Peripheral Nervous System - Anatomy

The sympathetic nervous system is typically associated with "fight or flight" responses in the body. When you think of stressful situations, it helps to think of adrenaline (or epinephrine) being used by the body. The postganglionic neurons of the sympathetic nervous system will release the neurotransmitter epinephrine or norepinephrine, which will then attach to adrenergic receptors on the effector organ to initiate the sympathetic action.

The neurons of the parasympathetic nervous system and the preganglionic neurons of the sympathetic nervous system use acetylcholine.

Norepinephrine is the neurotransmitter that is released by the postganglionic neuron, and stimulates sympathetic responses in effectors. Acetylcholine is the primary neurotransmitter in post ganglionic neurons in the parasympathetic nervous system. Dopamine, GABA, and glycine are other transmitters used by the nervous system, but are not the primary neurotransmitter of the postganglionic sympathetic neurons.

As one moves from the transverse colon to the descending colon, the sympathetic innervation changes from the thoracic splanchnics to the lumbar splanchnics.

Pelvic splanchnic nerves provide parasympathetic innervation of pelvic and genital organs, including the distal third of the transverse colon, the descending colon, the sigmoid colon, and the rectum rectum. The proximal two thirds of the transverse colon, and the rest of the proximal gastrointestinal tract receives parasympathetic innervation from the vagus nerve.

The superior mesenteric, inferior mesenteric, and iliac arteries provide blood supply for the large intestine. The celiac trunk does not provide blood supply to the large intestine. Branches of the superior mesenteric artery mainly perfuse the transverse colon, while branches of the inferior mesenteric artery mainly perfuse the descending colon. Venous drainage mirrors colonic arterial supply, with both the superior and inferior mesenteric veins joining the hepatic portal vein.

The sympathetic nervous system is one of two divisions of the autonomic nervous system; it is responsible for the "fight or flight" response and is involved in homeostasis. Some sympathetic fibers pass through the paravertebral ganglia of the sympathetic trunk, while other sympathetic fibers synapse there.

Parasympathetic nerves are associated with cranial nerves, and the dilation of blood vessels in skin of the back and limbs (the sympathetic system constricts these vessels). Preganglionic neurons originate in the thoracolumbar region of the spinal cord (T2 to L1) then travel to a paravertebral ganglion or prevertebral ganglion, where they synapse with a postganglionic neruon. The paravertebral ganglion are found throughout the length of the spinal cord, including the cervical, thoracic, lumbar, and sacral areas.

The autonomic nervous system can be divided into the sympathetic and parasympathetic nervous systems. Although both divisions have target organs and tissues, the parasympathetic postganglionic neurons are usually much closer to the effector organ than sympathetic postganglionic neurons.

The autonomic nervous system is under involuntary control, while the somatic nervous system is under voluntary control. Only sympathetic postganglionic neurons use epinephrine; most autonomic neurons use acetylcholine. Both divisions include neurons that originate from the spinal cord. The cranial nerves have both sensory and motor functions, meaning that they can be involved in either somatic or autonomic innervation.

The cranial nerves that are part of the visceral efferent parasympathetic in the PNS are III, VII, IX, and X. Cranial nerve III (oculomotor) travels to the ciliary ganglion and supplies the pupil of the eye for constriction. Cranial nerve VII (facial) travels to the pterygomandibular and submandibular ganglion and supplies the lacrimal and salivary glands. Cranial nerve IX (glossopharyngeal) travels to the otic ganglion and supplies the Parotid gland. Lastly, cranial nerve X (vagus) travels to the Prevertebral plexus and thoracic plexus in order to supply the gut, heart, and enteric system.

The pupillary sphincter muscles contain muscarinic cholinergic receptors that help the eyes to respond to parasympathetic tone. When this parasympathetic signal is blocked (antagonized) via atropine, you will observe a "sympathetic response" at the level of the pupil. In this scenario, you would observe the sympathetic response of pupil dilation. Hint: "aTROP'ine" and "seeing a TROPical setting" both cause your pupils to dilate nice and wide!

The parasympathetic nervous system conserves energy and is thus know as the "rest and digest" system. It slows heart rate, increases salivation, lacrimation, urination, deification, digestion, and sexual arousal. The nerves of the parasympathetic system arise in the CNS. Specific nerves include cranial nerves, including the oculomotor nerve, facial nerve, glossopharyngeal nerve, and vagus nerve.

In the sympathetic nervous system some sympathetic fibers pass through the paravertebral ganglia, and others synapse there.

The vagus nerve (CNX) provides the heart and digestive tract with parasympathetic ("rest and digest") control.

Cranial nerves with associated parasympathetic activity include CN III, VII, IX, and X. The sympathetic trunk and the white ramus communicans are associated with the sympathetic nervous system.

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.

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 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.