Chapter 4: Communication: Chemical and Electrical Signaling

1. Chapter 4:Communication: Chemical and Electrical Signaling

2. Chapter Objectives • The Nature of Communication • Identify the four components required for communication. • Chemical Signaling • Explain the difference between paracrine factors, hormones, neurotransmitters, and neurohormones. • Name three types of cell membrane receptors, and explain how they enable signaling molecules to affect intracellular events without entering the cell. • Explain why every cell may encounter a chemical signal, but not every cell responds to the chemical signal. • List the steps in signaling by lipophilic molecules at intracellular receptors.

3. Chapter Objectives • Electrical Signaling • Define membrane potential, and explain the difference between a membrane potential of – 100 mV and a membrane potential of +30mV. • Explain why the resting membrane potential is negative. • Sketch a neuron, labeling the body, dendrites, axon, and myelin. • Compare and contrast action potentials and graded potentials. • Name the four phases of the action potential, and discuss the involvement of sodium and/or potassium channels in each phase.

4. Chapter Objectives • Electrical Signaling (cont’d) • Compare action potential propagation in myelinated and unmylelinated neurons. • Using the example of gamma aminobutyric acid (GABA), list all the events that occur at a chemical synapse. • Caffeine and Communication: The Case of Andy M. • Explain why Andy’s synaptic activity is reduced when he drinks decaffeinated coffee. In your explanation, use the following terms: caffeine, adenosine, first and secondmessengers, endogenous ligands, and receptor antagonists; also explain the importance of changes in receptor number and action potential thresholds.

5. Chapter Objectives • Caffeine and Communication: The Case of Andy M. (cont’d) • Explain why Andy’s synaptic activity is reduced when he drinks decaffeinated coffee. In your explanation, use the following terms: caffeine, adenosine, first and secondmessengers, endogenous ligands, and receptor antagonists; also explain the importance of changes in receptor number and action potential thresholds. • Using examples from the case study, discuss the roles of senders, signals, mediums, and receivers in person-to-person and cell-to-cell communication.

6. Communication is the transmission and reception of information by a signal. • Requires a signal, a signal, a medium, and a receiver The Nature of Communication Back to chapter objectives

7. The Nature of Communication (cont’d) Communication What is the medium? Back to chapter objectives

8. Body signals are chemical or electrical • Chemical signals are proteins, lipids, and gases • Electrical signals are changes in the balance of negative and positive ions. The Nature of Communication (cont’d) Back to chapter objectives

9. Communication is critical in homeostasis • All cells participate in homeostatic signal loops. • They transmit and receive signals that help maintain health. • All physiological conditions have a set point • When a cell’s sensor detects a deviation, it generates a signal asking for an opposing change. • Problems with communication can cause disease. The Nature of Communication (cont’d) Back to chapter objectives

10. If a dolphin sends a sound wave signal to another dolphin, what is the medium? The Nature of Communication (Review) Back to chapter objectives

11. Answer: water The Nature of Communication (Review) Back to chapter objectives

12. Chemical signals are molecules that serve bodily communication. • Chemical signals are also known as ligands. • Regardless of their structure, all share the same travel itinerary. • Release from a secreting cell • Travel to a target cell • Affect activity of target cell Chemical Signaling Back to chapter objectives

13. Decoders of chemical signals are cell receptors. • Chemical signals exert an effect only on cells that have the correct receptor. Chemical Signaling (cont’d) Back to chapter objectives

14. Chemical signals are classified according to sender and medium. • Hormones—travel through the bloodstream and act on distant cells • Paracrine factors—act on nearby cells Chemical Signaling (cont’d) Back to chapter objectives

15. Chemical signals vary according to their lipid solubility and binding site • Only hydrophobic signals (lipid-soluble) can cross the cell membrane. • Hydrophilic signals usually bind membrane receptors. Chemical Signaling (cont’d) Back to chapter objectives

16. Hydrophobic chemical signals bind to intracellular receptors • Most common are steroid hormones • Hormone crosses cell membrane by simple diffusion • Hormone binds to receptor • Hormone-receptor complex enters nucleus through nuclear pore • Hormone-receptor complex binds to regulatory region of gene • Ribosomes translate mRNA into a protein Chemical Signaling (cont’d) Back to chapter objectives

17. Chemical Signaling (cont’d) Signaling: intracellular receptors Back to chapter objectives

18. Hydrophilic signal molecules bind to receptors on the cell surface • Three categories of cell membrane receptors • Ligand-gated channel receptors • Enzyme-linked receptors • G-protein-linked receptors Chemical Signaling (cont’d) Back to chapter objectives

19. Chemical Signaling (cont’d) Signaling: Cell membrane receptors Back to chapter objectives

20. Ligand-gated channel receptors modify ion flux • Act as gates to allow ions to cross the cell membrane • Binding of the ligand opens or closes the channel and moderates the flow of ions in and out of the cell. • Frequently convert a chemical signal into an electrical signal Chemical Signaling (cont’d) Back to chapter objectives

21. Enzyme-linked receptors generate active enzymes • Ligand binding to the extracellular side if the receptor activates the intracellular enzymes, which then activates other enzymes. • Chain reaction eventually induces functional change in the cell Chemical Signaling (cont’d) Back to chapter objectives

22. G Protein receptors activate intracellular second messengers • In some reactions, a ligand is a first messenger, which activates a second messenger. • The second messenger is a small molecule that transmits a cell surface signal to sites of action in the cell. Chemical Signaling (cont’d) Back to chapter objectives

23. G Protein Receptors Activate Intracellular Second Messengers • G protein-coupled receptors (GPCRs) are a class of membrane receptors that use second messengers to propagate an extracellular message within the cell. • The ligand binds with the GPCR, which activates the G protein. • The G protein regulates the production of the second messenger Chemical Signaling (cont’d) Back to chapter objectives

24. G protein receptors activate intracellular second messengers • Many GPCRs use cAMP (cyclic AMP) as the second messenger. • The hormone activates the cAMP pathway when the body needs more glucose. Chemical Signaling (cont’d) Back to chapter objectives

25. Chemical Signaling (cont’d) Second messenger and amplification Back to chapter objectives

26. A particular ligand can bind to more than one type of receptor • Adenosine binds type II receptors on blood vessels • Increases cAMP production, widens the vessel • Adenosine binds type I receptors in the brain • Decreases cAMP production, reduces electrical activity in the nerve cell Chemical Signaling (cont’d) Back to chapter objectives

27. Receptor activity can be modified by agonists and antagonists • Naturally occurring ligand for a particular receptor is called an endogenous ligand • An exogenous ligand exists outside the body. • An agonist is a ligand that mimics the effect of an endogenous ligand. • Antagonists bind to a receptor site and block the binding site • They can also bind to a related receptor and decrease the effect of an endogenous ligand. Chemical Signaling (cont’d) Back to chapter objectives

28. Chemical Signaling (cont’d) Ligands, Agonists, and Antagonists Back to chapter objectives

29. Cells Can Vary the Number and Sensitivity of Their Receptors • If a ligand is present in excess for a long time, cells decrease their responsiveness by reducing the number of receptors • The cell may alter the receptor’s structure so that it responds less strongly to the ligand. Chemical Signaling (cont’d) Back to chapter objectives

30. What is the name of a chemical signal released from a gland into blood in order to influence cells in a distant part of the body? Chemical Signaling (Review) Back to chapter objectives

31. Answer: hormone Chemical Signaling (Review) Back to chapter objectives

32. The inside and outside of a cell membrane have different electrical charges. • Electrical signals travel along the cell membrane as disturbances of these charges. Electrical Signaling Back to chapter objectives

33. Neurons signal electrically • Basic structure of neurons • Cell body–main mass of the neuron • Dendrites– branched, cytoplasmic extensions; direct signals toward the cell body • Axon–single, usually long, extension from the cell body; directs signals away from the cell body • Myelin–covers axons; and accelerates signal transmission Electrical Signaling (cont’d) Back to chapter objectives

34. Electrical Signaling (cont’d) Basic structure of a neuron What covers the axon and aids in signal transmission? Back to chapter objectives

35. Ions carry electrical signals • Basic electrical principles • Excess of protons or electrons creates ions that are positively or negatively charged • Similar charges repel • Opposite charges attract • Energy required to keep opposite charges apart Electrical Signaling (cont’d) Back to chapter objectives

36. Electrical signaling relies on the electrical gradient • Electrical gradient exists in every cell • Also known as membrane potential • Exists at cell membranes • Source of potential energy • Overall electrical charge of the body is neutral Electrical Signaling (cont’d) Back to chapter objectives

37. Electrical signaling relies on the electrical gradient (cont’d) • Electrical gradient exerts force inside and outside the cell • Insulating lipids of the cell membrane prevents ions from rushing in and out of the cell. • Ion channels exist within the cell membrane to control ion passage in and out of the cell. Electrical Signaling (cont’d) Back to chapter objectives

38. Electrical signaling relies on the electrical gradient (cont’d) • The value of the membrane potential in nerve and skeletal cells “at rest” is -70mV. • This means the strength of the electrical gradient is 70 mV. • It also means the cytosol contains excessive anions. Electrical Signaling (cont’d) Back to chapter objectives

39. Electrical Signaling (cont’d) Membrane Potential Back to chapter objectives

40. Sodium and concentration gradients exist in every cell • Extracellular fluid contains high concentration of Na+ • Extracellular fluid contains low concentration of K+ • Intracellular fluid has a high concentration of K+ • Intracellular fluid has a low concentration of Na+ • Sodium flows down its concentration gradient into the cell. • Potassium flows down its concentration gradient out of the cell. Electrical Signaling (cont’d) Back to chapter objectives

41. Sodium and concentration gradients exist in every cell (cont’d) • Both gradients are created and maintained by active transport. • Negative ions play supporting roles. Electrical Signaling (cont’d) Back to chapter objectives

42. Ions move down gradients using ion channels • Leak channels—always open allowing ions to leak down their concentration or electrical gradients • Ligand-gated channels—open in response to a specific chemical signal • Voltage-gated channels—open in response to a change in the membrane potential Electrical Signaling (cont’d) Back to chapter objectives

43. The Resting Membrane Potential is Determined by Potassium • The resting membrane potential is based on two elements: • K+ is more concentrated outside the cell • K+ leak channels are always open Electrical Signaling (cont’d) Back to chapter objectives

44. A Cell’s Electrical Signal is a Wave of Change in the Membrane Potential • In cell signaling, the sodium and potassium concentration gradients do not change significantly. • The membrane potential does change. When it does it travels as an electrical signal. Electrical Signaling (cont’d) Back to chapter objectives

45. Changes in membrane potential are depolarization, hyperpolarization, and repolarization • Depolarization—a change that reduces the strength of the electrical gradient • Hyperpolarization—a change that increases the strength of an electrical gradient • Repolarization—returns the cell to its original resting membrane potential Electrical Signaling (cont’d) Back to chapter objectives

46. Electrical Signaling (cont’d) Membrane potential changes If the membrane potential moves from –70 mV to –65 mV, is the membrane hyperpolarized or depolarized? Back to chapter objectives

47. Graded potentials remain localized and vary in strength • Graded potentials — short-lived changes in membrane potential that work locally over a small region • Magnitude of individual graded potential depends on how many channels in the cell membrane are open • Graded potentials spread to neighboring regions because negative and positive ions attract each other Electrical Signaling (cont’d) Back to chapter objectives

48. Action potentials are large changes that can travel long distances • Action potential — large change in membrane potential that travels the length of the cell, no matter the distance • Axons can be several feet long Electrical Signaling (cont’d) Back to chapter objectives

49. Action potentials have four phases • Resting state — inside of cell is negatively charged • Development of graded potential – depolarization of a membrane segment causes some sodium channels to open • Depolarization – more voltage-gated sodium channels open, causing inside of cell to become positively charged; Potassium channels then begin to open as sodium channels close. • Repolarization – open potassium channels allow potassium ions to flow out of the cell, causing cell to return to resting state Electrical Signaling (cont’d) Back to chapter objectives

50. Four phases of action potentials Electrical Signaling (cont’d) Back to chapter objectives