1. Neurons, Synapses, and Signaling37
Neurons, Synapses, and Signaling

2. do now: label the parts- hw q #1
Stimulus
Axon
hillock A
Cell
body
Presynaptic
cell C
Signal
direction D
Synaptic terminals
Figure 37.2 Neuron structure
Synaptic
terminals
Postsynaptic cell E 2

3. Neuron Structure and Function
Most of a neuron’s organelles are in the cell body (A)
Most neurons have dendrites (B), highly branched extensions that receive signals from other neurons
The single axon (C), a much longer extension, transmits signals to other cells
The cone-shaped base of an axon, where signals are generated, is called the axon hillock
Video: Dendrites 3

4. The branched ends of axons transmit signals to other cells at a junction called the synapse (D)
At most synapses, chemical messengers called neurotransmitters (E) pass information from the transmitting neuron to the receiving cell 4

5. Introduction to Information Processing
Nervous systems process information in three stages
Sensory input
Integration
Motor output 5

6. Processing center (CNS)
Figure 37.4
Sensory input
Integration
Sensor(PNS)
Motor output
Figure 37.4 Summary of information processing
Processing center (CNS)
Effector(PNS) 6

7. (information processing)
Figure 38.5
Central Nervous
System
(information processing)
Peripheral Nervous
System
Afferent neurons
Efferent neurons
Autonomic
nervous system
Motor
system
Sensory
receptors
Control of
skeletal muscle
Figure 38.5 Functional hierarchy of the vertebrate peripheral nervous system
Internal
and external
stimuli
Sympathetic
division
Parasympathetic
division
Enteric
division
Control of smooth muscles,
cardiac muscles, glands 7

8. Sensory neurons transmit information from eyes and other sensors that detect external stimuli or internal conditions (PNS)
This information is sent to the brain or ganglia, where interneurons integrate the information (CNS)
Neurons that extend out of the processing centers trigger muscle or gland activity
For example, motor neurons transmit signals to muscle cells, causing them to contract (PNS) 8

9. Workbook: page 235 Dendrites Axon Cell body Portion of axon ??? neuron
Figure 37.5 Structural diversity of neurons
Portion
of axon
??? neuron
???neuron
??? neuron 9

10. Structure of neurons: quick check
What is true regarding neurons:
1.Motor neurons have a cell body in the middle of the axon
2.Nerve signals travel from an axon to a dendrite
3.The interneuron in vertebrates are located in the CNS
4.The gap between two neurons is called a neurotransmitter

11. The communication between these neurons occurs because of concentration gradients…
The inside of a cell is negatively charged relative to the outside
This difference is a source of potential energy, termed membrane potential
The resting potential is the membrane potential of a neuron not sending signals
Hw q #2:Which ion is in high concentration outside the cell? Inside the cell?
Changes in membrane potential act as signals, transmitting and processing information 11

12. Key OUTSIDE Na OF CELL K Sodium- potassium pump Potassium channel
Figure 37.6
Key
OUTSIDE
OF CELL
Na K
Sodium-
potassium
pump
Figure 37.6 The basis of the membrane potential
Potassium
channel
Sodium
channel
INSIDE
OF CELL 12

13. Formation of the Resting Potential
K and Na play an essential role in forming the resting potential
In most neurons, the concentration of K is highest inside the cell, while the concentration of Na is highest outside the cell
Sodium-potassium pumps use the energy of ATP to maintain these K and Na gradients across the plasma membrane
Why is this important?
Animation: hw Q #3
Animation: Resting Potential 13

14. Ions Change in membrane potential (voltage) Ion channel
Figure 37.9
Ions
Change in
membrane
potential
(voltage)
Ion
channel
Figure 37.9 Voltage-gated ion channel
(a) Gate closed: No ions
flow across membrane.
(b) Gate open: Ions flow
through channel. 14

15. Hw q: 4,5 and 6: Based on these graphs: what is the definition of hyperpolarization and depolarization?
Stimulus
Stimulus
Strong depolarizing stimulus
50
50
50
Action
potential
Membrane potential (mV)
Membrane potential (mV)
Membrane potential (mV)
Threshold
Threshold
Threshold
−50
−50
−50
Resting
potential
Resting
potential
Resting
potential
Hyperpolarizations
Depolarizations
−100
−100
−100
Figure Graded potentials and an action potential in a neuron 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 6
Time (msec)
Time (msec)
Time (msec)
Which gates are open?
Sodium or potassium?
How do you know?
(b) Which gates are open?
Sodium or potassium?
How do you know?
© why is this the only situation
Where you have an action
Potential? 15

16. Action potentials are all or none
Action potentials occur whenever a depolarization increases the membrane potential to a particular value, called the threshold (-55mV)
Action potentials are all or none 16

17. Membrane potential: quick check:
Select the statement that is false:
At rest, there is a high concentration of Na+ outside the neuron
Ion channels are specific: for example K+ channels will only allow K+ to pass through
When sodium channels open the membrane potential becomes more positive
If there is a depolarization, this means that potassium channels must be open

18. Hw Q#7 Key Na K Rising phase of the action potential3
Rising phase of the action potential 4
Falling phase of the action potential
50
Action
potential 3
Membrane potential
(mV) 2 4
Threshold
−50 1 1 5
Resting potential 2
Depolarization
−100
Time
Figure The role of voltage-gated ion channels in the generation of an action potential
OUTSIDE OF CELL
Sodium
channel
Potassium
channel 5
Undershoot
INSIDE OF CELL
Inactivation loop 1
Resting state 18

19. Key Na K Resting state OUTSIDE OF CELL Sodium channel Potassium
Figure 37.11a
Key
Na K
OUTSIDE OF CELL
Sodium
channel
Potassium
channel
Figure 37.11a The role of voltage-gated ion channels in the generation of an action potential (part 1: resting state)
INSIDE OF CELL
Inactivation loop 1
Resting state 19

20. When stimulus depolarizes the membrane
Some gated Na+ channels open first and Na flows into the cell
During the rising phase, the threshold is crossed, and the membrane potential increases
During the falling phase, voltage-gated Na channels become inactivated; voltage-gated K channels open, and K flows out of the cell 20

21. Key Na K Depolarization 2 Figure 37.11b
Figure 37.11b The role of voltage-gated ion channels in the generation of an action potential (part 2: depolarization) 2
Depolarization 21

22. Rising phase of the action potential
Figure 37.11c
Key
Na K
Figure 37.11c The role of voltage-gated ion channels in the generation of an action potential (part 3: rising phase) 3
Rising phase of the action potential 22

23. Falling phase of the action potential
Figure 37.11d
Key
Na K
Figure 37.11d The role of voltage-gated ion channels in the generation of an action potential (part 4: falling phase) 4
Falling phase of the action potential 23

24. During the undershoot, membrane permeability to K is at first higher than at rest, and then voltage-gated K channels close and resting potential is restored 24

25. Key Na K Undershoot 5 Figure 37.11e
Figure 37.11e The role of voltage-gated ion channels in the generation of an action potential (part 5: undershoot) 5
Undershoot 25

26. Membrane potential (mV)
Figure 37.11f
50
Action
potential 3
Membrane potential
(mV) 2 4
Threshold
−50 1 1
Figure 37.11f The role of voltage-gated ion channels in the generation of an action potential (part 6: graph) 5
Resting potential
−100
Time 26

27. During the refractory period after an action potential, a second action potential cannot be initiated
Why might this be beneficial?
The refractory period is a result of a temporary inactivation of the Na channels
For most neurons, the interval between the start of an action potential and the end of the refractory period is only 1–2 msec 27

28. Conduction of Action Potentials
At the site where the action potential is initiated (usually the axon hillock), an electrical current depolarizes the neighboring region of the axon membrane
Action potentials travel only toward the synaptic terminals
Inactivated Na channels are behind the zone of movement
Why is this beneficial? 28

29. Axon Plasma Action membrane potential Cytosol 1 1 Na Figure 37.12-1
Figure Conduction of an action potential (step 1) 29

30. Axon Plasma Action membrane potential Cytosol Action potential 1 2 1
Figure
Axon
Plasma
membrane
Action
potential 1 1
Na
Cytosol
Action
potential K 2
Na
Figure Conduction of an action potential (step 2) K 30

31. Axon Plasma Action membrane potential Cytosol Action potential Action
Figure
Axon
Plasma
membrane
Action
potential 1
Na
Cytosol
Action
potential K 2
Na
Figure Conduction of an action potential (step 3) K
Action
potential K 3
Na K 31

32. Action potential: quick check
Identify the following statements as true or false
1. the threshold value for an action potential is -55mV
2. increased amounts of sodium entering causes the membrane potential to become negative.
3. potassium channels open at +35mv
4. the action potential moves forward and not backwards due to inactivated potassium channels

33. Label the parts: C A Axon ??? B cell B cell C Axon A Nucleus of B cell
Figure Schwann cells and the myelin sheath
0.1 m 33

34. Evolutionary Adaptations of Axon Structure
The speed of an action potential increases with the axon’s diameter
In vertebrates, axons are insulated by a myelin sheath
Why is this beneficial?
Myelin sheaths are produced by glia—oligodendrocytes in the CNS and Schwann cells in the PNS 34

35. A selective advantage of myelination is space efficiency
Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na channels are found
Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction
A selective advantage of myelination is space efficiency 35

36. Schwann cell Depolarized region (node of Ranvier) Cell body Myelin
Figure 37.14
Schwann cell
Depolarized region
(node of Ranvier)
Cell body
Myelin
sheath
Axon
Figure Saltatory conduction 36

37. Concept 37.4: Neurons communicate with other cells at synapses
At electrical synapses, the electrical current flows from one neuron to another
Most synapses are chemical synapses, in which a chemical neurotransmitter carries information from the presynaptic neuron to the postsynaptic cell 37

38. Hw q #8 K Ca2 Na Presynaptic cell Postsynaptic cell Axon
Synaptic vesicle
containing neurotransmitter 1
Synaptic
cleft
Postsynaptic
membrane
Presynaptic
membrane 3 4
Figure A chemical synapse K
Ca2 2
Ligand-gated
ion channels
Voltage-gated
Ca2 channel
Na 38

39. The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal
The arrival of the action potential causes the release of the neurotransmitter
The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell
Animation: Synapse
Animation: How Synapses Work 39

40. Generation of Postsynaptic Potentials
Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell
Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential
Q #8 from hw: what are the two kinds of post synaptic potentials? 40

41. Postsynaptic potentials fall into two categories
Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward threshold
Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold 41

42. The neurotransmitters clear quickly, in the following ways:
Some are recaptured into presynaptic neurons to be repackaged and reused
Some are recaptured into glia to be used as fuel
Others are removed by simple diffusion or hydrolysis of the neurotransmitter (ex: acetylcholinesterase for acetylcholine 42

43. Summation of Postsynaptic Potentials
The cell body of one postsynaptic neuron may receive inputs from hundreds or thousands of synaptic terminals
A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron 43

44. Integration
What is the difference between the way these two neurons are generating an action potential?

45. If two EPSPs are produced in rapid succession, an effect called temporal summation occurs
In spatial summation, EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together
The combination of EPSPs through spatial and temporal summation can trigger an action potential 45

46. Neurotransmitters
Signaling at a synapse brings about a response that depends on both the neurotransmitter from the presynaptic cell and the receptor on the postsynaptic cell
A single neurotransmitter may have more than a dozen different receptors
Acetylcholine is a common neurotransmitter in both invertebrates and vertebrates 46

47. Acetylcholine
Acetylcholine is vital for functions involving muscle stimulation, memory formation, and learning
Vertebrates have two major classes of acetylcholine receptor, one that is ligand gated and one that is metabotropic
Nicotine is a direct agonist. What does this mean?
Atropine and curare is an antagonist. What does this mean? 47

48. Table 37.2
Table 37.2 Major neurotransmitters 48

49. Amino Acids
Glutamate (rather than acetylcholine) is used at the neuromuscular junction in invertebrates
Gamma-aminobutyric acid (GABA) is the neurotransmitter at most inhibitory synapses in the brain
Glycine also acts at inhibitory synapses in the CNS that lies outside of the brain 49

50. Biogenic Amines Biogenic amines include
Norepinephrine and the chemically similar ephinephrine
Dopamine
Serotonin
They are active in the CNS and PNS
Biogenic amines have a central role in a number of nervous system disorders and treatments 50

51. Synapses: quick check
Which of the following statements is true regarding synapses:
1. a synapse can occur if two IPSP occur at a neuron
2. a synapse involves the release of Ca ions which cause the release of neurotransmitters
3. acetylcholine is a rare neurotransmitter
4. only a spatial summation can cause a synapse