Chapter 60:The Autonomic Nervous System and the Adrenal Medulla

 

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The Autonomic Nervous System

The autonomic nervous system regulates the body's visceral functions, including arterial pressure, gastrointestinal motility, glandular secretion, urinary bladder emptying, sweating, and body temperature. A defining feature of this system is its remarkable rapidity and intensity, capable of doubling arterial pressure or doubling heart rate within seconds. It operates through central control centers in the spinal cord, brain stem, and hypothalamus, as well as subconscious visceral reflexes.

Efferent signals diverge into two major subdivisions: the sympathetic nervous system and the parasympathetic nervous system. While sympathetic pathways originate from spinal segments T-1 through L-2, parasympathetic fibers exit via cranial nerves III, VII, IX, and X, along with lower sacral spinal segments. Both systems utilize preganglionic and postganglionic neurons, though parasympathetic ganglia are located directly in or near visceral effector organs.

Neurotransmitters and Receptors of the Autonomic System

Autonomic nerve fibers communicate via specific synaptic neurotransmitters categorized as either cholinergic, which secrete acetylcholine, or adrenergic, which secrete norepinephrine. All preganglionic neurons and almost all parasympathetic postganglionic neurons are cholinergic. Conversely, most postganglionic sympathetic fibers are adrenergic, with notable exceptions supplying sweat glands and piloerector muscles.

Effector organs respond to these transmitters through specialized membrane receptors. Acetylcholine activates muscarinic receptors on parasympathetic and sympathetic cholinergic effectors, as well as nicotinic receptors located at autonomic ganglia. Adrenergic receptors are divided into alpha and beta types, which mediate diverse excitatory and inhibitory responses depending on the specific target tissue.

Sympathetic and Parasympathetic Effects on Organs

Sympathetic and parasympathetic stimulation exert widely varying, and frequently reciprocal, physiological actions across different visceral organs. Sympathetic activation generally prepares the body for intense physical activity by accelerating heart rate, dilating bronchioles, constricting most systemic blood vessels, and releasing hepatic glucose stores. Adrenal medullary secretion of epinephrine and norepinephrine further prolongs and amplifies these widespread systemic responses.

In contrast, parasympathetic stimulation promotes vegetative restoration, conservation, and digestive housekeeping functions. It constricts pupils, contracts ciliary muscles for near vision, stimulates copious watery glandular secretions, slows heart rate, and enhances gastrointestinal peristalsis while relaxing sphincters. Most visceral organs are dominantly controlled by one specific autonomic subdivision.

Autonomic Tone and Denervation Supersensitivity

Both the sympathetic and parasympathetic systems maintain continuous basal activity known as autonomic tone. This background tonic discharge allows a single nervous system division to both increase and decrease an organ's functional activity relative to baseline states. For instance, sympathetic vasomotor tone maintains systemic arterioles at approximately half their maximum diameter.

When an autonomic nerve is severed, the target organ temporarily loses its tone, resulting in acute functional shifts like vasodilation. However, intrinsic smooth muscle adaptations and denervation supersensitivity caused by post-synaptic receptor up-regulation gradually restore function and enhance tissue responsiveness to circulating neurotransmitters.

Autonomic Reflexes and Central Integration

Visceral homeostasis is continuously regulated by automatic autonomic reflexes operating at various levels of the nervous system. Cardiovascular baroreceptor reflexes monitor arterial pressure and adjust sympathetic and parasympathetic outflow accordingly. Gastrointestinal and micturition reflexes coordinate propulsive motility and sphincter relaxation in response to organ distension.

Higher integration occurs within the brain stem, pons, and mesencephalon, which house vital autonomic control centers for respiration, vasomotor tone, and cardiac regulation. These lower centers are modulated extensively by the hypothalamus and limbic cortex, linking emotional states and behavioral stress directly to autonomic output.

Pharmacology of the Autonomic Nervous System

Pharmacological agents modify autonomic function by mimicking, enhancing, or blocking specific neurotransmitters and receptors. Sympathomimetic drugs like norepinephrine and epinephrine directly stimulate adrenergic receptors, whereas indirect agents cause vesicular release. Conversely, adrenergic blockers inhibit synthesis, block release, or antagonize alpha and beta receptors.

Parasympathomimetic drugs and anticholinesterase agents potentiate cholinergic activity, while antimuscarinic drugs like atropine block muscarinic receptors. Ganglionic blocking agents simultaneously interrupt impulse transmission across both sympathetic and parasympathetic ganglia, significantly reducing blood pressure.