how does the autonomic nervous system help to maintain homeostasis
- maintained via balance between ANS, endocrine and behavioral systems
- homeostatic control centers monitor various physiological processes-> hypothalamus, pons, medulla
- ANS responses can be influenced by emotions (limbic system) or cerebral cortex (behavioral state)
- some autonomic functions can occur without brain input
- ANS branches have antagonistic functions
explain how homeostasis reflects a balance between the autonomic branches
- homeostasis is a dynamic balance between the autonomic branches
- rest and digest: parasympathetic activity dominates
- fight or flight: sympathetic activity dominates
describe antagonistic control by the ANS and examples
- most internal organs are under antagonistic control
- one autonomic branch is excitatory and the other branch is inhibitory
- example - heart
parasympathetic = slows rate
sympathetic = increase rate and force of contraction - example - gastrointestinal tract
parasympathetic = promotes digestion
sympathetic = slows/halts digestion
what are some exceptions to antagonistic control
- blood vessels only have SNS input and are under tonic control
- regulated by increase/decrease of SNS activity
compare origin, ganglion location, and pathways for sympathetic and parasympathetic
sympathetic:
origin in CNS = thoracic and lumbar segments
ganglion location = close to spinal cord
pathway = short preganglionic, long postganglionic neurons
parasympathetic:
origin in CNS = brain stem and sacral segments
ganglion location = on or close to targets
pathway = long preganglionic, short postganglionic neurons
what is a ganglion
- cluster of cell bodies outside the CNS (in the peripheral nervous system)
what are the main anatomical differences between sympathetic and parasympathetic branches of the ANS
- CNS origin of the preganglionic neuron
- the location of the peripheral autonomic ganglia
- relative lengths of pre and post ganglionic neurons
- postganglionic neurotransmitter released
- receptors at target synapses
what are the neurotransmitters and receptors of the sympathetic nervous system
- uses acetylcholine and norepinephrine
- ACh onto nicotinic receptor = ligand gated ion channel (ionotropic response)
- norepinephrine onto adrenergic receptors= GPCRs on target tissue (metabotropic response)
what are the neurotransmitters and receptors of the parasympathetic nervous system
- uses acetylcholine
- ACh onto nicotinic receptors = ligand gated ion channel (ionotropic)
- ACh onto muscarinic receptor = GPCRs (metabotropic)
autonomic synapses at target are called
- neuroeffector junctions
how do neuroeffector junctions (synapses) work
- postganglionic neuron has specialized terminals
- autonomic varicosities are swollen regions of axon filled with neurotransmitter vesicles
- chains of varicosities lie across the surface of the target tissue
- varicosities release neurotransmitter over the surface of target cells
- 1 neuron can innervate a large area of tissue
- target receptors are not localized under varicosities
explain the norepinephrine cycle at a sympathetic varicosity
- action potential arrives at the varicosity
- depolarization opens voltage gated Ca2+ channels
- Ca2+ entry triggers exocytosis of synaptic vesicles
- NE binds to adrenergic receptor on target
- receptor activation ceases when NE diffuses away from the synapse
- NE is removed from the synapse
- NE can be taken back into synaptic vesicles for re-release
- NE is metabolized by monoamine oxidase (MAO)
what is the adrenal medulla
- part of the sympathetic branch
- located above the kidney
- secretes epinephrine
- is a modified sympathetic ganglion made up of chromaffin cells
what does the adrenal cortex do
- is a true endocrine gland and secretes hormones (cortisol) associated with osmoregulation and glucose metabolism
explain the process of how epinephrine is secreted as a neurohormone
- spinal cord (thoracolumbar) -> preganglionic sympathetic neuron -> ACh on adrenal medulla -> the chromaffin cell is modified postganglionic sympathetic neuron -> epinephrine is a neurohormone that enters the blood and goes to target tissues
explain how the nicotinic ACh receptor is a non-selective cation channel
- the nicotinic cholinergic receptor binds 2 ACh molecules, opening a nonspecific monovalent cation channel
- the open channel allows Na+ and K+ to pass
- net influx of Na+ depolarizes the muscle fiber
- under normal conditions, ACh binding to the nicotinic cholinergic receptor will always cause an EPSP on the post ganglionic neuron
- more Na+ flows in than K+ flowing out
what are adrenergic receptors
- sympathetic target cell receptors
- alpha and beta adrenergic receptors
- all GPCRs but are coupled to different G proteins
- different responses depend on G protein type
- each receptor has a different affinity for NE and E
what are the 5 adrenergic receptors and the G proteins associated with it
alpha 1 = Gq (Phospholipase C)
alpha 2 = Gi (decreases cAMP)
beta 1 = Gs (increase cAMP)
beta 2 = Gs (increase cAMP)
beta 3 = Gs (increase cAMP)
what is the sensitivity for each receptor and where are they found
alpha 1 = NE>E, sympathetic target tissues
alpha 2 = NE>E, GI tract and pancreas
beta 1 = NE=E, heart muscle, kidney
beta 2 = NE<E, blood vessels and smooth organs
beta 3 = NE>E, adipose tissue
what are the muscarinic cholinergic receptors
- 5 known subtypes of muscarinic ACh receptors
- M1,M2,M3,M4,M5
- all are GPCRs
- specific G protein will determine intracellular response
- M1,M3,M5 = Gq and activate phospholipase C (PLC)
-M2, M4 = Gi and inhibit adenylyl cyclase to decrease cAMP
what are the autonomic targets
- smooth and cardiac muscles
- some endocrine and exocrine glands
- some adipose tissue
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