Figure 48.1 Overview of a vertebrate nervous system

1. Figure 48.1 Overview of a vertebrate nervous system

2. Neuron

3. Conduction of the Nerve Impulse • Membrane Potential • Voltage measured across a membrane due to differences in electrical charge • Inside of cell is negative wrt outside • Resting potential of neuron = -70 mV

4. Figure 48.6 Measuring membrane potentials

6. Sodium-Potassium Pump Extracellular P Intracellular + ADP 1. Carrier in membrane binds intracellular sodium. ATP 6. Dephosphorylation of protein triggers change to original conformation, with low affinity for K+. K+ diffuses into the cell, and the cycle repeats. 2. ATP phosphorylates protein with bound sodium. K+ Na+ Pi Pi Pi 3. Phosphorylation causes conformational change in protein, reducing its affinity for Na+. The Na+ then diffuses out. Pi 5. Binding of potassium causes dephosphorylation of protein. 4. This conformation has higher affinity for K+. Extracellular K+ binds to exposed sites.

7. Excitable Cells • Neurons & muscle cells • Have gated ion channels that allow cell to change its membrane potential in response to stimuli

9. Gated Ion Channels • Some stimuli open K+ channels • K+ leaves cell • Membrane potential more negative • hyperpolarization • Some stimuli open Na+ channels • Na+ enters cell • Membrane potential less negative • depolarization

10. Gated Ion Channels • Strength of stimuli determines how many ion channels open = graded response

11. Nerve Impulse Transmission

12. Action Potentials • Occur once a threshold of depolarization is reached • -50 to –55 mV • All or none response (not graded) • Magnitude of action potential is independent of strength of depolarizing stimuli • Hyperpolarization makes them less likely

13. 3. Top curve 2. Rising Phase Maximum voltage reached Stimulus causes above threshold voltage Potassium gate opens K+ Na+ Na+ channel inactivation gate closes Sodium channel activation gate opens +50 0 Membrane potential (mV) –70 3 1 2 1. Resting Phase 4. Falling Phase Time (ms) Equilibrium between diffusion of K+ out of cell and voltage pulling K+ into cell Undershoot occurs as excess potassium diffuses out before potassium channel closes Potassium gate open Voltage-gated potassium channel Potassium channel gate closes Equilibrium restored Na+ channel inactivation gate closed Voltage-gated sodium channel Potassium channel Sodium channel activation gate closes. Inactivation gate opens.

14. Refractory Period • During undershoot the membrane is less likely to depolarize • Keeps the action potential moving in one direction

15. Propagation of Action Potential • Action potential are very localized events • DO NOT travel down membrane • Are generated anew in a sequence along the neuron

16. resting repolarized depolarized + + + + + + + + + – – – – – – – – – Cytoplasm + + + + + + + + + – – – – – – – – – Cell membrane – – + + + + + + + + + – – – – – – – Na+ + + + + + + + – – – – – – – – – + + K+ + + – – + + + + + – – + + – – – – – Na+ + + + + + – – + + – – – – – + + – – K+ K+ + + + + – – – + + – – – – +++ – – Na+ + + – – – + + + + – – +++ – – – – K+ K+ + + + + + + + – – – – – – – – – + + Na+ – – + + + + + + + + + – – – – – – – K+

17. Supporting Cells - Neuroglia • provide neurons with nutrients, remove wastes Two important types in vertebrates • Oligodendrocytes – myelin sheath in CNS • Schwann cells -myelin sheath in PNS

18. Myelin Sheath Formation

20. Saltatory Conduction

21. Transfer of Nerve Impulse to Next Cell • Synapse • the gap between the synaptic terminals of an axon and a target cell

22. Transfer of Nerve Impulse to Next Cell • Electrical synapses • Gap junctions allow ion currents to continue • Chemical synapses • More common • Electrical impulses must be changed to a chemical signal that crosses the synapse

23. Chemical Synapses