bodies grouped in sensory ganglia o Processes (afferent fibers) extend from sensory receptors to CNS 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) 12-2 Neurons • Motor Neurons o Carry instructions from CNS to peripheral effectors o Via efferent fibers (axons) 12-2 Neurons © 2015 Pearson Education, Inc. • Motor Neurons o Two major efferent systems 1. Somatic nervous system (SNS) o Includes all somatic motor neurons that innervate skeletal muscles 2. Autonomic (visceral) nervous system (ANS) o Visceral motor neurons innervate all other peripheral effectors Smooth muscle, cardiac muscle, glands, adipose tissue 12-2 Neurons • Motor Neurons o Two groups of efferent axons Signals from CNS motor neurons to visceral effectors pass synapses at autonomic ganglia dividing axons into: o Preganglionic fibers o Postganglionic fibers 12-2 Neurons • Interneurons o Most are located in brain, spinal cord, and autonomic ganglia Between sensory and motor neurons o Are responsible for: Distribution of sensory information Coordination of motor activity o Are involved in higher functions Memory, planning, learning 12-3 Neuroglia • Neuroglia o Half the volume of the nervous system o Many types of neuroglia in CNS and PNS 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. 12-3 Neuroglia • Ependymal Cells o Form epithelium called ependyma o 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 12-3 Neuroglia • Astrocytes o Maintain blood–brain barrier (isolates CNS) o Create three-dimensional framework for CNS o Repair damaged neural tissue o Guide neuron development o Control interstitial environment 12-3 Neuroglia • Oligodendrocytes o Myelination Increases speed of action potentials Myelin insulates myelinated axons Makes nerves appear white 12-3 Neuroglia • Oligodendrocytes o Nodes and internodes Internodes – myelinated segments of axon Nodes (also called nodes of Ranvier) o Gaps between internodes o Where axons may branch 12-3 Neuroglia • Myelination o White matter Regions of CNS with many myelinated nerves o Gray matter Unmyelinated areas of CNS 12-3 Neuroglia • Microglia o Migrate through neural tissue © 2015 Pearson Education, Inc. o Clean up cellular debris, waste products, and pathogens 12-3 Neuroglia • Neuroglia of the Peripheral Nervous System o Ganglia Masses of neuron cell bodies Surrounded by neuroglia Found in the PNS 12-3 Neuroglia • Neuroglia of the Peripheral Nervous System o Satellite cells Also called amphicytes Surround ganglia Regulate environment around neuron 12-3 Neuroglia • Neuroglia of the Peripheral Nervous System o Schwann cells Also called neurilemma cells Form myelin sheath (neurilemma) around peripheral axons One Schwann cell sheaths one segment of axon o Many Schwann cells sheath entire axon 12-3 Neuroglia • Neurons and Neuroglia o Neurons perform: All communication, information processing, and control functions of the nervous system o Neuroglia preserve: Physical and biochemical structure of neural tissue o Neuroglia are essential to: Survival and function of neurons 12-3 Neuroglia • Neural Responses to Injuries o Wallerian degeneration Axon distal to injury degenerates o Schwann cells Form path for new growth Wrap new axon in myelin © 2015 Pearson Education, Inc. 12-3 Neuroglia • Nerve Regeneration in CNS o Limited by chemicals released by astrocytes that: Block growth Produce scar tissue 12-4 Membrane Potential • Ion Movements and Electrical Signals o All plasma (cell) membranes produce electrical signals by ion movements o Membrane potential is particularly important to neurons 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 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 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 12-4 Membrane Potential • The Membrane Potential o Three important concepts 1. The extracellular fluid (ECF) and intracellular fluid (cytosol) differ greatly in ionic composition o Concentration gradient of ions (Na+, K+) 2. Cells have selectively permeable membranes 3. Membrane permeability varies by ion © 2015 Pearson Education, Inc. 12-4 Membrane Potential • Passive Forces Acting across the Plasma Membrane o Chemical gradients Concentration gradients (chemical gradient) of ions (Na+, K+) o Electrical gradients Separate charges of positive and negative ions Result in potential difference 12-4 Membrane Potential • Electrical Currents and Resistance o Electrical current Movement of charges to eliminate potential difference o Resistance The amount of current a membrane restricts 12-4 Membrane Potential • The Electrochemical Gradient o For a particular ion (Na+, K+) is: The sum of chemical and electrical forces o Acting on the ion across a plasma membrane A form of potential energy 12-4 Membrane Potential • Equilibrium Potential o The membrane potential at which there is no net movement of a particular ion across the cell membrane o Examples: K+ Na+ 66 mV 12-4 Membrane Potential • Active Forces across the Membrane o 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) 12-4 Membrane Potential • The Resting Potential o Because the plasma membrane is highly permeable to potassium ions: The resting potential of approximately 70 mV is fairly close to 90 mV, © 2015 Pearson Education, Inc. the equilibrium potential for K+ o 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 + 12-4 Membrane Potential • The Resting Potential o 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 o 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 12-4 Membrane Potential • Changes in the Membrane Potential o Membrane potential rises or falls In response to temporary changes in membrane permeability Resulting from opening or closing specific membrane channels 12-4 Membrane Potential • Sodium and Potassium Channels o Membrane permeability to Na+ and K+ determines membrane potential o They are either passive or active 12-4 Membrane Potential • • Passive Channels (Leak Channels) o Are always open o Permeability changes with conditions Active Channels (Gated Channels) o Open and close in response to stimuli o At resting potential, most gated channels are closed 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 12-4 Membrane Potential • Chemically Gated Channels o Open in presence of specific chemicals (e.g., ACh) at a binding site o Found on neuron cell body and dendrites 12-4 Membrane Potential • Voltage-gated Channels o Respond to changes in membrane potential o Have activation gates (open) and inactivation gates (close) o Characteristic of excitable membrane o Found in neural axons, skeletal muscle sarcolemma, cardiac muscle 12-4 Membrane Potential • Mechanically Gated Channels o Respond to membrane distortion o Found in sensory receptors (touch, pressure, vibration) 12-4 Membrane Potential • • Membrane Potential Exists across Plasma Membrane o Because: Cytosol and extracellular fluid have different chemical/ionic balance The plasma membrane is selectively permeable Membrane Potential o Changes with plasma membrane permeability o In response to chemical or physical stimuli 12-4 Membrane Potential • Graded Potentials o Also called local potentials o Changes in membrane potential That cannot spread far from site of stimulation o Any stimulus that opens a gated channel Produces a graded potential 12-4 Membrane Potential © 2015 Pearson Education, Inc. • Graded Potentials o The resting state Opening sodium channel produces graded potential o Resting membrane exposed to chemical o Sodium channel opens o Sodium ions enter the cell o Membrane potential rises o Depolarization occurs 12-4 Membrane Potential • Graded Potentials o Depolarization A shift in membrane potential toward 0 mV o Movement of Na+ through channel o Produces local current o Depolarizes nearby plasma membrane (graded potential) o Change in potential is proportional to stimulus 12-4 Membrane Potential • Graded Potentials o Whether depolarizing or hyperpolarizing, share four basic characteristics 1. The membrane potential is most changed at the site of stimulation, and the effect decreases with distance 2. The effect spreads passively, due to local currents 12-4 Membrane Potential • Graded Potentials o Whether depolarizing or hyperpolarizing, share four basic characteristics 3. The graded change in membrane potential may involve either depolarization or hyperpolarization The properties and distribution of the membrane channels involved determine the nature of the change o For example, in a resting membrane, the opening of sodium channels causes depolarization, whereas the opening of potassium channels causes hyperpolarization The change in membrane potential reflects whether positive charges enter or leave the cell 12-4 Membrane Potential • Graded Potentials o Whether depolarizing or hyperpolarizing, share four basic characteristics 4. The stronger the stimulus, the greater the change in the membrane © 2015 Pearson Education, Inc. potential and the larger the area affected 12-4 Membrane Potential • Graded Potentials o Repolarization When the stimulus is removed, membrane potential returns to normal o 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 12-4 Membrane Potential • Graded Potentials o Effects of graded potentials At cell dendrites or cell bodies o Trigger specific cell functions o For example, exocytosis of glandular secretions At motor end plate o Release ACh into synaptic cleft 12-5 Action Potential • Action Potentials o Propagated changes in membrane potential o Affect an entire excitable membrane o Link graded potentials at cell body with motor end plate actions 12-5 Action Potential • Initiating Action Potential o 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) 12-5 Action Potential • Initiating Action Potential o All-or-none principle If a stimulus exceeds threshold amount o The action potential is the same o No matter how large the stimulus Action potential is either triggered, or not © 2015 Pearson Education, Inc. 12-5 Action Potential • Four Steps in the Generation of Action Potentials o Step 1: Depolarization to threshold o Step 2: Activation of Na channels o Step 3: Inactivation of Na channels and activation of K channels o Step 4: Return to normal permeability 12-5 Action Potential • • Step 1: Depolarization to Threshold Step 2: Activation of Na Channels o Rapid depolarization o Na+ ions rush into cytoplasm o Inner membrane changes from negative to positive 12-5 Action Potential • Step 3: Inactivation of Na Channels and Activation of K Channels o At 30 mV o Inactivation gates close (Na channel inactivation) o K channels open o Repolarization begins 12-5 Action Potential • Step 4: Return to Normal Permeability o K+ channels begin to close When membrane reaches normal resting potential (70 mV) o K+ channels finish closing Membrane is hyperpolarized to 90 mV Membrane potential returns to resting level Action potential is over 12-5 Action Potential • The Refractory Period o The time period: From beginning of action potential To return to resting state During which membrane will not respond normally to additional stimuli 12-5 Action Potential • Absolute Refractory Period o Sodium channels open or inactivated o No action potential possible © 2015 Pearson Education, Inc. • Relative Refractory Period o Membrane potential almost normal o Very large stimulus can initiate action potential 12-5 Action Potential • Powering the Sodium–Potassium Exchange Pump o To maintain concentration gradients of Na+ and K+ over time Requires energy (1 ATP for each 2 K+/3 Na+ exchange) o Without ATP Neurons stop functioning 12-5 Action Potential • Propagation of Action Potentials o Propagation Moves action potentials generated in axon hillock Along entire length of axon o Two methods of propagating action potentials 1. Continuous propagation (unmyelinated axons) 2. Saltatory propagation (myelinated axons) 12-5 Action Potential • Continuous Propagation o Of action potentials along an unmyelinated axon o Affects one segment of axon at a time o Steps in propagation Step 1: Action potential in segment 1 o Depolarizes membrane to 30 mV o Local current Step 2: Depolarizes second segment to threshold o Second segment develops action potential 12-5 Action Potential • Continuous Propagation o Steps in propagation Step 3: First segment enters refractory period Step 4: Local current depolarizes next segment o Cycle repeats Action potential travels in one direction (1 m/sec) 12-5 Action Potential • Saltatory Propagation o Action potential along myelinated axon © 2015 Pearson Education, Inc. o o o o 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 12-6 Axon Diameter and Speed • Axon Diameter and Propagation Speed o Ion movement is related to cytoplasm concentration o Axon diameter affects action potential speed o The larger the diameter, the lower the resistance 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: o Diameter o Myelination o Speed of action potentials 12-6 Axon Diameter and Speed • Type A Fibers o Myelinated o Large diameter o High speed (140 m/sec) o Carry rapid information to/from CNS o For example, position, balance, touch, and motor impulses 12-6 Axon Diameter and Speed • Type B Fibers o Myelinated o Medium diameter o Medium speed (18 m/sec) o Carry intermediate signals o For example, sensory information, peripheral effectors 12-6 Axon Diameter and Speed • Type C Fibers o Unmyelinated o Small diameter o Slow speed (1 m/sec) © 2015 Pearson Education, Inc. o Carry slower information o For example, involuntary muscle, gland controls 12-6 Axon Diameter and Speed • Information o “Information” travels within the nervous system As propagated electrical signals (action potentials) o The most important information (vision, balance, motor commands) Is carried by large-diameter, myelinated axons 12-7 Synapses • Synaptic Activity o Action potentials (nerve impulses) Are transmitted from presynaptic neuron To postsynaptic neuron (or other postsynaptic cell) Across a synapse 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 12-7 Synapses • Electrical Synapses o Are locked together at gap junctions (connexons) o Allow ions to pass between cells o Produce continuous local current and action potential propagation o Are found in areas of brain, eye, ciliary ganglia 12-7 Synapses • Chemical Synapses o Are found in most synapses between neurons and all synapses between neurons and other cells o Cells not in direct contact o 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 12-7 Synapses • The Effect of a Neurotransmitter o On a postsynaptic membrane Depends on the receptor Not on the neurotransmitter o For example, acetylcholine (ACh) Usually promotes action potentials But inhibits cardiac neuromuscular junctions 12-7 Synapses • Cholinergic Synapses o 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 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 12-7 Synapses • Synaptic Delay o A synaptic delay of 0.2–0.5 msec occurs between: Arrival of action potential at synaptic terminal And effect on postsynaptic membrane o Fewer synapses means faster response o Reflexes may involve only one synapse © 2015 Pearson Education, Inc. 12-7 Synapses • Synaptic Fatigue o Occurs when neurotransmitter cannot recycle fast enough to meet demands of intense stimuli o Synapse inactive until ACh is replenished 12-8 Neurotransmitters and Neuromodulators • Other Neurotransmitters o At least 50 neurotransmitters other than ACh, including: Biogenic amines Amino acids Neuropeptides Dissolved gases 12-8 Neurotransmitters and Neuromodulators • Important Neurotransmitters o Other than acetylcholine Norepinephrine (NE) Dopamine Serotonin Gamma aminobutyric acid (GABA) 12-8 Neurotransmitters and Neuromodulators • • Norepinephrine (NE) o Released by adrenergic synapses o Excitatory and depolarizing effect o Widely distributed in brain and portions of ANS Dopamine o A CNS neurotransmitter o May be excitatory or inhibitory o Involved in Parkinson’s disease and cocaine use 12-8 Neurotransmitters and Neuromodulators • • Serotonin o A CNS neurotransmitter o Affects attention and emotional states Gamma Aminobutyric Acid (GABA) o Inhibitory effect o Functions in CNS Not well understood 12-8 Neurotransmitters and Neuromodulators © 2015 Pearson Education, Inc. • Chemical Synapse o The synaptic terminal releases a neurotransmitter that binds to the postsynaptic plasma membrane o Produces temporary, localized change in permeability or function of postsynaptic cell o Changes affect cell, depending on nature and number of stimulated receptors 12-8 Neurotransmitters and Neuromodulators • Many Drugs o Affect nervous system by stimulating receptors that respond to neurotransmitters o Can have complex effects on perception, motor control, and emotional states 12-8 Neurotransmitters and Neuromodulators • Neuromodulators o Other chemicals released by synaptic terminals o Similar in function to neurotransmitters o 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 12-8 Neurotransmitters and Neuromodulators • • Neuropeptides o Neuromodulators that bind to receptors and activate enzymes Opioids o Neuromodulators in the CNS o Bind to the same receptors as opium or morphine o Relieve pain 12-8 Neurotransmitters and Neuromodulators • Four Classes of Opioids 1. Endorphins 2. Enkephalins 3. Endomorphins 4. Dynorphins 12-8 Neurotransmitters and Neuromodulators • How Neurotransmitters and Neuromodulators Work © 2015 Pearson Education, Inc. o Direct effects on membrane channels For example, ACh, glycine, aspartate o Indirect effects via G proteins For example, E, NE, dopamine, histamine, GABA o Indirect effects via intracellular enzymes For example, lipid-soluble gases (NO, CO) 12-8 Neurotransmitters and Neuromodulators • Direct Effects o Ionotropic effects o Open/close gated ion channels 12-8 Neurotransmitters and Neuromodulators • Indirect Effects – G Proteins o Work through second messengers o Enzyme complex that binds GTP o Link between neurotransmitter (first messenger) and second messenger o Activate enzyme adenylyl cyclase Which produces second messenger cyclic-AMP (cAMP) 12-8 Neurotransmitters and Neuromodulators • Indirect Effects – Intracellular Receptors o Lipid-soluble gases (NO, CO) o Bind to enzymes in brain cells 12-9 Information Processing • Information Processing o At the simplest level (individual neurons) Many dendrites receive neurotransmitter messages simultaneously Some excitatory, some inhibitory Net effect on axon hillock determines if action potential is produced 12-9 Information Processing • • Postsynaptic Potentials o 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 o A neuron that receives many IPSPs: Is inhibited from producing an action potential Because the stimulation needed to reach threshold is increased Summation o To trigger an action potential: One EPSP is not enough EPSPs (and IPSPs) combine through summation 1. Temporal summation 2. Spatial summation 12-9 Information Processing • • Temporal Summation o Multiple times o Rapid, repeated stimuli at one synapse Spatial Summation o Multiple locations o Many stimuli, arrive at multiple synapses 12-9 Information Processing • Facilitation o A neuron becomes facilitated As EPSPs accumulate Raising membrane potential closer to threshold Until a small stimulus can trigger action potential 12-9 Information Processing • Summation of EPSPs and IPSPs o Neuromodulators and hormones Can change membrane sensitivity to neurotransmitters Shifting balance between EPSPs and IPSPs 12-9 Information Processing • Axoaxonic Synapses o Synapses between the axons of two neurons o Presynaptic inhibition Action of an axoaxonic synapse at a synaptic terminal that decreases the neurotransmitter released by presynaptic membrane o Presynaptic facilitation Action of an axoaxonic synapse at a synaptic terminal that increases the neurotransmitter released by presynaptic membrane © 2015 Pearson Education, Inc. 12-9 Information Processing • • Frequency of Action Potentials o Information received by a postsynaptic cell may be simply the frequency of action potentials received Rate of Generation of Action Potentials o Frequency of action potentials depends on degree of depolarization above threshold o Holding membrane above threshold level Has same effect as a second, larger stimulus Reduces relative refractory period 12-9 Information Processing • In the Nervous System o A change in membrane potential that determines whether or not action potentials are generated is the simplest form of information processing 12-9 Information Processing • Summary o Information is relayed in the form of action potentials In general, the degree of sensory stimulation or the strength of the motor response is proportional to the frequency of action potentials o The neurotransmitters released at a synapse may have either excitatory or inhibitory effects The effect on the axon’s initial segment reflects a summation of the stimuli that arrive at any moment The frequency of generation of action potentials is an indication of the degree of sustained depolarization at the axon hillock 12-9 Information Processing • Summary o Neuromodulators Can alter either the rate of neurotransmitter release or the response of a postsynaptic neuron to specific neurotransmitters o Neurons May be facilitated or inhibited by extracellular chemicals other than neurotransmitters or neuromodulators 12-9 Information Processing • Summary o The response of a postsynaptic neuron to the activation of a presynaptic neuron can be altered by: 1. The presence of neuromodulators or other chemicals that cause facilitation or inhibition at the synapse © 2015 Pearson Education, Inc. 2. Activity under way at other synapses affecting the postsynaptic cell 3. Modification of the rate of neurotransmitter release through presynaptic facilitation or presynaptic inhibition © 2015 Pearson Education, Inc.