Advanced Nervous Systems - General Trend In Evolution
In higher groups of animals you will observe clear trends towards centralisation of the nervous system, to maximize effectiveness. The diffuse peripheral network becomes less conspicuous or not recognizable giving place to nerves made up of nerve fibers. The motor neurons and association neurons become mainly concentrated and centralised into masses known as ganglia. The increasing dominance of the association neurons that are the links in reflex pathways, makes possible more complex central pathways that form the structural basis of advances in integration. The sensory neurons become relegated to the periphery. This results in differentiation of functional tracts of neurites and nerves by which stimuli are brought to and carried out of the central nervous system. Thus better association or integration of afferent sensory stimuli along with meaningful efferent motor outputs become possible, resulting in more intricate and capable behavioral patterns. Perikarya become limited to the rind or cortex of the ganglia, the interior being made up of nerve fiber mass called neuropile where conspicuous pathways or tracts already evident in the lower groups, become developed. The development of giant fiber system associated with the central nervous system further improves the efficiency resulting in enhanced speed of propagation of impulse and hence efficiency of response to stimuli. With one end of the animal always directed forward throughout locomotion, an anterior end and bilateral symmetry become established. The anterior end becomes more significant with most sensory organs becoming associated with this end. Naturally, the ganglia become better developed and much more highly organized in this region, giving rise to the structurally and physiologically more complicated brain dominating over the whole organism. This is the direct result of cephalisation.