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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.
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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.
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CENTRAL NERVOUS SYSTEM (CNS) AND PERIPHERAL NERVOUS SYSTEM (PNS)
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The central nervous system (CNS) consists of the brain, brainstem, cerebellum, and spinal cord (Fig. 3–1). The peripheral nervous system (PNS) includes the dorsal and ventral roots that enter (dorsal roots) and exit (ventral roots) the spinal cord and continue into the peripheral nerves. Before devoting the rest of this chapter to the internal and external structures of the brain, a brief orientation to the brainstem, spinal cord, and cerebellum is provided here. The brainstem is divided into three levels from superior to inferior: midbrain, pons, and medulla (see Ch. 9). The medulla transitions seamlessly into the spinal cord, which itself is divided into four levels: cervical, thoracic, lumbar, and sacral (see Ch. 5). The cerebellum lies posterior to the brainstem and inferior to the posterior aspects of the cerebral hemispheres, and is connected to the brainstem by way of the three cerebellar peduncles (see Ch. 8).
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