Neural Tissue. PowerPoint Lecture Presentations prepared by Jason LaPres. Lone Star College North Harris Pearson Education, Inc.

Transcription

1 12 Neural Tissue PowerPoint Lecture Presentations prepared by Jason LaPres Lone Star College North Harris

2 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

3 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

4 12-1 Divisions of the Nervous System Anatomical Divisions of the Nervous System Central nervous system (CNS) Peripheral nervous system (PNS)

5 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

6 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

7 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

8 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

9 12-1 Divisions of the Nervous System Functional Divisions of the PNS Receptors Detect changes or respond to stimuli Neurons and specialized cells Complex sensory organs (e.g., eyes, ears)

10 12-1 Divisions of the Nervous System Functional Divisions of the PNS The efferent division Somatic nervous system (SNS) Controls voluntary and involuntary (reflexes) muscle skeletal contractions

11 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

12 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

13 12-2 Neurons Nissl bodies Dense areas of RER and ribosomes Make neural tissue appear gray (gray matter)

14 12-2 Neurons Dendrites Highly branched Dendritic spines Many fine processes Receive information from other neurons 80 90% of neuron surface area

15 12-2 Neurons The axon Is long Carries electrical signal (action potential) to target Axon structure is critical to function

16 Figure 12-1a The Anatomy of a Multipolar Neuron Dendrites Perikaryon Nucleus Cell body Axon Telodendria This color-coded figure shows the four general regions of a neuron.

17 Figure 12-1b The Anatomy of a Multipolar Neuron Dendritic branches Nissl bodies (RER and free ribosomes) Mitochondrion Axon hillock Initial segment of axon Golgi apparatus Neurofilament Nucleus Nucleolus Dendrite Axolemma Axon Telodendria Synaptic terminals See Figure 12 2 PRESYNAPTIC CELL An understanding of neuron function requires knowing its structural components. POSTSYNAPTIC CELL

18 12-2 Neurons The Structure of Neurons The synapse Area where a neuron communicates with another cell

19 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

20 12-2 Neurons The Synapse The synaptic terminal Is expanded area of axon of presynaptic neuron Contains synaptic vesicles of neurotransmitters

21 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 synaptic terminal

22 12-2 Neurons Types of Synapses Neuromuscular junction Synapse between neuron and muscle Neuroglandular junction Synapse between neuron and gland

23 Figure 12-2 The Structure of a Typical Synapse Telodendrion Synaptic terminal Mitochondrion Endoplasmic reticulum Synaptic vesicles Presynaptic membrane Postsynaptic membrane Synaptic cleft

24 12-2 Neurons Structural Classification of Neurons Anaxonic neurons Found in brain and sense organs Bipolar neurons Found in special sensory organs (sight, smell, hearing) Unipolar neurons Found in sensory neurons of PNS Multipolar neurons Common in the CNS Include all skeletal muscle motor neurons

25 12-2 Neurons Anaxonic Neurons Small All cell processes look alike Bipolar Neurons Are small One dendrite, one axon

26 12-2 Neurons Unipolar Neurons Also called pseudounipolar neurons Have very long axons Fused dendrites and axon Cell body to one side Multipolar Neurons Have very long axons Multiple dendrites, one axon

27 Figure 12-3 A Structural Classification of Neurons Anaxonic neuron Bipolar neuron Unipolar neuron Multipolar neuron Dendritic branches Dendrite Dendrites Initial segment Axon Dendrites Cell body Cell body Cell body Axon Synaptic terminals Axon Cell body Axon Synaptic terminals Synaptic terminals

28 Figure 12-3a A Structural Classification of Neurons Anaxonic neuron Cell body Anaxonic neurons have more than two processes, but axons cannot be distinguished from dendrites.

29 Figure 12-3b A Structural Classification of Neurons Bipolar neuron Dendritic branches Dendrite Cell body Axon Synaptic terminals Bipolar neurons have two processes separated by the cell body.

30 Figure 12-3c A Structural Classification of Neurons Unipolar neuron Dendrites Initial segment Axon Cell body Axon Synaptic terminals Unipolar neurons have a single elongate process, with the cell body situated off to the side.

31 Figure 12-3d A Structural Classification of Neurons Multipolar neuron Dendrites Cell body Axon Synaptic terminals Multipolar neurons have more than two processes; there is a single axon and multiple dendrites.

32 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

33 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

34 12-3 Neuroglia Ganglia Masses of neuron cell bodies Surrounded by neuroglia Found in the PNS

35 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)

36 12-2 Neurons Motor Neurons Carry instructions from CNS to peripheral effectors Via efferent fibers (axons)

37 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

38 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

39 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

40 12-3 Neuroglia Neuroglia Half the volume of the nervous system Many types of neuroglia in CNS and PNS

41 12-3 Neuroglia Four Types of Neuroglia in the CNS 1. Ependymal cells 2. Astrocytes 3. Oligodendrocytes 4. Microglia

42 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

43 12-3 Neuroglia Astrocytes Create three-dimensional framework for CNS Repair damaged neural tissue

44 Figure 12-4 An Introduction to Neuroglia Neuroglia are found in Central Nervous System contains Ependymal cells Astrocytes Oligodendrocytes Microglia 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 dissolvedgas concentrations; absorb and recycle neurotransmitters; form scar tissue after injury Myelinate CNS axons; provide structural framework Remove cell debris, wastes, and pathogens by phagocytosis

45 Figure 12-5a Neuroglia in the CNS Central canal Ependymal cells Central canal of spinal cord LM 450 Gray matter Light micrograph showing the ependymal lining of the central canal of the spinal cord

46 Figure 12-5b Neuroglia in the CNS CENTRAL CANAL Ependymal cells Gray matter Neurons Microglial cell A diagrammatic view of neural tissue in the CNS, showing relationships between neuroglia and neurons

47 Figure 12-5b Neuroglia in the CNS Myelinated axons Internode White matter Myelin (cut) Axon Oligodendrocyte Axolemma Astrocyte Node Unmyelinated axon Basement membrane Capillary A diagrammatic view of neural tissue in the CNS, showing relationships between neuroglia and neurons

48 12-3 Neuroglia Oligodendrocytes Myelination Increases speed of action potentials Myelin insulates myelinated axons Makes nerves appear white

49 12-3 Neuroglia Oligodendrocytes Nodes and internodes Internodes - myelinated segments of axon Nodes

50 Figure 12-6a Schwann Cells and Peripheral Axons Axon hillock Axon Nucleus Myelinated internode Initial segment (unmyelinated) Dendrite Nodes Schwann cell nucleus Axon Neurilemma Axon Axolemma Myelin covering internode A myelinated axon, showing the organization of Schwann cells along the length of the axon. Also shown are stages in the formation of a myelin sheath by a single Schwann cell along a portion of a single axon.

51 Figure 12-6a Schwann Cells and Peripheral Axons Neurilemma Axons Myelin Myelin sheath TEM 20,600 A myelinated axon, showing the organization of Schwann cells along the length of the axon. Also shown are stages in the formation of a myelin sheath by a single Schwann cell along a portion of a single axon.

52 12-3 Neuroglia Myelination White matter Regions of CNS with many myelinated nerves Gray matter Unmyelinated areas of CNS

53 12-3 Neuroglia Microglia Migrate through neural tissue Clean up cellular debris, waste products, and pathogens

54 12-3 Neuroglia Neuroglia of the Peripheral Nervous System Satellite cells Surround ganglia Regulate environment around neuron

55 12-3 Neuroglia Neuroglia of the Peripheral Nervous System Schwann cells Form myelin sheath around peripheral axons One Schwann cell sheaths one segment of axon Many Schwann cells sheath entire axon

56 Figure 12-4 An Introduction to Neuroglia Neuroglia are found in Peripheral Nervous System contains Satellite cells Surround neuron cell bodies in ganglia; regulate O 2, CO 2, nutrient, and neurotransmitter levels around neurons in ganglia Schwann cells Surround all axons in PNS; responsible for myelination of peripheral axons; participate in repair process after injury

57 Figure 12-6b Schwann Cells and Peripheral Axons Schwann cell Schwann cell nucleus Schwann cell #1 Schwann cell #2 Neurilemma Axons Schwann cell #3 nucleus Axons The enclosing of a group of unmyelinated axons by a single Schwann cell. A series of Schwann cells is required to cover the axons along their entire length.

58 Figure 12-6a Schwann Cells and Peripheral Axons Neurilemma Axons Myelin Myelin sheath TEM 20,600 A myelinated axon, showing the organization of Schwann cells along the length of the axon. Also shown are stages in the formation of a myelin sheath by a single Schwann cell along a portion of a single axon.

59 Figure 12-6a Schwann Cells and Peripheral Axons Axon hillock Axon Nucleus Myelinated internode Initial segment (unmyelinated) Dendrite Nodes Schwann cell nucleus Axon Neurilemma Axon Axolemma Myelin covering internode A myelinated axon, showing the organization of Schwann cells along the length of the axon. Also shown are stages in the formation of a myelin sheath by a single Schwann cell along a portion of a single axon.

60 Figure 12-6b Schwann Cells and Peripheral Axons Schwann cell #3 nucleus Axons Neurilemma Axons Unmyelinated axons TEM 27,625 The enclosing of a group of unmyelinated axons by a single Schwann cell. A series of Schwann cells is required to cover the axons along their entire length.

61 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

62 12-3 Neuroglia Neural Responses to Injuries Axon distal to injury degenerates Schwann cells Form path for new growth Wrap new axon in myelin

63 12-3 Neuroglia Nerve Regeneration in CNS Limited by chemicals released by astrocytes that: Produce scar tissue

64 Figure 12-7 Peripheral Nerve Regeneration after Injury Fragmentation of axon and myelin occurs in distal stump. Axon Myelin Proximal stump Distal stump

65 Figure 12-7 Peripheral Nerve Regeneration after Injury Schwann cells form cord, grow into cut, and unite stumps. Macrophages engulf degenerating axon and myelin. Macrophage Cord of proliferating Schwann cells

66 Figure 12-7 Peripheral Nerve Regeneration after Injury Axon sends buds into network of Schwann cells and then starts growing along cord of Schwann cells.

67 Figure 12-7 Peripheral Nerve Regeneration after Injury Axon continues to grow into distal stump and is enclosed by Schwann cells.

68 12-4 Transmembrane Potential The Transmembrane 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

69 12-4 Transmembrane 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

70 Figure 12-9 The Resting Potential is the Transmembrane Potential of an Undisturbed Cell EXTRACELLULAR FLUID mv K + leak channel Cl 3 Na + Na + leak channel Plasma membrane Sodium potassium exchange pump CYTOSOL Protein 2 K + Protein Protein

71 12-4 Transmembrane 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

72 Figure 12-10a Electrochemical Gradients for Potassium and Sodium Ions Potassium Ion Gradients At normal resting potential, an electrical gradient opposes the chemical gradient for potassium ions (K + ). The net electrochemical gradient tends to force potassium ions out of the cell. Potassium chemical gradient Potassium electrical gradient Net potassium electrochemical gradient Resting potential 70 mv Plasma membrane K + Cytosol Protein

73 Figure 12-10b Electrochemical Gradients for Potassium and Sodium Ions Potassium Ion Gradients If the plasma membrane were freely permeable to potassium ions, the outflow of K + would continue until the equilibrium potential ( 90 mv) was reached. Potassium chemical gradient Potassium electrical gradient Equilibrium potential 90 mv Plasma membrane K + Cytosol Protein

74 Figure 12-10c Electrochemical Gradients for Potassium and Sodium Ions Sodium Ion Gradients At the normal resting potential, chemical and electrical gradients combine to drive sodium ions (Na + ) into the cell. Sodium chemical gradient Sodium electrical gradient Net sodium electrochemical gradient Resting potential 70 mv Plasma membrane Cytosol Protein

75 Figure 12-10d Electrochemical Gradients for Potassium and Sodium Ions Sodium Ion Gradients If the plasma membrane were freely permeable to sodium ions, the influx of Na + would continue until the equilibrium potential (+66 mv) was reached. Sodium chemical gradient Sodium electrical gradient Equilibrium potential +66 mv Plasma membrane Cytosol Protein

76 12-4 Transmembrane 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)

77 12-4 Transmembrane 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 +

78 12-4 Transmembrane 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

79 12-4 Transmembrane Potential Changes in the Transmembrane Potential Transmembrane potential rises or falls In response to temporary changes in membrane permeability Resulting from opening or closing specific membrane channels

80 12-4 Transmembrane Potential Sodium and Potassium Channels Membrane permeability to Na + and K + determines transmembrane potential They are either passive or active

81 12-4 Transmembrane 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

82 12-4 Transmembrane Potential Three Classes of Gated Channels 1. Chemically gated channels 2. Voltage-gated channels 3. Mechanically gated channels

83 12-4 Transmembrane Potential Chemically Gated Channels Open in presence of specific chemicals (e.g., ACh) at a binding site Found on neuron cell body and dendrites

84 12-4 Transmembrane Potential Voltage-gated Channels Respond to changes in transmembrane potential Have activation gates (open) and inactivation gates (close) Characteristic of excitable membrane Found in neural axons, skeletal muscle sarcolemma, cardiac muscle

85 12-4 Transmembrane Potential Mechanically Gated Channels Respond to membrane distortion Found in sensory receptors (touch, pressure, vibration)

86 Figure 12-11a Gated Channels Chemically gated channel Resting state Presence of ACh ACh Binding site Channel closed Gated channel Channel open A chemically gated Na + channel that opens in response to the presence of ACh at a binding site.

87 Figure 12-11b Gated Channels Voltage-gated channel 70 mv Channel closed Inactivation gate 60 mv Channel open +30 mv Channel inactivated

88 Figure 12-11c Gated Channels Mechanically gated channel Channel closed Applied pressure Channel open Pressure removed Channel closed

89 12-4 Transmembrane Potential Graded Potentials Also called local potentials Changes in transmembrane potential That cannot spread far from site of stimulation Any stimulus that opens a gated channel Produces a graded potential

90 12-4 Transmembrane Potential Graded Potentials The resting state Opening sodium channel produces graded potential Resting membrane exposed to chemical Sodium channel opens Sodium ions enter the cell Transmembrane potential rises Depolarization occurs

91 Figure Graded Potentials Initial segment Resting State Resting membrane with closed chemically gated sodium ion channels 70 mv EXTRA- CELLULAR FLUID CYTOSOL

92 12-4 Transmembrane Potential Graded Potentials Depolarization A shift in transmembrane potential toward 0 mv Movement of Na + through channel

93 Figure Graded Potentials Stimulus applied here Stimulation Membrane exposed to chemical that opens the sodium ion channels 65 mv

94 Figure Graded Potentials Graded Potential Spread of sodium ions inside plasma membrane produces a local current that depolarizes adjacent portions of the plasma membrane Local current 60 mv 65 mv 70 mv Local current

95 12-4 Transmembrane Potential Graded Potentials Repolarization When the stimulus is removed, transmembrane 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

96 Figure Depolarization, Repolarization, and Hyperpolarization Chemical stimulus applied Chemical stimulus removed Repolarization Chemical stimulus applied Chemical stimulus removed Transmembrane potential (mv) Depolarization Resting potential Hyperpolarization Return to resting potential

97 12-5 Action Potential Action Potentials Propagated changes in transmembrane potential Affect an entire excitable membrane

98 12-5 Action Potential Initiating Action Potential Initial stimulus A graded depolarization of axon large enough (10 to 15 mv) to change resting potential ( 70 mv) to threshold level of voltage-gated sodium channels ( 60 to 55 mv)

99 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

100 Figure Generation of an Action Potential 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 = Sodium ion = Potassium ion

101 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

102 12-5 Action Potential Step 1: Depolarization to threshold Step 2: Activation of Na channels Rapid depolarization Na + ions rush into cytoplasm Inner membrane changes from negative to positive

103 Figure Generation of an Action Potential Depolarization to Threshold 60 mv Local current KEY = Sodium ion = Potassium ion

104 Figure Generation of an Action Potential Activation of Sodium Channels and Rapid Depolarization +10 mv KEY = Sodium ion = Potassium ion

105 12-5 Action Potential Step 3: Inactivation of Na channels and activation of K channels At +30 mv Inactivation gates close (Na channel inactivation) K channels open Repolarization begins

106 Figure Generation of an Action Potential Inactivation of Sodium Channels and Activation of Potassium Channels +30 mv KEY = Sodium ion = Potassium ion

107 12-5 Action Potential Step 4: Return to normal permeability K + channels begin to close When membrane reaches normal resting potential ( 70 mv) K + channels finish closing Membrane is hyperpolarized to 90 mv Transmembrane potential returns to resting level Action potential is over

108 Figure Generation of an Action Potential Closing of Potassium Channels 90 mv KEY = Sodium ion = Potassium ion

109 Figure Generation of an Action Potential Transmembrane potential (mv) Sodium channels close, voltagegated potassium channels open D E P O L A R I Z A T I O N R E P O L A R I Z A T I O N Resting potential Voltage-gated sodium ion channels open Threshold All channels closed Graded potential causes threshold ABSOLUTE REFRACTORY PERIOD Cannot respond RELATIVE REFRACTORY PERIOD Responds only to a larger than normal stimulus Time (msec)

110 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)

111 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

112 Figure Continuous Propagation of an Action Potential along an Unmyelinated Axon Action potential As an action potential develops at the initial segment, the transmembrane potential at this site depolarizes to +30 mv. Na mv 70 mv Extracellular Fluid 70 mv Cell membrane Cytosol

113 Figure Continuous Propagation of an Action Potential along an Unmyelinated Axon As the sodium ions entering at spread away from the open voltage-gated channels, a graded depolarization quickly brings the membrane in segment to threshold. Graded depolarization 60 mv 70 mv

114 Figure Continuous Propagation of an Action Potential along an Unmyelinated Axon An action potential now occurs in segment while segment beings repolarization. Repolarization (refractory) Na mv 70 mv

115 Figure Continuous Propagation of an Action Potential along an Unmyelinated Axon As the sodium ions entering at segment spread laterally, a graded depolarization quickly brings the membrane in segment to threshold, and the cycle is repeated. 60 mv

116 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

117 Figure Saltatory Propagation along a Myelinated Axon An action potential has occurred at the initial segment. +30 mv 70 mv 70 mv Extracellular Fluid Na + Myelinated internode Myelinated internode Myelinated internode Plasma membrane Cytosol

118 Figure Saltatory Propagation along a Myelinated Axon A local current produces a graded depolarization that brings the axolemma at the next node to threshold. 60 mv 70 mv Local current

119 Figure Saltatory Propagation along a Myelinated Axon An action potential develops at node. Repolarization (refractory) +30 mv 70 mv Na +

120 Figure Saltatory Propagation along a Myelinated Axon A local current produces a graded depolarization that brings the axolemma at node to threshold. 60 mv Local current

121 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

122 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

123 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

124 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

125 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

126 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

127 12-7 Synapses Synaptic Activity Action potentials (nerve impulses) Are transmitted from presynaptic neuron To postsynaptic neuron (or other postsynaptic cell) Across a synapse

128 12-7 Synapses 1. Chemical synapses Signal transmitted across a gap by chemical neurotransmitters

129 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

130 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

131 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

132 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

133 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

134 Figure Events in the Functioning of a Cholinergic Synapse An action potential arrives and depolarizes the synaptic terminal Presynaptic neuron Synaptic vesicles ER Action potential EXTRACELLULAR FLUID Synaptic terminal AChE Initial segment POSTSYNAPTIC NEURON

135 Figure Events in the Functioning of a Cholinergic Synapse Extracellular Ca 2+ enters the synaptic terminal, triggering the exocytosis of ACh ACh Ca2+ Ca 2+ Synaptic cleft Chemically gated sodium ion channels

136 Figure Events in the Functioning of a Cholinergic Synapse ACh binds to receptors and depolarizes the postsynaptic membrane Initiation of action potential if threshold is reached at the initial segment Na + Na + Na + Na + Na +

137 Figure Events in the Functioning of a Cholinergic Synapse ACh is removed by AChE Propagation of action potential (if generated)

138 12-8 Neurotransmitters and Neuromodulators Important Neurotransmitters Other than acetylcholine Norepinephrine (NE) Dopamine Serotonin Gamma aminobutyric acid (GABA)

139 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

140 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

141 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