6 Axon guidance
6.1 The intricacies of neuron growth
Particular nerves, such as those sensory nerves that arise from the nasal retina (the side of the retina adjacent to the nose), cross the midline; other sensory nerves, such as those that arise from the temporal retina do not. This pattern is consistent between individuals, to such an extent that a map of the nervous system is good enough to be used to locate a particular nerve pathway in most people. (See Figure 14.)
The precision and consistency of connections raises questions about how these connections come about. The embryo, after all, just has clusters of neuroblasts in various places in the neural tube. As these neuroblasts continue to differentiate into, say, efferent neurons in the motor cortex, they must put out axons that grow and make links to their targets. In the case of a special type of cortical output neuron, Betz cells, the targets are the motor neurons that control the muscles in the arm or leg. Betz cells are either in the left or the right motor cortex, and they innervate motor neurons in the spinal cord on the opposite side of the body. In effect, Betz cells control the muscles in the contralateral limbs. Betz cells do not innervate the muscles at the back of the eye or the motor neurons in the spinal cord on the same side of the body, to control the ipsilateral limbs; they innervate only the motor neurons in the contralateral spinal cord. How do Betz cells do that; how do they cross the midline, from left to right (or right to left) and grow to the contralateral motor neurons and not to, say, the back of the eye? This question of whether to cross or not to cross the midline is addressed later (Section 6.4). The next two sections address the question of how do neurons grow (Section 6.2) and how do neurons know where to grow (Section 6.3).
There are two main reasons why these two particular questions are addressed here, rather than any of the myriad other possible questions about the development of the nervous system. First, axon growth continues postnatally, and so it is possible to visualise how postnatal environmental events, such as those discussed in earlier sections, might influence axon growth and hence behaviour. Second, axon growth is of major interest to medical science as damage to the nervous system, in particular to the spinal cord, is currently extremely difficult to repair.