21 - Autonomic Nervous System Flashcards
Tissues innervated by autonomic nervous system
All tissues except skeletal muscle fibres (careful: ANS innervates blood vessels within skeletal muscle)
What does the ANS need in order to maintain homeostasis?
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• Signalling from the periphery, i.e. specialised sensors,
visceral sensory nerves
• CNS circuits to
– ‘interpret’ the signals
– decide on the most suitable outcome
– activate the appropriate sympathetic or parasympathetic
nerves
Anatomical distinction between sympathetic and parasympathetic nervous systems
Sympathetic fibres emerge from thoraco-lumbar part of spinal cord.
Parasympathetic fibres emerge from the cranio-sacral parts of the spinal cord.
Parts of spinal cord where no autonomic pathways originate
Cervical spine or lumbar enlargements.
Location of sympathetic pre-ganglionic neuron cell bodies
Intermediolateral cell column (T1 to L3)
Connections of sympathetic preganglionic neurons
Sympathetic postganglionic neurons lie in two types of ganglia:
1) Paravertebral ganglia (sympathetic chain)
2) Prevertebral ganglia
Function of sympathetic paravertebral ganglia
Primary source of vasoconstrictor muscles
Function of sympathetic prevertebral ganglia
Primary source of neurons innervating non-vascular smooth muscle
Sympathoadrenal system
Sympathetic preganglionic neurons innervate adrenal medulla (release of adrenaline)
Parasympathetic brainstem nuclei
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Edinger-Westphal nucleus
Salivatory nuclei
Dorsal motor nucleus of vagus
and nucleus ambiguus
Edinger-Westphal nucleus role
To ciliary ganglion to control
sphincter pupillae, ciliary muscle
Salivatory nuclei role
To submandibular, sphenopalatine and otic ganglia to
control lacrimal, salivary, sublingual, nasal and palatine
glands
Dorsal motor nucleus of vagus and nucleus ambiguus role
To microganglia near and on outer surface of thoracic and abdominal organs: numerous functions
Sacral spinal parasympathetic preganglionic location
Intermediolateral cell column (S1 to S5)
Where do sacral postganglionic parasympathetic neurons connect?
Postganglionic neurons lie in pelvic plexus (inferior hypogastric plexus).
Form microganglia on target organs.
Pelvic ganglia
Pelvic ganglia contain many sympathetic and parasympathetic neurons:
i.e., these are mixed ganglia =
very unusual
Where do the sympathetic neurons which deliver sensory information to ANS travel?
In the same nerve bundles as motor ANS neurons.
How do ANS neurons connect with target organs?
Complex networks of unmyelinated axons within organs
More than one site of transmitter release per axon (axon varicosities)
Major CNS integrative centre of autonomic function
Nucleus tractus solitarius (nucleus of the solitary tract)
Type of neurons in the nucleus tractus solitarius
Second-order sensory neurons, which receive input from visceral afferents.
Functions of the nucleus tractus solitarius
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Information received from the periphery is distributed to either:
1. Provide feedback to reflexes that control organ/tissue function
- Provide information to higher centres to drive more complex responses (coordinated behaviour: visceral, somatic, endocrine; emotion)
Central coordinator of autonomic output
Hypothalamus
Examples of AND reflexes that don’t require the CNS
Enteric nervous system function.
What often accompanies sympathetic NS activation?
Adrenal medulla activation (sympathoadrenal system).
Why is sympathetic NS able to stimulate a large number of targets quickly?
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• Catecholamine hormones spread through the
bloodstream
• Each preganglionic neuron activates many
postganglionic neurons
Situations where sympathetic mass activation is necessary
Extreme, ‘fight or flight’ situations
Situations where subtle sympathetic control are necessary
Adjustments of blood pressure (postural), small temperature changes, reproductive, voiding
Examples of parasympathetic functions 1 2 3 4
- Decreased heart rate
- Decreased contractile force of heart
- Increased gut motility
- Constriction of airways
Why is the ability of the parasympathetic NS to ‘mass activate’ limited, compared to the sympathetic?
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- Much lower ratio of pre:postganglionic neurons
- No equivalent of adrenal gland, ie no circulating Ach to
affect all organs at once. - Therefore coordination has to occur by brain
mechanisms
Are the sympathetic and parasympathetic NSs often antagonistic?
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• There are very few cases where the two systems are
genuinely antagonistic at the cellular level.
• More commonly, actions are on different tissues to
cause different type of actions (synergistic but not
necessarily opposite).
• Some tissues only have one type of innervation.
Examples where sympathetic and parasympathetic NSs are genuinely antagonistic at the cellular level
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1) Heart rate (S increases heart rate, P decreases, acting on SA node)
2) Airways (S relaxes, P constricts)
Examples of functional antagonism between S and P
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Sympathetic
contract pupil dilator muscle
contract base of bladder (continence)
Parasympathetic
contract pupil sphincter muscle
contract body of bladder (voiding)
Examples of different but not opposite actions of S and P
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Sympathetic
serous salivary secretion - only S innervation
constriction of abdominal vasculature - only S innervation
lipolysis
—
Parasympathetic mucous salivary secretion -- -- Tear secretion (lacrimal glands) - only P innervation
Electroceuticals
Implanting an electrical device in the body which modulates nervous function
