Chapter 48

1. Chapter 48 Nervous Systems

2. Nerve net Figure 48.2a (a) Hydra (cnidarian) Organization of Nervous Systems • The simplest animals with nervous systems, the cnidarians • Have neurons arranged in nerve nets

3. Eyespot Brain Nerve cord Transversenerve Figure 48.2c (c) Planarian (flatworm) • In relatively simple cephalized animals, such as flatworms • A central nervous system (CNS) is evident

4. Brain Sensoryganglion Spinalcord (dorsalnerve cord) Figure 48.2h (h) Salamander (chordate) • In vertebrates • The central nervous system consists of a brain and dorsal spinal cord • The PNS connects to the CNS

5. Sensory input Integration Sensor Motor output Effector Central nervoussystem (CNS) Peripheral nervoussystem (PNS) Figure 48.3 Information Processing • Nervous systems process information in three stages • Sensory input, integration, and motor output

6. 2 6 5 4 1 3 Gray matter Sensory neurons convey the information to the spinal cord. Sensors detect a sudden stretch in the quadriceps. The sensory neurons communicate with motor neurons that supply the quadriceps. The motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward. Cell body of sensory neuronin dorsal root ganglion Sensory neurons from the quadriceps also communicate with interneuronsin the spinal cord. Quadricepsmuscle Hamstringmuscle White matter The interneurons inhibit motor neurons that supply the hamstring (flexor) muscle. This inhibition prevents the hamstring from contracting, which would resist the action of the quadriceps. Spinal cord(cross section) Sensory neuron Motor neuron The reflex is initiated by tapping the tendon connected to the quadriceps (extensor) muscle. Interneuron Figure 48.4 • The three stages of information processing • Are illustrated in the knee-jerk reflex (monosynaptic)

7. Dendrites Cell body Nucleus Synapse Signal direction Axon Axon hillock Presynaptic cell Postsynaptic cell Myelin sheath Synapticterminals Figure 48.5 Neuron Structure • Most of a neuron’s organelles • Are located in the cell body

8. Supporting Cells (Glia) • Glia are supporting cells • That are essential for the structural integrity of the nervous system and for the normal functioning of neurons

9. 50 µm Figure 48.7 • In the CNS, astrocytes • Provide structural support for neurons and regulate the extracellular concentrations of ions and neurotransmitters

10. Node of Ranvier Layers of myelin Axon Schwann cell Schwann cell Nodes of Ranvier Nucleus of Schwann cell Axon Myelin sheath 0.1 µm Figure 48.8 • Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) • Are glia that form the myelin sheaths around the axons of many vertebrate neurons

11. Concept 48.2: Ion pumps and ion channels maintain the resting potential of a neuron • Across its plasma membrane, every cell has a voltage • Called a membrane potential • The inside of a cell is negative • Relative to the outside • The resting potential • Is the membrane potential of a neuron that is not transmitting signals

12. TECHNIQUE APPLICATION Electrophysiologists use intracellular recording to measure the membrane potential of neurons and other cells. A microelectrode is made from a glass capillary tube filled with an electrically conductive salt solution. One end of the tube tapers to an extremely fine tip (diameter < 1 µm). While looking through a microscope, the experimenter uses a micropositioner to insert the tip of the microelectrode into a cell. A voltage recorder (usually an oscilloscope or a computer-based system) measures the voltage between the microelectrode tip inside the cell and a reference electrode placed in the solution outside the cell. Microelectrode –70 mV Voltage recorder Referenceelectrode Figure 48.9 • The membrane potential of a cell can be measured

13. Inner chamber Outer chamber Inner chamber Outer chamber –92 mV +62 mV + – + – 150 mMNaCl 150 mMKCL 5 mMKCL 15 mMNaCl Cl– + – + – K+ Na+ Cl– + – Potassiumchannel Sodium channel + – Artificialmembrane (b) Membrane selectively permeable to Na+ Figure 48.11a, b (a) Membrane selectively permeable to K+ • The concentration of Na+ is higher in the extracellular fluid than in the cytosol • While the opposite is true for K+

14. A neuron that is not transmitting signals • Contains many open K+ channels and fewer open Na+ channels in its plasma membrane • The diffusion of K+ and Na+ through these channels • Leads to a separation of charges across the membrane, producing the resting potential

15. Stimuli +50 0 Membrane potential (mV) Threshold –50 Restingpotential Hyperpolarizations –100 0 1 2 3 4 5 Time (msec) (a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K+. The larger stimulus producesa larger hyperpolarization. Figure 48.12a • Some stimuli trigger a hyperpolarization • An increase in the magnitude of the membrane potential • Harder to fire impulse • Further from threshold

16. Stimuli +50 0 Membrane potential (mV) –50 Threshold Restingpotential Depolarizations –100 0 1 2 3 4 5 Time (msec) (b) Graded depolarizations produced by two stimuli that increase membrane permeability to Na+.The larger stimulus produces alarger depolarization. Figure 48.12b • Other stimuli trigger a depolarization • A reduction in the magnitude of the membrane potential

17. Hyperpolarization and depolarization • Are both called graded potentials because the magnitude of the change in membrane potential varies with the strength of the stimulus

18. Stronger depolarizing stimulus +50 Actionpotential 0 Membrane potential (mV) Threshold –50 Restingpotential –100 0123456 Time (msec) (c) Action potential triggered by a depolarization that reaches the threshold. Figure 48.12c • A stimulus strong enough to produce a depolarization that reaches the threshold • Triggers a different type of response, called an action potential • all-or-none depolarization

19. Both voltage-gated Na+ channels and voltage-gated K+ channels • Are involved in the production of an action potential • When a stimulus depolarizes the membrane • Na+ channels open, allowing Na+ to diffuse into the cell

20. As the action potential subsides • K+ channels open, and K+ flows out of the cell • A refractory period (undershoot) follows the action potential (brief hyperpolarization) • During which a second action potential cannot be initiated

21. –  – –  – –  – –  – +  + +  + +  + +  + +  + +  + +  + +  + –  – –  – –  – –  – 3 4 Falling phase of the action potential 3 +  + +  + +  + +  + 2 4 –  – –  – –  – –  – 5 1 1 Depolarization 2 +  + +  + +  + +  + Activationgates Extracellular fluid Potassiumchannel –  – –  – –  – –  – +  + Plasma membrane –  – 5 Inactivationgate Resting state 1 • The generation of an action potential Na+ Na+ Na+ Na+ K+ K+ Rising phase of the action potential Depolarization opens the activation gates on most Na+ channels, while the K+ channels’ activation gates remain closed. Na+ influx makes the inside of the membrane positive with respect to the outside. The inactivation gates on most Na+ channels close, blocking Na+ influx. The activation gates on mostK+ channels open, permitting K+ effluxwhich again makesthe inside of the cell negative. +50 Actionpotential Na+ Na+ 0 Membrane potential (mV) Threshold Threshold –50 K+ Resting potential –100 Time A stimulus opens the activation gates on some Na+ channels. Na+ influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential. Na+ Na+ Na+ +  +  +  +  +  +  +  + +  + +  + K+ –  –  –  –  –  –  –  – –  – –  – Undershoot Both gates of the Na+ channelsare closed, but the activation gates on some K+channels are still open. As these gates close onmost K+ channels, and the inactivation gates open on Na+ channels, the membrane returns toits resting state. Cytosol Sodiumchannel K+ The activation gates on the Na+ and K+ channelsare closed, and the membrane’s resting potential is maintained. Figure 48.13

22. Conduction of Action Potentials • An action potential can travel long distances • By regenerating itself along the axon

23. – – + + + + + + – – + + + + + + Axon Actionpotential An action potential is generated as Na+ flows inward across the membrane at one location. 1 + + – – – – – – Na+ – – – – – – + + – – + + + + + + Actionpotential The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward. 2 K+ – – + – – – + – Na+ – – – – – – + + – – + + + + + + K+ Actionpotential The depolarization-repolarization process isrepeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon. 3 K+ – – – – + + + + – + + + + – – – Na+ – – – + + – + + Figure 48.14 – + + – – – + + K+ • At the site where the action potential is generated, usually the axon hillock • An electrical current depolarizes the neighboring region of the axon membrane

24. Conduction Speed • The speed of an action potential • Increases with the diameter of an axon • In vertebrates, axons are myelinated • Also causing the speed of an action potential to increase

25. Schwann cell Depolarized region(node of Ranvier) Myelin sheath – –– – – – ++ + Cell body ++ ++ + Axon – – – ++ + – – – Figure 48.15 • Action potentials in myelinated axons • Jump between the nodes of Ranvier in a process called saltatory conduction

26. Concept 48.4: Neurons communicate with other cells at synapses • In an electrical synapse • Electrical current flows directly from one cell to another via a gap junction • The vast majority of synapses however • Are chemical synapses

27. Postsynapticneuron Synapticterminalof presynapticneurons 5 µm Figure 48.16 • In a chemical synapse, a presynaptic neuron • Releases chemical neurotransmitters, which are stored in the synaptic terminal

28. Postsynaptic cell Presynapticcell Na+ Synaptic vesiclescontainingneurotransmitter Neuro-transmitter K+ Presynapticmembrane Postsynaptic membrane Ligand-gatedion channel Voltage-gatedCa2+ channel Ca2+ Postsynaptic membrane 3 Synaptic cleft Ligand-gatedion channels 5 4 6 1 2 Figure 48.17 • When an action potential reaches a terminal • Ca++ enters promoting vesicle fusion and NT release • The final result is the release of neurotransmitters into the synaptic cleft (gap)

29. Direct Synaptic Transmission • The process of direct synaptic transmission • Involves the binding of neurotransmitters to ligand-gated ion channels • Neurotransmitter binding • Causes the ion channels to open, generating a postsynaptic potential

30. After its release, the neurotransmitter • Diffuses out of the synaptic cleft • May be taken up by surrounding cells and degraded by enzymes • Unlike action potentials • Postsynaptic potentials are graded and do not regenerate themselves

31. Postsynaptic potentials fall into two categories • Excitatory postsynaptic potentials (EPSPs) • Inhibitory postsynaptic potentials (IPSPs)

32. Terminal branch of presynaptic neuron Postsynaptic neuron E1 Threshold of axon of postsynaptic neuron 0 Restingpotential Membrane potential (mV) –70 E1 E1 (a) Subthreshold, nosummation Figure 48.18a • Since most neurons have many synapses on their dendrites and cell body • A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron

33. E1 Axonhillock Actionpotential E1 E1 (b) Temporal summation Figure 48.18b • If two EPSPs are produced in rapid succession • An effect called temporal summation can occur

34. E1 E2 Actionpotential E1 + E2 (c) Spatial summation Figure 48.18c • In spatial summation • EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together

35. E1 I E1 I E1 + I (d) Spatial summationof EPSP and IPSP Figure 48.18d • Through summation • An IPSP can counter the effect of an EPSP

36. Indirect Synaptic Transmission • In indirect synaptic transmission • A neurotransmitter binds to a receptor that is not part of an ion channel • This binding activates a signal transduction pathway • Involving a second messenger in the postsynaptic cell, producing a slowly developing but long-lasting effect

37. Neurotransmitters • The same neurotransmitter • Can produce different effects in different types of cells

38. Table 48.1 • Major neurotransmitters

39. Acetylcholine • Acetylcholine • Is one of the most common neurotransmitters in both vertebrates and invertebrates • Can be inhibitory or excitatory (skeletal muscle)

40. Biogenic Amines • Biogenic amines • Include epinephrine, norepinephrine, dopamine, and serotonin • Are active in the CNS and PNS

41. Central nervous system (CNS) Peripheral nervous system (PNS) Brain Cranial nerves Spinal cord Ganglia outside CNS Spinal nerves Figure 48.19 • Concept 48.5: The vertebrate nervous system is regionally specialized • In all vertebrates, the nervous system • Shows a high degree of cephalization and distinct CNS and PNS components

42. The Peripheral Nervous System • The PNS transmits information to and from the CNS • And plays a large role in regulating a vertebrate’s movement and internal environment

43. Peripheral nervous system Somatic nervous system Autonomic nervous system Sympathetic division Parasympathetic division Enteric division Figure 48.21 • The PNS can be divided into two functional components • The somatic nervous system and the autonomic nervous system

44. The somatic nervous system • Carries signals to skeletal muscles • The autonomic nervous system • Regulates the internal environment, in an involuntary manner • Is divided into the sympathetic, parasympathetic, and enteric divisions

45. Parasympathetic division Sympathetic division Action on target organs: Action on target organs: Dilates pupil of eye Constricts pupil of eye Location of preganglionic neurons: brainstem and sacral segments of spinal cord Location of preganglionic neurons: thoracic and lumbar segments of spinal cord Inhibits salivary gland secretion Stimulates salivary gland secretion Sympathetic ganglia Neurotransmitter released by preganglionic neurons: acetylcholine Constricts bronchi in lungs Relaxes bronchi in lungs Neurotransmitter released by preganglionic neurons: acetylcholine Cervical Accelerates heart Slows heart Inhibits activity of stomach and intestines Thoracic Stimulates activity of stomach and intestines Location of postganglionic neurons: in ganglia close to or within target organs Location of postganglionic neurons: some in ganglia close to target organs; others in a chain of ganglia near spinal cord Inhibits activity of pancreas Stimulates activity of pancreas Stimulates glucose release from liver; inhibits gallbladder Stimulates gallbladder Lumbar Neurotransmitter released by postganglionic neurons: acetylcholine Neurotransmitter released by postganglionic neurons: norepinephrine Stimulates adrenal medulla Promotes emptying of bladder Inhibits emptying of bladder Promotes erection of genitalia Promotes ejaculation and vaginal contractions Sacral Synapse Figure 48.22 • The sympathetic and parasympathetic divisions • Have antagonistic effects on target organs

46. The sympathetic division • Correlates with the “fight-or-flight” response • The parasympathetic division • Promotes a return to self-maintenance functions • The enteric division • Controls the activity of the digestive tract, pancreas, and gallbladder

47. Forebrain Midbrain Hindbrain Midbrain Hindbrain Forebrain (a) Embryo at one month Embryonic Development of the Brain • In all vertebrates • The brain develops from three embryonic regions: the forebrain, the midbrain, and the hindbrain Embryonic brain regions Figure 48.23a

48. The Brainstem • The brainstem consists of three parts • The medulla oblongata, the pons, and the midbrain

49. The medulla oblongata • Contains centers that control several visceral functions • The pons • Also participates in visceral functions • The midbrain • Contains centers for the receipt and integration of several types of sensory information

50. Eye Input from ears Reticular formation Input from touch, pain, and temperature receptors Figure 48.24 Arousal and Sleep • A diffuse network of neurons called the reticular formation • Is present in the core of the brainstem