Different neurons respond differently to each neurotropin. A given neuron may be attracted, be repulsed, or not respond to a certain neurotropin. In many parts of the vertebrate nervous system, over half of the neurons die during normal development. A target tissue can regulate the number of neurons that innervate it by limiting the supply of a neurotropin. Neurotropins can function both as chemotactic factors and survival factors.
Different neurons require different neurotropins.
Once an axon finds its target, it forms a specialized junction called a synapse. The connection may be with other neurons, with specialized glands, or with muscles. Figure 13.25 depicts the formation of a neuromuscular synapse. Formation of the neuromuscular junction requires reciprocal interactions between the neuron and the muscle . Axon of motor neuron releases agrin at the junction with the muscle.