Week 8 the autonomic nervous system Flashcards
Autonomic nervous system
->Maintains internal environment
->Mostly involuntary
->Controls visceral functions
- circulation
- digestion
- excretion
->Modulates endocrine function
ANS Input (afferent)
Autonomic nervous system: afferent
Sensory neurones from peripheral organs to centres in e.g., hypothalamus, medulla
ANS Output (efferent)
->Sympathetic or Parasympathetic neurons
- Organs innervated by both (often)
- Generally opposing actions
May be Linked by reflex arcs
(afferent and efferent)
Outputs: What do ANS nerves control
Mainly smooth muscle, cardiac muscle and secretory glands
Divisions of the ANS
-Sympathetic
-Parasympathetic
Sympathetic
->’Fight or flight’
->Short term survival
->Generally increases energy availability/ capacity and usage e.g.,
– Increase heart rate
– Increase lung capacity
– Increase blood flow to some skeletal muscles
– Increase blood glucose
Parasympathetic
->‘Rest and digest’
->Long term survival
->Generally reduces energy availability/capacity and usage e.g.,
– Decrease heart rate
– Decrease lung capacity
– Increase blood flow to digestive system
– Increase fat/glycogen stores
Homeostasis
->SNS and PNS are active
constantly - modulating
DYNAMIC BALANCE
Autonomic nervous system functions
->Each organ controlled by
branches of both
parasympathetic and
sympathetic NS
->Stimulation of the
parasympathetic NS has
broadly opposite effect to stimulation of the sympathetic in that organ (and vice versa)
ANS neurons vs motor (somatic NS) neurons
-Neurons bundle together to form nerves
Anatomical differences in PNS and SNS
->Where nerves originate (leave CNS)
->Where the ganglia (cell
bodies) are, i.e., where 2
neurons synapse
->Differences in
neurotransmitters and
receptor subtypes
SNS and PNS
Each organ receives input from both the parasympathetic and the sympathetic nervous system -> Each organ is controlled by both divisions but each division has broadly opposite effects
Arrangement of PNS and SNS neurons
PNS is made up of spinal nerves that leave the spinal cord between the vertebra
SNS is made up of cranial nerves that leave the CNS below the base of the brain and the base of the spinal cord (sacral region)
Anatomical similarities in PNS and SNS
Consist of two neurons that exist in series
Autonomic ganglion
Where neurons synapse, lots of neurons synapse in this place
They leave the CNS at their relevant location, enter the periphery and synapse onto a second neuron, the point where they synapse is called an autonomic ganglion -> where the second neuron ends it will synapse onto an effector organ where you will see the effect
->1st neuron is called a preganglionic neuron (myelinated)
->2nd neuron is called a postganglionic neuron (unmyelinated)
Cranial nerves
10 cranial nerves -> one of these is the vagus nerve, critial parasympathetic nerve, cranial nerve 10, carries most of the parasympathetic messages to the body and controls most of our main organs
Neurotransmitters involved in SNS
1st neuron releases acetylcholine onto the 2nd neuron within the sutonomic ganglion
When acetylcholine is released it binds to receptors on the post synaptic membrane and stimulate the second neuron -> it stimulates the second neuron to release noradrenaline (norepinephrine) onto the smooth muscle (effector) to cause a contraction
Neurotransmitters involved in PNS
1st neuron releases acetylcholine onto the 2nd neuron, the 2nd neuron also releases acetylcholine
Adrenal gland exception
The adrenal gland in the SNS, exception to the neurotransmitters released by SNS neurons
1st neuron will release acetylcholine onto the adrenal gland but it stimulates the adrenal gland to release a epinephrine and norepinephrine which will travel in blood vessels and have an effect on the effector organ
Adrenal gland
Adrenal gland located: On top of kidneys
-Adrenal medulla (inside part) is a major organ of the sympathetic nervous system
-Synapse/ganglia in gland
-Can cause body-wide release
of epinephrine / adrenaline
and norepinephrine/
noradrenaline in an extreme
emergency (adrenaline “rush” or surge)
Receptors
Protein structures that receive and transduce signals
-Include ligand gated ion channel-linked (ionotropic) receptors
-G protein coupled (metabotropic) receptors
Ionotropic receptors
Binding site and channel
combined, have a binding site and a channel down the middle of the receptor that are combined, no downstream signalling molecules (no 2nd messenger) so you get a rapid response
Metabotropic receptors
GPCR
-Binding site not combined
with a channel, don’t have an obvious channel that ions can travel through associated with them but they do have a secondary messenger system
-2nd messenger/GPCR
-Slower response
May have another channel opening
PNS overview
Preganglionic nerves - cholinergic
Postganglionic nerves - cholinergic
Effector Neurotransmitter = ACETYLCHOLINE
Two types of acetylcholine receptors; (distrabuted differently in the ANS)
->Nicotinic
->Muscarinic
Nicotinic
Ionotropic ligand gated channel
-5 subunits
-2 ACh binding sites
-(Skeletal muscle), autonomic
ganglia, adrenal medulla
Muscarinic
GPCR
3 subtypes;
– M1 - brain, parietal cells (excitatory)
– M2 – heart (inhibitory)
– M3 - smooth muscle, glands,
(excitatory)
Post-synaptic receptors (targets) -> PNS
1st neuron releases acetylcholine onto nicotinic receptors
Nicotinic receptors on the dendrites of the second neuron in series
In the PNS, the receptor that sits on the smooth muscle of the effector, is a muscarinic receptor
Nicotinic receptor in the autonomic ganglion and a muscarinic receptor on the effector tissue
SNS overview
Preganglionic nerves - cholinergic
Postganglionic nerves - adrenergic
Effector Neurotransmitter = NORADRENALINE (NOREPINEPHRINE)
Post-synaptic receptors (targets) -> SNS
1st neuron releases acetylcholine onto 2nd neuron, we have a nicotinic receptor in the autonomic ganglion
On the smooth muscle we have adrenoceptors receptors -> respond to noradrenaline and adrenaline –» important for drug specificity, agonists can be designed to bind to specific receptors
Drug specificity
If you have a drug that binds to a nicotinic cholinergic receptor, you will either block or stimulate the sympathetic nervous system and the parasympathetic nervous system to the same extent -> problematic as if you stimulate both you would have no effect
Adrenoceptors
G-protein coupled receptors
(metabotropic)
-2 types: alpha (alpha 1 and alpha 2) and beta (beta 1 and beta 2)
-Respond to adrenaline,
noradenaline
-Have diverse actions depending on the target tissue, that depends on the downstream molecules that these receptors stimulate
-> Some of these receptors are inhibitory and some of these receptors are excitatory
Diverse actions of adrenoceptors
e.g., Some of these receptors have conflicting effects despite being SNS, for example the SNS overall causes the bronchi to dilate, but there are two types of adrenoceptors, alpha 1 and beta 2 receptors -> alpha causing constriction and beta causing dilation - most of the adrenoceptors are beta 2 receptors, so the overall effect on the bronchi by the SNS is dilation (there are some alpha 1 receptors)
PNS vs SNS
Organs have inputs from both PNS and SNS -> broadly opposite effects
Individual cells generally have inputs from both PNS and SNS
Autonomic: smooth muscle neuro muscular junction
Controls;
-Vascular smooth muscle
-Bronchial smooth muscle
-Intestinal peristalsis
Specialised neuro muscular junction
Where the second neuron of the ANS makes contact with the smooth muscle
Axons end with a series of swollen areas called varicosity contains vesicles filled with neurotransmitter -> release neurotransmitter
