3. Nurve Tissue Membrane Potential and Stimulus.pptx

Detail explanation of nervous system

1. Physiology of Nerve Tissue, Membrane Potential and Stimulus Arba Minch College of Medicine & Health Sciences Department of Pharmacy Medical Physiology Membrane Potential and Stimulus By Zelalem K (BSc; MSc in Medical Physiology)

2. Objectives: Apprehending the two classes of cells in the nervous system. Understanding how ion channels in the membrane of neurons generate Resting Membrane Potential. Perceiving the two signaling systems in the nervous system.

3. OUTLINE Introduction The neuron Ion channels Membrane potential Neural signaling The physiology of synaptic transmission

4. October 2023 Introduction I

5. Functions of NS Sensory detection Information processing Motor function… Nerve Physiology Introduction I 11/8/2023 5 Nerve Tissue, Membrane Potential and Stimulus

6. Human brain =  1.5kgs Neuron = 10 12 1 neuron forms = 10 3 synaptic junctions Possible synapses = 10 15 Volume of blood =  700ml/min ( 15% of CO ). O 2 consumption = 3.5ml/100g/min. ¿¿¿ Number of synapses Vs Intelligence??????…. I Nerve Physiology Introduction 11/8/2023 6 Nerve Tissue, Membrane Potential and Stimulus

7. The Physiology of Nerve Cells II Cells of the Nervous System October 2023

8. The Physiology of Nerve Cells The Neuron Part II The nervous system contains two main classes of cells: Neurons/nerve cells Glia/glial cells 11/8/2023 8 Nerve Tissue, Membrane Potential and Stimulus

9. The neuron a. Structural and functional unit of the NS. Wilhelm von Waldeyer coined the word ‘ neuron ’ ( 1891 ). Principle of dynamic polarization ( Cajal , 1911)- neurites of different neurons are not continuous with one another and must communicate by contact, not continuity Part II The Physiology of Nerve cells The Neuron 11/8/2023 9 Nerve Tissue, Membrane Potential and Stimulus

10. Structural components of a neuron Soma/cell body Neural processes ( neurites ) that extend from the cell body Dendrites Axon Part II The Physiology of Nerve cells The Neuron 11/8/2023 10 Nerve Tissue, Membrane Potential and Stimulus

11. The Physiology of Nerve cells The Neuron Part II 11/8/2023 11 Structural components of a neuron Nerve Tissue, Membrane Potential and Stimulus

12. The soma (= cell body ) Diameter =20 μm Gives rise to axons and dendrites Membranous organelles Mitochondria rER / Nissl bodies … ribosomes (protein synthesis) sER -without the ribosomes, regulate the concentrations of Ca + Golgi apparatus: sorting of proteins that are destined for delivery to different parts of the neuron, such as the axon and the dendrites. The Cytoskeleton Part II The Physiology of Nerve cells The Neuron 11/8/2023 12 Nerve Tissue, Membrane Potential and Stimulus

13. Functions of the cell body Protein synthesis Metabolic center of the neuron Synthesis of synaptic vesicle membrane components… Synthesis of enzymes ( choline acetyltransferase ) Synthesis of NTs (neuropeptides…) Reception + integration of input signals … Part II The Neurobiology of a Neuron The Neuron 11/8/2023 13 Nerve Tissue, Membrane Potential and Stimulus

14. b. Dendrites: tree of neurons I . Origin: soma Length ≤1mm ii. Components Voltage-gated Ca 2+ -Channels Voltage-gated Na + Channels Spines ( SA ) receive synaptic input Actin microfilaments (motile). Part II The Physiology of Nerve cells The Neuron 11/8/2023 14 Nerve Tissue, Membrane Potential and Stimulus

15. iii. Functions a. Receive the input signal from other neurons ( dendritic spines/tree ). As the antennae of the neuron, they are covered with thousands of synapses The dendritic membrane (the postsynaptic membrane) has many specialized protein molecules called receptors that detect the NTs in the synaptic cleft. Intelligence Vs. mental retardation with dendritic spines L oss of dendritic spines ; (aging, Down syndrome). b. Integration of the signal. Part II The Physiology of Nerve cells The Neuron 11/8/2023 15 Nerve Tissue, Membrane Potential and Stimulus

16. c. Axon Origin: soma Length: 0.1mm-2m long ONLY ONE/neuron θ = 0.2-20 μ m No rER , GA, ribosomes…Lysosomes… They have only sER (for Ca 2+ release), Microtubules The protein composition of the axon membrane is different from the soma membrane. ii. Special features: Axon hillock- axon origin—specialized for the initiation of AP Axon proper - the body of the axon- Voltage-gated Na + Channels Axon terminal - specialized to release NT on the arrival of an AP Myelin sheath- (+ axolemma + axoplasm) Nodes of ranvier : not covered by Myelin sheath Axon collaterals- side branches of axons Part II The Physiology of Nerve cells The Neuron 11/8/2023 16 Nerve Tissue, Membrane Potential and Stimulus

17. Part II The Physiology of Nerve cells The Neuron Axon terminal Swollen regions Axon comes in contact with other neurons (synapse) Cytoplasm of the axon terminal differs from the axon Microtubules do not extend. Terminal contains synaptic vesicles It has numerous mitochondria, indicating a high energy demand. 11/8/2023 17 Nerve Tissue, Membrane Potential and Stimulus

18. iii. Functions a . Initiation of AP at the axon hillock High density of voltage-gated ion channels of Na + , K + , Ca 2+  t hreshold. b. Impulse conduction : AP (100mV, 1 - 100m/s) Part II The Physiology of Nerve cells The Neuron 11/8/2023 18 Nerve Tissue, Membrane Potential and Stimulus

19. c. Axoplasmic Transport Materials enclosed within vesicles, which then “walk down” the microtubules of the the axon Occurs in the microtubules; ( ATP + Ca 2+ dependent) Purpose : Replacement of synaptic vesicles and enzymes responsible for NTs iii. Bidirectional : 1. Anterograde: Soma/Cell body  Nerve terminal The “legs” are provided by a protein called kinesin 2. Retrograde: Soma/Cell body  Nerve terminal The “legs” for retrograde transport are provided by a protein, dynein. Part II The Neurobiology of a Neuron The Neuron 11/8/2023 19 Nerve Tissue, Membrane Potential and Stimulus

20. 11/8/2023 20 A mechanism for the movement of material on the microtubules of the axon. Trapped in membrane enclosed vesicles, material is transported from the soma to the axon terminal by the action of the protein kinesin, which “walks” along microtubules at the expense of ATP.

21. . Classification of neurons 1. Based on function a. Sensory neurons (=afferent neurons) i. Transmit the information from receptors to the CNS . ii. Most have no dendrites (do not receive inputs from other neurons). iii. Cell body + long peripheral processes are outside the CNS . Only the short central process enters the CNS . v. Most of them are pseudo-unipolar The Physiology of Nerve cells The Neuron Part II 11/8/2023 21 Nerve Tissue, Membrane Potential and Stimulus

22. b. Motor neurons (= efferent neurons) Transmit information out of the CNS to effectors ( Muscles or glands ) Cell body + dendrites + small segment of the axon, in CNS. Most of the axon is outside the CNS. Part II The Physiology of Nerve cells The Neuron 11/8/2023 22 Nerve Tissue, Membrane Potential and Stimulus

23. c. Interneurons Function as integrators + signal changers (modulators or modifiers) Integrate groups of afferent + efferent neurons into reflex circuits. Lie entirely within CNS. Account for 99% of all neurons (A: E: I, 1:10:200,000). Part II The Physiology of Nerve cells The Neuron 11/8/2023 23 Nerve Tissue, Membrane Potential and Stimulus

24. 2. Based on the number of processes that originate from the cell body Unipolar neurons Single neurite Have a single primary process… Invertebrate nervous system In vertebrate: ANS (Dorsal root ganglia) Part II The Physiology of Nerve cells The Neuron 11/8/2023 24 Nerve Tissue, Membrane Potential and Stimulus

25. b. Bipolar neurons Two processes ( axon + dendrites ) Dendrite: conveys information from the periphery of the body. Axon: carries information toward the CNS. iii. Neurons of sensory cells : retina, auditory, vestibular, olfactory iv. Mechanoreceptors: touch, pressure, stretch and pain ( Pseudo-unipolar cells ) Part II The Physiology of Nerve cells The Neuron 11/8/2023 25 Nerve Tissue, Membrane Potential and Stimulus

26. d. Multipolar neurons Three or more neurites Predominate in the CNS . Single axon + many dendrites . Part II The Physiology of Nerve cells The Neuron 11/8/2023 26 Nerve Tissue, Membrane Potential and Stimulus

27. 11/8/2023 The Physiology of Nerve Cells 3. On the basis of electrical activity a. Silent Neurons Steady unchanging RMP in the absence of external stimulation. b. Pacing Neurons/Beating Neurons i . Fire repetitively at a constant frequency ( do not require continual synaptic activation or other external stimuli) . ii. External stimulation can change the firing rate of the cell or inhibit it altogether. The Physiology of Nerve Cells The Neuron Part II

28. 11/8/2023 The Physiology of Nerve Cells c. Bursting Neurons i . Fire spontaneously in the absence of external stimulation. ii. Do not fire at fixed regular intervals. iii. Generate regular bursts of action potentials that are separated by hyperpolarization of the membrane. iv. Significance: Generate rhythmic behaviors ( breathing, walking, swimming …) Secrete neurohormones (OXT, AVP) The Physiology of Nerve Cells The Neuron Part II

29. 11/8/2023 The Physiology of Nerve Cells 4. On the type of NT they synthesize and release Glutamatergic Cholinergic Adrenergic... 5. On the basis of their shape Pyramidal Granule Mitral... 6. On the basis of their location Cortical neurons Spinal neurons ... etc. 7. Projecting/Golgi type I Vs Intrinsic/Golgi type II The Physiology of Nerve Cells The Neuron Part II

30. Part II The Physiology of Nerve cells The Glia The glial cells The glia supporting cells of neuronal functions NOT directly involved in signal processing. Without glia, the brain could not function properly We have devoted most of our attention to the neurons, some neuroscientists consider glia to be the “sleeping giants” of neuroscience. 11/8/2023 30 The Glia Nerve Tissue, Membrane Potential and Stimulus

31. 11/8/2023 The Physiology of Nerve Cells Types: 1. Peripheral Schwann cells: Produce Myelin in PNS Myelin: Lipids (Cholesterol, Phospholipids, 70%) Proteins (Potent antigens, 30%) Envelope ONLY ONE AXON (Insulation→ speeds transmission of signals along nerves). b. Satellite cells: encapsulate dorsal root and cranial nerve ganglia. The Physiology of Nerve Cells The Neuroglia Part II

32. Part II The Physiology of Nerve cells The Glia Astrocytes Star-shaped The most numerous =abundant Versatile Fill the spaces between neurons that is not occupied by neurons and blood vessels 11/8/2023 32 Nerve Tissue, Membrane Potential and Stimulus Functions : Induce formation of the BBB Provide nourishment for neurons Interconnect blood vessels and nerve fibers Remove NTs from the synaptic cleft NT receptors, Neurogenesis, Detoxification Spatial buffering effect [K + ] Produce growth factors (BDNF, GDNF, FGF, PDGF, IGF-1, TGF, LIF…) 2. Central

33. Part II The Physiology of Nerve cells The Glia II. Oligodendrocytes Produce the myelin sheath which provides the electrical insulation for neurons in the CNS One cell form myelin sheath around many neurons. III. Polydendrocytes : stem cell pool within CNS (glial and neuronal cells). 11/8/2023 33 Nerve Tissue, Membrane Potential and Stimulus

34. IV. Ependymal cells Types: Ependymocytes, tanycytes , choroidal epithelial cells Line the ventricles in the brain They differentiate into choroid epithelial cells, forming part of the choroid plexus, which produces CSF Form the epithelium that separates CNS from CSF. Are ciliated; ciliary action helps circulate CSF Tanycytes : are specialized ependymal cells Transport substances between a blood vessel and a ventricle. The Physiology of Nerve cells The Glia Part II 11/8/2023 34 Nerve Tissue, Membrane Potential and Stimulus

35. VI. Microglia Scavenging: removing debris after infection, injury or neuronal death. Proliferating immune cells of CNS Contribute to BBB The Physiology of Nerve cells The Glia Part II 11/8/2023 35 Nerve Tissue, Membrane Potential and Stimulus

36. Ion Channels Basic Principles of Electrophysiology October 2023 III

37. Ion Channels Def .:  pores in the membrane to allow ion flow. Ion permeable pores whose rapid opening and closing mediate rapid and precise signaling in the nervous system. b. Signaling in neurons ( E m  RP, AP, SP) RP= Receptor potential SP= Sensory Potential The Physiology of Nerve Cells The Ion Channels Part III 11/8/2023 37 Nerve Tissue, Membrane Potential and Stimulus

38. b. Distribution Excitable cells : Neurons + muscle cells. c. Features : i. High permeation rate: 1000 x carrier systems ii. Passive: allow inorganic ions (Na + , K + , Ca 2+ and Cl - ). iii. Selective: ions of appropriate size and charge . iv. Conformation : fluctuate between open and closed states. v. Stimulus : electrical , chemical , thermal or mechanical . The Physiology of Nerve Cells The Ion Channels Part III 11/8/2023 38 Nerve Tissue, Membrane Potential and Stimulus

39. Mechanically-gated ion channels : Stretch-sensitive ion channels ( Cationic / anionic ) Voltage-gated ion channels Voltage-gated cationic channels (Na + , K + , Ca 2+ ) Voltage-gated anionic channels ( Cl -) Ligand-gated ion channels External ligand Internal ligand Excitatory cationic (Ach, Glutamate, Aspartate Inhibitory anionic (GABA, Glycine) Nucleotides Ions Gap junction channels d. Types : The Physiology of Nerve Cells The Ion Channels Part III 11/8/2023 39 Ion Channels Gated Leak/resting/ nongated (Na + , K + , Cl - ) Nerve Tissue, Membrane Potential and Stimulus

40. The Physiology of Nerve Cells The Ion Channels Part III A. A localized conformational change in one region Physical models for the opening and closing of ion channels 11/8/2023 40 Nerve Tissue, Membrane Potential and Stimulus

41. The Physiology of Nerve Cells The Ion Channels Part III B. Blocking particle 11/8/2023 41 Nerve Tissue, Membrane Potential and Stimulus

42. The Physiology of Nerve Cells The Ion Channels Part III A. Ligand gating Many types of stimuli control the opening & closing of ion channels. 11/8/2023 42 Nerve Tissue, Membrane Potential and Stimulus

43. The Physiology of Nerve Cells The Ion Channels Part III B. Phosphorylation gating 11/8/2023 43 Nerve Tissue, Membrane Potential and Stimulus

44. The Physiology of Nerve Cells The Ion Channels Part III c. Voltage gating 11/8/2023 44 Nerve Tissue, Membrane Potential and Stimulus

45. The Physiology of Nerve Cells The Ion Channels Part III d. Stretch/pressure gating 11/8/2023 45 Nerve Tissue, Membrane Potential and Stimulus

46. e. Functions: Cell type Functions ion channel i. Neuron & muscle cells Propagation of signals within a cell Ca 2+ -dependent transmitter release ii. Exocrine cells Regulation of Na + , K + , Cl - co-transporter iii. Endocrine cells Regulation of Ca 2+ - dep. hormone release iv. Oocytes Fast block polyspermy ???? The Physiology of Nerve Cells The Ion Channels Part III 11/8/2023 46 Nerve Tissue, Membrane Potential, and Stimulus

47. 47 How do ion channels discriminate different ions? Ionic diameter K + = 0.133nm Na + = 0.095nm 2. Cloud of water molecules Mobility of an ion in a solution  _________1__________ Size of ion + shell of H 2 O The Smaller the ion, the lower the mobility. Smaller the ion  the more highly localized its charge, the stronger its electrical field. Na + exerts a stronger attraction on its H 2 O of hydration The Physiology of Nerve Cells The Ion Channels Part III

48. 48 3. Channels have a selectivity filter/molecular sieve/recognition site Ions of appropriate size + charge. 4. Charge of inner lining of the pore i . Pore is lined with hydrophilic and charged amino acids. Cation channel : negative amino acids Anion channel: positive amino acids ii. Outer mouth of the pore: negatively charged carboxylic acid groups ( attracting cations , repelling anions ). The Physiology of Nerve Cells The Ion Channels Part III

49. 49 Na + channel : 2 gating particles a. m-gate : covers the EC side of the Na + channel. b. h-gate : covers the IC side of the Na + channel. m -gate open: activation of Na + channel h -gate close: inactivation of Na + channel c. Both m and h gates must be open for Na + to flow thru the Na + channel. The Physiology of Nerve Cells The Ion Channels Part IIII Closed/Activatable/Resting Open Closed/Inactivatable/Refractory

50. 50 K + channel : ONE gating particle : n-gate n -gate covers the IC side of the K + channel n -gate must be open for K + to flow thru the K + channel. n -gate open: activation of the K + channel K + channel does not have an inactivation gate. The Physiology of Nerve Cells The Ion Channels Part III

51. Membrane Potential and Stimulus October 2023 IV

52. Membrane Potential Def.  electrical charge difference between the inside and outside of the cell. E m = V in – V out , where V in = Potential on the inside of the cell V out = Potential on the outside E m = Membrane potential ( mV ) The Physiology of Nerve Cells The Membrane Potential Part IV 11/8/2023 52 Nerve Tissue, Membrane Potential, and Stimulus

53. All cells have membrane potentials . E m  charge separation across the membrane. The range of E m → -20 mV to -90mV Any change of a membrane’s permeability of ions causes a change in E m . The Physiology of Nerve Cells The Membrane Potential Part IV 11/8/2023 53 Nerve Tissue, Membrane Potential, and Stimulus

54. Resting Membrane Potential (RMP) Membrane potential at rest At rest, there is: Electro positivity out & Electro negativity inside the cell membrane of the neuron No net flow of ions across the PM →( influx = efflux ). The RMP is primarily determined by nongated ion channels . Always negative in nerve and muscle cells. The Physiology of Nerve Cells The Membrane Potential Part IV 11/8/2023 54 Nerve Tissue, Membrane Potential, and Stimulus

55. The Physiology of Nerve Cells The Membrane Potential Part IV Nonexcitable cells have RMP. RMP is necessary for the cell to fire an AP. RMP = E K 11/8/2023 55 Nerve Tissue, Membrane Potential, and Stimulus

56. The Physiology of Nerve Cells The Membrane Potential Part IV Measurement of the membrane potential of the nerve fiber using a microelectrode 11/8/2023 56 Nerve Tissue, Membrane Potential, and Stimulus

57. 57 What are the causes of the RMP? Outward diffusion of K + through K + leak channels. ECF is very high in Na + while ICF is very high in K + . As a result, K + is constantly leaking out of the cell . Na + /K + pump is constantly pumping 3 Na + ions out and 2 K + ions in for every ATP used. Thus more positive charge is leaving than entering. Protein anions Negatively charged proteins within the ICF that cannot travel through the PM. The Physiology of Nerve Cells The Membrane Potential Part IV 11/8/2023 Nerve Tissue, Membrane Potential, and Stimulus

58. Basic Electrophysiological Terms Excitability : the ability of the cell to generate the AP Excitable cells : cells that generate AP during excitation (muscle, nerve). Stimulus : a sudden change in the internal or external environmental condition of the cell. Threshold (intensity): the lowest or minimal intensity of the stimulus to elicit an AP. The Physiology of Nerve Cells The Membrane Potential Part IV 11/8/2023 58 Nerve Tissue, Membrane Potential, and Stimulus

59. The Physiology of Nerve Cells The Membrane Potential Part IV 11/8/2023 59 Polarization : a state in which membrane is polarized at rest, negative inside and positive outside . Depolarization : the membrane potential becomes less negative than the resting potential (-70 mV). Repolarization : when the membrane returns to the resting potential after depolarization. Hyperpolarization : membrane potential become more negative than the resting potential (-70 mV). Nerve Tissue, Membrane Potential, and Stimulus

60. Neural Signaling February 2023 Part V

61. 11/8/2023 61 . Types Neural Signaling: 1. Graded potentials 2. Action potentials 1. Graded potentials /depends up on grade of stimulus Def.:-  local changes in E m in either depolarizing or hyperpolarizing direction. The Physiology of Nerve Cells Neural Signaling Part V Nerve Tissue, Membrane Potential, and Stimulus

62. 11/8/2023 62 b. Features 1. Depolarizing or hyperpolarizing 2. Variable in amplitude and duration 3. Conducted decremental 4. Can be summed 5. Has no threshold 6. Has no refractory period 7. Propagation is passive The Physiology of Nerve Cells Neural Signaling Part V Nerve Tissue, Membrane Potential, and Stimulus

63. 11/8/2023 63 Graded Potentials Receptor Potentials Pacemaker Potentials Synaptic Potentials Excitatory Postsynaptic Potentials/EPSP Inhibitory Postsynaptic Potentials/IPSP Endplate Potentials/EPP Miniature Endplate Potentials/MEPP c. Types : The Physiology of Nerve Cells Neural Signaling Part V Nerve Tissue, Membrane Potential, and Stimulus

64. 11/8/2023 64 2. Action Potentials / membrane potential in action Def :  rapid , transient , self-propagating electrical excitation in the PM of excitable cells. b. Genesis/Initiation  ΔE m  AP Sequential opening of voltage-gated channels of Na + and K + . The Physiology of Nerve Cells Neural Signaling Part V Nerve Tissue, Membrane Potential, and Stimulus

65. 11/8/2023 65 c. Features: 1. All-or-none phenomenon * . Threshold depolarization  AP Supra-threshold depolarization  AP Stereotypic Sub-threshold  no AP 2. Has threshold * . 3. Amplitude and duration is κ 4. Always depolarizing. 5. Has refractory period * . 6. Nondecremental * . 7. Propagation is active. The Physiology of Nerve Cells Neural Signaling Part V Nerve Tissue, Membrane Potential, and Stimulus

66. 11/8/2023 66 d. Phases and ionic basis of AP 1. Threshold Potential Minimum value of E m at which an AP will occur. Potential at which all voltage-gated sodium channels opens AP occurs only when the NET movement of the positive charge is inward ( Na + influx > K + efflux ). The Physiology of Nerve Cells Neural Signaling Part V Nerve Tissue, Membrane Potential, and Stimulus

67. 11/8/2023 67 2. Depolarization Phase/ upstroke ↑ flow of Na + into the cell The Physiology of Nerve Cells Neuronal Signaling Part V Nerve Tissue, Membrane Potential, and Stimulus

68. 11/8/2023 68 3. Overshoot/peak of the AP   Portion of the AP during which the membrane is positive/reversed. ii. Magnitude: 30 to 40mV iii. Approaching E Na + The Physiology of Nerve Cells Neural Signaling Part V Nerve Tissue, Membrane Potential, and Stimulus

69. 11/8/2023 69 4. Repolarization/Downstroke i. Rapid return of the membrane towards its RMP. ii. Reasons: a. Time-limited nature of Na + permeability (g Na + is short-lived) b. ↑gK + (delayed opening of K + channels, n-gates  open, activated) The Physiology of Nerve Cells Neural Signaling Part V Nerve Tissue, Membrane Potential, and Stimulus

70. 11/8/2023 70 5. Afterpotentials/undershoot Membrane becomes more negative than its RMP at the end of the AP Hyperpolarization afterpotential (-70mV -72/-73mV, 40ms) n gates opened  ↑K + out of the cell  ↑ inside negativity   excitability. The Physiology of Nerve Cells Neural Signaling Part V Nerve Tissue, Membrane Potential, and Stimulus

71. 11/8/2023 71 e. Refractory period Def.  an interval during which it is more difficult to elicit an AP. ( ∵ Period of decreased excitability following AP ) . ii. Types: a. Absolute refractory period b. Relative refractory period The Physiology of Nerve Cells Neural Signaling Part V Nerve Tissue, Membrane Potential, and Stimulus

72. 11/8/2023 72 a. Absolute Refractory Period : Another AP can not be elicited, regardless of the strength of the stimulus. A Na + channel cannot be involved in another AP until the inactivation gate has been reset. The Physiology of Nerve Cells Neural Signaling Part V Nerve Tissue, Membrane Potential, and Stimulus

73. 11/8/2023 73 b. Relative Refractory Period Follows absolute refractory period. A second AP can be elicited if the stimulus is stronger stimulus . The Na + channels are still inactivated. The membrane potential stays hyperpolarized until the voltage-gated K channels close. Therefore, more depolarizing current is required to bring the membrane potential to threshold. Rationale: faithfulness! i. Ensures ONLY one-way of propagation of APs along an axon. ii. Imposes a limit on the maximum rate a neuron can fire. iii. Prevents APs from summating. The Physiology of Nerve Cells Neural Signaling Part V Nerve Tissue, Membrane Potential, and Stimulus

74. 11/8/2023 74 Propagation of AP If an AP is generated at the axon hillock, it will travel all the way down to the synaptic knob. The way it travels depends on whether the neuron is myelinated or unmyelinated. Types Continuous conduction Occurs in unmyelinated axons. The whole length of the membrane is depolarized Velocity of conduction is slow Consumes a large amount of energy The Physiology of Nerve Cells Neural Signaling Part V Nerve Tissue, Membrane Potential, and Stimulus

75. ii. Saltatory Conduction Occurs in myelinated nerve Recall that the myelin sheath is not complete. There exist myelin free regions along the axon, the nodes of Ranvier . Velocity is faster Economizes ATP The Physiology of Nerve Cells Neuronal Signaling Part V 11/8/2023 75 Nerve Tissue, Membrane Potential, and Stimulus

76. The Physiology of Synaptic Transmission Part VI February 2023

77. The Physiology of Synaptic Transmission Synaptic transmission : Communication among neurons, muscles and glands. Synapse: A site at which an impulse is transmitted from one cell to another. The Physiology of Synaptic Transmission The Synaptic transmission Part VI 11/8/2023 77 Nerve Tissue, Membrane Potential, and Stimulus

78. Components Terminals of presynaptic axon Receptor on the postsynaptic cell Zone of apposition/synaptic cleft The Physiology of Synaptic Transmission The synaptic transmission Part VI 11/8/2023 78 Nerve Tissue, Membrane Potential, and Stimulus

79. Types (based on the nature of the apposition) Chemical Electrical The Physiology of Synaptic Transmission The synaptic transmission Part VI 11/8/2023 79 Nerve Tissue, Membrane Potential, and Stimulus

80. Chemical Synapses (On morphological basis…) Axodendritic synapse ( 98% ) Axosomatic ( 2% ) Axoaxonic Dendrodendritic Somasomatic Dendrosomatic The Physiology of Synaptic Transmission The Synaptic transmission Part VI 11/8/2023 80 Nerve Tissue, Membrane Potential, and Stimulus

81. Structural components of a chemical synapse I. Presynaptic terminal NT synthesizing enzymes, synaptic vesicle, reuptake transporters … II. Synaptic cleft Width: 30nm (x = 20-50nm ) ECF Basal lamina NT degrading enzymes III. Postsynaptic terminal Ionotropic receptors/directly-gated, receptor channels, ligand-gated channel Metabotropic receptors…indirectly-gated… The Physiology of Synaptic Transmission The chemical synapse Part VI 11/8/2023 81 Nerve Tissue, Membrane Potential, and Stimulus

82. The Physiology of Synaptic Transmission The chemical synapse Part VI The components of a chemical synapse. 11/8/2023 82 Nerve Tissue, Membrane Potential, and Stimulus

83. Characteristics of chemical neurotransmission (chemical synapse) Unidirectional/ anterograde transmission. Amount of NT release  frequency of stimulation. Transmitter inactivating enzymatic system in the synaptic region. Synaptic delay (0.5ms) The Physiology of Synaptic Transmission The chemical synapse Part VI 11/8/2023 83 Sequence of events at chemical synapses Nerve Tissue, Membrane Potential, and Stimulus

84. Electrical Synapses The presynaptic terminal and postsynaptic cell are very closely apposed by a structure called gap junctions/ gap junction channels. Provide instantaneous signal transmission (stereotyped). The channel allows ions to pass directly from the cytoplasm of one cell to the cytoplasm of the other. The pore of most gap junction channels is relatively large, diameter = 1–2 nm, big enough for all the major cellular ions, and many small organic molecules, to pass through. Found in neurons: Neuroendocrine cells of hypothalamus ( Magnocellular + Parvocellualr cells ) Lateral vestibular nucleus, inferior olive, cerebellum, neocortex Thalamus, striatum, hippocampus, retina, olfactory bulb The Physiology of Synaptic Transmission The electrical synapse Part VI 11/8/2023 84 Nerve Tissue, Membrane Potential, and Stimulus

85. Found in non-neuronal cells Neuroglial cells ( astrocytes, Schwann cells ) Myocardial cells Smooth muscle cells Epithelial cells + hepatocytes e. Do not generally allow inhibitory actions The Physiology of Synaptic Transmission The electrical synapse Part VI 11/8/2023 85 Morphology of a gap junction Nerve Tissue, Membrane Potential, and Stimulus

86. 11/8/2023 86 (b) An enlargement showing gap junction channels, which bridge the cytoplasm of the two cells. Ions and small molecules can pass in both directions through these channels. (c) Six connexin subunits comprise one connexon, two connexons comprise one gap junction channel, and many gap junction channels comprise one gap junction.

87. Characteristics of electrical synapses A ΔE m in one cell is transmitted to the other cell by the direct flow of current. b. No synaptic delay (direct interactions between neighboring cells). c. Allow conduction in both directions (information flow is bidirectional). The Physiology of Synaptic Transmission The e lectrical synapse Part VI 11/8/2023 87 Nerve Tissue, Membrane Potential, and Stimulus

88. Chemical Vs electrical synapses Property Chemical synapse Electrical synapse a. Distance between presyn - postsyn 30-50nm 3-4nm b. Cytoplasmic continuity No yes c. Ultrastructural components Presynaptic active zones, vesicles, postsynaptic receptor Gap junctions d. Agent of transmission Chemical transmission Ionic current e. Synaptic delay 0.3ms (1-5ms) Virtually Ø f. Direction of transmission Unidirectional Bidirectional The Physiology of Synaptic Transmission The c hemical Synapse Vs electrical Synapse Part VI 11/8/2023 88 Nerve Tissue, Membrane Potential, and Stimulus

89. THE END OF THE PHYSIOLOGY OF THE NERVE TISSUE!!