Re: How does our brain read nerve messages?Is the nerve message read like DNA?

Question: How does our brain read nerve messages? Is the nerve message read like DNA?

From: Yan Ho

Yan-

Good question! The brain doesn’t read nerve messages the same way DNA is read. Where DNA is code made of basic building blocks that can be read to make proteins, brain cells (called neurons) don’t have a similar code that they can piece together. First, the fact that neurons can communicate with each other is key. Second, they have to rely on huge networks of many interconnecting cells to make sense of the world.

To understand how the brain as whole interprets messages, we need to first look at the individual cells. (See Figure 1) Neurons are specially designed to transmit information. The branched ends, called dendrites, receive information. The long end, called the axon, sends information – it can be up to 1 meter long! The ball in the middle, the cell body, sums up many inputs from the dendrites, and decides whether or not to send any info down the axon. (A note: a nerve is a bunch of axons from a few different neurons – like a bundle of sticks all stuck together). Cells are interconnected, with the axon of one cell contacting the dendrites of others. Information is passed from the dendrite to the cell body to the axon.

The point at which an axon touches a dendrite is called a synapse. (On Figure 1, they are the colored balls on the dendrites and at the end of the axon. I’ll explain the colors in a bit.) At this synapse, the axon can send a chemical message called a neurotransmitter to the dendrite. When the dendrite receives this message, it recognizes it and responds accordingly. This is the first important way that the brain interprets signals. There is some more written about this process here: htt p://www.madsci.org/posts/archives/dec2000/977272802.Ns.r.html

An important point is that one axon can contact many dendrites belonging to many other cells. Also, the dendrites of one cell are contacted by the axons from many different cells. You can see that this becomes a really complicated network of cells (See Figure 2). It’s similar to the telephone system – it lets you talk to lots of different people and lets lots of people talk to you. But it’s like having 1000 people on the line at once!

So we know that axons and dendrites can transmit and exchange information. But what is the information that is being passed down the axon and dendrites? What is really important to note is that axons can only send one type of signal. Let’s call it an “OK” signal (in technical terms, it’s called an action potential). At the end of the axon, this OK signal is converted into a “YES” or “NO” signal by releasing a neurotransmitter. (On Figure 1, YES is in green, and NO is red) Whether it is YES or NO depends on what kind of cell it is – there are cells that have either YES or NO neurotransmitters at the end of their axons. Dendrites, on the other hand, take both YES and NO signals (remember – they receive contacts from many different cells!). All of these signals are summed up at the cell body, and if there are more YES signals than NOs, then the axon sends an OK signal. And you can see that this will give a YES or NO signal at the end of the axon of this cell – and this YES or NO will be help or prevent the next cells send an OK signal. But if there are more NOs, then then there is no OK signal, and no neurotransmitter is released from the end of the axon. It’s not a NO signal, but the absence of a signal.

The really important point is that this big network of cells is what allows the brain to interpret signals from your nerves (like in Figure 3). Large, specialized networks of neurons in your brain are specially connected to perceive touch, and others sight, and others are wired up to let you move your muscles as you please. They are organized appropriately – the touch receptors from your arm aren’t directly connected to the parts of your brain that try to interpret vision. Certain parts of nerves will activate particular networks which respond to basic properties of a sensation, and this information will passed on to more complex networks. For instance, first you will independently establish that something is heavy, round, or smooth. Then this information will be passed into more networks, and you determine that you’re holding tell you that it’s a ball. Then that info will passed on through yet more networks, and you finally realize it’s a bowling ball!

These networks are critical in forming representations of the world in your brain. But how your nerves respond to the world is also encoded – for instance, a heavy weight on your arm may send A LOT of OK signals in a quick succession (a burst of “OK”s). So the network of cells in your brain recognizes that there is something on your arm. But since there such a huge number of signals in such a short time, your brain realizes that this weight is really heavy. In this example, a feather would barely elicit any OK signals – just enough to let you know that something is there. There is some more written about that here: http: //www.madsci.org/posts/archives/dec99/945634578.Ns.r.html

I apologize if this is a bit confusing – there is a lot of complexity in the system. We barely understand how the brain does what it does! Each level of complexity builds on the lower one. I’ve drawn 5 cells connected together in Figure 3. But the brain has 10 - 100 BILLION of these cells, each with up to 1000 connections – all contained in the top of your head!

So, in summary, to read a nerve signal, the brain utilizes the not only how often the nerve is sending a signal, but also how that signal affects the complicated networks of neurons in the brain!

-Alex G
cgoddard@fas.harvard.edu

A really good website that deals with some of the nuts and bolts of neuroscience can be found at:

http://faculty.was hington.edu/chudler/introb.html

It also includes lots of relevant links!