Question

4. Pages 1084-1085 of your text compare types of nervous networks found among various kinds of organisms and discuss the evolution of complex nervous systems. What have been some general trends in this evolution? (0.5 pts)

Answer 1

The evolution of nervous systems dates back to the first development of Simple nerve net seen in animals like Cnidarians evolved first, consisted of poly-modal neurons which serve a dual purpose in motor and sensory functions. If later organisms evolved from single-celled ancestors, then there must have been some system for the transmission of information from one evolutionary stage to the next. Means a 1. stable encoding, transmitting, and decoding characteristics from one generation to the next, 2. the possibility of alterations in the code taking place by evolutionary mutation, 3. and Selection of only those characteristics for transmission that is favorable for survival.

Though different phyla may show differences there is consistency in neurotransmitter present in different organism of particular phylum. In vertebrates the motor neurons are always cholinergic mean they secret the neurotransmitter acetylcholine. On the other hand in arthropods they are not cholinergic, although the sensory neuron do secrete acetylcholine.

Cnidarians can be compared to Ctenophores, which although are both jellyfish, have very different nervous systems. Unlike Cnidarians, Ctenophores have neurons that use electrochemical signaling. This was confusing because the phylum Ctenophora was considered to be more ancient than that of Porifera, which have no nervous system at all. This led to the rise of two theories which described how the early nervous system came about. Cnidarians and Ctenophora are related animals having diffused nerve nets rather than a central nervous system.

In most Cnidarians like jellyfish the nerve net is spread more or less evenly across the body whereas in Ctenophores like comb jellies it is concentrated near the mouth.

Human shows segmented bilateral body plan at nervous system. A spinal cord that contains a series of segmental ganglia that give rise to motor and sensory nerve. Other one is brain include forebrain, midbrain and hindbrain. This is the most complex nervous system pattern in an organism.