Understanding Neurons & Nervous Systems: Key Concepts and Functions

1. 34 Neurons, Sense Organs, and Nervous SystemsPart I

2. Chapter 34 Neurons, Sense Organs, and Nervous Systems • Key Concepts • 34.1 Nervous Systems Are Composed of Neurons and Glial Cells • 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • 34.3 Neurons Communicate with Other Cells at Synapses

3. Chapter 34 Neurons, Sense Organs, and Nervous Systems • Key Concepts • 34.4 Sensory Processes Provide Information on an Animal’s External Environment and Internal Status • 34.5Neurons Are Organized into Nervous Systems

4. Chapter 34 Opening Question • How might the star-nosed mole’s brain be specialized to process information from its nose?

5. Concept 34.1 Nervous Systems Are Composed of Neurons and Glial Cells • Animals need a way to transmit signals at high speeds from place to place within their bodies (e.g., to avoid danger). • Mammalian neurons transmit signals at 20–100 meters per second.

6. Figure 34.1 Banjo Plucking Dramatizes the Speed of Neuronal Signaling

7. Concept 34.1 Nervous Systems Are Composed of Neurons and Glial Cells • Neurons are excitable cells—they can generate and transmit electrical signals, called action potentials. • Cell membranes ordinarily have electrical polarity: the outside is more positive than the inside. • An impulse, or action potential, is a state of reversed polarity.

8. Concept 34.1 Nervous Systems Are Composed of Neurons and Glial Cells • In an excitable cell, an action potential generated at one point propagates over the whole membrane. • The region of depolarization moves along the cell membrane, and the membrane is said to “conduct” the impulse.

9. Concept 34.1 Nervous Systems Are Composed of Neurons and Glial Cells

10. Concept 34.1 Nervous Systems Are Composed of Neurons and Glial Cells • Neurons (nerve cells) are specially adapted to generate electric signals, usually in the form of action potentials. • Neurons must make contact with target cells. • Synapse: cell-to-cell contact point specialized for signal transmission • A signal arrives at the synapse by way of the presynaptic cell and leaves by way of the postsynaptic cell.

11. Concept 34.1 Nervous Systems Are Composed of Neurons and Glial Cells • Most neurons have four regions: • Dendrites—carry signals to the cell body • Cell body—contains nucleus and organelles • Axon—conducts action potentialsawayfrom the cell body; can be very long • Presynaptic axon terminals—make contact with other cells

12. Figure 34.2 A Generalized Neuron

13. Concept 34.1 Nervous Systems Are Composed of Neurons and Glial Cells • A neuron is said to innervate the cells that the axon terminals contact. • Axons of many neurons often travel together in bundles called nerves. • Nerve refers only to axon bundles outside the brain and spinal cord. • In the brain or spinal cord they are called tracts.

14. Concept 34.1 Nervous Systems Are Composed of Neurons and Glial Cells • Glial cells, or glia,are not excitable. They have several functions: • Help orient neurons toward their target cells during embryonic development • Provide metabolic support for neurons • Help regulate composition of extracellular fluids and perform immune functions • Assist signal transmission across synapses

15. Concept 34.1 Nervous Systems Are Composed of Neurons and Glial Cells • Oligodendrocytes are glia that insulate axons in the brain and spinal cord. • Schwann cells insulate axons in nerves outside of these areas. • The glial membranes form a nonconductive sheath called myelin. • Myelin-coated axons are white matter and areas of cell bodies are gray matter.

16. Figure 34.3 Electrically Insulating an Axon

17. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Current: flow of electric charges from place to place; in cells, current is based on flow of ions such as Na+ • Voltage, or electrical potential difference exists if positive charges are concentrated in one place and negative charges are concentrated in a different place. • Voltages produce currents because opposite charges attract and will move toward one another if given a chance.

18. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • No voltage differences exist within open solutions such as the intracellular fluids. • Voltage differences exist only across membranes such as the cell membrane.

19. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions

20. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • The voltage across a membrane is called membrane potential and is easily measured. • Resting neuron: membrane potential is the resting potential, typically –60 to –70 millivolts (mV) • Negative sign means the inside of the cell is electrically negative relative to the outside.

21. Figure 34.4 Measuring the Membrane Potential

22. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Membrane potential can change rapidly, and only relatively small numbers of positive charges need to move through the membrane for this change of membrane potential to occur. • Composition of the bulk solutions (the intra- and extracellular fluids) does not change.

23. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Ion redistribution occurs through membrane channel proteins and ion transporters in the membrane. • Sodium–potassium pump—uses energy from ATP to move 3 Na+ ions to the outside and 2 K+ to the inside; establishes concentration gradients of these ions

24. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Potassium channels are open in the resting membrane. • K+ ions diffuse out of the cell through leak channels and leave behind negative charges within the cell. • K+ ions diffuse back into the cell because of the negative electrical potential. • At this equilibrium point, there is no net movement of K+; called the equilibrium potential of K+.

25. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Diffusion of ions is controlled by concentration effect and electrical effect. When they are equal, electrochemical equilibriumis reached.

26. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Electrochemical equilibrium is called the equilibrium potential of the ion, calculated by the Nernst equation:

27. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Most ion channels are “gated”—they open and close under certain conditions. Most are closed in a resting neuron, which is why K+ leak channels determine resting membrane potential. • Voltage-gated channels open or close in response to changes in membrane potential • Stretch-gated channels respond to tension applied to cell membrane

28. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Ligand-gated channels open or close when a specific chemical (ligand) binds to the channel protein.

29. Figure 34.5 Three Types of Gated Ion Channels (Part 1)

30. Figure 34.5 Three Types of Gated Ion Channels (Part 2)

31. Figure 34.5 Three Types of Gated Ion Channels (Part 3)

32. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Opening and closing gated channels can alter membrane potential. • If Na+ channels open, Na+ diffuses into the neuron because it is more concentrated outside the cell, and the cell membrane is more negative on the inside. • When membrane becomes less negative on the inside, the membrane is depolarized.

33. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • The membrane is hyperpolarized if the charge on the inside becomes more negative.

34. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Membrane potential can be graded or all-or-none. • Graded membrane potentials are changes from the resting potential that are less than the threshold of –50 mV. • Graded means any value of the membrane potential is possible • Caused by various ion channels opening or closing

35. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Graded membrane potentials spread only a short distance.

36. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • If neuron depolarizes to the –50 mV threshold, an all-or-none event occurs: an action potential is generated. • Action potentials are not graded (always the same size) and do not become smaller, they stay the same in size as they propagate along the cell membrane.

37. Figure 34.6 Graded and All-or-None Changes in Membrane Potential

38. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Graded changes can give rise to all-or-none changes by being summed together; provides a mechanism for integrating signals. • A key area for this integration is the axon hillock, where action potentials are most often generated. • Graded changes resulting from multiple signals reaching the dendrites, spread to the axon hillock, where all the depolarizations and hyperpolarizations sum.

39. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • An action potential (nerve impulse) is a rapid, large change in membrane potential that reverses membrane polarity. • The membrane depolarizes from –65 mV at rest to about +40 mV (depolarization). • It is localized and brief but is propagated with no loss of size—an action potential at one location causes currents to flow that depolarize neighboring regions.

40. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • When membrane potential reaches threshold, many voltage-gated Na+ channels open quickly, and Na+ rushes into the axon. • The influx of positive ions causes more depolarization, and an action potential occurs.

41. Figure 34.7 Production of an Action Potential (Part 1)

42. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • The axon quickly returns to resting potential: • Voltage-gated Na+ channels close • Voltage-gated K+ channels open slowly and stay open longer—K+ moves out

43. Figure 34.7 Production of an Action Potential (Part 2)

44. Figure 34.7 Production of an Action Potential (Part 3)

45. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Positive feedback during depolarization: • When the membrane is partially depolarized, some Na+ channels open; as Na+ starts to diffuse into the cell, more depolarization occurs, opening more channels. • This continues until all voltage-gated Na+ channels open and maximum depolarization occurs.

46. Figure 34.8 Positive Feedback Plays a Key Role in the Generation of an Action Potential

47. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Action potential travels in only one direction: • After the action potential, Na+ channels cannot open again for a brief period (refractory period) and cannot depolarize. • Thus the action potential can only propagate in the direction of the axon terminals.

48. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions • Action potentials travel faster in larger diameter axons. • Myelination by glial cells also increases speed of action potentials. • The nodes of Ranvier are gaps where the axon is not covered by myelin. • Action potentials are generated only at the nodes and jump from node to node (saltatory conduction).

49. Concept 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions

50. Concept 34.3 Neurons Communicate with Other Cells at Synapses • Neurons communicate with other neurons or target cells at synapses. • Chemical synapse: a very narrow space between cells (synaptic cleft) that an action potential cannot cross • When an action potential arrives at the end of the presynaptic cell, a neurotransmitter is released that diffuses across the space.