Friday, June 25, 2010

Some Chapter Summaries, Practice Problems, Essay Questions...

Here are a few chapter summaries and practices problems,... I don't anticipate doing this for every quiz, but I figure that since we don't know what Professor Patrick's exams are going to be like I should give you as much material as I can.

Also, here are MY answers to the exam essays. These are NOT from an answer key made by Professor Patrick. I used my understanding of the questions and information from the text.

1) Compare propagation of the action potential in myelinated vs. non-myelinated neurons.

In a myelinated neuron, the axon is wrapped with an insulation called myelin. The myelin sheath consists of many layers of membrane provided by glial support cells—Schwann cells in the peripheral nervous system (outside the brain and spinal cord) and oligodendroglia in the CNS. Just as wrapping a leaky garden hose with duct tape facilitates water flow down inside the hose, myelin facilitates current flow down the inside of the axon, thereby increasing action potential conduction velocity.

The myelin sheath does not extend continuously along the entire length of the axon. There are breaks in the insulation where ions cross the membrane, to generate action potentials (Nodes of Ranvier). Action potential conduction without myelin is like walking down the sidewalk in small steps, heel-to-toe, using every inch of the sidewalk to creep along. Conduction with myelin, is like skipping down the sidewalk. In myelinated axons, action potentials skip from node to node and this is called salutatory conduction.

2) Describe the disagreement between Cajal and Golgi with regard to neuronal anatomy in the brain. How was this issue finally resolved?

Golgi believed that the neuritis of different cells are fused together to form a continuous reticulum, or network, similar to the arteries and veins of the circulatory system. According to this reticular theory, the brain is an exception to the cell theory, which states that the individual cell is the elementary functional unite of all animal tissues. Cajal, on the other hand argued forcefully that the neuritis of different neurons are not continuous with one another and must communicate by contact, not continuity. This idea that the neuron adhered to the cell theory came to be known as the neuron doctrine.

The development of the electron microscope offered proof of the neuron doctrine. With the increased resolving power of the electron microscope, it was finally possible to show that the neuritis of different neurons are not continuous with one another.

3) Why is the neuronal sodium-potassium pump so important for proper neuronal functioning?

The neuronal membrane potential depends on the ionic concentrations on either side of the membrane. It is important for K+ to be more concentrated on the inside and Na+ and Ca2+ are more concentrated on the outside. Ionic concentration gradients are established by the actions of ion pumps in the neuronal membrane. The sodium-potassium pump is an enzyme that breaks down the ATP in the presence of internal Na+. The chemical energy released by this reaction drives the pump, which exchanges internal Na+ for external K+. The actions of this pump ensure that K+ is more concentrated on the inside of the neuron and that Na+ is concentrated on the outside. This concentration gradient provides potential energy for the neuron to execute an action potential, much like rolling a ball up a hill gives it potential energy to roll back down.