Autonomic Nervous System Flashcards
Subdivisions of nervous system
Recap of somatic NS division
- Voluntary action
- Somatic motor = efferent fibres to striated (skeletal) muscles
- Somatic sensory = afferent fibres from receptors (e.g. skin receptors)
- All within 31 pairs of segmental spinal nerves (or cranial nerves)
How do somatic sensory fibres enter the spinal cord?
Via the dorsal root ganglion
What does the ANS control?
Contrals internal activities of organs and glands
ANS recap
- Largely involuntary
- Monitors conditions in the internal environment to maintain homeostasis
- Influenced rostrally by the hypothalamus
Where do autonomic (visceral) motor fibres go to?
Efferent fibres to smooth muscles, cardiac muscle or glands
Where do autonomic (visceral) sensory fibres come from?
afferent fibres from sensory receptors in an internal organ
Where do visceral efferent fibres have their cell bodies?
- Lateral grey horn of spinal cord –> only found in T1-L2 and S2-S4
- Use ventral root to exit the spinal cord
Where do autonomic visceral afferent fibres have their cell bodies?
Dorsal root ganglion then use dorsal root to enter spinal cord
Where do autonomic fibres (either afferent or efferent) only exist?
Segemental spinal nerves at either T1-L2 or S2-S4 OR specific cranial nerves
Autonomic (visceral) afferents recap:
- Autonomic (visceral) sensory = afferent fibres from sensory receptors in an internal organ, glands and blood vessels
- Conduct sensory impulses from viscera to CNS
- Responsible for referred pain
Which division of the NS is responsible for referred pain?
Autonomic (visceral) afferent
What is referred pain?
Pain sensations from visceral organs often perceived as regions of the body surface innervated by the same spinal nerves.
E.g. MI pain is referred to upper chest and limb
Referred pain explained
- Somatic afferent fibres (e.g. from skin T1-T4) use dorsal root to enter cord
- Visceral afferent fibres (e.g. from heart) follow same pathway and use dorsal root to enter cord
- Heart is innervated by afferent fibres between T1-T4
- Brain interprets pain as if its coming from T1-T4 regions of skin
- The pain is usually referred to dermatomes that are at the same spinal nerve level as the visceral afferent synapse.
Describe the 2 neuron chain for autonomic (visceral) efferents
- Preganglionic neuron in CNS
- Cell body in CNS (brainstem or spain cord)
- Pre and postganglionic neurons synapse at an autonomic ganglion in the PNS
- Postganglionic neuron in PNS
- Travels to target organ/tissue
The ANS: summary
- Innervates smooth muscle (organs), cardiac muscle (heart) and glands (e.g. salivary, lacrimal, sweat, etc)
- Visceral motor efferents and visceral afferents
- Two divisions, exert antagonistic effects
- Sympathetic –> Produces effects most apparent during stress or excitement: fright/flight/fight
- Parasympathetic –> Conserves and restores energy: rest and digest
- Most visceral organs receive both sympathetic and parasympathetic
Difference between the outflow of parasympathetic vs sympathetic information
Properties of the SNS and PSNS preganglionic neurons differ with respect to the spinal cord exit points:
- Sympathetic:
- Has thoracolumbar outflow, meaning that the neurons begin at the thoracic and lumbar (T1–L2) portions of the spinal cord.
- Parasympathetic:
- Has craniosacral outflow, meaning that the neurons begin at the cranial nerves (CN3, CN7, CN9, CN10) and sacral (S2–S4) spinal cord.
Sympathetic outflow summary
Thoracolumbar outflow –> T1-L2
Parasympathetic outflow summary
Craniosacral outflow:
- Brainstem CN III, VII, IX, X
- S2-S4
Difference between the ganglia of the sympathetic and parasympathetic NS
- Sympathetic:
- Organised ganglia in sympathetic chain/paravertebral chain
- Series of ganglia that extends whole length of spinal cord
- Prevertebral and paravertebral ganglia
- Parasympathetic:
- Ganglia located near the effector organ
Is the effect of the parasympathetic and sympathetic system widespread or localised?
Sympathetic –> widespread
Parasympathetic –> localised
Route of parasympathetic division; cranium and sacral origin
- Cranio-sacral origin (brainstem and S2-S4)
- Axons of preganglionic neurons from brainstem leave via cranial nerves III, VII, IX or X
- Axons of preganglionic neurons from sacral cord leave spinal cord via S2-S4 spinal nerve
- Synapse with postganglionic fibre in PNS (autonomic ganglia)
- Sends axon to effector organ
All preganglionic fibres of the ANS use what as their neurotransmitter?
Acetylcholine –> all fibres are cholinergic and are myelinated for faster transmission
What do the postganglionic fibres of the ANS use as their neurotransmitter?
Sympathetic division –> use noradrenaline so are adrenergic
Parasympathetic division –> use acetylcholine so are cholinergic
Describe the length of the preganglionic and postganglionic fibres of the parasympathetic division
2 neuron chain:
- Preganglionic fibre has long axon
- Postganglionic fibre has relatively short axon
- Autonomic ganglion (where these 2 synapse) is located close to effector organ
Parasympathetic fibres run within which 4 cranial nerves? Where do each of these cranial nerves travel to? What is the effect?
- CN III
- Travels to pupil
- Causes dilation of pupil
- CN VII
- Travels to salivary and lacrimal glands
- Increases salivary secretions / tears
- CN IX
- Travels to salivary glands
- Increases salivary secretions
- CN X
- Travels to cardiac, pulmonary and digestive systems
- Decreases heart rate
- Constricts bronchi
- Increases digestion
What is the main parasympathetic nerve of the viscera of the trunk?
Vagus nerve
Origin of the vagus nerve?
Medulla oblongata of brainstem –> posterolateral sulcus
how does the vagus nerve innervate target organs?
Via organ plexuses
What do S2-S4 parasympathetic fibres leave the spinal cord as? Effect on organs?
As pelvic splanchnic nerves
- Control over bladder and distal part of colon and erectile tissue
- Allows urination and defecation as well as erection
The axons of presynaptic parasympathetic neurons extend from the CNS into a ganglion that is either very close to or embedded in their target organ.
In the cranium, preganglionic fibers (cranial nerves III, VII, and IX) arise from the CNS and create a synapse at one of four parasympathetic ganglia.
What are these 4 parasympathetic ganglia of the cranium?
- Ciliary
- Pterygopalatine
- Otic
- Submandibular
How do fibres pass from their ganglia of the cranium to the respective organ (head and neck only)?
by “piggy-backing” on branches of the trigeminal nerve
Preganglionic and postganglionic neurotransmitter of the:
- sympathetic division?
- parasympathetic division?
- Sympathetic:
- Pre –> ACh
- Post –> noradrenaline (except sweat glands)
- Para:
- Pre –> ACh
- Post –> ACh
Origin of:
- Sympathetic division?
- Parasympathetic division?
- Thoraco-lumbar origin (T1-L2 spinal segments)
- Craniosacral origin (CN III, CN VII, CN IX, CN X) and S2-S4 spinal segments
Where are most of the ganglia of the sympathetic division located?
Mostly in the sympathetic chain
Describe the route of the sympathetic fibres after leaving spinal cord
- Preganglionic fibre has origin T1-L2 (cell body in CNS)
- Sends axon to synapse with postganglionic fibre in sympathetic chain
- ACh neurotransmitter
- Sends axon to synapse with postganglionic fibre in sympathetic chain
- Autonomic ganglia located in sympathetic chain
- Postganglionic fibre sends axon to target organ
- Noradrenaline neurotransmitter
What are the 2 reasons why the effects of the sympathetic division are widespread?
-
Neuronal divergence:
- One preganglionic neuron can synapse with many postganglionic neurons
-
Adrenal medulla:
- Some preganglionic sympathetic fibres leave spinal cord and travel directly to the adrenal medulla cells (these cells are modified postganglionic cells)
- Adrenal medulla synthesis and release noradrenaline and adrenaline into the bloodstream
- Widespread release of these hormones into the bloodstream when stimulated by preganglionic sympathetic fibres
Distribution of sympathetic chain explained:
- All preganglionic fibres leave cord in segmental spinal nerves T1-L2 ONLY
- Run to the sympathetic chains (entire cord) or prevertebral ganglia
- From sympathetic chain, fibres are distributed to the body
- Allows innervation of e.g. structures above T1
What are prevertebral ganglia?
?
Effect of sympathetic fibres on glands/organs?
- Dilates pupils
- Decreases salivary secretions
- Increases heart rate
- Dilates bronchi
- Inhibits digestion
- Liver increases availability of glucose
- Inhibits defection and urination
What is the sympathetic chain?
- Interconnected chain of autonomic ganglia located on either side of the entire vertebral column (paravertebral)
- Sites of synapse between sympathetic pre and post ganglionic neurons
- Permit pre ganglionic fibres ascend or descend beyond T1-L2
Sympathetic outflow options:
- Preganglionic neuron leaves spinal cord and synapses at entry level in the sympathetic trunk (paravertebral ganglia). Postganglionic neuron then travels to effector organ (mainly thorax, skin and body wall).
- Preganglionic neuron leaves spinal cord and passes through sympathetic trunk without synapsing, instead synapsing in prevertebral (collateral) ganglia or adrenal medulla. Postganglionic fibre then goes to effector organ (mainly in abdomen or pelvis)
- Preganglionic neuron leaves spinal cord and ascends or descends in the sympathetic trunk to synapse with another ganglion in the trunk closer to the target organ. Postganglionic fibre then goes to effector organ.
There are 2 rami communicantes associated with the sympathetic division. What are these? Function?
- White ramus communicans
- Gray ramus communicans
Provide a route for fibres to travel
What type of fibres does the white and gray ramus communicans contain? Function?
- White –> contains myelinated preganglionic sympathetic fibres
- Gray –> contains unmyelinated postganglionic fibres
Where are the white and gray ramus communicans located?
White –> only between T1-L2
Gray –> all levels of spinal cord
Function of white ramus communicans? Gray?
White: Carries presynaptic sympathetic fibers from the ventral rami to the sympathetic trunk and exist only between spinal cord segments T1 and L2.
- Preganglionic fibre leaves spinal cord via ventral root.
- Travels in white ramus communicans to enter sympathetic chain
- Then either:
- Synapse at entry level
- Pass through trunk without synapsing to collateral ganglia
- Ascend or descend
Gray: carries postsynaptic sympathetic fibers from the sympathetic trunk to all spinal nerves
- For fibres that want to rejoin the spinal nerves after synapsing, they do this via the gray ramus communicans
- Then travels to body wall via spinal nerves
Describe the sympathetic and parasympathetic effects on heart rate and respiratory volume
Sympathetic:
- Stimulates heart contraction
- Induces bronchodilation
Parasympathetic:
- Inhibits heart contraction
- Induces bronchoconstriction
Describe the sympathetic innervation of the heart (efferent fibres)
- Preganglionic fibres leave spinal cord from T2-T4 levels of cord
- Synapse at different levels of sympathetic chain
- Postganglionic fibres travel towards heart in cardiac nerves / cardiac plexus
- Reaches target organ –> sympathetic effects
Describe the length of pre and postganglionic sympathetic fibres
Pre –> short (short distance to enter sympathetic chain)
Post –> long
Which spinal cord levels do sympathetic fibres innervating the heart originate?
T2-T4
Describe the sympathetic innervation of the lungs (afferent fibres)
- Preganglionic fibres leave spinal cord at T2-T4
- Enter symapthetic chain and synapse
- Postganglionc fibres travel as pulmonary plexus to the lungs
- Sympathetic innervation of lungs
Describe the parasympathetic innervation of the heart/lungs (efferent fibres)
- CN X leaves medulla
- An associated parasympathetic nucleus in the brainstem contains the preganglionic parasympathetic cell bodies
- Axons leave via vagus nerve and descend to the thorax
- Synapse with postganglionic parasympathetic fibre at level of target organ (heart or lungs)
- Travels to effector organ –> parasympathetic effects
Describe the innervation of the carotid sinus and carotid body
- The carotid sinus nerve carries visceral sensory (afferent) fibres to the medulla via the glossopharyngeal nerve
- Sensory informaltion on BP and O2
- Vagus nerve can then e.g. lower BP
What is micturition?
Urination
Describe 3 types to nervous supply of the urinary bladder. Which nerve? Effects?
- Sympathetic
- Hypogastric nerve (T12 – L2)
- It causes relaxation of the detrusor muscle, promoting urine retention.
- Parasympathetic
- Pelvic nerve (S2-S4)
- Increased signals from this nerve causes contraction of the detrusor muscle, stimulating micturition.
- Somatic
- Pudendal nerve (S2-S4)
- It innervates the external urethral sphincter, providing constant voluntary control over micturition.
In addition to the efferent nerves supplying the bladder, there are sensory (afferent) nerves that report to the brain. What do these signal?
They are found in the bladder wall and signal the need to urinate when the bladder becomes full.
Describe the sympathetic innervation of the urinary bladder
- Preganglionic fibre leaves spinal cord at T11-L2 and enters sympathetic chain
- Descends down chain to synapse with postganglionic fibre
- Postganglionic fibres then travels towards bladder via hypogastric plexuses
- Sympathetic innervation to bladder –> relaxes detrusor muscle and contracts internal uretheral sphincter and bladder fills with urine
What is the bladder stretch reflex?
The bladder stretch reflex is a primitive spinal reflex, in which micturition is stimulated in response to stretch of the bladder wall. It is analogous to a muscle spinal reflex, such as the patella reflex.
During toilet training in infants, this spinal reflex is overridden by the higher centres of the brain, to give voluntary control over micturition.
Describe the reflex arc for urinary bladder
- Bladder fills with urine, and the bladder walls stretch –> under sympathetic control
- Sensory (afferent) nerves detect stretch and transmit this information to the spinal cord.
- Interneurons within the spinal cord relay the signal to the parasympathetic efferents (the pelvic nerve).
- The pelvic nerve acts to contract the detrusor muscle, and stimulate micturition.
Describe the parasympathetic innervation of the urinary bladder
- Parasympathetic innervation of urinary bladder via pelvic splanchnic nerve (S2-S4)
- Stimulates contraction of detrusor muscle and relaxation of internal uretheral sphincter
BUT urination cannot happen without somatic innervation (via pudendal nerve)
Describe the action of urinating in infants vs adults
- Infants –> bladder reflex arc, parasympathetic innervation (when bladder is full) causes micturition
- Adults –> parasympathetic innervation FOLLOWED BY somatic innervation causes micturition
What nervous supplies are involved in defecation? Function?
- Sympathetic –> prevents rectum emptying
- Parasympathetic –> induces rectum emptying
- Somatic –> voluntary inhibition of rectum emptying via pudendal nerve
Describe the sympathetic innervation of defecation
- Sympathetic preganglionic fibres leave spinal cord between L1-L2
- Pass straight through sympathetic trunk to synapse at a prevertebral/collateral ganglion (the inferior mesenteric ganglion)
- Postganglionic sympathetic fibres travel towards distal colon and rectum
- Sympathetic innervation –> prevents peristalsis
Describe the parasympathetic innervation of defecation
- Preganglionic parasympathetic fibres leave via S2-S4
- Synapse with postganglionic fibres
- Parasympathetic innervation –> induce peristalsis and emptying
Describe the somatic innervation of defecation
- Pudendal nerve controls default state for defecation –> contraction of anal sphincter preventing defecation
- During defecation:
- Voluntary inhibition of sympathetic NS and parasympathetic NS takes over –> emptying
Stretch receptors also present here (info carried to brain via sensory (afferent) fibres)
What syndrome can injury to sympathetic supply to head and neck (sympathetic trunk in neck or upper thoracic segments) lead to?
Horner syndrome:
- Anhidrosis
- Miosis
- Ptosis
What are the sympathetic effects of an injury to the spinal cord or cauda equina?
Disrupt sympathetic control to bladder
What can injuries to cranial nerves (III, VII, IX, X) result in? Is this a sympathetic or para injury?
Parasympathetic injury –> dilation of pupil, loss of pupillary light reflex, impaired lacrimation, salivation, etc)
What are the parasympathetic effects of an injury to the spinal cord or cauda equina?
Disrupt bladder, rectum, sexual functions
The bladder has important clinical considerations when it comes to spinal cord lesions. There are two different clinical syndromes, depending on where the damage has occurred.
What are these 2 syndromes?
- Reflex Bladder – Spinal Cord Transection Above T12
- Flaccid Bladder – Spinal Cord Transection Below T12
What is the effect on the urinary bladder of a spinal cord lesion above T12?
- Reflex bladder
- The afferent signals from the bladder wall are unable to reach the brain, and the patient will have no awareness of bladder filling
- There is also no descending control over the external urethral sphincter, and it is constantly relaxed.
- BUT there is a functioning spinal reflex, where the parasympathetic system initiates detrusor contraction in response to bladder wall stretch.
- RESULT: Thus, the bladder automatically empties as it fills – known as the reflex bladder.
What is the effect on the urinary bladder of a spinal cord lesion below T12?
- Flaccid bladder
- At this level, the parasympathetic outflow to the bladder is damaged
- The detrusor muscle will be paralysed, unable to contract –> The spinal reflex does not function.
- In this scenario, the bladder will fill uncontrollably, becoming abnormally distended until overflow incontinence occurs.
How is the hypothalamus separated from the thalamus?
Hypothalamic sulcus
What is the hypothalamus bordered anteriorly and ventrally by?
Anteriorly –> lamina terminalis
Ventrally –> optic chiasma
Inferiorly and posteriorly –> brainstem
What is the function of the hypothalamus regarding the ANS?
The hypothalamus, just above the brain stem, acts as an integrator for autonomic functions, receiving ANS regulatory input from the limbic system to do so.
I.e. controlling influence upon the activity of the ANS
Levels of ANS control of functioning
- Cerebrum –> emotional responses (limbic system) through connection with hypothalamus e.g. memories can trigger responses (salivation)
- Hypothalamus –> overall control and regulation and influences:
- Brainstem centres –> CVS, respiratory and swallowing control
- Spinal cord centres –> urination, defecation, erection and ejactulation
