Neurons+MV

Molly Vaughn 13 October 2008

Neurons

Neuron Structure 1. Insert an image of the basic neuron structure that includes cell body, dendrites, and axon. p. 249

Myelin Sheath 2. Insert an image of a neuron with a myelin sheath.

3. What is the myelin sheath made of? Where is it located? What is its function? p. 249 - Many axons are covered by a protective myelin sheath. In the PNS, this covering is formed by a type of neuroglia called Schwann cells, which contain myelin (a lipid substance) in their plasma membranes. In the CNS, oligodendrocytes perform this function. The myelin sheath develops when these cells wrap themselves around an axon many times. Because each neuroglia cell covers only a portion of an axon, the myelin sheath is interrupted. The gaps where there is no myelin sheath are called nodes of Ranvier. Long axons tend to have myelin sheath, but short axons do not. 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. In the PNS, myelin gives nerve fibers their white, glistening appearance and serves as an excellent insulator. The myelin sheath also plays an important role in nerve regeneration within the PNS. If an axon is accidently severed, the myelin sheath remains and serves as a passageway for new fiber growth.

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 - Multiple sclerosis is caused by loss of myelin from the axons. It is believed to be caused by an attack on the myelin by the body’s immune system.

Nerve Impulse p.250-252 5. Where are the sodium and potassium ions when the axon is at rest? What is the charge inside the axon? - The resting potential correlates with a difference in ion distribution on either side of the axonal membrane. The concentration of sodium ions is greater outside the axon than inside, and the concentration of potassium ions is greater inside the axon than outside. The unequal distribution of these ions is due to the action of the sodium-potassium pump that actively transports sodium out of and potassium into the axon. The work of the pump maintains the unequal distribution of sodium and potassium across the membrane.

6. Which ions can cross the membrane and enter the axon? potassium or sodium? - sodium

7. What happens to the charge inside the axon when sodium gates open? Where does sodium go? - When the action potential occurs, the gates of sodium channels open first, and sodium flows into the axon. As sodium moves to inside the axon, the membrane potential changes from -65 mV to +40 mV. This is a depolarization because the charge inside the axon changes from negative to positive.

8. What happens to the charge inside the axon when the potassium gates open? Where does potassium go? - Second, the gates of potassium channels open, and potassium flows to outside the axon. As potassium moves to outside the axon, the action potential changes from +40 mV back to -65 mV. This is a repolarization because the inside of the axon resumes a negative charge as potassium exits the axon.

9. Look at the graph on page 251. What do you think depolarization means? What do you think repolarization means? - As the time passes depolarization starts to occur, which causes the voltage to sore and then as the potassium moves to outside the axon, the voltage decreases and the repolartization starts, which in a way means recharging.

10. What causes depolarization of the axon? - Depolarization is caused when the sodium moves inside the axon.

11. What causes repolarization of the axon? - Repolarization is caused when potassium moves to outside the axon.

12. What is an action potential? - The conduction of an action potential is an all-or-none event-that is, either an axon conducts a nerve impulse or it does not. The intensity of a message is determined by how many nerve impulses are generated within a given time span. An axon can conduct a volley of exchanged with each impulse. As soon as an impulse has passed by each successive portion of an axon, it undergoes a refractory period, during which the sodium gates are unable to open. The ensures that the action potential cannot move backward and instead always moves down an axon toward its branches.

13. How does the sodium potassium pump reestablish the resting potential during the refractory period? - During the refractory period, the sodium gates are unable to open. This ensures that the action potential cannot move backward and instead always moves down an axon toward its branches.

14. What is a synapse? - A synapse is the junction between neurons consisting of the presynaptic (axon) membrane, the synaptic cleft, and the postsynaptic membrane.

15. What is a synaptic cleft? - A synaptic cleft is a small gap between presynaptic and postsynaptic membranes of synapse.

16. What is a neurotransmitter? - A neurotransmitter is a chemical stored at the ends of axons that is responsible for transmission across a synapse.

17. Since neurons don't touch, how is the signal from one neuron transmitted to another neuron? - 1) Nerve impulses traveling along an axon reach an axon terminal. 2) Calcium ions enter the terminal, and the stimulate synaptic vesicles to merge with the sending membrane. 3) Neurotransmitter molecules are released into the synaptic cleft, and they diffuse across the cleft to the receiving membrane, where they bind with specific receptor proteins.

18. Insert an image showing an action potential.

19. Insert an image showing a synapse.