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The Nervous System

The Nervous System. Neurons - Chapter 7. The Nervous System. Rapid Communication and Control Sensation receives info. on environmental changes Integration interprets the changes, integrates signals from multiple signals Response induces action from of muscles or glands.

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The Nervous System

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  1. The Nervous System Neurons - Chapter 7

  2. The Nervous System • Rapid Communication and Control • Sensation • receives info. on environmental changes • Integration • interprets the changes, integrates signals from multiple signals • Response • induces action from of muscles or glands

  3. Nervous System Organization:General Anatomy • Central Nervous System (CNS) • Brain + Spinal Cord • control center (integration) • Peripheral Nervous System (PNS) • cranial nerves and spinal nerves • connects CNS to sensory receptors, muscles and glands

  4. Cell Types • Neurons • conduct electrical signals • Neuroglia • 80% of all NS cells • support neurons Figs 7.4, 7.5

  5. Cell Body nucleus and organelles Dendrites receive information Axon conduct electrical signals (action potentials) axon hillock - site where AP’s originate axon terminals - where chemical signals are released Neurons Figs 7.1, 7.2

  6. Types of Neurons • Sensory (Afferent) Neuron - input • part of the PNS • transmit electrical signals from tissues and organs to CNS • detect changes in environment

  7. Types of Neurons • Motor (Efferent) neuron - output • part of the PNS • transmit signals from CNS to effector tissues (muscle, gland cells) • somatic motor neurons • skeletal muscle contraction • both voluntary and reflexive • autonomic motor neurons • smooth muscle, cardiac muscle, and glands • involuntary

  8. Types of Neurons • Interneurons = processors & integrators • 99% of all neurons • connect afferent to efferent • located entirely in the CNS

  9. Types of Neuroglia • Schwann Cells • surround axons of all PNS neurons • form myelin sheath around axons • gaps = Nodes of Ranvier • presence increases the speed nerves conduct signals Figs 7.2, 7.5

  10. Types of Neuroglia • Oligodendrocytes • form myelin sheaths around CNS axons • white matter • area of CNS with high density of axons • grey matter • area of CNS with mostly cell bodies and dendrites (no myelin) Fig 7.7

  11. Types of Neuroglia • Microglia • phagocytose (eat) foreign and degenerated material

  12. Types of Neuroglia • Astrocytes • cover capillaries within the brain • control permeability of capillaries • regulate exchange of material between blood and cerebrospinal fluid • “blood-brain barrier” • Deliver nutrients directly to neurons • Uptake excess neurotransmitter and control ion concentrations at synapses. Fig 7.9

  13. Types of Neuroglia • Ependymal cells • form epithelial lining of brain and spinal cord cavities • produce cerebrospinal fluid

  14. Types of Neuroglia • Satellite Cells (Ganglionic Gliocytes) • Form capsules around cell neuron cell bodies in ganglia • Support and protect cell bodies

  15. Electrical Activity of Neurons:Resting Potential • Due to differences in permeability of membrane to charged particles • completely impermeable to A- • relatively permeable to K+ • relatively impermeable to Na+ • Inside of cell negative relative to the outside (-70 mV) • At resting potential, neither K+ nor Na+ are in equilibrium Fig 6.23

  16. Electrical Activity of Neurons:Electrical Signals • Electrical signals • changes in membrane potential • due to changes in membrane permeability and increased flow of charged particles • changes in permeability are due to increased number of open membrane channels. • Allows ions to flow along electrochemicalgradient

  17. Depolarization and Hyperpolarization • Depolarize • reduce charge difference • Hyperpolarize • increase charge difference See also Fig 7.10

  18. Membrane Proteins Involved in Electrical Signals • Non-gated ion channels • Always open • specific for a particular ion See Fig 7.11

  19. Membrane Proteins Involved in Electrical Signals • Gated Ion channels • open only under particular conditions (stimulus) • voltage gated – changes in membrane potential • chemically gated – binding of a chemical messenger • physically gated – stretching/distortion of the membrane See Fig 7.11

  20. Membrane Proteins Involved in Electrical Signals • Na+/K+ pump • active (require ATP) • Na+ pumped out, K+ pumped in (3 Na+ per 2 K+) See Fig 6.17

  21. Types of Electric Signals: Graded Potentials • occur in dendrites and cell body • small, localized change in membrane potential • change of only a few mV • opening of chemically-gated or physically-gated ion channels • changes permeability of membrane • travels only a short distance (mm) Fig 7.1

  22. Types of Electric Signals: Graded Potentials • a triggered event (requires stimulus) • e.g. - light, touch, chemical messengers • graded •  stimulus intensity → change in membrane potential

  23. Types of Electric Signals:Action Potentials • begins at the axon hillock, travels down axon • brief, rapid reversal of membrane potential • Large change (~70-100 mV) • Opening of voltage-gated Na+ and K+ channels • self-propagating - strength of signal maintained • transmits electrical signals over long distances Fig 7.1

  24. Types of Electric Signals:Action Potentials • triggered • membrane depolarization at axon hillock • not graded = "All or none" • axon hillock must be depolarized a minimum amount (threshold) • if depolarized to threshold, AP will occur at maximum strength • if threshold not reached, no AP will occur

  25. Action Potential: Depolarization Phase • Triggering event causes membrane to depolarize • slow increase until threshold is reached • voltage-gated Na+ channels open quickly (K+ channels slowly) • Na+ enters cell • further depolarization • more channels open • further depolarization • membrane depolarizes to 0 mV, but continued flow of Na+ in leads to reversed polarity (+30 mV) Figs 7.12 and 7.13

  26. Action Potential: Repolarization Phase • At +30 mV, voltage-gated Na+ channels close • Slow opening of voltage-gated K+ channels • reach peak K+ permeability as Na+ channels close • K+ rushes out of the cell • membrane potential restored • K+ channels close @ threshold • [Na+] and [K+] restored by the Na+-K+ pump Figs 7.12 and 7.13

  27. Action Potentials • response of the nerve cell to the stimulus is “all or none” • Amt of depolarization (amplitude) always the same • differences in stimulus intensity are detected by • The number of neurons undergoing AP in response to the stimulus • The frequency of action potential generation Fig 7.14

  28. Refractory Period • time that must pass before the neuron segment can undergo a second action potential • absolute refractory period • neuron segment is undergoing AP • cannot respond to a second stimulus • channels enter an inactive state • relative refractory period • neuron segment is repolarizing • action potential may be produced if a stronger stimulus is applied Fig 7.15 and 7.11

  29. Action Potential Propagation • Na+ moving into one segment of the neuron quickly moves laterally inside the cell • Depolarizes adjacent segment to threshold Fig 7.16

  30. Action Potential Propagation:Myelinated Axons • Saltatory conduction - increased speed of the AP produced by myelination of the axon • myelin = lipid insulator (PM of Schwann cells or oligodendrocytes) • nodes of Ranvier =contain lots of Na+ channels • signals “jump” from one node to the next • AP conduction speed Fig 7.17

  31. Synapses • Synapse • functional connection between a neuron and either an effector cell or another neuron • allow information to pass from one cell to the next

  32. Electrical Synapses (Gap Junctions) • Present in cardiac and smooth muscle, and some neurons • Series of channels crossing membranes of both cells • Allow flow of ions from one cell to the next • Electrical signals move quickly from one cell to the next Fig 7.19

  33. Chemical Synapses • Unidirectional info. flow • presynaptic neuron • synaptic terminal bouton • contains synaptic vesicles filled with neurotransmitter • synaptic cleft • space in-between cells • postsynaptic neuron • Subsynaptic membrane • Receptor proteins for neurotransmitter Figs 7.20 and 7.21

  34. Chemical Synapses • Many voltage-gated Ca2+ channels in the terminal bouton • Ca2+ is in higher conc. in the ECF than the ICF • AP in bouton opens Ca2+ channels • Ca2+ rushes in. • Ca2+ causes vesicles to fuse to plasma membrane and release contents • Transmitter diffuses across synaptic cleft and binds to receptors on subsynaptic membrane Figs 7.20 and 7.21

  35. Chemical Synapses • Specific ion channels in subsynaptic membrane open • chemically-gated ion channels • Ions enter postsynaptic cell • graded potential forms • If depolarizing graded potential is strong enough to reach threshold, action potential generated in postsynaptic cell Figs 7.23 and 7.25

  36. Types of Chemical Synapse • Excitatory chemical synapse: • excitatory postsynaptic potentials (EPSPs) • Transmitter binding opens Na+ channels in the postsynaptic membrane • Small depolarization of postsynaptic neuron • More positive inside the cell • closer to threshold Fig 7.26

  37. Types of Chemical Synapse • Inhibitory chemical synapse: • inhibitory postsynaptic potentials (IPSPs) • Transmitter binding opens K+ or Cl- ion channels • K+ flows out or Cl- flows in down gradients • Small hyperpolarization of postsynaptic neuron • More negative inside cell • further from threshold Fig 7.29

  38. Neurotransmitters • Chemicals that carries the message of the A.P. from one cell to the next • Acetylcholine • somatic MNs – skeletal muscle contraction • autonomic MNs – slow HR, gland secretion etc. • Norepinephrine • autonomic MNs – mental alertness, increases blood pressure and HR, etc. • Seratonin + Dopamine • interneurons – behavioral effects

  39. Neurotransmitters • types vary between synapses • response depends on postsynaptic membrane • e.g. acetylcholine • produces EPSPs when applied to skeletal muscle • produced IPSPs when applied to cardiac muscle

  40. Synaptic Integration • Multiple synaptic events have an additive effect on membrane potential • Sum of inputs determines whether axon hillock depolarized enough for AP to form.

  41. Spatial Summation • numerous presynaptic fibers may converge on a single postsynaptic neuron • additive effects of numerous neurons inducing EPSPs and IPSPs on the postsyn. neuron Figs 7.29 - 7.31

  42. Temporal Summation • additive effects of EPSPs and IPSPs occurring in rapid succession • next synaptic event occurs before membrane recovers from previous event

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