Functioning of the ANS Flashcards
what are the sensory components of the autonomic NS?
Receptors:
- Viscera (internal organs of the abdominal, thoracic, and pelvic cavities) have sensory receptors
- NOT consciously perceived (u can’t sense them)
- They are responsible for internal monitoring
E.g. Baroreceptors (mechanoreceptors) allow us to monitor BP. If blood pressure falls outside normal range, the sensory receptors are activated and bring about changes via a reflex action to allow BP to enter normal range again. (w/o us being consciously aware of it)
Somatosensory receptors:
Monitor external environment
E.g. vision, hearing, touch and detection of joints and limbs
Both receptors carry sensory information to the CNS which then produces an autonomic/somatic response
how does the somatic & autonomic NS interact to respond to sensory info?
There is significant somatic and autonomic integration e.g. a single piece of sensory information can produce both somatic and autonomic outputs.
example of sensory info that influences both autonomic and somatic efferent activity:
Focusing the eyes on a near object:
ANS - pupil/lens adjustments
Somatic - eye movements to look at object
Response to cold:
ANS – vasoconstriction of blood vessels the skin to maintain heat
Somatic – shivering, rhythmic contraction of skeletal muscle.
what differences do the autonomic motor components have compared to somatic?
Differences compared to somatic motor components:
- The effectors innervated
- Number of neurones between CNS and effector
- Nature of Neurotransmitter release
- Neurotransmitters released
what are the differences in the effectors innervated in somatic and autonomic?
somatic - skeletal muscle
autonomic - cardiac and smooth muscle
what are the differences in number of neurones between CNS and effectors in somatic and autonomic?
For somatic NS an uninterrupted axon is sent from the cell body in CNS to the effector (direct)
Therefore, motor units can bring about concise and precise control of motor fibres in skeletal muscle
For ANS there is a 2-neuron chain between CNS and effector tissue
Preganglionic neurones may branch to become in contact with multiple post ganglionic neurones.
results in the widespread activation of multiple postganglionic neurones and therefore activation of many different types of cells at once (from one pre- ganglionic neurone) e.g. smooth and cardiac muscle or glandular cells within the target tissue.
This allows for synchronised but widespread activity within effector
what is the difference in the nature of neurotransmitter release in somatic and autonomic? (somatic)
SOMATIC NS:
There is a NMJ.
DISCRETE release of NT which innervates skeletal muscles fibres.
This is because we can control what individual muscle fibres are doing through the activation of the specific motor neurones that supply those muscle fibres
This allows for precise coordination of control over a small area.
what is the difference in the nature of neurotransmitter release in somatic and autonomic? (autonomic)
AUTONOMIC:
No NMJ.
DIFFUSE release of neurotransmitter
This is done over a large area via a branching network created by the axon of the post ganglionic neurone.
The axon has periodic swelling (varicosities). This is where the neurotransmitter is contained in vesicles and when stimulated will be released. (arrangement can be described as beads on a string)
This results in NT release at multiple sites e.g. lots of smooth muscle cells.
This has strong functional implications.
- It means a small number of postganglionic neurones can influence a large number of effector cells e.g. a large number of smooth muscle cells
- This allows for high level of coordination between the smooth muscle cells.
- This means in an organ (e.g. smooth muscle cells in a blood vessel) we can organise the change in activity through activation of a small number of post ganglionic neurones.
what is the difference in the neurotransmitter released in somatic and autonomic?
Somatic neurones:
Ach is always produced at the NMJ
Autonomic neurones:
Sympathetic:
Pre-ganglionic – ACh –> nAChR
Post-ganglionic - noradrenaline
Parasympathetic
Pre-ganglionic – ACh –> nAChR
Post-ganglionic – ACh –> mAChR
what would an implication for drug action be if we wanted to target a sympaNS and paraNS at the same time?
If we wanted to target both sympathetic and parasympathetic at the same time, we would use a drug that altered cholinergic transmission.
If we just wanted to alter activity in the sympathetic NS, we would use a drug targeting noradrenaline in noradrenergic transmission
what is an example of an exception that doesn’t follow the usual pattern of neurotransmitters released?
whilst noradrenaline is the most common NT to be released by the post ganglionic neurone in SympaNS it is not the only one.
Sweat glands:
- Have a 2-neurone chain and are part of the sympathetic division.
- However, both ganglia produce the neurotransmitter ACh.
- Are classified as sympathetic overall due to their embryological origins despite parasympathetic neurological appearance
why is the adrenal medulla considered an endocrine cell?
because they release adrenaline and some noradrenaline into the blood.
therefore, the adrenal medulla is described as a neuroendocrine organ.
what is another exception that doesn’t follow the usual pattern of neurotransmitters released? (adrenal medulla)
The chromaffin cells of the adrenal medulla receive input by a sympathetic preganglionic neurone.
Ach is released to act on the adrenal medulla cells.
Functional consequence:
- Adrenaline and Noradrenaline released is able to activate non-innervated receptors called adrenergic receptors.
- These can be distant from nerve terminals (this is why they are described as non-innervated)
- This means these receptors are activated by circulating NA and A.
- These receptors are located in smooth muscle of bronchioles and some blood vessels
What do the SNS and PNS do?
Have a major role in homeostasis, vision and reproduction
SympaNS is responsible for when we are alert and active (not only active during fight or flight!)
- It is active in maintaining homeostasis
- It is active when there is an emergency:
E.g. defence response - fight/fright/flight
ParaNS is responsible for rest and digest
- Involved in energy conservation when we are sleeping or relaxing
- Involved in gaining energy through digestion
- Involved in emptying (bowel and bladder)
how does the paraNS and sympNS work to carry out action?
MOST organs
Have dual innervation – both sympathetic and parasympathetic innervation
Actions are antagonistic - actions of both autonomic divisions oppose each other –> leads to control via autonomic tone
Example:
Heart: SNS: ↑ heart rate PNS: ↓ heart rate
[Therefore heart rate determined by a combination of the sympathetic and parasympathetic activity]
GIT: SNS: ↓ motility PNS: ↑ motility
[motility increased or decreased due to action of either NS on the smooth muscle cells of the GI tract wall]
give an example of dual innervation where its antagonistic but you have different muscles controlling each action?
antagonistic BUT different muscles
e.g. smooth muscle surrounding pupil of the eye
The smooth muscle is in the iris and determines pupil diameter.
These smooth muscle cells are arranged in 2 bands – the inner and outer band:
- outer band of muscle = constrictor (circular muscle)
- inner band = dilator (radial muscle)
They are different due to the orientation of the cells which allows them to have different effects when they contract.
PUPIL CONSTRICTION:
- PNS innervates circular muscle (sphincter pupillae)
- causes it to contract leading to constriction of pupil.
PUPIL DILATION:
- SNS innervates Radial Muscle (dilator pupillae)
- causes it to contract leading to the pupil being pulled open (dilation)
Therefore, pupil diameter is regulated by a combined action by the SNS and PNS.
how are MOST blood vessels controlled?
MOST (not all) BV have single innervation - SNS only
Increase in SNS activity –> vasoconstriction
Decrease in SNS activity –> vasodilation
SNS is always on, only the degree of input varies.
This is in contrast with somatic nervous system which is more of an on off switch (skeletal muscle is either contracting or relaxing).
explain dual innervation that is present in the exocrine glands
Present in exocrine glands in the head and neck e.g. salivary glands
NOT antagonistic
SNS input to salivary gland promotes production of saliva and SNS input to blood vessel (in head or neck) causes vasoconstriction.
This reduces blood flow to the salivary gland.
This means less raw product (e.g. fluid, ions etc) are delivered to the salivary gland.
This results in production of low volume sticky, viscous saliva.
The SNS is associated with alertness so we do not need much saliva. This is why people have a dry mouth when they are anxious e.g. before performance. Their mouth is not dry, it is just less wet with thick and sticky salvia.
The PNS > SNS effect on the salivary gland when you are at rest.
PNS input to salivary gland also promotes production of saliva but PNS input to the blood vessels (in head or neck) causes vasodilation.
This means more raw material goes to salivary gland and thus a large volume of watery saliva is produced.
PNS is associated with rest and digest so this makes sense. As the saliva begins digestion process and lubricates the food as it begins journey though GI tract.
what is the mechanism in which blood vessel diameter is regulated?
E.g. blood vessels inside the skeletal muscle and inside the cardiac muscle of the heart.
SNS body-wide activation can lead to vasodilation due to influence of adrenaline
This contradicts that SNS activity also leads to vasoconstriction
Mechanism:
Most blood vessels:
Increase in body wide SNS activity causes innervation of the blood vessels of the skin.
This causes NA to be released at SNS terminals.
NA works on alpha receptors (sub-type of androgenic receptors).
When noradrenaline acts on these receptors it causes smooth muscle to constrict so causes vasoconstriction.
However, increase in body wide SNS activity will cause innervation of adrenal medulla causing adrenaline to be released into blood stream.
Adrenaline binds to beta receptors (sub-type of androgenic receptor).
When beta receptors are activated on smooth muscle it causes relaxation of smooth muscle causing vasodilation.
So, there is a balance of noradrenaline mediated vasoconstriction and adrenaline mediated vasodilation.
Most blood vessels primarily express alpha receptors and hence vasoconstrict
However, the blood vessels within the skeletal muscle or within the cardiac muscle express high density of beta receptors so they will vasodilate.
All of this works in combination, simultaneously.
give a summary of the different actions of the SNS and PNS on various organs
