L2 ANS Neurotransmission Flashcards
Describe the structural organization of the ANS
Sensory/afferent originate from sensors in internal organs, smooth muscles
Motor/efferent is response due to sensory input, modulates organ activity
Describe the functional organization of the ANS
Two main functions:
- Maintain homeostasis: viscera are effectors, innervates smooth and cardiac muscle and glands, makes adjustments to ensure optimal activities, involuntary
- Respond to external stimuli: fight or flight response
How do the PS and S branches differ?
PS is craniosacral, rest and digest
S is thoracolumbar, fight or flight
PS and S are complementary, but produce opposing effects
Main ANS neurotransmitters
NE and ACh
Types of Autonomic receptors
Adrenergic: a1, a2, B1, B2 (metabotropic)
Cholinergic: nicotinic (ionotropic), muscarinic (metab.)
a1 receptors
Target: GI sphincter, bladder sphincter, iris muscle, renal and splanchnic
Mechanism of Action: increase IP3 and intracell. Ca
a2 receptors
Target: GI wall, presynaptic adrenergic neurons
Mechanism of Action: inhibit adenylate cyclase, decrease cAMP
B1 receptors
Target: Heart, salivary glands, adipose, kidney
Mech. of Action: stimulate adenylate cyclase, increase cAMP
B2 receptors
Target: Vascular smooth muscle of skeletal muscle, GI wall, bladder wall, bronchioles
Mech. of Action: stimulate adenylate cyclase, increase cAMP
Nicotinic receptors
Target: Skeletal muscle, motor end plate, all postgang. neurons, adrenal medulla
Mech. of Action: EPSP, Na and K current, depolarization
Muscarinic receptors
Target: All effector organs (PS), sweat glands (S)
Mech. of Action: increase IP3, increase intracellular Ca
What is the same about all preganglionic neurons in ANS?
All secrete ACh, which acts on postganglionic nicotinic receptors
Adrenal gland
EXCEPTION to the preganglionic rule- synapse occurs in the gland, there is no postgang. neuron
-Direct activation of gland causes global release of Epi and NE into blood (for fight or flight)
How much Epi and NE is the adrenal medulla responsible for?
80% of Epi and 20% of NE into circulation
Sweat glands
EXCEPTION to preganglionic and sympathetic rule- innervated by symp. branch, but postganglionic neurons are muscarinic receptors that are activated by ACh
Location of preganglionic somas: S vs. PS
S: T1-T12, L1-L3
PS: Brainstem and sacral spinal cord
Location of postganglionic somas: S vs. PS
S; Paravertebral ganglia (lateral to SC) and prevertebral ganglia (in abdomen)
PS: Ganglia located near or in target organ
Length of preganglionic fibers: S vs. PS
S: Short
PS: Long
Length of postganglionic fibers: S vs. PS
S: Long
PS: Short
Preganglionic fiber NT: S vs. PS
ACh for both!
Postganglionic fiber NT: S vs. PS
S: NE and Epi (except for sweat glands)
PS: ACh
Receptor type at target organ: S vs. PS
S: Adrenergic receptor, slow metabotropic (a1, 2, B1, 2)
PS: Muscarinic receptor, slow metabotropic (M1-5)
Feedback Loop for S and PS activation
Sensors in viscera go back to integrative system (adjusting point) via SC or CNS, cause adjustment of condition in response to any factor altering homeostasis
Difference between ANS and CNS synapse
ANS: post-synaptic neuron is metabotropic, synapses by passing through fat varicosities
CNS: ionotropic postsynaptic neuron, creates E/IPSPs
-Neuromuscular jxns only happen in skeletal muscle
Adrenergic Transmission
NE made and transported into vesicles
AP arrives and vesicles released
NE free flowing in cleft binds to a1 receptors (smooth muscle) or back to neuron via a2 receptors
Immediate destruction or reuptake of NE
What happens to NE in adrenergic transmission when bound to a1 receptor?
Endocytosed by muscle and inactivated by MAO (monoamine oxidase) and COMT (catechol-O-methyl transferase)
What happens to NE in adrenergic transmission when bound to a2 receptor?
Uptake by axon and destroyed by MAO in mitochondria
Adrenergic vs. Cholinergic vesicles
Adrenergic= small, granular vesicles Cholinergic= agranular
Cholinergic Transmission
ACh released, short lived- binds to muscarinic receptor or destroyed by AChE
-Once destroyed, get free choline which is reuptaken by axon for reuse
Two Neuron Pathway
Preganglionic neuron goes from SC to ganglia/nucleus nad synapses there (ionotropic receptor, need fast response)
Postganglionic neuron goes to target organ and synapses (receptor is metabotropic)
Which nerves do pain and reflex signals travel with?
Pain and reflex fibers are sensory afferent fibers.
Pain travels with sympathetic nerves to the SC or CNS
Reflexes travel with PS nerves
Baroreceptor Reflex Control
Increase in BP= stretch in baroreceptors at carotid sinus and aortic arch= mechanoreceptors open= increase in firing= signal travels to medulla via CN IX and X= increase PS to decrease HR, decrease Symp. to decrease HR & stroke volume and increase vasodilation= BP returns to normal
Autonomic centers in brain
Reticular formation (brainstem)- most direct influence over ANS
Medulla- respiration, intake
Pons- micturition
Hypothalamus- master of master gland, temp regulation, food, fight, flee, reproduction
Amygdala- learned fear, fight or flight
Cortex- some modulatory control
