1. Neurons, Synapses, and Signaling CHAPTER 48

2. Figure 48.1 Overview of a vertebrate nervous system

3. NERVOUS SYSTEM Central nervous system (CNS) – brain and spinal cord Peripheral nervous system (PNS) – nerves that communicate motor and sensory signals between CNS and rest of body

4. NEURON Functional unit of nervous system Relatively large cell body Processes: –Dendrites – convey signals from tips to cell body; often branched –Axons – conduct signals away from body and toward tip; often single Myelin sheath – protective, insulating layer that covers many axons

5. Axon ends at synaptic terminals –Synapse – site of contact between synaptic terminal and target cell (neuron or effector cell – for example a muscle cell) –Neurotransmitter – chemical messengers between neurons and other cells

6. Figure 48.2 Structure of a vertebrate neuron

7. Figure 48.0 A neuron on a microprocessor

8. Figure 48.0x1 Aplysia neuron

9. Figure 48.5 Schwann cells

10. ORGANIZATION OF NEURONS Sensory neurons – communicate sensory information from eyes and other senses and internal conditions –Senses, blood pressure, muscle tension, CO 2 levels) Interneurons – integrate sensory input and motor output; communicate only between neurons; make up vast majority of brain neurons Motor neurons – convey impulses from CNS to effector cells (muscles and glands)

11. Ganglion – cluster of nerve cell bodies in PNS Nuclei – clusters of nerve cell bodies in brain –Both allow activities without entire nervous system involved –Knee-jerk reflex Reflex – automatic reaction to stimuli mediated by spinal cord and lower brain

12. Figure 48.3 The knee-jerk reflex

13. MEMBRANE POTENTIAL Voltage measured across the membrane (like a battery) Inside of cell more negative Typically –50 to –80 mV (resting potential) Sodium-potassium pump keeps ionic gradient (3Na + out, 2K + in)

14. Figure 8.15 The sodium-potassium pump: a specific case of active transport

15. Figure 48.6 Measuring membrane potentials

16. Figure 48.7 The basis of the membrane potential

17. Charges Across Membranes Neurons have ability to generate changes in their membrane potential Resting potential – membrane potential of cell at rest (-60mV to -80mV) Gated ion channels control membrane potential – open to different stimuli –Hyperpolarization – increase in electrical gradient Open K + channel (K + moves out) Cell becomes more negative No action potential because it makes it harder to depolarize

18. –Depolarization – decrease in electrical gradient Open Na + channel (Na + moves in) Cell becomes more positive Action potential generated if threshold is reached (-50mV to -55mV) –Massive change in voltage Threshold causes all-or-none event

19. Figure 48.8 Graded potentials and the action potential in a neuron

20. Figure 48.9 The role of voltage-gated ion channels in the action potential

21. ROLE OF GATED CHANNELS Depolarizing – Na + gates open rapidly so Na + moves into cell Repolarizing – K + gates finally open and K + moves out; Na + gates close Undershoot (Refractory Period) - K + still open (they are slower to close) and Na + still closed so cell becomes even more negative than resting and cannot be depolarized Stronger stimuli result in greater frequency of action potentials and NOT from stronger action potentials Propagation –Action potentials move in one direction due to refractory period

22. Propagation of the action potential Na + moves into cell starting action potential. Depolarization spreads and K + repolarizes initial area. Prevents action potential on that side.

23. Figure 48.11 Saltatory conduction Voltage leaps from node to node

24. SYNAPSES Presynaptic cell – transmitting cell Postsynaptic cell – receiving cell Two types of synapses –Electrical Need gap junctions (channels between neurons) No delays –Chemical Narrow gap, synaptic cleft, between cells More common than electrical in vertebrates and most invertebrates Require neurotransmitters (chemical intercellular messengers)

25. Depolarization of presynaptic membrane causes influx of Ca 2+ Increased Ca 2+ in cell causes synaptic vesicles to fuse to cell membrane and release neurotransmitters via exocytosis Neurotransmitters diffuse to postsynaptic cell Postsynaptic membrane has gated channels that open when neurotransmitters bond to specific receptors

26. Figure 48.12 A chemical synapse

27. A single neuron may receive many inputs simultaneously Neurotransmitters cause 2 different responses depending on the gates that are opened –Inhibitory (hyperpolarization) –Excitatory (depolarization) Neurotransmitters are quickly degraded Excitatory postsynaptic potential (EPSP) – Na + in and K + out = depolarization Inhibitory postsynaptic potential (IPSP) - K + out or CL - in = hyperpolarization

28. Figure 48.13 Integration of multiple synaptic inputs

29. Figure 48.14 Summation of postsynaptic potentials

30. NEUROTRANSMITTERS Acetylcholine –one of the most common –can excite skeletal muscle and inhibit cardiac muscle Epinephrine and norepinephrine –also function as hormones

31. Dopamine –Usually excitatory –Excess dopamine can cause schizophrenia –Lack of dopamine can cause Parkinson’s Sertonin –Usually inhibitory Endorphins –Natural painkillers (morphine and opium mimic endorphins shape) Nitric Oxide (NO) –Released during sexual arousal (increasing blood flow) –Nitroglycerin used to treat chest pain

32. Nervous System Chapter 49

33. NERVOUS SYSTEM Verebrate Nervous Systems –All have brain and spinal cord –Brain is integrative –White matter – axons with white meylin in CNS –Gray matter – dendrites and cell bodies in CNS

34. CNS The brain and spinal cord contain fluid filled spaces (called ventricles in the brain) The spinal cord is hollow and has a central canal

35. Figure 48.16 The nervous system of a vertebrate

36. Figure 48.16x Spinal cord

37. Cerebrospinal fluid –Fills canal and ventricles –Brain filtered blood –Contains nutrients and WBC –Circulated and eventually empties into veins –Major function is as a shock absorber

38. PNS –Cranial nerves – innervate organs of head and upper body –Spinal nerves – innervate entire body Mammals have 12 pairs of cranial and 31 pairs of spinal nerves –Hierarchy of PNS Afferent Division – convey info to CNS Efferent Division – convey info from CNS

39. Efferent Division Motor –controls skeletal muscles (voluntary) Autonomic – controls smooth and cardiac muscle (involuntary) Sympathetic – arousal and energy generation (flight or fight) Parasympathetic - calming and self- maintenance (rest and digest) Enteric – digestive tract, pancreas, and gall bladder

40. Functional hierarchy of the peripheral nervous system

41. Figure 48.18 The main roles of the parasympathetic and sympathetic nerves in regulating internal body functions

42. BRAIN Brainstem –Medulla oblongata – breathing, heart rate, swallowing, vomiting, digestion –Pons – breathing –Midbrain – receives sensory information

43. Figure 48.19 Embryonic development of the brain

44. Cerebellum –Coordination of movement, hand-eye coordination, learning and remembering Diencephalon –Hypothalamus, thalamus, and epithalamus Hypothalamus - regulates hunger, thirst, sexual response, mating behaviors, fight or flight, biological clock –Contains the Suprachiasmatic nuclei – make proteins in response to light/dark (biological clock)

45. Cerebrum –Most complex integration –Controls learning, emotion, memory, and perception –Divided into right and left hemispheres –Cerebral cortex Most complex, most evolved, and surface area is 0.5 m 2 which is ~80% of total brain mass –Corpus callosum – connects hemispheres

46. Figure 48.20 The main parts of the human brain

47. Figure 48.20x1 Cerebral cortex, gray and white matter

48. Neural Plasticity The overall organization of the brain is developed in the embryo; however neural plasticity can change Neural plasticity – ability of brain to be remodeled –Synaptic connections can increase or decrease –Formation of memory