Axons are the appendages that originate in the cell body of a neuron. They are responsible for signal transmission. Their growth is regulated by growth cones, enlargements of the axons tips that actively search their environment while searching for their precise destination. Many specific molecules that help guide growth cones have been identified. The binding of these molecules tells the growth cone to move forward, stop, recoil or change direction.

These molecules include proteins with names such as cadherin, netrin, semaphorins, ephrin, neuropilin and plexin. In most cases, these are families of related molecules. For example, there are at least 15 semaphorins and at least 10 ephrins.

So Similar

The most remarkable finding about these molecules is that most of these proteins are common to worms, insects and mammals. Each family is smaller in worms or flies than in mice or people, but their functions are strikingly similar. Therefore, it is possible to use the ‘simpler’ animals to gain knowledge that can be directly applied to human beings.

For example, the first netrin was discovered in a worm and shown to guide neurons around the worm’s nerve ring. A little while later, vertebrate netrins were found to guide axons around the mammalian spinal cord.

Reaching the Target

Synapses are formed once the axons reach their target, permitting electrical signals to jump from the axon to the next cell, where they can either provoke or prevent the generation of a new signal. The regulation of this transmission and the integration of the input from thousands of synapses each neuron receives, are responsible for the incredible information processing capacity of the brain.

For this processing to occur accurately, the connections must be highly specific. Some of this specificity arises from the mechanisms that guide the axons to their proper target. Additional molecules mediate target recognition, whereby the axon chooses the right neuron and often the proper part of the target, once it arrives at its destination. Few of these molecules have been identified so far.

Contact

There has been more success, however, in identifying the ways in which the synapse forms once contact has been made. The tiny portion of the axon that contacts the dendrite becomes specialized for the release of neurotransmitters, and the tiny portion of the dendrite that receives the contact becomes specialized to receive and respond to the signals.

Special molecules pass between the sending and receiving cells to ensure that the contact is formed properly and that the receiving and sending specializations are precisely opposed to each other so that transmission can be fast and efficient.