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Nervous system

Nervous system. Ch 48. Nervous system. Central Nervous system – Brain & spinal cord Peripheral nervous system- nerves that communicate motor & sensory signals through the body. Neurons. Sensory neuron – input from external stimuli Interneuron – integration: analyze & interprets input

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Nervous system

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  1. Nervous system Ch 48

  2. Nervous system • Central Nervous system – • Brain & spinal cord • Peripheral nervous system- nerves that communicate motor & sensory signals through the body

  3. Neurons • Sensory neuron – input from external stimuli • Interneuron – integration: analyze & interprets input • Motor neuron– signal sent to muscle or gland cells

  4. Dendrites Parts of a neuron Stimulus Axon hillock Nucleus Cellbody Presynapticcell Axon Signaldirection Synapse Synaptic terminals Synapticterminals Postsynaptic cell Neurotransmitter

  5. Node of Ranvier Layers of myelin Axon Schwanncell Schwanncell Nodes ofRanvier Nucleus ofSchwann cell Axon Myelin sheath

  6. Nerve Signals • Membrane potential - the electrical charge difference across a membrane • Due to different concentrations of ions in & out of cell • Resting potential – the membrane potential of an unstimulated neuron • About -70 mV (more negative inside)

  7. Key Na K Sodium-potassiumpump OUTSIDEOF CELL Potassiumchannel Sodiumchannel

  8. Maintaining resting potential To keep sodium & potassium in the right gradients, the sodium-potassium pump uses ATP to maintain gradients The sodium-potassium pump pumps 2K+ in and 3Na+ out each time.

  9. Types of ion channels: • Ungated ion channels – always open • Gated ion channels – open or close in response to stimuli • Ligand gated ion channels (chemically gated)–in response to binding of chemical messenger(i.e. neurotransmitter) • Voltage gatedion channels – in response to change in membrane potential • Stretch gated ion channels – in response to mechanical deformation of plasma membrane

  10. Hyperpolarization Stimulus 50 • When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative 0 Membrane potential (mV) Threshold 50 Restingpotential Hyperpolarizations

  11. Depolarization Stimulus 50 • Opening other types of ion channels triggers a depolarization, a reduction in the magnitude of the membrane potential • For example, depolarization occurs if gated Na+ channels open and Na+ diffuses into the cell 0 Membrane potential (mV) Threshold 50 Restingpotential Depolarizations 100 0 3 4 5 1 2

  12. Action Potentials • Signals conducted by axons, transmitted over long distances • Occur as the result of gated ion channels that open or close in response to stimuli • - “All or nothing”

  13. Action Potential 50 Actionpotential • Steps: • 1) resting state • 2) threshold • 3) depolarization phase • 4) repolarization phase • 5) undershoot 0 Membrane potential (mV) Threshold 50 Restingpotential 100 0 3 5 6 4 1 2

  14. 1 5 4 3 2 1 5 4 3 1 2 Key Na K Falling phase of the action potential Rising phase of the action potential 50 Actionpotential 0 Membrane potential(mV) Threshold 50 Depolarization Resting potential 100 Time OUTSIDE OF CELL Sodiumchannel Potassiumchannel INSIDE OF CELL Inactivation loop Resting state Undershoot

  15. Action potential • https://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html

  16. How do action potentials “travel” along a neuron? • Where action potential is generated (usually axon hillock), the electrical current depolarizes the neighboring region of membrane • Action potentials travel in one direction – towards synaptic terminals

  17. 2 1 3 Axon Plasma membrane Actionpotential Cytosol Na Actionpotential K Na K Actionpotential K Na

  18. Why doesn’t it travel backwards? • The refractory period is due to inactivated Na+ channels, so the the depolarization can only occur in the forward direction.

  19. Speed of action potentials • Speed is proportional to diameter of axon, the larger the diameter, the faster the speed • Several cm/sec – thin axons • 100 m/sec in giant axons of invertebrates such as squid and lobsters

  20. Nerveswith giant axons Ganglia Brain Arm Eye Mantle Nerve http://www.youtube.com/watch?v=omXS1bjYLMI

  21. Speeding up Action potential in vertebrates Schwann cell • Myelination (insulating layers of membranes) around axon • Myelin is deposited by Schwann cells or oligodendrocytes. Depolarized region(node of Ranvier) Cell body Axon

  22. Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na+ channels are found • Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatoryconduction

  23. Synapses • Neurons communicate with other cells at synapses • Electrical synapse- • Direct communication from pre to post synaptic cell • Gap junctions connect cells and ion currents flow between cells

  24. Chemical Synapse • Much more common in vertebrates & most invertebrates • 1) Action potential reaches synaptic terminal • 2) This depolarization causes Ca+ to rush into neuron through voltage gated calcium channels

  25. 3) Synaptic vesicles fuse with presynaptic membrane and release neurotransmitters. • 4) Neurotransmitter diffuses across synaptic cleft and binds to ligand gated ion channels in second neuron. • 5) Ligand gated ion channels open, generating a post-synaptic potential • 6) Neurotransmitter is removed quickly – by enzymes or by surrounding cells uptake

  26. 2 1 3 4 Presynapticcell Postsynaptic cell Axon Synaptic vesiclecontainingneurotransmitter Postsynapticmembrane Synapticcleft Presynapticmembrane K Ca2 Voltage-gatedCa2 channel Ligand-gatedion channels Na

  27. Excitatory synapses • Some synapses are excitatory – they increase the likelihood that the axon of the postsynaptic neuron will generate an action potential • Opens channel for both Na+ & K+ - allows Na+ to enter & K+ to leave cell, so this depolarizes the membrane • EPSP – excitatory postsynaptic potential

  28. Inhibitory synapses • Some synapses are inhibitory – they make it more difficult for the postsynaptic neuron to generate an action potential • Opens channel that is permeable for only K+ or Cl-, so this hyperpolarizes the membrane • IPSP – inhibitory postsynaptic potential

  29. Summation of postsynaptic responses • A single EPSP is usually not enough to produce an action potential • Summation = the additive effect of postsynaptic potentials • The axon hillock is the neuron’s integrating center • Temporal summation • Spatial summation

  30. Neurotransmitters • Many different types – 5 main groups: • Acetylcholine • biogenic amines • amino acids • Neuropeptides • gases • One neurotransmitter can have more than a dozen different receptors

  31. Acetylcholine • - One of the most common neurotransmitters in vertebrates and invertebrates • - Can be inhibitory or excitatory • - Released at neuromuscular junctions, activates muscles • - inhibits cardiac muscle contraction • -also involved in memory formation, and learning

  32. Biogenic Amines • Biogenic amines are derived from amino acids • They include • Norepinephrine – excitatory neurotransmitter in the autonomic nervous system • Dopamine – rewards increase dopamine levels • Serotonin - helps regulate mood, sleep, appetite, learning and memory • They are active in the CNS and PNS

  33. Endorphins • - Decrease our perception of pain • - Inhibitory neurotransmitters • - produced during times of physical or emotional stress – i.e. childbirth, exercise • Opiates (i.e. morphine & heroin) bind to the same receptors as endorphins and can be used as painkillers

  34. Vertebrate brain specialization Cerebrum – 2 hemispheres, higher brain functions such as thought & action

  35. Brain Hemispheres

  36. Vertebrate brain specialization Cerebellum – helps coordinate movement, posture, balance

  37. Vertebrate brain specialization Brainstem – controls homeostatic functions such as breathing rate, heart rate, blood pressure. Conducts sensory & motor signals between spinal cord & higher brain centers

  38. Allan Jones: A map of the brain http://www.ted.com/talks/allan_jones_a_map_of_the_brain.html The mysterious workings of the adolescent brain http://www.ted.com/talks/sarah_jayne_blakemore_the_mysterious_workings_of_the_adolescent_brain.html

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