Module 3: The Autonomic Nervous System (Efferent Division)

What does the autonomic nervous system influence? What can it be further broken down into?

It influences the heart, smooth muscles, and glands. (also controls enteric nervous system, reproduction, and thermoregulation)

• does so through the sympathetic and parasympathetic systems → they’re essential in the maintenance of homeostasis and are constantly active to maintain a dynamic equilibrium

Where does the output of the autonomic nervous system come from? Where does it go?

hypothalamus, brainstem, and spinal cord. It’s sent to the periphery via the sympathetic and and parasympathetic pathways.

What is the sympathetic nervous system’s main role? What is the parasympathetic nervous system’s main role?

Sympathetic

• main role: stimulate fight or flight response

• constantly active

• adrenal medulla produces hormones epinephrine and norepinephrine

Parasympathetic:

• main role: body’s rest/digest activities (processes occurring when body’s at rest, such as digestion, urination or salivation)

Which of the following effects is mediated by the parasympathetic nervous system?

• stimulation of the digestive system

• bronchodilation

• increased blood flow to the lungs '

• secretion of epinephrine and norepinephrine by the adrenal glands

Which of the following effects is mediated by the sympathetic nervous system?

• pupil constriction

• inhibition of the adrenal glands

• increased heart rate

• decreased lung capacity

1. stimulation of digestive system

2. increased heart rate

The autonomic nerve pathways all involve a two neuron chain that connects the CNS to the effector. Describe this process.

Cell body of the 1st neuron is in the CNS.

• the preganglionic fibre (cell body’s axon) synapses with the cell body of the 2nd neuron.

Cell body of 2nd neuron is located in ganglion (cluster of neuronal cells)

• the postganglionic fibre (cell body’s axon) innervates (stimulates) the effector organ

Nerve fibres for sympathetic and parasympathetic systems have differing characteristics. For each state where the nerve fibres originate, their lengths and where they terminate.

Sympathetic:

• Origin

  • thoracic and lumbar regions of spinal cord

• Lengths

  • preganglionic = short

  • postganglionic = long

• Termination Location

  • preganglionic = ganglia on both sides of spinal cord

  • postganglionic = effector organs

Parasympathetic

• Origin

  • brain or lower spinal cord

• Lengths

  • preganglionic = long

  • postganglionic = short

• Termination Location

  • preganglionic = ganglia (terminal gagnlia) that are close to effector organ (why postganglionic are short)

  • postganglionic = effector organ

All preganglionic fibres use the neurotransmitter _________. However, the sympathetic and parasympathetic nervous systems have different neurotransmitters for postganglionic fibres. Name all of the neurotransmitters involved and what the associated fibres are now named.

All preganglionic use Acetycholine (ACh)

Sympathetic Postganglionic

• norepinephrine (NE) but some use epinephrine (Epi)

• fibres called adrenergic

Parasympathetic Postganglionic

• acetylcholine (ACh)

• fibres called cholinergic

Let’s Review! Fill in the values for both the Sympathetic and Parasympathetic Nervous System:

• preganglionic fibre

• postganglionic fibre

• neurotransmitter released from preganglionic fibre

• neurotransmitter released fro postganglionic

• fibre type

preganglionic fibre

• SNS → short

• PNS → long

postganglionic fibre

• SNS → long

• PNS → short

neurotransmitter released from preganglionic fibre

• SNS → ACh

• PNS →ACh

neurotransmitter released fro postganglionic

• SNS → NE, Epi

• PNS → ACh

fibre type

• SNS → adrenergic

• PNS → cholinergic

What is dual innervation? Is it applicable to all organs? How does this apply to afferent nerve traffic?

Almost all effector organs receive efferent input from both the sympathetic and parasympathetic nervous systems.

• some don’t have direct innervation from both, i.e., kidneys, adrenal gland

• most afferent nerve traffic from visceral organs/activities (ex. digestion, sweating, circulation) never reach consciousness and instead are regulated by autonomic efferent output.

When we talk about the autonomic innervation of organs, how do the sympathetic and parasympathetic systems contribute to regulation? Is this general rule applicable in every situation?

Sympathetic = excitatory, Parasympathetic = inhibitory.

• ex. sympathetic increases heart rate, parasympathetic decreases heart rate.

Not applicable in all cases

• ex. digestive system → sympathetic decreases gastric mobility and parasympathetic increases the gastric mobility

MAIN TAKEAWAY: both systems have opposite regulatory actions that allow for precise regulation of homeostatic parameters.

For the following effector organs, explain the effect that sympathetic stimulation and parasympathetic stimulation would have on them.

• heart, eye, digestive tract, blood vessels, lungs, urinary bladder

1. Heart

   • SNS → increased heart rate, increased force of contraction on whole heart

   • PNS → decreased heart rate, decreased force of contraction in the atria only

2. Eye

   • SNS → pupil dilation, adjusting eye for further vision

   • PNS → adjusting eyes for near vision

3. Digestive Tract (anomaly)

   • SNS → decreases motility, contracts sphincters to prevent the forward movement of waste, inhibits digestive secretion

   • PNS → increased motility, relax sphincters to allow forward movement of waste, stimulates digestive secretion

4. Blood Vessels

   • SNS → constriction

   • PNS → dilation of vessels supplying to the clitoris and penis

5. Lungs

   • SNS → dilation of bronchioles, inhibit mucous secretion

   • PNS→ constricts airways, stimulates mucous secretions

6. Urinary Bladder

   • SNS → relaxation

   • PNS → contraction (empty bladder)

At any given time, both the sympathetic and parasympathetic systems are active to some degree. Their relative contributions = __________. When one system gets more activated than its regular activity, what does the other system do>

Relative contributions = tone (sympathetic tone or parasympathetic tone).

• when one system gets more activated than its typical tonic activity, the other system tends to decrease their firing so that one system becomes the dominant influence on an organ.

There’s times when its advantageous to have a system be the dominant influence on an effector organ. When is it good to have sympathetic dominance? What about parasympathetic dominance?

Sympathetic dominance

• good in stressful/emergency situations

• ex. encountering a bear, the SNS without conscious thought, would take over and elevate the body’s preparedness and elevate our ability to run/fight

Parasympathetic dominance

• occurs during rest and actions opposite of the SNS

• ex. turning around and seeing that the bear’s gone, the PNS would take over and begin the relaxing process to help body recover from SNS effects (basically returning your body back to its homeostatic state).

If you encountered a bear in the woods, your sympathetic nervous system would become dominant. What would this outflow look like in your body? If you turn around and see that the bear is now gone, your parasympathetic nervous system would take over, What would this look like in your body?

• think heart, eye, digestive tract, blood vessels, urinary tract, lungs or other bodily functions involved

SNS

• increased heart rate and force of heart contraction

• constrict arterioles to increase blood pressure

• dilate respiratory airways to bring in more oxygen

• dilate the pupils + adapt for far vision

• decreased motility (break down carbs and fat to make sure theres glucose for your muscles) and inhibit digestion/renal function

• increased sweating to prepare for heat generation of physical activity

PNS

• decreased heart rate and force of heart contraction

• dilate arterioles to decrease blood pressure

• constrict airways; not as much O2 needed

• liver stops producing glucose and focuses on digestion

• promote digestion/renal function

• pupils constrict back to normal

• adrenal glands stop producing NE and Epi

What are some examples of dual innervation exceptions? What do they receive primary stimulation of?

1. Most arterioles and veins

   • sympathetic stimulation only

   • regulated by increasing or decreasing this sympathetic activity

   • only penis and clitoris have dual

2. Most sweat glands

   • sympathetic stimulation only

   • these sympathetic postganglionic fibres actually release ACh instead of NE.

3. Salivary glands

   • receive dual innervation, however, both systems can stimulate salivary secretion, rather than on being excitatory and the other being inhibitory.

When discussing the sympathetic nervous system, it’s essential to mention the adrenal glands. There are two. Each are closely associated with a kidney and have 2 distinct regions ____ and _____. How does this gland function?

Adrenal Gland: outer cortex and inner medulla

• adrenal medulla functions like a sympathetic ganglia (innervated by a sympathetic preganglionic fibre but doesn’t give rise to a postganglionic fibre).

• instead it releases chemical transmitters into the blood (these qualify as hormones because they’re released into circulation).

• About 20% are norepinephrine and 80% are epinephrine

• upon sympathetic stimulation the adrenal medulla acts as a global amplifier of the sympathetic system

Although ACh, NE and Epi are the neurotransmitters of the ANS, it’s the receptors of the ANS for these chemicals that defines how a tissue/organ will respond to ANS stimulation. There are two main classes of receptors. What are they? Briefly explain each of them.

1. Cholinergic receptor

   • on membrane of cells

   • responds to acetylcholine (ACh)

   • two types identified: muscarinic and nicotinic.

2. Adrenergic receptor

   • G-protein coupled receptor in the membrane of cells.

   • responds to catecholamines (NE and Epi).

   • these are what truly define how an organ will respond (more so than cholinergic)

   • classified as alpha or beta receptors and further sub-classified as a₁, a₂, B₁, B₂, B₃

   • once receptors are activated they have different cellular actions (depending on diff subclassifications and if NE or Epi)

Cholinergic receptors can be further divided into two classes: muscarinic and nicotinic. How do these receptor types work?

1. Muscarinic

   • activated by mushroom poison muscarine

   • found on the effector cells’ membranes

   • responds to ACh released by the PNS’ postganglionic fibres.

   • Binding of ACh or muscarine to these receptors trigger a G-protein coupled reaction that results in opening cation channels.

   • thus creating a depolarization potential (could lead to an AP)

2. Nicotinic

   • activated by nicotine

   • found on cell bodies of postganglionic cells in all autonomic ganglia

   • bind ACh released from both the PNS and SNS postganglionic fibres.

   • binding of ACh or nicotine to these receptors on cation channels leads to them opening, leading to response.

We can further classify adrenergic receptors as alphas or beta receptors. Once receptors are activated they have different cellular actions, so compare the release and binding of these receptors to norepinephrine/epinephrine.

Alpha receptors:

• both a₁ and a₂ have a greater sensitivity to norepinephrine than epinephrine

• recall that all adrenergic receptors activate the G-protein

• but a₂ activation suppresses cAMP
  pathway (2nd messanger system).

• while a₁ activation activates calcium second messenger system.

Beta receptors:

• B₂ has a greater affinity for epinephrine than B₁ does

• B₁ responds equally to norepinephrine and epinephrine

• recall that all adrenergic receptors activate G-proteins

• but both B₁ and B₂ enhance cAMP pathway.

The effects of sympathetic stimulation are determined by the _____ and _____ of adrenergic receptors in target tissues and organs. Give examples of this.

determined by number and type

• alpha receptors

  • a₁ receptors are almost always excitatory (expressed in smooth muscles of blood vessels, thus their stimulation causes contraction when stimulated by Epi)

  • in contrast, a₂ receptors are primarily expressed in smooth muscle cells of digestive system which causes a decrease in contraction when activated by Epi (recall they suppress cAMP pathway so not excitatory)

• beta receptors

  • stimulation of B₁ is usually excitatory (primarily found in the heart).

  • stimulation of B₂ is usually inhibitory and primarily found in the smooth muscles of arterioles and respiratory airways.

Describe what would happen to a blood vessel (arteriole) when stimulated by epinephrine.

If it binds to an a₁ receptor, the blood vessel will contract and constrict.

If it binds to a B₂ receptor, the blood vessel will dilate.

Recall: Stimulation of B₁ receptors is stimulatory and found in the heart, while stimulation of B₂ receptors is generally inhibitory and found in smooth muscles cells of the arterioles and respiratory airway.

Salbutamol is often used to treat asthma. Why is it effective?

it’s a bronchodilator meaning it activates B₂ receptors. It opens the airways with very little effect on the heart.

For each question choose the neurotransmitter and receptor that you think is associated with each fight or flight response. Some questions will have two correct options.

1. Increased heart rate and force of contraction

   • Epi or NE

   • a₁, a₂, B₁, B₂

2. Constriction of Arterioles

   • Epi or NE

   • a₁, a₂, B₁, B₂

3. Dilation of respiratory airways to bring in more oxygen

   • Epi or NE

   • a₁, a₂, B₁, B₂

4. Inhibition of digestion or renal function

   • Epi or NE

   • a₁, a₂, B₁, B₂

1. NE, Epi, B₁

2. NE, a₁

3. Epi, B₂

4. a₂, NE