Lecture 6 - The Autonomic Nervous System Flashcards
What kind of movements does the autonomic nervous system control? include examples
- involuntary motor (effernt)/unconscious processes
- innervates smooth & cardiac muscle, viscera, and glands
- ex: heart rate, respiration, blood pressure, sweating, digestion
What is the main goal of the ANS?
maintain homeostasis throuhgout the body
What are the three systems thats branch off of the ANS?
- sympathetic
- parasympathetic
- enteric
What is the sympathetic nervous system (SNS)?
- stimulates metabolism; most active during times of exertion, stress, or emergency
- “fight or flight”
- ex: increase heart rate, force of contration and blood pressure & pauses digestion & dilates blood vessels in skeletal muscles
What is the parasympathetic nervous system (PSNS)?
- promotes relaxation, nutrient uptake, energy storage; most active under resting conditions
- “rest and digest”
- ex: decrease heart rate, force of contration and blood pressure & stimulates digestion, dilates blood vessels in GI tract
SNS vs PSNS
- most organs receive both sympathetic and parasympathetic innervation
- they are functionally and anatomically distinct
- effects are generally opposing
What is a neruon and what is their anatomy?
- basic functional unit of the nervous system; highly specialized cells that carry electric signals
- neuronal cell body
-> axon
What surrounds the axon of nerve cells and what does it do?
- Myelin sheath (lipid-rich sheath)
- acts as an insulating layer
Efferent: Somatic vs Autonomic
Somatic:
- motor neuron (ventral horn)
- voluntary control
- one neuron between CNS and target
- 1 nerve fiber
- innervates skeletal muscle
Autonomic:
- visceral motor neuron (spinal cord; or brainstem nuclei)
- involuntary control
- two neurons between CNS and target
- preganglionic neuron -> autonomic ganglion -> postganglionic neuron
- innervates cardiac muscle, smooth muscle, glands, viscera
Location of neuronal cell bodies of pregranglionic vs postganglionic neuron
- pre: brainstem or spinal cord
- post: autonomic ganglion
Is the preganglionic neuron myelinated?
yes
Is the postganglionic neuron myelinated?
no
What is a ganglion? and what is the DRG?
- collection of neuronal cell bodies found in the PNS
- DRG: sensory (afferent) ganglion of the somatic nervous system
What does the ganglion function as?
a relay station between preganglionic and postganglionic neurons of the ANS
What are the regions of the SNS?
- “thoraco-lumbar”
- T1-L2
What are the regions of the PSNS?
- “cranio-sacral”
- cranial nerves:
- CNIII:
Occulomotor - CNVII: Facial
- CN IX:
Glossopharyngeal - CNX: Vagus
- CNIII:
- sacrial regions: S2-S4
What are the three types of prevertebral ganglia?
- celiac
- superior mesentreric
- inferior mesenteric
What is the sympathetic chain?
- sympathetic ganglia
- aka sumpathetic trunk or paraverterbral ganglia
- paired sympathetic ganglia linked together to form a chain along the length of the vertebral colum (C1-S5)
Why are the ganglia connected to each other in the sympathetic chain?
it coordinates and spreads ympathetic responses to many target tissues simultaneously
What is the prevertebral ganglia?
- situated anterior to the vertebral column, along the aorta
- named relative to blood vessels branching from the abdomincal aorta
Where are sympathetic ganglia located?
close to the vertebral colum
Is preganglionic ganglia long or short? Postganglionic
- pre: short
- post: long
what are Rami communicans?
- connectin between the spinal nerve and sympathetci trunk
What are the two types of communicantes?
- white and gray ramus communicans
Where are white rami? Grey?
- white: T1-L2
- grey: at all levels along the sympathetic chain
Where does sympathetic outflow originate?
in the lateral horn of the spinal cord, from segments T1-L2
Where do preganglionic (myelinated) neurons exit the spinal cord?
- via the ventral horn and root to enter the spinal nerve
Where do preganglionic neurons enter the sympathetic chain?
- via the white ramus communicans “on ramp”
What are the 3 main roots of preganglionic neurons upon entering the sympathetic chain?
- synapse within the chain at the same level
- change level within the chain and then synapse
- formation of the splanchnic nerve
Synapse within the chain at the same level
- preganglionic neuron synapses with postganglionic neuron at the same level & exits via the grey ramus, travelling with spinal nerve to target
- ex: sympathetic innervation of blood vessels & sweat glabdss of the body wall & limbs
Same level
preganglionic neurons located between T1-L2
Nerve target
- body wall
- abdominal and thoracic viscera
- limbs
What is a dermatomes?
strip of skin supplied by a spinal nerve
preganglionic neuron
- located in the lateral horn (T1-L2)
- myelinated preganglionic axon exits spinal cord via the ventral horn and ventral root to enter the spinal nerve
- preganglionic axon then leaves spinal nerve, and enters the sympathetic trunk via white ramus communicans (white communicating ramus) to synapse on the postganglionic neuron
- communicating ramus is called ‘white’ because the preganglionic fibers it carries are myelinated
postganglionic neuron
- unmyelinated postganglionic axon passes through the gray ramus communicans to enter the spinal nerve
- communicating ramus is ‘gray’ because the postganglionic fibers it carries are unmyelinated
- postganglionic fiber travels within spinal nerve to innervate target
Change level: preganglionic fiber can ascend or descend within the sympathetic chain
- sympathetic distribution is from T1-L2
- preganglionic nerve fibers can ascend or descend above/below/within this distribution before synapsing
ascending nerves target… (3)
- head and neck viscera
- thoracic viscera
- upper limbs
descending nerves target… (2)
- abdomino pelvic viscera
- lower limbs
viscera of the thorax
- orgasn in the thorax are supplied to various plexuses
- postganglionic contribute to autonomic plexuses including cardiac plexus (heart), pulmonary plexus (lungs)
splanchnic nerves aka visceral nerves
- typically associated with abdominopelvic viscera
When do splanchnic nerve form?
when autonomic fibers do not re-join the spinal nerve; instead fibers form splanchnic nerve
where do preganglionic fibers bypasses the sympathetic chain completely and synapses in?
A peripheral (prevertebral) ganglion near the target organ
Abdominopelvic viscera
- ## preganglionic nerve fibers pass through sympathetic trunk without synapsing and form splanchnic nerves
originate from thoracic and lumbar vertebral levels of the spinal cord, as sets of paired splanchnic nerves, including:
- greater splanchnic
- lesser splanchnic
- least/lumbar splanchnic
sympathetic splanchnic nerves synapse in prevertebral ganglia:
- celiac ganglion
- superior mesentric ganglion
- inferior mesenteric ganglion
throacic & lumbar splanchnic nerves:
- greater (T5-9)
- lesser (T10-11)
- least/lumbar (T12-L2)
synapse in prevertebral ganglia
- celiac
- superior mesenteric
- inferior mesenteric
Cranial Outflow: Head & Neck
- CN III, VII , IX
- preganglionic neurons leave brainstem from parasympathetic nuclei
- synapse on post-ganglionic neuron within parasympathetic ganglia
- preganglionic neurons leave the brainstem via CN X (vagus nerve) which contributes to autonomic plexuses
- supplied parasympathetics to pharynx, larynx, thorax & midgut structures (to transverse colon)
- preganglionic neurons synapse on ganglia within or close to target oragn
- preganglionic neuronal cell bodies located in spinal segments S2-S4
- axons travel via pelvic splanchnic nerves which contribute to autonomic plexuses
- preganglionic fibers synapse within ganglia in or near effector organ to supply targets
visceral afferent fibers conduct sensory information from autonomic targets to the CNS (ex: organs)
visceral affeent fibers are distinct from the autonomic nervous system (visceral motor) and are not considered sympathetic or parasympathetic
afferent fibers
carry sensory information to the CNS
visceral reflexes
are the simplest functional units in the autonomic nervous system
visceral afferent fibers and visceral reflexes
- visceral reflex arcs consist of a sensory nerve (visceral afferent), and visceral motor neurons
- ex: shinning a light in the eye triggers a visceral reflex that constricts the pupils
enteric nervous system
- the ANS includes a third divison called the enteric nervous system
- network of neurons located in the wlls of the digestive tract
enteric nervous system receives regulatory signals via sympathetic and parasympathetic divisions
- however, many complex visceral reflexes are initiated and coordinated independently of sympathetic and parasympathetic NS
