Figure 41.1 illustrates some of the types of nervous systems found in various animals. Note the varying levels of complexity from a nerve net to the complex mammalian system. Functions, Cell types, and Structures Neurons are the functional cells of the nervous system.
Sensors receive and convert information.
Effectors carry out functions at the command of the nervous system. Ganglia are clusters of neuron cell bodies in primitive animals' nervous systems and are clusters of neuron cell bodies in the PNS of more advanced organisms. Glial cells support the neurons in many ways. Astrocytes create the blood-brain barrier that keeps unwanted substances out of the brain tissue. Other glial cells supply nutrients, consume debris, or maintain the ionic environment of the nervous system.