The Sympathetic Nervous System

by Jason

The sympathetic nervous division is the most dominant in stressful situations, which would include anger, fear or anxiety, as well as during exercise. This is often referred to as a "Fight or Flight" response.

The sympathetic division of the CENTRAL NERVOUS SYSTEM (CNS) brings out various and widespread responses to many different organs and organ systems (see table 1, below). Before I talk about the body systems, I would like to discuss three basic neurotransmitters:

1) ACETYCHOLINE - a transmitter released by all the pre-ganglionic neurons,
2) NOREPINEPHRINE - a transmitter released by adrenergic post-ganglionic neurons, and
3) EPINEPHINE - a hormone that is secreted by the adrenal medulla that stimulates many responses to enable the body to react to a stressful situation.

The sympathetic stimulation causes dilation of the coronary blood vessles, along with the constriction of the peripheral vessels, ensuring that increased oxygen demands of the heart are met by increasing blood flow and oxygen supply. By doing so, hormones are released. The hormones that are responsible for this are Epinephrine and Norepinephrine, and they are secreted into circulating blood (usually during stress or physical exertion).

Epinephrine has almost the same effect on the cardiac muscle as norepinephrine and therefore increases the rate, force and contraction of the heart.

The respiratory system is affected by the sympathetic nervous system by the dilation of the bronchioles, as well as the relaxation of the tracheal and bronchial muscles in order to take in more air and create a greater tidal volume.

The skeletal system is affected by all the blood vessels in this system dilating causing blood and oxygen to flood into your muscles so that you are prepared and ready to go.

The eyes are affected by the sympathetic nervous system as the ciliary muscles relax and the puplis dilate for better vision at a long distance.

The digestive system is affected by the salivary glands secreting little or no mucus, decreasing of the esophagus and stomach involuntary muscle contractions, as well as stomach acids and enzymes decrease. During this process the anal sphincter contracts to prevent defecation.

During a sympathetic reaction the liver changes glycogen into glucose, anticipating the body to work hard and supply it with energy.

The sweat glands increase production during a sympathetic reaction to rid the body of excess waste and regulate body temperature, caused by blood vessels in the skin to contract and the pale skin color due to vasoconstriction.

To summarize, it is important to note that the body shunts blood from the peripherals to the core of the body for the important organs in times of stress, and how easily this system can be overlooked when making a provisional diagnosis as it is usually at work in some form in every type of injury or illness that we will see.

The Parasympathetic Nervous System

Also known as the "Feed or Breed" or "Rest and Digest" response, the main nerves of the parasympathetic system are the tenth cranial nerves, the vagus nerves. They originate in the medulla oblongata. Other preganglionic parasympathetic neurons also extend from the brain as well as from the lower tip of the spinal cord.

Each preganglionic parasympathetic neuron synapses with just a few postganglionic neurons, which are located near - or in - the effector organ, a muscle or gland. Acetylcholine (ACh) is the neurotransmitter at all the pre- and many of the postganglionic neurons of the parasympathetic system. However, some of the postganglionic neurons release nitric oxide (NO) as their neurotransmitter.

Parasympathetic stimulation causes

• slowing down of the heartbeat
• lowering of blood pressure
• constriction of the pupils
• increased blood flow to the skin and viscera
• peristalsis of the GI tract

In short, the parasympathetic system returns the body functions to normal after they have been altered by sympathetic stimulation. In times of danger, the sympathetic system prepares the body for violent activity. The parasympathetic system reverses these changes when the stress has passed.

Table 1: Responses of major organs to autonomic nerve impulses

Organ	Sympathetic Stimulation	Parasympathetic Stimulation
Heart	Increased heart rate beta1 (& beta2)	Decreased heart rate
	Increased force of contraction beta1 (& beta2)	Decreased force of contraction
	Increased conduction velocity	Decreased conduction velocity
Arteries	Constriction (alpha1)	Dilation
	Dilation (beta2)
Veins	Constriction (alpha1)
	Dilation (beta2)
Lungs	Bronchial muscle relaxation (beta2)	Bronchial muscle contraction
		Increased bronchial gland secretions
Gastro- intestinal tract	Decreased motility (beta2)	Increased motility
	Contraction of sphincters (alpha)	Relaxation of sphincters
Liver	Glycogenolysis (beta2 & alpha)	Glycogen synthesis
	Gluconeogenesis (beta2 & alpha)
	Lipolysis (beta2 & alpha)
Kidney	Renin secretion (beta2)
Bladder	Detrusor relaxation (beta2)	Detrusor contraction
	Contraction of sphincter (alpha)	Relaxation of sphincter
Uterus	Contraction of pregnant uterus (alpha)
	Relaxation of pregnant and non-pregnant uterus (beta2)
Eye	Dilates pupil (alpha)	Constricts pupil
		Increased lacrimal gland secretions
Submandibular & parotid glands	Viscous salivary secretions (alpha)	Watery salivary secretions