Cranial Nerves | Autonomic Nervous System
- There are twelve cranial nerves. They exit the brain in an orderly fashion. Two, olfactory and optic, are located in the telencephalon. Two, occulomotor and trochlear, are located in the midbrain. Four, trigeminal, abducens, facial and acoustovestibular, are located in the pons. The remaining four, glossopharyngeal, vagus, accessory and hypoglossal are located in the medulla.
- It is most useful however to discuss them, not related to location, but rather as to function.
- There are three cranial nerves with primarily sensory function. Link to Sensory. Cranial nerve I, Olfactory, modulates smell, cranial nerve II, Optic,modulates vision. Cranial nerve VIII, Acoustovestibular, modulates hearing and balance.
- There are four cranial nerves with primarily motor function. Link to Corticobulbar. CN IV, Trochlear, innervates the superior oblique muscle of the eyeball. CN VI, Abducens, innervates the lateral rectus muscle of the eyeball and abducts the eye. CN XI, Accessory, innervates the trapezius muscle in the back and shoulders. CN XII, Hypoglossal, innervates the muscles of the throat and enables us to swallow.
- Five cranial nerves have mixed sensory, motor and parasympathetic function. These five will be discussed in detail here. The mixed cranial nerves are CN III Occulomotor, CN V Trigeminal, CN VII Facial, CN IX Glossopharyngeal and CN X Vagus.
- The occulomotor nerve exits the brain at the midbrain in the middle of the cerebral peduncle. As such it is compressed whenever there is swelling of the brain and herniation through the tentorium. This results in a dilated pupil which is unreactive to light. A clinical phenomenon known as a "blown" pupil. This signals dire consequences and brain death unless the condition is quickly treated.
- The occulomotor nerve supplies four of the six eyeball muscles. Link to corticobulbar. It also carries Parasympathetic input to the ciliary ganglion to constrict pupil. Link to Autonomic.
- The trigeminal is a large important nerve which carries Sensory input from the face, cornea, mouth, nose and temporomandibular joint. Nerve impulses travel from the face to the trigeminal ganglion which is located at the base of the skull, to the nucleus in the medulla. The stimulus finally travels to the Ventral Posterior Medial thalamus.
- The trigeminal nerve also supplies motor innervation to the muscles of mastication. Link to corticobulbar.
- The Facial nerve carries taste sensation from the anterior 2/3 tongue. Link to taste. It also supplies Motor function to the muscles of facial expression. Link to corticobulbar. Paralysis of the facial nerve causes a facial droop. Finally it carries Parasymapathetic input to the salivary and lacrimal glands. Link to autonomic.
- The Glossopharyngeal nerve carries taste sensation from the posterior 1/3 tongue. It also carries general sensation from the auditory tube and pharynx. This is why ear problems commonly cause funny sensations in the throat. It also supplies Parasympathetic input to the parotid gland. Link to autonomic.
- Last, but certainly not least, is the Vagus. It supplies general sensory and motor function to the larynx and pharynx. Link to corticobulbar.
- Perhaps most importantly it supplies Parasympathetic input to the heart and lungs and gastro intestinal tract to the level of the splenic flexure of the colon. Link to autonomic.
- The Hypothalamus controls temperature, sleep, water metabolism, pituitary gland, blood pressure, hunger, sympathetic and parasympathetic balance. It is a tiny structure of immense importance. It is bounded superiorly by the Hypothalamic sulcus, rostrally by the Optic chiasm and caudally by the Mamillary bodies.
- The Reticular System is also immensely important. The Descending reticular system transmits to both the sympathetic and parasympathetic nervous system and relays extrapyramidal output to voluntary muscles.
- The Ascending Reticular System also called the Reticular Activating System carries input from all sensory organs. It projects to the thalamus and cortex. This system is important for such important functions as arousal from sleep by an alarm clock, bright lights or movement.
last update Jan 21, 2002