1. Neural Signaling
Chapter 40

2. Learning Objective 1
Describe the processes involved in neural signaling: reception, transmission, integration, and action by effectors

3. Neural Signaling 1 (1) Reception of information
by a sensory receptor
(2) Transmission by an afferent neuron
to the central nervous system (CNS)
(3) Integration by interneurons
in the central nervous system (CNS)

4. Neural Signaling 2 (4) Transmission by an efferent neuron
to other neurons or effector
(5) Action by effectors
the muscles and glands

5. Peripheral Nervous System (PNS)
Made up of
sensory receptors
neurons outside the CNS

6. Response to Stimulus

7. (e.g., vibration, movement, light, odor) Internal stimulus
External stimulus
(e.g., vibration, movement,
light, odor)
Internal stimulus
(e.g., change in blood
pH or blood pressure)
RECEPTION
Detection by external
sense organs
Detection by internal
sense organs
Figure 40.1: Response to a stimulus.
Whether a stimulus originates in the outside world or inside the body, information must be received; transmitted to the central nervous system; integrated; and then transmitted to effectors, muscles or glands that carry out some action, the actual response.
TRANSMISSION
Sensory (afferent) neurons transmit information
Fig. 40-1a, p. 846

8. Central Nervous System (brain and spinal cord)
INTEGRATION
Interneurons sort and interpret information
TRANSMISSION
Motor (efferent) neurons transmit impulses
ACTION BY EFFECTORS (muscles and glands)
Figure 40.1: Response to a stimulus.
Whether a stimulus originates in the outside world or inside the body, information must be received; transmitted to the central nervous system; integrated; and then transmitted to effectors, muscles or glands that carry out some action, the actual response.
e.g., animal
runs away
e.g., espiration
rate increases;
blood pressure
rises
Fig. 40-1b, p. 846

9. (e.g., vibration, movement, light, odor) Internal stimulus
External stimulus
(e.g., vibration, movement,
light, odor)
Internal stimulus
(e.g., change in blood
pH or blood pressure)
RECEPTION
Detection by external
sense organs
Detection by internal
TRANSMISSION
Sensory (afferent) neurons transmit information
Central Nervous System (brain and spinal cord)
INTEGRATION
Interneurons sort and interpret information
TRANSMISSION
Motor (efferent) neurons transmit impulses
Figure 40.1: Response to a stimulus.
Whether a stimulus originates in the outside world or inside the body, information must be received; transmitted to the central nervous system; integrated; and then transmitted to effectors, muscles or glands that carry out some action, the actual response.
ACTION BY EFFECTORS (muscles and glands)
e.g., animal
runs away
e.g., espiration
rate increases;
blood pressure
rises
Stepped Art
Fig. 40-1, p. 846

10. KEY CONCEPTS
Neural signaling involves reception, transmission, integration, and action by effectors

11. Learning Objective 2 What is the structure of a typical neuron?
Give the function of each of its parts

12. Neurons Specialized to Cell body receive stimuli
transmit electrical and chemical signals
Cell body
contains nucleus and organelles

13. Dendrites Many branched dendrites extend from cell body of neuron
specialized to receive stimuli and send signals to the cell body

14. Axons 1 A single long axon Transmits signals into terminal branches
extends from neuron cell body
forms branches (axon collaterals)
Transmits signals into terminal branches
which end in synaptic terminals

15. Axons 2 Myelin sheath Schwann cells surrounds many axons insulates
form the myelin sheath in the PNS

16. Axons 3 In the CNS Nodes of Ranvier
sheath is formed by other glial cells
Nodes of Ranvier
gaps in sheath between successive Schwann cells

17. Neuron Structure

18. Dendrites covered with dendritic spines Cytoplasm of Schwann cell
Synaptic terminals
Axon
Axon collateral
Cell body
Nucleus
Myelin sheath
Nucleus
Figure 40.2: Structure of a multipolar neuron.
The cell body contains most of the organelles. Many dendrites and a single axon extend from the cell body. Schwann cells form a myelin sheath around the axon by wrapping their own plasma membranes around the neuron in jelly-roll fashion.
Axon
Nodes of Ranvier
Schwann cell
Terminal branches
Fig. 40-2, p. 847

19. Nerves and Ganglia Nerve Ganglion several hundred axons
wrapped in connective tissue
Ganglion
mass of neuron cell bodies in the PNS

20. Nerve Structure

21. Ganglion Cell bodies Myelin sheath Vein Axon Artery (a)
Figure 40.3: Structure of a nerve.
Vein
Axon
Artery
(a)
Fig. 40-3a, p. 848

22. Figure 40.3: Structure of a nerve.
100 µm
(b)
Fig. 40-3b, p. 848

23. Learn more about the structure of neurons and nerves by clicking on the figures in ThomsonNOW.

24. Learning Objective 3 Name the main types of glial cells
Describe the functions of each

25. Glial Cells Support and nourish neurons
Are important in neural communication

26. Glial Cell Types 1 Astrocytes physically support neurons
regulate extracellular fluid in CNS (by taking up excess potassium ions)
communicate with one another (and with neurons)
induce and stabilize synapses

27. Glial Cell Types 2 Oligodendrocytes Schwann cells
form myelin sheaths around axons in CNS
Schwann cells
form sheaths around axons in PNS

28. Glial Cell Types 3 Microglia Ependymal Cells Phagocytic cells
line cavities in the CNS
contribute to formation of cerebrospinal fluid
serve as neural stem cells

29. KEY CONCEPTS
Neurons are specialized to receive stimuli and transmit signals; glial cells are supporting cells that protect and nourish neurons and that can modify neural signals

30. Learning Objective 4
How does the neuron develop and maintain a resting potential?

31. Neural Signals Electrical signals transmit information
along axons
Plasma membrane of resting neuron (not transmitting an impulse) is polarized
Inner surface of plasma membrane is negatively charged
relative to extracellular fluid

32. Resting Potential Potential difference of about -70 mV
across the membrane
Magnitude of resting potential
(1) differences in ion concentrations (Na+, K+) inside cell relative to extracellular fluid
(2) selective permeability of plasma membrane to these ions

33. (a) Measuring the resting potential of a neuron.
Axon 40 20
–20
–40
–60
–70 mV
–80
Time
Amplifier
Figure 40.4: Resting potential.
Plasma membrane
Electrode placed inside the cell
Electrode placed outside the cell + + + – + – + + – – – – – – – + – + – + – + + +
(a) Measuring the resting potential of a neuron.
Fig. 40-4a, p. 850

34. Ions Pass through specific passive ion channels
K+ leak out faster than Na+ leak in
Cl- accumulate at inner surface of plasma membrane
Large anions (proteins)
cannot cross plasma membrane
contribute negative charges

35. Sodium–Potassium Pumps
Maintain gradients that determine resting potential
transport 3 Na+ out for each 2 K+ in
Require ATP

36. + + + + + + – – – – – – Extracellular fluid 3 Na+ CI– Na+ Na+ CI– Na+
Diffusion out K+
Na+ K+ K+
Na+
Plasma membrane + + + + + + – – – – – –
Na /K pump K+ K+
Na+
Figure 40.4: Resting potential. K+ K+ K+
CI–
Diffusion in K+
2 K+
Na+ A_ A_ A_
CI–
CI–
CI–
Cytoplasm
(b) Permeability of the neuron membrane.
Fig. 40-4b, p. 850

37. KEY CONCEPTS
The resting potential of a neuron is maintained by differences in concentrations of specific ions inside the cell relative to the extracellular fluid and by selective permeability of the plasma membrane to these ions

38. Learning Objective 5
Compare a graded potential with an action potential
Describe the production and transmission of each

39. Membrane Potential Membrane is depolarized Membrane is hyperpolarized
if stimulus causes membrane potential to become less negative
Membrane is hyperpolarized
if membrane potential becomes more negative than resting potential

40. Graded Potential A local response Varies in magnitude Fades out
depending on strength of applied stimulus
Fades out
within a few millimeters of point of origin

41. Action Potential 1 Action potential is a wave of depolarization
that moves down the axon
Generated when
voltage across the membrane declines to a critical point (threshold level)
voltage-activated ion channels open
Na+ ions flow into the neuron

42. Voltage-Activated Ion Channels

43. (b) Potassium channels.
Extracellular fluid
Activation gate
Figure 40.6: Voltage-activated ion channels.
Cytoplasm
Inactivation gate
(a) Sodium channels.
(b) Potassium channels.
Fig. 40-6, p. 852

44. Voltage-Activated Ion Channels During an Action Potential

45. Spike (a) Action potential. Depolarization Repolarization
Membrane potential (mV)
Threshold level
Resting state
Figure 40.7: State of voltage-activated ion channels during an action potential.
Time (milliseconds)
(a) Action potential.
Fig. 40-7a, p. 853

46. Return to resting state. Repolarization.
Axon
Extracellular fluid
Sodium channel
Potassium channel
Figure 40.7: State of voltage-activated ion channels during an action potential.
Cytoplasm 1
Resting state. 2
Depolarization.
Return to resting state. 3
Repolarization. 4
(b) The action of the ion channels in the plasma membrane determines the state of the neuron.
Fig. 40-7b, p. 853

47. Action Potential 2 An all-or-none response
no variation in strength of a single impulse
either membrane potential exceeds threshold level or it does not
Once begun, an action potential is self-propagating

48. Repolarization
As an action potential moves down an axon, repolarization occurs behind it

49. Transmission of an Action Potential

50. Area of depolarization
Stimulus
Axon
Area of depolarization
Potassium channel
Sodium channel
Figure 40.8: Transmission of an action potential along the axon.
When the dendrites (or cell body) of a neuron are stimulated sufficiently to depolarize the membrane to its threshold level, an action potential is generated.
Action potential
(1) Action potential is transmitted as wave of depolarization that travels down axon. At region of depolarization, Na+ diffuse into cell.
Fig. 40-8a, p. 854

51. Area of repolarization Area of depolarization
Action potential
Figure 40.8: Transmission of an action potential along the axon.
When the dendrites (or cell body) of a neuron are stimulated sufficiently to depolarize the membrane to its threshold level, an action potential is generated.
(2) As action potential progresses along axon, repolarization occurs quickly behind it.
Fig. 40-8b, p. 854

52. Refractory Periods
During depolarization, the axon enters an absolute refractory period
when it can’t transmit another action potential
When enough gates controlling Na+ channels have been reset, the neuron enters a relative refractory period
when the threshold is higher

53. Learn more about ion channels and action potentials by clicking on the figures in ThomsonNOW.

54. KEY CONCEPTS
Depolarization of the neuron plasma membrane to threshold level generates an action potential, an electrical signal that travels as a wave of depolarization along the axon

55. Learning Objective 6
Contrast continuous conduction with saltatory conduction

56. Continuous Conduction
Involves entire axon plasma membrane
Takes place in unmyelinated neurons

57. Saltatory Conduction
Depolarization skips along axon from one node of Ranvier to the next
more rapid than continuous conduction
takes place in myelinated neurons
Nodes of Ranvier
sites where axon is not covered by myelin
Na+ channels are concentrated

58. Saltatory Conduction

59. Area of action potential Saltatory conduction Nodes of Ranvier Axon
Figure 40.9: Saltatory conduction.
This sequence of diagrams (steps ●1 through ●4) illustrates transmission of an action potential in a myelinated neuron.
Axon 1
Schwann cell 2
Fig. 40-9a, p. 855

60. Direction of depolarization3
Figure 40.9: Saltatory conduction.
This sequence of diagrams (steps ●1 through ●4) illustrates transmission of an action potential in a myelinated neuron. 4
Direction of depolarization
Fig. 40-9b, p. 855

61. Learning Objective 7
Describe the actions of the neurotransmitters identified in the chapter

62. Synapses Junctions between two neurons Most synapses are chemical
or between a neuron and effector
Most synapses are chemical
some are electrical synapses

63. Synaptic Transmission
A presynaptic neuron releases neurotransmitter (chemical messenger) from its synaptic vesicles

64. Neurotransmitters 1 Acetylcholine Biogenic amines
triggers contraction of skeletal muscle
Biogenic amines
norepinephrine, serotonin, dopamine
important in regulating mood
dopamine is also important in motor function

65. Neurotransmitters 2 Some amino acids Neuropeptides (opioids)
glutamate (excitatory neurotransmitter in brain)
GABA (widespread inhibitory neurotransmitter)
Neuropeptides (opioids)
endorphins (e.g. beta-endorphin)
enkephalins

66. Neurotransmitters 3 Nitric oxide (NO) gaseous neurotransmitter
transmits signals from postsynaptic neuron to presynaptic neuron (opposite direction from other neurotransmitters)

67. Learning Objective 8
Trace the events that take place in synaptic transmission
Draw diagrams to support your description

68. Synaptic Transmission
Calcium ions cause synaptic vesicles to fuse with presynaptic membrane
releases neurotransmitter into synaptic cleft
Neurotransmitter diffuses across the synaptic cleft
combines with specific receptors on a postsynaptic neuron

69. Synaptic Transmission

70. Plasma membrane of postsynaptic neuron
Synaptic vesicles
Plasma membrane of postsynaptic neuron
Figure 40.10: Synaptic transmission.
0.25 µm
(a) The TEM shows synaptic terminals filled with synaptic vesicles.
Fig a, p. 858

71. Figure 40.10: Synaptic transmission.
Fig bc, p. 858

72. (b) How a neural impulse is transmitted across a synapse.
Axon of presynaptic neuron
Synaptic terminal
Voltage-gated Ca2+ channel 1
Synaptic vesicle
Ca2+ 2 3
Neuro- transmitter molecule
Figure 40.10: Synaptic transmission. 4
Ligand-gated channels 5
Postsynaptic membrane
(b) How a neural impulse is transmitted across a synapse.
Receptor for neurotransmitter
Postsynaptic neuron
Fig b, p. 858

73. Synaptic terminal Synaptic cleft
Ca2+
Synaptic terminal
Presynaptic membrane
Synaptic cleft
Na+
Postsynaptic membrane
Figure 40.10: Synaptic transmission.
(c) Neurotransmitter binds with receptor. Ligand-gated channel opens, resulting in depolarization.
Fig c, p. 858

74. Neurotransmitter Receptors
Many are proteins that form ligand-gated ion channels
Others work through a second messenger such as cAMP

75. Learn more about synaptic transmission by clicking on the figure in ThomsonNOW.

76. KEY CONCEPTS
Neurons signal other cells by releasing neurotransmitters at synapses

77. Learning Objective 9
Compare excitatory and inhibitory signals and their effects

78. Binding of Neurotransmitter to a Receptor
Binding causes either
excitatory postsynaptic potential (EPSP)
or inhibitory postsynaptic potential (IPSP)
Depending on the type of receptor

79. EPSPs and IPSPs EPSPs IPSPs bring neuron closer to firing
move neuron farther away from its firing level

80. Learning Objective 10 Define neural integration
Describe how a postsynaptic neuron integrates incoming stimuli and “decides” whether or not to fire

81. Neural Integration Process of summing (integrating) incoming signals
Summation
process of adding and subtracting incoming signals

82. Summation Each EPSP or IPSP is a graded potential
vary in magnitude
depending on strength of stimulus applied
Summation of several EPSPs
brings neuron to critical firing level

83. Temporal Summation
Occurs when repeated stimuli cause new EPSPs to develop before previous EPSPs have decayed

84. Spatial Summation
Occurs when several closely spaced synaptic terminals release neurotransmitter simultaneously
stimulating postsynaptic neuron at several different places

85. Neural Integration

86. Threshold level Resting potential
Postsynaptic membrane potential (mV)
Resting potential
Figure 40.11: Neural integration.
E1 and E2 are excitatory stimuli. I represents an inhibitory stimulus.
Time (msec)
(a) Subthreshold (no summation).
(b) Temporal summation.
(c) Spatial summation.
(d) Spatial summation of EPSPs and IPSPs.
Fig , p. 860

87. KEY CONCEPTS
During integration, incoming neural signals are summed; temporal and spatial summation can bring a neuron to threshold level

88. Learning Objective 11
Distinguish among convergence, divergence, and reverberation
Explain why each is important

89. Neural Circuits
Complex neural circuits are possible because of associations such as convergence and divergence

90. Convergence
A single neuron is affected by converging signals from two or more presynaptic neurons
Allows CNS to integrate incoming information from various sources

91. Divergence
A single presynaptic neuron stimulates many postsynaptic neurons
allowing widespread effect

92. Neural Circuits

93. Figure 40.12: Neural circuits.
(a) Convergence of neural input. Several presynaptic neurons synapse with one postsynaptic neuron.
(b) Divergence of neural output. A single presynaptic neuron synapses with many postsynaptic neurons.
Fig , p. 861

94. Reverberating Circuits
Important in
rhythmic breathing
mental alertness
short-term memory
Depend on positive feedback
new impulses generated again and again until synapses fatigue

95. Reverberating Circuits

96. 1 2
(a) Simple reverberating circuit. An axon collateral of the second neuron turns back on its own dendrites, so the neuron continues to stimulate itself.
Figure 40.13: Reverberating circuits.
Fig a, p. 861

97. Interneuron Axon collateral 1 2 3
(b) Reverberating circuit with interneuron. An axon collateral of the second neuron synapses with an interneuron. The interneuron synapses with the first neuron in the sequence. New impulses are triggered again and again in the first neuron, causing reverberation.
Figure 40.13: Reverberating circuits.
Fig b, p. 861