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Introduction to the somatic and autonomic nervous systems

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Whether you’re playing volleyball with friends or just watching TV, your nervous system is always operating, making sure that the right organs function at the right time. The nervous system is structurally divided into two regions, called the central nervous system, or CNS, and the peripheral nervous system, or PNS. The peripheral nervous system can be further subdivided functionally into the somatic nervous system, and the autonomic nervous system.

Okay, let’s start with the somatic nervous system, which describes a set of nerve fibers that control voluntary actions and convey sensory information from the skin, skeletal muscles and joints. Somatic sensory fibers allow transmission of sensory information such as touch, pain, temperature, and proprioception. For example, somatic sensory fibers inform our CNS that our cup of coffee is too hot. Then there are somatic motor fibers, which only innervate skeletal muscle and control voluntary actions of the body, like putting the coffee cup back on the table until it cools down.

On the flip side, the autonomic nervous system controls the involuntary activities within the body. This system consists of visceral motor fibers that carry motor signals to smooth muscle, such as that found in the intestinal walls that allow for peristalsis to occur, as well as cardiac muscle, and glandular tissue.

We also have visceral sensory fibers, which are not typically defined as part of the autonomic nervous system, but they act in conjunction with the visceral motor fibers of the autonomic nervous system to control visceral function. Visceral sensory fibers travel with the visceral motor fibers carrying sensory information from the viscera back to the CNS, where visceral motor fibers will act in response to this sensory information. For example, they provide information about things such as the amount of oxygen in your blood, your arterial blood pressure, and the level of distention of your stomach after that big meal! This visceral sensory information is continuously regulating the activity of the visceral motor neurons of the autonomic nervous system - even while you sit here and watch this osmosis video!

Now, there is also a unique set of sensory neurons we want to mention called special sensory fibers, which relay sensory information related to our special senses, such as smell, sight, taste, hearing and balance back to the CNS for interpretation. These fibers are not really classified as part of the somatic or autonomic nervous system. However this information can modulate the activity of the somatic motor system, such as sending signals to skeletal muscles to turn your head in response to a sound, or the autonomic nervous system, such as stimulating salivary glands in response to the smell of a freshly baked apple pie!

Okay, now, let’s dive a bit more into the autonomic nervous system - or ANS. The ANS has two major subdivisions: the sympathetic nervous system – or SNS - and the parasympathetic nervous system - or PSNS. In addition, some classify the enteric nervous system – which is the intrinsic nervous system of the GI tract - as the third subdivision of the ANS. Now let’s start by discussing the SNS and PSNS, which, for the most part, have opposite effects in the body.

All right, now you’ve probably heard about the ‘fight or flight’ response, which kicks in when one encounters an alarming situation, like seeing a bear on your hike, or before taking an exam. These situations activate the SNS, which prepares the body to respond to extreme or stressful situations. For example, the sympathetic nervous system will cause dilation of the pupils to improve vision, increased heart rate and blood pressure, and diversion of blood flow to the organs that will help with our fight or flight response, like skeletal muscles and the brain. On the flip side, organs that are not urgently needed to address the immediately stressful situation, like the kidneys and the gastrointestinal tract, are toned down.

Ok, so once that bear is gone, or the exam is over, the parasympathetic nervous system kicks in to calm things down. It does so by decreasing your heart rate and blood pressure, increasing gut motility and digestive secretions and by allowing blood to flow back to the organ systems that have been put on hold by the sympathetic nervous system, like the digestive system. These effects can be summarized as the body’s ‘rest and digest’ response. In a nutshell, you can think of these two divisions of the ANS like a seesaw, where they work together to balance each other out.

Alright, as a quick break, let’s see if you can answer the following questions: What are the main divisions of the peripheral nervous system? How about the different components of the autonomic nervous system?

Ok, now, let’s discuss how information from the sympathetic and parasympathetic nervous systems communicate through the central nervous system. Remember that a group of cell bodies in the central nervous system is called a nucleus, while a group of cell bodies in the peripheral nervous system is called a ganglion.

Now, information that’s conveyed in the autonomic nervous system passes from the CNS, to its target organ through a two neuron system. The first neuron sending a signal from the central nervous system is called a preganglionic - or presynaptic - neuron, and the second neuron that the preganglionic neuron synapses with is called a postganglionic - or postsynaptic - neuron. This second neuron is usually the effector neuron, meaning that it then directly synapses on the organ that it acts on.

Now, there are other differences between the sympathetic or parasympathetic divisions of the ANS based on the location of the preganglionic neuron cell bodies and the route that the pre- and postganglionic neurons take to reach their effector organs. Let’s start by talking about the neurons of the sympathetic nervous system. So, the cell bodies of the preganglionic neurons of the sympathetic division are located in the intermediolateral nuclei which make up the lateral horns of the spinal cord’s gray matter in the thoracolumbar spinal cord segments.Specifically, the preganglionic neurons arise from the T1-L3 spinal cord segments.

Now, let’s follow the path of the preganglionic neurons more closely. So, from the intermediolateral nucleus, all of the preganglionic sympathetic fibers exit the spinal cord via the anterior root and enter the anterior rami of spinal nerves T1-L3. Shortly after, these fibers leave the anterior rami to enter the sympathetic chain via the white rami communicantes. The sympathetic chain - or trunk - is a paired structure that runs parallel on either side of the spinal cord, and it resembles an interconnected string of pearls. The sympathetic chain consists of a bundle of nerve fibers and neuron cell bodies. The neuron cell bodies within the sympathetic chain form the paravertebral ganglia.

Now, when the preganglionic fibers enter the sympathetic chain there are 4 possible routes for these fibers to take to synapse with their postganglionic counterparts. First, a preganglionic fiber can enter and immediately synapse with the postganglionic cell body in the paravertebral ganglion at its respective vertebral level. Second, it can also pass through the paravertebral ganglion to ascend, or, third, to descend the sympathetic chain before synapsing at a paravertebral ganglia at another vertebral level.

Fourth and finally, the preganglionic fibers can pass through the paravertebral ganglia without synapsing and instead, they continue via splanchnic nerves to one of a number of prevertebral ganglia, located anterior to the aorta, and synapse there. These ganglia are named according to the main aortic branch that they’re positioned near. The most superior are the paired celiac ganglia, found around the celiac trunk, just below are one or more superior mesenteric ganglia, followed by the inferior mesenteric ganglion.