08-02-22 - Structure of the ANS Flashcards
Learning outcomes
- Identify the two main divisions of the ANS and their relationship to the rest of the nervous system.
- Characterise each branch of the ANS by their basic anatomical location, pathways and targets organs.
- Know the general physiological consequences of sympathetic or parasympathetic innervation.
- Propose the effects of stimulating or antagonising each branch of the ANS based on their generalised functions.
- Recognise the two-neuron system of innervation and the main neurotransmitters involved in both branches of the ANS.
- To know that drugs which affect the synthesis, storage, release and termination of acetylcholine effects at the skeletal neuromuscular junction also affect the ANS.
Describe the 3 steps in the function of a somatic systems.
Describe the steps 3 in the function of autonomic (visceral) systems
• Steps in the function of somatic systems
1) Upper motor neuron descends from motor cortex of brain
2) It synapses to lower motor neuron int eh brain stem or spinal cord
3) Lower motor neuron goes on to have its effect on skeletal muscle
• Steps in the function of autonomic (visceral) systems
1) Hypothalamus sends neurons down to nuclei in the brainstem or spinal cord
2) From the spinal cord, there is a pre-ganglionic neuron, that synapses in a ganglion
3) A post-synaptic ganglion then goes to an effector
Where do pre-ganglionic fibres of the ANS originate from?
Where do post-ganglionic fibres from the ANS originate from?
What is the length of pre and post ganglionic fibres in the Sympathetic and Parasympathetic nervous system?
Where do these fibres originate?
- Pre-ganglionic fibres originate from the CNS
- Post-ganglionic fibres of the ANS originate from autonomic ganglia
- The sympathetic nervous system has short pre-ganglionic and long post-ganglionic neurons
- The parasympathetic nervous system has long pre and short post-ganglionic neurons
- Sympathetic pre-ganglionic fibres have (leave CNS at) thoracolumbar origin
- Sympathetic post-ganglion fibres have paravertebral chain/prevertebral ganglia origin
- Parasympathetic pre-ganglionic fibres leave the CNS at the brainstem and sacral spinal cord (craniosacral origin)
- Parasympathetic post-ganglionic neurons origin in or near the target organ
What does the vagus nerve supply?
What does this include?
- The vagus nerve (cranial nerve 10) supplies pre-ganglionic innervation everything outside the neck, all the way down to 2/3rds of the way around the transverse colon
- This includes the heart, lungs and upper GI tract
What happens to the pre-ganglionic neurons of the sympathetic nervous system?
What happens at this point?
What structures connect ganglia to spinal nerves?
What can happen to nerves emerge that emerge anteriorly?
- Pre-ganglionic fibres of the sympathetic nervous system emerge, and very quickly enter the paravertebral chain of connected ganglion
- Here, they synapse with post-ganglionic fibres, which will go on to have their effects on viscera
- Rami communicans connect ganglia to spinal nerves
- Nerves that emerge anteriorly can go to synapse on pre-vertebral ganglia
Where does each sympathetic trunk extend as far as?
Where do they all fuse with each other?
What are the 3 different types of cervical ganglion?
What does the latter use?
- Each sympathetic trunk extends as far as the atlas
- They all fuse with each other in the ganglion impar, which is opposite the coccyx
- There are superior, middle, and inferior cervical ganglion
- The latter often fuses with the T1 ganglion to from the stellate ganglion, that lies on the neck of the fist rib
What are the options in what a pre-ganglionic sympathetic neuron does when it leaves the CNS?
• Options in what a pre-ganglionic sympathetic neuron does:
1) Go up and synapse
• It can leave the CNS and enter the sympathetic chain via white rami communicans
• It can then ascend to the sympathetic chain towards the cervical ganglia
• In cervical region, it can synapse and become a post-ganglionic neuron
2) Synapse at the same level
• A pre-synaptic neuron can leave the CNS and go into the synaptic chain and synapse at the level it comes out
• E.g Comes out in T2 of the spinal cord and synapses in T2 ganglion, and the post-synaptic neuron will pass back into the spinal nerve, then off into its distribution of T2 (around the upper thorax and armpit)
• There are cardiopulmonary splanchnic nerves controlling the heart and lungs
• These tend to come out in the cervical chain, an come down outside of its chain
3) Go down and synapse
• Enters the synaptic chain, and instead of synapsing, we descend to a lower level
• E.g coming out of T12, descending through the sympathetic chain to synapse in sympathetic ganglion around L4, then traveling back out of the L4 neuron to supply sweat glands
4) Pass through without synapsing
• Pre-synaptic neuron leaves the CNS, and enters the sympathetic chain, but doesn’t synapse
• Instead, we pass straight out of he front of the sympathetic ganglion in a thoracic splanchnic nerve
• We then come down to the abdomen, where we synapse onto a preaortic or prevertebral ganglion
• The post-ganglionic neuron then goes out to supply the gut tube
How do autonomic visceral reflexes occur?
What are 2 examples of this?
Why is it called visceral?
How do autonomic motor reflexes occur?
What is an example of this?
- Autonomic visceral reflexes occur when visceral afferents travel back to the CNS carrying information in order to initiate a response e.g heart rate and blood pH
- It is called visceral, as it unconscious
- In autonomic motor reflexes, neurons will follow back along their path of parasympathetic innervation
- E.g upper part of gut tube, sensory afferents will travel back along the same direction the vagus nerve came in
What do visceral pain afferents follow?
What do visceral reflex afferents follow?
When does referred pain occur?
What is referred pain?
What is an example of this?
- Visceral pain afferents tend to follow the path of sympathetic supply
- Visceral reflex afferents follow the path of parasympathetic supply
- Referred pain occurs when we get visceral pain heading back to the CNS
- Referred pain is when our visceral pain afferent synapses onto the same second order sensor neurone as a somatic afferent
- This results in pain being felt in the skin e.g visceral pain being felt as somatic pain in the skin due to referred pain
- An example of referred pain is when heart pain can be felt in he skin down the neck/left arm
What is parasympathetic and sympathetic nervous supply for?
What do they each innervate?
- Parasympathetic nervous supply is for rest and digest
* Sympathetic nervous supply is for fight or flight
What effects do parasympathetic and sympathetic innervation have on the following tissues: • Heart • Digestive tract • Lungs • Urinary Bladder • Pupil of the eye • Male sexual organs • Blood vessels • Salivary glands • Sweat glands • Liver • Pilomotor
What is the sympathetic and parasympathetic innervation in the eye?
What are regions responsible for drainage in the eye?
How can drainage be stopped?
How can this be corrected?
- The sympathetic nervous system innervates the pupillary dilator muscle in the iris
- When this muscle constricts, It causes mydriasis (opening of the pupil)
- The parasympathetic nervous system innervates the pupillary constrict muscle in the iris
- When this muscle contracts, it causes miosis (closing of the pupil)
- Canals of Schlemm are responsible for drainage in the eye
- Drainage can be stopped by overreactive dilator muscles
- To treat this, stimulating the eye with sympathetic antagonists can prevent the problem
What part of the ANS predominates at different levels of fluid in the bladder?
- When the bladder is filling, sympathetic control predominates
- Relaxation of detrusor muscle
- Contraction of internal sphincter muscle
- When the bladder is full, parasympathetic control predominates
- Contraction of detrusor muscle
- Relaxation of internal sphincter muscle
What are 3 different principles of sympathetic and parasympathetic innervation?
What are the general effects of ANS innervation?
What do autonomic reflex arcs exist for?
How can referred pain be explained
• Sympathetic and parasympathetic innervation:
1) May oppose or compliment
2) May (rarely) have the same effect (e.g saliva)
3) Some tissues only innervated by one
- General effects of parasympathetic innervation:
- Parasympathetic – rest and digest
- Sympathetic – fight or flight
- Autonomic reflex arcs exist for visceral control
- Path of afferent fibres can explain referred pain
Where do varicosities exist in the ANS? What occurs here?
- Chain like networks of varicosities exist where neurotransmitters are released onto the surface of smooth muscle cells
- Here, there is synapse between post-ganglionic fibres and target tissues
Describe the 3 steps in the neuron network in the somatic efferent system?
What is unique about the stimulation of the adrenal medulla in the sympathetic system?
• In the somatic efferent system:
1) a single neuron extends from the CNS that synapses on a skeletal muscle fibre
2) This acts on a Nicotinic ACh receptor
3) This causes the contraction of the muscle
• Stimulation of the adrenal medulla is done by a long pre-ganglionic fibre that releases ACh, which stimulates release of adrenaline into circulation, which travels around the body and effects tissues
In the parasympathetic system:
1) Pre-ganglionic fibres release ACh as a NT
2) This binds to ACh receptor on the postganglionic cell, causing depolarisation and an action potential to go down the post-ganglionic fibre
3) Post-ganglionic fibres also release ACh as a NT onto the target organ, which acts on muscarinic ACh receptors
Where do pre-ganglionic fibres synapse on the adrenal medulla?
What do chromaffin cells release?
- On the adrenal medulla, sympathetic fibres synapse on chromaffin cells
- Chromaffin cells release adrenaline and noradrenaline into systemic circulation
