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
Steps of a visceral reflex
1. Receptors detect internal changes (e.g., stretch, tissue damage, blood chemistry, temperature)
2. Afferent visceral sensory neurons transmit signals to the CNS
3. Integration center processes input
- May involve interneurons in the dorsal horn, preganglionic neurons, or local circuits in the organ walls (e.g., enteric nervous system)
4. Efferent pathway: Signal is relayed via a two-neuron motor chain (preganglionic and postganglionic neurons)
5. Visceral effector (smooth muscle, cardiac muscle, glands) responds accordingly
Integration centers may be located in:
- The spinal cord
- The brainstem
- The walls of the target organs (e.g., enteric reflexes in the gut)
Note: Most sources limit the term “ANS” to the efferent motor pathways, though some include visceral sensory components as well.
Describe Baroreflex
A fast, autonomic negative feedback loop that maintains stable blood pressure by adjusting heart rate and vessel diameter.
1. Receptor activation
- Baroreceptors in the internal carotid artery and aortic arch detect changes in vessel wall stretch (i.e., blood pressure)
2. Sensory signal transmission
- Signals sent via CN IX (glossopharyngeal) and CN X (vagus) to the medulla oblongata
3. Integration and motor response
- Medulla oblongata integrates signals and adjusts autonomic output
When blood pressure is too high:
- Increased baroreceptor firing
- Parasympathetic output increases via the vagus nerve (CN X) → slows heart rate (SA node inhibition)
- Sympathetic output is suppressed → vasodilation and reduced cardiac contractility
- Result: Heart rate ↓, vessel diameter ↑ → blood pressure decreases
When blood pressure is too low:
- Decreased baroreceptor firing
- Sympathetic output increases → raises heart rate, increases contractility, and causes vasoconstriction
- Parasympathetic tone is reduced -> When vagal tone decreases (i.e., less parasympathetic input), this removes the brake on the SA node → heart rate rises passively
- Result: Heart rate ↑, vessel diameter ↓ → blood pressure increases
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 → sympathetic reflexes
- Brainstem cranial nerve nuclei → 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, body wall, 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 to synapse in a ganglion above (e.g., cervical ganglia)
3. Descend 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
Spinal Cord Structure
Posterior median sulcus: A shallow groove on the dorsal side of the spinal cord that marks the separation between the left and right dorsal columns.
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.
Spinal nerves: Mixed nerves that carry both sensory and motor information between the spinal cord and the body.
Anterior white column: Contains ascending and descending tracts of nerves that transmit motor and sensory information.
Lateral white column: Contains both ascending sensory and descending motor pathways.
Posterior white column: Contains ascending sensory tracts that transmit information about touch, pressure, and proprioception to the brain.
central canal: A fluid-filled channel in the center of the spinal cord that contains cerebrospinal fluid (CSF) and is involved in cushioning and protecting the spinal cord.
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 or abdominal viscera (e.g., heart, lungs, GI tract)
Key point:
- Postganglionic fibers that travel via sympathetic nerves serve visceral organs in the thoracic and abdominal cavities (e.g., heart, lungs, GI tract)
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 and sensory fibers
- 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.
Parasympathetic Neurotransmitter Release
-
Preganglionic axons are long
- Release ACh
- Stimulate nicotinic receptors on ganglionic neurons near the target
- Effect is always excitatory
-
Postganglionic axons are short
- Release ACh at neuroeffector junctions
- Stimulate muscarinic receptors on target
- Effect can be excitatory or inhibitory
- Parasympathetic stimulation is usually brief and localized
-
Parasympathetic nerves innervate:
- Head, thoracic, abdominal, and pelvic organs
Compare anatomical differences of sympathetic and parasympathetic division
Enteric NS
- Considered a third division of the autonomic nervous system
- Does not arise from brainstem or spinal cord
- Innervates smooth muscle and glands in the digestive tract
- Composed of ~100 million neurons
- Found in walls of esophagus, stomach, intestines (especially small intestine)
- Has its own reflex arcs
- Can function independently of CNS, though normally regulated by it
-
Controls:
- Motility (movement of digestive tract)
- Secretion of digestive enzymes and acid
-
Regulated by:
- Sympathetic system → inhibits digestion
-
Parasympathetic system via:
- Vagus nerve (CN X) → foregut and midgut
- Pelvic splanchnic nerves (S2–S4) → hindgut
Autonomic Neurotransmitters and Duration of Effects
- All preganglionic fibers of both sympathetic and parasympathetic divisions release acetylcholine (ACh)
-
Postganglionic fibers of the parasympathetic division also release ACh
- ACh is rapidly broken down by acetylcholinesterase
- → Effects are brief and localized - Most postganglionic fibers of the sympathetic division release norepinephrine (NE)
- NE has three possible fates:
- Reuptake into the neuron → broken down by monoamine oxidase (MAO)
- Diffusion into nearby tissues → degraded by catechol-O-methyltransferase (COMT)
- Entry into bloodstream → eventually broken down by the liver
Functional result:
- Because NE is not rapidly degraded, sympathetic effects last longer and are more widespread than parasympathetic effects
Acetylcholine receptors
-
Nicotinic receptors
- Found on:
- All postganglionic neurons in the ANS.
- Adrenal medulla (sympathetic system)
- Skeletal muscle (somatic motor system)
- Always excitatory
- ACh binding opens Na⁺ channels → depolarization → action potential -
Muscarinic receptors
- Found on:
- Cardiac muscle, smooth muscle, and glands (primarily targets of parasympathetic postganglionic fibers)
- Also found on sweat glands (innervated by sympathetic postganglionic fibers that release ACh)
- Can be excitatory or inhibitory depending on receptor subtype
- May open Na⁺ channels (excitatory)
- Or K⁺ or Cl⁻ channels (inhibitory)
Norepinephrine receptors
Alpha-adrenergic receptors
- Usually excitatory
- Includes α₁ and α₂ subtypes
- Use different second messengers
Beta-adrenergic receptors
- Usually inhibitory
- Includes β₁ and β₂ subtypes
- Both act through cyclic AMP (cAMP) → leads to signal amplification
Dual Innervation
Most viscera receive input from both the sympathetic and parasympathetic divisions.
Antagonistic effects
- Occur when the two divisions produce opposing outcomes
- May act on:
- The same effector cell (e.g., heart rate: sympathetic ↑, parasympathetic ↓)
- Different effectors within the same organ (e.g., pupil: sympathetic dilates, parasympathetic constricts)
Cooperative effects
- Occur when both divisions act on different effectors to produce a coordinated result
- Example: Salivation
- Parasympathetic → stimulates serous cells → watery, enzyme-rich saliva (dominates at rest)
- Sympathetic → stimulates mucous cells → thick, sticky mucus (dominates under stress)
- Both contribute to forming effective saliva for digestion and swallowing
Summary:
- In cooperative effects, both divisions are active but do different things toward the same functional goal
- The balance of their activity shifts depending on rest vs. stress
Control without dual innervation
Some organs receive input from only one autonomic division:
Sympathetic-only innervation only:
- Adrenal medulla
- Arrector pili muscles
- Sweat glands
- Most blood vessels
Vasomotor tone
- Baseline sympathetic tone keeps blood vessels partially constricted
- ↑ Sympathetic activity → vasoconstriction
- ↓ Sympathetic activity → vasodilation
- Allows redistribution of blood flow based on body demands
Examples:
Fight or Flight
- ↑ Sympathetic tone → blood shunted to heart and skeletal muscles
Rest and Digest
- ↓ Sympathetic tone → blood directed to GI tract and skin
Central control of ANS
The ANS is regulated by multiple levels of the CNS, ranging from voluntary influence to automatic control:
Cerebral cortex
- Voluntary modulation possible (indirect)
- Emotions like fear, anger, anxiety influence ANS via the limbic system → hypothalamus
- Allows conscious influence over autonomic functions (e.g., breath control, defecation delay)
Hypothalamus
- Involuntary
- The main integration center for visceral motor output
- Controls homeostatic drives:
- Hunger, thirst, sex drive, temperature regulation, circadian rhythm
Brainstem (midbrain, pons, medulla)
- Involuntary
- Houses nuclei for:
- Cardiac and vasomotor control
- Swallowing, salivation, pupillary responses
- Sweating and urinary reflexes
Spinal cord
- Involuntary, but can be overridden by higher centers (e.g., in toilet training)
- Coordinates autonomic reflexes like:
- Defecation
- Micturition (urination)
- These reflexes operate even if the spinal cord is disconnected from the brain
Summary:
- Voluntary influence = mainly via cortex → limbic system → hypothalamus
- Unconscious autonomic control = primarily via hypothalamus, brainstem, spinal cord
Drugs and the ANS
-
Sympathomimetics
- Enhance sympathetic activity
- Stimulate receptors or increase norepinephrine release
- Example: Cold medicines → dilate bronchioles or constrict nasal blood vessels -
Sympatholytics
- Suppress sympathetic activity
- Block receptors or inhibit norepinephrine release
- Example: Beta blockers → lower BP by blocking effects of epinephrine/norepinephrine on heart and vessels -
Parasympathomimetics
- Enhance parasympathetic activity -
Parasympatholytics
- Suppress parasympathetic activity -
Other CNS-acting drugs
- Prozac: blocks serotonin reuptake → prolongs mood-elevating effects
- Caffeine: blocks adenosine receptors (adenosine promotes sleepiness)
Summary of Autonomic Synapses
