Autonomic Nervous System Notes Review

A set of flashcards reviewing key concepts and vocabulary related to the autonomic nervous system and its functions.

Autonomic Nervous System (ANS)

The involuntary branch of the peripheral nervous system, primarily responsible for the subconscious regulation of internal organ functions such as heart rate, digestion, respiration, and blood pressure to maintain homeostasis. It operates without conscious control, adjusting body systems based on internal and external cues.

Sympathetic Division

Often referred to as the 'fight or flight' system, this division of the ANS prepares the body for stressful or emergency situations. Its widespread activation leads to effects like increased heart rate, elevated blood pressure, bronchodilation, pupil dilation, redirection of blood flow from digestive organs to skeletal muscles, and release of glucose from the liver.

Parasympathetic Division

Known as the 'rest and digest' system, this division of the ANS promotes energy conservation and non-emergency functions. Its activation typically slows heart rate, lowers blood pressure, enhances digestion and absorption of nutrients, increases glandular secretions, and causes pupil constriction.

Visceral Reflex Arc

A neural pathway governing involuntary visceral responses to stimuli, analogous to a somatic reflex arc. It consists of five components: a receptor (detects stimuli), a sensory neuron (transmits afferent signal to CNS), an integration center (in CNS, processes signal, often involving interneurons), a motor neuron (transmits efferent signal via a two-neuron chain), and an effector (smooth muscle, cardiac muscle, or gland that responds).

Dual Innervation

The phenomenon where most visceral organs receive nerve fibers from both the sympathetic and parasympathetic divisions of the ANS. These divisions often exert opposing, or antagonistic, effects, allowing for precise control and fine-tuning of organ activity to maintain homeostasis.

Cholinergic Receptors

Protein receptors on target cells that bind to acetylcholine (ACh) as their neurotransmitter. They are broadly classified into two subtypes: nicotinic receptors (ion channels, always excitatory) and muscarinic receptors (G-protein coupled, can be excitatory or inhibitory depending on subtype and location).

Adrenergic Receptors

Protein receptors on target cells that bind to norepinephrine (NE) and epinephrine (Epi) as their neurotransmitters (or hormones). These receptors are found at sympathetic effector sites and are categorized into two main types: alpha (\alpha) receptors and beta (\beta) receptors, each with specific subtypes and physiological effects.

Myelinated Neurons

Neurons whose axons are insulated by a myelin sheath, a fatty layer that significantly increases the speed of nerve impulse conduction (saltatory conduction). In the ANS, all preganglionic neurons are myelinated, while postganglionic neurons are typically unmyelinated or sparsely myelinated.

Neural Divergence

A characteristic feature of the sympathetic division where a single preganglionic neuron synapses with multiple postganglionic neurons (often 1:10 to 1:20 or more) and frequently extends to several different ganglia. This widespread projection allows for a diffuse, generalized sympathetic response, affecting many organs simultaneously.

Atropine

A parasympatholytic drug that acts as a competitive antagonist at muscarinic acetylcholine receptors. It blocks the effects of acetylcholine at parasympathetic postganglionic synapses, leading to effects like pupil dilation (mydriasis), increased heart rate, decreased glandular secretions (e.g., salivation, sweating), and relaxation of smooth muscle in the GI and urinary tracts.

Phenylephrine

An adrenergic agonist that selectively activates alpha (\alpha)-1 adrenergic receptors, mimicking the effects of norepinephrine. It causes vasoconstriction and is commonly used as a nasal decongestant (to constrict blood vessels in the nasal mucosa), to increase blood pressure, or to dilate pupils.

Vagus Nerve

The tenth cranial nerve (X), which provides the most extensive parasympathetic innervation to the thoracic and abdominal organs, including the heart, lungs, and most of the digestive tract. It plays a crucial role in regulating heart rate, respiratory rate, and digestive activity through its long preganglionic fibers.

Homeostasis

The dynamic process by which the body maintains a relatively stable internal environment despite continuous fluctuations in external and internal conditions. The autonomic nervous system is a primary regulator of many physiological parameters, such as body temperature, blood pressure, and blood glucose, that are essential for maintaining homeostasis.

Visceral Effectors

The involuntary target organs and tissues controlled by the ANS, which include: smooth muscle (e.g., in blood vessels, digestive tract, airways), cardiac muscle (the heart), and glands (e.g., salivary, sweat, adrenal, pancreatic). These effectors respond involuntarily to autonomic nerve impulses.

Splanchnic Nerves

Nerves that originate from sympathetic preganglionic neurons in the thoracic or lumbar spinal cord. These fibers pass through the sympathetic chain ganglia without synapsing and instead terminate in distant collateral (prevertebral) ganglia (e.g., celiac, superior mesenteric, inferior mesenteric ganglia) located near major abdominal arteries, from where postganglionic fibers innervate abdominal and pelvic viscera.

Ganglia

Clusters of nerve cell bodies and their synapses located outside the central nervous system. In the ANS, ganglia serve as relay stations where preganglionic neurons synapse with postganglionic neurons. Sympathetic ganglia (chain and collateral) are typically closer to the spinal cord, while parasympathetic ganglia (terminal) are located in or near the effector organs.

Hypothalamus

A vital region of the diencephalon in the brain that serves as the main integration center for the ANS and is the overall boss of the autonomic system. It coordinates both sympathetic and parasympathetic activities, regulating body temperature, hunger, thirst, sleep-wake cycles, and emotional responses through its direct and indirect control over autonomic output.

Antagonistic Effect

A common outcome of dual innervation where the sympathetic and parasympathetic divisions have opposing actions on the same target organ. For example, the sympathetic division increases heart rate, while the parasympathetic division decreases it, allowing for precise control over the heart's activity.

Cooperative Effect

An outcome of dual innervation where both sympathetic and parasympathetic divisions work together to produce a unified, complementary response in an organ, often leading to a complex physiological outcome. For instance, in salivation, the parasympathetic division stimulates watery saliva production, while the sympathetic division stimulates thicker, mucus-rich saliva, both contributing to the overall digestive process.

Vasomotion

The regulation of blood vessel diameter, primarily controlled by sympathetic nervous system input. Increased sympathetic stimulation causes vasoconstriction (narrowing of blood vessels), increasing peripheral resistance and blood pressure. Decreased sympathetic stimulation leads to vasodilation (widening of blood vessels), often occurring without significant direct parasympathetic innervation for most blood vessels.

Enteric Nervous System

A complex, semi-independent network of neurons located entirely within the walls of the gastrointestinal tract, extending from the esophagus to the anus. It can function autonomously to regulate digestive processes like motility and secretion, although its activity is also modulated by both the sympathetic and parasympathetic divisions of the ANS.

Neurotransmitters of the ANS

The primary chemical messengers released by autonomic neurons to transmit signals. Acetylcholine (ACh) is released by all preganglionic neurons (sympathetic and parasympathetic) and all parasympathetic postganglionic neurons. It's also released by some sympathetic postganglionic neurons (e.g., to sweat glands). Norepinephrine (NE) is released by most sympathetic postganglionic neurons. Epinephrine and Norepinephrine are also secreted as hormones by the Adrenal Medulla.

Adrenal Medulla

A modified sympathetic ganglion located on top of the kidneys. Unlike other sympathetic ganglia, its postganglionic neurons lack axons and instead secrete catecholamines – primarily epinephrine (adrenaline) and lesser amounts of norepinephrine (noradrenaline) – directly into the bloodstream upon sympathetic stimulation, acting as hormones to intensify and prolong the 'fight or flight' response throughout the body.

Autonomic Tone

The constant, baseline level of activity maintained by both the sympathetic and parasympathetic divisions in most visceral organs. This continuous, low-level stimulation allows for the precise regulation of organ function by either increasing or decreasing the activity of one or both divisions from this resting tone, enabling fine-tuned control over body systems.

Nicotinic Receptors

A type of cholinergic receptor that responds to acetylcholine. These are ligand-gated ion channels, and their binding with ACh always results in an excitatory postsynaptic potential. They are prominently found on: 1. All postganglionic neurons (in both sympathetic and parasympathetic ganglia). 2. The adrenal medulla. 3. Skeletal muscle cells at the neuromuscular junction (though this is part of the somatic nervous system, it highlights ACh action).

Muscarinic Receptors

A type of cholinergic receptor that responds to acetylcholine. These are G-protein coupled receptors, and their binding with ACh can lead to either excitatory or inhibitory effects, depending on the specific receptor subtype and the target organ. They are primarily found on all parasympathetic target organs and a few sympathetic target organs (e.g., sweat glands).

Alpha (\alpha) Adrenergic Receptors

A class of G-protein coupled adrenergic receptors that are generally more sensitive to norepinephrine than epinephrine. Subtypes include \alpha-1 (primarily mediates vasoconstriction, pupil dilation, contraction of internal urethral sphincter) and \alpha-2 (often presynaptic, inhibiting NE release, or in pancreatic islets, inhibiting insulin release).

Beta (\beta) Adrenergic Receptors

A class of G-protein coupled adrenergic receptors that are generally more sensitive to epinephrine than norepinephrine. Subtypes include eta-1 (increases heart rate and contractility), eta-2 (causes bronchodilation, vasodilation in skeletal muscle and coronary arteries, glycogenolysis), and eta-3 (stimulates lipolysis in adipose tissue).

Preganglionic vs. Postganglionic Neurons (ANS)

The two-neuron chain characteristic of the ANS motor pathway: 1. Preganglionic Neuron: Originates in the CNS (brainstem or spinal cord). It has a myelinated axon that extends to an autonomic ganglion, where it synapses with a postganglionic neuron. Always releases acetylcholine. 2. Postganglionic Neuron: Has its cell body in an autonomic ganglion. It has an unmyelinated axon that extends from the ganglion to the effector organ. Releases either acetylcholine (parasympathetic and some sympathetic) or norepinephrine (most sympathetic).

Sympathetic Chain Ganglia (Paravertebral Ganglia)

A paired chain of autonomic ganglia running vertically along both sides of the spinal column, extending from the cervical to the sacral region. These ganglia are the primary sites where sympathetic preganglionic neurons synapse with postganglionic neurons for innervation of structures in the head, neck, torso, and limbs. Some preganglionic fibers may simply pass through to collateral ganglia.

Cranial Nerves in Parasympathetic Division

Several cranial nerves carry parasympathetic preganglionic fibers from the brainstem to target organs: * Oculomotor Nerve (III): Innervates intrinsic eye muscles (pupil constriction, lens accommodation). * Facial Nerve (VII): Innervates lacrimal, nasal, and salivary glands (submandibular and sublingual). * Glossopharyngeal Nerve (IX): Innervates the parotid salivary gland. * Vagus Nerve (X): Provides extensive innervation to visceral organs in the thoracic and abdominal cavities, accounting for ~90% of all preganglionic parasympathetic fibers.