Neurons, Synapses, and Signaling: Lines of Communication

1. Chapter 48 Neurons, Synapses, and Signaling

2. Overview: Lines of Communication • The cone snail kills prey with venom that disables neurons • Neuronsare nerve cells that transfer information within the body • Neurons use two types of signals to communicate: electrical signals (long-distance) and chemical signals (short-distance)

3. The transmissionof information depends on the path of neurons along which a signal travels • Processingof information takes place in simple clusters of neurons called gangliaor a more complex organization of neurons called a brain

4. Introduction to Information Processing • Nervous systems process information in three stages: sensory input, integration, and motor output

5. Sensory receptors collect information about the world outside the body as well as processes inside the body. Ex: rods and cones of the eyes; pressure receptors in the skin. They send this information along sensory neurons to the brain or ganglia. • Here interneuronsconnect sensory and motor neurons or make local connections in the brain and spinal cord. • Motor output leaves the brain or ganglia via motor neurons, which transmit signals to effectors, such as muscle cells and glands, to trigger a response.

6. Many animals have a complex nervous system which consists of: • A central nervous system (CNS) where integration takes place; this includes the brain and a nerve cord • A peripheral nervous system (PNS), which brings information into and out of the CNS

7. Fig. 48-3 Sensory input Integration Sensor Motor output Central nervous system (CNS) Effector Peripheral nervous system (PNS)

8. Neuron Structure and Function • A neuronis a the functional cell unit of the nervous system. • It is composed of a cell body, which contains the nucleus and organelles; dendrites, which are cell extensions that receive incoming messages from other cells; and axons, which is a much longer extension that transmit messages to other cells. • A synapseis a junction between an axon and another cell

9. Fig. 48-4 Dendrites Stimulus Presynaptic cell Nucleus Axon hillock Cell body Axon Synapse Synaptic terminals Postsynaptic cell Neurotransmitter

10. The synaptic terminal of one axon passes information across the synapse in the form of chemical messengers called neurotransmitters • Neurotransmitters will diffuse across the synapse and bind to receptors on the neuron, muscle fiber, or gland across the synapse, effecting a change in the second cell. • Examples of neurotransmitters include acetylcholine, dopamine, and serotonin.

11. Information is transmitted from a presynaptic cell (a neuron) to a postsynaptic cell(a neuron, muscle, or gland cell) • Most neurons are nourished or insulated by cells called glia

12. Fig. 48-5 Dendrites Axon Cell body Portion of axon 80 µm Cell bodies of overlapping neurons Sensory neuron Interneurons Motor neuron

13. Concept 48.2: Ion pumps and ion channels maintain the resting potential of a neuron • Every cell has a voltage(difference in electrical charge) across its plasma membrane called a membrane potential • Messages are transmitted as changes in membrane potential • The resting potential is the membrane potential of a neuron at rest. It exists because of differences in the ionic composition of the extracellular and intracellular fluids across the plasma membrane.

14. Formation of the Resting Potential • In a mammalian neuron at resting potential, the concentration of K+ is greater inside the cell, while the concentration of Na+ is greater outside the cell • Sodium-potassium pumps use the energy of ATP to maintainthese K+ and Na+gradientsacross the plasma membrane • These concentration gradients represent chemical potential energy

15. Fig. 7-16-7 EXTRACELLULAR FLUID Na+ [Na+] high Na+ [K+] low Na+ Na+ Na+ Na+ Na+ Na+ ATP [Na+] low P Na+ P [K+] high CYTOPLASM ADP 2 3 1 K+ K+ K+ K+ K+ P K+ P 6 5 4

16. The opening of ion channels in the plasma membrane converts chemical potential to electrical potential • A neuron at resting potential contains many open K+ channels and fewer open Na+ channels; K+ diffuses out of the cell • Anionstrapped inside the cell contribute to the negative charge within the neuron Animation: Resting Potential

17. Fig. 48-6a OUTSIDE CELL [Na+] 150 mM [Cl–] 120 mM [K+] 5 mM [A–] 100 mM [K+] 140 mM INSIDE CELL [Na+] 15 mM [Cl–] 10 mM (a)

18. Fig. 48-6b Key Sodium- potassium pump Na+ Potassium channel Sodium channel K+ OUTSIDE CELL INSIDE CELL (b)

19. Concept 48.3: Action potentials are the signals conducted by axons • Neurons contain gated ion channelsthat open or close in response to stimuli • Membrane potential changes in response to opening or closing of these channels • When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative • This is hyperpolarization, an increase in magnitude of the membrane potential

20. Fig. 48-9a Stimuli +50 0 Membrane potential (mV) –50 Threshold Resting potential Hyperpolarizations –100 1 5 2 3 4 0 Time (msec) (a) Graded hyperpolarizations

21. Other stimuli trigger a depolarization, a reduction in the magnitude of the membrane potential • For example, depolarization occurs if gated Na+ channels open and Na+ diffuses into the cell • Graded potentials are changes in polarization where the magnitude of the change varies with the strength of the stimulus

22. Fig. 48-9b Stimuli +50 0 Membrane potential (mV) Threshold –50 Resting potential Depolarizations –100 0 1 5 2 3 4 Time (msec) (b) Graded depolarizations

23. Production of Action Potentials • Voltage-gatedNa+ and K+ channels respond to a change in membrane potential • When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell • The movement of Na+ into the cell increases the depolarization and causes even more Na+ channels to open • A strong stimulus results in a massive change in membrane voltage called an action potential

24. Fig. 48-9c Strong depolarizing stimulus +50 Action potential 0 Membrane potential (mV) –50 Threshold Resting potential –100 0 2 4 5 6 1 3 Time (msec) (c) Action potential

25. An action potential occurs if a stimulus causes the membrane voltage to cross a particular threshold • An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane • Action potentials are signals that carry information along axons

26. Generation of Action Potentials: A Closer Look • A neuron can produce hundreds of action potentials per second • The frequency of action potentials can reflect the strength of a stimulus • An action potential can be broken down into a series of stages

27. At resting potential • Most voltage-gated Na+ and K+ channels are closed, but some K+ channels (not voltage-gated) are open

28. Fig. 48-10-1 Key Na+ K+ +50 Action potential 3 0 Membrane potential (mV) 2 4 Threshold –50 1 1 5 Resting potential Depolarization –100 Time Extracellular fluid Sodium channel Potassium channel Plasma membrane Cytosol Inactivation loop Resting state 1

29. When an action potential is generated • Voltage-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

30. Fig. 48-10-2 Key Na+ K+ +50 Action potential 3 0 Membrane potential (mV) 2 4 Threshold –50 1 1 5 Resting potential Depolarization 2 –100 Time Extracellular fluid Sodium channel Potassium channel Plasma membrane Cytosol Inactivation loop Resting state 1

31. Fig. 48-10-3 Key Na+ K+ Rising phase of the action potential 3 +50 Action potential 3 0 Membrane potential (mV) 2 4 Threshold –50 1 1 5 Resting potential Depolarization 2 –100 Time Extracellular fluid Sodium channel Potassium channel Plasma membrane Cytosol Inactivation loop Resting state 1

32. Fig. 48-10-4 Key Na+ K+ Falling phase of the action potential 4 Rising phase of the action potential 3 +50 Action potential 3 0 Membrane potential (mV) 2 4 Threshold –50 1 1 5 Resting potential Depolarization 2 –100 Time Extracellular fluid Sodium channel Potassium channel Plasma membrane Cytosol Inactivation loop Resting state 1

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

34. Fig. 48-10-5 Key Na+ K+ Falling phase of the action potential 4 Rising phase of the action potential 3 +50 Action potential 3 0 Membrane potential (mV) 2 4 Threshold –50 1 1 5 Resting potential Depolarization 2 –100 Time Extracellular fluid Sodium channel Potassium channel Plasma membrane Cytosol Inactivation loop Undershoot 5 Resting state 1

35. During the refractory period after an action potential, a second action potential cannot be initiated • The refractory period is a result of a temporary inactivation of the Na+ channels BioFlix: How Neurons Work Animation: Action Potential

36. Conduction of Action Potentials • An action potential can travel long distances by regenerating itself along the axon • At the site where the action potential is generated, an electrical current depolarizes the neighboring region of the axon membrane

37. Inactivated Na+ channels behind the zone of depolarization prevent the action potential from traveling backwards • Action potentials travel in only one direction: toward the synaptic terminals

38. Fig. 48-11-1 Axon Plasma membrane Action potential Cytosol Na+

39. Fig. 48-11-2 Axon Plasma membrane Action potential Cytosol Na+ Action potential K+ Na+ K+

40. Fig. 48-11-3 Axon Plasma membrane Action potential Cytosol Na+ Action potential K+ Na+ K+ Action potential K+ Na+ K+

41. Conduction Speed • The speedof an action potential increases with the axon’s diameter • In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase • Myelin sheaths are made by glia— oligodendrocytesin the CNS and Schwann cellsin the PNS

42. Fig. 48-12 Node of Ranvier Layers of myelin Axon Schwann cell Schwann cell Nucleus of Schwann cell Nodes of Ranvier Axon Myelin sheath 0.1 µm

43. 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 jumpbetween the nodes of Ranvier in a process called saltatory conduction

44. Fig. 48-13 Schwann cell Depolarized region (node of Ranvier) Cell body Myelin sheath Axon

45. Concept 48.4: Neurons communicate with other cells at synapses • At electrical synapses, the electrical current flows from one neuron to another • At chemical synapses, a chemical neurotransmitter carries information across the gap junction • Most synapses are chemical synapses

46. Fig. 48-14 Postsynaptic neuron Synaptic terminals of pre- synaptic neurons 5 µm

47. The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicleslocated in the synaptic terminal • 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

48. Fig. 48-15 5 Na+ K+ Synaptic vesicles containing neurotransmitter Presynaptic membrane Voltage-gated Ca2+ channel Postsynaptic membrane Ca2+ 1 4 6 2 3 Synaptic cleft Ligand-gated ion channels

49. 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

50. 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