The Nervous System (cont.)

Today in 8th and 9th period AP bio we discussed the Nervous System. I'm going to start off with reviewing some of yesterdays lesson because it will help lead into today's lesson.

Well, first you want to know how a nerve impulse travels. An impulse starts with a stimulus and then the nerve is stimulated. The cell is originally negative on the inside and positive on the outside. Once it reaches threshold potential it opens up the channels in the cell membrane. The example we used in class was Na+ and K+. Once the Na+ channels in the membrane open up, the cations diffuse into the membrane. This causes the cell to become depolarized. That point on the neuron then becomes positive inside the cell, and negative outside because the charge of the sodium is positive. This needs to continue down the cell so in order for this to happen the rest of the gates need to open. The change in charge opens up the next Na+ gates down the line. These are called voltage-gated channels. This wave that moves down the neuron, the nerve impulse, is called action potential. Once this occurs the cell has to be re-set to the negative charge inside and positive outside. This is changed by a 2nd wave. This time the K+ channels open and the potassium ions diffuse out of the cell. This makes the inside of the cell negative, again and the outside positive. The combined waves travel down the neuron and in one direction. The wave is an active potential/nerve impulse, that travels from our brain to fingertips in milliseconds. The voltage-gated channels open and close in response to changes in charge across the membrane. The sodium channels open much faster than the potassium channels. But they both close slowly.
After this a neuron needs to re-set it. This happens by the Na+ moves back out and then the K+ moves back in. They are both moving against concentration gradients. In order to do this they need a pump. The sodium-potassium pump requires ATP. This pump allows 3 Na+ ions to be pumped out and 2 K+ ions to be pumped in. Now the neuron is ready to fire again.

Heres a video on this process:



Here is another video:






At the end of the axon the signal needs to make it to the next nerve cell. In order for this to happen the impulse needs to jump the synapse. The synaptic terminal releases neurotransmitters and the diffusion of chemicals across the synapse conducts the signal across the synapse. At the synapse the action potential depolarizes the membrane, which then allows the entrance of Calcium, which allows neurotransmitter vesicles to move and fuse with the membrane and release it to the synaptic cleft. The neurotransmitter than binds with a protein receptor, the ion-gated channels open and the neurotransmitter is degraded or reabsorbed. In the next nerve cell the chemical signal opens the ion-gated channels and Na+ diffuses into the cell and K+ out of the cell.

Myelin Sheath
The axon of nerve cells is wrapped with a set of cells, Schwann cells. These cells insulate the axon and causes the signal, sent to the nerve, to hop from node to node. The hoping from node to node is called saltatory conduction. This increases the rate 30x.
There are some times when the myelin sheath is attacked by the immune system. This cause a loss in signal and is a disease called Multiple Sclerosis.

Anything that affects neurotransmitters or mimics then affects the functions of the nerves. Gases, mood altering drugs, hallucinogenic drus, Prozac and poisons all affect nerve function.

We also discussed the some of the different kinds of neurotransmitters. Acetylcoline-transmits signal to skeletal muscle
Epinephrine and norepinephrine- fight or flight response
Dopamine- affects sleep, mood, attention and learning. Lack of dopamine-associated with Parkinson's disease. Too much-schizophrenia
Serotonin- affects sleep, mood, attention and learning

One more note: Acetylcholinesterase is an enzyme which breaks down acetylcoline neurotransmitter.

That concludes my sherpa report.
Tomorrows sherpa will be.....Kelly Prince.