Autonomic and Somatic Nervous System: Key Concepts for ANS Review
Nervous System: Somatic vs Autonomic
Overview: The speaker connects themes from anatomy and physiology to show how nervous system organization supports survival. Emphasis on seeing the nervous system as an integrated system across body parts, not isolated pieces.
Two broad functional divisions to organize the nervous system beyond anatomical location and impulse direction:
Somatic nervous system (SNS): sensory input leads to a voluntary motor response.
Autonomic nervous system (ANS): subconscious, self-regulating control of internal functions.
This session does not cover sense organs yet; those are in Chapter 10. The upcoming webinars will cover sense organs in two sessions.
Core idea: The nervous system has complementary functions that are essential for an animal’s survival, and understanding their organization helps explain how the body responds to stimuli.
The Somatic Nervous System (SNS)
Definition: Sensory input results in a voluntary motor response.
Example: Smell grilled cheese -> want to move toward it -> brain sends a voluntary signal to skeletal muscles to move.
Key components:
Voluntary control of skeletal muscles.
Neuromuscular junction (NMJ): a synapse between a motor neuron and a muscle fiber; there is a small gap, and neurotransmitters cross this gap to trigger muscle contraction.
Neuron-to-muscle signaling is the basis for voluntary movement.
Important terminology introduced previously: neuron as the nervous system unit connecting to muscles via NMJ; neurons release neurotransmitters to muscles.
The Autonomic Nervous System (ANS)
Definition: Subconscious body functions; automatic regulation of organs and glands.
Division into two subsystems:
Sympathetic nervous system (SNS): fight or flight; activates survival responses to external threats.
Parasympathetic nervous system (PNS): rest and digest (also called rest and restore by some sources); maintains baseline functions and digestion when not threatened.
The ANS links sensory input to automatic responses and coordinates with the CNS to support survival.
The two subsystems have distinct neurotransmitters at their effector sites and different receptors, leading to opposing or complementary effects on target organs.
Sympathetic Nervous System (SNS) — Anatomy and Function
Core concept: Fight or flight response; prepares the body to respond to threats.
Origin and organization:
Efferent pathways originate from the thoracic and lumbar spinal segments (thoracolumbar system).
Spinal nerves exit the spinal cord and connect to a sympathetic ganglionic chain that runs lateral to the spinal cord on both sides.
Ganglia can be located in the chain or as separate, non-chain ganglia in other body regions.
Ganglionic chain details:
The sympathetic chain consists of a bilateral series of ganglia (one on each side of the spinal cord).
Postganglionic neurons emerge from these ganglia and innervate multiple target organs; short preganglionic neurons synapse with the chain, and long postganglionic neurons extend to organs.
The chain allows simultaneous widespread activation of multiple organs and muscles, facilitating rapid, coordinated responses.
Lightning up the body for action:
Activation leads to rapid physiological changes to support quick movement and survival.
Adrenal involvement: The adrenal medulla releases catecholamines (epinephrine and norepinephrine) into the bloodstream, potentiating the fight-or-flight response systemically.
When SNS is activated, the brain’s higher cognitive processing may be reduced or downregulated to prioritize automatic, rapid actions (survival mode).
Neurotransmitters and receptors (SNS):
Postganglionic neurotransmitters are primarily norepinephrine (noradrenaline) and epinephrine (adrenaline) in the bloodstream.
Adrenergic receptors (receptors that respond to epi/norepi) include: oldsymbol{ ext{α}1}, oldsymbol{ ext{α}2}, oldsymbol{ ext{β}1}, oldsymbol{ ext{β}2}
Receptors play specific roles in different organs; an example mapping (conceptual):
α1 receptors on vascular smooth muscle (note: standard physiology states α1 causes vasoconstriction; the speaker’s slides describe a dilation pattern in some contexts—see correction note below).
α2 receptors: can modulate sympathetic output; in veterinary pharmacology, α2 agonists produce sedation and analgesia.
β1 receptors: primarily in the heart; increase heart rate and force of contraction.
β2 receptors: in the lungs and some vascular beds; bronchodilation and increased airflow.
Adrenergic receptors are named for the neurotransmitter they respond to and the effect on target organs; the term “adrenergic” refers to receptors sensitive to epinephrine/norepinephrine.
Important caveat (textbook nuance): Alpha-1 receptors are classically associated with vasoconstriction in many vascular beds; the slides mention dilation in some contexts, which is an inconsistency with standard physiology. The correct general statement is that α1 activation typically causes vasoconstriction (with local exceptions depending on tissue and receptor distribution). This distinction is noted for exam accuracy.
Parasympathetic antagonism/opposition is common: sympathetic actions are often opposed by parasympathetic actions on the same organs to regulate homeostasis.
Parasympathetic Nervous System (PNS) — Anatomy and Function
Core concept: Rest and digest; supports maintenance, digestion, and conservation of energy.
Origin and organization:
Craniosacral origin: nerves arise from brainstem (cranial nerve nuclei) and sacral spinal cord (sacral outflow).
Long preganglionic neurons originate in the brainstem or sacral region and synapse on postganglionic neurons that are located close to or on the target organ.
Postganglionic neurons are relatively short because the ganglia are near or on the organ.
Anatomical contrasts with SNS:
SNS: thoracolumbar origin; short preganglionic and long postganglionic neurons; ganglia in a chain near the spinal cord.
PNS: craniosacral origin; long preganglionic and short postganglionic neurons; ganglia near or on the organ.
Functional outcomes:
Slows heart rate; supports digestion and resting metabolic functions.
Keeps body in a relaxed, steady state under non-stress conditions.
Neurotransmitters and receptors (PNS):
The primary postganglionic neurotransmitter is acetylcholine (ACh).
Receptors are cholinergic: two main types
Nicotinic receptors (nAChR): typically at the neuromuscular junction and autonomic ganglia (postganglionic neuron to the next cell).
Muscarinic receptors (mAChR): located on target organs (e.g., heart, GI tract, glands).
The preganglionic neuron also releases acetylcholine to activate nicotinic receptors on the postganglionic neuron.
Neurotransmitters, Receptors, and Pharmacology (Autonomic focus)
General principle: neurotransmitter type and receptor shape govern the action at the target cell; receptor–ligand compatibility is like a plug fitting into a wall outlet—shape-specific binding triggers the response.
Sympathetic transmitters & receptors:
Neurotransmitters: norepinephrine (noradrenaline) and epinephrine (adrenaline).
Receptors: adrenergic receptors with subtypes α and β: oldsymbol{ ext{α}1}, oldsymbol{ ext{α}2}, oldsymbol{ ext{β}1}, oldsymbol{ ext{β}2}.
Pharmacology note: drugs can mimic or block these receptors; e.g., beta blockers block β receptors to slow heart rate and reduce force of contraction.
Adrenal involvement: the adrenal gland releases epinephrine/norepinephrine into the bloodstream, producing widespread effects beyond direct neural release.
Parasympathetic transmitters & receptors:
Neurotransmitter: acetylcholine (ACh).
Receptors: cholinergic receptors, subdivided into nicotinic (nAChR) and muscarinic (mAChR).
Preganglionic vs postganglionic transmitter roles:
In both SNS and PNS, preganglionic neurons primarily release acetylcholine onto nicotinic receptors.
Postganglionic neurotransmitters differ by division: sympathetic postganglionic neurons typically release norepinephrine, while parasympathetic postganglionic neurons release acetylcholine.
Adrenal gland interaction:
The sympathetic system activates the adrenal gland to release adrenaline (epinephrine) and noradrenaline (norepinephrine) into the bloodstream, producing widespread organ effects.
Conceptual to practical pharmacology:
The same receptor types are targets for drugs in veterinary and human medicine.
Toxins and drugs can mimic receptor effects; the saying “the dose makes the poison” applies because the same mechanism can be therapeutic at low doses and harmful at high doses.
A note on receptor architecture in pharmacology:
Beta blockers illustrate receptor-targeted therapy by occupying adrenergic receptors, preventing adrenaline/noradrenaline from binding and thereby reducing heart rate and contractility.
The drug interaction can be visualized as the blocker occupying the receptor site and preventing endogenous neurotransmitters from binding.
Quick cross-reference to a learning tool:
A supplementary video demonstrates receptor interactions and beta-blocker action, reinforcing the conceptual model of receptor–neurotransmitter binding (note that the video is human-focused but the principles carry over to veterinary contexts).
Organ and Systemic Effects: Sympathetic vs Parasympathetic (Compared Points)
Heart rate and force:
Sympathetic: ↑ heart rate and force of contraction.
Parasympathetic: ↓ heart rate; minimal direct effect on contraction force.
Bronchioles (airways):
Sympathetic: ↑ diameter (bronchodilation) to increase air flow for rapid activity.
Parasympathetic: ↓ diameter (bronchoconstriction).
Pupils:
Sympathetic: ↑ pupil diameter (mydriasis) to expand visual field.
Parasympathetic: ↓ pupil diameter (miosis).
GI motility and secretions:
Sympathetic: ↓ motility and secretions; reduces digestive activity during high-threat situations.
Parasympathetic: ↑ motility and secretions; supports digestion and nutrient absorption.
Blood vessel diameter and distribution:
Sympathetic: constriction of many skin and visceral vessels; relative redirection of blood to muscles and vital organs; skin blood flow often reduced.
Parasympathetic: no broad systemic vasodilatory/svasoconstrictive action; effects are more targeted to specific organs.
Kidney blood flow:
Sympathetic: decreased diameter of kidney vessels (reduced renal blood flow) during stress to conserve energy and redirect blood to muscles.
Parasympathetic: generally no prominent direct effect noted in the material.
Summary emphasis:
The two systems act in opposition to regulate body state for either high activity or maintenance/rest, maintaining homeostasis and enabling survival in changing environments.
Reflexes: Quick, Protective Neural Loops
Definition: Reflexes are rapid, automatic responses to stimuli designed to protect the body.
Scope: Can be somatic (skeletal muscle) or autonomic (smooth muscle, cardiac muscle, glands).
All reflexes share a reflex arc: a pathway that enables a fast response, often bypassing the brain when speed is essential.
Reflex arc components:
Sensory (afferent) input enters via a sensory neuron and travels to the spinal cord ( dorsal horn ).
Interneurons in the spinal cord process the signal and coordinate the response.
Motor (efferent) output leaves via a motor neuron from the ventral horn to the target muscle or gland.
The brain may or may not be involved; for hot stimuli, the response can be spinally mediated to maximize speed and avoid delay.
Ipsilateral vs contralateral:
Ipsilateral reflex: the sensory input and the motor output occur on the same side of the body.
Contralateral reflex: the response involves opposite sides (input and output cross to the other side via the spinal cord pathways).
Educational note: The spinal cord houses interneurons that mediate many simple reflexes; more complex reflexes may involve brain input.
Future topics (preview): The next session will cover specific reflexes, trauma-related spinal cord injury, and anesthesia-related reflex considerations.
Cross-Species Considerations and Practical Implications
Species differences:
The core ANS organization is conserved, but anatomical placement and exact functional nuances vary across mammals, birds (avians), and reptiles.
The material emphasizes mammals commonly encountered in veterinary practice (dog, cat, horse, cow, pig) and notes there are differences to be expected in avian/reptile physiology; additional details are planned in a subsequent anatomy course.
Real-world relevance:
Anxiety and the sympathetic response can affect exam performance by increasing sympathetic output and suppressing higher cognitive function; this provides a bridge between physiology and practical human/animal welfare.
Understanding autonomic balance helps explain why animals may urinate when nervous (fight/flight arousal) and the two leading hypotheses (abdominal pressure from muscle contraction and amplification of signals in a heightened state).
Pharmacology and clinical applications (conceptual):
Drugs can target adrenergic or cholinergic receptors to modify heart rate, airway function, digestion, sedation, analgesia, etc.
Understanding receptor subtypes (α1, α2, β1, β2; nicotinic, muscarinic) helps predict organ-specific effects and potential side effects.
The role of the adrenal gland in providing systemic catecholamines under sympathetic activation explains why a neural signal can have whole-body consequences.
The dose–response relationship is central: beneficial therapeutic effects can become harmful if doses are too high or misapplied (the classic pharmacology adage: the dose makes the poison).
Teaching strategy and study tips:
Visualize the sympathetic chain as a bilateral relay system along the spinal column with short preganglionic and long postganglionic neurons.
Remember the craniosacral origin for the parasympathetic system and the organ-proximal nature of its postganglionic connections.
Use the receptor names to anchor pharmacologic expectations: adrenergic (α, β) vs cholinergic (nicotinic, muscarinic).
Link nervous system activity to real-world scenarios (e.g., running from a predator, test anxiety) to internalize how physiology translates to behavior and health.
Quick Reference: Key Terms and Concepts (Recap)
Somatic nervous system (SNS): senses to voluntary movement; skeletal muscles; NMJ with a gap where neurotransmitters act to trigger muscle contraction.
Autonomic nervous system (ANS): subconscious regulation of internal organs; two subsystems:
Sympathetic: fight or flight; thoracolumbar origin; adrenergic receptors; prepares body for rapid action.
Parasympathetic: rest and digest; craniosacral origin; cholinergic receptors; promotes maintenance and digestion.
Neurotransmitters and receptors:
Sympathetic: norepinephrine and epinephrine on adrenergic receptors (α1, α2, β1, β2).
Parasympathetic: acetylcholine on cholinergic receptors (nicotinic, muscarinic).
Pre- vs postganglionic: preganglionic neurons typically release acetylcholine in both systems; postganglionic neurons release norepinephrine (SNS) or acetylcholine (PNS).
Adrenal gland involvement: the adrenal medulla releases adrenaline/nor adrenaline into the bloodstream to amplify the sympathetic response systemically.
Receptor pharmacology: drugs can mimic or block receptor effects; e.g., beta blockers inhibit β receptors to reduce heart rate; alpha-2 agonists can provide sedation/analgesia in veterinary contexts.
Reflex arcs: rapid, automatic responses; involve sensory input, dorsal horn, interneurons in the spinal cord, and ventral motor output; can be ipsilateral or contralateral.
Note on exam readiness: If you have questions on specifics (e.g., exact receptor distributions or organism-specific differences), ask in the questions box or consult the upcoming anatomy modules that cover avian and reptilian autonomic differences. The material here emphasizes foundational concepts and general mammalian patterns, with acknowledgment of species-specific variation.