Organ systems - autonomic control

What are the two principal ways the nervous system influences physiology?

By releasing blood-borne hormones from the pituitary gland and through autonomic neurons whose axons leave the CNS.

What does the autonomic nervous system control?

Visceral functions including arterial pressure, GI motility and secretion, bladder emptying, sweating, and body temperature.

Why is the nervous system involved in physiological regulation?

It can predict future events and prepare adjustments, and it can produce reflex-mediated adjustments in response to afferent inputs.

What is an example of a predictive autonomic response?

A potentially harmful situation can increase heart rate and blood pressure, widen pupils, and cause piloerection for fight-or-flight.

What is an example of a reflex-mediated autonomic response?

A fall in blood pressure decreases baroreceptor afferent firing, causing increased heart rate and peripheral vasoconstriction.

Which CNS regions are especially important for physiological regulation?

The hypothalamus and several brainstem regions, mainly in the medulla.

What is the role of the hypothalamus in autonomic regulation?

It integrates neural inputs, influences brainstem autonomic regions, and has chemical influences on pituitary hormonal release.

What are the three divisions of the autonomic nervous system?

Sympathetic, parasympathetic, and enteric.

What does the enteric nervous system control?

Activity in the gastrointestinal tract.

What is the nucleus tractus solitarius (NTS)?

A region in the dorsomedial caudal medulla that receives visceral afferents and participates in most reflex-mediated autonomic adjustments.

What does the rostral ventrolateral medulla do?

It helps control sympathetic output neurons involved particularly in cardiovascular regulation.

How many neurons are in the peripheral sympathetic and parasympathetic pathways?

Two: a preganglionic neuron and a postganglionic neuron.

Where is a preganglionic autonomic neuron's cell body?

In the CNS.

Where is a postganglionic autonomic neuron's cell body?

In a peripheral autonomic ganglion.

What do autonomic postganglionic neurons typically innervate?

Smooth muscle or cardiac muscle and other visceral targets.

Where are sympathetic preganglionic neuron cell bodies located?

In the intermediolateral cell column/horn of the thoracic and upper lumbar spinal cord.

How do sympathetic preganglionic fibers leave the spinal cord?

Through the ventral/anterior root and spinal nerve, then through a white ramus.

What are the possible courses of sympathetic preganglionic fibers in the sympathetic chain?

Synapse in the entry ganglion, ascend/descend and synapse elsewhere in the chain, or pass through to prevertebral ganglia.

Where are sympathetic postganglionic neuron cell bodies located?

In paravertebral/sympathetic chain ganglia or prevertebral ganglia.

What do gray rami carry?

Sympathetic postganglionic fibers from the sympathetic chain back into spinal nerves.

Which targets receive sympathetic postganglionic fibers through skeletal nerves?

Arterioles, sweat glands, and piloerector muscles.

Approximately what percentage of fibers in an average skeletal nerve are postganglionic fibers?

About 8%.

What is special about the adrenal medulla?

Sympathetic preganglionic fibers reach it without synapsing; adrenal medulla cells act in some ways like postganglionic neurons.

What does the adrenal medulla release?

Epinephrine and norepinephrine into the bloodstream.

Where are parasympathetic preganglionic neurons located?

In the brainstem, mainly the nucleus ambiguus and dorsal motor nucleus of the vagus, and in the sacral spinal cord.

Which cranial nerves carry parasympathetic preganglionic axons?

CN III, VII, IX, and X.

Approximately what percentage of parasympathetic preganglionic axons are in the vagus nerve?

About 75%.

What organs receive vagal parasympathetic fibers?

Heart, lungs, esophagus, stomach, small intestine, proximal colon, pancreas, and upper ureters.

What does parasympathetic CN III innervate?

Sphincter pupillae and ciliary muscles.

What does parasympathetic CN VII innervate?

Lacrimal, nasal, and submandibular glands.

What does parasympathetic CN IX innervate?

The parotid gland.

What do sacral parasympathetic nerves innervate?

Distal colon, rectum, bladder, lower ureters, and external genitalia.

What is a cholinergic fiber?

A fiber that releases acetylcholine.

What is an adrenergic fiber?

A fiber that releases norepinephrine.

What neurotransmitter do all autonomic preganglionic neurons release?

Acetylcholine.

What do most parasympathetic postganglionic neurons release?

Acetylcholine.

What do most sympathetic postganglionic neurons release?

Norepinephrine.

Which sympathetic postganglionic fibers are cholinergic exceptions?

Those innervating sweat glands, piloerector muscles, and a few blood vessels in some species.

What are varicosities?

Bulbous swellings along autonomic postganglionic fibers that contain transmitter substances near effector cells.

How is acetylcholine synthesized?

From acetyl-CoA and choline by choline acetyltransferase.

How is acetylcholine inactivated?

By acetylcholinesterase.

How is norepinephrine mainly inactivated after release from nerve terminals?

By rapid reuptake into nerve fibers.

What are the two major acetylcholine receptor types?

Nicotinic and muscarinic.

Where are nicotinic acetylcholine receptors located?

At neuromuscular junctions and in autonomic ganglia.

Where are muscarinic acetylcholine receptors located?

At targets of parasympathetic postganglionic neurons and cholinergic sympathetic postganglionic neurons.

How do nicotinic receptors produce effects?

By opening specific cation channels.

How do muscarinic receptors produce effects?

Through G-protein-mediated intracellular effects involving enzymes/second messengers or gated potassium channels.

What are the two major subclasses of norepinephrine receptors?

Alpha and beta.

Which adrenergic receptors are found on sympathetic targets?

α1, β1, and β2; α2 receptors are also found on sympathetic nerve terminals.

Which catecholamine has greater affinity for α1/α2 receptors?

Norepinephrine (NE > epinephrine).

Which catecholamine has greater affinity for β1 receptors?

Neither; NE and epinephrine have approximately equal affinity.

Which catecholamine has greater affinity for β2 receptors?

Epinephrine (epinephrine > NE).

What does α1 activation do?

Activates phospholipase C and increases intracellular Ca2+.

What does α2 activation do?

Inhibits adenylate cyclase and decreases cAMP.

What does β receptor activation do?

Activates adenylate cyclase and increases cAMP.

What is the usual role of α2 receptors?

They are mostly presynaptic autoreceptors that reduce norepinephrine release.

What is the sympathetic effect of β1 activation at the SA node?

Increased heart rate.

What is the parasympathetic effect at the SA node?

Decreased heart rate.

What is the sympathetic effect of β1 activation at the AV node?

Increased conduction velocity.

What is the parasympathetic effect at the AV node?

Decreased conduction velocity.

What is the sympathetic effect of β1 activation in the ventricles?

Increased contractility.

What is the effect of β2 activation on coronary arterioles?

Dilation.

What is the sympathetic effect of α1 activation on skin and mucosal vessels?

Constriction.

What is the sympathetic effect of β2 activation on skeletal muscle vasculature?

Dilation.

What is the sympathetic effect of α1 activation on abdominal visceral vessels?

Constriction.

What is the sympathetic effect of α1 activation on veins?

Constriction.

What is the sympathetic effect on the eye's radial muscle?

α1-mediated contraction, widening the pupil.

What is the parasympathetic effect on the pupillary sphincter?

Contraction, constricting the pupil.

What does sympathetic β2 activation do to the ciliary muscle?

Relaxes it, flattening the lens for far vision.

What does parasympathetic activation do to the ciliary muscle?

Contracts it, making the lens more convex.

What does sympathetic β2 activation do to bronchial smooth muscle?

Relaxes it.

What does parasympathetic activation do to bronchial smooth muscle?

Contracts it.

What does sympathetic α1 activation do to GI sphincters?

Contracts them.

What does parasympathetic activation do to GI sphincters?

Relaxes them.

What does sympathetic β2 activation do to the urinary bladder wall?

Relaxes it.

What does parasympathetic activation do to the urinary bladder wall?

Contracts it.

What does sympathetic α1 activation do to the urinary bladder sphincter?

Contracts it.

What does parasympathetic activation do to the urinary bladder sphincter?

Relaxes it.

What does sympathetic cholinergic activation do to sweat glands?

Causes generalized secretion.

What does sympathetic cholinergic activation do to pilomotor muscles?

Causes contraction and piloerection.

What percentage of adrenal medulla secretion is epinephrine?

Approximately 80%.

What percentage is norepinephrine?

Approximately 20%.

How does norepinephrine generally affect vascular resistance?

It predominantly activates α1 receptors, causing widespread vasoconstriction.

Why does epinephrine have concentration-dependent vascular effects?

It activates both α1 and β2 receptors, which have opposing effects; β2 has higher affinity for epinephrine.

What does low circulating epinephrine do to skeletal muscle vasculature?

β2-mediated vasodilation predominates, increasing blood flow to active muscle.

What does high circulating epinephrine do?

α1-mediated vasoconstriction becomes more prominent and total peripheral resistance increases.

What is sympathetic tone?

Tonic sympathetic activity that helps maintain baseline tone, such as vascular smooth-muscle tone.

Which vascular bed is rich in β2 receptors?

Skeletal muscle vasculature.

How does epinephrine affect skin, mucosa, and splanchnic beds?

It generally causes α1-mediated vasoconstriction.

How does epinephrine affect coronary vasculature?

It causes coronary arteriolar dilation through β2 receptors.

How does epinephrine affect renal vasculature?

It generally causes α1-mediated vasoconstriction and reduced renal blood flow during stress.

What was Walter Cannon's classical view?

Sympathetic and parasympathetic systems act antagonistically for homeostasis; sympathetic activity produces fight-or-flight and parasympathetic activity produces rest-and-digest.

What did Cannon mean by sympathetic 'mass discharge'?

He proposed that all sympathetic efferents discharge together during stress to produce a global response.

What is the modern view of sympathetic control?

Sympathetic control can be precise and patterned, with selected efferents activated while others are inhibited.

What are non-adrenergic, non-cholinergic transmitters mentioned in the lecture?

Peptide transmitters such as neuropeptide Y and VIP.

What is nitric oxide (NO)?

A gaseous endogenous vasodilator that is also used as a neurotransmitter by certain nerve cells.

Where is vascular NO produced?

In the endothelial lining of blood vessels.

How does NO cause vasodilation?

It diffuses into adjacent smooth muscle and causes relaxation.

How does NO relax smooth muscle intracellularly?

It activates guanylate cyclase, increasing cGMP; cGMP promotes calcium removal and inhibits actin-myosin interaction.

How does sildenafil work?

It inhibits PDE5, reducing cGMP degradation and enhancing NO-mediated smooth-muscle relaxation.