A. Concept of membrane potential--A cell membrane has a charge associated with it. This is due to the fact that even if the cell is in osmotic balance, it is not in ionic balance. That is, there will be a difference of charge between the interior of the cell and the exterior of the cell. This is associated with the fact that large molecules, such as proteins, contribute to the osmolarity of the cell interior (or organism interior). If the ions were equally distributed in and outside of the cell, then the organism would be in osmotic imbalance. Therefore, there is an unequal balance of ions between the interior and the exterior and since the ions have a charge associated with them, there is a difference of charge between the interior and exterior. This constitutes the membrane potential

B. Concept of information--The concept of information maybe slightly analogous to the question as to if a tree falls in a forest and there is no one there to observe it, does it make a noise. Even thing has an information content, that is, how much information does it take to specify the attributes of an item. The question though is, if that content doesn't change how do we know what that information content is. When we think of nervous systems, we can think of them as systems that process information. An organism needs to know whether its environment, both internal and external, is changing, so it can respond to those changes. If a cell has a membrane potential and that potential is changed, i.e., it is depolarized, it is a change of state and in a way that cell because of its depolarization, has now detected a change of state or a change in the information associated with its environment. The membrane potential is changed because something has caused a flux in the ions associated with the cell. To restore the original membrane potential, an organism will now have to pump ions either into or out of its interior. The change of state has caused the organism to use energy.

C. The nature of nervous systems

1. In its simplest sense, any organism in which the cells change membrane potential, the cells are functioning as a nervous system. One can assume that as one cell changes state, due to its contact with adjacent cells that information will be decrementally passed along (decremental because it spreads radially from the source).

2. Actual nerve cells have processes leading into (dendrites) and out of(axons) the central area (called the cell body). With respect to the axon, information is passed down its length, because localized waves of depolarization pass down its length, causing changes in its membrane polarity. The rate of conduction is related to the cross-sectional diameter of the nerve fiber, with larger fibers faster and smaller slower. This is a major difference between invertebrate and vertebrate nerves, as vertebrate nerves are myelinated (that is, they have a sheath around the fiber that causes the impulse to jump from node of Ranvier to node of Ranvier --a process called saltation). Collections of process are called nerves

3. Information is carried across the gap between the end of the axon and the next cell or dendrite either by "electral sparking" (rare) or by chemical mediation via a neurotransmitter (most common)

a. direct contact--

b. chemical mediation--end of axon secretes a substance, often acetylcholine, that diffuses across the gap and causes the dendrite or cell to depolarize. If the acetylcholine were to stay in the gap, the nervous would constantly be telling a story to the next unit, an undesirable feature. Therefore, what happens is that a short time after the axon releases acetylcholine, acetylcholine esterase is released the immobilizes the activity of acetylcholine. This stops the transmission of information across the gap. This also makes the gap a one-way system, as the dendrite or cell can not secrete acetylcholine in the direction of the axon.

4. When cell bodies are collected in one spot, they are called a ganglion--The earliest systems, such as what we see in sponges or cnidaria simply involve cell contact to cell contact, where information is spread in a decremental manner. In flatworms, there is a ladder type system with two parallel nerve cords with cross connections between cords. Because of bilaterallity, there is a collection of nerve cell bodies at the anterior end and we call this structure a brain.

5. Another difference between invertebrates and vertebrates is that invertebrates often have many ganglia in many parts of the body, that all function as integrators (localized brains). Vertebrates typically only have one brain.

D. Some general features of action

1. Sensory structures to detect changes in external or internal millieus.

2. Nerves to conduct information to centers of integration and decision making

3. Ganglia (brains) to make decisions. Intelligence as a maximizing of the number of interconnections.

4. Nerves to carry directions to body organs and appendages

5. Motor structures to carry out the work activities