Autonomic Nervous System: Pathways, Receptors, and Organ Effects

What is the autonomic nervous system (ANS)?

The involuntary efferent PNS division that regulates internal organs and helps maintain the body's internal environment.

What tissues and functions does the ANS control?

- Cardiac and smooth muscle, glands

- Heart rate, circulation, breathing, digestion, and secretion.

What is the parasympathetic system's main role?

"Rest and digest"

- Supports maintenance and restoration

- Generally slows the heart

- Constricts pupils and airways

- Promotes digestion

What is the sympathetic system's main role?

"Fight or flight"

- Prepares the body for activity

- Generally increases heart activity

- Dilates pupils and airways

- Reduces digestion.

What ANS activity is integrated by the spinal cord?

Autonomic reflexes not subject to higher control

e.g. urination and defecation

What ANS functions do the medulla and pons regulate?

- Cardiovascular

- Respiratory

- Digestive functions.

How does the hypothalamus regulate the ANS?

It influences heart rate, blood pressure, and respiration via the medulla, and hormone secretion through the HPA axis.

What is the basic ANS pathway from CNS to effector?

CNS → preganglionic neuron → ganglion → postganglionic neuron → effector organ.

What is an autonomic ganglion?

A cluster of neural cell bodies outside the CNS, where pre- and postganglionic neurons connect.

Where do parasympathetic nerves originate, and what are their fibre lengths and transmitters?

Cranial (including vagus) and sacral outflow; long preganglionic and short postganglionic fibres; both release ACh.

Where do sympathetic nerves originate, and what are their fibre lengths and transmitters?

Thoracolumbar outflow (T1-L3); short preganglionic fibres release ACh, and long postganglionic fibres usually release noradrenaline (NA).

Which receptor does preganglionic ACh activate in both ANS divisions?

Nicotinic NN (N2) receptors on postganglionic cell bodies.

What is a nicotinic ACh receptor?

An ionotropic, fast, excitatory ion channel permeable to Na+, Ca2+, and K+; found in autonomic ganglia (NN) and skeletal muscle junctions (NM).

What is a muscarinic ACh receptor?

A metabotropic, G-protein-coupled receptor; parasympathetic postganglionic ACh activates it on effector organs. Effects may stimulate or inhibit.

What do M2 and M3 muscarinic receptors do?

M2 slows/inhibits the heart; M3 stimulates glandular secretion and GI smooth-muscle contraction.

What are adrenergic receptors?

Metabotropic receptors activated by noradrenaline or adrenaline; main types are α1, α2, β1, and β2.

What does α1 receptor activation do?

Contracts iris radial muscle, dilating the pupil.

What does α2 receptor activation do in the digestive tract?

Reduces GI smooth-muscle contraction and motility; sympathetic activity also inhibits digestive secretions.

What does β1 receptor activation do?

Increases heart rate and force of cardiac contraction.

What does β2 receptor activation do?

Relaxes airway smooth muscle and dilates bronchioles.

How do sympathetic and parasympathetic inputs affect the heart?

Sympathetic β1 increases heart rate and force; parasympathetic M2 slows heart rate and reduces atrial contraction force.

How do the two ANS divisions affect the lungs?

Sympathetic β2 dilates bronchioles; parasympathetic M3 constricts them.

How do the two ANS divisions affect the digestive tract?

Sympathetic α2 reduces motility and secretion; parasympathetic M3 increases motility and secretion.

How do the two ANS divisions affect the pupil?

Sympathetic α1 dilates it; parasympathetic M3 constricts it.

How do the two ANS divisions affect the bladder?

Parasympathetic activity promotes bladder contraction; sympathetic activity relaxes the bladder.

What is dual reciprocal innervation?

Most organs receive both ANS divisions, which usually have opposing effects for rapid, precise control.

What is tonic ANS activity?

Partial activity of both sympathetic and parasympathetic systems at rest; the balance helps set organ activity.

When does each ANS division usually dominate?

Sympathetic during stress or emergencies; parasympathetic during quiet, relaxed states.

How are heart rate and contraction force controlled at rest?

Both ANS divisions are active: the SA node sets heart rate, myocytes generate force, and the balance of inputs determines cardiac activity.

What does tachycardia suggest about ANS activity?

Increased sympathetic activity.

What is the adrenal medulla's role in sympathetic signalling?

A modified sympathetic structure stimulated directly by preganglionic ACh; it has no postganglionic fibres and releases hormones into blood.

What does the adrenal medulla release?

Approximately 80% adrenaline and 20% noradrenaline.

What is an agonist versus an antagonist?

An agonist activates a receptor; an antagonist blocks it.

What are key ANS drug examples?

Atropine blocks muscarinic receptors, reducing salivary/bronchial secretions and potentially increasing heart rate; salbutamol is a β2 agonist that dilates bronchioles; β1 blockers lower heart rate.

What are two sympathetic innervation exceptions?

Most arterioles and veins are sympathetic-only; most sweat glands are also sympathetic-only, but their nerves release ACh rather than NA.