The cranial nerves provide the sensory and motor interfaces

between the brain and the structures of the head. They supply the sensory inputs from our more than five senses and the motor (effector) innervation of muscles and glands. Like spinal nerves, the cranial nerves have sensory, or afferent, components that innervate structures in the head as well as the viscera of the thorax and abdomen and motor, or efferent, components that innervate muscles and glands in the head and the viscera. Three additional "special" components of cranial nerves are commonly recognized that spinal nerves lack; however, insights into the embryological derivation of sensory structures and muscles in the head allow us to discard this special category. Considering the sensory cranial nerves, humans arguably have at least 13 different senses, but even so we lack some additional senses that are present in other vertebrates. Some tetrapods, including most mammals, have an accessory olfactory (vomeronasal) system, for example, that is present in humans only transiently during embryological development, and humans (along with other mammals) entirely lack the lateral line mechanoreceptive and electroreceptive systems of most aquatic vertebrates. We do not possess the infrared-receptive system of snakes or the independently evolved electroreceptive and mechanore-ceptive systems via the trigeminal nerve that platypuses have. With regard to these other systems, we are, as William Wordsworth wrote, "creature[s] moving about in worlds not realized.'' Nevertheless, our set of sensory and motor, cranial and spinal nerves is the essential and only connection that the human central nervous system has with the external world. Via these nerves, all the sensory stimuli that one can detect are brought into the brain, and all the motor actions that one makes are commanded.

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