Sympathetic Nervous System

The nervous system that controls your responses in stress conditions like facing a dog, having an accident or being afraid of an approaching deadline is the sympathetic nervous system. It is a division of the autonomic nervous system that prepares the body for fight or flight response.

It is a network of interconnected neurons that prepares the body to deal with the conditions of stress and fear. The sympathetic nervous system not only initiates responses required to deal with the stress condition but also makes necessary changes in the functioning of the body organs.

Although the sympathetic system is activated in the conditions of stress, small sympathetic tone is always present in the body to regulate the vital functions.

In this article, we will talk about the organization of the sympathetic nervous system, its neurotransmitters and receptors, and its effects on different organs of the body.

In the end, we will talk about some disorders of the sympathetic nervous system. We will also discuss some drugs that act on this system.

Anatomy and Organization:

The sympathetic nervous system is also a subdivision of the peripheral nervous system. Like other subdivisions of the PNS, it consists of nerve fibers leading to or originating from the brain and spinal cord. It also has neuronal cell bodies in the form of ganglia.

Nerve fibers

The nerve fibers are of two types.

Pre-ganglionic nerve fibers: These are the nerve fibers that originate from the central nervous system and terminate at the ganglia. In the sympathetic nervous system, the pre-ganglionic fibers are short.

Post-ganglionic nerve fibers: They originate from the ganglia of the sympathetic nervous system and terminate at the target organs. The post-ganglionic fibers are long in the case of sympathetic nervous system.

The sympathetic system has thoracolumbar outflow. The pre-ganglionic nerve fibers originate from the thoracic and lumbar segments of the spinal cord and terminate in the ganglia.

Ganglia

They are the collections of neuronal cell bodies located outside the CNS. The ganglia of the sympathetic nervous system are present in the form of chains along the spinal cord.

As stated earlier, the sympathetic system has thoracolumbar outflow with short pre-ganglionic fibers. So, the ganglia of the sympathetic system are also present along the thoracic and lumbar segments of the spinal cord.

Neurotransmitters

These are the chemicals released by the axons at the nerve terminals. They bind to the specific receptors present on the target cells and initiate chemical responses.

There are three types of neurotransmitters present in the sympathetic nervous system:

Acetylcholine: It is released by the preganglionic fibers at the ganglia. It binds to the receptors present on the neuronal cell bodies or their dendrites. The function of acetylcholine is to transmit signals from the preganglionic fibers to the cell bodies located in the ganglia.

Nor-epinephrine: It is released by the postganglionic fibers at the target tissues. Norepinephrine binds to the receptors present on the target tissue and initiates responses. The response generated by a neurotransmitter is dependent on the receptor it binds.

Epinephrine: It also acts on the target tissues by binding to the receptors of the sympathetic nervous system. It is only rarely released by the post-ganglionic fibers. However, a large amount of epinephrine is released by the adrenal medulla in the blood. This circulating epinephrine binds on the receptors and generates response depending on the receptor.

Receptors

The receptors of the sympathetic nervous system are divided into two categories:

Ganglionic receptors: They are present on the ganglia. They are the nicotinic receptors activated by acetylcholine.

Target receptors: These receptors are present on the target organs and are activated by either epinephrine or nor-epinephrine. The target receptors are of four types:

Alpha-1 receptors: They are the stimulatory receptors present on blood vessels, eyes, urinary bladder, kidney, etc. Activation of these receptors causes contraction of the smooth muscles.

Alpha-2 receptors: These are the inhibitory receptors present on the nerve terminals. These receptors are involved in the feedback mechanism. Activation of these receptors prevents further release of neurotransmitters from the axons present at nerve terminal.

Beta-1 receptors: These are the stimulatory receptors present in the heart. Activation of beta-1 receptors increases heart rate, force of contractions, automaticity and ejection fraction.

Beta-2 receptors: They are also stimulatory in nature. They are present in smooth muscles of blood vessels, bronchi, sphincter, gastrointestinal tract, urinary bladder, etc. Activation of beta-2 receptors causes smooth muscle relaxation, an effect opposite to the alpha-1 receptor stimulation.

Effects

As stated earlier, the sympathetic nervous system is activated in conditions of stress. However, sympathetic stimulation of the organs to a small extent is always present in the body, even in resting state.

Below we will discuss how activation of sympathetic nervous system regulates different vital functions of the body.

Blood Pressure:

Sympathetic system plays the most important role in the regulation of blood pressure. It is especially important when the blood pressure falls below normal in conditions like hemorrhage, blood loss, etc.

In response to decreased blood pressure, the sympathetic system is activated. It causes constriction of the blood vessels. As a result, the total peripheral resistance increases, and the blood pressure goes up.

Heart Rate:

Heart rate is also controlled by the sympathetic nervous system. The beta-1 receptors present on the heart are activated in conditions like blood loss, decreased circulatory volume, decreased blood pressure, heart failure, etc.

Activation of beta-1 receptors causes an increase in heart rate as well as contractility of the cardiac muscles and the ejection fraction. All these responses prevent the body from going into the condition of circulatory shock.

Breathing

Beta-1 receptors are present in the smooth muscles of the respiratory system. The activation of these receptors causes relaxation of smooth muscles and opening of the airways. Thus, the activation of the sympathetic system promotes the process of breathing.

Digestion

Activation of the sympathetic system decreases the contraction of smooth muscles in the walls of the digestive tract. Thus, it inhibits the processes of digestion. Contrary to this, the smooth muscles present in sphincters are activated. Thus, the sphincters of the digestive tract remain closed that further hinders the process of digestion.

Urination:

The process of urination is inhibited by the sympathetic system. It causes contraction of the smooth muscle present in sphincters. As a result, it causes urinary retention.

Sexual Response:

The activation of sympathetic system causes ejaculation.

Pupillary Response:

Alpha-1 receptors are present in the radial muscles of the eye. Activation of the sympathetic system causes contraction of radial muscles. It results in dilated pupils.

Disorders affecting the activity of Sympathetic system

Let us talk a little bit about the disorders that change the activity of the sympathetic nervous system.

Heart failure: In heart failure, the heart rate and cardiac contractility decreases. As a result, there is increased firing of the sympathetic system. However, this increased sympathetic activity can further exacerbate the condition leading towards increased risk of mortality.

Hypertension: In hypertension, there is decreased firing of the sympathetic system. It is a protective response that tends to lower the blood pressure by decreasing the total peripheral resistance.

Adrenal medullary tumors: In tumors of adrenal medulla, there is an increased release of epinephrine in the blood. Epinephrine binds to the receptors of the sympathetic system and can result in increased blood pressure, increased heart rate etc. even when the sympathetic system in dormant.

Drugs

The drugs acting on the sympathetic system are used to treat a number of conditions like heart failure, arrhythmias, hypertension, asthma, benign prostate hyperplasia, etc.

These drugs are divided into two sub-categories:

Sympathomimetic drugs:

As the name indicates, they mimic the activity of sympathetic system. They bind to the receptors and activate them, increasing the activity of sympathetic system. Every receptor has its own agonists. The drugs in this category include phenylephrine, methyldopa, ibuterol, salbutamol, etc.

Sympatholytic drugs:

These drugs decrease the activity of sympathetic system. They are the antagonists which bind to the receptors and inhibit their activation. These include prazosin, tamsulosin, propranolol, esmolol, etc.

Conclusion/Summary:

Sympathetic system is a division of autonomic nervous system that prepares the body for stress conditions. Although it is activated in the stress conditions, a small amount of sympathetic activity is present in the body every time which is essential to regulate different vital body functions.

Sympathetic system consists of short pre-ganglionic fibers and long post-ganglionic fibers. The ganglia of the sympathetic system are present in a line along the thoracolumbar segments of the spinal cord.

The ganglia of the sympathetic system have nicotinic receptors. However, four different types of receptors are present in the target organs. These include:

  • Alpha-1
  • Alpha-2
  • Beta-1
  • Beta-2

The ganglionic receptors are activated by acetylcholine. The receptors at the target organs are activated by either epinephrine or nor-epinephrine.

Many vital functions are under the control of the sympathetic system. These include:

  • Blood pressure
  • Heart rate
  • Breathing
  • Digestion
  • Urination
  • Sexual responses
  • Pupillary response

The drugs that act on the sympathetic system either increase or decrease its activity. They mainly act by binding to the receptors, facilitating or inhibiting their activation.

References

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