Autonomic Nervous System Divisions Compared

Posted on May 2, 2025 in Biology

Somatic vs. Autonomic Nervous Systems

Somatic Nervous System (SNS)

  • Single axonal pathway from CNS.
  • Thick myelin fibers.
  • Target organs: skeletal muscles.
  • Effect of neurotransmitter: excitatory.
  • Neurotransmitter at target site: always acetylcholine.
  • No ganglia present.

Autonomic Nervous System (ANS)

  • Preganglionic and ganglionic neurons.
  • Effectors are glands and involuntary muscles.
  • At least one ganglion located outside the CNS.
  • Thin or unmyelinated fibers.
  • Transmitted effect may be excitatory or inhibitory.

Shared Characteristics (Somatic & Autonomic)

  • Efferent motor pathways.
  • Motor neuron cell bodies located in the CNS.
  • Neural pathways are in cranial and spinal nerves.
  • Under control of higher brain centers (e.g., hypothalamus).

Autonomic Nervous System Divisions

Sympathetic Division (SNS)

  • Short preganglionic and long postganglionic fibers.
  • Called the thoracolumbar division.
  • Ganglia close to the CNS (sympathetic chain ganglia).
  • Gray and white rami communicantes.
  • Associated with ‘fight or flight’ response.
  • Effects include dry mouth, dilated bronchi.
  • Increases blood glucose level.
  • Arrector pili muscles contract.

Parasympathetic Division (PNS)

  • Slower conduction.
  • Travel within cranial and sacral nerves.
  • Long preganglionic and short postganglionic fibers.
  • No fibers in rami communicantes.
  • Called the craniosacral division.
  • Minimal branching of preganglionic fibers.
  • Terminal ganglia close to visceral organs.
  • Associated with ‘rest and digest’ or normal control.
  • Specific, localized control.
  • Effects include constricted eye pupils, decreased heart rate.
  • Conserves body energy.
  • Increases digestive tract motility.

Organ Effects and Innervation

  • Adipose Tissue: Sympathetic innervation via spinal sympathetic nerves; increases lipolysis.
  • Adrenal Gland: Sympathetic innervation via splanchnic nerve; increases Norepinephrine (NE) / Epinephrine (Epi) release.
  • Arterioles: Both Sympathetic (spinal sympathetic nerves) and Parasympathetic (Cranial Nerve X) innervation; effects include constriction and dilation.
  • Bladder: Both Sympathetic (spinal nerves – relaxes detrusor muscle, contracts internal sphincter muscle) and Parasympathetic (pelvic nerves – contracts detrusor muscle, relaxes internal sphincter muscle) innervation.
  • Blood: Sympathetic innervation; increases platelet coagulation.
  • Esophagus: Both Sympathetic (celiac plexus – decreases smooth muscle contraction) and Parasympathetic (Cranial Nerve X – increases smooth muscle contraction) innervation.
  • Gallbladder: Both Sympathetic (celiac plexus – relaxation) and Parasympathetic (Cranial Nerve X – contraction) innervation.
  • Gastric Glands: Both Sympathetic (splanchnic nerves – decreases secretion) and Parasympathetic (Cranial Nerve X – increases secretion) innervation.
  • Gastrointestinal Tract (Motility): Both Sympathetic (decreases smooth muscle contractions) and Parasympathetic (increases smooth muscle contractions) innervation.

Questions and Concepts

  • Q1: The parasympathetic branch of the ANS arises from the brainstem and lower regions of the spinal cord, with long preganglionic fibers.
  • Q2: Tachycardia in a woman may be associated with Guillain-Barré Syndrome.
  • Q3: Sympathetic chain ganglia are found along the entire spinal cord and carry sympathetic postganglionic fibers to target tissues.
  • Q4: Sympathetic control of arrector pili smooth muscle is via spinal nerves.
  • Q5: Tissues innervated by the sympathetic nervous system include the kidney and liver.
  • Q6: Mushroom poisoning can stimulate target cells innervated by postganglionic parasympathetic fibers.
  • Q7: Chest symptoms in a 54-year-old man may be related to overproduction of epinephrine.
  • Q8: Adrenergic Receptor Effects: α1 = smooth muscle contraction; α2 = inhibit insulin release; β1 = amylase release; β2 = increase insulin release; β3 = increase lipolysis.
  • Q9: How SNS reflexes differ from ANS reflexes: Hypothalamus involvement in ANS reflexes.
  • Q10: ANS Reflexes: Tissues innervated by both branches often have opposite effects.
  • Q11: What does NOT describe the ANS: A system that innervates all muscle cells.
  • Q12: Effect NOT directly controlled by the ANS: Skeletal muscles.
  • Q13: Cranial nerve containing parasympathetic preganglionic fibers: Cranial Nerve X (Vagus).
  • Q14: Where you would NOT find an autonomic ganglion: Lymph nodes.
  • Q15: Plexus NOT an origin for preganglionic fibers of the Vagus nerve: Cervical plexus.
  • Q16: Approximately 90% of parasympathetic preganglionic fibers are derived from Cranial Nerve X.
  • Q17: NOT a plexus of the Vagus nerve: Celiac plexus.
  • Q18: Sympathetic preganglionic fibers may leave the spinal cord at which vertebra: 1st thoracic (T1).
  • Q19: Fibers that enter and leave the sympathetic chain without synapsing: Splanchnic nerves.
  • Q20: Where you would NOT find cholinergic nicotinic receptors: All parasympathetic target organs.
  • QQ: Incorrect statement comparing ANS and SNS: The neurotransmitter utilized at the effector in both systems is Acetylcholine (ACH).
  • QQ: Incorrect statement about Norepinephrine: Sympathetic preganglionic neurons secrete NE.
  • QQ: Correct statement about ANS: Sympathetic chain ganglia are also called paravertebral ganglia.
  • QQ: SNS and ANS differ in all the following ways EXCEPT: Origin of nerve cell bodies.
  • QQ: Incorrect pairing: Acetylcholine (ACH) dilates the pupil.
  • QQ: NOT a result of parasympathetic stimulation: Dilatation of pupils.
  • QQ: Smooth muscle of the digestive viscera is served by: Cranial Nerve X (Vagus).
  • QQ: Parasympathetic ganglion serving the eye: Ciliary ganglion.
  • QQ: Secretions of the adrenal medulla act to supplement the effect of: Sympathetic stimulation.
  • QQ: Incorrect statement relative to the parasympathetic division of the ANS: Crying and salivation is not parasympathetic innervation but a sympathetic response.
  • QQ: Incorrect statement relative to sympathetic activity: Inhibits orgasm.
  • QQ: Results in acceleration of heart rate: Epinephrine.
  • QQ: Parasympathetic functions include: Accommodation for close vision.
  • QQ: Beta blocker effects: Decrease heart rate and blood pressure.
  • QQ: Which adrenergic receptor plays a major role in heart activity: β1.