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.
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.
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 adrenaline and noradrenaline, which are collectively called catecholamines.