Module 2 Section 3 (Sympathetic Nervous System) Flashcards
What is the sympathetic nervous system?
The sympathetic nervous system is a second division of the autonomic nervous system, alongside the parasympathetic nervous system.
In response to a perceived threat, the body specifically activates the sympathetic nervous system and prepares for fight or flight.
What occurs when the SNS is activated?
- Increased heart rate
- Increased blood pressure
- Increased blood supply to the tissues
- Increased rate of cell metabolism
- Increased blood glucose
Where do the neurons of the SNS originate?
The neurons of the SNS originate from the thoracic and lumbar regions of the spinal cord. These two sections constitute the middle portions of the spinal cord.
Describe the structure of the preganglionic neurons of the SNS. What do the fibres release?
The sympathetic nervous system has short preganglionic neurons that release ACh at the ganglia.
What happens when ACh binds to and activated NN receptors?
The acetylcholine binds to and activates NN receptors at the sympathetic ganglia, conducting the signal to the long postganglionic neurons of the sympathetic nervous system.
Describe the structure of the postganglionic neurons of the SNS. What do the fibres release?
The sympathetic postganglionic neurons predominantly release norepinephrine at the target organ, which binds to alpha (α) or beta (β) receptors.
- Exceptions to this rule are the sympathetic postganglionic neurons that innervate sweat glands and renal vascular smooth muscle, as they release acetylcholine (binds to M receptors) and dopamine (binds to D receptors) respectively.
List the characteristics and neurotransmitters relating to sympathetic regulation.
Preganglionic fibre
- SNS:
Postganglionic fibre
- SNS:
Neurotransmitters released from preganglionic fibre
- SNS:
Neurotransmitters released from postganglionic fibre
- SNS:
Fibre type
- SNS:
Preganglionic fibre
- SNS: short
Postganglionic fibre
- SNS: long
Neurotransmitters released from preganglionic fibre
- SNS: ACh
Neurotransmitters released from postganglionic fibre
- SNS: NE (mostly) and Epi
Fibre type
- SNS: adrenergic (G-protein coupled receptors)
What are the autonomic nerve origins for the SNS?
SNS
- Fibres originate: thoracid and lumbar regions of the spinal cord
- Preganglionic fibres = short and end in ganglia located in chains down both sides of the spinal cord. Some, however, end in ganglia that’s located halfway b/w the CNS and effector organ
- Postganglionic fibres = long and end on the effector organs.
What organs are influenced by the SNS?
- Sweat glands
- Cardiac muscle
- Smooth muscle
- Nerve terminals
- Gland cells
- Adrenal medulla
What is the role of the adrenal medulla?
The adrenal medulla is a specialized organ that essentially functions as a sympathetic autonomic ganglion.
The adrenal medulla is the centre portion of the adrenal gland, surrounded by the adrenal cortex.
What is the adrenal medulla innervated by? What occurs when it is activated?
It is innervated by sympathetic preganglionic fibres.
When these sympathetic preganglionic fibres are activated, they release acetylcholine, which binds to NN receptors on the adrenal medulla. Activation of these NN receptors results in the release of predominantly epinephrine (also known as adrenaline; 80%), but also norepinephrine (20%) from the adrenal medulla.
- The released Epi and NE travel through the blood and interact with α and β receptors throughout the body, causing varying effects.
- Since the neurotransmitters released by the adrenal medulla act via the circulation at distant sites, they are considered to act like hormones.
True or false: stress can stimulate the adrenal medulla, releasing epinephrine and norepinephrine, leading to a prolonged sympathetic effect.
True
Which one of the following effects listed is mediated by increased activity of the SNS?
a) Inhibition of the adrenal glands
b) Increased digestion
c) Increased heart rate
d) Constriction of the pupils
c) Increased heart rate
What are the subtypes of alpha receptors? What occurs when they’re activated?
α1 receptors are located post-synaptically, predominantly on smooth muscle (e.g. blood vessels, GI muscle, and uterus).
- Activation of α1 receptors predominantly leads to contraction of the muscle.
- Multiple subtypes of α1receptors exist which are organ selective and drugs can be designed to target a specific subclass of α1 receptors, allowing for drug selectivity
α2 receptors are located post-synaptically on smooth muscle as well as pre-synaptically (auto-receptors) on the neuronal membrane.
- The effects listed for activation of α1receptors also apply to activation of α2 receptors that are located post-synaptically.
- Activation of α2 autoreceptors, however, leads to a decrease in the release of NE from the presynaptic nerve, thereby decreasing sympathetic activation. These receptors are only targeted in specific situations.
What are the subtypes of beta receptors? What occurs when they’re activated?
β1 receptors are found predominantly in the heart and GI muscle, and when activated, increase the force and rate of contraction of the heart, and relax GI smooth muscle.
β2 receptors are found in the lungs, blood vessels, GI muscle, and uterus. Activation of these receptors leads to muscle relaxation.
Which receptor corresponds to the primary locations listed?
a) Pre-synpatic neuronal membrane
b) Heart and gastrointestinal muscle
c) Lungs, blood vessels, gastrointestinal muscle, and uterus
a) Pre-synpatic neuronal membrane = a2
b) Heart and gastrointestinal muscle = β1
c) Lungs, blood vessels, gastrointestinal muscle, and uterus = β2
Which receptor corresponds to the responses listed?
a) Relaxes smooth muscle
b) Contracts smooth muscle in the vessels and uterus and decreases motility and tone in the gastrointestinal tract
c) Increases heart rate and force of contraction
d) Decreases the release of norepinephrine from the presynaptic neuron
a) Relaxes smooth muscle = β2
b) Contracts smooth muscle in the vessels and uterus and decreases motility and tone in the gastrointestinal tract = a1
c) Increases heart rate and force of contraction = β1
d) Decreases the release of norepinephrine from the presynaptic neuron = a2
True or false: Norepinephrine binds to α1or β receptors in the postsynaptic membrane to exert its action.
True
How is the activity of NE terminated?
The activity of norepinephrine is terminated by reuptake back into the presynaptic neuron followed by enzyme degradation within the neuron.
Drugs acting directly or indirectly on the SNS can mimic the actions of NE, thereby appearing to increase the activity of the sympathetic nervous system. What are the 3 mechanisms that allow this to occur? Provide examples.
1) Direct stimulation: the drug binds directly to the receptor and produces an effect.
- Ex: epinephrine, which works by directly binding to alpha and beta receptors.
2) Indirect stimulation: the drug increases the release of NE from the presynaptic neuron.
- Ex: Amphetamines act in this way.
3) Combination of the two: the drug binds directly to the receptor AND increases release of NE. Drugs can stimulate one or both of alpha and beta receptors.
- Ex: the drug ephedrine which is used as a decongestant.
True or false: many drugs that activate the sympathetic nervous system target more than one receptor subtype.
True
True or false: clinically, drugs that activate processes normally controlled by the SNS are primarily used for their effects on the heart, blood pressure, bronchial tree, and nasal passages.
True
These drugs produce similar effects to the anticholinergics, but due to the receptor subtypes (α1, β1,β2), the actions of these drugs are more specific and predictable depending on which receptor subtype is activated.
What are the clinical indications for adrenergic drugs? (5)
Anaphylaxis:
- The treatment for these severe allergic reactions isepinephrine (e.g. EpiPen), which causes bronchodilation and constriction of blood vessels, relieving the symptoms.
- Epinephrine will also increase heart rate and the force of contraction of the heart, thereby increasing blood pressure if the person happens to go into shock.
Cardiac applications:
- Adrenergic drugs, such as epinephrine, increase heart rate and the force of contraction of the heart by activating β receptors.
- These drugs can be used for temporary emergency management of complete heart block or cardiac arrest.
Nasal congestion:
- α1 agonists constrict blood vessels and are therefore useful in the treatment of nasal congestion.
- Ex: Phenylephrine and pseudoephedrine
Ophthalmic:
- When given in the eye, α1 agonists such as phenylephrine dilate the pupil, facilitating retinal examination.
Pulmonary:
- β2-selective drugs, such as salbutamol, produce bronchodilation, which is an effective treatment for someone suffering from asthma.
- Both short-acting and long-acting β2 agonists are available. These drugs can also be used to treat chronic obstructive pulmonary disease (COPD).
Which one of the conditions listed is the correct clinical indication for a beta 2 receptor agonist?
a) Congestive heart failure
b) Asthma
c) Nasal congestion
d) Benign prostatic hyperplasia
b) Asthma