<P> In reality, this straightforward schema is subject to numerous complications . Although for the simplest reflexes there are short neural paths from sensory neuron to motor neuron, there are also other nearby neurons that participate in the circuit and modulate the response . Furthermore, there are projections from the brain to the spinal cord that are capable of enhancing or inhibiting the reflex . </P> <P> Although the simplest reflexes may be mediated by circuits lying entirely within the spinal cord, more complex responses rely on signal processing in the brain . For example, when an object in the periphery of the visual field moves, and a person looks toward it many stages of signal processing are initiated . The initial sensory response, in the retina of the eye, and the final motor response, in the oculomotor nuclei of the brain stem, are not all that different from those in a simple reflex, but the intermediate stages are completely different . Instead of a one or two step chain of processing, the visual signals pass through perhaps a dozen stages of integration, involving the thalamus, cerebral cortex, basal ganglia, superior colliculus, cerebellum, and several brainstem nuclei . These areas perform signal - processing functions that include feature detection, perceptual analysis, memory recall, decision - making, and motor planning . </P> <P> Feature detection is the ability to extract biologically relevant information from combinations of sensory signals . In the visual system, for example, sensory receptors in the retina of the eye are only individually capable of detecting "points of light" in the outside world . Second - level visual neurons receive input from groups of primary receptors, higher - level neurons receive input from groups of second - level neurons, and so on, forming a hierarchy of processing stages . At each stage, important information is extracted from the signal ensemble and unimportant information is discarded . By the end of the process, input signals representing "points of light" have been transformed into a neural representation of objects in the surrounding world and their properties . The most sophisticated sensory processing occurs inside the brain, but complex feature extraction also takes place in the spinal cord and in peripheral sensory organs such as the retina . </P> <P> Although stimulus - response mechanisms are the easiest to understand, the nervous system is also capable of controlling the body in ways that do not require an external stimulus, by means of internally generated rhythms of activity . Because of the variety of voltage - sensitive ion channels that can be embedded in the membrane of a neuron, many types of neurons are capable, even in isolation, of generating rhythmic sequences of action potentials, or rhythmic alternations between high - rate bursting and quiescence . When neurons that are intrinsically rhythmic are connected to each other by excitatory or inhibitory synapses, the resulting networks are capable of a wide variety of dynamical behaviors, including attractor dynamics, periodicity, and even chaos . A network of neurons that uses its internal structure to generate temporally structured output, without requiring a corresponding temporally structured stimulus, is called a central pattern generator . </P>

Explain the importance of the nervous system to the different body systems