Sympathetic and parasympathetic divisions

The small lateral horn, present from T1 to L2, holds autonomic motor cell bodies. The dorsal horn is sensory and the ventral horn holds somatic motor neurons.

• Dorsal horn:
• Ventral horn:
• Correct: Lateral horn:

A ganglion is a cluster of neuron cell bodies in the PNS. Inside the CNS the same kind of cluster is called a nucleus, and a bundle of axons is a tract.

• Correct: A cluster of neuron cell bodies outside the CNS:
• A bundle of axons inside the CNS:
• A cluster of neuron cell bodies inside the CNS:

The vagus nerve (wandering nerve) leaves the brainstem and runs down to the heart, lungs and digestive organs.

• Glossopharyngeal nerve (IX):
• Correct: Vagus nerve (X):
• Accessory nerve (XI):

Sympathetic preganglionic cell bodies sit in the lateral horn of the thoracic and upper lumbar cord, T1–L2. That is why the division is called thoracolumbar.

• In the brainstem and in sacral segments S2–S4: The brainstem and S2–S4 are the parasympathetic (craniosacral) origins.
• Correct: In the lateral horn of the spinal cord, segments T1–L2: Correct. The lateral horn of T1–L2 holds every sympathetic preganglionic cell body.
• In the chain ganglia beside the vertebral column: The chain ganglia hold the cell bodies of postganglionic neurons. Preganglionic cell bodies are inside the CNS.
• In the ventral horn at every spinal segment: The ventral horn holds somatic motor neurons that supply skeletal muscle, and it runs the whole cord. Sympathetic output starts only at T1–L2.

The oculomotor (III), facial (VII), glossopharyngeal (IX) and vagus (X) nerves carry the cranial parasympathetic outflow. The rest of the parasympathetic outflow leaves from S2–S4.

• Correct: III, VII, IX and X: Correct. Remember them as 3, 7, 9 and 10.
• V, VII, IX and X: The trigeminal nerve (V) is sensory to the face and motor to the chewing muscles. It carries no parasympathetic preganglionic fibers of its own.
• III, IV, VI, VII and X: Nerves IV and VI are purely somatic motor nerves to eye muscles. Of the eye-movement nerves, only nerve III carries parasympathetic fibers.
• I, II, VIII and X: Nerves I, II and VIII are sensory nerves for smell, vision, and hearing and balance. They carry no motor output at all.

The parasympathetic division is craniosacral, uses terminal or intramural ganglia in or beside its targets, so its preganglionic fibers are long and postganglionic fibers short, and about three quarters of its fibers travel in the vagus nerve. It has no chain and no gray rami.

• Correct: Preganglionic fibers leave in cranial nerves and from S2–S4: Correct. Nerves III, VII, IX and X, plus S2–S4.
• Correct: Ganglia sit near or inside the target organ: Correct. These are the terminal and intramural ganglia.
• Postganglionic fibers reach the skin through gray rami communicantes: Gray rami carry sympathetic postganglionic fibers. Parasympathetic fibers do not reach the skin.
• Correct: Preganglionic fibers are long and postganglionic fibers are short: Correct. The ganglion sits at the target, so the first neuron covers almost the whole distance.
• A preganglionic neuron branches to many chain ganglia: Branching through the chain is the sympathetic pattern. Parasympathetic preganglionic neurons contact only a few postganglionic neurons, and there is no parasympathetic chain.
• Correct: The vagus nerve carries most of its preganglionic fibers: Correct. The vagus carries about three quarters of all parasympathetic preganglionic fibers.

Gray rami carry unmyelinated sympathetic postganglionic fibers from the chain back into spinal nerves. Their preganglionic neurons sit in the upper thoracic cord; the fibers climbed the chain to a cervical ganglion before synapsing.

• A sympathetic preganglionic fiber entering the chain: Preganglionic fibers enter the chain through white rami, and there are no white rami at C6 because no sympathetic fibers leave the cord there.
• A parasympathetic postganglionic fiber heading to the arm: The parasympathetic division does not supply the limbs and has no fibers in the gray rami.
• A somatic motor fiber heading to an arm muscle: Somatic motor fibers leave in the ventral root and stay in the spinal nerve. They do not pass through the rami communicantes.
• Correct: An unmyelinated sympathetic postganglionic fiber returning from the chain: Correct. A gray ramus at any level carries postganglionic sympathetic fibers back into the spinal nerve.

The sympathetic supply to the whole head leaves the cord at T1–T2 and climbs the chain to the superior cervical ganglion. A tumor at the lung apex can compress the chain there. The eye muscle that widens the pupil, a small smooth muscle in the eyelid and the facial sweat glands all lose their sympathetic input, so the pupil is small, the lid droops and the skin stays dry.

• The right oculomotor nerve: Oculomotor damage would remove the parasympathetic fibers that narrow the pupil, so the pupil would be large, not small. It would also paralyze several eye movements.
• Correct: The right sympathetic chain: Correct. Losing sympathetic input to the head produces a small pupil, a slightly drooping lid and loss of facial sweating on that side.
• The right vagus nerve: The vagus nerve supplies the organs of the chest and abdomen. It does not supply the pupil, the eyelid or the sweat glands of the face.
• The right facial nerve: The facial nerve's parasympathetic fibers go to the tear and saliva glands, and its motor fibers to the muscles of facial expression. Facial sweat glands get only sympathetic fibers.

Preganglionic fibers synapse directly on chromaffin cells in the adrenal medulla, which release epinephrine and some norepinephrine into the blood.

• Correct: Chromaffin cells of the adrenal medulla: Correct. Chromaffin cells stand in for postganglionic neurons.
• Postganglionic neurons in the celiac ganglion: Fibers to the adrenal medulla pass through the celiac ganglion without synapsing. If they synapsed there, the second neuron would release its transmitter onto an organ, not into the blood.
• A gray ramus communicans: Gray rami carry postganglionic fibers back to spinal nerves, toward the skin and limbs. They are not on the route to the adrenal gland.
• Cells of the outer layer of the adrenal gland: The outer layer of the adrenal gland is controlled by hormones, not by sympathetic fibers. Epinephrine comes from the medulla.

Skeletal muscle contracts only when a somatic motor neuron excites it, so cutting the nerve paralyzes it. Cardiac muscle, smooth muscle and many glands have their own built-in activity. Autonomic nerves adjust that activity up or down rather than starting it.

• Correct: Autonomic effectors have their own activity, which the nerves speed up or slow down: Correct. The heart beats without nerves; its autonomic supply only changes the rate and strength.
• Autonomic signals travel through two neurons, so one cut neuron is replaced by the other: The two neurons are in series, not in parallel. Cutting either one breaks the pathway.
• Autonomic nerves regrow within hours after being cut: Cut peripheral axons regrow slowly, over weeks to months, and after a transplant the heart stays largely without nerves for a long time.
• Heart muscle is controlled by somatic motor neurons that are not cut in surgery: Cardiac muscle is an autonomic effector. No somatic motor neurons supply it.

Parasympathetic fibers do not travel in the gray rami or spinal nerves, so they never reach the skin or most blood vessels, and they do not supply the adrenal medulla. The heart and the iris get fibers from both divisions.

• Correct: Sweat glands in the skin: Correct. Sweat glands get sympathetic fibers through the gray rami.
• Correct: Arrector pili muscles: Correct. The arrector pili muscles are supplied only by sympathetic fibers.
• Correct: The adrenal medulla: Correct. Only sympathetic preganglionic fibers reach the adrenal medulla.
• The heart: The heart gets sympathetic fibers from the upper thoracic chain and parasympathetic fibers from the vagus.
• Correct: Smooth muscle in most blood vessel walls: Correct. Most blood vessels receive only sympathetic fibers.
• The iris: The iris gets parasympathetic fibers from nerve III, which narrow the pupil, and sympathetic fibers, which widen it.