15.2 Autonomic Reflexes and Homeostasis

The autonomic nervous system regulates involuntary bodily functions through complex reflex arcs. These reflexes involve smooth muscle, cardiac muscle, and glands, with pathways that include interneurons and autonomic ganglia outside the central nervous system.

Sympathetic reflexes prepare the body for "fight or flight," while parasympathetic reflexes promote "rest and digest" functions. Short reflexes involve local circuits within organs, while long reflexes engage the central nervous system and multiple organs to maintain homeostasis.

Autonomic Nervous System Reflexes

Somatic vs autonomic reflex arcs

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• Somatic reflex arcs involve skeletal muscle effectors, with the afferent neuron synapsing directly with the efferent somatic motor neuron in the spinal cord or brain stem, which then synapses with the skeletal muscle
• Autonomic reflex arcs involve smooth muscle, cardiac muscle, or glandular effectors and have a more complex pathway: the afferent neuron synapses with interneurons in the spinal cord or brain stem, which then synapse with preganglionic autonomic neurons, followed by postganglionic neurons in autonomic ganglia outside the central nervous system that finally synapse with the effector tissues

Sympathetic vs parasympathetic reflexes

• Sympathetic reflexes prepare the body for "fight or flight" responses by increasing heart rate and contractility, dilating bronchioles, decreasing digestive activity, constricting blood vessels in the skin and digestive organs, dilating blood vessels in skeletal muscles, and stimulating glucose release from the liver
• Parasympathetic reflexes promote "rest and digest" functions by decreasing heart rate and contractility, constricting bronchioles, increasing digestive activity, dilating blood vessels in the skin and digestive organs, constricting blood vessels in skeletal muscles, and stimulating glucose storage in the liver

Autonomic Reflex Pathways and Regulation

Short vs long autonomic reflexes

• Short reflexes involve local circuits within a single organ or tissue, such as the myenteric reflex in the digestive tract, where stretching of the intestinal wall stimulates local enteric neurons to promote peristalsis without involving the central nervous system
• Long reflexes involve the central nervous system and multiple organs or tissues, like the baroreceptor reflex, where changes in blood pressure detected by baroreceptors (aorta and carotid arteries) are transmitted by afferent neurons to the cardiovascular center (medulla oblongata), which then sends efferent signals to adjust heart rate, contractility, and blood vessel diameter to maintain stable blood pressure (homeostasis)

Autonomic regulation of organ systems

• Acetylcholine (ACh) is released by preganglionic neurons in both sympathetic and parasympathetic pathways and by postganglionic neurons in parasympathetic pathways, binding to nicotinic receptors on postganglionic neurons and muscarinic receptors on effector tissues
• Norepinephrine (NE) is released by most postganglionic neurons in sympathetic pathways, binding to $\alpha$-adrenergic receptors (typically mediating vasoconstriction and smooth muscle contraction) and $\beta$-adrenergic receptors (typically mediating vasodilation, bronchodilation, and increased heart rate and contractility)
• Other signaling molecules include ACh released by some sympathetic postganglionic neurons (sweat glands and blood vessels in skeletal muscles) and epinephrine and NE released by the adrenal medulla into the bloodstream as hormones, amplifying sympathetic effects

Drug impacts on autonomic function

• Cholinergic drugs:
  1. Muscarinic agonists (pilocarpine) mimic ACh at muscarinic receptors, enhancing parasympathetic effects and potentially causing bradycardia, bronchoconstriction, and increased digestive activity
  2. Anticholinesterases (physostigmine) inhibit ACh breakdown, prolonging its effects, and can be used to treat myasthenia gravis (muscle weakness)
• Adrenergic drugs:
  1. Sympathomimetics (ephedrine) mimic NE and epinephrine at adrenergic receptors, enhancing sympathetic effects and potentially causing tachycardia, bronchodilation, and vasoconstriction
  2. $\beta$-blockers (propranolol) block $\beta$-adrenergic receptors, reducing sympathetic effects on the heart and lungs, and can be used to treat hypertension, angina, and arrhythmias
• Autonomic blockers:
  1. Muscarinic antagonists (atropine) block ACh at muscarinic receptors, reducing parasympathetic effects and potentially causing tachycardia, bronchodilation, and decreased digestive activity
  2. $\alpha$-blockers (phentolamine) block $\alpha$-adrenergic receptors, reducing sympathetic effects on blood vessels, and can be used to treat hypertension and peripheral vascular disease

Homeostatic Mechanisms and Feedback Systems

Negative feedback

• Negative feedback mechanisms work to counteract changes and maintain a stable internal environment
• Example: When body temperature rises, the hypothalamus triggers sweating to cool the body down, returning temperature to its set point

Positive feedback

• Positive feedback mechanisms amplify changes and are less common in biological systems
• Example: During childbirth, contractions stimulate oxytocin release, which further intensifies contractions