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
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
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
- 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)
- Synthesis of Norepinephrine begins in the axoplasm of the terminal nerve endings of adrenergic fibers and is completed inside the vesicles
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
- Muscarinic receptors
- 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
- Alpha receptors
