Neurons 1. 2. Cell body: produces all the proteins needed for whole neurons Axon: main conducting unit of neuron Myelin sheath: speeds up message Dendrites: receive signals Axon terminal: contains neurotransmitters to continue message onto next neuron/muscle/gland. Schwann cell: produces myelin Node of ranvier: allows message to “re-energize” itself. Neuron at rest 3. 4. The sodium-potassium pump pushes 3 Na and 2 K against their concentration gradients using 1 ATP. It restores and maintains the resting potential by pushing more Na out of neuron and pushing more K into neuron. (But overall, it pushes more positive charges out of the cell than it brings in.) 5. The resting potential is the difference in charge between the inside and the outside of the neuron. Because there are fewer (positive) charges inside the cell, the voltage is -70mV. Stimulating the Neuron 6. Sensory neurons are stimulated by the receptors (example: when rhodopsin breaks in photoreceptors, they release energy used to depolarize the neuronal membrane). Motor neurons are stimulated by synapses with other neurons in the brain or spinal cord. 7. The all-or-none principle conveys the idea that the neuron’s message is always the same – the voltage changes the same way every time. The neuron cannot send a “strong” or a “weak” message – once there is enough stimulus to start the message (it has hit the “threshold”), the message goes through the neuron. The neuron cannot send slower or faster messages. For the brain to tell between strong stimuli and weak stimuli, it relies on the number of different types of receptor activated (different receptors require different amounts of stimuli to reach threshold) and the frequency of messages. Depolarization 8. During depolarization, the Na+ channels open and Na+ floods into the neuron, while the K+ channels close briefly. 9. The Na+ floods in, changing the voltage to +40mV. 10. Action potential is the scientific name for the message moving in a neuron. Repolarization 11. When the voltage becomes positive, the Na+ channels close and the K+ channels open, so K+ floods out of the cell. 12. K+ floods out of the neuron during repolarization. This brings the voltage down to 100mV. 13. The refractory period is the time until a neuron can fire another message. During this time, the Na/K pump is resetting the proper concentrations for resting potential. Until the Na/K pump has achieved this, the neuron cannot depolarize. Myelin Sheath 14. 2m/s 15. The myelin sheath is a fatty layer that surrounds the axon (outside the neuronal membrane) in the peripheral nervous system. 16. Nodes of Ranvier are areas of the axon not covered by myelin sheath. They occur regularly along the axon. 17. The myelin sheath insulates the axon, so that when the axon terminal depolarizes, the voltage change speeds along to the first node of ranvier (much quicker than if the membrane wasn’t insulated), where it opens the sodium gates, depolarizing that section of the axon. The voltage change will jump to the next node of ranvier and so on. Synapse 18. A synapse is a gap between the dendrite of one neuron and the axon terminal of another. 19. Presynaptic is the neuron before the gap, carrying the message. Postsynaptic is the neuron after the gap, receiving and possibly carrying on the message. 20. The dendrite contains vesicles full of chemicals called neurotransmitters. When the action potential (wave of depolarization) reaches the dendrite, the voltage change causes calcium channels to open. The influx of calcium starts the process of exocytosis – the vesicles fuse with the membrane and release the neurotransmitters into the synapse. They diffuse across the gap and attach to receptors on the post-synaptic axon terminal membrane and these receptors cause the sodium gates or the potassium gates to open. 21. In an excitatory response, the sodium gates are opened and the action potential continues in the post-synaptic neuron. In an inhibitory response, the potassium gates are opened and the action potential stops there.