05.01 Sensory Basics

Okay guys in this lesson we're going to talk about the basics of our senses and how they work

So what the heck are senses anyways? A sense is just a neural pathway that receives a signal and sends it up to the central nervous system for interpretation. So you get some sort of signal - like feeling hot water in your skin. That sensory nerve sends a signal to the spinal cord or through the spinal cord to the brain where the signal gets interpreted. That’s your senses. Now this is just part of the process - the second half is the motor or reflex process, but for this lesson we’re just going to focus on the sensory part.

So everything starts down in the sensory receptors. These are just modified parts of the sensory neuron that is designed to receive and respond to a stimulus. With most sensory neurons we actually see something called specificity - which means these receptors are designed to detect and respond to just one specific kind of stimulus. Once it detects that stimulus, it starts these little local depolarizations - we talk about those in the neuro section. As those start summating or adding together, we’ll be able to create an action potential and off we go. The signal will propagate or conduct down the neuron towards the central nervous system so our brain or spinal cord can react to it!

There are 5 major senses - touch, which we are going to talk about in detail in this lesson. Vision, which is controlled by cranial nerve 2 and hearing which is cranial nerve 8. We also have taste, or gustation - which is cranial nerves 7 and 9. Basically the chemicals in our food get dissolved in our saliva and our taste buds are able to sense and respond to those chemicals. Taste buds are just a cluster of taste cells and we have 5 types. Salty, sweet, bitter, sour, and umami - umami is the flavor of meat proteins. And last we have smell or olfaction. This is cranial nerve 1 - the olfactory nerve. The way it works is there are volatile chemicals in the air. Volatile just means the chemicals evaporate and we can breathe them in. Then the sensory neurons and receptors in the epithelium in our nose will detect and respond to those chemicals and send the signals through that olfactory nerve into the brain to interpret. Now I’ve mentioned sensory receptors, so let’s look a little closer at those receptors.

There are three main types of receptors. Exteroreceptors respond to external stimuli - things like sight, sound, smell, taste, and touch - these are the main things we think of when we think of senses. This could be things in our skin and mucus membranes - cutaneous receptors - or pain receptors - and we’ll look at that in more detail in just a second. We also have proprioceptors which are responsible for helping us interpret position and movement. Basically it helps us know our position in the world - physically. Like … am I hanging upside down? Am I spinning in circles? Turning left? Turning right? etc. Then finally we have interoreceptors which help us to detect internal stimuli that comes from within our bodies. So let’s look a little closer at these two, exteroreceptors and interoreceptors.

So remember exteroreceptors respond to external stimuli and that includes cutaneous or receptors in our skin and mucus membranes. There are 4 different things they respond to - and I’ve given you details about those receptors in your outline. We see tactile receptors that respond to light touch and help us perceive the difference between things like sharp, dull, and soft. We also have receptors in our subcutaneous tissue to help us detect deep pressure. Free nerve endings can help us detect temperature changes and determine hot versus cold. And finally we have other kinds of free nerve endings called nocireceptors that help us to detect pain in our skin.

We also have sensory receptors that help us to detect pain and there are 4 types of pain you need to know about. First is superficial somatic - this is physical pain felt in our skin or tongue. This would be like getting a cut or an ulcer or a burn. Deep somatic pain is felt in our muscles, tendons, ligaments, and bones - it’s obviously deeper and usually more intense than superficial somatic pain. This might be from a fracture or torn ligament. Then we have visceral pain - any time you see visceral, think ‘organ’. So visceral pain is pain felt from the internal organs - this could be like stretching your stomach when it’s too full, or lack of blood flow - which is what ischemia means. It’s very internal and sometimes hard to pinpoint. And the last kind is unique - it’s called Referred pain. This is when pain from one area is actually felt in another. Let me give you an example. The heart and the left arm’s sensory neurons come together and come into the spinal cord through the same spinal nerve. So the theory here is that when something happens to the heart, like a heart attack - and it sends that signal. The brain gets confused because it hasn’t seen that signal before. So it interprets it as being left arm pain - because it understands that. So a heart attack may present with left arm or jaw pain. We can also see liver damage present with right shoulder pain - again it’s called Referred pain.

Lastly we have interoreceptors - remember these are responding to internal stimuli. I’m not going to go into super deep detail here, because in other lessons we talk about how these actually impact the body - let’s just run through them quickly. Baroreceptors respond to changes in blood pressure - baro means pressure. Chemoreceptors respond to chemical changes in the blood or body fluids like a lack of oxygen, too much CO2, a change in pH or changes in the gastric acid concentration. Osmoreceptors detect and respond to the concentration of blood and how much water is in it.

Temperature receptors respond to changes in internal body temperature and the response to that is controlled by the hypothalamus. We see nocireceptors in internal organs again detecting ischemia or stretch in the organs - that’s that visceral pain. And finally we see stretch receptors in the lungs that detect changes in lung volume so your body can respond to that. So, again, these are sensory receptors that respond to internal stimuli in the body.
So let’s recap quickly. Sensory receptors function to detect and respond to specific types of stimuli and send those signals to the central nervous system for interpretation. We have 5 main senses - touch, vision, hearing, taste, and smell. We have exteroreceptors to respond to external stimuli, both pain related and through the skin and mucus membranes. Proprioceptors help us determine our position and movement - so you know if you’re upside down even if you can’t see. And we have interoreceptors to respond to internal stimuli like blood pressure or chemical changes, visceral pain, temperature, and stretch of the lungs.

So that’s it for the basics of how our bodies detect and respond to stimuli, both externally and internally. Make sure you check out all the resources attached to this lesson. Now, go out and be your best self today. And, as always, happy nursing!