Sympathetic and Parasympathetic Action

This answer is oriented to dentistry students.

Autonomic Centers in the CNS

 * Mechanisms related to maintaining homeostasis acts on three major systems
 * Endocrine System
 * Autonomic Nervous System
 * Behavioral homeostatic mechanisms

Spinal Cord, Brain Stem
Autonomic centers integrate signals from autonomic and somatic receptors, and from the higher brain centers (hypothalamus, limbic system)
 * Spinal cord → Intermediolateral cell column (thoracolumbar & sacral division)
 * Brain stem → Cranial nuclei and autonomic centers
 * Respiratory centers
 * Vasomotor & cardioregulatory centers
 * Control of pupil diameter center
 * Micturition center
 * Sexual reflexes center
 * GIT motility & secretion center

Hypothalamus

 * Control functions
 * Energy balance, food intake
 * Body fluid homeostasis, water balance, water intake
 * Thermoregulation
 * Sexual function
 * Autonomic control (respiration, vasomotor reactions, activity of heart)
 * Body rhythms
 * Mechanisms of immunity
 * Emotional behavior


 * Receptor functions
 * Glucoreceptors
 * Osmoreceptors
 * Thermoreceptors
 * Receptors for hormones


 * Effector functions
 * Hormone production
 * Neural control of the ANS
 * Neural control of the brain activity (modulatory neural pathways)
 * Behavior (sexual, feeding, thermoregulatory behavior)

Epithalamus, Pineal Gland

 * Body rhythms → Circadian, annual cycles

Basic characteristics of Sympathetic & Parasympathetic functions

 * Synaptic transmitters
 * All preganglionic neurons are cholinergic (acetylcholine, nicotinic) in both SYM & PAR fibers
 * The postganglionic neurons of PAR are cholinergic (acetylcholinergic, muscarinic)
 * The postganglionic neurons of SYM are adrenergic (Norepinephrine)
 * Post sympathetic neurons to sweat glands, piloerector muscle and some few blood vessels are cholinergic
 * Post sympathetic neurons to sweat glands, piloerector muscle and some few blood vessels are cholinergic
 * Acetylcholine
 * Is synthesized and stored in the terminal endings of cholinergic nerve fibers
 * Secreted acetylcholine is split to acetate & choline by enzyme (acetylcholinesterase) in the local connective tissue
 * Choline is then transported back into the terminals and used for the synthesis of new Acetylcholine
 * Norepinenphrine
 * Synthesis of Norepinephrine begins in the axoplasm of the terminal nerve endings of adrenergic fibers and is completed inside the vesicles
 * Tyrosine → DOPA → Dopamine → Transport into vesicles → Norepnienphrine
 * After secretion Norepinephrine is removed
 * Reuptake into the adrenergic nerve terminal (70%)
 * Diffusion and removal by blood
 * Destruction by enzymes (MAO, COMT)
 * Norepinephrine secreted into a tissue remains active for few seconds
 * Norepinephrine secreted into the blood remain active for 10 to 30 seconds (up to several minutes) and then it is destroyed (mainly in liver)

Receptors of acetylcholine & catecholamines

 * The receptor is usually on the outside of the cell membrane. When the transmitter binds, it causes conformational change in the structure of protein molecule, causing:
 * A change in the cell membrane permeability to one or more ions (Ca2+, Na2+ - depolarization, K+ - hyperpolarization)
 * Activation of an enzyme attached to the other and of the receptor protein protruding in the interior of the cell (Adenylcyclase – cAMP)
 * Acetylcholine activates two different types of receptors
 * Muscarinic receptors
 * In all effector cells of the postganglionic PAR neurons and postganglionic cholinergic neurons of the SYM system
 * Nicotinic receptors
 * In synapses between the pre- & post ganglionic neurons of both the SYM & PAR system
 * Neuromuscular junctions of the skeletal muscle
 * The are two major types of adrenergic receptors
 * Alpha receptors
 * Excited by epinephrine, and strongly sensitive to norepinephrine
 * Certain alpha functions are excitatory, other are inhibitory
 * Beta receptors
 * Excited be epinephrine, weakly sensitive to norepinephrine
 * Certain beta functions are excitatory, other are inhibitory