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