Glossary

15.1 Divisions of the Autonomic Nervous System

3 min read•june 18, 2024

The autonomic nervous system controls our body's unconscious functions. It's split into two parts: the sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) systems. These work together to regulate vital processes like heart rate, digestion, and blood pressure.

Understanding the autonomic nervous system is key to grasping how our body maintains balance. It uses different neurotransmitters and has unique connections that allow for both widespread and localized effects on our organs and tissues.

Divisions of the Autonomic Nervous System

Components of autonomic nervous system

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    • originates in thoracic and lumbar regions of spinal cord
    • have cell bodies in of spinal cord and short axons that synapse in ganglia near spinal cord
    • have cell bodies in ganglia and long axons that extend to target organs
    • () are located on either side of spinal cord and interconnected by nerve fibers to form ()
    • are located anterior to spinal cord and include celiac, superior mesenteric, and
    • is directly innervated by preganglionic fibers and secretes and into bloodstream (endocrine function)
    • originates in brain (cranial nerves III, VII, IX, X) and sacral region of spinal cord
    • Preganglionic neurons have cell bodies in brainstem nuclei and sacral spinal cord with long axons that synapse in ganglia near or within target organs ()
    • Postganglionic neurons have cell bodies in terminal ganglia and short axons that innervate target tissues

Sympathetic vs parasympathetic connections

  • Sympathetic division
    • Preganglionic axons are short, synapsing in ganglia near spinal cord (paravertebral and prevertebral ganglia)
    • Postganglionic axons are long, extending to target organs throughout the body
    • Divergence occurs as one preganglionic neuron synapses with multiple postganglionic neurons, allowing for widespread effects
    • releases and into bloodstream, contributing to widespread effects
  • Parasympathetic division
    • Preganglionic axons are long, synapsing in ganglia near or within target organs (terminal ganglia)
    • Postganglionic axons are short, innervating target tissues directly
    • Less divergence compared to sympathetic division, resulting in more localized and specific effects on target organs
    • No equivalent of adrenal medulla in parasympathetic division

Neurotransmitters in autonomic communication

    • Released by all preganglionic neurons in both sympathetic and parasympathetic divisions
    • Released by postganglionic neurons in parasympathetic division and select sympathetic postganglionic neurons innervating sweat glands, skeletal muscle blood vessels, and some pelvic organs
    • are ligand-gated ion channels found on postganglionic neurons
    • are G protein-coupled receptors found on target tissues innervated by postganglionic parasympathetic neurons (heart, smooth muscle, glands)
  • Norepinephrine (NE)
    • Released by most postganglionic neurons in sympathetic division (except those releasing ACh)
    • α1\alpha_1, α2\alpha_2, β1\beta_1, and are G protein-coupled receptors found on target tissues innervated by postganglionic sympathetic neurons (blood vessels, heart, lungs, liver, adipose tissue)
  • Epinephrine
    • Released along with norepinephrine from adrenal medulla into bloodstream
    • Acts on the same receptors as norepinephrine (α\alpha and β\beta receptors) throughout the body
    • Prolonged and widespread effects due to circulation in the bloodstream (increases heart rate, dilates bronchioles, stimulates )

Autonomic regulation and responses

  • Hypothalamus plays a crucial role in coordinating autonomic functions and integrating them with other physiological processes
  • of many organs by both sympathetic and parasympathetic divisions allows for fine-tuning of organ function
  • refers to the baseline level of activity in both sympathetic and parasympathetic divisions, maintaining homeostasis
  • is activated by the sympathetic nervous system in response to stress or perceived threats
  • is mediated by the parasympathetic nervous system, promoting relaxation and digestive processes

Key Terms to Review (68)

$ ext{alpha}_1$ receptors: $ ext{alpha}_1$ receptors are a type of adrenergic receptor that are primarily found on the smooth muscle cells of blood vessels, the gastrointestinal tract, and the urinary bladder. These receptors play a crucial role in the autonomic nervous system's control of various physiological functions.
$ ext{alpha}_2$ receptors: $ ext{alpha}_2$ receptors are a class of adrenergic receptors that are activated by the neurotransmitter norepinephrine. They are involved in various physiological processes, including the regulation of the autonomic nervous system, which is the focus of the 15.1 Divisions of the Autonomic Nervous System topic.
$\beta_1$ Receptors: $\beta_1$ receptors are a type of adrenergic receptor that are primarily found in the heart and are responsible for mediating the effects of the sympathetic nervous system on cardiac function. They are a key component in the regulation of the autonomic nervous system.
$\beta_2$ receptors: $\beta_2$ receptors are a type of adrenergic receptor that are primarily found in the smooth muscle cells of the lungs, blood vessels, and other tissues. These receptors play a crucial role in the autonomic nervous system's regulation of various physiological processes.
Acetylcholine: Acetylcholine is a neurotransmitter that plays a crucial role in the communication between neurons, the activation of muscle fibers, and the regulation of various physiological processes in the body. It is a key player in the functioning of the nervous system, muscle tissues, and the autonomic nervous system.
Acetylcholine (ACh): Acetylcholine is a neurotransmitter in the nervous system that plays a crucial role in stimulating muscle contractions and is involved in various brain functions including memory and learning. In the context of skeletal muscle, it is essential for transmitting nerve signals to muscle cells, leading to muscle movement.
Adrenal medulla: The adrenal medulla is the inner part of the adrenal glands, which are located above each kidney. It secretes important hormones such as adrenaline (epinephrine) and noradrenaline (norepinephrine) in response to stress.
Adrenal Medulla: The adrenal medulla is the inner portion of the adrenal gland, responsible for the production and secretion of the hormones epinephrine (adrenaline) and norepinephrine (noradrenaline). These hormones play a crucial role in the body's response to stress and in the regulation of various physiological processes.
Adrenergic: Adrenergic relates to the part of the autonomic nervous system that uses adrenaline (epinephrine) or noradrenaline (norepinephrine) as neurotransmitters. It primarily involves the sympathetic nervous system, which is responsible for the body's 'fight or flight' response during stressful situations.
Alpha (α)-adrenergic receptor: Alpha-adrenergic receptors are proteins located on the surface of cells in various tissues that respond to adrenaline and noradrenaline by triggering vasoconstriction, pupil dilation, and other physiological responses. They play a crucial role in the sympathetic division of the autonomic nervous system, helping regulate cardiovascular function, pupil size, and more.
Autonomic Tone: Autonomic tone refers to the resting state or baseline activity of the autonomic nervous system, which controls involuntary bodily functions like heart rate, breathing, and digestion. It represents the balance between the sympathetic and parasympathetic divisions of the autonomic nervous system in the absence of any external stimuli.
Beta (β)-adrenergic receptor: Beta-adrenergic receptors are proteins located on the surface of cells within various tissues, such as the heart, lungs, and muscles, that bind to epinephrine and norepinephrine, triggering physiological responses related to the fight or flight response. These receptors play a crucial role in modulating cardiovascular function, respiratory rates, and metabolic processes under the influence of the sympathetic nervous system.
Bronchodilation: Bronchodilation is the expansion of the bronchial air passages in the lungs. It occurs when the muscles surrounding these passages relax, allowing for increased airflow to and from the lungs.
Bronchodilation: Bronchodilation refers to the dilation or widening of the bronchial tubes, the passageways that carry air in and out of the lungs. This process helps to increase airflow and reduce airway resistance, making it easier to breathe. Bronchodilation is an important mechanism in the context of the autonomic nervous system and drugs that affect the autonomic system.
Celiac Ganglia: The celiac ganglia are a pair of large autonomic ganglia located in the abdomen, near the celiac trunk. They are an important part of the sympathetic division of the autonomic nervous system and play a crucial role in regulating various visceral functions in the body.
Celiac ganglion: The celiac ganglion is a major ganglion of the abdominal cavity that provides sympathetic innervation to the upper abdominal organs including the stomach, liver, spleen, and kidneys. It acts as a relay station for autonomic signals going to and from these organs.
Central neuron: A central neuron is a nerve cell located within the central nervous system (CNS) that integrates and processes information received from the autonomic nervous system's peripheral parts. It plays a key role in transmitting signals to various body parts, coordinating involuntary physiological functions such as heart rate and digestion.
Cholinergic: Cholinergic refers to cells or receptors that use acetylcholine as their neurotransmitter. These are prevalent in the divisions of the autonomic nervous system, influencing various bodily functions like heart rate and digestion.
Chromaffin cells: Chromaffin cells are specialized cells located in the adrenal medulla, part of the adrenal gland, which release adrenaline and noradrenaline into the bloodstream in response to stress signals from the autonomic nervous system. These hormones prepare the body for a 'fight or flight' response by increasing heart rate, blood pressure, and blood glucose levels.
Ciliary ganglion: The ciliary ganglion is a small, parasympathetic ganglion located in the orbit of the eye, which plays a crucial role in controlling the pupil’s constriction and lens accommodation for focusing. It acts as a relay station between the brain and the eye muscles that adjust for close vision.
Collateral Ganglia: Collateral ganglia are small, peripheral nervous system structures that serve as relay stations, allowing communication between the central nervous system and the autonomic nervous system. They play a crucial role in the regulation and coordination of various bodily functions.
Craniosacral system: The craniosacral system is a physiological system that includes the membranes and cerebrospinal fluid that surround and protect the brain and spinal cord, playing a crucial role in the functioning of the autonomic nervous system. It primarily interacts with the parasympathetic division to help regulate the body's rest-and-digest or feed-and-breed activities.
Craniosacral System: The craniosacral system is a physiological system in the human body that influences the functioning of the central nervous system, including the brain and spinal cord. It is closely connected to the autonomic nervous system, which regulates many vital bodily functions.
Dual innervation: Dual innervation refers to the phenomenon where a single organ or tissue receives nerve supply from both divisions of the autonomic nervous system: the sympathetic and parasympathetic systems. This interaction allows for a balanced regulation of various physiological processes, ensuring that the body can maintain homeostasis and respond appropriately to changing conditions.
Edinger–Westphal nucleus: The Edinger–Westphal nucleus is a collection of neurons located in the midbrain, functioning as the primary parasympathetic control center for the eye, responsible for pupil constriction and lens accommodation. It plays a crucial role in the reflexive responses of the eye such as adjusting to different levels of light and focusing on objects at varying distances.
Endogenous: Endogenous refers to processes, substances, or actions that originate from within an organism, cell, or system. In the context of the autonomic nervous system, it emphasizes internal sources of signals or changes rather than those coming from external environmental factors.
Epinephrine: Epinephrine, also known as adrenaline, is a hormone and neurotransmitter produced by the adrenal glands that plays a crucial role in the body's fight-or-flight response by increasing heart rate, muscle strength, blood pressure, and sugar metabolism. It prepares the body for rapid action in situations perceived as stressful or dangerous.
Epinephrine: Epinephrine, also known as adrenaline, is a hormone and neurotransmitter produced by the adrenal glands. It plays a crucial role in the body's stress response, preparing the body to handle emergency situations by increasing heart rate, blood pressure, and blood sugar levels.
Exogenous: In the context of anatomy and physiology, particularly relating to the autonomic nervous system, exogenous refers to substances or influences that originate from outside the body. These can affect the body's functions by interacting with various physiological processes, including those controlled by the autonomic nervous system.
Fight-or-flight response: The fight-or-flight response is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. It triggers a chain of rapid changes within the body, such as increased heart rate and adrenaline release, preparing an individual for either confronting the threat or fleeing from it.
Fight-or-flight response: The fight-or-flight response is a physiological reaction that occurs in response to a perceived threat, preparing the body to either confront or flee from danger. This response is primarily regulated by the sympathetic division of the autonomic nervous system, which triggers a cascade of hormonal and physical changes that enhance the body's ability to respond effectively to stressors.
G protein–coupled receptor: G protein-coupled receptors are a large family of cell surface receptors that play a key role in transmitting signals from outside the cell to the inside, affecting cellular responses. They are involved in various physiological processes, including those in the autonomic nervous system where they mediate responses to neurotransmitters.
Gray rami communicantes: Gray rami communicantes are nerve fibers that connect the sympathetic ganglia to spinal nerves, allowing sympathetic nerve signals to travel to various parts of the body. They contain mostly unmyelinated postganglionic fibers, facilitating the distribution of autonomic signals.
Greater splanchnic nerve: The greater splanchnic nerve is a bundle of nerve fibers that arises from the thoracic segment of the spinal cord and carries sympathetic nervous impulses to the abdominal organs. It plays a critical role in the autonomic regulation of abdominal visceral functions, including influencing the heart rate, digestive processes, and blood flow to various organs.
Inferior Mesenteric Ganglia: The inferior mesenteric ganglia are a collection of nerve cell bodies located in the lower abdomen that are part of the autonomic nervous system. They play a crucial role in the regulation of various functions within the gastrointestinal tract and pelvic organs.
Inferior mesenteric ganglion: The inferior mesenteric ganglion is a cluster of nerve cells (neurons) located in the abdomen that provides sympathetic innervation to the large intestine, rectum, and other pelvic organs. It plays a crucial role in regulating digestive processes such as movement of the bowel and fluid absorption.
Intramural ganglia: Intramural ganglia are collections of nerve cells (neurons) located within the walls of organs, specifically in the gastrointestinal tract, that are part of the autonomic nervous system. They play a crucial role in regulating the physiological processes of these organs, such as digestion and blood flow.
Lateral Horn: The lateral horn is a distinct region within the gray matter of the spinal cord that is part of the autonomic nervous system. It is responsible for the integration and relay of autonomic nervous system signals, playing a crucial role in the regulation of involuntary bodily functions.
Lesser splanchnic nerve: The lesser splanchnic nerve is a bundle of nerve fibers that arises from the thoracic spinal cord, specifically from the ninth and tenth thoracic nerves, and plays a role in innervating the abdominal viscera. It carries sympathetic fibers to parts of the digestive system and adrenal glands, influencing various involuntary bodily functions like gastrointestinal motility.
Ligand-gated cation channel: A ligand-gated cation channel is a type of ion channel in cell membranes that opens or closes in response to the binding of a specific chemical messenger (ligand), allowing positively charged ions (cations) to pass through. These channels are crucial in transmitting signals across nerve cells and muscles within the autonomic nervous system.
Lipolysis: Lipolysis is the metabolic process by which triglycerides in fat cells are broken down into glycerol and free fatty acids, providing energy for the body. It occurs in the adipose tissue and is regulated by hormones such as insulin and adrenaline.
Lipolysis: Lipolysis is the process of breaking down stored fat, or triglycerides, into free fatty acids and glycerol. This process is crucial for providing the body with an alternative energy source when glucose is not readily available, and it occurs primarily in adipose tissue under the regulation of the autonomic nervous system and endocrine hormones.
Mesenteric plexus: The mesenteric plexus is a network of nerves located in the mesentery, which supplies nerve fibers to the intestines, primarily aiding in the control and regulation of gastrointestinal functions. It is part of the autonomic nervous system, specifically involved in managing digestion through modulating smooth muscle contraction, glandular secretion, and blood flow within the gut.
Muscarinic Receptors: Muscarinic receptors are a class of G protein-coupled receptors that are activated by the neurotransmitter acetylcholine. These receptors play a crucial role in the parasympathetic division of the autonomic nervous system, regulating various physiological processes throughout the body.
Nicotinic Receptors: Nicotinic receptors are a type of cholinergic receptor that are activated by the neurotransmitter acetylcholine. These receptors play a crucial role in the functioning of the autonomic nervous system, particularly in the regulation of physiological processes and homeostasis.
Norepinephrine: Norepinephrine is a neurotransmitter and hormone produced by the adrenal glands that plays a key role in the body's fight or flight response, increasing heart rate, blood pressure, and blood sugar levels. It also helps to regulate sleep cycles, attention, and memory functions.
Norepinephrine: Norepinephrine, also known as noradrenaline, is a neurotransmitter and hormone produced by the adrenal glands. It plays a crucial role in the body's sympathetic nervous system, triggering physiological responses to stress and preparing the body for 'fight-or-flight' situations.
Parasympathetic Nervous System: The parasympathetic nervous system is one of the two divisions of the autonomic nervous system, responsible for the body's resting, digesting, and restorative functions. It works in opposition to the sympathetic nervous system, promoting relaxation and conserving energy.
Paravertebral ganglia: Paravertebral ganglia are a series of small, segmental nerve cell clusters located along the spinal column that are part of the autonomic nervous system. They serve as relay stations for the transmission of sympathetic nervous system signals between the central nervous system and the peripheral organs and tissues.
Postganglionic fiber: A postganglionic fiber is a type of nerve fiber that carries signals from a ganglion in the autonomic nervous system to a target organ. These fibers are part of the pathway through which the brain and spinal cord communicate with the body's involuntary systems, such as the heart, lungs, and glands.
Postganglionic neurons: Postganglionic neurons are the second-order neurons in the autonomic nervous system that transmit signals from the autonomic ganglia to the target organs. They serve as the final relay in the autonomic pathways, responsible for the efferent control of various physiological functions.
Preganglionic fiber: A preganglionic fiber is a type of nerve fiber that connects the central nervous system (CNS) to a ganglion in the autonomic nervous system. These fibers are responsible for transmitting motor impulses from the CNS to an autonomic ganglion.
Preganglionic Neurons: Preganglionic neurons are a type of neuron found in the autonomic nervous system that transmit signals from the central nervous system to the autonomic ganglia, where they synapse with postganglionic neurons. These neurons play a crucial role in regulating involuntary bodily functions.
Prevertebral ganglia: Prevertebral ganglia are a group of autonomic nervous system ganglia located in front of the vertebral column. They play a crucial role in the regulation of various bodily functions by serving as relay stations for the sympathetic nervous system.
Rest and digest: Rest and digest is a phrase used to describe the state in which the body's parasympathetic nervous system promotes activities that conserve energy, enhance digestion, and reduce stress. It triggers responses such as decreased heart rate, increased intestinal activity, and relaxed muscles in the gastrointestinal tract.
Rest-and-Digest Response: The rest-and-digest response, also known as the parasympathetic nervous system response, is the physiological state of the body when it is resting, digesting, and recovering. It is the opposite of the fight-or-flight response, which is driven by the sympathetic nervous system.
Superior cervical ganglion: The superior cervical ganglion is the largest and most cranial of the cervical ganglia, part of the sympathetic nervous system, located near the top of the spinal column in the neck. It controls various physiological responses such as pupil dilation, sweating, and blood vessel constriction in the head and neck.
Superior Mesenteric Ganglia: The superior mesenteric ganglia are a collection of nerve cell bodies located in the abdomen that are part of the autonomic nervous system. They play a crucial role in regulating the function of the digestive organs within the abdominal cavity.
Sympathetic Chain: The sympathetic chain, also known as the paravertebral ganglia or sympathetic trunk, is a series of ganglia (clusters of nerve cell bodies) located on either side of the vertebral column. It is a crucial component of the sympathetic division of the autonomic nervous system, responsible for the body's fight-or-flight response.
Sympathetic Nervous System: The sympathetic nervous system is a division of the autonomic nervous system responsible for the body's immediate physiological response to stressful or emergency situations. It prepares the body for the 'fight-or-flight' response by increasing heart rate, blood pressure, and respiration rate, while also diverting blood flow away from the digestive system.
Sympathetic Trunk: The sympathetic trunk is a paired structure of the autonomic nervous system that runs alongside the vertebral column and coordinates the body's sympathetic responses. It serves as the main pathway for transmitting sympathetic nerve impulses from the central nervous system to the peripheral organs and tissues.
Target effector: A target effector in the context of the autonomic nervous system (ANS) is an organ, muscle, or gland that responds to signals from the ANS to perform a specific function. These responses include activities like heart rate modulation, digestion, and perspiration.
Terminal Ganglia: Terminal ganglia are collections of nerve cell bodies located at the end of the autonomic nervous system pathways. They play a crucial role in the regulation and control of various bodily functions by serving as the final relay point between the central nervous system and the target organs or tissues.
Thoracolumbar system: The thoracolumbar system, part of the autonomic nervous system, consists of the sympathetic nerves that emerge from the thoracic and lumbar regions of the spinal cord. It is responsible for the body's fight or flight response, preparing the body to respond to stress or danger.
Thoracolumbar System: The thoracolumbar system is a component of the autonomic nervous system that integrates the thoracic and lumbar regions of the spinal cord. It is responsible for regulating various involuntary physiological processes, including cardiovascular function, respiratory control, and gastrointestinal activity.
Vasoconstriction: Vasoconstriction is the narrowing of the blood vessels due to the contraction of the muscular walls of the vessels, primarily in small arteries and arterioles. This process decreases blood flow and increases blood pressure within the vessel.
Vasoconstriction: Vasoconstriction is the physiological process by which blood vessels, specifically arteries and arterioles, narrow or constrict in response to various stimuli. This reduction in the diameter of the blood vessels results in decreased blood flow to the affected area, which can have significant implications for the body's overall function and homeostasis.
White rami communicantes: White rami communicantes are myelinated nerve fibers that connect the spinal nerves to the sympathetic chain ganglia in the autonomic nervous system. They primarily carry preganglionic sympathetic signals from the spinal cord to the ganglia.
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