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Questions and Answers
Which of the following is a physiological effect of the Autonomic Nervous System (ANS) on body functions?
Which of the following is a physiological effect of the Autonomic Nervous System (ANS) on body functions?
What is the primary function of the sympathetic nervous system?
What is the primary function of the sympathetic nervous system?
Where do the parasympathetic neurons originate?
Where do the parasympathetic neurons originate?
What is the role of the hypothalamus in the Autonomic Nervous System (ANS)?
What is the role of the hypothalamus in the Autonomic Nervous System (ANS)?
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Which of the following is a characteristic of the parasympathetic nervous system?
Which of the following is a characteristic of the parasympathetic nervous system?
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Which of the following describes a tissue innervated by both the sympathetic and parasympathetic nervous systems?
Which of the following describes a tissue innervated by both the sympathetic and parasympathetic nervous systems?
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Which of the following is a neurotransmitter commonly associated with the sympathetic nervous system?
Which of the following is a neurotransmitter commonly associated with the sympathetic nervous system?
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Which of the following correctly describes the pathway of the parasympathetic nervous system?
Which of the following correctly describes the pathway of the parasympathetic nervous system?
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Which of the following enzymes is responsible for the metabolism of NE to inactive intermediates? (Select all that apply)
Which of the following enzymes is responsible for the metabolism of NE to inactive intermediates? (Select all that apply)
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Which of the following is a product of NE metabolism?
Which of the following is a product of NE metabolism?
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In the heart, which of the following receptors are stimulated by NE, leading to increased heart rate?
In the heart, which of the following receptors are stimulated by NE, leading to increased heart rate?
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Which of the following effects does NE have on the lungs? (Select all that apply)
Which of the following effects does NE have on the lungs? (Select all that apply)
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Which of the following effects does NE have on the urinary bladder? (Select all that apply)
Which of the following effects does NE have on the urinary bladder? (Select all that apply)
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What effect does sympathetic stimulation have on the cerebral blood vessels?
What effect does sympathetic stimulation have on the cerebral blood vessels?
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Which of the following is a consequence of sympathetic stimulation on the salivary gland?
Which of the following is a consequence of sympathetic stimulation on the salivary gland?
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Which of the following correctly describes the effect of sympathetic stimulation on the heart?
Which of the following correctly describes the effect of sympathetic stimulation on the heart?
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What is the initial effect of sympathetic stimulation on the coronary vessels?
What is the initial effect of sympathetic stimulation on the coronary vessels?
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Which of the following accurately reflects the sympathetic effect on the eye?
Which of the following accurately reflects the sympathetic effect on the eye?
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Which of the following is a physiological effect of sympathetic stimulation on the liver?
Which of the following is a physiological effect of sympathetic stimulation on the liver?
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What is the primary effect of sympathetic stimulation on the urinary bladder?
What is the primary effect of sympathetic stimulation on the urinary bladder?
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Which of the following is a physiological effect of sympathetic stimulation on the skeletal muscle?
Which of the following is a physiological effect of sympathetic stimulation on the skeletal muscle?
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How does sympathetic stimulation affect the spleen?
How does sympathetic stimulation affect the spleen?
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Which of the following is a physiological effect of sympathetic stimulation on the rectum?
Which of the following is a physiological effect of sympathetic stimulation on the rectum?
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What is the primary function of the sympathetic nervous system in relation to the adrenal medulla?
What is the primary function of the sympathetic nervous system in relation to the adrenal medulla?
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Which of the following accurately depicts the effect of sympathetic stimulation on the vas deferens and associated structures?
Which of the following accurately depicts the effect of sympathetic stimulation on the vas deferens and associated structures?
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Which of the following statements about the sympathetic nervous system is incorrect?
Which of the following statements about the sympathetic nervous system is incorrect?
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In which of the following locations are parasympathetic preganglionic neuron cell bodies found?
In which of the following locations are parasympathetic preganglionic neuron cell bodies found?
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What is the role of acetylcholinesterase (AChE) in the parasympathetic nervous system?
What is the role of acetylcholinesterase (AChE) in the parasympathetic nervous system?
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Which of the following statements accurately describes the divergence of sympathetic versus parasympathetic pathways?
Which of the following statements accurately describes the divergence of sympathetic versus parasympathetic pathways?
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Which of the following scenarios is MOST likely to be triggered by the activation of the parasympathetic nervous system?
Which of the following scenarios is MOST likely to be triggered by the activation of the parasympathetic nervous system?
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What type of receptor is found on the postganglionic neuron in both the sympathetic and parasympathetic nervous systems?
What type of receptor is found on the postganglionic neuron in both the sympathetic and parasympathetic nervous systems?
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Which of the following is a similarity between the sympathetic and parasympathetic nervous systems?
Which of the following is a similarity between the sympathetic and parasympathetic nervous systems?
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Which of the following is an example of a sympathetic postganglionic neuron target tissue?
Which of the following is an example of a sympathetic postganglionic neuron target tissue?
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Which of the following is NOT a response to cholinergic stimulation of the sinoatrial node?
Which of the following is NOT a response to cholinergic stimulation of the sinoatrial node?
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What is the receptor subtype responsible for increased watery secretion in salivary glands?
What is the receptor subtype responsible for increased watery secretion in salivary glands?
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Which of the following correctly describes the role of acetylcholine in parasympathetic neurotransmission?
Which of the following correctly describes the role of acetylcholine in parasympathetic neurotransmission?
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How does parasympathetic stimulation influence the heart?
How does parasympathetic stimulation influence the heart?
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Which of the following is a direct effect of parasympathetic stimulation on the gastrointestinal tract?
Which of the following is a direct effect of parasympathetic stimulation on the gastrointestinal tract?
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Which of the following effects contribute to the parasympathetic response of miosis (pupil constriction)?
Which of the following effects contribute to the parasympathetic response of miosis (pupil constriction)?
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How does acetylcholine influence the synthesis of its own precursor molecule?
How does acetylcholine influence the synthesis of its own precursor molecule?
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Which of the following statements accurately describes the role of cholinergic receptors in the transmission of parasympathetic nerve impulses?
Which of the following statements accurately describes the role of cholinergic receptors in the transmission of parasympathetic nerve impulses?
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Study Notes
Autonomic Nervous System (ANS)
- The Autonomic Nervous System (ANS) is a complex network that controls a wide range of involuntary and subconscious functions within the body, primarily those essential for survival and maintaining life.
- This system plays a critical role in regulating the internal environment of the body to ensure stability and balance, a concept known as homeostasis. Homeostasis involves various physiological parameters such as temperature, pH levels, hydration, and electrolyte balance, all crucial for optimal cellular function.
- The ANS is categorized as part of the peripheral nervous system, specifically as an involuntary branch of the peripheral efferent division, which means it transmits signals from the central nervous system (CNS) to the effector organs without requiring conscious thought.
- One key function of the ANS is its ability to innervate the heart, which involves cardiac muscle stimulation, increasing blood pumping to compensate for drops in blood pressure during events like physical exertion or sudden changes in posture.
- Additionally, the ANS affects the gastrointestinal system by innervating the stomach's smooth muscle. It strategically delays the emptying of the stomach until the intestines are prepared to process the incoming food, thereby optimizing the digestive process.
- Another important role of the ANS is in thermoregulation; it innervates sweat glands to initiate sweating in response to heat exposure, which helps cool the body through evaporative cooling mechanisms.
- The ANS also has a significant impact on metabolic processes through its innervation of the endocrine pancreas. After meals, it increases insulin secretion, facilitating the uptake of glucose and regulation of blood sugar levels.
- Interestingly, the two major branches of the ANS, the sympathetic and parasympathetic systems, often work oppositely yet harmoniously. This duality enables the fine-tuning of various bodily functions to maintain homeostasis even amid physiological stress or crisis.
- Overall, the ANS is regulated by higher brain centers, including the hypothalamus, pons, and medulla oblongata, along with spinal reflexes that provide rapid local responses to stimuli.
- The ANS utilizes a two-neuron pathway comprising the preganglionic neuron, which extends from the CNS to autonomic ganglia located outside the CNS, and the postganglionic neuron, which transmits impulses from the ganglion to the target organ or tissue.
Learning Objectives
- Explain the function of the sympathetic and parasympathetic systems, emphasizing their contrasting roles in the regulation of body functions.
- Discuss the physiological effects of the ANS on various body functions, including cardiovascular, respiratory, gastrointestinal, and thermoregulatory systems.
- Identify and explain the various neurotransmitters and receptors involved in the ANS, elaborating on their roles and mechanisms.
- Examine the tissues and organs that are exclusively innervated by the sympathetic and parasympathetic systems, as well as those that receive dual innervation from both branches.
Anatomical Distribution of ANS
- Sympathetic neurons originate primarily from the lateral horns situated in the thoracic and lumbar regions of the spinal cord, specifically between segments T1 and L2. This anatomical placement highlights their origin in the so-called thoracolumbar division of the autonomic nervous system.
- Conversely, parasympathetic neurons stem from the midbrain, medulla oblongata, and sacral region of the spinal cord. This origin refines the understanding of the parasympathetic division, often referred to as the craniosacral division, as these neurons travel through cranial nerves and sacral nerves to reach their target tissues.
Two Motor Neurons
- The first neuron in the ANS pathway, known as the preganglionic neuron, has its cell body situated within the CNS, either in the brain or in specific segments of the spinal cord. This positioning is vital for the initiation of autonomic signals that will influence various bodily responses.
- In the sympathetic nervous system, preganglionic neurons are localized in the lateral gray horns of the thoracolumbar region.
- In the parasympathetic nervous system, preganglionic neurons reside in the nuclei of the brain stem or in the lateral gray horns located in the sacral region.
- The second motor neuron, known as the postganglionic neuron, has its cell body located within an autonomic ganglion, which serves as a synaptic relay point for the transmission of signals from the preganglionic neuron to the target effector tissue.
- These ganglionic fibers are responsible for sending impulses to the target organ, thereby eliciting appropriate physiological responses based on the signals received from preganglionic neurons.
- The effects on target organs vary considerably depending on the types of neurotransmitters released and the specific receptor types present on the effector cells. These interactions underline the complexity of the ANS and its influence over multiple body systems.
Preganglionic vs Postganglionic Neurons
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Sympathetic:
- Preganglionic neurons in the sympathetic division are characterized as being short, myelinated, and cholinergic, meaning they release acetylcholine at their synapses.
- In contrast, postganglionic neurons are long, unmyelinated, and use norepinephrine as their primary neurotransmitter, classifying them as noradrenergic.
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Parasympathetic:
- Within the parasympathetic division, preganglionic neurons are notably long, myelinated, and also cholinergic, releasing acetylcholine at synapses.
- Postganglionic neurons, on the other hand, are short, unmyelinated, and likewise cholinergic, releasing acetylcholine, but targeting different effector receptors.
- The divergence of sympathetic neurons is high, allowing for widespread and coordinated responses throughout the body, while the divergence of parasympathetic neurons is notably low, leading to more localized and specific responses.
- In terms of target tissues, sympathetic divisions influence a wide range of structures including smooth muscles, cardiac muscles, various endocrine glands, and adipose (fat) tissue.
- Meanwhile, parasympathetic target tissues encompass smooth muscles, cardiac muscles, and exocrine glands, as well as fatty tissues, reflecting their roles in promoting restorative and preparatory activities in the body.
Sympathetic vs Parasympathetic Similarities
- Both divisions of the ANS function as efferent (motor) systems and are collectively recognized as "visceromotor" systems due to their influence over visceral organs and functions.
- They share the common characteristic of regulating internal physiological processes that occur outside of conscious control, thus earning the designation of "autonomous" control.
- Moreover, both systems employ two-neuron pathways that include synapses in peripheral ganglia, thereby innervating a variety of glands, smooth muscles, and cardiac muscle tissues effectively and efficiently.
Sympathetic vs Parasympathetic Differences
- Location of preganglionic neurons: In the sympathetic division, preganglionic neurons extend from the spinal cord segments T1 to L2, while in the parasympathetic division, they originate primarily from the brainstem and sacral segments S2 to S4.
- Location of postganglionic neurons: In sympathetic pathways, these neurons are located either close to or within the target organs, or accumulated in chains adjacent to the spinal cord. In contrast, parasympathetic postganglionic neurons are located much closer to or within the target organs themselves, allowing for localized responses.
- Neurotransmitter released by preganglionic neuron: Both sympathetic and parasympathetic preganglionic neurons release acetylcholine, establishing a commonality in their initial signaling mechanisms.
- Neurotransmitter released by postganglionic neuron: The sympathetic division predominantly releases norepinephrine from postganglionic neurons, acting on various adrenergic receptors. In comparison, parasympathetic postganglionic neurons also release acetylcholine, but target muscarinic receptors on effector cells, demonstrating divergent signaling pathways within the same basic framework.
Parasympathetic Nervous System
- Both preganglionic and postganglionic chromatophore neurotransmitters in the parasympathetic division are acetylcholine, underscoring their uniformity in signaling.
- Acetylcholine, once released into the synaptic cleft, is rapidly metabolized by the enzyme acetylcholinesterase (AChE), ensuring that signals are promptly terminated to prevent overstimulation.
- The release of ACh from preganglionic fibers activates nicotinic receptors located on the target postganglionic neurons, facilitating transmission of impulses within the parasympathetic signaling pathway.
- The subsequent depolarization of postganglionic fibers results in the release of additional ACh, which subsequently activates muscarinic receptors on effector cells to create a physiological response tailored to the situation.
Parasympathetic Nervous System - Cholinergic Stimulation
- The cell bodies of preganglionic neurons involved in the parasympathetic division are located in the cranial nerve nuclei situated within the brainstem, emphasizing their central origin.
- Preganglionic fibers exit the CNS through cranial nerves, specifically cranial nerves III (Oculomotor), VII (Facial), IX (Glossopharyngeal), and X (Vagus), which play important roles in parasympathetic functions.
- The oculomotor nerve (CN III) is pivotal for several functions including miosis (pupil constriction), facilitating near vision focusing, and controlling various other aspects of eye movement.
Parasympathetic - Sacral Outflow
- In the case of the urinary bladder, parasympathetic stimulation results in the contraction of the bladder wall while simultaneously relaxing the sphincter. This coordination leads to micturition, or urination, signifying a functional response to fluid intake.
- For the seminal vesicles and prostate, parasympathetic activity promotes the secretion of fluids, essential for reproductive functions and the provision of a conducive environment for sperm.
- Regarding the rectum and descending colon, parasympathetic innervation results in wall contractions while relaxing the sphincters, facilitating defecation, an essential bodily function for waste elimination.
- In erectile tissues, parasympathetic stimulation causes vasodilation, resulting in engorgement and erection, demonstrating the system's role in enhancing reproductive capabilities.
Sympathetic Pathway of the Medulla
- The sympathetic pathway involves norepinephrine secretion from sympathetic neurons, which acts on target cells to exert various physiological effects closely associated with the body’s stress response.
- Notably, the sympathetic nervous system influences the adrenal medulla, where its stimulation leads to the primary secretion of epinephrine. This hormone acts on distant target cells, leading to systemic responses crucial for fight-or-flight reactions during stressful situations. In contrast, norepinephrine acts locally on target cells at the site where it is released, showcasing the distinct roles of these catecholamines within autonomic regulation.
Adrenal Medullary Hormones
- The adrenal medulla is responsible for the secretion of catecholamines, primarily epinephrine (commonly known as adrenaline) and norepinephrine (noradrenaline), as well as a minor amount of dopamine, all of which play critical roles in the body's response to stress or emergencies.
- The enzymatic conversion of dopamine to norepinephrine is facilitated by the enzyme dopamine β-hydroxylase (DBH), ensuring that these catecholamines can be formed and released effectively when needed.
- Furthermore, norepinephrine can be converted into epinephrine predominantly through the action of phenylethanolamine-N-methyltransferase (PNMT), illustrating the biochemical pathways that lead to the generation of these important hormones.
- The secretion of these hormones is essential as it diverts blood flow away from non-essential organs and redirects it toward vital areas such as the brain, heart, and skeletal muscles, thereby preparing the body for rapid action in response to perceived threats.
- Moreover, the amino acid tyrosine is critically involved in the biosynthesis of both epinephrine and norepinephrine, emphasizing its importance in the overall functioning of the adrenergic system.
Epinephrine and Norepinephrine
- Epinephrine, accounting for approximately 80% of the catecholamines released, acts as a potent stimulator, enhancing heart rate, contractility, and a myriad of metabolic activities, effectively preparing the body for dynamic activity.
- Norepinephrine, which comprises about 20% of the catecholamines, significantly influences peripheral vascular resistance and is crucial in regulating blood pressure through vasoconstriction of blood vessels.
Mechanism of Norepinephrine Release and Recycling
- The process of norepinephrine release commences when an action potential arrives at the varicosity, the swelling within a neuron where neurotransmitter storage occurs, allowing for the conversion of electrical signals into chemical messages.
- Upon depolarization, voltage-gated Ca2+ channels open, permitting calcium ions to enter the neuron, a pivotal step that drives the cascade of neurotransmitter release.
- This calcium influx triggers exocytosis of synaptic vesicles, which contain norepinephrine, thus releasing the neurotransmitter into the synaptic cleft where it can bind to receptors on target cells.
- Norepinephrine acts on adrenergic receptors found on the target tissues, eliciting various physiological responses. The cessation of activity occurs when norepinephrine diffuses away from the synaptic cleft, ensuring that the signals do not persist longer than necessary.
- Following this action, norepinephrine is taken back into the axon through reuptake mechanisms, whereby it can be recycled for future neurotransmission.
- Notably, recycled norepinephrine can also be repackaged into synaptic vesicles, emphasizing the efficiency of neurotransmitter systems. Additionally, norepinephrine is metabolized by monoamine oxidase (MAO), thereby regulating its levels in circulation.
Metabolism of Catecholamines
- The degradation of catecholamines utilizes two primary enzymes: monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT), each playing a role in breaking down excess neurotransmitters within the body.
- Assessments of catecholamine production and metabolism often involve the measurement of urinary vanillylmandelic acid (VMA) and metanephrine, which are metabolites derived from catecholamines, indicating levels of adrenal function and activity.
Pheochromocytoma
- Pheochromocytoma refers to a tumor that originates from chromaffin cells within the adrenal medulla. These tumors can be benign or malignant and significantly affect catecholamine production.
- Common symptoms associated with pheochromocytoma include episodic hypertension (periods of elevated blood pressure), palpitations (irregular heartbeats), headaches, excessive sweating (diaphoresis), and pallor (pale skin), which are indicative of excessive catecholamine release into the system.
- Clinical evaluations for pheochromocytoma often reveal elevated levels of urinary VMA and plasma catecholamines, providing insight into the presence and activity of this condition and guiding treatment options.
Sympathetic Cervical Division
- Eye: Components of the sympathetic cervical division lead to pupil dilation (mydriasis), widening of the palpebral fissure, exophthalmos (protrusion of the eyeball), vasoconstriction of ocular blood vessels, and relaxation of the ciliary muscle, facilitating far vision.
- Salivary gland: This division influences trophic secretion (maintaining glandular function), induces vasoconstriction, and stimulates salivary secretion, particularly in response to the perception of food.
- Lacrimal gland: Autonomic regulation leads to trophic secretion and simultaneous vasoconstriction in the lacrimal glands, influencing tear production.
- Sweat secretion: The sympathetic system prompts an increase in sweat production, crucial for thermoregulation during emotional and physical stress.
- Face skin blood vessels: Vasoconstriction occurs within the blood vessels of facial skin, impacting the blood flow and thermoregulation in response to stress.
Sympathetic Cardiopulmonary Division
- Heart: Activation leads to enhanced cardiac properties, including increased contraction strength, rhythmicity, excitability, and overall conductivity, thus facilitating a "fight or flight" response.
- Bronchi: Sympathetic stimulation causes bronchodilation, reduces bronchial secretions, and induces vasoconstriction of pulmonary blood vessels, optimizing airflow during periods of increased physical demand.
- Coronary vessels: Initially, sympathetic activation causes vasoconstriction of coronary vessels, but this is followed by vasodilation due to the accumulation of metabolic byproducts, ensuring adequate blood flow to the heart.
- Hair: The system prompts erection of hair follicles due to contraction of the erector pili muscles, resulting in piloerection, a physiological reaction commonly associated with fear or cold stimuli.
Sympathetic Splanchnic Division
- Stomach and Intestine: Sympathetic activation leads to sphincter contraction, retaining food within the gastrointestinal tract during times of stress and redirecting blood flow away from digestive processes.
- Liver: This division stimulates glycogenolysis, releasing glucose into the bloodstream and thereby increasing blood glucose levels, a crucial energy source during stress responses.
- Spleen: Activation results in the contraction of the spleen’s capsule, facilitating the evacuation of red blood cells into circulation, which is vital during acute stress responses.
- Urinary Bladder: The internal urethral sphincter contracts, alongside relaxation of the bladder wall, resulting in urine retention, which reflects the sympathetic focus on survival during stressful situations.
- Genital Organs: Sympathetic pathways induce vasoconstriction in genital organs, leading to shrinkage of erectile tissues such as the penis and clitoris, emphasizing their role in modulating sexual functions during stress.
- Rectum: Relaxation of the distal colon, rectum wall, and internal anal sphincter occurs, leading to fecal retention during periods of sympathetic dominance, reflecting the body's prioritization of survival functions.
- Vas Deferens: In males, the sympathetic response involves contraction of the vas deferens, contributing to ejaculation, which emphasizes the system’s role in reproductive functions during the fight-or-flight response.
Sympathetic-Somatic Division
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Skin: Activating the sympathetic division results in vasoconstriction in the skin's vasculature and stimulates sweat glands, facilitating thermoregulation and the body’s ability to respond to heat.
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Skeletal Muscle: The response includes vasodilation driven by cholinergic effects, alongside adrenergic vasoconstriction, ensuring that skeletal muscles receive adequate blood flow while managing stress levels.
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Adrenal Medulla: The adrenal medulla is stimulated by preganglionic sympathetic fibers to secrete epinephrine and a small amount of norepinephrine, reinforcing the body's capacity to respond effectively under stress.
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. This commonality underscores their shared origin from the central nervous system.
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Neurotransmitter released by the postganglionic neuron: The sympathetic division typically releases norepinephrine, which binds to adrenergic receptors on target organs, while the parasympathetic division primarily releases acetylcholine, activating muscarinic receptors in target tissues, resulting in differing physiological effects.
Parasympathetic Nervous System
- In both preganglionic and postganglionic divisions, the primary neurotransmitter employed is acetylcholine, highlighting a consistent mechanism for communication within the parasympathetic system.
- Acetylcholine itself is rapidly metabolized by the enzyme acetylcholinesterase (AChE), ensuring that its action is short-lived and tightly controlled, thus preventing overstimulation of target organs.
- The release of acetylcholine from preganglionic fibers activates nicotinic receptors located on postganglionic neurons, facilitating the transmission of signals within the autonomic pathway.
- Following activation, the depolarization of postganglionic fibers leads to a subsequent release of acetylcholine, which then interacts with muscarinic receptors on effector cells, influencing their activity in various organ systems.
Parasympathetic Nervous System - Cholinergic Stimulation
- Preganglionic cell bodies of the parasympathetic system are primarily located in cranial nerve nuclei within the brain stem, emphasizing the system's central control in autonomic regulation.
- Preganglionic fibers associated with this system utilize cranial nerves (CN) III, VII, IX, and X to relay messages to their target organs, showcasing their extensive reach throughout the body.
- The oculomotor nerve (CN III) specifically regulates functions such as miosis (pupil constriction), the ability to focus on near objects, and various eye movements, contributing to visual acuity and ocular response.
Parasympathetic - Sacral Outflow
- In the urinary bladder, parasympathetic activity leads to the contraction of the bladder wall and relaxation of the internal sphincter muscle, facilitating the process of micturition (urination).
- In the seminal vesicles and prostate gland, the parasympathetic stimulus promotes the secretion of reproductive fluid, which is essential for normal fertility processes.
- For the rectum and descending colon, parasympathetic activation causes the contraction of the walls and relaxation of the internal anal sphincter, leading to coordinated movements that result in defecation.
- In erectile tissue, the parasympathetic nervous system induces vasodilation, which is crucial for achieving penile erection and overall sexual function.
Sympathetic Pathway of the Medulla
- The sympathetic pathway includes norepinephrine secretion from sympathetic neurons, which acts on target cells to facilitate localized physiological responses, crucial for the immediate adaptive reactions of the body.
- Moreover, the sympathetic nervous system exerts effects on the adrenal medulla, which primarily secretes epinephrine into the bloodstream. This systemic response plays a vital role in preparing the body for action, as epinephrine influences multiple distant target cells, amplifying the fight-or-flight response.
Adrenal Medullary Hormones
- The adrenal medulla secretes catecholamines, specifically adrenaline (epinephrine) and noradrenaline (norepinephrine), along with a small amount of dopamine, all of which are crucial for regulating stress responses and metabolic activities.
- Dopamine is converted to norepinephrine through the enzymatic activity of dopamine β-hydroxylase (DBH), facilitating the production of major catecholamine hormones.
- Norepinephrine is predominantly transformed into epinephrine by phenylethanolamine-N-methyltransferase (PNMT), reflecting the metabolic processes that produce these vital hormones.
- The secretion of these hormones results in the diversion of blood flow towards critical regions of the body, including the brain, heart, and skeletal muscles, particularly during times of high demand.
- Additionally, the amino acid tyrosine is a precursor involved in the biosynthesis of both epinephrine and norepinephrine, highlighting the intricate biochemical pathways that sustain hormonal production.
Epinephrine and Norepinephrine
- Epinephrine, constituting approximately 80% of catecholamine release, serves as a powerful stimulator of cardiovascular and metabolic processes, elevating heart rate and enhancing energy availability during stress.
- Norepinephrine, making up about 20% of catecholamine output, plays a significant role in peripheral vasoconstriction. This action is crucial for maintaining blood pressure and ensuring adequate blood supply to essential organs during times of challenge.
Mechanism of Norepinephrine Release and Recycling
- The sequential process begins as an action potential arrives at the varicosity of the sympathetic neuron, a structure where neurotransmitter release occurs.
- Upon arrival, depolarization of the axon terminal opens voltage-gated Ca2+ channels, allowing calcium ions to flow into the neuron.
- The influx of Ca2+ triggers the exocytosis of synaptic vesicles containing norepinephrine into the synaptic cleft, resulting in neurotransmitter release.
- Norepinephrine then binds to adrenergic receptors located on the surface of target cells, initiating various physiological responses that affect organ function.
- After completing its action, the activity of norepinephrine ceases as it diffuses away from the synapse, reducing its concentration and thus terminating its effects.
- Norepinephrine is then transported back into the presynaptic axon, a process known as reuptake, where it can either be repackaged into synaptic vesicles for future use or metabolized.
- The recycling of norepinephrine ensures efficient use of neurotransmitters, with the remaining norepinephrine being broken down by the enzyme monoamine oxidase (MAO) within the neuron.
Metabolism of Catecholamines
- Degradation of catecholamines involves two primary enzymes: monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT), both of which play pivotal roles in maintaining neurotransmitter balance.
- An assessment of catecholamine production can be performed using measures of urinary vanillylmandelic acid (VMA) and metanephrine, substances that serve as metabolites indicating catecholamine levels and activity.
Pheochromocytoma
- Pheochromocytoma is a type of tumor that arises from chromaffin cells in the adrenal medulla, potentially leading to severe and sometimes malignant forms. Its presence can significantly affect the body's hormonal regulation.
- Symptoms associated with pheochromocytoma may include episodic hypertension (high blood pressure), palpitations, severe headaches, excessive sweating, and pallor, indicating a hyperactive sympathetic response.
- Testing for elevated urinary VMA and plasma catecholamine levels are commonly used diagnostic markers that can indicate this condition, with increased levels suggesting an overproduction of catecholamines by the tumor.
Sympathetic Cervical Division
- Eye: The sympathetic nervous system causes pupil dilation (mydriasis), widening of the palpebral fissure (the opening between eyelids), exophthalmos (protrusion of the eyeball), vasoconstriction of ocular blood vessels, and relaxation of ciliary muscles, enhancing the ability to see distant objects.
- Salivary gland: There is stimulation of trophic secretion which aids in maintaining salivary gland health, along with vasoconstriction to modulate blood flow and constriction during active salivation, thereby squeezing secretion into the oral cavity.
- Lacrimal gland: The sympathetic activation induces trophic secretion as well as vasoconstriction, which may affect tear production and overall ocular lubrication.
- Sweat secretion: The sympathetic system stimulates copious secretion, allowing a critical thermoregulatory response during instances of heat stress or exercise.
- Face skin blood vessel: Vasoconstriction occurs, directing blood flow away from the skin to prioritize perfusion of vital organs during moments of stress or danger.
Sympathetic Cardiopulmonary Division
- Heart: The sympathetic division increases cardiac properties such as the force and rate of contraction, rhythmicity, excitability, and conductivity, critical for enhancing cardiac output during periods of increased physical demand.
- Bronchi: It facilitates bronchodilation, resulting in widened airways for enhanced airflow, while decreasing bronchial secretions and inducing vasoconstriction of pulmonary blood vessels to optimize respiratory function.
- Coronary vessels: During sympathetic activation, there's an initial vasoconstriction followed by vasodilation, particularly influenced by metabolic byproducts, which demonstrates the body’s need for increased blood flow to the heart during heightened activity.
- Hair: Erection of hair is induced due to the contraction of erector pili muscles, a phenomenon often associated with the body's fight-or-flight response.
Sympathetic Splanchnic Division
- Stomach and Intestine: Sphincter muscles contract in this division, leading to food retention within the digestive tract, allowing optimal time for digestion and absorption of nutrients.
- Liver: The activation of sympathetic pathways triggers glycogenolysis, increasing blood glucose concentration to provide essential energy during stress responses.
- Spleen: The sympathetic stimulation causes contraction of the splenic capsule, leading to evacuation of blood, which increases blood circulation during an emergency scenario.
- Urinary Bladder: Contraction of the internal urethral sphincter paired with relaxation of the bladder wall culminates in urine retention, allowing the body to exert control over the timing of urination.
- Genital Organs: The sympathetic system induces vasoconstriction, leading to a reduction in size of erectile tissues such as the penis and clitoris, impacting sexual function during stress.
- Rectum: It facilitates relaxation of the distal colon and rectal wall, opening the internal anal sphincter and leading to feces retention which can be crucial during emergencies.
- Vas Deferens: Contraction of this structure is stimulated, resulting in ejaculation, an important reproductive function influenced by sympathetic activation.
Sympathetic-Somatic Division
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Skin: The sympathetic nervous system enhances vasoconstriction in cutaneous blood vessels while simultaneously stimulating sweat gland function, aiding in thermoregulation during stress.
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Skeletal Muscle: The sympathetic activation results in a mixed response, causing vasodilation due to cholinergic effects, while adrenergic effects lead to vasoconstriction, which allows for the regulation of blood flow depending on the body's current demands.
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Adrenal Medulla: The adrenal medulla releases epinephrine and a smaller portion of norepinephrine, stimulated by preganglionic sympathetic fibers, facilitating the body’s readiness for stress-induced activities by maintaining heightened alertness and energy availability.
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nic neurons, which acts on nicotinic receptors present on postganglionic neurons, ensuring effective transmission of signals to downstream targets.
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Neurotransmitter released by postganglionic neuron: In the sympathetic system, postganglionic neurons predominantly release norepinephrine and activate adrenergic receptors on target tissues, while parasympathetic postganglionic neurons release acetylcholine to activate muscarinic receptors on their effector cells.
Parasympathetic Nervous System
- Both preganglionic and postganglionic neurons in the parasympathetic system rely on the neurotransmitter acetylcholine for signaling, maintaining clarity and uniformity in the transmission of messages throughout the system.
- Acetylcholine is quickly metabolized by the enzyme acetylcholinesterase (AChE), which ensures that its action is brief and tightly regulated in order to prevent overstimulation of target tissues.
- During activation, preganglionic fibers release acetylcholine (ACh), which subsequently activates nicotinic receptors located on postganglionic neurons to transmit the signal further.
- Once the postganglionic fibers are depolarized, they release acetylcholine to activate muscarinic receptors on effector cells, leading to physiological responses such as decreased heart rate and enhanced digestive activity.
Parasympathetic Nervous System - Cholinergic Stimulation
- The preganglionic cell bodies for the parasympathetic division are situated in cranial nerve nuclei located in the brainstem, particularly within specific nuclei associated with cranial nerves.
- The preganglionic fibers travel through a series of cranial nerves—specifically cranial nerves III (Oculomotor), VII (Facial), IX (Glossopharyngeal), and X (Vagus)—to reach their target organs.
- The oculomotor nerve (CN III) has specific roles in visual function, leading to processes such as miosis (constriction of the pupil), adjustments for near vision, and the control of eye movement.
Parasympathetic - Sacral Outflow
- Within the pelvic region, parasympathetic stimulation of the urinary bladder causes contraction of the bladder wall, while simultaneously relaxing the sphincter, facilitating the process of micturition or urination.
- In males, the parasympathetic system promotes secretion of fluid from seminal vesicles and the prostate gland, aiding in the reproductive process.
- The rectum and descending colon respond to parasympathetic activation by contracting their walls and relaxing the anal sphincter, thereby allowing for the process of defecation.
- In erectile tissue, which is pertinent for sexual function, parasympathetic activation results in vasodilation, ultimately leading to penile or clitoral erection, showcasing the system's role in reproductive health.
Sympathetic Pathway of the Medulla
- The sympathetic pathway illustrated on the right highlights the secretion of norepinephrine from sympathetic neurons as it acts on designated target cells, contributing significantly to local tissue responses.
- In contrast, the pathway illustrated on the left presents the effects of the sympathetic nervous system on the adrenal medulla, which primarily secretes epinephrine (adrenaline) into the bloodstream. This release acts on distant target cells, resulting in systemic effects essential for the body's quick response to stressors.
Adrenal Medullary Hormones
- The adrenal medulla is responsible for secreting catecholamines, which include adrenaline (epinephrine) and noradrenaline (norepinephrine), along with smaller quantities of dopamine. These hormones are integral in facilitating the body's acute stress response.
- Dopamine produced within the adrenal medulla is subsequently converted into norepinephrine through the action of the enzyme dopamine β-hydroxylase (DBH), which enhances the availability of norepinephrine for action.
- In a similar fashion, a substantial proportion of norepinephrine is converted into epinephrine by the enzyme phenylethanolamine-N-methyltransferase (PNMT), which further amplifies the systemic impact of sympathetic activation.
- The secretion of these catecholamines leads to essential physiological changes, including the diversion of blood flow towards critical organs such as the brain, heart, and skeletal muscles, thereby preparing the body for rapid responses in emergency situations.
- Tyrosine, an amino acid, serves as a precursor in the biosynthesis of both epinephrine and norepinephrine, underlining the biochemical foundations that support the ANS’s regulatory capabilities.
Epinephrine and Norepinephrine
- Epinephrine, which makes up approximately 80% of the total catecholamine output, serves as a powerful stimulant, particularly enhancing cardiac and metabolic activities crucial during the fight-or-flight response.
- Norepinephrine, accounting for the remaining 20%, plays significant roles in peripheral vasoconstriction and regulation of blood pressure, demonstrating its importance in maintaining hemodynamic stability during stress.
Mechanism of Norepinephrine Release and Recycling
- The process begins when an action potential arrives at the varicosity of the sympathetic neuron, leading to depolarization of the neuron's membrane.
- This depolarization results in the opening of voltage-gated calcium (Ca2+) channels, allowing Ca2+ ions to enter the neuron, an essential step for neurotransmitter release.
- The influx of Ca2+ triggers exocytosis of synaptic vesicles filled with norepinephrine, causing neurotransmitter release into the synaptic cleft.
- Once released, norepinephrine binds to and activates adrenergic receptors located on target cells, which initiates downstream effects related to the sympathetic response.
- Activity at the synapse ceases once norepinephrine diffuses away from the synaptic cleft or is transported back into the presynaptic neuron through norepinephrine transporters, thus terminating the signal.
- Norepinephrine can be repackaged into synaptic vesicles for recycling or be broken down by the enzyme monoamine oxidase (MAO), a key process for maintaining neurotransmitter balance in the nervous system.
Metabolism of Catecholamines
- Two primary enzymes are involved in the degradation of catecholamines: monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT), which further regulate the levels of these neurotransmitters in the synaptic cleft.
- Measurement of urinary vanillylmandelic acid (VMA) and metanephrine serves as clinical markers to assess catecholamine production in the body, providing insights into the functioning of the adrenal medulla and potential pathologies.
Pheochromocytoma
- Pheochromocytoma is a type of tumor that originates from chromaffin cells within the adrenal medulla and can sometimes be malignant, leading to significant clinical implications.
- Common symptoms of this condition include episodic hypertension (high blood pressure), palpitations (irregular heartbeats), headaches, excessive sweating, and pallor, all indicative of excessive catecholamine release.
- Elevated levels of urinary VMA and plasma catecholamines are often diagnostic indicators of pheochromocytoma, emphasizing the importance of these biomarkers in clinical diagnostics.
Sympathetic Cervical Division
- Eye: Activation of the sympathetic pathways leads to pupil dilation (mydriasis), widening of the palpebral fissure, exophthalmos (protrusion of the eyeball), vasoconstriction of ocular blood vessels, and relaxation of the ciliary muscle, facilitating distance vision.
- Salivary gland: Sympathetic stimulation results in trophic secretion and vasoconstriction, alongside squeezing mechanisms that regulate salivary secretion.
- Lacrimal gland: The sympathetic system influences the lacrimal glands leading to trophic secretion and vasoconstriction, impacting tear production.
- Sweat secretion: The sympathetic nervous system stimulates profuse sweating as a thermoregulatory response during stressful situations.
- Face skin blood vessels: There is vasoconstriction of blood vessels supplying the skin of the face, promoting a pale complexion during sympathetic activation.
Sympathetic Cardiopulmonary Division
- Heart: Sympathetic activation enhances several properties of the cardiac muscle, including increased force of contraction (inotropism), improved rhythmicity, heightened excitability, and enhanced conductivity of electrical impulses.
- Bronchi: Sympathetic stimulation leads to bronchodilation, allowing for increased airflow to the lungs, decreased secretion of mucus within bronchial tubes, and vasoconstriction of pulmonary blood vessels to redirect blood flow.
- Coronary vessels: Initial vasoconstriction of coronary vessels occurs during sympathetic activation; however, a subsequent vasodilation may occur due to the accumulation of metabolites produced during increased metabolic activity.
- Hair: The sympathetic activation induces contraction of erector pili muscles, resulting in hair standing at attention—a response commonly associated with the 'fight or flight' mode.
Sympathetic Splanchnic Division
- Stomach and Intestine: The sympathetic division influences gastrointestinal activity by promoting sphincter contraction, which substantially aids in food retention during stress.
- Liver: In the liver, sympathetic activation induces glycogenolysis, a process that releases glucose into the bloodstream, thus increasing blood glucose levels during energy-demanding situations.
- Spleen: Sympathetic stimulation causes contractions of the splenic capsule, effectively leading to evacuation of blood reserves into circulation.
- Urinary Bladder: In terms of urinary control, the sympathetic response results in contraction of the internal urethral sphincter while relaxing the bladder wall, which prevents urination and retains urine during stress.
- Genital Organs: In male genital organs, sympathetic activation causes vasoconstriction leading to a decrease in size, while in females, similar effects can be observed in the clitoris.
- Rectum: The sympathetic division facilitates relaxation of the distal colon and rectal wall, contributing to fecal retention in stressful conditions.
- Vas Deferens: Contraction of the vas deferens during sympathetic activation leads to ejaculation, showcasing the system's role in reproductive function.
Sympathetic-Somatic Division
- Skin: The sympathetic system leads to vasoconstriction of dermal blood vessels while simultaneously stimulating sweat glands, facilitating thermoregulation.
- Skeletal Muscle: Sympathetic activation causes a dual effect on skeletal muscle blood flow, inducing vasodilation thanks to cholinergic effects, while also prompting vasoconstriction through adrenergic influences, effectively managing blood distribution based on demands.
- Adrenal Medulla: The adrenal medulla is stimulated by preganglionic sympathetic fibers to secrete epinephrine and a small amount of norepinephrine, preparing the body for quick, widespread responses to stressors.
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Test your knowledge about the Autonomic Nervous System (ANS) with this quiz. It covers physiological effects, functions of sympathetic and parasympathetic systems, and roles of neurotransmitters. Perfect for students studying biology or preparing for exams.
