Autonomic control and visceral reflexes

The hypothalamus, below the thalamus, sets body temperature, hunger, thirst and water balance, and controls the autonomic nervous system.

• Cerebellum:
• Correct: Hypothalamus:
• Thalamus:

Both kinds of reflex have a sensory receptor, an afferent path, an integration center, an efferent path and an effector. They differ in the effector: skeletal muscle for somatic, cardiac or smooth muscle and glands for visceral.

• Correct: A somatic reflex acts on skeletal muscle; a visceral reflex acts on cardiac muscle, smooth muscle or glands:
• A somatic reflex has no integration center; a visceral reflex has one:
• A somatic reflex is always conscious; a visceral reflex is always spinal:

Parasympathetic postganglionic neurons release acetylcholine onto muscarinic receptor proteins, which slow the heart by opening potassium channels.

• Beta-1:
• Nicotinic:
• Correct: Muscarinic:

Removing a tone reveals how much it was doing. Blocking the vagus nerve's effect frees the heart from a large brake, so the rate jumps. Blocking beta-1 removes only a small sympathetic push, so the rate falls a little.

• Correct: At rest, vagal tone on the heart is much stronger than its sympathetic tone: Correct. Vagal tone dominates the resting heart.
• The heart carries no beta-1 receptor proteins while the body is at rest: The heart carries beta-1 receptor proteins all the time; exercise and fright act through them. At rest they are simply receiving little norepinephrine.
• Atropine directly stimulates beta-1 receptor proteins: Atropine blocks muscarinic receptor proteins. It has no action on beta-1 receptor proteins.
• The sympathetic division is silent at rest: The sympathetic division keeps a low resting tone, which is why a beta blocker lowers the rate at all.

Immediately, phenylephrine narrows vessels through alpha-1 receptor proteins and blood pressure rises, with no direct effect on the heart. The blood pressure reflex then increases vagal tone and lowers sympathetic tone, so the heart rate falls. Negative feedback corrects only part of the rise, so pressure stays somewhat high while the drug acts.

• Diameter of skin blood vessels, direct drug effect: down. Phenylephrine activates alpha-1 receptor proteins on vessel smooth muscle, which contracts and narrows the vessels.
• Blood pressure, direct drug effect: up. Narrower vessels throughout the body raise the pressure the heart must push against, so arterial pressure rises.
• Heart rate, direct drug effect: no change. Phenylephrine does not act on beta-1 receptor proteins, so it does not directly change the heart rate.
• Vagal output to the heart, a minute later: up. Stretch-sensitive sensory receptors in the walls of the large arteries fire more as pressure rises. The medulla oblongata responds by increasing vagal output and reducing sympathetic output.
• Heart rate, a minute later: down. More vagal tone, acting through muscarinic receptor proteins on the heart, slows the heart below its starting rate.
• Blood pressure, a minute later: up. The reflex slows the heart and trims sympathetic tone, but the drug is still holding alpha-1 receptor proteins active, so pressure is pulled back only part of the way and stays above its starting value.

Vasomotor nerves keep finger skin vessels partly constricted at rest. Firing less lets the smooth muscle relax, and the vessels widen. One division with a resting tone gives two-way control.

• It sends acetylcholine to them through the vagus nerve: The vagus nerve does not supply skin vessels, and most blood vessels have no parasympathetic fibers at all.
• Correct: It lowers sympathetic tone, so the vessels constrict less: Correct. Withdrawing sympathetic tone widens the vessels.
• It raises sympathetic tone, which widens them through alpha-1 receptor proteins: Alpha-1 receptor proteins make vessel smooth muscle contract. More sympathetic tone narrows skin vessels.
• It cannot widen them; only local heat can: The hypothalamus widens these vessels through the nervous system, by withdrawing sympathetic tone. Local warmth helps, but it is not the only route.

Her right retina and optic nerve still send the light signal to the midbrain, which drives both Edinger–Westphal nuclei. The left oculomotor nerve is intact, so the left pupil narrows. The right oculomotor nerve cannot carry the parasympathetic output, so the right pupil stays wide.

• Both pupils narrow normally: The right pupil needs the right oculomotor nerve to narrow, and that nerve is compressed.
• Neither pupil narrows: The sensory side works and the midbrain sends the signal to both sides, so the left pupil still responds.
• The right pupil narrows, but the left pupil stays wide: The right pupil's efferent pathway is the damaged one, so the right pupil is the one that fails.
• Correct: The left pupil narrows but the right does not: Correct. The input is intact and the midbrain crosses the signal, but only the left efferent pathway works.

Sympathetic activation narrows skin vessels (alpha-1), widens the airways (beta-2, mostly epinephrine), releases glucose from the liver (beta-2), and drives sweating (muscarinic, through cholinergic sympathetic fibers). It widens the pupils and slows the gut.

• Correct: Blood vessels in the skin narrow: Correct. Norepinephrine from the vasomotor nerves acts on alpha-1 receptor proteins and contracts the smooth muscle of skin vessels.
• Correct: The liver releases glucose into the blood: Correct. Epinephrine on beta-2 receptor proteins makes the liver break down glycogen.
• The pupils narrow: Narrowing the pupils is a parasympathetic effect. Sympathetic activity widens them through alpha-1 receptor proteins.
• Correct: Sweating increases: Correct. Sympathetic cholinergic fibers drive the eccrine sweat glands.
• Gut movement speeds up: Speeding the gut is a parasympathetic effect. Sympathetic activity slows it.
• Correct: The small airways widen: Correct. Epinephrine relaxes airway smooth muscle through beta-2 receptor proteins.

The hypothalamus compares temperature, water balance and energy stores with their set points, receives emotional input from the limbic system, and sends commands to the brainstem and spinal cord autonomic centers.

• Cerebellum: The cerebellum coordinates movement. It is not the integrator of autonomic output.
• Correct: Hypothalamus: Correct. The hypothalamus sits at the top of the autonomic control hierarchy below the cerebrum.
• Primary motor cortex: The primary motor cortex controls voluntary movement through somatic motor pathways.
• Thalamus: The thalamus relays sensory signals to the cortex. It does not integrate autonomic output.

Brain control of all sympathetic output runs down the cord to T1–L2, below the cut. Without sympathetic tone, vessels widen (low pressure, warm pink skin) and the heart loses its sympathetic push. The vagus nerve leaves the brainstem above the cut, so vagal tone continues unopposed and slows the heart.

• His vagus nerve was cut, so vagal tone is lost: The vagus nerve leaves the brainstem and never enters the spinal cord, so a cord injury does not cut it. Losing vagal tone would speed the heart.
• His adrenal medulla is releasing too much epinephrine: Epinephrine would speed the heart and constrict skin vessels, the opposite of these findings. The adrenal medulla also depends on T-segment preganglionic neurons cut off from the brain.
• Correct: He has lost sympathetic tone while vagal output continues: Correct. Lost sympathetic tone plus intact vagal tone explains all three findings.
• His skeletal muscles are paralyzed, so they need less of the blood flow: Paralysis does not explain a slow heart or warm feet. The findings come from lost sympathetic tone.

Blocking every ganglion silences both divisions. On the resting heart, vagal tone is the larger influence, so removing both leaves a net loss of braking: the heart rises toward its own rate of about 100 beats a minute.

• It falls, because sympathetic input to the heart is removed: Sympathetic input is removed, but the larger vagal tone is removed too, so the net effect is a rise.
• It stays the same, because the two effects cancel exactly: The two tones are not equal at rest; vagal tone dominates.
• Correct: It rises, because removing both tones takes away the larger vagal brake: Correct. The net loss is of the vagal brake.
• It stops, because the heart needs autonomic input to beat: The heart beats on its own; autonomic nerves only adjust its rate.