Structure and Functions of the Autonomic Nervous System
Objectives
By the end of this section students will be able to:
- Compare the structural and functional differences between the somatic and autonomic parts of the nervous system.
Somatic vs. Autonomic Nervous Systems
As you have previously learned, the nervous system is quite extensive and composed of two types of cells: neurons and neuroglia. Neurons are the functional unit of the nervous system and are in charge of receiving, processing, and transmitting information in the form of action potentials. Neuroglia, on the other hand, come in many forms and have a variety of jobs that fall under the broad designation of supporting the neuron and allowing it to perform its job effectively and efficiently. The nervous system (composed of these two categories of cells) can be organized into two main branches: the central nervous system or CNS which is composed of the brain and spinal cord, and the peripheral nervous system or PNS which contains everything else. That phrase "everything else" is not only vague but incredibly broad so we further breakdown the PNS into three branches that differ the most in the effector receiving the motor output.
The first branch is the enteric nervous system or ENS whose motor output is sent to the organs of digestive system. Because of this, the ENS is often thought of as the little brain of your gut. You will learn that the digestive system is very complicated and organ motility and secretion are continually monitored and adjusted based on diet and meal frequency. If your cerebrum was in charge of monitoring this activity, conscious thought would be required to move food through the digestive system and your brain would not be able to perform this monitoring simultaneously with the rest of your body functions.
The second branch of the PNS is the somatic nervous system or SNS whose motor output is sent to the skeletal muscles. You may remember this system from the muscle module and how action potentials were sent down somatic motor neurons to the neuromuscular junction.
The last branch of the PNS is the autonomic nervous system or ANS whose motor output is sent to smooth muscles, cardiac muscles, glands, and adipose tissue. This branch is the focus of the module. To help you visualize the breakdown of the nervous system please view the flow chart below.
We must first compare and contrast the branch that we have already studied, the SNS, and the new branch we wish to explore, the ANS.
Let's start with similarities:
- Both branches are a part of the PNS.
- Both branches control muscle movement (albeit the SNS controls skeletal muscles while the ANS controls smooth and cardiac movement).
- Both branches may be used in response to a single stimulus. For example, if you are hiking through the woods and you see a bear, what do you do? Most likely you will freeze initially and then begin backing away before getting out of the bear's line of sight and running at world record speed until you are safely locked back in your car. Both systems were activated by the sight of the bear – the skeletal muscles used to run away, the cardiac muscles that increased your heart rate, and the smooth muscle that directed blood flow to the essential organs.
- Both branches utilize the neurotransmitter acetylcholine (ACh).
That is where the similarities between the branches end; we now move into discussing the differences that exist. The first and most notable difference is that the SNS is consciously controlled, meaning that the skeletal muscle movement is voluntary and controlled by thought or intention. This means that if you think about wanting to flex your arm you will do so. The ANS on the other hand cannot be consciously controlled to any great degree. Simply put this means that you cannot think the command "Pupils Constrict" and it will happen. It takes a stimulus like a light shown in the eye to cause the pupil constriction. This is why ANS responses are tested in a lie detector and used to determine if the person is lying. Since we do not have a great degree of control over the response, a spike in heart rate or temperature could signal deception but it could also signal an emotional response. As you will see later the autonomic nervous system is tied not only to external stimuli but also emotional responses like anxiety or embarrassment.
So, conscious control is the largest difference between the SNS and ANS but other differences exist between these branches and they are found primarily in the efferent pathway. There is a single motor neuron in the SNS that carries the signal for contraction extending from the cerebrum to the effectors. With the ANS division, the efferent pathway consists of 2 motor neurons. One that carries the signal to contract from the spinal cord out to an integrating structure called the autonomic ganglion. This motor neuron is before the ganglion and is therefore termed the preganglionic neuron. The signal to contract is then passed from this preganglionic neuron to the second motor neuron within the autonomic ganglion. The second motor neuron is called the postganglionic neuron because it carries the information out of the autonomic ganglion to the effector. See the figure below for a visual of the one versus two motor neuron motor pathways.
Figure 1: The afferent inputs to somatic and visceral reflexes are essentially the same, whereas the efferent branches are different. Visceral (ANS) reflexes, for instance, involve a projection from the central spinal cord motor neuron (in the lateral horn) to a ganglion, followed by a second postganglionic neuron from the ganglion to the target effector. Somatic (SNS) reflexes involve a direct connection from the ventral horn of the spinal cord to the skeletal muscle.
Lastly, the neurotransmitters used in these two systems vary as well. As you may remember, the SNS uses ACh at the neuromuscular junction to excite the skeletal muscle to contract. The SNS only needs this one neurotransmitter because the only result of excitation by the somatic motor neuron is skeletal muscle contraction. A lack of signal will result in skeletal muscle relaxation. I think of this like a cup of coffee – coffee being the signal to contract. After drinking a cup of coffee we have lots of energy to contract and move about our day, a lack of coffee leaves us fatigued and in a relaxed state.
