Organization of the nervous system (CNS vs PNS, and subdivisions)
CNS: brain + spinal cord
PNS splits into:
Somatic NS (SNS) → voluntary control of skeletal muscle
Autonomic NS (ANS) → involuntary control of:
smooth muscle
cardiac muscle
glands
ANS further divides into:
Sympathetic (fight/flight)
Parasympathetic (rest/digest)
Somatic vs Autonomic nervous system (structure + function, in terms of ganglions present too)
Somatic NS:
Voluntary
Controls skeletal muscle
1 neuron pathway (CNS → muscle)
No ganglia
Autonomic NS:
Involuntary
Controls visceral effectors
2 neuron chain:
Preganglionic neuron
Postganglionic neuron
Synapse occurs in autonomic ganglion
What are autonomic/visceral effectors?
Targets of the autonomic nervous system:
Smooth muscle
Cardiac muscle
Glandular epithelial tissue
👉 These receive impulses from autonomic neurons
Structure of autonomic pathways (preganglionic vs postganglionic)
Preganglionic neuron:
Cell body in CNS
Axon goes to ganglion
Postganglionic neuron:
Cell body in ganglion
Axon goes to target organ
👉 Always a 2-neuron chain (unlike somatic)
Sympathetic nervous system (origin + structure)
Origin: T1–L2 (thoracic + lumbar)
Preganglionic fibers: short
Ganglia:
Sympathetic chain ganglia (near spinal cord)
Collateral ganglia
Can stimulate adrenal medulla → releases epinephrine + norepinephrine into blood
Postganglionic fibers: long
:
👉 Preganglionic fibers are short → go to the sympathetic chain ganglia → then postganglionic fibers go to the organs
Sympathetic nervous system effects (“fight or flight”)
↑ mental alertness
↑ metabolic rate
↑ heart rate + blood pressure
Bronchodilation (increase airflow)
↑ sweating
↓ digestion + urinary activity
Mobilizes energy (glucose + fat)
👉 Prepares body for stress/emergency
Sympathetic neurotransmitters
Preganglionic: ACh (acetylcholine)
Postganglionic: usually NE (norepinephrine)
Adrenal medulla: releases epinephrine + norepinephrine into bloodstream
Parasympathetic nervous system (origin + structure)
Origin: Brainstem + S2–S4 (sacral)
Preganglionic fibers: long
Ganglia:
Near or inside target (intramural)
Postganglionic fibers: short
Parasympathetic nervous system effects (“rest and digest”)
↓ heart rate + blood pressure
↓ metabolic rate
↑ salivation + digestion
↑ GI motility + blood flow
↑ urination + defecation
👉 Promotes energy conservation + maintenance
Parasympathetic neurotransmitters
Preganglionic: ACh
Postganglionic: ACh
👉 Entire system uses acetylcholine
Sympathetic vs Parasympathetic (key structural differences, origin, ganglia, preganglionic, postganglionic, nuerotransmitters)
SYMPATHETIC
origin –> T1-L2
ganglia –> near spinal cord
preganglionic –> short
postganglionic –> long
neurotransmitters –> NE
PARASYMPATHETIC
origin –> brainstem + S2-S4
ganglia –> near/in organis
preganglionic –> long
postganglionic –> short
NT –> ACh
Sympathetic vs Parasympathetic effects on major organs (eyes, heart, lungs, digestion)
Eye:
Symp → dilates
Para → constricts
Heart:
Symp → ↑ HR + force
Para → ↓ HR
Lungs:
Symp → bronchodilation
Para → bronchoconstriction
Digestive system:
Symp → ↓ digestion
Para → ↑ digestion
Sympathetic vs Parasympathetic in metabolism + excretion
Liver/Pancreas:
Symp → ↑ glucose, ↓ insulin
Para → ↑ insulin, glycogen storage
Urinary system:
Symp → inhibits urination
Para → stimulates urination
Reproductive:
Symp → ejaculation
Para → erection
What is autonomic tone?
Continuous baseline activity of ANS
Both divisions always active at low levels
👉 Example:
At rest → parasympathetic dominates
Heart rate controlled by balance between both systems
👉 It means your organs are ALWAYS being slightly controlled — not just turned on/off
ANS role in homeostasis
Maintains internal stability
Pathway:
Stimulus
Sensory receptors
CNS processing
Motor output:
Somatic → skeletal muscle
Autonomic → visceral organs
Visceral reflexes (definition + types)
Automatic responses controlling internal organs
Types:
Short reflex
No CNS involvement
occurs in ganglion
Long reflex
Involves CNS
more complex control
Front:
Examples of visceral reflexes
Cardioacceleratory → ↑ heart rate
Vasomotor → blood vessel diameter
Pupillary reflex → light adjustment
Gastric reflex → digestion
Defecation reflex
Urination reflex
Swallowing reflex
Coughing reflex
Baroreceptors (function + locations)
Detect pressure/stretch
Locations:
Carotid sinus
Aortic sinus
Digestive organs
Bladder
Colon
👉 Example:
↓ BP → reflex ↑ heart rate
Chemoreceptors (function + what they detect)
Detect chemical changes in blood:
pH
CO₂
O₂
Locations:
Medulla
Carotid bodies
Aortic bodies
👉 Adjust respiration + cardiovascular activity
Big picture of autonomic nervous system
ANS = involuntary control of internal organs
Two divisions:
Sympathetic → stress, energy use
Parasympathetic → recovery, energy storage
Work in opposition + balance (autonomic tone)
Maintain homeostasis through reflexes and feedback systems
