CHAPTER 17: Cranial Nerves

OVERVIEW OF THE CRANIAL NERVES

BIG PICTURE

Cranial nerves (CNN) emerge through openings in the skull and are covered by tubular sheaths of connective tissue derived from the cranial meninges. Twelve pairs of cranial nerves are 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.

RAPID REVIEW OF THE NERVOUS SYSTEM

To best understand the cranial nerves the following information is helpful to remember:

• 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).

CRANIAL NERVE MODALITIES

The 12 pairs of cranial nerves may have one or a combination of the following sensory and motor modalities (Figure 17-1; Table 17-1):

• Sensory (afferent) neurons. Carry information from the body tissues to the CNS.

  • General sensory (general somatic afferent) neurons. Transmit sensory information (e.g., pain, temperature, and touch), carried mainly by CN V but also by CNN VII, IX, and X.

  • Special sensory (special visceral afferent) neurons. Include special sensory neurons (e.g., smell, vision, taste, hearing, and equilibrium) that are carried by CNN I, II, VII, VIII, IX, and X.

  • Visceral sensory (general visceral afferent) neurons. Transmit visceral sensory information (e.g., stretch and baroreceptors) from the heart, lungs, GI tract, carotid body, and carotid sinus mainly within CNN IX and X.

• Motor (efferent) neurons. Carry 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 CNN III, IV, VI, and XII.

  • 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 CNN V, VII, IX, X, and XI.

  • 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 preganglionic neurons originate in the brainstem and synapse outside the brain in parasympathetic ganglia. The postganglionic neurons innervate smooth muscles and glands via CNN III, VII, IX, and X.