Lecture 13: The Autonomic System Flashcards
What spinal fibres contain parasympathetic fibres?
S2-S4
What is contained in the lateral grey horn?
Cell bodies of visceral motor afferents
What is referred pain?
Pain from visceral organs often perceived as regions of the body surface innervated by the same spinal nerves. E.g. in an MI, pain is referred to the upper chest and left arm.
Give functional and anatomical differences between the PNS and SNS.
PNS:
- Craniosacral outflow
- Ganglia are near the effect organ so there is a long preganglionic neurone and a short post ganglionic neurone
- Has a localised effect
SNS:
- Thoracolumbar outflow
- Ganglia are extended through the sympathetic cain and prevertebral ganglia - so there is a short preganglionic neurone and a long post ganglionic neurone
- Widespread effect
What neurotransmitter is used in the PNS?
ACh
What cranial nerves contain PNS fibres?
- CN III - to the pupil
- CN VII - to the salivary and lacrimal gland
- CN IX - to the parotid gland
- CN X - to the cardiac, pulmonary and digestive system
What spinal nerves contain SNS fibres? What neurotransmitter do they use?
T1-L2
- Preganglionic neurone sends its axon towards the peripherally carrying Acetylcholine (ACh).
- Postganglionic neurotransmitter is noradrenaline/norepinephrine (except sweat glands)
How does the SNS create a widespread effect?
- 1 preganglionic neurone that can innervate many post ganglionic neurones
- The other reason preganglionic sympathetic fibres travel directly to the adrenal medulla. They synapse directly with the adrenal medulla. The cells of the adrenal medulla are analogous to post-ganglionic cells (modified postganglionic cells). The cells release norepinephrine and epinephrine into the bloodstream. We then get widespread release into the blood stream exerting a widespread effect.
What is the sympathetic chain?
Interconnected chain of autonomic ganglia located on either side of the entire vertebral column (paravertebral). It is the site of synapse between sympathetic pre and post ganglionic neurons that permit pre ganglionic fibres ascend or descend beyond T1-L2.
What are the outflow pathways out of the sympathetic chain?
- The preganglionic neurone leaves the cord and synapses near the level of entry at a paravertebral ganglion with the post-ganglionic neurone. A post ganglionic neurone from here will travel to its target organs - mainly to organs in the thorax and glands and smooth muscle near the body wall
2. The preganglionic neurone leaves the cord and bypasses the sympathetic ganglia. The preganglionic neurone synapses at a collateral ganglia (prevertebral ganglia). The post-ganglionic neurone form here will go to a specific organ. This tends to be organs in the abdomen or the pelvis.
3. The preganglionic neurone can leave the cord - either really close to T1 or L2 and It ascends/descend a couple of levels and synapses with another ganglia in the sympathetic chain closes to the target organs e.g. to the head and neck will use a ganglia in the chain higher up and sending a post-ganglionic neurone to the organs in the head and neck. This is the same if it were going to the pelvis. It would descend to find a ganglia lower down.
What is the grey and white rams communicans?
White rami communicantes carry presynaptic sympathetic fibers from the ventral primary rami to the sympathetic trunk and exist only between spinal cord segments T1 and L2. Myelinated fibres.
Gray rami communicantes carry postsynaptic sympathetic fibers from the sympathetic trunk to all spinal nerves. It is non-myelinated.
What is the ANS control over heart rate and respiratory volume?
Sympathetic fibres, preganglionic fibres leave the chord from T1-L2. They meet a postganglionic neurone at different levels of the sympathetic chain. From here sympathetic fibres travel in cardiac nerves/plexus. There are long post ganglionic neurones. They will act to stimulate heart contraction and induce bronchodilation (leaving via pulmonary plexus’).
The parasympathetic effects: the chief parasympathetic supply to these organs is via the vagus nerve.
What is the ANS control over micturtion?
- Somatic innervation forms the default state. In the adult, the default state is that the external urethral sphincter is under constant contract by the pudendal nerve. It prevents voiding.
- When we want to void, stretch receptors in the bladder sense the level of distension of the muscle. When the level of urine reaches a certain level, the impulses from the sensory fibres increase. This leads to a reflex arc. If we wish to urinate the parasympathetic nervous stem takes over.
What is the ANS control over defection?
Sympathetic input leaves form the lumbar cord. They bypass the sympatric trunk to meet the inferior mesenteric ganglion (Collateral ganglion). There are post-ganglionic neurones to supply the distal colon and rectum. During situation of stress we prevent peristalsis and emptying of the rectum.
The preganglionic sympathetic fibres leave from S2,3 and 4. The pudendal nerve gives the default voluntary state. It allows contraction of the external anal sphincter to prevent defecation. We can have a voluntary inhibition of the contraction of the external anal sphincter. The parasympathetic system will then take over induce peristalsis and emptying of the rectum. We have stretch receptors in the rectum that sense the level of distension by increased level of stool. This voluntary control is again not present in infants.
How do injuries to the SNS present?
- Injury to sympathetic supply to head and neck (sympathetic trunk in neck or upper thoracic segments) e.g. sympathetic trunk at the level of the cervical spinal cord or upper thoracic cord
→ Horner syndrome - characterised by miosis, ptosis, anhidrosis and redness
- Injury to cord or cauda equina
→Disrupt sympathetic control to bladder
