By Catherine A. Sanderson, Amherst College
The nervous system is typically divided into the central nervous system, which includes not only the brain, but also the spinal cord, and the peripheral nervous system that includes all of the nerves outside of the brain and spinal cord. It is the spinal cord, however, which is the core of the entire nervous system, central and peripheral.
Reflexes
The spinal cord is really a long tube containing bundles of nerve fibers and has two main functions. The first of this is initiating certain automatic behaviors on its own. These involuntary behaviors are called reflexes, as the response to the incoming stimuli is automatically sent to the spinal cord and then reflected back to the relevant muscles.
Hence, reflexes are basically a shortcut that allows our body to immediately react since the signals never pass through the brain. We’re all born with numerous reflexes—such as blinking in response to a puff of air delivered to the eyes, or the gag reflex in response to an object touching the back of the throat.
When the doctor taps our knee just below the kneecap, our leg automatically kicks out. This knee jerk reflex is actually an example of what is known as a deep tendon reflex. The doctor is tapping on a tendon, which then stretches the tendon and the muscle in the thigh that connects to it.
If the nervous system is working properly, that stretch sends a message to the spinal cord and the spinal cord very quickly sends a message back to the muscle telling it to contract, which is what causes the lower leg to suddenly kick out.
Carrier of Information
Another important function which the spinal cord performs is that it carries information from the entire body into and out of the brain. The spinal cord works together with the peripheral nervous system which carries information to and from the central nervous system of the brain and spinal cord to the body’s sense receptors, muscles, and glands.
The peripheral nervous system includes all the nerves outside the brain and the spinal cord. It is subdivided into two distinct parts that control different types of processes: the somatic nervous system and the autonomic nervous system.
Somatic Nervous System
The somatic nervous system carries sensory information to the brain and spinal cord and then carries messages from the brain and spinal cord to muscles.
We can better understand the working of our somatic nervous system through an example. Suppose we are outside, running on a cold winter morning and we see ice in front of us. Our visual system recognizes ice and sends this information to our brain. Our brain then sends signals to our muscles telling them to take action, to shift our body to a different part of the path so that we can avoid the ice, and a potentially dangerous fall. This is all voluntary and is done by the somatic nervous system.
Autonomic Nervous System
The other part of the peripheral nervous system is the autonomic nervous system, which controls involuntary tasks such as heart rate, digestion, breathing. This system (as its name implies) operates at an automatic level. We don’t have to regularly remind ourselves to breathe or digest food.
The autonomic nervous system also further divides into two distinct branches: the sympathetic and the parasympathetic, which work in opposition to each other.
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Sympathetic and the Parasympathetic Nervous Systems
These two branches regulate multiple activities within the body, including breathing, heart rate, digestion, and sexual arousal. These two systems balance each other out.
The sympathetic nervous system helps the body respond quickly to mobilize resources during times of stress; this emergency response is often described as the fight-or-flight response.
The parasympathetic nervous system works to calm the body and conserve energy, including the freeze response. So, the two systems together are sometimes called flight, fight, or freeze.
Handling a Crisis
To understand how crucial a role they play in handling a crisis, let’s take a hypothetical situation. Suppose one is walking in the woods and sees a rattlesnake, coiled and ready to strike. in such a situation, our sympathetic nervous system would immediately kick in and prepare us to run or fight back if running away was not possible.
Our heart rate and breathing would increase, our digestive system would stop working, and cortisol (a stress hormone) would be released into the bloodstream. All of these physiological responses would help get more blood and energy to our muscles, allowing us to take quick action.
Flight, Fight, or Freeze
If we froze, or after we had successfully escaped from the threat, the parasympathetic nervous system would kick in to help return our body to a calmer state. Our heart rate would slow down, our blood pressure would lower, and our digestive process would return to normal.
Hence, although they work in opposition and have opposite effect on our body—whether causing us to flight, fight, or freeze—together they help bail us out of immediate danger.
Common Questions about the Human Nervous System
When the doctor taps our knee just below the kneecap, our leg automatically kicks out. This knee jerk reflex is actually an example of what is known as a deep tendon reflex.
The central nervous system includes the peripheral nervous system. It includes all the nerves outside the brain and the spinal cord.
The sympathetic nervous system helps the body respond quickly to mobilize resources during times of stress; this emergency response is often described as the fight-or-flight response.
