When starting a project in AI, robotics, or motion control, I like to get a head start by checking what solutions Nature might have already worked out. Neurons are organized within an organism according to a few basic plans. Each plan introduces an order of magnitude increase in complexity and increasingly complex behavior. Within each plan, organisms exhibit a gradation of complexity.
Nerve Net
The most primitive organization is homogeneous. Animals like the hydra, jellyfish, and anemone have hundreds of neurons scattered throughout the body and connected in a network.
The neurons embody mostly autonomic regulatory behaviors such as the production of chemicals and hormones necessary to sustain life. Sensory input available to the neurons is limited to primitive cells for the detection of light levels, sense hairs, balance, chemical levels, and hormonal levels. Neurons are generally clustered around the mouth, but there are no clusters of neurons with specialized functionality.
Ring Organization
The ring organization of the starfish enables basic behavior. Since the spatial organization of the network is patterned, the neurons can coordinate in a patterned way to generate mass action among the neurons to coordinate motion.
There is no central control in the thousands of neurons making up a ring organized nervous system. Beyond the capabilities of the more primitive creatures, sensory input includes light sensing dots, external chemical sensors for identifying food, and pressure sensors on tentacles. This is enough complexity to allow these simple creatures to negotiate their environment and engage in various survival and reproductive activities.
Nerve Cord
The tens of thousands of neurons found in some invertebrates such as leeches and worms are organized as a nerve cord studded with large groups of cells known as ganglia. The cord structure matches the physical structure of the organism - there is a ganglion dedicated to the control of each segment of the animal, and a thickening in the head of the animal called the cerebral ganglion. Each of the ganglia along the nerve code has a preferred mode of operation – a ganglion associated with locomotor muscles in a segment might tend to spontaneously oscillate in a pattern that advances the organism forward. The cerebral ganglion moderates the behavior of the ganglia along the nerve cord through selective inhibition – it suppresses the natural oscillations of the nerve cord ganglia, and releases them to affect a behavior, such as crawling, or pushing food through the gut.
New sensory capabilities include sensory hairs on the skin, chemical sensors to guide the animal towards food, and occasionally primitive eyespots.
Neural Ladder
The insects and arthropods demonstrate the next level of sophistication above the protocordate. The insect's nervous system has hundreds of thousands of neurons, organized into a bilateral ladder, topped by a brain. The rungs of the ladder correspond to the insect's physical structure, with a pair of ganglion in each segment. Neighboring ganglia are connected, both across the segment to the other ganglion in the pair, and to the neighboring segments forward and aft.
The brain is called the corpora pedunculata, or mushroom body, due to its shape. A simpler insect will have larger ganglia in the segments along the ladder and a small brain, whereas a more sophisticated insect will have simpler ganglia with more functionality migrated towards the head of the insect. As in the the worms and leeches, behaviors are governed by the brain through the mechanism of selective inhibition.
Improved senses include mechanical sensors in joints for proprioception (self-sensing), antenna for sensing airborne chemicals and vibrations, and facetted eyes.
Subsystems
The cephalopods - the squid, octopus, and so on - have a nervous system comprised of millions of neurons. The neurons are arranged in lobes and tracts. The major improvement made by the cephalopod brain is the functional subsystem -specialized cortices and nuclei exist for motor control, sensory integration, and so on.
Cephalopod tentacles have somewhat independent nervous systems, reducing the behavioral and motor control burden on the central nervous system. Each tentacle is roughly as intelligent as a protocordate like a worm. The cephalopod nervous system reaches its highest development in the octopus, which can have as many as three hundred million neurons.
The cephalopods enjoy an improved eye design with a lens, iris, and retina. Their tentacles have sophisticated organs of touch.
Vertebrate Nervous System
Vertebrates have even more complex nervous systems, the highest development being found in the primates and cetacea. The human brain has billions of cells, commonly estimated to number on the order of ten to the tenth. The vertebrate brain is stereotypical from species to species and has specializations to suit an organism's particular niche. As an example, the bat has an extremely well developed auditory cortex.
The nervous system is comprised of hundreds of systems organized by function. The primary divisions are the cerebral cortex, the limbic system, and the autonomic nervous system. One significant improvement introduced in the vertebrate nervous system is that sheets of generic processing cells govern the specialized subsystems, and process signals from sensory organs and even between various systems within the nervous system.
The cerebral cortex is where we imagine thinking and memory take place. It is subdivided into functional subsystems, such as the visual cortex, the motor cortex, and the association cortex or frontal lobes. The cerebral cortex is organized as thin wrinkled sheets of similar neurons, highly interconnected.
Underlying the cortex is the limbic system containing large functional subsystems. The limbic system governs learning, conditioning, hormonal production, and behavioral strategy.
The cerebellum is another layered system of identical regularly connected cells. It sits at the base of the skull, and helps coordinate movement for the motor cortex. All sensory information to the brain (except for smell) goes through another regular structure, the thalamus, often called the gateway to the brain.
The spinal chord routes information from all over the body through to the cerebellum and the thalamus, and also governs many reflex actions such as jerking away from fire.
Other major nerve centers not located in the brain itself are the solar plexus and the spinal ganglia which have a great deal to do with unconscious and involuntary reactions, and how the organism feels at any particular time.