Explain what happens whena. filopodium contacts attractive substrateb. filopodium contacts a repulsive substrate

a. When a filopodium contacts an attractive substrate, it initiates a process known as axon guidance. This is a complex mechanism that involves the growth cone, a specialized structure at the tip of the growing axon. The filopodium, which is a slender cytoplasmic projection, senses the attractive substrate and responds by stabilizing and growing towards it. This is facilitated by the binding of guidance molecules present in the attractive substrate to their corresponding receptors on the filopodium. This binding triggers intracellular signaling pathways that lead to the reorganization of the cytoskeleton, promoting growth towards the attractive substrate.

b. When a filopodium contacts a repulsive substrate, the opposite occurs. The repulsive substrate contains guidance molecules that bind to their corresponding receptors on the filopodium, triggering intracellular signaling pathways that lead to the collapse of the growth cone. This is achieved by the destabilization and retraction of the filopodium, steering the growth cone and hence the growing axon away from the repulsive substrate. This mechanism ensures that the axon grows in the correct direction, avoiding areas that are not conducive to its function.