Organization of Sensory Pathways: Study with Video Lessons, Practice Problems & Examples

The somatosensory system processes sensory information through three levels: receptor, circuit, and perceptual. Sensation begins when a stimulus excites a receptor, generating graded potentials—either generator or receptor potentials. These potentials lead to action potentials that travel via afferent pathways to the brain. At the perceptual level, the cortex interprets this information, allowing conscious awareness of stimuli, including their intensity and location. Understanding these processes is crucial for grasping how we perceive touch, pain, and proprioception, which are essential for interacting with our environment.

Somatosensory System

Hey there. So in this video, we're going to be talking about the organization of our sensory pathways. Now, the somatosensory system is the specific part of the sensory system serving the body wall and the limbs. And when we're thinking about senses, there's an important distinction to make. There are two types of senses. So first, we have our special senses, and these are probably the ones that you're most familiar with. This is when we have receptors actually located inside complex sense organs. Complex sense organs are things like your ears, your nose, your tongue, or your eyes. This pertains to senses like vision, hearing, taste, smell, and equilibrium. And you can see that these are all concentrated around our head typically. And then we have our general senses. The majority of sensory receptors actually belong to our general senses, and these receptors are also called simple receptors. And this includes pretty much everything involving touch. So the sense of touch. So pain, temperature, vibration, pressure, and proprioception all fall under general senses. Some of these you are very consciously aware of—things like pain, temperature, pressure, for example—as well as things that you are less consciously aware of but are still very important, like proprioception. Your ability to know where your body is in space and how to move it appropriately through that space. Those are the general senses. Now, we're going to have an entire chapter on special senses for you. So we are not going to focus on those in this chapter. This chapter is going to be entirely about the general senses. Alright. So now that we got that distinction out of the way, the somatosensory system operates at three levels of neural integration. So first, we have the receptor level and this involves our sensory receptors. So if you look down here at this green part of our image, we have our receptors and you don't have to know any details about these. Our examples here for you are muscle spindles and these joint kinesthetic receptors. But all you have to know is that these receptors are in your skin, they're in your muscles, they're in your joints, wherever they are, and they are sensing information about your internal and external environment, and they are sending that information up to the circuit level. And the circuit level is the processing that we see happening in ascending pathways. In other words, this is where they're going up the afferent nerves. So we have ascending, afferent, and they are going to arrive up in the brain in the CNS. Right? And that is the perceptual level. So this is where we have processing in cortical areas. So now we are in the central nervous system. We're no longer in that peripheral nervous system. And at this perceptual level, the information will get passed through the thalamus, our sensory relay station, and it will get processed in that somatosensory cortex. And your brain will decide what information to make you consciously aware of, what information to kind of keep on the unconscious level, and it's going to react to whatever it is perceiving and change your behavior accordingly. So those are our three levels that we see in our somatosensory system, and I will see you in our next video to talk about that receptor level in a bit more detail. See you there.

Receptor Level

Okay. So now we're going to dive into that receptor level. So, very broadly speaking, sensation requires two things. First, a stimulus has to excite a receptor, and then an action potential has to actually reach our central nervous system. And there are a couple of requirements in generating that signal successfully. So first, the stimulus energy has to match the specificity of the receptor. So you'll see this in an upcoming video, but basically, different types of receptors respond to different types of energy. For example, the photoreceptors in your eye respond to light energy. So our energy has to match our receptor. Next, our stimulus has to be applied within the receptive field, and the receptive field is just the area that that receptor monitors. So basically, our stimulus has to be close enough to our receptor for it to actually detect it. And then finally, a graded potential has to reach threshold. So our old friend, the graded potential, has returned. Very exciting. So receptors can actually produce two types of graded potentials, and we're going to cover those now.

So first, we have generator potentials, and these are pretty straightforward. Here the receptor region for these is part of a sensory neuron, and this is very common in general sense receptors. So in our general senses, and basically, what happens is that it's going to generate an action potential right there in the sensory neuron. So if we look down to our figure here, this one here on the top is depicting our generator potential. And you can see we have our sensory neuron. Let's just say that that's the sensory neuron in my fingertip, and we have our stimulus. Let's say that's pressure from my pen right there, and what is going to happen is that that stimulus is going to trigger a graded potential right there in that sensory neuron and then assuming that that hits the threshold, it will trigger an action potential, then that will get sent up to our central nervous system. So it's the exact same process that we've already learned about. So if you need a refresher on any of that, feel free to go back and watch my video on graded potentials. We cover all of that in lots of detail.

Alright. And next, we have a receptor potential, and this one's a little bit different. So a receptor potential actually involves two cells. So we have a separate receptor cell and our sensory neuron. So here the receptor region is actually in that separate receptor cell, and this is very common in the special senses. And if you're thinking, Hannah, you just told me that we're focusing on general senses. Why are we even learning this? Fantastic question. So you are going to have an entire chapter on special senses coming up, but they are not going to cover these graded potentials in detail. So we're going to kind of get this out of the way for you, and by the time you get there, you will understand all of this perfectly. Alright? So a nice example of this receptor potential is the rods and cones in your eye. So your eye has these specialized cells called rods and cones, and they are going to communicate with sensory neurons and that is what's happening here. So basically, what happens is that this graded potential will change the amount of neurotransmitter that gets released by that receptor cell onto the sensory neuron. So if we look down at our figure here, we have our receptor potential here at the bottom and you can see we have two cells. We have our receptor cell and our sensory neuron. So this receptor cell again could be like a cone in your eye and the stimulus would come in. In our example, it's going to be light stimuli and that's going to create a graded potential right there in that receptor cell and then that will trigger the release of neurotransmitters across the synapse, and then those neurotransmitters will create a graded potential in our sensory neuron. Again, assuming it's big enough, that will trigger an action potential and that signal will get sent up to the brain. Alright. So that is how a receptor potential works. So very broadly, the difference between the two is that in our generator potential, that graded potential will get generated right there in that sensory neuron and in our receptor potential, it's going to be generated in the receptor cell and it will then create an effect on our sensory neuron. Alright, so that is our receptor level and I'll see you in our next video to cover those circuit and perceptual levels. So, I'll see you there.

Organization of Sensory Pathways Example 1

Okay. So this example reads, the retina of the eye contains specialized photoreceptor cells, rods and cones. When light hits these cells it creates a blank potential. So, is this a receptor or a generator potential? Now keep in mind that rods and cones themselves are not sensory neurons, right? Those are specialized sensory cells, and so we must be dealing with a receptor potential. Remember, generator potentials happen right there in the sensory neuron. Receptor potentials have to kind of have that intermediate step of that specialized cell. So, if we're thinking about this, what we would have here is we would have our rod and then we would have our sensory neuron over here, and that light stimulus would hit our rod. The rod would have a graded potential. It would then release some neurotransmitters. Those neurotransmitters would cause our sensory neuron to then have a graded potential, and then, assuming that was strong enough, we would have that action potential, and then it would be sent up to the brain. So, in this case, we are dealing with a receptor potential, and I'll see you in the next one. Bye bye.

How does the strength of a sensory stimulus affect the generator potential in sensory receptors?

• A. Stronger stimuli result in smaller generator potentials.
• B. The strength of a stimulus is correlated with the strength of the receptor potential.
• C. Stronger stimuli result in larger generator potentials.
• D. Generator potentials are not influenced by stimulus strength, just by the presence of a stimulus.

Circuit Level & Perceptual Level

Okay. So in this video, we're going to be talking about the circuit and the perceptual levels. Now, the circuit level pretty much consists of impulses that are being delivered to the appropriate region of the cerebral cortex. So this is basically those action potentials that we talked about actually getting up, going up those ascending pathways toward the brain. So I always think of this as, like, you know, in cartoons when characters make little phones out of tin cans and they're, like, connected by a string? I always imagine we have, like, the sense receptors holding 1 tin can and the brain is holding the other one, and that little string is basically the circuit level. It's the actual circuitry that connects them. And these ascending pathways, or the circuitry, consist of neurons. Now the number of neurons can differ. On average, it's about 3, but there can be fewer than 3, so just keep that in mind. So once this signal actually gets up to the brain, we have the perceptual level. So at this level, the sensory input is processed and interpreted by the cortex. So here we're going to have our conscious awareness of the stimulus including all types of information such as the location of the stimulus, the intensity of the stimulus. So you know, are these lights very bright? Is this food very flavorful? Information like that. And then pretty much any other properties of the stimulus. So here we're going to start to understand the more kind of fine-tuned detailed information. So if we're thinking about touch this might be more detailed textural information. So, like, wool is soft and kind of scratchy. If it's food, we're getting kind of more fine-tuned understandings of the flavors. My tea is bitter. It's also sweet. So all of that detailed information kind of reaches our conscious level. So to give you a further example we have this little cartoon here depicting the circuit level over here. So she put her foot into her boot and, unfortunately, there is a little spider, although he's a very cute little spider to be fair. And but he's hanging out in her boot and so she had that information coming in from her toe, and it triggered this action potential that gets sent up these ascending pathways to her brain. And then she has this perceptual understanding of, oh my gosh, there's a spider in my boot, which is a poor thing. That's, like, my worst nightmare. Hopefully, she handles it better than I would. But so, yeah, that is the general organization of our sensory pathways from the moment that we detect a stimulus and our receptor responds right up to when we have conscious perception of it. And now that we understand that a little better, we are going to dive into sensory receptors in some more detail in our next video. So I will see you there.

Organization of Sensory Pathways Example 2

Okay. So this one reads, a reflex can be defined as a rapid automatic response to a stimulus. Which of the three levels of processing does not necessarily need to occur for a reflex to be carried out? So, let's kind of move through these and think about this sort of conceptually. Now at the receptor level, this absolutely has to happen. Right? Our sense receptors have to pick up something that's going to initiate that reflex. Maybe pain, a sudden temperature change, a change in our body position, maybe we start falling. Whatever it is, our sensory receptors have to pick up some kind of stimulus that's going to initiate this entire thing. So that one needs to be there. Alright. And similar to that, the circuit level also kind of has to be there because that signal has to make it at least to our spinal cord, if not, all the way up to our brainstem. Right? So the circuit level has to be there. The perceptual level, however, is not necessary. Remember, these are automatic responses. They do not need conscious perception of what is happening in order for a reflex to be initiated. Think about if you've ever accidentally touched a hot pot or something. You probably jerked your hand away before you even started feeling pain in your finger. Right? And that's because we don't need perception. Perception usually comes after that reflex actually happens, possibly during it, but it's not necessary for the reflex to take place. So the one that is not necessary is the perceptual level, and I'll see you guys in the next one. Bye bye.

Spatial discrimination is the ability to distinguish between and precisely identify two or more stimuli. In terms of touch, which of these body parts do you think would have the lowest degree of spatial discrimination?

• A. Tongue.
• B. Lower back.
• C. Fingertips.
• D. Lips.

Here’s what students ask on this topic:

What are the three levels of neural integration in the somatosensory system?

The somatosensory system operates at three levels of neural integration: receptor, circuit, and perceptual. At the receptor level, sensory receptors in the skin, muscles, and joints detect stimuli and generate graded potentials. These graded potentials can be either generator potentials or receptor potentials. The circuit level involves the transmission of action potentials through ascending pathways (afferent nerves) to the brain. Finally, at the perceptual level, the sensory input is processed and interpreted by the cerebral cortex, allowing for conscious awareness of the stimulus, including its location, intensity, and other properties.

What is the difference between generator potentials and receptor potentials?

Generator potentials and receptor potentials are two types of graded potentials. Generator potentials occur in general sense receptors where the receptor region is part of a sensory neuron. When a stimulus is detected, it generates an action potential directly in the sensory neuron. Receptor potentials, on the other hand, involve two cells: a separate receptor cell and a sensory neuron. This is common in special senses. The receptor cell generates a graded potential in response to a stimulus, which then causes the release of neurotransmitters. These neurotransmitters create a graded potential in the sensory neuron, which can then trigger an action potential if the threshold is reached.

How does the somatosensory system process sensory information?

The somatosensory system processes sensory information through three levels: receptor, circuit, and perceptual. At the receptor level, sensory receptors detect stimuli and generate graded potentials. These potentials lead to action potentials that travel via afferent pathways to the brain at the circuit level. The circuit level involves the transmission of these action potentials through ascending pathways to the cerebral cortex. At the perceptual level, the cortex interprets the sensory input, allowing for conscious awareness of the stimuli, including their intensity, location, and other properties. This process enables us to perceive touch, pain, and proprioception, which are essential for interacting with our environment.

What role does the thalamus play in the somatosensory system?

The thalamus acts as a sensory relay station in the somatosensory system. When sensory information reaches the brain, it first passes through the thalamus. The thalamus processes and filters this information before relaying it to the appropriate cortical areas, such as the somatosensory cortex. This ensures that the brain receives and interprets the most relevant sensory information, allowing for conscious awareness and appropriate behavioral responses. The thalamus plays a crucial role in determining which sensory inputs reach our conscious perception and which remain at an unconscious level.

What is the significance of the receptive field in sensory perception?

The receptive field is the specific area monitored by a sensory receptor. For a stimulus to be detected, it must be applied within this receptive field. The size and location of the receptive field determine the sensitivity and specificity of the receptor. Smaller receptive fields allow for more precise localization of stimuli, while larger fields may detect stimuli over a broader area but with less precision. Understanding receptive fields is crucial for comprehending how sensory information is processed and perceived, as it influences the accuracy and detail of the sensory input that reaches the brain.