The cranial nerves are 12 pairs of nerves that connect directly to the brain, unlike most other nerves which connect to the spinal cord. The nerves going out to the body may be voluntary, such as those controlling skeletal muscles, or they may be involuntary, or autonomic. The autonomic nervous system is composed of a sympathetic system responsible "fight and flight" types of activities, and a parasympathetic nervous system responsible for "rest and digest" activities.
Cranial Nerve III
Cranial nerve III, or the oculomotor nerve, exits the brain to serve some functions of the eye. This nerve carries motor neurons, which are nerves that control voluntary muscles, and parasympathetic fibers. The motor neurons control several of the muscles responsible for eye movement. The parasympathetic fibers lead to the lens and iris, and cause constriction of the lens for vision involving close objects. They also cause constriction of the iris as a result of stimulation from light. If damaged, the eye will not be able to focus on close objects and will be dilated.
Cranial Nerve VII
Cranial nerve VII, or the facial nerve, is largely responsible for moving the muscles of facial expression through motor neurons. The parasympathetic fibers travel to three different places. First, they go to some of the salivary glands in the mouth, specifically the submandibular glands and sublingual glands, under the jaw and tongue, respectively. Second, they travel to the nose to signal for nasal secretions. Then they travel to the lacrimal gland, or tear duct.
Cranial Nerve IX
Cranial nerve IX, also known as the glossopharyngeal nerve, provides sensation to the back of the throat and tongue, including touch and taste. It also senses the blood pressure in the major arteries of the neck called the carotid arteries. Its motor function is to help lift the throat during speech and swallowing. The parasympathetic fibers from the glossopharyngeal nerve run to the parotid gland, which are large salivary glands at the back of the cheeks.
Cranial Nerve X
Cranial nerve X is known as the vagus nerve. It helps move muscles in the mouth and throat, including some of the muscles of speech, and some sensation of touch in the ear canal. The parasympathetic fibers actually travel down in the body through the chest and into the abdomen, where they are responsible for the rest and digest functions in many of the organs, including the kidneys, pancreas, gallbladder, liver, stomach, intestine, heart and lungs. Examples of their function include slowing the heart rate, stimulating digestion and secreting mucus in the airways.
References
- Encyclopaedia Brittanica: Human Nervous System: Parasympathetic Nervous System
- "Neuroscience"; Dale Purves; 2007
