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Cranial nerves (CNN) emerge through openings in the skull and are covered by tubular sheaths of connective tissue derived from the cranial meninges. There are 12 pairs of cranial nerves, numbered I to XII, from rostral to caudal, according to their attachment to the brain. The names of the cranial nerves reflect their general distribution and function. Like spinal nerves, cranial nerves are bundles of sensory and motor neurons that conduct impulses from sensory receptors and innervate muscles or glands.
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Rapid Review of the Nervous System
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To best understand the cranial nerves, it is helpful to remember the following information:
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- Neuron versus nerve. A neuron is a single sensory or motor nerve cell, whereas a nerve is a bundle of neuronal fibers (axons). Cranial nerves have three types of sensory and three types of motor neurons, known as modalities. Therefore, a nerve may be composed of a combination of sensory or motor neurons (e.g., the facial nerve possesses sensory and motor neurons).
- Ganglion. A ganglion is a collection of nerve cell bodies in the peripheral nervous system.
- Nucleus. A nucleus is a collection of nerve cell bodies in the central nervous system (CNS).
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Cranial Nerve Modalities
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The 12 pairs of cranial nerves may possess one or a combination of the following sensory and motor modalities (Figure 17-1; Table 17-1):
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- Sensory (afferent) neurons. Conduct information from the body tissues to the CNS.
- General sensory (general somatic afferent). Transmit sensory information (e.g., touch, pain, and temperature), conducted mainly by CN V but also by CNN VII, IX, and X.
- Special sensory (special visceral afferent). Include special sensory neurons (e.g., smell, vision, taste, hearing, and equilibrium), mainly conducted by the olfactory, optic, and vestibulocochlear nerves (CNN I, II, and VIII, respectively) as well as by CN VII and CN X.
- Visceral sensory (general visceral afferent). Convey sensory information from the viscera, including the gastrointestinal tract, trachea, bronchi, lungs, and heart, as well as the carotid body and sinus. Visceral sensory neurons course within CN IX and CN X.
- Motor (efferent) neurons. Conduct information from the CNS to body tissues.
- Somatic motor (general somatic efferent) neurons. Innervate skeletal muscles derived from somites, including the extraocular and tongue muscles. Innervation is accomplished via the oculomotor, trochlear, abducens, and hypoglossal nerves (CNN III, IV, VI, and XII, respectively).
- Branchial motor (special visceral efferent) neurons. Innervate skeletal muscles derived from the branchial arches, including the muscles of mastication and facial expression and the palatal, pharyngeal, laryngeal, trapezius and sternocleidomastoid muscles. Innervation is accomplished via the trigeminal, facial, glossopharyngeal, vagus, and spinal accessory nerves (CNN V, VII, IX, X, and XI, respectively).
- Visceral motor (general visceral efferent) neurons. Innervate involuntary (smooth) muscles or glands, including visceral motor neurons that constitute the cranial outflow of the parasympathetic division of the autonomic nervous system. The ...