41 Functional divisions of the nervous system

Imagine driving a car. You have to be mindful of the steering wheel, your mirrors, brakes, the radio…there is so much to be cognisant of. Now imagine on top of that, you have to manually spin the wheels, inject fuel into the engine, file your taxes and cook dinner. Simply being human can be like driving – there is so much to be aware of, yet there is so much that goes on within our bodies to ensure we can function.

The functional divisions of the nervous system:

Our bodies can function within homeostasis – whether it is controlling heart rate, digestion or breathing – through the autonomic nervous system (ANS). It operates automatically to keep our body’s internal environment stable, without us having to think about it. Within the body there are two branches of the autonomic nervous system – the parasympathetics and the sympathetic. However, we are given voluntary control over some of our daily rhythms, and this is accomplished through the somatic nervous system. Think back to our example of eating an apple. To bring that apple to your mouth your somatic nervous system must allow your hand to bring the fruit to your mouth. Yet once you swallow, your voluntary control is over, and your ANS takes over to digest the apple. It’s like your somatic nervous system is a trained pilot and the ANS is the autopilot.

Division of the somatic nervous system:

Remember learning to play an instrument for the first time? Perhaps it was the guitar, and you were trying to strum a chord or switch between notes? You had to look at your fingers, feel the strings beneath them, listen to the sound, and carefully move your hand in just the right way. That process of sensing, interpreting, and acting all happened because of the close coordination between your somatosensory and motor systems of the somatic nervous system.

Somatosensory Pathways:

The somatosensory pathway is the sensory division of the somatic nervous system which help bring in information about the outside world, like the texture of the strings or the pressure of your fingertips.

• First-order neurons are like your fingertips sending the first message up the line. Their cell bodies sit in the dorsal root ganglia (DRG), clusters of sensory-neuron cell bodies located just outside the spinal cord, which carry raw (unprocessed) sensory information from your skin, muscles, or joints to the spinal cord or brainstem to become refined.
• Second-order neurons are the next step in the relay, located within the spinal cord or brainstem. They receive the input and then pass it along to the communications hub of the brain, the thalamus.
• Third-order neurons, based in the thalamus, act like messengers delivering the final signal to the primary somatosensory cortex in the parietal lobe, where you become consciously aware of the sensation, like recognizing that you’re pressing too hard on a string or not quite in the right spot.

The Motor Response

Once the brain has processed what you have felt, heard and seen, it decides what to do next, like adjusting your finger placement or softening your grip.

• Upper motor neurons (UMNs) start the plan. Their cell bodies are in the primary motor cortex of the frontal lobe, and they send instructions down the brainstem or spinal cord.
• Interneurons, located in the spinal cord or brainstem, help coordinate and refine this movement. They pull in extra information, from your sensory system or other motor areas, to make sure your fingers land just right.
• Lower motor neurons (LMNs) are the ones that actually carry out the command. Their cell bodies are in the spinal cord or brainstem, and their axons travel through cranial or spinal nerves to activate your skeletal muscles, allowing your hand to move smoothly and deliberately.

Together, these sensory and motor systems are constantly in conversation. While reflexes like jerking your hand away from something hot happen automatically, voluntary actions like tuning your guitar or playing a chord rely on a careful back-and-forth between input and output. The somatosensory cortex, motor cortex, and cerebellum work together to interpret information, plan movements, and make sure your actions are precise and well-coordinated.

Divisions of the ANS:

• The sympathetic nervous system (SNS) is like the body’s emergency response system. When we are placed in a fight or flight situation, the SNS kicks in to automatically prepare the body for action whether it is through increasing heart rate, dilating the pupils or constricted blood vessels.
• The parasympathetic nervous system (PNS) is like the body’s maintenance routine. When the day winds down and you simply want to relax, the PNS relaxes the body, digests food, and conserves energy after stressors are gone.

Here are some real-life examples of tasks which your autonomic nervous system:

How do they interact?

While there are two branches of the autonomic nervous system, how do they interact with each other? What happens if one system is too strong? What would it be like to be stressed 24/7?…oh wait you’re already studying anatomy. In short, these two systems work in a kind of push-pull relationship to keep your body in homeostasis. This is achieved through the presence of neurotransmitters. The SNS is activated through the presence of epinephrine, whereas the PNS is activated through acetylcholine (AcH).

You may have noticed each system can have opposing influences on an organ – the SNS dilates the pupils, whereas the PNS constricts them. In life’s daily rhythms these two systems regulate each other, so while your SNS may dilate your pupils through release of epinephrine, your PNS will regulate how dilated they will get by releasing minimal amounts of AcH.