Sympathetic Nervous System: Function & Response

NeurologyNursing/NCLEXUSMLE
Written By Simplico

Sympathetic Nervous System: This lecture reviews the sympathetic nervous system definition, function, fight or flight response, activation, neurotransmitters involved, examples, and more!

Sympathetic Nervous System

Sympathetic Nervous System - Quick Notes:

  • Definition:
    The sympathetic nervous system is a branch of the autonomic nervous system that prepares the body for dangerous, stressful, critical, or fearful situations, etc.

  • Function:
    The sympathetic nervous system functions to produce physiological responses that prepare the body for perceived threats or fear, or in critical or emergent situations, or during physical activity, etc.

  • Response:
    The sympathetic nervous system response is called the fight or flight response.

  • Activation:
    The sympathetic nervous system is activated by stressful, dangerous, fearful, emergent, critical situations, etc.

  • Neurotransmitters:
    The neurotransmitters and hormones involved in producing the final sympathetic response are mainly norepinephrine and epinephrine.

  • Examples:
    Examples of the sympathetic nervous system response include increased heart rate, increased cardiac output, increased blood pressure, bronchodilation, mydriasis (pupil dilation), etc.

  • Symptoms:
    Sympathetic nervous system symptoms include rapid heart rate, rapid breathing, increased sweating (diaphoresis), etc.

  • Disorders:
    Conditions that can affect the sympathetic nervous system include diabetes mellitus (especially type 2), autoimmune disorders, infections, etc.

Simplified: The sympathetic nervous system increases body functions critically important for survival, while downregulating less critical body functions.

Examples:

Activation of the sympathetic nervous system will increase (upregulate):

  • Heart rate

  • Cardiac output

  • Blood pressure

  • Bronchodilation

  • Metabolism

  • Gluconeogenesis (process of making glucose)

  • Pupil dilation (mydriasis)

  • Diaphoresis

  • Activation of the renin-angiotensin-aldosterone system

Activation of the sympathetic nervous system will decrease (downregulate):

  • Salivation

  • Lacrimation

  • Defecation

  • Digestion

  • Urination

Let’s now review the sympathetic nervous system in more detail.

Sympathetic Pathway

The sympathetic pathway can be divided into 3 main parts:

  1. Preganglionic Neurons

  2. Sympathetic Ganglia

  3. Postganglionic Neurons

The sympathetic neurons exiting the spinal cord and terminating in the sympathetic ganglia are called preganglionic neurons.

The neurons originating in the sympathetic ganglia and terminating at their target organs are called postganglionic neurons.

Preganglionic neurons synapse with postganglionic neurons in the sympathetic ganglia.

Let’s review each step below.

1. Preganglionic Sympathetic Neurons

Preganglionic Sympathetic Neurons - Quick Notes:

  • Arise from the central nervous system at the thoracolumbar level

  • Have short axons as they terminate nearby at the paravertebral ganglia (sympathetic chain) or prevertebral ganglia

  • Release a neurotransmitter called acetylcholine

  • Are cholinergic neurons, since they release acetylcholine

Preganglionic sympathetic neurons arise from the central nervous system at the thoracolumbar region, specifically T1-L2.

They exit the spinal cord and travel to the sympathetic ganglia, where they synapse with postganglionic neurons (see below).

Preganglionic neurons release acetylcholine onto postganglionic neurons.

The acetylcholine released by preganglionic neurons binds to nicotinic cholinergic receptors on postganglionic cell bodies.

When acetylcholine binds to nicotinic receptors, an action potential is generated through the postganglionic neuron (see below).

Important: Some preganglionic sympathetic neurons bypass the sympathetic ganglia and terminate directly onto the adrenal medulla, which acts as a modified sympathetic ganglion.

This triggers the release of epinephrine and norepinephrine from the adrenal medulla into the bloodstream (see below).

2. Sympathetic Ganglia

Preganglionic neurons synapse with postganglionic neurons in sympathetic ganglia.

The sympathetic ganglia are collections of postganglionic cell bodies that receive neuronal information from the preganglionic neurons.

The sympathetic ganglia are found in 2 main locations:

  1. Paravertebral Ganglia

    1. A chain of ganglia that run adjacent/parallel to the vertebral column

    2. Also called the sympathetic chain or sympathetic trunk

  2. Prevertebral Ganglia

    1. Located in the abdominal cavity, anterior to the vertebral column and abdominal cavity

Preganglionic neurons release acetylcholine onto postganglionic neurons in the sympathetic ganglia.

This initiates an action potential (neuronal signal) through the postganglionic neurons to target organs (see below).

The preganglionic sympathetic neurons that terminate directly onto the adrenal medulla also release acetylcholine.

3. Postganglionic Sympathetic Neurons

Postganglionic Sympathetic Neurons - Quick Notes:

  • Originate in the sympathetic ganglia (paravertebral or prevertebral)

  • Have longer axons compared to the preganglionic neurons, as they travel longer distances to their target organs

  • Release a neurotransmitter called norepinephrine

    • Exception: sweat glands (see below)

  • Are adrenergic neurons, since they release norepinephrine

    • Exception: sweat glands (see below)

Postganglionic sympathetic neurons originate in sympathetic ganglia, specifically within the paravertebral and prevertebral ganglia.

They travel further distances compared to preganglionic neurons and synapse on target organs.

Postganglionic neurons release norepinephrine onto target tissues and organs.

The norepinephrine released by postganglionic neurons binds to alpha adrenergic and beta adrenergic receptors on target organs.

When norepinephrine binds to a target organ, it causes a sympathetic response in that particular organ.

Norepinephrine is both a neurotransmitter (released by postganglionic sympathetic neurons) and a hormone (released by the adrenal medulla) - see below.

Important: Adrenal Medulla

Epinephrine is another key hormone in generating the sympathetic response.

As previously mentioned, some preganglionic sympathetic neurons directly terminate on the adrenal medulla.

This stimulates the adrenal medulla to secrete both epinephrine and norepinephrine into the bloodstream.

The norepinephrine released by postganglionic sympathetic neurons, and the circulating epinephrine and norepinephrine released by the adrenal medulla, bind to adrenergic receptors on target organs.

This generates a sympathetic response.

Norepinephrine and epinephrine are examples of catecholamines, a group of hormones and neurotransmitters involved in the sympathetic response.

Important Exception: Sweat glands

Postganglionic sympathetic neurons that terminate on sweat glands release acetylcholine instead of norepinephrine.

The acetylcholine binds to muscarinic cholinergic receptors on sweat glands.

The sweat glands are one exception in that postganglionic sympathetic fibers release acetylcholine onto muscarinic cholonergic receptors on the sweat glands rather than releasing norepinephrine onto adrenergic receptors.

Let’s review example sympathetic responses below.

Sympathetic Nervous System - Function & Response

Postganglionic sympathetic neurons release norepinephrine onto adrenergic receptors on target cells/organs (see above).

The adrenal medulla secretes epinephrine and norepinephrine into the bloodstream, which binds to adrenergic receptors on target organs (see above).

Activation of adrenergic receptors will generate a sympathetic nervous system response.

Example Sympathetic Nervous System Responses:

  • Heart

    • Activation of adrenergic receptors in the heart (primarily β1 receptors) augments cardiac action potentials

    • This results in tachycardia (increased heart rate), increased cardiac contractility, and increased cardiac output

    • This allows the heart to pump more blood to the body during situations of danger, stress, physical activity, etc.

  • Lungs

    • Activation of adrenergic receptors in the lungs (primarily β2 receptors) leads to bronchodilation

    • Of note, β2 adrenergic receptors have a higher affinity for epinephrine than norepinephrine

    • In fact, there are few postganglionic sympathetic neurons (which release norepinephrine) that terminate on the lungs

    • Instead, bronchodilation primarily occurs from the circulating epinephrine released by the adrenal medulla

    • Bronchodilation allows the airways to open during situations of danger, stress, physical activity, etc.

  • Blood Vessels

    • Activation of adrenergic receptors on blood vessels (primarily α1 receptors) leads to vasoconstriction

    • This increases systemic vascular resistance (SVR)

    • Increased SVR and cardiac output (see above) increase blood pressure

    • Vasoconstriction and increased blood pressure allow for quick delivery of oxygen to vital organs (heart, muscles, brain, etc.) during situations of danger, stress, physical activity, etc.

    • Blood flow to less critical areas (such as the digestive system) is reduced

  • Kidneys

    • Activation of adrenergic receptors on the juxtaglomerular cells of the kidneys (primarily β1 receptors) stimulates the renin-angiotensin-aldosterone system (RAAS)

    • Stimulation of the RAAS, along with vasoconstriction and increased cardiac output (see above), increase blood pressure

    • Increased blood pressure helps deliver oxygen and nutrients to essential organs during situations of danger, stress, physical activity, etc.

  • Pupils

    • Activation of adrenergic receptors on the iris dilator muscle (primarily α1 receptors) leads to pupil dilation, called mydriasis

    • Pupil dilation allows for more light to enter the eye, which can improve vision during situations of perceived danger or threat

  • GI Tract and Bladder

    • Activation of adrenergic receptors in the gastrointestinal tract or bladder decrease digestion and urination, respectively.

    • Digestion and urination are not critical functions for immediate survival, and therefore are reduced during a sympathetic response

  • Liver and Gluconeogensis

    • Activation of adrenergic receptors in the liver (primarily β2 receptors) promotes gluconeogesis (process of making glucose) and glycogenolysis (breakdown of glycogen to glucose)

    • This increases blood glucose levels during a sympathetic response

    • Glucose provides energy for the body, which is important in times of danger, stress, physical activity, etc.

  • Sweat Glands

    • Activation of muscarinic receptors on sweat glands (primarily M3 receptors) leads to sweat secretion (diaphoresis)

    • Reminder: The postganglionic sympathetic neurons to sweat glands are different from other postganglionic sympathetic neurons, in that they release acetylcholine onto muscarinic receptors (instead of norepinephrine onto adrenergic receptors)

    • Diaphoresis during a sympathetic response allows for thermoregulation

    • During a sympathetic response, metabolism increases which produces body heat

    • Diaphoresis reduces and controls the excess body heat

Visit the alpha adrenergic receptor and beta adrenergic receptor lectures for more information about:

  • The different types of adrenergic receptors

  • Where they are located

  • The different effects they produce, etc.

Resources

https://www.kenhub.com/en/library/anatomy/sympathetic-nervous-system

Simplico