1. © 2015 Pearson Education, Inc.

2. An Introduction to the Nervous System
Includes all neural tissue in the body
Neural tissue contains two kinds of cells
Neurons
Cells that send and receive signals
Neuroglia (glial cells)
Cells that support and protect neurons
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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
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4. 12-1 Divisions of the Nervous System
Anatomical Divisions of the Nervous System
Central nervous system (CNS)
Peripheral nervous system (PNS)
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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
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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
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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
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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
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9. 12-1 Divisions of the Nervous System
Functional Divisions of the PNS
Receptors and effectors of afferent division
Receptors
Detect changes or respond to stimuli
Neurons and specialized cells
Complex sensory organs (e.g., eyes, ears)
Effectors
Respond to efferent signals
Cells and organs
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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) skeletal muscle contractions
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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
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12. Figure 12-1 A Functional Overview of the Nervous System.
Organization of the Nervous System
Central Nervous System (CNS) (brain and spinal cord)
Integrate, process, and coordinate sensory data and motor commands
Sensory information
within
afferent division
Motor commands
within
efferent division
Peripheral Nervous System (PNS) (neural tissue outside the CNS)
includes
Somatic nervous system (SNS)
Autonomic
nervous system (ANS)
Parasympathetic division
Sympathetic division
Receptors
Effectors
Smooth muscle
Cardiac muscle
Glands
Adipose tissue
Special sensory receptors
Visceral sensory receptors
Somatic sensory receptors
monitor smell, taste, vision, balance, and hearing
monitor internal organs
monitor skeletal muscles, joints, and skin surface
Skeletal muscle
Skeletal muscle

13. 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
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14. 12-2 Neurons The Cell Body Large nucleus and nucleolus
Perikaryon (cytoplasm)
Mitochondria (produce energy)
RER and ribosomes (produce neurotransmitters)
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15. 12-2 Neurons The Cell Body Cytoskeleton Nissl bodies
Neurofilaments and neurotubules in place of microfilaments and microtubules
Neurofibrils: bundles of neurofilaments that provide support for dendrites and axon
Nissl bodies
Dense areas of RER and ribosomes
Make neural tissue appear gray (gray matter)
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16. 12-2 Neurons Dendrites Highly branched Dendritic spines
Many fine processes
Receive information from other neurons
80–90 percent of neuron surface area
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17. 12-2 Neurons The axon Is long
Carries electrical signal (action potential) to target
Axon structure is critical to function
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18. 12-2 Neurons Structures of the Axon Axoplasm Axolemma
Cytoplasm of axon
Contains neurofibrils, neurotubules, enzymes, organelles
Axolemma
Specialized cell membrane
Covers the axoplasm
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19. 12-2 Neurons Structures of the Axon Axon hillock Initial segment
Thick section of cell body
Attaches to initial segment
Initial segment
Attaches to axon hillock
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20. 12-2 Neurons Structures of the Axon Collaterals Telodendria
Branches of a single axon
Telodendria
Fine extensions of distal axon
Axon terminals
Tips of telodendria
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21. Figure 12-2a The Anatomy of a Multipolar Neuron.
Dendrites
Perikaryon
Cell body
Telodendria
Nucleus
Axon
This color-coded figure shows the four general regions of a neuron. a

22. Figure 12-2b The Anatomy of a Multipolar Neuron.
Nissl bodies (RER and free ribosomes)
Dendritic branches
Mitochondrion
Axon hillock
Initial segment of axon
Axolemma
Axon
Telodendria
Direction of action potential
Golgi apparatus
Axon terminals
Neurofilament
Nucleolus
Nucleus
Dendrite
See Figure 12–3
Presynaptic cell b
An understanding of neuron function requires knowing its structural components.
Postsynaptic cell

23. 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
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24. 12-2 Neurons The Synapse The synaptic terminal
Is expanded area of axon of presynaptic neuron
Contains synaptic vesicles of neurotransmitters
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25. 12-2 Neurons Neurotransmitters Are chemical messengers
Are released at presynaptic membrane
Affect receptors of postsynaptic membrane
Are broken down by enzymes
Are reassembled at axon terminal
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26. 12-2 Neurons Types of Synapses Neuromuscular junction
Synapse between neuron and muscle
Neuroglandular junction
Synapse between neuron and gland
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27. Figure 12-3 The Structure of a Typical Synapse.
Telodendrion
Axon terminal
Mitochondrion
Synaptic vesicles
Presynaptic membrane
Postsynaptic membrane
Synaptic cleft

28. 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
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29. Figure 12-4 Structural Classifications of Neurons.
Anaxonic neuron
Bipolar neuron
Unipolar neuron
Multipolar neuron
Dendrites
Dendrites
Initial segment
Cell body
Dendritic branches
Axon
Dendrite
Cell body
Cell body
Axon
Axon
Cell body
Axon
Axon terminals
Axon terminals
Axon terminals a
Anaxonic neurons have more than two processes, and they are all dendrites. b
Bipolar neurons have two processes separated by the
cell body. c
Unipolar neurons have a single elongated process, with the cell body located off to the side.
Multipolar neurons have more than two processes; there is a single axon and multiple dendrites. d

30. 12-2 Neurons Three Functional Classifications of Neurons
Sensory neurons
Afferent neurons of PNS
Motor neurons
Efferent neurons of PNS
Interneurons
Association neurons
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31. 12-2 Neurons Functions of Sensory Neurons
Monitor internal environment (visceral sensory neurons)
Monitor effects of external environment (somatic sensory neurons)
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32. 12-2 Neurons Three Types of Sensory Receptors Interoceptors
Monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive)
Internal senses (taste, deep pressure, pain)
Exteroceptors
External senses (touch, temperature, pressure)
Distance senses (sight, smell, hearing)
Proprioceptors
Monitor position and movement (skeletal muscles and joints)
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33. 12-2 Neurons Motor Neurons Two major efferent systems
Somatic nervous system (SNS)
Includes all somatic motor neurons that innervate skeletal muscles
Autonomic (visceral) nervous system (ANS)
Visceral motor neurons innervate all other peripheral effectors
Smooth muscle, cardiac muscle, glands, adipose tissue
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34. 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
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35. 12-3 Neuroglia Four Types of Neuroglia in the CNS Ependymal cells
Astrocytes
Oligodendrocytes
Microglia
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36. Figure 12-5 An Introduction to Neuroglia (Part 1 of 2).
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 dissolved gas concentrations; absorb and recycle neurotransmitters; form scar tissue after injury
Myelinate CNS axons; provide structural framework
Remove cell debris,
wastes, and pathogens by phagocytosis

37. 12-3 Neuroglia Ependymal Cells Secrete cerebrospinal fluid (CSF)
Have cilia or microvilli that circulate CSF
Monitor CSF
Contain stem cells for repair
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38. Figure 12-6a Neuroglia in the CNS.
Light micrograph showing the ependyma of the central canal of the spinal cord
Ependymal cells
Central canal
Gray matter
White matter
Gray matter
Central canal of spinal cord
LM × 450

39. 12-3 Neuroglia Astrocytes Maintain blood–brain barrier (isolates CNS)
Create three-dimensional framework for CNS
Repair damaged neural tissue
Guide neuron development
Control interstitial environment
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40. 12-3 Neuroglia Oligodendrocytes Myelination
Increases speed of action potentials
Myelin insulates myelinated axons
Makes nerves appear white
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41. 12-3 Neuroglia Oligodendrocytes Nodes and internodes
Internodes – myelinated segments of axon
Nodes (also called nodes of Ranvier)
Gaps between internodes
Where axons may branch
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42. 12-3 Neuroglia Myelination White matter Gray matter
Regions of CNS with many myelinated nerves
Gray matter
Unmyelinated areas of CNS
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43. 12-3 Neuroglia Microglia Migrate through neural tissue
Clean up cellular debris, waste products, and pathogens
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44. 12-3 Neuroglia Neuroglia of the Peripheral Nervous System Ganglia
Masses of neuron cell bodies
Surrounded by neuroglia
Found in the PNS
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45. Figure 12-5 An Introduction to Neuroglia (Part 2 of 2).
are found in
Peripheral Nervous System
contains
Satellite cells
Schwann cells
Surround neuron
cell bodies in ganglia; regulate O2, CO2, nutrient, and neurotransmitter levels around neurons in ganglia
Surround all axons in PNS; responsible for myelination of peripheral axons; participate in repair process after injury

46. 12-3 Neuroglia Neuroglia of the Peripheral Nervous System
Satellite cells
Also called amphicytes
Surround ganglia
Regulate environment around neuron
Schwann cells
Also called neurilemma cells
Form myelin sheath (neurilemma) around peripheral axons
One Schwann cell sheaths one segment of axon
Many Schwann cells sheath entire axon
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47. segment (unmyelinated)
Figure 12-7a Schwann Cells, Peripheral Axons, and Formation of the Myelin Sheath (Part 1 of 2).
Axon hillock
Nucleus
Myelinated internode
Initial
segment (unmyelinated)
Dendrite
Nodes
Axon
Axolemma
Myelin covering internode
A myelinated axon, showing the organization of Schwann cells along the length of the axon. a

48. 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
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49. 12-3 Neuroglia Nerve Regeneration in CNS
Limited by chemicals released by astrocytes that:
Block growth
Produce scar tissue
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50. 12-4 Membrane Potential
Five Main Membrane Processes in Neural Activities
Resting potential
The membrane potential of resting cell
Graded potential
Temporary, localized change in resting potential
Caused by stimulus
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51. 12-4 Membrane Potential
Five Main Membrane Processes in Neural Activities
Action potential
Is an electrical impulse
Produced by graded potential
Propagates along surface of axon to synapse
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52. 12-4 Membrane Potential
Five Main Membrane Processes in Neural Activities
Synaptic activity
Releases neurotransmitters at presynaptic membrane
Produces graded potentials in postsynaptic membrane
Information processing
Response (integration of stimuli) of postsynaptic cell
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53. 12-4 Membrane Potential
Passive Forces Acting across the Plasma Membrane
Chemical gradients
Concentration gradients (chemical gradient) of ions (Na+, K+)
Electrical gradients
Separate charges of positive and negative ions
Result in potential difference
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54. 12-4 Membrane Potential Electrical Currents and Resistance
Movement of charges to eliminate potential difference
Resistance
The amount of current a membrane restricts
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55. Figure 12-10a Electrochemical Gradients for Potassium and Sodium Ions.
Potassium Ion Gradients a
At normal resting membrane potential, an electrical gradient opposes the chemical gradient for potassium ions (K+). The net electrochemical gradient tends to force potassium ions out of the cell.
Potassium chemical gradient
Potassium electrical gradient
Net potassium electrochemical gradient
Resting membrane potential + + + + +
–70 mV + + + + + +
Plasma membrane – – – – – – – + + + + + + + – – – – + + K+ + + + – –
Protein –
Cytosol

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

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

58. Figure 12-10d Electrochemical Gradients for Potassium and Sodium Ions.
Sodium Ion Gradients d
If the plasma membrane were freely permeable to sodium ions, the influx of Na+ would continue until the equilibrium potential (+66 mV) was reached. Note how different it is from the resting membrane potential.
Sodium chemical gradient
Sodium electrical gradient
Equilibrium potential + + + + + +
+66 mV + + + + + + + + + + – – – – – – –
Plasma membrane + + + + + + + – + + + + + + + + – – – + + – + – + + + – +
Protein
Cytosol +

59. 12-4 Membrane Potential Active Forces across the Membrane
Sodium–potassium ATPase (exchange pump)
Is powered by ATP
Carries 3 Na+ out and 2 K+ in
Balances passive forces of diffusion
Maintains resting potential (70 mV)
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60. 12-4 Membrane Potential Sodium and Potassium Channels
Membrane permeability to Na+ and K+ determines membrane potential
They are either passive or active
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61. 12-4 Membrane Potential Passive Channels (Leak Channels)
Are always open
Permeability changes with conditions
Active Channels (Gated Channels)
Open and close in response to stimuli
At resting potential, most gated channels are closed
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62. 12-4 Membrane Potential Three States of Gated Channels
Closed, but capable of opening
Open (activated)
Closed, not capable of opening (inactivated)
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63. 12-4 Membrane Potential Three Classes of Gated Channels
Chemically gated channels
Voltage-gated channels
Mechanically gated channels
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64. 12-4 Membrane Potential Chemically Gated Channels
Open in presence of specific chemicals (e.g., ACh) at a binding site
Found on neuron cell body and dendrites
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65. 12-4 Membrane Potential Voltage-gated Channels
Respond to changes in membrane potential
Have activation gates (open) and inactivation gates (close)
Characteristic of excitable membrane
Found in neural axons, skeletal muscle sarcolemma, cardiac muscle
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66. Table 12-2 Graded Potentials.

67. 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
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68. 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
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69. 12-5 Action Potential The Refractory Period The time period:
From beginning of action potential
To return to resting state
During which membrane will not respond normally to additional stimuli
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70. Figure 12-14 Generation of an Action Potential (Part 9 of 9).
ABSOLUTE REFRACTORY PERIOD
RELATIVE REFRACTORY PERIOD 3
Sodium channels close, voltage-gated potassium channels open, and potassium ions move out of the cell. Repolarization begins.
+30
DEPOLARIZATION
REPOLARIZATION 2
Resting potential
Potassium channels close, and both sodium and potassium channels return to their
normal states.
Voltage-gated sodium channels open and sodium ions move into the cell. The membrane potential rises to +30 mV.
Membrane potential (mV)
–40
Threshold
–60
–70 1 4
A graded depolarization brings an area of excitable membrane to threshold
(–60 mV).
During the absolute refractory period, the membrane cannot respond to further stimulation.
During the relative refractory period, the membrane can respond only to a larger-than-normal stimulus. 1 2
Time (msec)

71. 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
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72. 12-6 Axon Diameter and Speed
Three Groups of Axons
Type A fibers
Type B fibers
Type C fibers
These groups are classified by:
Diameter
Myelination
Speed of action potentials
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73. 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
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74. 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
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75. 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
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76. 12-7 Synapses Two Classes of Neurotransmitters
Excitatory neurotransmitters
Cause depolarization of postsynaptic membranes
Promote action potentials
Inhibitory neurotransmitters
Cause hyperpolarization of postsynaptic membranes
Suppress action potentials
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77. 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
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78. 12-7 Synapses Events at a Cholinergic Synapse
Action potential arrives, depolarizes synaptic terminal
Calcium ions enter synaptic terminal, trigger exocytosis of ACh
ACh binds to receptors, depolarizes postsynaptic membrane
ACh removed by AChE
AChE breaks ACh into acetate and choline
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79. 12-8 Neurotransmitters and Neuromodulators
Other Neurotransmitters
At least 50 neurotransmitters other than ACh, including:
Biogenic amines
Amino acids
Neuropeptides
Dissolved gases
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80. 12-8 Neurotransmitters and Neuromodulators
Important Neurotransmitters
Other than acetylcholine
Norepinephrine (NE)
Dopamine
Serotonin
Gamma aminobutyric acid (GABA)
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81. 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
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82. 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
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83. 12-8 Neurotransmitters and Neuromodulators
Other chemicals released by synaptic terminals
Similar in function to neurotransmitters
Characteristics of neuromodulators
Effects are long term, slow to appear
Responses involve multiple steps, intermediary compounds
Affect presynaptic membrane, postsynaptic membrane, or both
Released alone or with a neurotransmitter
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84. 12-8 Neurotransmitters and Neuromodulators
Neuropeptides
Neuromodulators that bind to receptors and activate enzymes
Opioids
Neuromodulators in the CNS
Bind to the same receptors as opium or morphine
Relieve pain
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85. 12-8 Neurotransmitters and Neuromodulators
Four Classes of Opioids
Endorphins
Enkephalins
Endomorphins
Dynorphins
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