Neurons+MC

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3. What is the myelin sheath made of? Where is it located? What is its function? p. 249 it’s formed by a type of neuroglia called Schwann cells, which contain myelin (a lipid substance) in their plasma membranes. Myelin gives nerve fibers their white, glistening appearance and serves as an excellent insulator. It also plays a role in nerve regeneration within the PNS. If an axon is accidentally severed, the myelin sheath remains and serves as a passageway for new fiber growth. The myelin sheaths are located on top of the axons as a protective layer. 4. What is the difference between the "gray matter" and "white matter" in the brain? p.249 The gray matter of the CNS is gray because it contains no myelinated axons; the white matter of the CNS is white because it does. 5. What causes multiple sclerosis? P.249 it’s caused by a loss of myelin from the axons. MS is believed to be caused by an attack on the myelin by the body’s immune system. 6. Where are the sodium and potassium ions when the axon is at rest? What is the charge inside the axon? When the axon is at rest, there’s more potassium inside the axon, and there’s more sodium outside the axon. The charge in the inside of the axon is negative. 7. Which ions can cross the membrane and enter the axon? potassium or sodium? When action potential occurs, the gates of sodium channels open first, and Sodium flows into the axon, while potassium exits to the outside of the axon as the gates of potassium open. 8. What happens to the charge inside the axon when sodium gates open? Where does sodium go? Sodium moves into the inside of the axon. It’s called depolarization because the charge inside the axon changes from negative to positive. 9. What happens to the charge inside the axon when the potassium gates open? Where does potassium go? Potassium exits the axon to the outside of it. It’s called repolarization because the inside of the axon resumes a negative charge as potassium exits the axon. 10. Look at the graph on page 251. What do you think depolarization means? What do you think repolarization means? Depolarization is when the charge inside the axon changes from negative to positive. Repolarization is when the inside of the axon resumes a negative charge as Potassium exits the axon. 11. What causes depolarization of the axon? It occurs when Sodium gates open and Sodium moves to the inside the axon. 12. What causes repolarization of the axon? It occurs when Potassium gates open and potassium moves to outside the axon. 13. What is an action potential? It’s a rapid change in polarity across an axonal membrane as the nerve impulse occurs. If a stimulus causes the axonal membrane to depolarize to a certain level, called threshold, an action potential occurs in an all-or-none manner. The strength of an action potential does not change; an intense stimulus can cause an axon to fire more often in a given time interval. 14. How does the sodium potassium pump reestablish the resting potential during the refractory period? After an action potential has passed by, the sodium-potassium pump restores the resting potential by moving the potassium back to the inside and sodium back to the outside. 15. What is a synapse? It’s a junction between neurons consisting of the presynaptic (axon) membrane, the synaptic cleft, and the postsynaptic (usually dendrite) membrane. 16. What is a synaptic cleft? It’s a small gap that separates the sending neuron from the receiving neuron. The nerve impulse is unable to jump the cleft, and therefore, another means is needed to pass the nerve impulse from the sending neuron to the receiving neuron. 17. What is a neurotransmitter? Transmission across a synapse is carried out by molecules, called neurotransmitters, which are stored in synaptic vesicles in the axon terminals; a chemical stored at the ends of axons that is responsible for transmission across a synapse. 18. Since neuron's don't touch, how is the signal from one neuron transmitted to another neuron? Neurons integrate these incoming signals. If a neuron receives enough excitatory signals to outweigh the inhibitory ones, chances are the axon will transmit a nerve impulse. On the other hand, if a neuron receives more inhibitory than excitatory signals, the summing up of these signals may prohibit the axon from firing. 19.

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