Various Mechanisms Used by the Nervous System to Guide Axonal Growth - Essay Example

Congress 1990). The end of each growing axon is called a growth cone, and functions as a sensing device, interacting with molecules in the environment in order to reach the correct target (Schatten 2005; Bean 2012). The growth cones can be described as expanded “sensory-motile” tips of the growing axons that respond to various attractive and repulsive molecular signals in order to direct and guide axons to their synaptic targets (Myers & Gomez 2011, p. 13585). Thus, axonal growth is guided with the help of guidance cues in the environment.

As the nervous system matures, about half the neurons die and only those that have grown properly and are connected to the available synaptic sites survive (U.S. Congress 1990). A study of the mechanisms behind axon guidance to the appropriate targets is necessary in order to better understand the development of the nervous system and to elucidate the formation of the neural network. Mechanisms Underlying Axonal Guidance The guidance of axonal growth is a result of interplay of four types of guidance forces, namely contact attraction, contact repulsion, chemoattraction and chemorepulsion (Tessier-Lavigne & Goodman 1996). . At the leading end of the growth cone, actin is polymerized and then moved to the rear for depolymerisation (Bean 2012).

This continuous cycling of polymerised actin from the leading end to the rear end and then back to the leading end leads to a “tank-tread” kind of movement that enables the guiding of the growth cone (Bean 2012). Santiago Ramon y Cajal first described the movement of growth cones in 1880 (Zheng 2000; Bean 2012). He described that growth cones move towards their synaptic targets in a circuitous route, thus providing evidence that growth cones themselves have an active role in the guidance process (Bean 2012).

However, the first live microscopic observations of growth cones were not made until Ross Harrison developed modern tissue culture (Vitriol & Zheng 2012). Later on, Roger Sperry upon observing the optic nerve lesions of frogs showed that retinotectal axons could regenerate and find their targets, subsequently making precise synaptic connections (Bean 2012). It thus became evident that specific surface markers exist and are used by growth cones for the recognition of targets. This was further demonstrated through studies on zebrafish, drosophila, grasshopper and chick, which showed that guidance of axon growth followed specific and similar pathways in different species, indicating that the mechanisms are conserved across species (Bean 2012).

Studies by Gomez et al., Renaudin et al., and Robles and Gomez have shown that point contacts are formed by growth cones using various focal adhesion-related molecules like ?1-integrins, paxillin, FAK and vinculin (Myers & Gomez 2011). Extrinsic Environment-Related Mechanisms Apart from the intrinsic mechanism of growth cone movement, extrinsic