13.4 The peripheral nervous system  (Page 2/62)

Nerve structure

Close-up of nerve trunk

Cranial nerves

The nerves attached to the brain are the cranial nerves, which are primarily responsible for the sensory and motor functions of the head and neck (one of these nerves targets organs in the thoracic and abdominal cavities as part of the parasympathetic nervous system). There are twelve cranial nerves, which are designated CNI through CNXII for “Cranial Nerve,” using Roman numerals for 1 through 12. They can be classified as sensory nerves, motor nerves, or a combination of both, meaning that the axons in these nerves originate out of sensory ganglia external to the cranium or motor nuclei within the brain stem. Sensory axons enter the brain to synapse in a nucleus. Motor axons connect to skeletal muscles of the head or neck. Three of the nerves are solely composed of sensory fibers; five are strictly motor; and the remaining four are mixed nerves.

Learning the cranial nerves is a tradition in anatomy courses, and students have always used mnemonic devices to remember the nerve names. A traditional mnemonic is the rhyming couplet, “On Old Olympus’ Towering Tops/A Finn And German Viewed Some Hops,” in which the initial letter of each word corresponds to the initial letter in the name of each nerve. The names of the nerves have changed over the years to reflect current usage and more accurate naming. An exercise to help learn this sort of information is to generate a mnemonic using words that have personal significance. The names of the cranial nerves are listed in [link] along with a brief description of their function, their source (sensory ganglion or motor nucleus), and their target (sensory nucleus or skeletal muscle). They are listed here with a brief explanation of each nerve ( [link] ).

The olfactory nerve    and optic nerve    are responsible for the sense of smell and vision, respectively. The oculomotor nerve    is responsible for eye movements by controlling four of the extraocular muscles    . It is also responsible for lifting the upper eyelid when the eyes point up, and for pupillary constriction. The trochlear nerve    and the abducens nerve    are both responsible for eye movement, but do so by controlling different extraocular muscles. The trigeminal nerve    is responsible for cutaneous sensations of the face and controlling the muscles of mastication. The facial nerve    is responsible for the muscles involved in facial expressions, as well as part of the sense of taste and the production of saliva. The vestibulocochlear nerve    is responsible for the senses of hearing and balance. The glossopharyngeal nerve    is responsible for controlling muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva. The vagus nerve    is responsible for contributing to homeostatic control of the organs of the thoracic and upper abdominal cavities. The spinal accessory nerve    is responsible for controlling the muscles of the neck, along with cervical spinal nerves. The hypoglossal nerve    is responsible for controlling the muscles of the lower throat and tongue.