Nervous System MUIC Part 1 PDF 2024

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The document and media are for educational purposes only within the program of the Faculty of Medicine Siriraj Hospital. Any other use is prohibited, and violators may be prosecuted according to the Copyright Act. September 21, 2024 Nervous System ICBI 305 Human Anatomy I Adisorn Ratanayotha [email protected]  Topics After the session, students should be able to describe Organization of the Nervous System Nervous tissue: Structure and Function Central nervous system (CNS) Peripheral nervous system (PNS) Autonomic nervous system (ANS)  Topics After the session, students should be able to describe Organization of the Nervous System ① Nervous tissue: Structure and Function ② Central nervous system (CNS) Peripheral nervous system (PNS) ③ Autonomic nervous system (ANS)  Topics After the session, students should be able to describe Organization of the Nervous System ① Nervous tissue: Structure and Function Central nervous system (CNS) Peripheral nervous system (PNS) Autonomic nervous system (ANS)  7 The Nervous System Nervous System WHAT What? As the primary control system of the body, the nervous How? system provides for higher mental HOW function and emotional expression, maintains homeostasis, and Communication by regulates the activities of the nervous system involves muscles and glands. a combination of electrcial and chemical signals. WHY Why? All body systems are under the control or regulation INSTRUCTORS of the nervous system. If the nervous New Building systems stops functioning, the body Vocabulary Coaching can stay alive only with the Activities for this assistance of life-supporting chapter are assignable machines. in Elaine Marieb and Suzanne Keller. Essen als of Human Anatomy & Physiology, 12th Ed, 2018: p.225 ti  Basic Functions of the Nervous System 226 Essentials of Human Anatomy and Physiology 1. Sensory input: Central Perception Nervous System of stimuli Sensory input (brain and spinal cord) from extl and intl environments. Integration Sensory receptor 2. Integration: Analysis and processing of sensory info to generate Peripheral Nervous System (cranial and spinal nerves) commands. Motor output 3. Motor output: Commanding and Brain and spinal cord regulating organ functions. Effector Figure 7.1 The nervous system’s functions. Sensory Motor Nervous system, together with endocrine system, (afferent)maintain body homeostasis (efferent) (1) It uses its millions of sensory receptors to monitor changes occurring both inside and out- side the body. These changes are called stimuli, Elaine Marieb and Suzanne Keller. Essen als of Human Anatomy & Physiology, 12th Ed, 2018: p226 ti  Organization of the NS n Anatomy and Physiology nput Central Nervous System (brain and spinal cord) 1. Central nervous system (CNS) - Brain Integration - Spinal cord Peripheral Nervous System (cranial and spinal nerves) utput Brain and spinal cord 2. Peripheral nervous system (PNS) ystem’s functions. Sensory (afferent) Motor (efferent) - Cranial nerves - Spinal nerves of sensory receptors to ng both inside and out- anges are called stimuli, - mation is called sensory nd interprets the sensory should be done at each Sense Somatic Autonomic Ganglia (collection of neurons in PNS) organs (voluntary) (involuntary) d integration. (3) It then ect, by activating muscles Skeletal Cardiac and motor output. muscles smooth muscle, glands ervous system functions ack loop (Chapter 1, p. 19). Structural Divisions oop, a receptor receives nds to the brain (control egration); the brain then and determines the appro- Parasympathetic Sympathetic s to a motor response. ➔ Figure 7.2 Organization of the nervous system. trate how these functions As the flowchart shows, the central nervous system Elaine Marieb and Suzanne Keller. Essen als of Human Anatomy & Physiology, 12th Ed, 2018: p.226 ou are driving and see a receives input via sensory fibers and issues commands ti  Organization of the NS n Anatomy and Physiology nput Central Nervous System (brain and spinal cord) 2. Peripheral nervous system (PNS) - Sensory division Integration - Motor division Peripheral Nervous System (cranial and spinal nerves) utput Brain and spinal cord - Somatic (voluntary) ystem’s functions. Sensory (afferent) Motor (efferent) - Autonomic (involuntary) - Sympathetic NS of sensory receptors to ng both inside and out- anges are called stimuli, - mation is called sensory nd interprets the sensory should be done at each Sense Somatic Autonomic Parasympathetic NS organs (voluntary) (involuntary) d integration. (3) It then ect, by activating muscles Skeletal Cardiac and motor output. muscles smooth muscle, glands ervous system functions ack loop (Chapter 1, p. 19). Functional Divisions oop, a receptor receives nds to the brain (control egration); the brain then and determines the appro- Parasympathetic Sympathetic s to a motor response. ➔ Figure 7.2 Organization of the nervous system. trate how these functions As the flowchart shows, the central nervous system Elaine Marieb and Suzanne Keller. Essen als of Human Anatomy & Physiology, 12th Ed, 2018: p.226 ou are driving and see a receives input via sensory fibers and issues commands ti  PERIPHERAL NERVOUS SYSTEM CENTRAL NERVOUS SYSTEM PERIPHERAL NERVOUS SYSTEM (Sensory division) (Brain and spinal cord) (Motor division) SENSORY OR MOTOR OR AFFERENT EFFERENT NEURONE NEURONE Sensory receptors Effector organs Senses: Internal environment Somatic Autonomic sight (autonomic) e.g.: (voluntary): (involuntary): hearing chemoreceptors skeletal muscle cardiac muscle smell baroreceptors smooth muscle taste osmoreceptors glands touch Sympathetic Parasympathetic division division Anne Waugh and Allison Grant. Ross and Wilson Anatomy and Physiology in Health and Illness, 12th Ed, 2014: p.145  Topics After the session, students should be able to describe Organization of the Nervous System ① Nervous tissue: Structure and Function Central nervous system (CNS) Peripheral nervous system (PNS) Autonomic nervous system (ANS)  Nervous Tissue 8 Overview of the Nervous System Principal cells the NS Ventricle Ependyma Microglial cell - Neuron Tanycyte - Neuroglia (supporting cells) - Astrocyte CNS Neuron - Oligodendrocyte - Ependymal cell Oligodendrocyte - Microglia Axon - Satellite cell PNS - Schwann cell Astrocyte Astrocyte foot processes Perivascular Pia mater pericyte Capillary David Felten, et al. Ne er’s Atlas of Neuroscience, 3rd Ed, 2016: p.8 tt  CNS. Figure 49.9 provides an overview of the major types For suggested answers, see Appendix A. Neuroglia. Figure 49.9 Glia in the vertebrate nervous system. CNS PNS VENTRICLE Neuron Cilia Lisa Urry, et al. Campbell Biology, 12th Ed, 2020: p.1090 Ependymal cells Ependymal cells line the ventricles - of the Line thebrain brainand have cilia that ventriclescircu- promote - lation Use cilia of to the cerebrospinal fluid circulate that CSF fills these Capillary compartments. Astrocytes (from the Greek astron, star) have Oligodendrocytes Microglia are Schwann cells Astrocytes numerous functions in the CNS. They facilitate Oligodendrocytes myelinate axons in Microglia immune cells in the Schwann cells myelinate axons - Optimize the information neuronal transfer, environment. regulate extracellular ion - Form the CNS.myelin Myelination - Actthat CNS as phagocytes. protect - Form in myelin the PNS. concentrations, promote blood flow to greatly increases the against pathogens. - Supporthelp neurons, ef cient neuron function. form the blood-brain barrier, and sheaths wrapping conduction speed of - Engulf pathogens, sheaths wrapping - Contribute act to the as stem cells blood-brain to replenish barrier. certain neurons. around action nerve bers potentials. foreign particles, around nerve bers in the CNS. and cellular debris. in the PNS. fi fi fi  Astrocyte Nerve Neuroglia fibers (a) Astrocytes are the most abundant (d) Oligodendrocytes have processes that form and versatile neuroglia. myelin sheaths around CNS nerve fibers. CNS PNS Myelin sheath Satellite Process of cells Cell body of neuron oligodendrocyte Neuron Schwann cells Microglial Nerve (forming myelin sheath) te cell fibers Nerve fiber (d) Oligodendrocytes have processes that form myelin (b) sheaths Microglial around cells CNS nervethat are phagocytes fibers. (e) Satellite cells and Schwann cells (which form Oligodendrocytes defend CNS cells. Schwann myelin) surround cells neurons in the PNS. form Myelin sheath in CNS form Myelin sheath in PNS Satellite cells Fluid-filled cavity Cell body of neuron Ependymal Schwann cells cells al (forming myelin sheath) Brain or Elaine Marieb and Suzanne Keller. Essen als of Human Anatomy & Physiology, 12th Ed, 2018: p.228 ti  Myelin Sheath 18 Overview of the Nervous System CENTRAL NERVOUS SYSTEM PERIPHERAL NERVOUS SYSTEM Protection, insulation Sensory neuron Satellite - CNS: Oligodendrocyte cell body cells Pia mater Oligo- dendrocyte - PNS: Schwann cell Chapter 4 / The Cells of the Nervous System 89 Schwann cells associated with myelin sheaths of myelinated axons A Myelination in the central nervous system B Myelination in the peripheral nervous system Capillary Astrocyte Boutons of association neurons synapsing Postganglionic neuron of Sc Cyt with preganglionic autonomic neuron of sympathetic or parasympathetic brainstem or spinal cord ganglion Oligodendrocyte Ax Ml IM Oligo- OM Schwann cells associated with Satellite cells SM dendrocyte myelin sheaths of myelinated axons 1 µm C Development of myelin sheath in the peripheral nervous system Boutons of association neurons 1 2 3 4 synapsing with somatic Node of Ranvier motor neurons of brain or spinal cord Axon Inner mesaxon Axons terminating on motor end plates of Outer striated (voluntary) muscle mesaxon Figure 4–13 Myelin insulates the axons of both central and become compact. (Reproduced, with permission, from Dyck peripheral neurons. et al. 1984.) 1.15 MYELINATION OF CNS AND PNS AXONS A. Axons in the central nervous system are wrapped in several C. A peripheral nerve fiber is myelinated by a Schwann cell in CLINICAL POINT David Felten, etmyelination Central al. Ne er’s Atlas of axons of Neuroscience, is provided 3rd by oligodendroglia. Ed, 2016: p.18 The integrity of the myelin sheath is essential for proper neuronal func- Eric Kandel, et al. Principles of the Neural Science, 5th Ed, 2013: p.89 layers of myelin produced by oligodendrocytes. Each oligo- dendrocyte can myelinate many axons. (Reproduced, with several stages. In stage 1 the Schwann cell surrounds the axon. In stage 2 the outer aspects of the plasma membrane have Each oligodendroglial cell can myelinate a single segment of tion in both the CNS and the PNS. Disruption of the myelin sheath tt  some small fibres in the central nervous system are unmyelinated. In this type a number of axons are Myelin Sheath embedded in one Schwann cell (Fig. 7.3B). The adjacent Schwann cells are in close association and there is no Insulates nerve bers, enhancing the speed of neural signal transmission Schwann cell Node of Ranvier cytoplasm Schwann cell Myelin sheath nucleus Neurilemma Nucleus of Axon Schwann cell Myelin Neurilemma sheath (sheath of Schwann cell) A B C Axolemma Axon Figure 7.3 Arrangement of myelin. A.Unmyelinated ber Myelinated neurone. B. Unmyelinated Myelinated neurone. C. Length of myelinated axon. ber Anne Waugh and Allison Grant. Ross and Wilson Anatomy and Physiology in Health and Illness, 12th Ed, 2014: p.146 fi fi fi  Neuron Chapter 11 Fundamentals of the Nervous System and Nervous Tissue 427 Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) 1. Cell body - Nucleus, euchromatin, nucleolus - Nissl body (rER), neuro lament, mitoch, etc 2. Process; nerve ber Neuron cell body - Dendrite: Carries impulses toward the cell body Dendritic spine - Nucleus Axon: Transmits impulses away from the cell body - Axon hillock, Axon initial segment, Axon terminal - Myelin (b) sheath, node of Ranvier Initial Axon Nucleolus segment (impulse-generating Neuronal properties of axon and conducting Chromatophilic region) Myelin sheath gap substance (rough endoplasmic Axon terminals reticulum) Axon hillock Schwann cell (secretory region) - Irritability Impulse - Conductivity direction (a) Terminal branches 11 Figure 11.5 Structure of a motor neuron. (a) Illustration. (b) Digital reconstruction of Elaine Marieb and Katja Hoehm. a neuron Human Anatomy & Physiology, 11th Ed, 2019: p.427 (10003). fi fi  Neuron 2.1 The Cells of the Nervous System | 25 aNeuron, the vestibular area of the brain b Hippocampal neuron cNeuron, mouse DRG of the spinal cord Cultured d neuron, DRG of an embryonic rat e Pyramidal neuron, brain f Multipolar neuron cell body, human cerebral cortex Michael Gazzaniga, et al. Cogni FIGURE ve Neuroscience: 2.2 Mammalian The Biology neurons of the Mind,anatomical show enormous 5th Ed, 2019: p.25 variety. ti  Neuron Classi ed by the number of nerve bers Unipolar A Unipolar cell Bipolar B Bipolar cell Pseudo-unipolar C Pseudo-unipolar cell Axon terminals Dendrites Dendrites Peripheral axon to skin and muscle Axon Cell body Single bifurcated Cell body process Dendrites Central Axon axon Cell body Axon terminals Invertebrate neuron Bipolar cell of retina Ganglion cell of dorsal root D Three types of multipolar cells Eric Kandel, et al. Principles of the Neural Science, 5th Ed, 2013: p.25 fi fi  Cell body process Neuron Dendrites Central Axon axon Cell body Axon terminals Classi ed by the number of nerve bers Invertebrate neuron Bipolar cell of retina Ganglion cell of dorsal root D Three types of multipolar cells Multipolar Dendrites Apical dendrite Cell body Cell body Basal dendrite Dendrites Cell body Axon Axon Axon Motor neuron of spinal cord Pyramidal cell of hippocampus Purkinje cell of cerebellum Figure 2–3 Neurons are classified as unipolar, bipolar, or axons; one extends to peripheral skin or muscle, the other multipolar Eric Kandel, et al. Principles according of the to the Neural Science, 5thnumber Ed, 2013:of processes that p.25 to the central spinal cord. (Adapted, with permission, from fi fi  Neuron Studying the Nervous System Classi ed by neuronal function Sensory (afferent) FIGURE 1.7 The knee-jerk axon response, a simple reflex circu Muscle sensory Formally known as the myotatic receptor 3A Cross section of spinal cord reflex, this response illustrates se eral points about the functional Extensor organization of neural circuits. muscle 2B Stimulation of peripheral sensor (a muscle stretch receptor in thi 2A case) initiates receptor potentia 1 that trigger action potentials tha Flexor travel centrally along the afferen muscle 3B axons of the sensory neurons. Th information stimulates spinal mo tor neurons by means of synapt Motor (efferent) 2C Interneuron 4 axons contacts. The action potentials triggered by the synaptic poten - Sensory neuron in motor neurons travel periphe ally in efferent axons, giving rise - Motor neuronmuscle contraction and a beha ioral response. One of the purpo - Interneuron es of this particular reflex is to he maintain an upright posture in Hammer tap 2A Sensory neuron synapses Motor neuron conducts the face of unexpected change 1 6th Ed, 2018: p.11 Dale Purves et al. Neuroscience, 3A stretches tendon, with and excites motor action potential to (such as tripping). fi  1. Microtubules do not extend into the terminal. 2. The terminal contains numerous small bubbles of membrane, called Neural transmission synaptic vesicles, that measure about 50 nm in diameter. Axon hillock 3. The inside surface of the membrane that faces the synapse has a par- ticularly dense covering of proteins. 4. The axon terminal cytoplasm has numerous mitochondria, indicating a 42 PART ONE FOUNDATIONS Action potential & Synapse high energy demand. Axon collaterals Irritability ▲ FIGURE 2.15 appears as a swollenConductivity disk (Figure 2.16). The terminal is a site where the The axon and axon collaterals.axon The comes in contact with other neurons (or other cells) and passes in- - Response to stimuli. Dendrites axon functions like and formation a telegraph wire to on to them.- This Transmits point of nerve contact is called the synapse, a word dendritic derived sites from the Greek, meaning “to fasten together.” Sometimes axons spines send electrical impulses to distant - Converts stimuli into in the nervous system. The arrows signals indi-many short branches have to other at their ends, cells. and each branch forms a syn- cate the direction of information flow. nerve signals. apse on dendrites or cell bodies in the same region. These branches are collectively called the terminal arbor. Sometimes axons form synapses at swollen regions along their length and then continue on to terminate elsewhere (Figure 2.17). Such swellings are called boutons en passant Presynaptic axon terminal (“buttons in passing”). In either case, when a neuron makes synaptic contact with another cell, it is said to innervate that cell, or to provide innervation. Mitochondria Synapse The cytoplasm of the axon terminal differs from that of the axon in several ways: 1. Microtubules do not extend into the terminal. 2. The terminal contains numerous small bubbles of membrane, called Synaptic synaptic vesicles, that measure about 50 nm in diameter. vesicles Axon hillock 3. The inside surface of the membrane that faces the synapse has a par- ticularly dense covering of proteins. 4. The axon terminal cytoplasm has numerous Postsynaptic dmitochondria, rit e indicating a end high energy demand. Synaptic cleft Axon Receptors collaterals ▲ FIGURE 2.16 ▲ FIGURE 2.15 Axon The axon and axon and axon collaterals collaterals. The Axon terminal and synapse The axon terminal and the synapse. Axon terminals form synapses with the dendrites or somata of other neurons. When a nerve impulse arrives in the pre- axon functions like a telegraph wire to synaptic axon terminal, neurotransmitter molecules are released from synaptic send electrical impulses to distant sites vesicles into the synaptic cleft. Neurotransmitter then binds to specific receptorthe Brain, 4th Ed, 2016: p.42 in the nervous system. The arrows indi- Mark Bear, et al. Neuroscience: Exploring proteins, causing the generation of electrical or chemical signals in the postsyn- Two factors generate the resting membrane potential: differ- than the extracellular fluid. Negatively c ences in the ionic composition of the intracellular and extracel- teins (not shown) help to balance the po lular fluids, and differences Resting Membrane in the permeability of the Potential plasma + cellular cations (primarily K ). In the membrane to those ions. + positive charges of Na and other cations The resting state of a neuron − chloride ions (Cl ). Although there are m Voltmeter cose, urea, and other ions) in both fluids the most important role in generating the RMP ~ (-70) mV Membrane potential Differences in Plasma The electrical voltage difference Membrane Pe Plasma Ground electrode Next, let’s consider the differential perm membrane between the inside and outside of the cell. outside cell 1 1 1 1 1 brane to various ions (Focus Figure 11. Microelectrode membrane is impermeable to the larg inside cell proteins, very slightly permeable to sod 1 1 1 1 1 times more Resting statepermeable to potassium tha Axon permeable Outside to chloride is positive, insideions. These restin is negative. the properties of the leakage ion chann Potassium ions diffuse out of the cell alo gradient much more easily than sodium + Neuron along theirs. K flowing out of the cell ca + Na+-K+ ATPase more negative inside. Na trickling into K leakage channel + just slightly more positive than it would Therefore, at resting membrane potentia Figure 11.8 Measuring membrane potential in neurons. The and Katjaof Elaine Marieb the Human Hoehm. cell isAnatomy due to& Physiology, a much10th greater ability Ed, 2016: p.419  The big picture Action Potential (Nerve Impulses) What does this graph show? During the course of an action potential (below), voltage changes over time at a given point within the axon. Voltage-gated Na+ Channel (NaV) Voltage-gated K+ Channel (KV) 2 Depolarization is 3 Repolarization is caused by Na+ flowing caused by K+ flowing into the cell. out of the cell. +30 4 Hyperpolarization is caused by K+ continuing Membrane potential (mV) 3 to leave the cell. 0 Voltage-gated K+ Channel (KV) 2 ถูกกระ น –55 Threshold –70 1 1 1 Resting state. No ions move 4 through voltage-gated channels. 0 1 2 3 4 Na+-K+ ATPase Time (ms) K+ leakage channel Elaine Marieb and Katja Hoehm. Human Anatomy & Physiology, 10th Ed, 2016: p.424 ตุ้  are so slight that they dissipate long before threshold × time). Strongisstimuli reached. away depolarize thefrom its point membrane to of threshold isolated axon origin. (If an channels is stimulated and triggers an action potential there (Figure 11.11). An AP is an all-or-none phenomenon: It either quickly. Weakerhappens must beby com- stimuli an electrode, applied for longertheperiods nerve impulse to will move the Because away areafrom thethe AP originated has just generated where pointflow. of stimulus Very weakinstimuli both dodirectionsan along AP, thethe axon.)channels sodium In the in that area are inactivated and no Action Potential (Nerve Impulses) pletely or doesn’t happen at all. We can compare provide the generation the crucial amount of current The big picture not trigger an AP because the local current flows they produce new AP is generated there. For this reason, the AP propagates What does this graph show? During 11 the course are so slight of an action that they dissipate potential long before threshold is reached. away from its point of origin. (If an isolated axon is stimulated by an electrode, the nerve impulse will move away from the Membrane potential (mV) (below), voltage changes over time at a given Depolarization → Outside becomes point within the axon. at 2 ms negative, inside point becomes positive An AP is an all-or-none phenomenon: VoltageIt either happens com- 130 pletely or doesn’t happen at all. We can compare the generation of stimulus in both directions along the axon.) In the Voltage 11 2 Depolarization is 3 Repolarization is Voltage Membrane potential (mV) at 0 ms Voltage caused by Na+ flowing caused by + K flowing at 2 ms at 4 ms 270 130 into the cell. out of the cell. Voltage Voltage Recording at 0 ms at 4 ms electrode 270 22 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1111112221111111 1111111111122211 112 2 2 2 2 2 2 2 2 2 2 2 2 2 +230 222221112222222 2222222222211122 4 Hyperpolarization is caused by K+ continuing Membrane potential (mV) Recording electrode 3 to leave the cell. 112 2 2 2 2 2 2 2 2 2 2 2 2 2 2222221112222222 2222222222211122 221 1 111111111111 1 101 1 1 1 2 2 2 1 1 1 1 1 1 1 1111112221111111 1111111111122211 221 1 1 1 1 1 1 1 1 1 1 1 1 1 1111111111122211 112 2 2 2 2 2 2 2 2 2 2 2 2 2 2222221112222222 2222222222211122 AP propagation (a) Time = 0 ms. Action potential has (b) Time = 2 ms. Action potential 1 1 2 2 2peak 2 2 reaches 2 2 2 2the 2 2recording 222 2 (c) Time = 4 ms. Action potential 2 2 2 2 2peak 211 12 2 2 2the 22 2 2222222222211122 not yet reached the recording has passed recording electrode. 2 2 1 1 1electrode. 11111111111 1 1 1 1 1electrode. 12221 Membrane 1 1 1 1 1at1the 1111111111122211 recording electrode is still Resting potential hyperpolarized. Peak of action potential (a) Time = 0 ms. Action potential has –55 Threshold (b) Time = 2 ms. Action potential (c) Time = 4 ms. Action potential not yet reached the recording peak reaches the recording peak has passed the recording Hyperpolarization electrode. electrode. electrode. Membrane at the –70 1 1 recording electrode is still Figure 11.11 1 Resting state.ofNo Propagation an ions move action Resting potential potential (AP). Recordings at three successive 4 hyperpolarized. times as anthrough voltage-gated channels. Nerve impulses move in one direction along the axon, AP propagates along an axon (fromPeak left to right).potential of action The arrows show the direction of local current flow generated by the movement of positive ions. This Hyperpolarization 0 current brings 1 the 3 24 resting membrane at the leading edge of the AP to threshold, propagating the AP forward. Time (ms) from the from the origin to the axon terminal. Figure 11.11 Propagation of an action potential (AP). Recordings at three successive times as an AP propagates along an axon (from left to right). The arrows show the direction of local current flow generated by the movement of positive ions. This current brings the resting membrane at the leading edge of the AP to threshold, Elainepropagating Marieb andthe AP forward. Katja Hoehm. Human Anatomy & Physiology, 10th Ed, 2016: p.426  Myelin Sheath & Nerve Conduction A. A. Myelinated Myelinated fibers nerve bers Site where action potential is reinitiated Myelinated nerve bers " ! " ! " ! Impulse Transmit nerve signals faster ! " ! (Saltatory conduction) " ! " Node of Ranvier Axolemma Myelin sheath B. B. Unmyelinated Unmyelinated fibers nerve bers Axoplasm " " " " " " " " ! " " " " " " " " " " " " " ! ! ! ! ! ! ! ! " ! ! ! ! ! ! ! ! ! ! ! ! ! Unmyelinated nerve bers ! ! ! ! ! ! ! ! " #! ! ! ! ! ! ! ! ! ! ! ! ! Transmit nerve signals more slowly " " " " " " " " ! " " " " " " " " " " " " " Demyelination causes nerve bers to transmit signals much more slowly David Felten, et al. Ne er’s Atlas of Neuroscience, 3rd Ed, 2016: p.26 tt fi fi fi fi fi  Synapse 86 86 Chapter5 5 Chapter Nerve impulses reach the axon terminal → Synapse (A)(A) Electrical synapse Electrical Electrical synapse synapse (C)(C) Chemical synapse Chemical Chemical synapse synapse Microtubules Microtubules Electrical synapse Cytoplasm Cytoplasm - Specialized gap junction Presynaptic Presynaptic Presynaptic Presynaptic neuron neuron neuron neuron Synaptic Synaptic vesicle vesicle Mitochondrion Mitochondrion - Connexon Chemical synapse ** Gap Gap junction junction Postsynaptic Postsynaptic neuron neuron Postsynaptic Postsynaptic neuron neuron - Neurotransmitters (B)(B) IonIon current current flows flows through through (D)(D) - Presynaptic neuron connexon connexon channels. channels. Neurotransmitter Neurotransmitter released released Synaptic Synaptic Presynaptic Presynaptic membrane membrane vesicle vesicle fusing fusing Presynaptic Presynaptic membrane membrane - Presynaptic terminals - Presynaptic vesicles Synaptic Synaptic Extracellular Extracellular cleft cleft - Synaptic cleft Postsynaptic Postsynaptic - Postsynaptic neuron neurotransmitter neurotransmitter Postsynaptic Postsynaptic - Neurotransmitter receptor Postsynaptic Postsynaptic Ions Ions flow flow through through membrane membrane membrane membrane Connexons Connexons receptor receptor postsynaptic postsynaptic channels. channels. FIGURE 5.1 FIGURE 5.1 Electrical Electricaland chemical and chemicalsynapses differ synapses differ synapses, synapses, there there is no is nointercellular continuity, intercellular and continuity, andthus nono thus fundamentally fundamentally Dale in in Purves et al. their transmission their Neuroscience, 6thmechanisms. transmission mechanisms. Ed, (A)(A) 2018: p.86 AtAt direct flow direct flowof of current current from frompre- to to pre- postsynaptic postsynapticcell. (D) cell. Syn- (D) Syn- electrical synapses, electrical gap synapses, junctions gap occur junctions between occur pre- between and pre- and aptic apticcurrent current flows flowsacross across the postsynaptic the postsynaptic membrane membrane only only  the first time, the presence of synaptic vesicles in presyn- necessary to show that each fused vesicle produce aptic terminals. Putting these two Chemical discoveries Synapse together, Katz and others proposed that synaptic vesicles loaded quantal event in the postsynaptic cell. This challe met in the late 1970s, when John Heuser, Tom Re with transmitter are the source of Neuro-muscular junction the quanta. Subsequent colleagues correlated measurements of vesicle fus the quantal content of EP (A) Schwann cell (B) neuromuscular junction RestingResting