January 05, 2005

Development of Synaptic Connections

Synapses. I like them. They are small and funky, like Christina. Unlike Christina, synapses permit signalling between an axon terminal and another neuron or cell type (for example a muscle fibre). When you decide you want to move your booty, for example, the signal passes down from your brain to your booty muscles via your spinal cord. Now, there is not one neuron that goes all the way from up there to down there. There are a few that form a chain. 'Synapses' are the gaps between two neurons. Or they can be thought of as the structure that includes the end of the first neuron, the gap, and the beginning of the second neuron. Or they are the end of the first neuron, the gap, and the bit of the muscle they are stimulating. Here is a picture of the brain-booty pathway.

Before we move on, have another look at this diagram. The first few synapses are connecting one neuron to another neuron, right? These are called CNS synapses. CNS means central nervous system. Again: neuron-to-neuron connections are CNS synapses. Now look at the last synapse. It connects a neuron to a booty muscle. Where a synapse is connects a neuron to a muscle, we call this a neuromuscular junction (or NMJ). Because it is a junction between a neuron and a muscle.

You're with me so far. I'm going to speed it up a little bit because I want to talk about the development of synaptic connections. Synapse formation takes place in a series of steps, and these steps are controlled by some crazy-but-impressive internal signalling between two nerves (for a CNS synapse), or between a nerve and a muscle (for a neuromuscular junction, NMJ). Once these connections have been formed, they are then refined by external sensory input. So basically, a bunch of connections are just splurged everywhere at first, and then, depending on how they are used, some die (or are killed off) and some remain. It sounds cruel, and let me be quite frank; it is. The nervous system is a dog-eat-dog-eat-synapse world. For the rest of this post, and in most literature you read, people will often only tell you about NMJ synapses and say: "er, this is probably true for CNS synapses, too." This is because it is much easier to study NMJ synapses than CNS synapses, so loads of research has already been done on it.

Look at the emphasis above: synapse formation controlled by internal signalling and refined by external sensory input. Those are the two stages in synaptic development and those are the two sections in this post. Let's start with section 1.

Section 1: Intrinsic Factors influencing Synaptic Development

Ok, so axons grow out from wherever they have originated from and head towards their target by a beautiful and still quite mysterious system of signalling methods. We are not concerned with how they get to how they get to where they are going, but are more concerned with how they make a synapse at their target. So how does the axon synapse onto the muscle at a NMJ?

How?

It does it in only three steps!

(For legal reasons I must point out that although it does technically do it in only three steps, these three steps can fill most of a book. I will not fill most of a book. Scroll down and check if you are worried about being forced to fill most of a book.)

Here are the three steps if you are in a rush and need to quickly tell someone this. Maybe you are on a gameshow right now. Here is the answer but you should know this is a very stupid gameshow.

The three easy yet eyebrow-raising steps for synapse formation

1. The formation of some selective connections between the growing axon and its target (e.g. the muscle)

2. The shocking transformation of the tip of the axon (axonal growth cone) into a nerve terminal

3. The development of the amazing machinery in the target cell that will be able to the recognise signals given off by the axon.

Nice and vague. Science, eh? Who needs it?

The next post will go into these in more detail. My back hurts from typing so I am going to go to bed.

Posted at 9:36 pm | 1 comments