The autonomic nervous system controls our body's unconscious functions. It's split into two parts: the sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) systems. These work together to regulate vital processes like heart rate, digestion, and blood pressure.

Understanding the autonomic nervous system is key to grasping how our body maintains balance. It uses different neurotransmitters and has unique connections that allow for both widespread and localized effects on our organs and tissues.

Divisions of the Autonomic Nervous System

Components of autonomic nervous system

• Sympathetic nervous system
  • Thoracolumbar system originates in thoracic and lumbar regions of spinal cord
  • Preganglionic neurons have cell bodies in lateral horn of spinal cord and short axons that synapse in ganglia near spinal cord
  • Postganglionic neurons have cell bodies in ganglia and long axons that extend to target organs
  • Collateral ganglia (paravertebral ganglia) are located on either side of spinal cord and interconnected by nerve fibers to form sympathetic chain (sympathetic trunk)
  • Prevertebral ganglia are located anterior to spinal cord and include celiac, superior mesenteric, and inferior mesenteric ganglia
  • Adrenal medulla is directly innervated by preganglionic fibers and secretes epinephrine and norepinephrine into bloodstream (endocrine function)
• Parasympathetic nervous system
  • Craniosacral system originates in brain (cranial nerves III, VII, IX, X) and sacral region of spinal cord
  • Preganglionic neurons have cell bodies in brainstem nuclei and sacral spinal cord with long axons that synapse in ganglia near or within target organs (terminal ganglia)
  • Postganglionic neurons have cell bodies in terminal ganglia and short axons that innervate target tissues

Sympathetic vs parasympathetic connections

• Sympathetic division
  • Preganglionic axons are short, synapsing in ganglia near spinal cord (paravertebral and prevertebral ganglia)
  • Postganglionic axons are long, extending to target organs throughout the body
  • Divergence occurs as one preganglionic neuron synapses with multiple postganglionic neurons, allowing for widespread effects
  • Adrenal medulla releases epinephrine and norepinephrine into bloodstream, contributing to widespread effects
• Parasympathetic division
  • Preganglionic axons are long, synapsing in ganglia near or within target organs (terminal ganglia)
  • Postganglionic axons are short, innervating target tissues directly
  • Less divergence compared to sympathetic division, resulting in more localized and specific effects on target organs
  • No equivalent of adrenal medulla in parasympathetic division

Neurotransmitters in autonomic communication

• Acetylcholine (ACh)
  • Released by all preganglionic neurons in both sympathetic and parasympathetic divisions
  • Released by postganglionic neurons in parasympathetic division and select sympathetic postganglionic neurons innervating sweat glands, skeletal muscle blood vessels, and some pelvic organs
  • Nicotinic receptors are ligand-gated ion channels found on postganglionic neurons
  • Muscarinic receptors are G protein-coupled receptors found on target tissues innervated by postganglionic parasympathetic neurons (heart, smooth muscle, glands)
• Norepinephrine (NE)
  • Released by most postganglionic neurons in sympathetic division (except those releasing ACh)
  • $\alpha_1$, $\alpha_2$, $\beta_1$, and $\beta_2$ receptors are G protein-coupled receptors found on target tissues innervated by postganglionic sympathetic neurons (blood vessels, heart, lungs, liver, adipose tissue)
• Epinephrine
  • Released along with norepinephrine from adrenal medulla into bloodstream
  • Acts on the same receptors as norepinephrine ($\alpha$ and $\beta$ receptors) throughout the body
  • Prolonged and widespread effects due to circulation in the bloodstream (increases heart rate, dilates bronchioles, stimulates lipolysis)

Autonomic regulation and responses

• Hypothalamus plays a crucial role in coordinating autonomic functions and integrating them with other physiological processes
• Dual innervation of many organs by both sympathetic and parasympathetic divisions allows for fine-tuning of organ function
• Autonomic tone refers to the baseline level of activity in both sympathetic and parasympathetic divisions, maintaining homeostasis
• Fight-or-flight response is activated by the sympathetic nervous system in response to stress or perceived threats
• Rest-and-digest response is mediated by the parasympathetic nervous system, promoting relaxation and digestive processes