Autonomic Nervous System Physiology MCQs: Questions on Sympathetic and Parasympathetic Control

In both the sympathetic and parasympathetic divisions of the autonomic nervous system (ANS), preganglionic neurons release acetylcholine to transmit signals:

• Target receptors: Acetylcholine binds to nicotinic receptors on the postganglionic neurons in the peripheral ganglia.
• Role: Acts as the primary neurotransmitter to relay signals from the CNS to the peripheral ganglia, initiating the next step in the autonomic pathway.

This mechanism is crucial for coordinating involuntary responses in both divisions.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Autonomic Nervous System

The sympathetic nervous system (SNS), known as the “fight or flight” system, responds to stress by:

• Functions:
  • Increasing heart rate and blood pressure.
  • Mobilizing energy reserves to prepare the body for action.
• Neurotransmitters:
  • Primarily uses norepinephrine at target organs, except in specific cases like sweat glands, where acetylcholine is utilized.
• Origin:
  • The SNS originates from the thoracolumbar regions of the CNS, not the craniosacral regions.

This system is vital for preparing the body to handle acute stressors.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Nervous System

The parasympathetic preganglionic neurons originate from the:

• Brainstem: Associated with cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus).
• Sacral spinal cord (S2–S4): Supplies pelvic organs and lower gastrointestinal structures.

This distribution is why the parasympathetic division is referred to as the “craniosacral system.”

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Nervous System

Postganglionic sympathetic neurons predominantly release norepinephrine to mediate the effects of the sympathetic nervous system (SNS):

• Receptor activation:
  • Norepinephrine binds to adrenergic receptors (alpha and beta) on target organs.
• Physiological effects:
  • Increased heart rate (beta-1 receptors).
  • Vasoconstriction in certain vessels (alpha-1 receptors).
• Exceptions:
  • Sweat glands release acetylcholine instead of norepinephrine.

This neurotransmitter is crucial for executing the “fight or flight” response.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Neurotransmitters

The parasympathetic nervous system is often called the “rest and digest” system because it supports body functions during relaxation and recovery by:

• Slowing heart rate: Reduces energy expenditure.
• Promoting digestion: Enhances gastrointestinal activity and increases glandular secretions for nutrient processing.
• Conserving energy: Focuses resources on restoring the body rather than preparing for stress.

This system maintains homeostasis during restful states.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Nervous System

Beta-2 adrenergic receptors mediate bronchodilation during sympathetic stimulation to improve oxygen delivery.

• Mechanism:
  • Activation of beta-2 receptors on bronchial smooth muscles causes relaxation of these muscles, resulting in bronchodilation.
• Purpose:
  • Increases airflow to the lungs during stress or exercise, ensuring adequate oxygen supply to meet the body’s increased demands.

This response is crucial during the “fight or flight” reaction.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Autonomic Receptors

The adrenal medulla is unique in that it receives only sympathetic innervation.

• Mechanism:
  • Preganglionic sympathetic neurons release acetylcholine, which activates chromaffin cells in the adrenal medulla.
  • These cells secrete epinephrine and norepinephrine directly into the bloodstream, functioning as hormones to amplify the sympathetic response.
• Purpose:
  • This system supports the “fight or flight” response by increasing heart rate, blood pressure, and energy availability.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Nervous System

Visceral afferent neurons are sensory neurons that play a critical role in autonomic reflex arcs by:

• Function: Transmitting sensory input from internal organs (e.g., blood pressure, chemical changes, stretch) to the central nervous system (CNS).
• Integration: The CNS processes this information and generates motor responses via autonomic efferent neurons to maintain homeostasis.

These neurons are essential for regulating internal organ function and responding to physiological changes.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Autonomic Reflexes

Alpha-1 adrenergic receptors are primarily responsible for vasoconstriction during sympathetic activation.

• Mechanism: Norepinephrine binds to alpha-1 receptors on vascular smooth muscle, causing contraction of the muscle fibers.
• Effect: Vasoconstriction increases vascular resistance and blood pressure, ensuring adequate blood flow to critical organs during stress.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Adrenergic Receptors

During sympathetic activation, blood flow to the kidneys is reduced to prioritize perfusion of vital organs like the brain and heart.

• Mechanism: The renal arteries are vasoconstricted via activation of alpha-1 adrenergic receptors, reducing blood flow to the kidneys.
• Purpose: This redistribution of blood flow ensures sufficient oxygen delivery to essential organs during stress or emergencies.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Renal Blood Flow Regulation

Beta-1 adrenergic receptors are located primarily in the heart and are responsible for:

• Increasing heart rate (chronotropy): Stimulation by norepinephrine or epinephrine accelerates the heart’s pacing.
• Enhancing force of contraction (inotropy): Improves myocardial contraction strength, enabling the heart to pump more blood during stress or exercise.

These actions prepare the body to meet increased energy and oxygen demands during the “fight or flight” response.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Adrenergic Receptors

The vagus nerve (cranial nerve X) is the primary parasympathetic nerve innervating the thoracic and abdominal organs.

• Digestive functions:
  • Increases gastric secretions to aid in digestion.
  • Enhances intestinal motility for efficient food movement.
  • Relaxes gastrointestinal sphincters to facilitate nutrient absorption.
• Other roles: The vagus nerve also slows heart rate and supports other “rest and digest” activities.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Function

The sympathetic division of the autonomic nervous system (ANS) activates the “fight or flight” response to prepare the body for stressful or emergency situations.

• Physiological changes:
  • Increased heart rate (tachycardia): To enhance blood delivery to vital organs.
  • Dilated pupils (mydriasis): To improve vision by allowing more light into the eyes.
  • Redistribution of blood flow: Redirects blood to skeletal muscles and away from non-essential systems like digestion.
• Purpose: These changes enhance physical readiness and energy availability for survival.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Nervous System

Muscarinic acetylcholine receptors are located on target organs of the parasympathetic nervous system and mediate its “rest and digest” functions.

• Responses mediated by muscarinic receptors:
  • Reduced heart rate (bradycardia): Slows the heart to conserve energy.
  • Increased glandular secretions: Enhances salivary, gastric, and intestinal secretions to support digestion.
  • Pupil constriction (miosis): Helps focus on near vision tasks.
• Mechanism: Activated by acetylcholine released from parasympathetic postganglionic neurons.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Receptors

Sympathetic stimulation reduces gastrointestinal activity to conserve energy for critical functions during stress:

• Decreased motility: Reduces smooth muscle contractions, slowing the movement of food through the digestive system.
• Sphincter contraction: Prevents the flow of digestive contents to focus resources on “fight or flight” responses.
• Blood flow redistribution: Diverts blood away from the GI tract to skeletal muscles and vital organs like the heart and brain.

This allows the body to prioritize immediate survival over digestion.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Effects on GI Function

Alpha-2 adrenergic receptors act as negative feedback regulators in the sympathetic nervous system:

• Location: Found on presynaptic terminals of sympathetic neurons.
• Function:
  • When activated, these receptors inhibit further release of norepinephrine, modulating the intensity of sympathetic stimulation.
  • Prevents excessive norepinephrine levels, maintaining balance in sympathetic responses.

This mechanism helps regulate the duration and strength of sympathetic activity.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Adrenergic Receptors

In response to sympathetic stimulation, the adrenal medulla releases epinephrine into the bloodstream.

• Effect of epinephrine:
  • Acts on beta-2 adrenergic receptors in the lungs, causing bronchodilation, which increases airflow.
  • Supports the “fight or flight” response by improving oxygen delivery to vital organs and tissues.
• Additional effects:
  • Increases heart rate and blood glucose levels to provide energy for immediate action.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Adrenal Medulla

The enteric nervous system (ENS) is a network of neurons located within the walls of the gastrointestinal (GI) tract and functions to:

• Regulate digestion:
  • Controls peristalsis (coordinated muscle contractions for food movement).
  • Stimulates secretion of digestive enzymes and regulates nutrient absorption.
• Autonomous function: The ENS operates independently of the brain and spinal cord but is modulated by the sympathetic and parasympathetic systems.

This “second brain” ensures efficient digestive function and gut homeostasis.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Enteric Nervous System

The autonomic nervous system (ANS) typically provides dual innervation to visceral organs, with the sympathetic and parasympathetic divisions exerting opposing effects:

• Examples:
  • Heart rate:
    • Sympathetic stimulation increases heart rate.
    • Parasympathetic stimulation decreases heart rate.
  • Digestive activity:
    • Sympathetic stimulation decreases motility.
    • Parasympathetic stimulation increases motility.
• Purpose: This balance ensures precise control of organ function depending on the body’s needs (e.g., stress vs. rest).

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Dual Innervation

Nicotinic acetylcholine receptors play a critical role in the autonomic nervous system (ANS) by:

• Location: Found on the cell bodies of postganglionic neurons in autonomic ganglia of both the sympathetic and parasympathetic divisions.
• Function: When acetylcholine binds to these receptors, it causes an excitatory response, resulting in the activation of the postganglionic neuron.

This mechanism ensures the effective transmission of signals from the CNS to peripheral target organs.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Nicotinic Receptors

Beta-2 adrenergic receptors, when activated by epinephrine or norepinephrine, contribute to the “fight or flight” response by:

• Bronchodilation: Relaxing smooth muscle in the airways to increase airflow and oxygen delivery to the lungs.
• Vasodilation in skeletal muscle: Enhancing blood flow to active muscles, improving oxygen and nutrient supply during stress or exercise.

These effects optimize respiratory and muscular performance during heightened sympathetic activity.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Adrenergic Receptors

The baroreceptor reflex is critical for maintaining stable blood pressure during positional changes, such as standing up:

• Mechanism:
  • Baroreceptors in the carotid arteries and aortic arch detect changes in blood pressure.
  • When blood pressure drops (e.g., due to gravity when standing), the reflex triggers sympathetic activation.
• Effects:
  • Increased heart rate (tachycardia): To maintain cardiac output.
  • Vasoconstriction: To elevate vascular resistance and stabilize blood pressure.

This reflex prevents dizziness and fainting due to inadequate cerebral blood flow during postural changes.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Baroreceptor Reflex

During sympathetic stimulation, the bladder adjusts for urine storage by:

• Relaxing the detrusor muscle: Reduces bladder wall tension to accommodate more urine.
• Contracting the internal sphincter: Prevents urine leakage during stress or physical activity.

This response ensures efficient urine retention, allowing the body to focus on “fight or flight” functions.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Control of the Bladder

The adrenal medulla, located on top of the kidneys, is a specialized structure that plays a key role in the sympathetic nervous system:

• Hormone release:
  • Epinephrine (adrenaline) and norepinephrine are released into the bloodstream.
• Effect:
  • These hormones act on multiple tissues, enhancing the “fight or flight” response by increasing heart rate, bronchodilation, and energy mobilization.

The adrenal medulla functions as an endocrine amplifier of sympathetic activity.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Adrenal Medulla

Parasympathetic stimulation causes pupil constriction (miosis) by:

• Mechanism: Activates the sphincter pupillae muscle in the iris, reducing pupil size.
• Purpose: Protects the retina by limiting the amount of light entering the eye, particularly in bright environments.

This response is part of the rest and digest functions mediated by the parasympathetic system.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Effects on the Eye

Alpha-1 adrenergic receptors mediate vasoconstriction during sympathetic activation:

• Mechanism:
  • Norepinephrine or epinephrine binds to alpha-1 receptors on vascular smooth muscle, causing contraction.
• Effect:
  • Increases vascular resistance, raising blood pressure to ensure adequate blood flow to vital organs during the “fight or flight” response.

This action prioritizes blood flow to critical areas like the heart and muscles while reducing flow to non-essential regions.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Adrenergic Receptors

The adrenal medulla is unique because it:

• Receives only sympathetic innervation:
  • It is controlled by preganglionic sympathetic neurons, which release acetylcholine to stimulate chromaffin cells.
• Function:
  • Releases catecholamines (epinephrine and norepinephrine) directly into the bloodstream to amplify the “fight or flight” response.

In contrast, the parasympathetic system does not directly influence the adrenal medulla.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Adrenal Medulla

The enteric nervous system (ENS) is a specialized network of neurons located within the walls of the gastrointestinal (GI) tract:

• Primary role:
  • Regulates digestion, motility, and secretions to maintain efficient GI function.
• Modulation:
  • Functions autonomously but is influenced by sympathetic (inhibitory) and parasympathetic (stimulatory) divisions.

The ENS is often referred to as the “second brain” due to its autonomy and critical role in gut function.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Enteric Nervous System

Muscarinic cholinergic receptors, located in the heart, are activated by acetylcholine released from parasympathetic postganglionic neurons.

• Effect on the heart:
  • Decreases heart rate (negative chronotropy).
  • Reduces the force of cardiac contractions (negative inotropy).
• Purpose:
  • Helps maintain a calm and restful state by counteracting the effects of sympathetic activation.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Receptors

The parasympathetic nervous system, known for its “rest and digest” functions, reduces heart rate:

• Mechanism:
  • Parasympathetic neurons release acetylcholine, which binds to muscarinic receptors in the heart.
• Effect:
  • Slows the heart rate, conserving energy during periods of rest.

This response supports bodily recovery and homeostasis by counterbalancing the effects of sympathetic activation.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Nervous System

During sympathetic stimulation, beta-2 adrenergic receptors in the liver are activated, leading to:

• Glycogenolysis: Breakdown of glycogen into glucose, increasing blood glucose levels.
• Purpose: Provides a quick energy source to support the “fight or flight” response.

This mechanism ensures the body has sufficient glucose for energy-intensive activities during stress.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Adrenergic Receptor Function

The sympathetic nervous system regulates pupil dilation (mydriasis) via alpha-1 adrenergic receptors:

• Mechanism:
  • Stimulation of alpha-1 receptors in the radial muscles of the iris causes them to contract, enlarging the pupil.
• Purpose:
  • Allows more light to enter the eye, improving vision during low-light conditions or the “fight or flight” response.

This response enhances visual awareness during stress or emergencies.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Effects on the Eye

The parasympathetic nervous system controls bronchial smooth muscle tone via muscarinic receptors:

• Mechanism:
  • Activation of muscarinic receptors causes contraction of the bronchial smooth muscle, leading to bronchoconstriction.
• Effect:
  • Reduces airflow in the lungs, which is suitable for restful conditions when oxygen demand is lower.

This response is part of the “rest and digest” functions.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Effects on the Respiratory System

During sympathetic activation, beta-2 adrenergic receptors in blood vessels supplying skeletal muscles mediate:

• Vasodilation:
  • Increases blood flow to skeletal muscles.
  • Enhances oxygen and nutrient delivery during “fight or flight” situations.
• Purpose:
  • Prepares muscles for increased activity and ensures optimal performance during stress or exercise.

This action supports the body’s readiness for physical exertion during emergencies.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Adrenergic Receptor Functions

The parasympathetic nervous system enhances digestive functions as part of the “rest and digest” response:

• Increased motility: Activates smooth muscles in the GI tract, facilitating food movement through peristalsis.
• Secretion of digestive enzymes: Promotes the release of enzymes and gastric juices for efficient nutrient breakdown.

This activity optimizes digestion and nutrient absorption during restful states.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Effects on Digestion

The sympathetic nervous system modulates salivary gland activity to produce thick, mucous-rich saliva under stress:

• Mechanism: Activation of alpha-1 adrenergic receptors stimulates the secretion of mucous-rich saliva.
• Purpose: Lubricates the mouth during stress, but less effective for digestion compared to the watery saliva promoted by parasympathetic activation.

This response is part of the “fight or flight” mechanism, where energy is redirected from digestion to other vital functions.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Effects on Salivary Glands

Acetylcholine is the neurotransmitter released by preganglionic neurons in both the sympathetic and parasympathetic nervous systems:

• Function: Binds to nicotinic receptors on postganglionic neurons in autonomic ganglia.
• Purpose: Relays signals from the CNS to postganglionic neurons, which then activate target organs.

This neurotransmitter is critical for transmitting signals in the autonomic nervous system.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Autonomic Neurotransmitters

Sympathetic stimulation activates alpha-1 adrenergic receptors in the arrector pili muscles, resulting in:

• Piloerection: Contraction of these muscles causes hairs on the skin to stand on end.
• Purpose:
  • Part of the “fight or flight” response, piloerection may help animals appear larger or trap insulating air in cold conditions.

This reflex is a characteristic feature of the body’s stress response.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Effects on the Skin

Parasympathetic stimulation promotes insulin release from the pancreas by activating muscarinic cholinergic receptors:

• Mechanism: Acetylcholine released from parasympathetic postganglionic neurons binds to muscarinic receptors in pancreatic beta cells.
• Function:
  • Enhances insulin secretion to regulate blood glucose by facilitating glucose uptake into cells.

This response supports the “rest and digest” state by promoting nutrient storage and energy balance.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Regulation of the Pancreas

Alpha-2 adrenergic receptors inhibit insulin secretion from pancreatic beta cells during sympathetic activation:

• Mechanism: Norepinephrine binds to alpha-2 receptors, suppressing insulin release.
• Purpose:
  • Conserves glucose for essential organs like the brain and skeletal muscles during the “fight or flight” response, ensuring sufficient energy for immediate needs.

This regulation is vital for maintaining blood glucose levels under stress.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Effects on the Pancreas

Sweat glands are unique among sympathetic targets because they are stimulated by acetylcholine, which binds to muscarinic receptors:

• Mechanism: Sympathetic postganglionic fibers release acetylcholine instead of norepinephrine to activate sweat glands.
• Effect:
  • Promotes sweat production, aiding in heat loss and body temperature regulation.

This process is vital for thermoregulation, especially during elevated body temperature or physical exertion.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Effects on Sweat Glands

During a stress response, the sympathetic nervous system reduces blood flow to the skin by causing vasoconstriction of cutaneous blood vessels:

• Mechanism: Activation of alpha-1 adrenergic receptors leads to contraction of vascular smooth muscle in the skin.
• Purpose:
  • Redirects blood toward vital organs like the heart, muscles, and brain to prioritize survival during the “fight or flight” response.

This adaptation helps the body manage physical stress and prepare for action.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Regulation of Blood Flow

The parasympathetic nervous system controls lacrimal gland secretion to promote tear production:

• Mechanism: Acetylcholine is released from parasympathetic postganglionic neurons and binds to muscarinic receptors on lacrimal glands.
• Effect:
  • Increases tear production, which lubricates and protects the surface of the eye.

This response is part of the “rest and digest” functions of the parasympathetic system.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Regulation of Lacrimal Glands

During sympathetic activation, saliva production is reduced or altered:

• Saliva characteristics:
  • Sympathetic stimulation promotes thicker, mucous-rich saliva via alpha-1 adrenergic receptors, not the increased watery saliva typical of parasympathetic activation.
• Other effects of sympathetic activation:
  • Increased heart rate, bronchodilation, pupil dilation (mydriasis), and decreased gastrointestinal motility are hallmarks of the “fight or flight” response.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic and Parasympathetic Effects

The parasympathetic nervous system facilitates micturition (urination) by:

• Contraction of the detrusor muscle: Increases pressure within the bladder, pushing urine toward the urethra.
• Relaxation of the internal sphincter: Opens the pathway for urine to exit the bladder.

This coordinated action allows urine to be expelled efficiently during the “rest and digest” state.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Regulation of Micturition

During sympathetic activation, the body mobilizes energy to meet increased demands by:

• Glycogenolysis: Breaking down glycogen into glucose to provide an immediate energy source.
• Gluconeogenesis: Producing glucose from noncarbohydrate sources, such as amino acids, to sustain energy supply.
• Purpose:
  • Ensures adequate glucose levels for critical organs and skeletal muscles during the “fight or flight” response.

This metabolic regulation prepares the body to respond effectively to stress.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Metabolic Effects

Alpha-1 adrenergic receptors mediate vasoconstriction in vascular smooth muscle during sympathetic activation:

• Mechanism: Norepinephrine binds to alpha-1 receptors, causing smooth muscle contraction in blood vessel walls.
• Effect:
  • Increases vascular resistance and blood pressure, ensuring adequate blood flow to vital organs during the “fight or flight” response.

This is a key function of the sympathetic nervous system during stress or emergencies.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Adrenergic Receptor Effects

The parasympathetic nervous system influences the airways by:

• Mechanism: Acetylcholine binds to muscarinic receptors in bronchial smooth muscle, causing contraction.
• Effect: Bronchoconstriction reduces airflow when oxygen demand is low, such as during rest.

This response is part of the “rest and digest” functions, conserving energy in non-stressful conditions.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Effects on the Respiratory System

Sympathetic stimulation activates the adrenal medulla, causing it to release epinephrine and norepinephrine into the bloodstream:

• Effects of catecholamines:
  • Increased heart rate (chronotropy): Enhances blood flow to critical organs.
  • Airway dilation: Facilitates improved oxygen intake.
  • Energy mobilization: Promotes glycogenolysis and lipolysis to fuel muscles.
• Purpose: Supports the “fight or flight” response, preparing the body to handle stress.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Activation of the Adrenal Medulla

Beta-1 adrenergic receptors are located in the heart and are activated by norepinephrine during sympathetic stimulation:

• Effects on the heart:
  • Increased heart rate (chronotropy): Enhances the pacing of the sinoatrial (SA) node.
  • Increased contractility (inotropy): Strengthens cardiac contractions to improve cardiac output.
• Purpose: Optimizes blood delivery to muscles and vital organs during the “fight or flight” response.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Adrenergic Receptors in Cardiac Function

The parasympathetic nervous system reduces heart rate via muscarinic (M2) receptors in the heart:

• Mechanism:
  • Acetylcholine, released by parasympathetic postganglionic neurons, binds to M2 receptors on the sinoatrial (SA) node.
  • This slows the firing rate of the SA node and reduces cardiac contractility.
• Effect: Heart rate decreases, conserving energy during restful states.

This is part of the “rest and digest” response to maintain homeostasis.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Effects on the Heart

Sympathetic stimulation promotes lipolysis, the breakdown of stored fat, to provide energy during stress or exercise:

• Mechanism:
  • Activation of beta-adrenergic receptors in adipose tissue by norepinephrine stimulates enzymes that convert triglycerides into free fatty acids and glycerol.
• Purpose:
  • These byproducts are used as energy sources to fuel vital organs and muscles during the “fight or flight” response.

This process ensures the body can meet its increased energy demands during stress.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Metabolic Effects

The parasympathetic nervous system promotes digestive activity by stimulating enzyme secretion from the pancreas:

• Mechanism: Acetylcholine, released from parasympathetic postganglionic neurons, binds to muscarinic receptors in the pancreas.
• Effect: Enhances the secretion of digestive enzymes, which aids in the breakdown and absorption of nutrients following food intake.

This response supports the “rest and digest” functions of the parasympathetic system.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Regulation of Digestion

Parasympathetic stimulation controls the eyes to regulate light entry and focus:

• Mechanism: Activation of muscarinic receptors causes contraction of the sphincter pupillae muscle, leading to pupil constriction (miosis).
• Purpose: Protects the retina from excessive light exposure, especially in bright environments.

This response optimizes visual performance in conditions requiring less light intake.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Effects on the Eye

During sympathetic stimulation, beta-2 adrenergic receptors in the blood vessels of skeletal muscles mediate vasodilation:

• Mechanism: Activation by epinephrine or norepinephrine causes relaxation of vascular smooth muscle.
• Effect: Increases blood flow to skeletal muscles, enhancing oxygen and nutrient delivery during stress or physical activity.

This ensures optimal muscle performance during the “fight or flight” response.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Adrenergic Receptor Functions

The parasympathetic nervous system promotes glycogen synthesis in the liver through muscarinic receptor activation:

• Mechanism: Acetylcholine binds to muscarinic receptors, signaling the liver to convert glucose into glycogen.
• Purpose: Restores glycogen stores after meals, conserving glucose for later use and supporting the “rest and digest” state.

This process ensures energy is stored efficiently during restful periods.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Parasympathetic Effects on the Liver

Sympathetic stimulation regulates gastrointestinal activity to prioritize energy for vital functions during stress:

• Preparation: The body temporarily inhibits digestive processes by activating alpha-1 adrenergic receptors located on the smooth muscle of the gastrointestinal sphincters.
• Mechanism: Norepinephrine, released during sympathetic activation, binds to these receptors, causing contraction of the sphincters.
• Effect: Prevents the movement of food or waste, conserving energy and redirecting resources to critical systems like the heart and skeletal muscles.

This action supports the “fight or flight” response by optimizing the body’s physiological state for immediate action.

References: Cunningham’s Textbook of Veterinary Physiology, 6th Edition, Chapter 11, Sympathetic Regulation of GI Function