Autonomic Nervous System Lecture Flashcards
Describe the basic anatomy of the autonomic nervous system (ANS) (Sympathetic and parasympathetic nervous system).
Sympathetic Nervous System
Anatomy:
Origin: The sympathetic fibers originate from the thoracic and lumbar regions of the spinal cord (T1 to L2).
Ganglia: Sympathetic ganglia are primarily located in two chains known as the sympathetic trunk or paravertebral ganglia, running alongside the spinal column. There are also prevertebral ganglia located in front of the vertebral column (e.g., celiac, superior mesenteric, and inferior mesenteric ganglia).
Neurotransmitters: The primary neurotransmitter at the postganglionic synapse is norepinephrine (noradrenaline).
Function:
Prepares the body for “fight or flight” responses, increasing heart rate, dilating pupils, and redirecting blood flow to muscles.
Parasympathetic Nervous System
Anatomy:
Origin: Parasympathetic fibers emerge from the brainstem (cranial nerves III, VII, IX, and X) and the sacral spinal cord (S2 to S4).
Ganglia: Parasympathetic ganglia are located close to or within the target organs, making them often referred to as terminal or intramural ganglia.
Neurotransmitters: The primary neurotransmitter at both preganglionic and postganglionic synapses is acetylcholine.
Function:
Promotes “rest and digest” activities, decreasing heart rate, stimulating digestive processes, and conserving energy.
Understand the main structural and functional difference between the autonomic versus somatic nerve arc (number and location of its peripheral ganglia, length of preganglionic vs postganglionic fibres of autonomic nervous system etc.).
Autonomic Nervous System (ANS)
Structure:
Ganglia: The ANS contains peripheral ganglia. In the sympathetic division, ganglia are primarily located in the sympathetic trunk (paravertebral ganglia) and prevertebral ganglia. In the parasympathetic division, ganglia are located near or within the target organs (terminal or intramural ganglia).
Preganglionic Fibers: These fibers are generally long in the parasympathetic division (due to the location of the ganglia) and short in the sympathetic division.
Postganglionic Fibers: These fibers are typically short in the parasympathetic division and longer in the sympathetic division.
Function:
The ANS regulates involuntary functions such as heart rate, digestion, and respiratory rate, often working unconsciously to maintain homeostasis.
Somatic Nervous System
Structure:
Ganglia: The somatic nervous system does not have peripheral ganglia associated with its motor pathways. Instead, motor neurons project directly from the spinal cord to skeletal muscles without synapsing in ganglia.
Preganglionic Fibers: There are no preganglionic fibers in the somatic nervous system, as it operates through single motor neurons.
Postganglionic Fibers: Not applicable, as the somatic system uses a direct pathway to skeletal muscle.
Function:
The somatic nervous system controls voluntary movements, enabling conscious control over skeletal muscles, such as walking or picking up objects.
Describe outflow routes of ANS from the brain and spinal cord (thoracolumbar - sympathetic and craniosacral - parasympathetic).
Sympathetic Nervous System (Thoracolumbar Outflow)
Origin:
The sympathetic outflow originates from the thoracic and lumbar regions of the spinal cord, specifically from the spinal segments T1 to L2 (or L3).
Pathway:
Preganglionic Neurons: The cell bodies of preganglionic sympathetic neurons are located in the lateral horn of the spinal cord. These neurons exit the spinal cord through the ventral roots and enter the spinal nerves.
Sympathetic Chain: After exiting, the preganglionic fibers typically travel a short distance to the sympathetic trunk (paravertebral ganglia) located alongside the vertebral column. Here, they can synapse with postganglionic neurons.
Prevertebral Ganglia: Some fibers pass through the sympathetic trunk without synapsing and continue to prevertebral ganglia (such as the celiac and mesenteric ganglia), where they synapse with postganglionic neurons that innervate abdominal and pelvic organs.
Postganglionic Neurons: The postganglionic fibers then extend to their target organs throughout the body.
Parasympathetic Nervous System (Craniosacral Outflow)
Origin:
The parasympathetic outflow originates from the brainstem and the sacral region of the spinal cord, specifically through cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus), as well as from sacral spinal segments S2 to S4.
Pathway:
Cranial Outflow:
Cranial Nerves: Preganglionic neurons in the brainstem travel via the mentioned cranial nerves to their respective ganglia. For example:
Oculomotor Nerve (III): Controls pupil constriction and lens shape.
Facial Nerve (VII): Stimulates salivary and lacrimal glands.
Glossopharyngeal Nerve (IX): Also stimulates salivation.
Vagus Nerve (X): Provides extensive innervation to thoracic and abdominal organs.
Sacral Outflow:
Preganglionic Neurons: Originate from the lateral horn of spinal segments S2 to S4 and exit via the sacral spinal nerves.
Ganglia: Preganglionic fibers travel to terminal ganglia located near or within the walls of target organs, where they synapse with postganglionic neurons.
Postganglionic Neurons: The postganglionic fibers then innervate their target organs, promoting “rest and digest” functions.
Describe the connections of the sympathetic nervous system to the sympathetic chain (via the white and grey rami communicantes).
The sympathetic nervous system has a specific organizational structure that facilitates its communication with the sympathetic chain, also known as the sympathetic trunk. This structure is crucial for the distribution of sympathetic signals throughout the body. Here’s a detailed overview of the connections involving the white and gray rami communicantes:
Sympathetic Chain Overview
The sympathetic trunk runs alongside the vertebral column and consists of a series of interconnected ganglia (the sympathetic ganglia). It extends from the base of the skull to the coccyx, allowing sympathetic fibers to be distributed to various organs and tissues.
Connections via Rami Communicantes
White Rami Communicantes:
Structure: These are myelinated fibers that connect the spinal nerves to the sympathetic ganglia.
Location: White rami are present only in the thoracic and upper lumbar regions (T1 to L2) where the sympathetic outflow originates.
Function: Preganglionic sympathetic fibers (which are myelinated) exit the spinal cord via the ventral roots and enter the corresponding spinal nerve. They then travel a short distance to the white rami communicantes, where they synapse in the sympathetic chain ganglia. This pathway allows the preganglionic fibers to reach the sympathetic trunk.
Gray Rami Communicantes:
Structure: These are unmyelinated fibers that connect the sympathetic ganglia back to the spinal nerves.
Location: Gray rami are present at every spinal level (from cervical to sacral) and are found at the exit points of the sympathetic ganglia.
Function: After synapsing in the sympathetic ganglia, the postganglionic sympathetic fibers travel back to the spinal nerve through the gray rami communicantes. From there, they can enter the spinal nerves and be distributed to target organs via the peripheral nerves.
Pathway Summary
Preganglionic Fiber Pathway:
Originates in the lateral horn of the spinal cord (T1-L2).
Exits through the ventral root of the spinal nerve.
Enters the spinal nerve and then travels to the sympathetic trunk via the white ramus communicans.
Synapses in the sympathetic ganglion.
Postganglionic Fiber Pathway:
After synapsing in the sympathetic ganglion, the postganglionic fiber exits through the gray ramus communicans.
Re-enters the spinal nerve.
Travels to the target organ, supplying sympathetic innervation.
Functional Importance
This system allows the sympathetic nervous system to efficiently transmit signals throughout the body. The use of white and gray rami communicantes enables the integration of sympathetic responses across different regions, facilitating the “fight or flight” response when needed.
Describe the general distribution of each division (sympathetic and parasympathetic) to
the target organs.
Sympathetic Nervous System
General Distribution:
Widespread Effects: The sympathetic division has a broad distribution, affecting many organs simultaneously, which is essential for the “fight or flight” response.
Target Organs:
Heart: Increases heart rate and force of contraction.
Blood Vessels: Constricts blood vessels in non-essential areas (like the skin and digestive organs) while dilating those supplying skeletal muscles.
Lungs: Dilates bronchioles to increase airflow.
Digestive System: Decreases digestive activity (e.g., reduced salivation, decreased peristalsis).
Eyes: Dilates pupils to enhance vision.
Adrenal Medulla: Stimulates the release of adrenaline (epinephrine) into the bloodstream, amplifying the overall sympathetic response.
Sweat Glands: Activates sweat production.
Kidneys: Reduces urine production and promotes the release of renin.
Pathway:
Preganglionic fibers originate in the thoracolumbar region (T1-L2) and synapse in the sympathetic ganglia. Postganglionic fibers then travel to target organs.
Parasympathetic Nervous System
General Distribution:
Localized Effects: The parasympathetic division generally targets specific organs and promotes “rest and digest” functions, supporting conservation of energy and bodily maintenance.
Target Organs:
Heart: Decreases heart rate.
Lungs: Constricts bronchioles to reduce airflow.
Digestive System: Increases digestive activity (e.g., stimulates salivation, promotes peristalsis, and increases enzyme secretion).
Eyes: Constricts pupils for near vision (accommodation).
Urinary Bladder: Promotes bladder contraction to facilitate urination.
Sexual Organs: Enhances sexual arousal by increasing blood flow.
Pathway:
Preganglionic fibers arise from the craniosacral regions (brainstem and sacral spinal cord, S2-S4). They synapse near or within the target organs in terminal ganglia, leading to postganglionic fibers that have short lengths due to their proximity to the target organs.
