Autonomic Nervous System Notes Review
Chapter 15: Autonomic Nervous System
Overview of the Autonomic Nervous System (ANS)
The ANS is a subconscious system primarily involved in visceral motor functions, controlling processes like heart rate, digestion, respiration, pupillary response, and glandular secretions.
Main Functions:
Controls reflexes related to internal organs, such as blood pressure regulation via baroreflexes.
Regulates bodily functions without conscious control, ensuring internal homeostasis.
Major Divisions of the ANS
Sympathetic Division
Known for its role in "fight or flight" responses, preparing the body for stressful situations by increasing heart rate, dilating pupils, inhibiting digestion, and redirecting blood flow to skeletal muscles.
Parasympathetic Division
Responsible for "rest and digest" activities, promoting energy conservation, digestion, and normal bodily maintenance by decreasing heart rate, constricting pupils, and stimulating digestive processes.
Each division is in charge at different times based on physiological needs, exhibiting reciprocal control over most visceral organs.
Learning Objectives
Identify the ANS as the visceral motor branch of the Peripheral Nervous System (PNS).
Understand and compare the visceral reflex arc to the somatic reflex arc.
Describe both divisions of the ANS—sympathetic and parasympathetic.
Explain the parasympathetic division's effects on the digestive system and heart rate, contrasting with sympathetic effects on heart rate, blood pressure, and breathing rates.
Detail the structure of ANS:
Compare thoracolumbar and craniosacral origins.
Location of various ganglia such as paravertebral, collateral, and terminal ganglia.
Discuss the role of the vagus nerve in the parasympathetic response and the role of the adrenal medulla in the sympathetic response.
Explain dual and single innervations with examples of cooperative and antagonistic effects. Provide sympathetic vasomotion as an example of single innervation.
List neurotransmitters of the ANS, defining adrenergic and cholinergic.
Describe the action of at least one ANS agonist and antagonist, including examples used in the text: phenylephrine, beta-blockers, pilocarpine, and atropine.
Explain the hypothalamus's role in regulating the ANS, while mentioning optional roles of the cerebral cortex, brainstem, and spinal cord.
Somatic Nervous System (SNS)
Definition: Associated with skin and skeletal muscles, functioning both consciously (e.g., voluntary muscle movement) and subconsciously (e.g., diaphragm control during respiration).
Structure and Function: It involves a single motor neuron extending from the CNS directly to skeletal muscle.
Upper motor neurons originate in the primary motor cortex in the cerebrum.
These neurons synapse in the CNS, specifically in the brainstem or spinal cord.
Lower motor neurons extend from the CNS to the effector (skeletal muscles), are heavily myelinated, and release acetylcholine (ACh) to excite skeletal muscle fibers at the neuromuscular junction (NMJ).
Visceral Motor System
Definition: Originates from motor nuclei in the hypothalamus and other brainstem regions.
Neuronal Pathway: This system utilizes a two-neuron chain to reach its targets.
Preganglionic neurons: Cell bodies are located within the CNS, their axons are myelinated, and they exit the CNS to synapse with postganglionic neurons in peripheral ganglia.
Postganglionic neurons: Cell bodies are in the peripheral ganglia, their axons are unmyelinated, and they project to effectors (smooth muscle, glands, cardiac muscle, adipocytes).
Neuronal Characteristics
Myelination: The degree of myelination and the neurotransmitters involved differ between somatic and autonomic pathways.
Somatic motor neurons are heavily myelinated, allowing for rapid signal conduction, and release acetylcholine (ACh) to specifically excite skeletal muscle at the NMJ.
In the autonomic pathway:
Preganglionic neurons are lightly myelinated and release ACh, which acts on nicotinic receptors to excite postganglionic neurons.
Postganglionic neurons are unmyelinated and may release either ACh (in the parasympathetic division and some sympathetic targets like sweat glands) or norepinephrine (NE) (in the sympathetic division), which can excite or inhibit their respective effectors depending on the specific receptor subtype present on the target cell.
Effector Synapse Differences: The way neurotransmitters are released at the effector target is distinct.
Somatic Nervous System: The NMJ (neuromuscular junction) connects one motor neuron to one specific skeletal muscle cell in a highly specialized, one-to-one fashion, ensuring precise and rapid contraction.
Autonomic Nervous System: Postganglionic axons contain varicosities, which are numerous swellings along the axon terminal. These varicosities release neurotransmitters diffusely into a wide synaptic cleft, affecting multiple smooth muscle cells or glandular cells simultaneously, leading to a slower, more generalized, and prolonged response.
ANS Receptors
Cholinergic Receptors: These receptors bind to acetylcholine (ACh).
Nicotinic Receptors: Found on all postganglionic neurons (of both sympathetic and parasympathetic divisions), the adrenal medulla, and skeletal muscle cells at the NMJ. Stimulation by ACh always results in an excitatory postsynaptic potential.
Muscarinic Receptors: Found on all parasympathetic target organs and some sympathetic targets (e.g., sweat glands). Their effects can be either excitatory or inhibitory, depending on the specific muscarinic receptor subtype ( M1, M2, M3, M4, M_5 ) and the target organ.
Adrenergic Receptors: These receptors bind to norepinephrine (NE) and epinephrine (EPI).
Alpha (\alpha) Receptors: Primarily respond to NE.
\alpha_1 receptors: Generally excitatory; found in smooth muscle of blood vessels (causing vasoconstriction), pupils (causing dilation), and visceral organ sphincters.
\alpha_2 receptors: Often inhibitory; found on presynaptic terminals where they inhibit NE release, and in some smooth muscle cells.
Beta (\beta) Receptors: Respond to NE and EPI.
\beta_1 receptors: Primarily excitatory; found mainly in the heart, increasing heart rate and contractility.
\beta_2 receptors: Primarily inhibitory; found in smooth muscle of bronchioles (causing bronchodilation), some blood vessels (causing vasodilation), and the uterine wall.
\beta_3 receptors: Mostly excitatory; found in adipose tissue (stimulating lipolysis) and smooth muscle of the detrusor muscle of the bladder (causing relaxation).
Visceral Reflexes
Components: Visceral reflexes are involuntary, unconscious stereotyped responses to stimuli.
Receptors: Specialized sensory endings that detect changes in the internal environment, such as stretch receptors (baroreceptors in blood vessels, mechanoreceptors in hollow organs), chemoreceptors (sensing blood O2, CO2 , pH), osmoreceptors (sensing osmolarity), and nociceptors (pain).
Afferent sensory neurons: Transmit signals from the receptors to the CNS.
Integration: Occurs in the CNS, involving interneurons within the spinal cord (short reflexes) or more complex processing in the brainstem and hypothalamus (long reflexes).
Efferent pathways: Consist of the two-neuron chain (preganglionic and postganglionic neurons) of the ANS, concluding in effectors (smooth muscle, cardiac muscle, glands, or adipocytes).
Denervation Effects
Somatic Denervation: Results from damage to somatic motor neurons, leading to paralysis, flaccidity (loss of muscle tone), and severe muscle atrophy due to the complete loss of neural stimulation.
Visceral Denervation: Leads to hypersensitivity of the denervated organ. While organs can still function without ANS input (exhibiting autonomy), they become much more responsive to circulating hormones or even minimal neural stimulation, often causing exaggerated responses to normal stimuli (e.g., denervated blood vessels become more sensitive to circulating catecholamines), a phenomenon known as denervation supersensitivity.
Homeostasis Concept
Homeostasis is a dynamic equilibrium, constantly maintained by the intricate balance between the sympathetic and parasympathetic divisions of the ANS.
Rest-and-Digest: Dominated by parasympathetic activity, characterized by physiological states promoting calmness, energy conservation, slowed heart rate, pupil constriction, increased gastrointestinal motility and secretion, and bladder contraction.
Fight-or-Flight: Dominated by sympathetic activity, occurring during states of exercise, trauma, arousal, competition, anger, and fear. This state is characterized by increased heart rate, bronchodilation, decreased digestive activity, redirection of blood flow from viscera to skeletal muscles, pupillary dilation, release of stored glucose from the liver, and sweating.
Anatomical Origins of the ANS
Sympathetic Division: The thoracolumbar division.
Originates from the thoracolumbar region of the spinal cord (segments T1-L2).
Preganglionic fibers are short and myelinated, emerging from the spinal cord to synapse in ganglia close to the spinal cord. They release ACh.
Postganglionic fibers are long and unmyelinated, extending from the ganglia to the target organs. They primarily release NE (except for sweat glands, which release ACh).
Ganglia locations:
Sympathetic chain ganglia (paravertebral ganglia): Form a chain on either side of the vertebral column, allowing preganglionic fibers to synapse at the same level, ascend, or descend to synapse at different levels.
Collateral (prevertebral) ganglia: Located anterior to the vertebral column, primarily innervating abdominopelvic viscera (e.g., celiac, superior mesenteric, inferior mesenteric ganglia).
Parasympathetic Division: The craniosacral division.
Arises from the craniosacral region, specifically from nuclei in the brainstem (associated with cranial nerves III, VII, IX, X) and the sacral spinal cord (segments S2-S4).
Preganglionic fibers are long and myelinated, extending almost all the way to the target organs. They release ACh.
Postganglionic fibers are very short and unmyelinated, located within or very close to the target organs' walls. They release ACh.
Ganglia locations:
Terminal ganglia (intramural ganglia): Situated close to or, more commonly, within the wall of target organs.
The vagus nerve (cranial nerve X) is particularly significant, accounting for approximately 90% of all preganglionic parasympathetic fibers, innervating most of the thoracic and abdominal viscera up to the distal half of the large intestine.
The Autonomic Nervous System (ANS) is identified as the visceral motor branch of the Peripheral Nervous System (PNS), overseeing subconscious internal functions like heart rate and digestion.
Visceral Reflex Arc vs. Somatic Reflex Arc
Visceral Reflex Arc: These are involuntary, unconscious responses to internal stimuli. They typically involve:
Receptors: Detect internal changes (e.g., baroreceptors, chemoreceptors, osmoreceptors, nociceptors).
Afferent sensory neurons: Transmit signals to the CNS.
Integration: Occurs in the CNS, from spinal cord (short reflexes) to brainstem and hypothalamus (long reflexes).
Efferent pathways: A two-neuron chain (preganglionic and postganglionic neurons of the ANS) leading to effectors (smooth muscle, cardiac muscle, glands, adipocytes).
Somatic Reflex Arc: Involves a single motor neuron from the CNS directly to skeletal muscle, often consciously controlled but also subconscious (e.g., diaphragm). Somatic motor neurons are heavily myelinated and release acetylcholine (ACh) to excite skeletal muscle at the neuromuscular junction (NMJ).
Key Differences:
Neurons: Somatic uses one heavily myelinated motor neuron; visceral uses a two-neuron chain (preganglionic lightly myelinated, postganglionic unmyelinated).
Neurotransmitters: Somatic always releases ACh to excite skeletal muscle; visceral preganglionic neurons release ACh, while postganglionic neurons release either ACh (parasympathetic, some sympathetic) or norepinephrine (NE) (sympathetic), which can excite or inhibit.
Effector Synapse: Somatic has a precise, one-to-one NMJ; visceral postganglionic axons have varicosities that release neurotransmitters diffusely, affecting multiple target cells for a slower, more generalized response.
Divisions of the ANS
Sympathetic Division: Known for "fight or flight" responses, preparing the body for stress. It increases heart rate, blood pressure, and breathing rate, dilates pupils, inhibits digestion, and redirects blood flow to skeletal muscles.
Parasympathetic Division: Responsible for "rest and digest" activities, promoting energy conservation, digestion, and normal maintenance. It decreases heart rate, promotes digestion (increased gastrointestinal motility and secretion), and constricts pupils.
ANS Structure: Origins and Ganglia
Sympathetic Division (Thoracolumbar):
Origin: Thoracolumbar region of the spinal cord (T1-L2).
Preganglionic fibers: Short and myelinated, synapse in ganglia close to the spinal cord.
Postganglionic fibers: Long and unmyelinated, extend to target organs.
Ganglia Locations:
Paravertebral ganglia (sympathetic chain ganglia): Form a chain on either side of the vertebral column.
Collateral (prevertebral) ganglia: Located anterior to the vertebral column, innervating abdominopelvic viscera (e.g., celiac, superior mesenteric, inferior mesenteric ganglia).
Parasympathetic Division (Craniosacral):
Origin: Craniosacral region (brainstem nuclei/cranial nerves III, VII, IX, X and sacral spinal cord segments S2-S4).
Preganglionic fibers: Long and myelinated, extending almost to the target organs.
Postganglionic fibers: Very short and unmyelinated, located within or very close to the target organs.
Ganglia Locations:
Terminal ganglia (intramural ganglia): Situated close to, or within, the wall of target organs.
Role of Vagus Nerve and Adrenal Medulla
Vagus Nerve (Cranial Nerve X): Accounts for approximately 90% of all preganglionic parasympathetic fibers, innervating most thoracic and abdominal viscera up to the distal half of the large intestine, playing a crucial role in "rest and digest" functions.
Adrenal Medulla: Functions as a modified sympathetic ganglion. When stimulated by sympathetic preganglionic neurons (which release ACh acting on nicotinic receptors), it releases epinephrine (EPI) and norepinephrine (NE) directly into the bloodstream, acting as hormones to exert widespread and prolonged "fight or flight" effects throughout the body.
Dual and Single Innervation
Dual Innervation: Most visceral organs receive innervation from both sympathetic and parasympathetic divisions, which can have:
Antagonistic Effects: The most common. For example, the sympathetic division increases heart rate, while the parasympathetic division decreases it.
Cooperative Effects: When both divisions produce different effects that together lead to a unified outcome. An example is the salivary glands: parasympathetic stimulation produces watery, serous saliva, while sympathetic stimulation produces thicker, mucous saliva; both contribute to saliva production for digestion.
Single Innervation: Some organs receive innervation from only one division (usually sympathetic). An example is sympathetic vasomotion, where most blood vessels are primarily innervated by sympathetic fibers. Increased sympathetic tone causes vasoconstriction through \alpha_1 receptors, while decreased sympathetic tone leads to vasodilation, regulating blood pressure and flow.
ANS Neurotransmitters and Receptors
Neurotransmitters: The primary neurotransmitters of the ANS are acetylcholine (ACh) and norepinephrine (NE).
ACh: Released by all preganglionic neurons (both sympathetic and parasympathetic), all parasympathetic postganglionic neurons, and some sympathetic postganglionic neurons (e.g., those innervating sweat glands).
NE: Primarily released by most sympathetic postganglionic neurons.
Adrenergic: Pertains to nerves that release norepinephrine (and epinephrine) and the receptors that bind to them (e.g., alpha and beta receptors).
Cholinergic: Pertains to nerves that release acetylcholine and the receptors that bind to them (e.g., nicotinic and muscarinic receptors).
Mechanism of Action of Agonists and Antagonists
Phenylephrine (Agonist): An \alpha1 adrenergic agonist. It binds to and stimulates \alpha1 receptors (e.g., in vascular smooth muscle), causing vasoconstriction and pupillary dilation. Used as a decongestant.
Beta-blockers (Antagonist): Such as propranolol, are \beta adrenergic antagonists. They block \beta receptors (especially \beta_1 receptors in the heart), decreasing heart rate and contractility. Used to treat hypertension and arrhythmias.
Pilocarpine (Agonist): A muscarinic cholinergic agonist. It stimulates muscarinic receptors (e.g., in the eye), causing pupillary constriction and increased tear production. Used to treat dry mouth or glaucoma.
Atropine (Antagonist): A muscarinic cholinergic antagonist. It blocks muscarinic receptors, leading to effects like increased heart rate, pupillary dilation, and reduced glandular secretions. Used to treat bradycardia or to dilate pupils during eye exams.
Role of the Hypothalamus in ANS Regulation
The hypothalamus is the main integration center of the ANS, serving as the boss that orchestrates visceral control. It exerts its control over the ANS by influencing brainstem and spinal cord centers. It integrates visceral and emotional information, controlling essential body functions such as body temperature, hunger, thirst, and stress response by coordinating sympathetic and parasympathetic activities. Optional roles include the cerebral cortex (conscious control over some visceral functions, e.g., voluntary bladder control), the brainstem (centers for specific visceral reflexes like cardiovascular and respiratory control), and the spinal cord (mediates many visceral reflexes independently).