Autonomic Nervous System - Lecture Flashcards
General Characteristics Autonomic nervous system
- Self-governed: Functions independently of voluntary control; largely involuntary and unconscious
- Maintains homeostasis: Regulates vital internal functions—cardiovascular, respiratory, digestive, urinary, and reproductive systems
Primary targets of ANS control
1. Glands
2. Cardiac muscle
3. Smooth muscle (e.g., in blood vessels, airways, and digestive tract)
Primary organs regulated by the ANS
1. Viscera of the thoracic and abdominal cavities (e.g., heart, lungs, stomach, intestines)
2. Structures of the body wall, including:
- Cutaneous blood vessels
- Sweat glands
- Piloerector muscles
viscera = The soft internal organs of the body, including the lungs, the heart, and the organs of the digestive, excretory, and reproductive systems.
Subdivisions of NS
-
Central Nervous System (CNS)
Brain and spinal cord
Processes and integrates information -
Peripheral Nervous System (PNS)
Connects CNS to the body
➤ Sensory Division (Afferent)
- Somatic sensory: Signals from skin, joints, muscles
- Visceral sensory: Signals from internal organs
- special sensory receptors: provide sensation of smell, taste, vision, balance and hearing
➤ Motor Division (Efferent)
Carries signals from CNS to effectors
- Somatic Motor Division
Voluntary control of skeletal muscles - Visceral Motor Division / Autonomic Nervous System
Involuntary control of smooth muscle, cardiac muscle, glands
Sympathetic: “Fight or flight”
Parasympathetic: “Rest and digest”
Enteric: Digestive system regulation (sometimes considered part of autonomic)
Describe general visceral reflex
- Automatic, fast, local responses to internal stimuli
- Typically do not require input from the cerebral cortex or hypothalamus
- Help regulate homeostasis via feedback loops
- Fast, automatic responses to internal changes that help maintain homeostasis
Steps of a visceral reflex:
1. Receptor detects internal change (e.g., stretch, pH, temperature)
2. Afferent sensory neuron (primary afferent neuron) carries signal to CNS
3. Integration center processes input (spinal cord, brainstem, or organ wall)
4. Efferent pathway uses a two-neuron chain: preganglionic → postganglionic
5. Effector (smooth muscle, cardiac muscle, or gland) carries out the response
Integration centers
- Usually located in the spinal cord or brainstem
- The sensory neuron may synapse on an interneuron or directly on a preganglionic autonomic neuron (in the lateral horn)
- In some reflexes (e.g., enteric reflexes), integration occurs in local ganglia within the organ wall (e.g., myenteric plexus)
Note: Most sources limit the term “ANS” to the efferent motor pathways, though some include visceral sensory components as well.
Describe Baroreflex
- Baroreceptors in the carotid sinus (CN IX) and aortic arch (CN X) detect stretch from blood pressure changes
- Sensory signals travel via CN IX and X to the nucleus of the solitary tract (NTS) in the medulla
→ These primary sensory neurons synapse on interneurons in the NTS - Interneurons in the NTS relay the signal to:
- Parasympathetic preganglionic neurons nuclei in medulla.
- Sympathetic control centers in the medulla, project to the lateral horn of the thoracic spinal cord (T1–T4) to influence sympathetic preganglionic neurons
- If BP is high:
→ Interneurons excite parasympathetic preganglionic neurons (CN X) → ↓ heart rate
→ Inhibit descending sympathetic output → ↓ sympathetic tone → vasodilation, ↓ contractility - If BP is low:
→ Reduced input to NTS → less activation of vagal motor nuclei → ↓ parasympathetic tone → ↑ heart rate
→ Less inhibition of sympathetic centers → ↑ activation of sympathetic preganglionic neurons in T1–T4 → vasoconstriction, ↑ heart rate & contractility
Key point:
- Only the sympathetic branch controls blood vessel diameter (via vasoconstriction or vasodilation)
- The vagus nerve is parasympathetic only and slows heart rate; it does not affect vessels
Autonomic tone
- The baseline level of activity in the autonomic nervous system (ANS)
- Reflects the balance between sympathetic and parasympathetic output at any given time
- Allows continuous, subconscious regulation of body functions
Sympathetic tone
- Maintains partial vasoconstriction of blood vessels
- Helps sustain resting blood pressure
- Loss of sympathetic tone can cause a drop in vascular resistence -> sudden drop in BP
Parasympathetic tone
- Maintains smooth muscle tone in the intestines
- Holds resting heart rate at ~70–80 bpm
- Without parasympathetic tone, the SA node would fire at ~100 bpm
- The vagus nerve (CN X) mediates this effect by slowing heart rate
- Note: The vagus nerve only lowers heart rate; it does not raise it
Divisions of ANS
Parasympathetic Division
- Also called the craniosacral division
- Preganglionic neurons originate in the:
1. Brainstem nuclei associated with cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus)
2. - Lateral gray horn of sacral spinal cord (S2–S4)
Main effects:
1. Slows heart rate
2. Constricts pupils and narrows airways
3. Stimulates digestion (e.g., salivation, peristalsis, enzyme secretion) and urination
4. Promotes reproductive functions
Sympathetic Division
- Also called the thoracolumbar division
- Preganglionic neurons arise from the thoracic and upper lumbar spinal cord (T1–L2)
- Dominates during fight-or-flight responses (stress, physical activity, emergency)
Main effects:
1. Increases heart rate and respiratory rate -> A
2. Dilates pupils and expands airways
3. Inhibits digestion
4. Stimulates glucose release from liver and adrenaline secretion from adrenal medullaion
HR can increase by relaxing of paraysmpathetic tone (passive) and by increase of sympathetic (active)
Central vs Local control Autonomic Output
Central control
- The hypothalamus is the main integration center for autonomic function
- Receives visceral sensory input and sends descending autonomic fibers (e.g., via the hypothalamospinal tract)
- These fibers synapse on preganglionic neurons located in:
- The brainstem and sacral spinal cord (S2–S4) → parasympathetic output
- The thoracolumbar spinal cord (T1–L2) → sympathetic output
- Coordinates complex responses involving emotion, stress, thermoregulation, hormonal regulation, and more
Local autonomic reflexes
- Involve visceral sensory input entering the CNS and forming a reflex arc
- The input may:
- Synapse directly on a preganglionic autonomic neuron, or
- First activate an interneuron, which then stimulates the preganglionic neuron
Sites of local reflex integration:
- Lateral horn of the spinal cord (T1-L2) → sympathetic reflexes
- Brainstem cranial nerve nuclei / Lateral horn of spinal cord (s2-s4) → parasympathetic reflexes
Functions regulated locally:
- Blood pressure
- Heart rate
- Gut motility
- Bladder control
- These reflexes often function independently of hypothalamic input
Compare Somatic vs Autonomic NS efferent pathways
Varicosities are swellings along the axons of postganglionic autonomic neurons that function as neurotransmitter release sites.
Found in postganglionic sympathetic and parasympathetic fibers. The parasympathetic division can also form more classical synapses (like in the somatic nervous system), especially in certain target tissues.
Instead of forming a single synaptic terminal, these axons have multiple varicosities along their length
Each varicosity contains vesicles with neurotransmitters (like acetylcholine or norepinephrine)
Overview Sympathetic Division of ANS
- Origin: Preganglionic neurons are located in the lateral gray horns of spinal cord segments T1–L2.
These neurons are relatively short. They exit the spinal cord via ventral roots (through spinal nerves) and synapse with postganglionic neurons in the sympathetic chain ganglia, collateral ganglia, or adrenal glands - Preganglionic fibers release acetylcholine (ACh), which binds to nicotinic receptors on postganglionic neurons (always excitatory).
- Ganglia: Sympathetic ganglia are located in one of three locations, sympathetic chain ganglia, collateral ganglia, and adrenal glands
- Postganglionic neurons are relatively long, mostly release norepinephrine (NE).
- NE effects vary by target tissue, depending on adrenergic receptor type (α or β) — can be excitatory or inhibitory.
- Response: Activates the fight-or-flight response.
- Increases heart rate, dilates airways and pupils, slows digestion, and mobilizes energy.
Preganglionic sympathetic neurons are excited by: Descending autonomic fibers from the hypothalamus (especially in response to stress, temperature, or emotion) Visceral sensory input (e.g., baroreceptors, nociceptors) Local spinal cord interneurons in reflex arcs
Define Sympathetic Chain Ganglia
- A chain of ganglia running parallel to the spinal cord on both sides
- Also called paravertebral or lateral ganglia
- Contains postganglionic sympathetic neuron cell bodies
- Controls effectors in the head, neck, thorax (heart / longs), body wall (sweat glands, erector pilae, blood vessels), and limbs
- Postganglionic axons are long and travel to target organs
Preganglionic neurons:
- Originate in the lateral horn of the spinal cord (T1–L2) / aka cell bodies are in lateral horn.
- Their axons exit via ventral roots, enter the sympathetic chain via white rami communicantes
Once inside the sympathetic chain, preganglionic fibers may:
1. Synapse immediately in the ganglion at the same level
2. Ascend via sympathetic trunk to synapse in a ganglion above (e.g., cervical ganglia)
3. Descend via sympathetic trunk to synapse in a ganglion below (e.g., lumbar or sacral levels)
Key point:
- The sympathetic chain extends from the neck to the coccyx, even though preganglionic output only arises from T1–L2
- This allows sympathetic innervation to reach the entire body
*Where are the key spinal cord cell bodies for sensory and motor pathways
- Primary sensory neurons: Dorsal root ganglia (PNS)
- Secondary sensory neurons: Dorsal horn of spinal cord
- Lower motor neurons (somatic): Anterior (ventral) horn
- Preganglionic sympathetic neurons: Lateral horn (T1–L2)
-
Preganglionic parasympathetic neurons:
- Brainstem nuclei (CN III, VII, IX, X)
- Sacral spinal cord (S2–S4)
Spinal Cord Structure
Anterior median fissure: A deep groove on the ventral side of the spinal cord that divides it into right and left halves.
Ventral root: Contains motor neuron axons that transmit motor impulses from the spinal cord to the muscles.
Dorsal root: Contains sensory neuron axons that transmit sensory information from the periphery to the spinal cord
Dorsal root ganglia: Contain cell bodies of sensory neurons that carry signals to the spinal cord.
Lateral horn:
A region of gray matter in the spinal cord found in segments T1–L2 (thoracic and upper lumbar).
It contains preganglionic sympathetic neuron cell bodies, which send their axons out through the ventral root to synapse in sympathetic ganglia.
It is part of the autonomic (motor) pathway and is not present in cervical or most sacral segments.
Spinal nerves: Mixed nerves that carry both sensory and motor information between the spinal cord and the body.
- Sympathetic nerve:
Carries postganglionic sympathetic fibers directly to thoracic organs (e.g., heart, lungs) from the sympathetic chain
-
Ventral ramus:
Carries:- Postganglionic sympathetic fibers to the limbs and anterior/lateral body wall
- Somatic motor fibers to skeletal muscles of the limbs and trunk
- Somatic sensory fibers from the skin of the anterior/lateral trunk and limbs
-
Dorsal ramus:
Carries:- Postganglionic sympathetic fibers to the skin and deep muscles of the back
- Somatic motor fibers to intrinsic back muscles
Spinal Cord Horns
- Posterior Gray Horn: contains somatic and visceral sensory nuclei.
- Lateral Gray Horn: Contains Visceral Motor nuclei
- Anterior Gray Horn: Contains somatic Motor nuclei
Visceral motor path - chain root ganglion
- Visceral motor neurons (preganglionic sympathetic neurons) originate in the lateral gray horn (intermediolateral nucleus) of the spinal cord (T1–L2)
- The axon exits the spinal cord via the ventral root
- It joins with the dorsal root to form the spinal nerve
- The axon travels through the white ramus communicans, entering the sympathetic chain ganglion
- Once inside the chain ganglion, the preganglionic fiber may:
- Synapse immediately at the same level
- Ascend to a higher-level ganglion (e.g., cervical)
- Descend to a lower-level ganglion (e.g., lumbar/sacral)
- Pass through to a prevertebral ganglion (e.g., celiac) without synapsing (not shown here)
- After synapsing, the postganglionic neuron exits the ganglion and reaches the effector organ via one of two routes:
-
Gray ramus communicans → re-enters the spinal nerve and then joins the dorsal or ventral ramus to reach:
- Skin, body wall, or limbs
- Targets include: sweat glands, arrector pili muscles, and blood vessels
-
Sympathetic nerves → bypass spinal nerves and travel directly to:
- thoracic viscera
Rami of Spinal Chord
-
Dorsal Ramus
- Carries : - Somatic motor fibers → to deep back muscles (epaxial muscles)
- Somatic sensory fibers ← from skin of the back
- Postganglionic sympathetic fibers → to sweat glands, blood vessels, arrector pili in the back
- Innervates the back (skin and deep muscles)
-
Ventral Ramus
- Carries: - Somatic motor fibers → to muscles of the limbs and anterolateral body wall
- Somatic sensory fibers ← from skin of the anterior and lateral trunk and limbs
- Postganglionic sympathetic fibers → to sweat glands, blood vessels, and arrector pili in the front and limbs
-Innervates limbs and anterolateral body wall -
Rami Communicantes
- Caries visceral motor afibers
- White ramus communicans: Carries preganglionic sympathetic fibers from the spinal nerve into the sympathetic chain ganglion. Present only at T1–L2.
- Gray ramus communicans: Carries postganglionic fibers out of the sympathetic chain back to the spinal nerve. Present at all levels
Visceral motor path - Collateral ganglion
- visceral motor neurons (preganglionic sympathetic neuron) in lateral gray horn exits the spinal cord via the ventral root.
- It joins with the dorsal root to form the spinal nerve.
- The axon travels through the white ramus communicans, a branch of the spinal nerve.
- It passes through the sympathetic chain ganglion without synapsing, forming a splanchnic nerve that continues toward the abdomen.
- The preganglionic fiber synapses in a collateral ganglion (celiac, superior mesenteric, or inferior mesenteric ganglions).
- The postganglionic neuron’s axon travels directly to abdominal or pelvic organs, such as the stomach, intestines, liver, kidneys, or bladder.
These fibers do not re-enter the spinal nerve, and there is no gray ramus involved in this pathway.
Visceral motor path - Suprarenal (Adrenal) Glands (ganglion)
- visceral motor neurons (preganglionic sympathetic neuron) in lateral gray horn exits the spinal cord via the ventral root.
- It joins with the dorsal root to form the spinal nerve.
- The axon travels through the white ramus communicans, a branch of the spinal nerve.
- It enters the sympathetic chain ganglion but does not synapse, instead forming a splanchnic nerve.
- The splanchnic nerve travels toward the abdomen and passes through collateral (prevertebral) ganglia (e.g., celiac ganglion) without synapsing.
- preganglionic nerve synapses directly on chromaffin cells in the adrenal medulla (part of adrenal gland), which function as modified postganglionic sympathetic neurons.
- These cells release epinephrine and norepinephrine into the bloodstream, producing a widespread sympathetic (fight-or-flight) response.
Name collateral ganglia, state function
-
Celiac ganglion
- Targets: Stomach, liver, pancreas, adrenal medulla, spleen, kidneys
- Functions:
- ↓ Digestive gland activity
- ↑ Glucose release from glycogen reserves in liver
- ↑ Lipid release from adipose tissue
- Vasoconstriction to upper abdominal organs
- Stimulates adrenal medulla to release epinephrine & norepinephrine -
Superior mesenteric ganglion
- Targets: Small intestine, proximal large intestine, kidneys
- Functions:
- ↓ Intestinal activity
- ↓ Urine formation at kidneys
- Vasoconstriction to digestive organs -
Inferior mesenteric ganglion
- Targets: Distal colon, rectum, bladder, reproductive organs
- Functions:
- Relaxation of urinary bladder wall -> inhibits urination
- Vasoconstriction to pelvic organs
- Controls sexual function — Ejaculation is a sympathetic reflex
Function of chain root ganglion
- Mediate sympathetic responses targeting:
- ↓ Blood flow to skin (via vasoconstriction)
- ↑ Heart rate and ↑ blood flow to skeletal muscle
- ↑ Energy production from skeletal muscles and subcutaneous fat stores
- Activate arrector pili muscles and sweat glands
- Dilate pupils (mydriasis)
- Dilate bronchioles in lungs (bronchodilation)
Suprarenal (Adrenal) glands
Adrenal Cortex
- Secretes steroid hormones
Adrenal Medulla
- Functions as a modified sympathetic ganglion
- Made of chromaffin cells — modified postganglionic sympathetic neurons that lack axons and dendrites
- Stimulated by preganglionic sympathetic fibers (T5–T9) that release acetylcholine
- Secretes epinephrine (75–80%) and norepinephrine (20–25%) directly into the bloodstream
Effects of adrenal medulla output:
- Prolongs and amplifies sympathetic (fight-or-flight) response
- ↑ Heart rate, blood pressure, respiratory rate
- ↑ Glucose release from liver
- ↑ Fat breakdown in adipose tissue
- Bronchodilation in lungs
- ↑ Alertness and metabolic activity in most tissues
Sympathetic Activation and NT release
Preganglionic fibers
- Originate in the lateral horn (T1–L2)
- Are short and myelinated
- Release acetylcholine (ACh) nicotinic cholinergic synapses
Postganglionic fibers
- Are long and unmyelinated
- Form varicosities (small swellings along the axon) that store vesicles with neurotransmitters
- Release:
1. Norepinephrine (NE) at most neuroeffector junctions → adrenergic signaling
2. Acetylcholine (ACh) in specific cases: Muscarinic
- Sweat glands (in most of the body)
- Some blood vessels in skeletal muscle
Varicosities and signal transmission
- Varicosities are not aligned with individual target cells
- Instead, they release neurotransmitter diffusely into surrounding tissue
- This allows norepinephrine to activate multiple smooth muscle cells simultaneously
Functional result:
- Creates a slow, broad, and sustained sympathetic response
- Contrasts with the precise, rapid signaling seen at somatic neuromuscular junctions
Divergence
Sympathetic division
- considerable divergence
- One preganglionic neuron synapses with 10–20 postganglionic neurons.
- Allows widespread activation of multiple target organs.
- Results in highly diffuse and coordinated sympathetic responses.
Parasympathetic division
- little divergence
- one preganglionic fiber reaches target organ and stimulates fewer than five postganglionic cells
parasympathetic division general
Activation results in:
- Relaxation, food processing, and energy absorption
Major effects:
- Pupil constriction (miosis)
- Secretion of digestive enzymes from:
- Salivary glands
- Gastric glands
- Duodenal glands
- Pancreas
- Liver
- ↑ Smooth muscle activity in digestive tract
- Secretion of hormones that promote nutrient absorption
- Stimulation and coordination of defecation
- Contraction of urinary bladder
- Constriction of respiratory passages (bronchoconstriction)
- ↓ Heart rate
- Promotes sexual arousal
Neuronal structure:
- Preganglionic neurons originate in the brainstem and sacral spinal cord (S2–S4)
- Have long axons
- Postganglionic neurons are located near or within target organs
- Have short axons- Activation results in: relaxation, food processing, energy absorption
Parasympathetic Cranial Outflow – Nerves and Associated Functions
Parasympathetic preganglionic neurons originate in the brainstem and sacral spinal cord (S2–S4).
- In the brainstem, fibers arise from nuclei associated with cranial nerves III, VII, IX, and X
- In the sacral cord, fibers exit as pelvic splanchnic nerves (S2–S4)
- These preganglionic fibers synapse in parasympathetic ganglia, and postganglionic neurons then innervate the target organs.
CN III – Oculomotor nerve
- Carries fibers that control:
- Pupil constriction
- Lens focusing
CN VII – Facial nerve
- Carries fibers that stimulate:
- Lacrimal glands (tear production)
- Nasal mucosa glands
- Submandibular and sublingual salivary glands
CN IX – Glossopharyngeal nerve
- Carries fibers that stimulate:
- Parotid salivary gland
CN X – Vagus nerve
- Carries fibers to:
- Heart (cardiac plexus)
- Lungs (pulmonary plexus)
- Esophagus (esophageal plexus)
- Abdominal organs as far as the proximal half of the colon
S2–S4 – Pelvic splanchnic nerves
- Innervate:
- Distal colon, rectum, urinary bladder, reproductive organs
Note:
While parasympathetic fibers reach these organs via cranial or sacral routes, sympathetic postganglionic fibers also innervate many of the same targets — traveling from the sympathetic chain or prevertebral ganglia via plexuses or blood vessels.
