11-03-06, Nervous system and Transportation

This whole exercise was one in cellular transportation. We talked about transportation being a function of (this is just review) and that transportation is a function of entropy. Entropy is the ability or tendency of a system to break down and “become disorder” and the disorderliness of that system is based on the molecules moving from an area of high concentration to an area of low concentration.

And we also talked about how water molecules can effect that concentration relative to the cellular membrane. In order to understand the movement of those materials across the cell membrane is certainly influenced by the structure of the cell membrane. Remember the structure of the cell membrane: phospholipids, three different kinds of integral proteins, carbohydrates, the steroid cholesterol. We talked about cell-to-cell communication, active transport, passive transport, and today we are going to talk about the transmission of messages from one cell to another cell and how that all comes together in the nervous system.

To begin with, you need to understand the structure of the nerve cell. Like all specialized cells, they will have the basic cell of the tissue and that cell is called the neuron. Neurons are composed of three parts: bumpy-little dendrites, the body of the cell, which eventually narrows down into some long, singular axon.

Dendrites are short and stumpy and receive molecular messages. The axons transmit the messages onwards. The body of the cell converts dendrite messages to axon messages.

Neurons are cells that transfer electrical signals throughout the body. Read up on the myelin sheath and the nodes of Ranvier, also about axon terminals and Schwann cells.

Alright, so we will roll over the image to learn more. The cell body is in the center. The dendrites are the shorter ones. The axons are the longer ones. There are sections on the axons. Look at the axon. Axons are covered by this sheath that is made of concentrated lipids and it is called the Myelin sheath. The Myelin sheath prevents random transmission. It isolates the nerve message from other cells. It keeps it from jumping around.

Read about root canals in humans. Going into the back teeth of the adult male and taking the pick and scrambling around the nerve to the tooth. “He said it would be a little uncomfortable, and really it was just as comfortable as the guillotine is scary.” The electrical shocks would put Ragsdale to his knees when the nerve ends would shock and fire off. As you destroy the Myelin sheath the nerve cells are sending out messages to find out where they are and trying to find neurons to connect to. The Myelin sheath is an insulator and they do not pass electrical message. It restricts the message to the axon.

Those gaps that make the axon look like an individual cell are indeed individual cells. They are like hotdog lengths put together. Each of those are covered by a Schwann cell. Schwann cells are very specialized cells that increase the rate of transmission along the length of the axon. You will understand this better when you learn about the transmission of the nerve cell along an action potential. Basically, the Schwann cell bumps those messages across. They increase the rate of the action potential. Odd how this `action potential' is mentioned in neuroscience and then there are laws of least action in quantum physics and so on.

The points on which the Shwann cells touch the axons are called the intersections between Schwann cells and the axons - these are the nodes of Ranvier. Those are related to the gates for ionization that let ions filter through and so on.

Neurons have an electrical charge that differs between the fluid around them. The resting potential is the electrical potential across the cells when not active. The active potential is done when there is some action of the cell that facilitates the transmission not electrical impulses. First of all, the potential itself is the difference in the electrical charges. There is a normalcy to the nerve cell. You have thousands of nerve cells at the moment resting and not doing anything. The sound of Ragsdale's voice is stimulating specific sets of neurons and as that stimulus arrives at the nerve cell there is a difference between the positive and negative ions in and out of the cell and that difference is called the resting potential. The resting potential is the electrical potential across the cell membrane. (Bryan's note - electrical signals from the electromagnetic spectrum and so on are amplified by the resources in the human body and not the original stimulus). The resting potential is reversed (the polarity is reversed) when a signal is received. That creates an action potential. When you get action potential you get transmission of a message along the axon.

The axon is a membrane-bound structure and transmits action potentials away from the cell body. The Myelin sheath is a bilayer. The gaps in the Myelin sheath are known as the nodes of Ranvier. Neurons are negatively charged on the inside when in their resting state. There is a greater concentration of potassium ions inside the resting-state neuron. This resting potential is maintain by the sodium potassium pump which actively pumps potassium in and sodium out. Sodium ions actively move into the axon causing the inside to become more positively charged than the outside. The sodium gates are opened in the next gate is activated and in this way the Myelin sheath is able to conduct the electrical signal. After this, the cell enters the refractory period and the outer membrane once again becomes positive and the inside-membrane returns to its original negative state. Sodium-potassium pumps resume “potassium in, sodium out” and these pumps require constant supplies of ATP and consume a great deal of the body's energy.

The neuron is not able to fire in that refractory period. The neuron is ready for another action potential when the sodium potassium pumps have again been working for some time. Try hitting the knee at that Tellar tendon and you would think that if you hit it a bunch of times that there would be the same movement. Not the case. The neurons only fire off every so often. There is the refractory period. Nerve cells firing off all of the time are not good things because it uses way too much energy.

You need to understand that the resting potential of the cell is when the negative ions of the cell outnumber the positive number of ions on the outside. That balance is maintained by the sodium-potassium pump which trades potassium ions on the outside and the sodium ions on the outside and this is a 2:1 ratio. This is the resting potential. The body of the cell is slightly negative in this resting potential state. The negative ions are inside of the cell and they are potassium ions on the inside that outnumber the charge of the sodium ions on the outside. That is the constant flow. Go back to the notes on active transport. There are ionic pumps. Active transport requires ATP energy. Ionic pumps require ATP energy. This figure may be slightly exaggerated but it is pretty amazing (never quit learning)—Ragsdale read or heard someplace last week that as much as 85% of the energy that you use in your body is used in delivering action potentials from one nerve cell to another nerve cell and that energy is basically wrapped up in the way that the sodium potassium pumps work. So when you hear that sodium potassium pumps are using lots of energy, they are using tons of energy.

Diet and thinking is strongly correlated. They found that elementary school students that consume tons of sugar do not think as well as children on a well-balanced diet. The problem is that when you overload things do not get stored in the right places. Those sugar highs make you feel shaky and that's because your body is using up energy in the wrong places and not storing it in the right places and that makes it inaccessible. One of the fallacies of athletics is that you really need to give kids cokes and candy bars during half time. That's not true. Researchers found out that when you overload them with simple sugars that are immediately converted to glucose that there is this burst of energy and then there is this loss of energy that is greater than the use of the bursts. You have read about carb-loading and blood-doping. Blood-doping is where people take endrophines and reinject them into the body to get people `high' again. You do want to go very high and crash.

The nerve impulse triggers the reversal of polarity and this is called the action potential. The reverse in polarity is produced from the additional number of sodium potassium pumps that open through the cell membrane. There is a sequential basis to the opening of those pumps. There is the sequential opening of the pumps. The action potential creates the sequential nature of the opening and closing sodium potassium pumps. As they travel across the length, the message is carried electrically along the length of the cell membrane.

The rise in voltage opens up the sodium gates in the adjacent segment. This is kind of like a flame running across some rope. You can actually read the voltage levels. Frog legs were used in one of the earlier experiments on the nervous system. Frogs have primitive nervous systems and so they tend to not die when you put frog legs in the pan.