Sympathetic nervous system

Definitions & Key takeaways

The sympathetic nervous system is a part of the autonomic nervous system (ANS) that helps regulate the body's involuntary functions such as breathing, digestion, heart rate, and blood pressure. It is based on a preganglionic or cholinergic neuron that releases acetylcholine and a postganglionic neuron that releases either catecholamines or acetylcholine to a target cell.

The sympathetic nervous system is often referred to as the fight or flight system because it helps prepare the body for action in response to a threatening or stressful situation. In emergency situations, the activity of the sympathetic nervous system increases and diverts blood away from the organs that are not necessary for survival, like the gastrointestinal tract and the bladder, and increases blood flow to muscles and organs like the brain. The sympathetic nervous system works in opposition to the parasympathetic nervous system, which helps the body to relax and conserve energy.

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Introduction0:00–2:12

The nervous system is divided into the central nervous system, so the brain and spinal cord, and the peripheral nervous system, which includes all the nerves that connect the central nervous system to the muscles and organs.
The peripheral nervous system can be divided into the somatic nervous system, which controls voluntary movement of our skeletal muscles, and the autonomic nervous system, which is further divided into the sympathetic and the parasympathetic, and controls the involuntary movement of the smooth muscles and glands of our organs.
The sympathetic and parasympathetic nervous systems have opposite effects on the body. The sympathetic nervous system controls functions like increasing the heart rate and blood pressure, as well as slowing digestion.
All of this maximizes blood flow to the muscles and brain, and can help you either run away from a threat or fight it, which is why it’s also called the fight-or-flight response.
The parasympathetic nervous system instead slows the heart rate and stimulates digestion - the effects can be summarized as 'rest and digest'.
Now, neurons are the main cells of the nervous system. They’re composed of a cell body, which contains all the cell’s organelles, and nerve fibers, which are projections that extend out from the neuron cell body.
Nerve fibers are either dendrites that receive signals from other neurons, or axons that send signals along to other neurons.
Where two neurons come together is called a synapse, and that’s where one end of an axon sends neurotransmitters to the dendrites or directly to the cell body of the next neuron in the series.
Now the autonomic nervous system - so both the sympathetic and parasympathetic nervous system - is made up of a relay that includes two neurons.
And when there’s a group of neuron cell bodies that are next to each other in the central nervous system, the whole thing is called a nucleus, while a group of neuron cell bodies that are located outside of the central nervous system is called a ganglion.
We’ll focus on just the sympathetic nervous system. Signals for the autonomic nervous system start in the hypothalamus, which is a portion of the brain located at the base of the brain.

Sympathetic fibers and ganglia2:12–4:21

Hypothalamic neurons have really long axons - up to 1.4 meters or 4.5 feet - and they carry signals all the way down to the spinal cord nuclei where they synapse with preganglionic neuron cell bodies.
The sympathetic spinal cord nuclei start at the first thoracic vertebrae or T1 and runs all the way down to the second segment of the lumbar spinal cord or L2.
From there, the signal goes from the preganglionic neurons down it’s relatively short axon, exits the spinal cord, and reaches the nearby sympathetic ganglion, which is made up of lots of postganglionic neuron cell bodies.
Sympathetic ganglia are divided into two major groups, paravertebral and prevertebral ganglia, based on their location. Paravertebral ganglia run alongside the spinal cord, and they’re interconnected forming a sympathetic chain, which is like a string of pearls where the nerve fibers make up the string, and the paravertebral ganglia are the pearls.
Even though all preganglionic axons enter the sympathetic chain, only some of them actually synapse with postganglionic neurons there.
The rest of the preganglionic axons pass through the sympathetic chain without synapsing, and continue on to reach the prevertebral ganglia to synapse with the postganglionic neuron cell bodies here.
There are three pairs of prevertebral ganglia, the celiac ganglia located on the wall of the aorta right above where the renal arteries branch off, the superior mesenteric ganglia close to the origin of the superior mesenteric artery, and inferior mesenteric ganglia near where the inferior mesenteric artery branches off the abdominal aorta.
Finally, from both paravertebral and prevertebral ganglia, the axons of the postganglionic neurons leave the ganglia to reach the internal organs, where they synapse with the cells of the target organs.

Sympathetic synapses4:21–6:23

Now let’s zoom into the synapses of the sympathetic nervous system. The preganglionic and postganglionic neurons release different neurotransmitters, which are the tiny molecules that nerve cells use to communicate with one another.
The preganglionic neurons release the neurotransmitter acetylcholine, and that’s why the preganglionic neurons are called cholinergic neurons.
Acetylcholine binds to nicotinic receptors on the cell membrane of postganglionic neuron cell bodies. Nicotinic receptors are ion channels that open when acetylcholine binds to them; and they allow positive ions like sodium and calcium to cross the cell membrane, activating the postganglionic neurons.
Most postganglionic neurons are called adrenergic neurons because they release the neurotransmitters noradrenaline, which are collectively called catecholamines.
Catecholamines activate the adrenergic receptors on the cells of the target organs. There are two main groups of adrenergic receptors, alpha and beta receptors.
Alpha receptors have two subtypes: alpha1 and alpha2, while beta receptors have three subtypes: beta1, beta2 and beta3. Each of these adrenergic receptors are G-protein-coupled receptors, meaning that when catecholamines bind, they activate intracellular proteins, called G proteins.
These G proteins ultimately enable cells to change in a number of ways, and that’s how the sympathetic nervous system creates change at the cellular level.
Some postganglionic neurons are cholinergic neurons because like the preganglionic neurons, they also release acetylcholine.
This time, however, the acetylcholine binds to muscarinic receptors on the cells of target organs. Muscarinic receptors are also G-protein-coupled receptors, so once again, its the G proteins that facilitate change at the cellular level.

Sympathetic Effects6:23–8:34

The sympathetic nervous system affects each organ system differently. In blood vessels, it acts on alpha1 adrenergic receptors, which cause vasoconstriction in some tissues like the gastrointestinal tract and the bladder, and acts on beta2 adrenergic receptors, which cause vasodilation in other tissues like the skeletal muscles and brain.
In the heart, it acts on alpha1 and beta1 adrenergic receptors, which increase the heart rate and contractility, and that helps to pump out more blood.
In the lungs it acts on beta2 adrenergic receptors, which cause bronchodilation and that increases oxygen delivery to cells.
In the salivary glands, it works on beta2 adrenergic receptors to decrease saliva secretion. In the liver, it works on beta2 adrenergic receptors so that more glucose is released into the blood, since the body has increased energy needs.
In the pupils, it acts on alpha1 adrenergic receptors, triggering mydriasis or dilation to help with vision. In the bladder, it acts on alpha1 adrenergic receptors to cause sphincter constriction, as well as on alpha2 adrenergic receptors to relax the bladder muscle, called the detrusor muscle.
In adipose tissue, it acts on beta3 adrenergic receptors, to trigger lipolysis or breakdown of fat to obtain energy from it.
In the skin, it works on muscarinic receptors, to increase sweat production. Finally, in females, it acts on beta adrenergic receptors to cause uterine relaxation, and in males, it acts on alpha adrenergic receptors to trigger ejaculation.
These organs are controlled by both the sympathetic and parasympathetic, with opposite effects, except for the sweat glands and blood vessels, which are only controlled by the sympathetic in an on-off manner.
The sympathetic nervous system is active all the time at a low level, and in extreme or dangerous situations, the level of activation increases, triggering the fight or flight response.
The net effect is that there’s an increased cardiac output and that blood is diverted away from organs of the gastrointestinal tract, and towards the muscles which are needed to fight or run away, as well as organs like the brain to help think more clearly and be alert.

Review8:34–9:12

Alright, as a quick recap, the sympathetic nervous system is one of the divisions of the autonomic nervous system. The entire system is based on a preganglionic or cholinergic neuron that releases acetylcholine and a postganglionic neuron that releases either catecholamines or acetylcholine to a target cell.
In emergency situations, the activity of the sympathetic nervous system increases, and that helps with the fight or flight response because it diverts blood away from the organs that are not necessary for survival, like the gastrointestinal tract and the bladder, and increase blood flow to muscles and organs like the brain.