Neural Physiology - Anatomy

Cell bodies of preganglionic neurons are located in the central nervous system (CNS); they synapse onto autonomic ganglia. Parasympathetic ganglia are located in or near the effector organs giving them long preganglionic fibers and short postganglionic fiber. Sympathetic ganglia are located in the paravertebral chain, thus they have short preganglionic fibers and long postganglionic fibers.

The parasympathetic nervous system acts oppositely to the sympathetic nervous system, by signaling the activation of relaxation and digestion (increased blood flow to digestive system, pupil constriction, lower heart rate and blood pressure). The parasympathetic nervous system signal is transmitted to the body through a preganglionic neuron and a postganglionic neuron. Both of these use acetylcholine in the parasympathetic nervous system.

There are twelve cranial nerves. They are given in order below, with their associated functional classes.

CN I - olfactory nerve - sensory

CN II - optic nerve - sensory

CN III - occulomotor nerve - motor

CN IV - trochlear nerve - motor

CN V - trigeminal nerve - both sensory and motor

CN VI - abducens nerve - motor

CN VII - facial nerve - both sensory and motor

CN VIII - vestibulocochlear nerve - sensory

CN IX - glossopharyngeal nerve - both sensory and motor

CN X - vagus nerve - both sensory and motor

CN XI - accessory nerve - motor

CN XII - hypoglossal nerve - motor

The twelve cranial nerves and their origins are given below:

I-Olfactory nerve; origin: frontal lobe

II-Optic nerve; origin: thalamus

III-Ophthalmic nerve; origin: midbrain

IV-Trochlear nerve; origin: midbrain

V-Trigeminal nerve; origin: pons

VI-Abducens nerve; origin: pons

VII-Facial nerve; origin: pons

VIII-Vestibulocochlear nerve; origin: pons

IX-Glossopharyngeal nerve; origin: medulla

X-Vagus nerve; origin: medulla

XI-Spinal Accessory nerve; origin: medulla

XII-Hypoglossal nerve; origin: medulla

The nerves to originate in the pons are V, VI, VII, and VIII (the trigeminal, abducens, facial, and vestibulocochlear nerves).

The parasympathetic nervous system is responsible for the "rest and digest" mechanism. When the body is at rest the following occurs: pupils constrict (ophthalmic branch of the trigeminal nerve), lacrimation and salivation occur (facial nerve and glossopharyngeal nerve), heart rate decreases, respiration rate decreases, and digestion increases (vagus nerve).

In addition to the cranial nerves, sacral nerves 2-4 (pelvic splanchnic nerves) carry parasympathic fibers that control erections, the bladder, and the bowels.

The facial nerve (CN VII) is responsible for the sensation of taste on the anterior two-thirds of the tongue. CN IX, the glossopharyngeal nerve, is responsible for posterior sensation. CN V, the trigeminal nerve, is also involved in certain sensations throughout the tongue.

Cranial nerves 3, 7, 9, and 10 are all responsible for parasympathetic nuclei of the brainstem. Oculomotor CN 3 contains the Edinger-Westphal nucleus which controls the eye smooth muscles (which if damaged would result in a dilated pupil that would not constrict in bright light). Facial CN 7 controls the superior salivatory nucleus which controls the submandibular, sublingual, and lacrimal gland secretion. Glossopharyngeal CN 9 innervates the parotid gland secretions. The vagus CN 10 innervates the heart and lungs down into the chest cavity. Helpful hint** - Students often confuse the superior and inferior salivatory nuclei, remember that on the brainstem the facial nerve exits superior to the glossopharyngeal nerve and thus how it is properly named for their nuclei.

The six extraoccular muscles of the eye are innervated by three cranial nerves. CN III, the occulomotor nerve, innervates the superior rectus muscle, medial rectus muscle, inferior rectus muscles, and the inferior oblique muscle. CN IV, the trochlear nerve, innervates the superior oblique muscle. CN VI, the abducens nerve, innervates the lateral rectus muscle.

CN V, the trigeminal nerve, is responsible for sensory information from some regions of the face and motor function of the jaw.

Cranial nerve VIII is also called the vestibulocochlear nerve. It is a sensory nerve responsible for bringing information from the cochlea and semicircular canals to the brain. The cochlea supplies auditory information, while the semicircular canals give information about the orientation of the head.

Cranial nerve I is the olfactory nerve, which relays sensory information about smell from hair cells in the nose. Cranial nerve II is the optic nerve, which relays visual sensory information. Cranial nerve X is the vagus nerve, which is highly functional in the parasympathetic nervous system.

Cranial nerve V is also called the trigeminal nerve. It is associated with sensory receptors on the face, as well as stimulation of the chewing muscles.

Cranial nerves I, II, and VIII are associated with smell, sight, and hearing respectively. Cranial nerve I is the olfactory nerve, cranial nerve II is the optic nerve, and cranial nerve VIII is the vestibulocochlear nerve. Cranial nerves VII (facial nerve) and IX (glossopharyngeal nerve) provide the sense of taste.

Face, nose, and mouth sensations, as well as mastication (or chewing), are controlled by the trigeminal nerve.

The facial nerve is associated with taste, facial expression, and production of tears and saliva. Sensation and swallowing in the pharynx are controlled by the glossopharyngeal nerve. The vagus nerve supplies the pharynx, larynx, and the viscera of the thorax and abdomen. Eye movement is controlled by the trochlear nerve.

The vagus nerve supplies the viscera of the thorax and abdomen, including the liver, stomach, kidneys, and lungs. The accessory nerve is a motor nerve supplying the neck muscles, pharynx, and larynx. Eye movement is controlled by the abducens nerve. The trigeminal nerve is responsible for face, nose, and mouth sensations and for chewing. Eye movement is controlled by the trochlear nerve.

CN XI only has motor function (innervation of the trapezius and sternocleidomastoid muscles). The other nerves listed have the following parasympathetic functions: pupil constriction (CN III), salivation (CN VII and CN IX) and parasympathetic modulation over multiple internal organs (CN X).

CN XI (accessory nerve) is responsible for head turning and shoulder shrugging via its innervation of sternocleidomastoid and trapezius muscles. CN I (olfactory) is involved in olfaction. CN VI (abducens) allows for eye movement via its innervation of lateral rectus. CN VIII (vestibulocochlear) functions in hearing and balance. CN XII (hypoglossal) is responsible for tongue movement.

Cranial nerve III is the oculomotor nerve, which controls most of the muscles needed for eye movement. Also, this nerve controls the ciliary muscle and is responsible for pupillary constriction via parasympathetic innervation. The cranial nerve that makes the eye look to the side is the abducens nerve or cranial nerve VI. The muscle that helps roate the eye down and out is the trochlear nerve or cranial nerve IV. The nerve that carries visual information from the back of the eyeballs to the brain is the optic nerve or cranial nerve II.

Cranial nerve (CN) VIII is the vestibulocochlear nerve. It is purely sensory in function, and is involved in audition, and proprioception. CN VII is the facial nerve. CN X is the vagus nerve. CN II is the optic nerve.

The vagus nerve (cranial nerve X) innervates the larynx, and is involve in speech production. The accessory nerve innervates the muscles in the neck and upper back. Cranial nerve IX is the glossopharyngeal nerve and innervates the pharynx. Cranial nerve XIII is the hypoglossal nerve that innervates the tongue.

The inferior alveolar nerve passes through the mandibular foramen. It has motor and sensory components. It serves to give the lower face and jaw sensation and is also responsible for stimulating the muscles for movement of the jaw.

The trigeminal nerve (cranial nerve V) is responsible for facial sensation and motor functions, such as biting and chewing. The trigeminal nerve has three major branches (hence: tri-): the ophthalmic nerve (V1), the maxillary nerve (V2), and the mandibular nerve (V3). The opthalmic and maxillary nerves are purely sensory, and the mandibular nerve has both sensory and motor functions. VI provides sensation to the forehead, upper eyelids, and the bridge and tip of the nose. V2 provides sensation lateral to the eyelids, the cheeks, nostrils, and upper lip. V3 provides sensation to the lower lip, chin, skin over the jaw, and pre-auricular area.