Nervous Tissue and Function

1. Nervous Tissue and Function Function of the Nervous System Organization (Structural and Functional) Supporting Cells of the Nervous System Anatomy of a Neuron Classification of Neurons by Function Graded and Action Potentials Myleination and MS Reflexes Synapses EPSPs and IPSPs Neurotransmitters

2. Functional Classification of the Peripheral Nervous System

3. Organization of the Nervous System and reflexes inhibitory stimulatory

4. Peripheral nervous system (PNS) Central nervous system (CNS) Cranial nerves and spinal nerves Brain and spinal cord Communication lines between the CNS and the rest of the body Integrative and control centers Sensory (afferent) division Motor (efferent) division Somatic and visceral sensory nerve fibers Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Conducts impulses from receptors to the CNS Somatic sensory fiber Autonomic nervous system (ANS) Somatic nervous system Skin Visceral motor (involuntary) Somatic motor (voluntary) Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Conducts impulses from the CNS to skeletal muscles Visceral sensory fiber Stomach Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Parasympathetic division Mobilizes body systems during activity Conserves energy Promotes house- keeping functions during rest Sympathetic motor fiber of ANS Heart Structure Function Sensory (afferent) division of PNS Bladder Parasympathetic motor fiber of ANS Motor (efferent) division of PNS Figure 11.2

5. Nervous Tissue and Function Function of the Nervous System Organization (Structural and Functional) Supporting Cells of the Nervous System Anatomy of a Neuron Classification of Neurons Neuron Function Graded and Action Potentials Myleination and MS Reflexes Synapses EPSPs and IPSPs Neurotransmitters

6. Nervous Tissue: Support Cells (Neuroglia) • Astrocytes • Abundant, star-shaped cells • Brace neurons • Form barrier between capillaries and neurons • Control the chemical environment of the brain • Help transfer nutrients between capillaries and neurons • Mop up and recapture potassium ions and neurotransmitters "Star cells connect and protect"

7. Nervous Tissue: Support Cells • Microglia • Spider-like phagocytes • Dispose of debris by phagocytosis • Monitor neuron health "Tiny garbage spiders"

8. Ependymal Cells (literally, “wrapping garment” cells) • Range in shape from squamous to columnar • May be ciliated • Line the central cavities of the brain and spinal column • Separate the CNS interstitial fluid from the cerebrospinal fluid in the cavities Fluid-filled cavity Ependymal cells Brain or spinal cord tissue Figure 11.3c

9. Nervous Tissue: Support Cells • Oligodendrocytes • Produce myelin sheath around nerve fibers in the central nervous system "Oligos insulate"

10. Nervous Tissue: Support Cells • Schwann cells • Form myelin sheath in the peripheral nervous system • Satellite cells • Surround neuron cell bodies in the PNS Satellite cells Cell body of neuron Schwann cells (forming myelin sheath) Nerve fiber

11. Nervous Tissue and Function Function of the Nervous System Organization (Structural and Functional) Supporting Cells of the Nervous System Anatomy of a Neuron Classification of Neurons Neuron Function Graded and Action Potentials Myleination and MS Reflexes Synapses EPSPs and IPSPs Neurotransmitters

12. Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Nucleolus Axon (impulse generating and conducting region) Impulse direction Nucleus Node of Ranvier Nissl bodies (Rough ER) Axon terminals (secretory region) Axon hillock Schwann cell (one inter- node) Neurilemma (b) Terminal branches Figure 11.4b

13. Neuron Cell Body Names and Locations

14. Axons and Connection(s) to Other Neurons Synapse

15. Nerve Fiber Coverings • In the PNS, Schwann cells produce myelin sheaths in jelly-roll like fashion • Nodes of Ranvier – gaps in myelin sheath along the axon • In the CNS, oligodendrocytes produce myelin sheaths and have no neurilemma (so cannot regenerate if damaged). • In multiple sclerosis (MS) the myelin sheaths are destroyed (autoimmunity) leading to poorly functioning muscles

16. Nervous Tissue and Function Function of the Nervous System Organization (Structural and Functional) Supporting Cells of the Nervous System Anatomy of a Neuron Classification of Neurons by Function Graded and Action Potentials Myleination and MS Reflexes Synapses EPSPs and IPSPs Neurotransmitters

17. Functional Classification of Neurons • Sensory (afferent) neurons • Carry impulses from the sensory receptors • Cell bodies outside the CNS • Cutaneous sense organs • Proprioceptors – detect stretch or tension From skin receptors, muscles and tendons • Motor (efferent) neurons • Carry impulses from the central nervous system • Cell bodies inside the CNS • Interneurons (association neurons) • Found in neural pathways in the central nervous system • Connect sensory and motor neurons • Cell bodies inside the CNS

18. Nervous Tissue and Function Function of the Nervous System Organization (Structural and Functional) Supporting Cells of the Nervous System Anatomy of a Neuron Classification of Neurons by Function Graded and Action Potentials Myleination and MS Reflexes Synapses EPSPs and IPSPs Neuraltransmitters

19. Principles of Electricity • Opposite charges attract each other • Energy is required to separate opposite charges across a membrane • Energy is liberated when the charges move toward one another • If opposite charges are separated, the system has potential energy

20. Definitions • Voltage (V): measure of potential energy generated by separated charges • Potential difference: the voltage or difference in charge measured between two points • Current (I): the flow of electrical charge (ions) between two points • Resistance (R): hindrance to charge flow (provided by the plasma membrane) • Insulator: substance with high electrical resistance • Conductor: substance with low electrical resistance • Intracellular fluid (ICF) - cytoplasm of neuron • Extracellular fluid (ECF) - fluid outside a neuron cell

21. Role of Membrane Ion Channels (Protein “Gates”) • Two main types of ion channels: • Leakage (nongated) channels —always open • Gated channels (three types): • Chemically gated (ligand-gated) channels—open with binding of a specific neurotransmitter • Voltage-gated channels —open and close in response to changes in membrane potential • Mechanically gated channels —open and close in response to physical deformation of receptors Receptor Neurotransmitter chemical attached to receptor Na+ Na+ Na+ Chemical binds K+ K+ Membrane voltage changes Closed Open Na+ Na+ Closed Open

22. Potential difference across the membrane of a resting cell (= Resting Potential) Voltmeter Ground electrode outside cell Plasma membrane Electrochemical gradient or potential difference is established by the powered pumping of more positive ions in ECF than ICF. Microelectrode inside cell Intracellular fluid (ICF) Extracellular fluid (ECF) Axon Neuron Figure 11.7

23. Resting Membrane Potential • Differences in ionic makeup • ICF has lower concentration of Na+ and Cl– than ECF • ICF has higher concentration of K+ and negatively charged proteins (A–) than ECF K+ Na+ Na+ Na+ Na+ Na+ Na+ Cl– Na+ Na+ ECF Cl– Na+ Na+ Na+ Na+ Na+ K+ K+ K+ K+ A- ICF K+ K+ A- Cl– A- • Differential permeability of membrane • Impermeable to A– • Slightly permeable to Na+ (through leakage channels) • 75 times more permeable to K+ (more leakage channels) • Freely permeable to Cl– Cl– Na+ ECF ICF K+ Cl– A- • Negative interior of the cell is due to much greater diffusion of K+ out of the cell than Na+ diffusion into the cell • Sodium-potassium pump stabilizes the resting membrane potential by maintaining the concentration gradients for Na+ and K+ + + + + + + + ECF + + + + + + + + + ICF K+

24. To Recap…. The concentrations of Na+ and K+ on each side of the membrane are different. Outside cell The Na+ concentration is higher outside the cell. Na+ (140 mM) K+ (5 mM) The K+ concentration is higher inside the cell. K+ (140 mM) Na+ (15 mM) Na+-K+ ATPases (pumps) maintain the concentration gradients of Na+ and K+ across the membrane. Inside cell The permeabilities of Na+ and K+ across the membrane are different. Suppose a cell has only K+ channels... K+ loss through abundant leakage channels establishes a negative membrane potential. K+ leakage channels K+ K+ Cell interior –90 mV K+ K+ Now, let’s add some Na+ channels to our cell... Na+ entry through leakage channels reduces the negative membrane potential slightly. K+ K+ Na+ Cell interior –70 mV K K+ Na+ Finally, let’s add a pump to compensate for leaking ions. Na+-K+ ATPases (pumps) maintain the concentration gradients, resulting in the resting membrane potential. Na+-K+ pump K+ K+ Na+ K+ K+ Cell interior –70 mV Na+ Figure 11.8

25. Membrane Potentials That Act as Signals • Membrane potential changes when: • Concentrations of ions across the membrane change • Permeability of membrane to ions changes • Changes in membrane potential are signals used to receive, integrate and send information

26. Membrane Potentials That Act as Signals • Two types of signals • Graded potentials • Incoming short-distance signals • Short-lived, localized changes in membrane potential • Depolarizations or hyperpolarizations • Graded potential spreads as local currents change the membrane potential of adjacent regions • Act to enhance or limit chances of an action potential but does not send an electrical “message” • Action potentials • Long-distance signals of axons Graded potentials

27. Graded Potential: Depolarization • Stimulus causes gated ion channels to open • E.g., receptor potentials, generator potentials, postsynaptic potentials • Magnitude varies directly (graded) with stimulus strength • Decrease in magnitude with distance as ions flow and diffuse through leakage channels Depolarizing stimulus Inside positive Inside negative Depolarization Resting potential Time (ms) (a) Depolarization: The membrane potentialmoves toward 0 mV, the inside becoming less negative (more positive). Increases the probability of producing a nerve impulse. Figure 11.9a

28. Graded Potential: Hyperpolarization Hyperpolarizing stimulus Resting potential Hyper- polarization Time (ms) (b) Hyperpolarization: The membranepotential increases, the inside becomingmore negative. Decreases the probability of producing a nerve impulse. Figure 11.9b

29. Active area (site of initial depolarization) Membrane potential (mV) –70 Resting potential Distance (a few mm) (c) Decay of membrane potential with distance: Because current is lost through the “leaky” plasma membrane, the voltage declines with distance from the stimulus (the voltage is decremental). Consequently, graded potentials are short-distance signals. Figure 11.10c

30. Membrane Potentials That Act as Signals • Two types of signals • Graded potentials • Incoming short-distance signals • Short-lived, localized changes in membrane potential • Depolarizations or hyperpolarizations • Graded potential spreads as local currents change the membrane potential of adjacent regions • Action potentials • Long-distance signals of axons

31. Action Potential (AP) • Brief reversal of membrane potential with a total amplitude of ~100 mV • Occurs in muscle cells and axons of neurons • Does not decrease in magnitude over distance • Principal means of long-distance neural communication

32. Anatomy of an Action Potential • Na+ channel slow inactivation gates close • Membrane permeability to Na+ declines to resting levels • Slow voltage-sensitive K+ gates open • K+ exits the cell and internal negativity is restored • Depolarizing local currents open voltage-gated Na+ channels • Na+ influx causes more depolarization • Only leakage channels for Na+ and K+ are open • All gated Na+ and K+ channels are closed 1 3 2 Resting state Depolarization Repolarization • Some K+ channels remain open, allowing excessive K+ efflux • This causes after-hyperpolarization of the membrane (undershoot) Action potential 3 3 4 Hyperpolarization Na+ permeability Membrane potential (mV) 2 Action potential Relative membrane permeability 2 K+ permeability Threshold 1 1 1 4 Channel gating (online animation) 4 Time (ms) Figure 11.11 (1 of 5)

33. Voltage Change at Point in Neuron as Action Potential Passes By Voltage at 0 ms Recording electrode (a) Time = 0 ms. Action potential has not yet reached the recording electrode. Resting potential Peak of action potential Figure 11.12a Hyperpolarization

34. Voltage Change at Point in Neuron as Action Potential Passes By Voltage at 2 ms (b) Time = 2 ms. Action potential peak is at the recording electrode. Figure 11.12b

35. Voltage Change at Point in Neuron as Action Potential Passes By Voltage at 4 ms Action potential online (c) Time = 4 ms. Action potential peak is past the recording electrode. Membrane at the recording electrode is still hyperpolarized. Figure 11.12c

36. Threshold Stimulus • Subthreshold stimulus—weak local depolarization that does not reach threshold • Threshold stimulus—strong enough to push the membrane potential toward and beyond threshold (Membrane is depolarized by 15 to 20 mV) • AP is an all-or-none phenomenon—action potentials either happen completely, or not at all • All action potentials are alike and are independent of stimulus intensity

37. Action potentials Stimulus Threshold Time (ms) Figure 11.13

38. Refractory Periods Absolute refractory period Relative refractory period ARP • Time from the opening of the Na+ channels until the resetting of the channels • Ensures that each AP is an all-or-none event • Enforces one-way transmission of nerve impulses RRP • Most Na+ channels have returned to their resting state • Some K+ channels are still open • Repolarization is occurring • Threshold for AP generation is elevated • Exceptionally strong stimulus may generate an AP Depolarization (Na+ enters) Repolarization(K+ leaves) After-hyperpolarization Stimulus Time (ms) Figure 11.14

39. AP Velocity a Function of Axon Diameter and Myelination Size of voltage Stimulus (a) In a bare plasma membrane (without voltage-gatedchannels), as on a dendrite, voltage decays becausecurrent leaks across the membrane. Continuous conduction Voltage-gated ion channel Stimulus (b) In an unmyelinated axon, voltage-gated Na+ and K+channels regenerate the action potential at each pointalong the axon, so voltage does not decay. Conduction is slow because movements of ions and of the gatesof channel proteins take time and must occur beforevoltage regeneration occurs. Saltatory conduction Stimulus Node of Ranvier Myelin sheath 1 mm Myelin sheath (c) In a myelinated axon, myelin keeps current in axons(voltage doesn’t decay much). APs are generated onlyin the nodes of Ranvier and appear to jump rapidlyfrom node to node, about 30 times faster than a bare axon. Figure 11.15

40. Multiple Sclerosis (MS) • Nature • An autoimmune disease that mainly affects young adults • Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence • Myelin sheaths in the CNS become nonfunctional scleroses • Shunting and short-circuiting of nerve impulses occurs • Impulse conduction slows and eventually ceases • Treatment • Some immune system–modifying drugs, including interferons and Copazone: • Hold symptoms at bay • Reduce complications • Reduce disability

41. Nervous Tissue and Function Function of the Nervous System Organization (Structural and Functional) Supporting Cells of the Nervous System Anatomy of a Neuron Classification of Neurons by Function Graded and Action Potentials Myleination and MS Reflexes Synapses EPSPs and IPSPs Neurotransmitters

42. The Reflex Arc

43. Types of Reflexes and Regulation Autonomic reflexes Smooth muscle regulation Heart and blood pressure regulation Regulation of glands Digestive system regulation Somatic reflexes Activation of skeletal muscles

44. Nervous Tissue and Function Function of the Nervous System Organization (Structural and Functional) Supporting Cells of the Nervous System Anatomy of a Neuron Classification of Neurons by Function Graded and Action Potentials Myleination and MS Reflexes Synapses EPSPs and IPSPs Neurotransmitters

45. Two Kinds of Synapses • Electrical Synapses • Less common than chemical synapses • Neurons are electrically coupled (joined by gap junctions) • Communication is very rapid, and may be unidirectional or bidirectional • Are important in: • Embryonic nervous tissue • Some brain regions • Chemical Synapses • Specialized for the release and reception of neurotransmitters • Typically composed of two parts • Axon terminal of the presynaptic neuron, which contains synaptic vesicles • Receptor region on the postsynaptic neuron

46. How Neurons Communicate at Synapses Irritability – ability to respond to stimuli Conductivity – ability to transmit an impulse` Events at the Synapse (online animation) Narrated synapse (online) SodiumPotassium pump (online animation)

47. Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters. Presynapticneuron Presynapticneuron Postsynapticneuron 1 Action potentialarrives at axon terminal. Mitochondrion Ca2+ Ca2+ Ca2+ Ca2+ Synapticcleft Axonterminal Synapticvesicles Postsynapticneuron Figure 11.17, step 1

48. Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters. Presynapticneuron Presynapticneuron Postsynapticneuron 1 Action potentialarrives at axon terminal. 2 Voltage-gated Ca2+channels open and Ca2+enters the axon terminal. Mitochondrion Ca2+ Ca2+ Ca2+ Ca2+ Synapticcleft Axonterminal Synapticvesicles Postsynapticneuron Figure 11.17, step 2

49. Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters. Presynapticneuron Presynapticneuron Postsynapticneuron 1 Action potentialarrives at axon terminal. 2 Voltage-gated Ca2+channels open and Ca2+enters the axon terminal. Mitochondrion Ca2+ Ca2+ Ca2+ Ca2+ Synapticcleft 3 Ca2+ entry causesneurotransmitter-containing synapticvesicles to release theircontents by exocytosis. Axonterminal Synapticvesicles Postsynapticneuron Figure 11.17, step 3

50. Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters. Presynapticneuron Presynapticneuron Postsynapticneuron 1 Action potentialarrives at axon terminal. 2 Voltage-gated Ca2+channels open and Ca2+enters the axon terminal. Mitochondrion Ca2+ Ca2+ Ca2+ Ca2+ Synapticcleft 3 Ca2+ entry causesneurotransmitter-containing synapticvesicles to release theircontents by exocytosis. Axonterminal Synapticvesicles 4 Neurotransmitterdiffuses across the synapticcleft and binds to specificreceptors on thepostsynaptic membrane. Postsynapticneuron Figure 11.17, step 4