The nervous system serves three main functions: perception, cognition, and action. Perception is the translation of the outer world into electrochemical signals that can be interpreted by the brain. For example, light information is converted by the retina and then sent to the brain by the optic nerves (cranial nerve 2); sound is transformed by the inner ear apparatus and transmitted to the brain via the auditory nerves (cranial nerve 8). Action is the brain’s way of allowing the organism to interact with the environment by moving the body (and in the case of humans and some other animals, by using movements of the vocal apparatus to communicate). Cognition includes all of the operations that interpret perceptual input to understand the external environment, and plan the interaction with the environment through action.
In neuroanatomic terms, perception is carried out by the input to the nervous system (afferent pathways), action is the output (efferent pathways), and cognition arises from interconnections within and between perceptual modalities, as well as between perception and action. Perception begins with the sense organs (skin, eyes, ears, nose, mouth) and travels in peripheral nerves (including cranial nerves for the structures of the head), ultimately transmitting information to the sensory cortices of the cerebral hemispheres (e.g., somatosensory cortex, visual cortex, auditory cortex). Motor output is controlled by the motor cortex, whose signals travel by way of the motor pathways to ultimately reach the peripheral nerves that will command muscles to move (see Ch. 4). The motor cortex collaborates with adjacent structures (premotor and supplementary motor cortices) and participates in circuits involving the basal ganglia (see Ch. 7) and cerebellum (see Ch. 8), all of which work to coordinate and execute movements.