Lecture 6: Autonomic Nervous System Flashcards
Nervous system divisions
NS = CNS + PNS
PNS = somatic + autonomic
Autonomic = sympathetic + parasympathetic + enteric
What does the autonomic nervous system do?
Involuntary innervation of tissues other than skeletal muscle/GI tract (enteric)
How do autonomic nerves connect to the CNS?
2 neurons in series synapsing at the autonomic ganglion outside the CNS. Pre-ganglionic cell body is in the CNS.
How are sympathetic neurons/ganglia placed?
Sympathetic ganglia run in 2 chains close alongside the length of spinal cord in sympathetic trunks or are closer to organs in the abdominal cavity (collateral ganglia). Sympathetic nerves leave CNS at thoracic/lumbar spinal cord.
Sympathetic trunks (paravertebral ganglia)
Bundled nerve fibers running alongside the vertebral column containing the sympathetic ganglia. Allows pre-ganglionic sympathetic nerves to travel up/down before synapsing
How are parasympathetic nerves/ganglia placed?
Parasympathetic ganglia are placed very close to or within innervated organs. Parasympathetic nerves leave CNS at brain stem/sacral spinal cord.
Collateral (pre-vertebral) ganglia
Sympathetic ganglia in the abdominal cavity placed closer to innervated organs compared to other sympathetic ganglia. Specifically the celiac and superior/inferior mesenteric ganglia.
Autonomic nervous divisions
-Sympathetic (AKA thoracolumbar) division
-Parasympathetic (AKA craniosacral) division
-Enteric NS
Pattern of sympathetic innervation
Sympathetic NS typically affects targets in concert due to higher pre-ganglionic divergence and circulating epi/norepi.
Pattern of parasympathetic innervation
Parasympathetic NS typically affects targets individually in a finely-tailored pattern due to less pre-ganglionic divergence.
General somatic NS efferent pathway
Large diameter, myelinated axons of motor neurons directly innervate skeletal muscle with ACh; faster transmission
General autonomic NS efferent pathway
Medium diameter, myelinated pre-ganglionic neurons release ACh onto small, unmyelinated post-ganglionic neurons which innervate targets with NE (symp.) or ACh (parasymp.), or synapse onto adrenals. Apparently no evolutionary need for faster transmission.
Autonomic innervation of the adrenal medulla
The adrenal medulla acts like a ganglion. Chromaffin cells receive ACh from pre-ganglionic fibers and then release NE and epi into circulation.
Primary autonomic neurotransmitters
In general, pre-ganglionic fibers release ACh and post-ganglionic release NE (sympathetic/adrenal medulla) or ACh (parasympathetic).
Cholinergic receptor types
Named for agonists.
Nicotinic: skeletal muscle, autonomic post-ganglionic dendrites
Muscarinic: parasympathetic targets + sweat glands/blood vessels (sympathetic)
Antagonists for cholinergic receptors
Curare = somatic nicotinic antagonist
Hexamethonium = autonomic ganglia nicotinic antagonist
Atropine = muscarinic antagonist
Adrenergic receptor types
Adrenergic receptors are not named for agonists.
α1, α2, β1, β2 on smooth/cardiac muscle and glands (autonomic)
Agonist
Substance that binds a receptor and activates it
Inverse agonist
Substance that binds a receptor and decreases its (basal) activity in the absence of ligands
Antagonist
Substance that reduces the efficacy of agonists/inverse agonists
Reciprocal innervation
Most autonomic targets are innervated in opposition by both sympathetic and parasympathetic nerves. Exceptions include skin hair, sweat glands, adrenal medulla, some blood vessels
Reciprocal innervation exceptions
Likely allows separate regulation separate from circulating NE/epi. Piloerectors, sweat glands, adrenals, some blood vessels receive ACh and NE from sympathetic post-ganglionic nerves.
Nonadrenergic, noncholinergic neurons
Many autonomic post-ganglionic neurons use other NTs/factors like NO for many functions.
Sympathetic and parasympathetic purposes
Sympathetic = fight or flight, gas pedal
Parasympathetic = rest and digest, brakes
(in general)
Varicosities
AKA “en passant” synapses. Swellings along axon branches form many autonomic synaptic junctions (rather than axon terminals)
Patterns of somatic innervation
Somatic motor neurons are inactive until stimulated, and initiate activity in otherwise inactive muscle
Patterns of autonomic innervation
Autonomic neurons typically maintain a basal tonic activity and modulate targets that have autogenic activity.
Horner’s syndrome
Caused by a sympathetic defect -> inactivating several muscles, resulting in partial eyelid droop (ptosis) and uneven pupils (miosis)
