Video: Functional divisions of the nervous system

Brrr! It's a freezing cold winter's morning! The remedy? A warm cup of coffee to melt those icy fingers and get you ready for a rapid-fire physiology session. But, the act of grabbing and taking a sip of a hot cup of coffee is not a simple thing. Although you've probably never given it a second thought, this small task involves a complex symphony of millions of neurons firing in succession across your nervous system. You see, before doing anything, you first have to see that cup of coffee before you, then recognize it for what it is, decide what to do with it, and finally, respond with the appropriate action of picking it up and taking a nice glug. Sensation, integration, and response – that's what I'm talking about.

We can understand this symphony of the nervous system as the functional divisions of the nervous system.

The nervous system can be categorized according to two main classifications – structural and functional. The structural classification of the nervous system describes the organization and different parts of the nervous system based on their physical location or anatomy. It has two main divisions – the central nervous system, which covers the brain and spinal cord, and the peripheral nervous system, which is basically everything else outside of that.

To approach the nervous system differently, we can also describe it according to its functions. These generally fall into three main categories or divisions – sensation, integration, and response.

Before you grab that cup of coffee, you see it. This marks the initial step in the sensation response pathway. Then, as you pick up the cup, you sense its warmth against your cold hands. This is part of the sensory division, also known as the sensory or afferent nervous system. Sensation involves the excitation of sensory receptors by stimuli from the internal and external environment followed by the transmission of this information to the central nervous system for processing.

A receptor cell is a molecule, often a protein, that receives and transduces stimuli. These stimuli are then converted into electrical signals.

There are various types of receptors, specific to different modalities or types of sensations. So for instance, photoreceptors respond to light, while nociceptors detect stimuli which are potentially harmful or noxious and often result in pain. There are also thermoreceptors that, as you can probably guess by their name, responds to changes in temperature.

Sensory inputs are carried by the afferent division of the peripheral nervous system, meaning, sensation is carried towards the central nervous system. Now depending on where they originate and what type of information is involved, sensory or afferent signals can be defined according to three classifications – somatic sensory, visceral sensory, and special sensory.

The somatic sensory division consists of neurons that carry signals generally associated with the sense of touch, temperature, pain, and body position from various parts of the body, such as muscles, bones, skin, and joints, carrying signals like "Ouch, that coffee is really hot", "Something just slithered over my foot", or knowing your hands are in front of your face even when your eyes are closed.

This visceral sensory division comprises neurons transmitting signals from internal organs like your lungs, stomach, urinary bladder, and any other internal organ you can think of. We're talking about sensations like, "My bladder is really full", or, "I ate far too much for dinner."

Receptors in the special sensory division are located in specific organs like the oral cavity, nose, eyes, and ears, and transmit more specialized sensorial information about the special senses – the site of the coffee cup, the taste of the espresso shot, the smell of the coffee shop, the sound of the coffee grinder, and your ability to balance your body while walking towards the coffee counter.

I will note that some resources alternatively list the special senses of sight, hearing, balance, and smell as components of the somatic sensory division with taste belonging to the visceral sensory division – just to mention, in case this alternative classification comes across your path sometime.

So those are the three main divisions under sensation. Let's move on to integration.

Once we have seen the coffee cup, neurons in the brain then have the task of filtering and processing this information to create a coherent picture in order to respond appropriately. This process is called integration. Generally speaking, there are two types of integration – lower-order and higher-order. To keep it simple, lower-order integration mainly occurs in the brainstem, cerebellum, and certain structures like the spinal cord, hypothalamus, and amygdala. These processes handle subconscious activities such as maintaining balance, coordinating movement, basic emotional response, as well as regulating fundamental bodily functions like temperature, hunger, and breathing.

Higher-order integrative processes, on the other hand, occur primarily in the cerebral cortex and involve critical thinking, decision making, memory recall based on past experiences, and executive functions like planning and attention. These processes contribute to logical reasoning and goal-directed behavior.

Try to think of integration like an advanced plane, navigating the skies of life. The flight management computer represents lower-order integrative processes, responsible for deciding upon basic routine tasks to keep the plane on course and maintain its stability through altitude control, speed regulation, and basic navigation similar to how lower parts of the brain function to maintain basic bodily functions which are essential for survival without conscious effort.

While the flight management computer functions to keep the plane flying smoothly, the pilot actively flying engages with the environment, making decisions based on their broader understanding of the situation. They can deviate from the predetermined path, respond to unforeseen challenges, and make strategic choices to ensure the overall safety of the aircraft, just like how our higher-order integrative processes evaluate the environment around us, shape our responses, and ensure adaptability.

Finally, we have the moment we've all been waiting for – the response motor output – to finally pick up, smell, and sip the warm cup of coffee. Mmm…

The motor or efferent division consists of efferent neurons that carry signals from the brain to target structures known as effectors in order to effect the responses generated by integrative processes. Neural output is most often considered in the context of movement or motor control; for example, the contraction of skeletal muscle along your upper limb to move the cup in your hand to your mouth. However, neural outputs also include other signals like glandular control, stimulating your exocrine and endocrine glands to release substances, so they control salivation which starts in your mouth when you smell that coffee.

Similar to the sensory division, the efferent or motor division can also be classified into two main categories – the somatic motor and visceral motor divisions. The somatic motor division comprises neurons responsible for transmitting signals to skeletal muscles which are generally under conscious control, hence, often termed the voluntary motor division.

In contrast, the visceral motor division, more commonly referred to as the autonomic nervous system, regulates functions like glandular secretion, smooth muscle contractions in organ walls, and regulation of cardiac muscle contractions in the heart. It is consistently carrying signals which result in involuntary, fine-tuned adjustments to the organs across your body based on the signals transmitted by its counterpart, the visceral sensory system. It comprises two physiologically distinct, mutually antagonistic parts – the sympathetic and parasympathetic nervous systems.

The sympathetic nervous system is your fight or flight, get up and go-getter, which prepares your body for rapid, intense physical activity, usually as a response to stress or fear, while the parasympathetic nervous system is the counterbalance to this – your rest and digest, chilled out mellow fellow who promotes activities that occur when the body is at rest or during non-stressful situations.

So that's all three parts of the functional divisions of the nervous system, let's quickly recap.

Sensation involves the stimulation of receptors across our body and the transmission of sensory or afferent signals to the central nervous system. These signals can be somatic, visceral, or special sensory in nature. They are then integrated by lower and higher-order integrative processes which make sense of all of this information and make decisions about what to do or not to do next, which leads on to responses involving efferent or motor signals which act on our effectors. These again can be classified as somatic or visceral in nature.

Before your coffee even had time to cool, we managed to learn the functional divisions of the nervous system.

Bye for now!