Okay. So now, let's consider the concept of sensory transduction. So, you've already been introduced to this idea. This is the concept of how is the energy of the environment converted into electrical signals. Well, for the mechanosensory system, this happens when physical deformation of ion channels leads to the opening of those channels and the influx of current-carrying ions. So, for our somatic sensory system, this happens as channels that are permeable to sodium ions are forced open. So, the physical pressure forces open these ion channels. They are activated by physical forces, not by voltage, not by neurotransmitters, but by physical forces here. And that leads to the influx of sodium ions and so as these sodium ions enter, as you should now appreciate, they carry with them their positive charge, and that leads to some measure of depolarization. So, if you open a small number of these sodium channels, you're going to end up with a small depolarization of that axon terminal. So, this depolarization is called a receptor potential, because it originates with some kind of receptor element at the end of the sensory axon. If you provide a stronger stimulus, you'll get an even stronger receptor potential. But if that potential fails to reach threshold for generating an action potential, this signal will simply decay away and you'll never know it because there will not be an electrical signal that can propagate from the receptor ending into the central nervous system. That only happens when the stimulus is sufficiently strong to reach the threshold for generating an action potential. So, once that receptor potential hits threshold, now our familiar voltage-gated ion channels kick in and we end up firing an action potential. So, the stretch-gated channels, they get us to threshold. That's the receptor potential. Sometimes, we call that a generator potential. But the firing of action potential is where our voltage-gated sodium and potassium channels come in. That's what gets us from a receptor potential in the end of a sensory axon, all the way into the nervous system with the potential of actually experiencing that sensation if the signal makes into the cerebral cortex. I've been alluding to the morphological specializations at the end of our mechanosensory fibers. Now, let's have a look at them. And we see really, a beautiful array of specializations even within our skin surfaces. So, what we see depicted in this figure is an illustration of our endings of these sensory axons, and what happens to them. Let me first highlight those receptors that are responsible for our sense of light touch. So, go ahead and, and rub your fingers together, just very lightly, or perhaps pass your fingertips over a tabletop surface that you may be near. And that very light touch sensation that you may be feeling on your skin surfaces, that's due to the activation of Meissner's corpuscles and our Merkel cell-neurite complex. These are, are both exquisitely sensitive to mechanical deformation, but they operate in slightly different way. The Meissner's corpuscle is a specialization of the nerve ending with schwann cells that form a layer structure and constitute what we call a corpsucle. This is found as the dermis interdigitates with the epidermis. And we find these structures oriented in sort of a radial direction relative to the skin's surface. That makes them acutely sensitive to a small patch of skin that might be deformed. Now, contrast that with our Merkel cell-neurite complex, this is a specialized arrangement of nerve endings with cells that sit between these dermal papilae and there's actually neurotransmission that occurs here. So, a receptor potential is going to be generated in these specialized Merkel cells, a neurotransmitter will be released and that will activate the nerve terminal of the first-order axon. So together, the Meissner corpuscle and the Merkel cell-neurite complex are responsible for most of what we think about as light touch signals, okay? So you'll notice that there are some odd-looking receptors that are found a little bit deeper in the skin, in the dermis, and on into the subcutaneous layer. These are two different kinds of encapsulated receptors. One is called the Ruffini corpuscle. And this is a rather strange structure, we actually don't understand its physiology quite as well as some of the others. But it seems to be sensitive to stretch. So, if you were to extend the digits in your fingers and sort of wave out, fan out your fingers, you're stretching those skin surfaces and that sense of tugging in your skin is probably derived from these Ruffini corpuscles. And then, finally, deeper in the dermis or in the subcutaneous tissues, we have these, these beautiful structures called Pacinian corpuscles. So, this is a, a nerve ending that's encapsulated with many layers of schwann cell lamellae. And it's almost like the layers of an onion around a nerve ending and these layers are, are buffered with a fluid, an extracellular fluid. And so, what happens is that, as there is some depression, there is a very quick and rapid activation of that sensory ending. But very quickly, the fluids redistribute and that deformation rebounds and we end up restoring, very quickly, this rings of onion type of structure around that nerve ending. So, so, these are, are endings that are going to be sensitive to vibration but they're going to act only very physically as soon as than deformation is applied. Well, to speak to the physiology of these various receptors, I'm going to show you a table from the book which truly has more information than I want you to know. So, let me just tell you what about this table, I think is important for you to take away. I certainly want you to recognize our four major types of superficial receptors that we've been talking about. And I want you to notice that there are two, our Merkel and Meissner structures that have small receptive fields, which means, that they're going to be especially sensitive to spatial position. And then there are two, those that are deeper, our Pacinian and Ruffini corpusles, that have larger receptive fields. So, pay attention to the extent of skin surface that can activate that receptor. I would also invite you to focus in on the function that's listed here in this table. So, notice that the Merkel and Meissners have to do with light touch and perhaps even different aspects of light touch, whereas, our deeper receptors have to do with broader sensations that have less spatial detail in them. In the case of the Pacinian mainly sensitivity to vibration. And as I mentioned for Ruffini corpsucles a change in shape that would be consistent with a stretch of the skin. And lastly, I would encourage you to pay attention to the innervation density, which relates to the spatial acuity that is provided by this receptor system. Notice that the innervation density is quite high for our, our Merkel cell-neurite complex. And, and really extremely high for our Meissner's carpuscles that contributes to the pretty high spatial acuity of this system. Whereas, our deeper receptors, our Pacinian corpuscles, and our Ruffini corpuscles are present at much lower density, and that's consistent with them being sensitive to a broader surface of the skin. Okay, let's move on and let's consider some additional types of receptors that are found in deeper tissues in the body. One very important proprioceptor that we'll talk about in some detail when we talk about spinal cord circuits for motor control, is something called the muscle spindle. And so the muscle spindle is formed by specialized muscle fibers and certain types of receptor endings that innervate those specialized muscle fibers. So, what we see here is an illustration of a muscle. So, this is a muscle that is part of a musculoskeletal unit, and within that muscle, we find a fusiform-shaped spindle that is bounded by a capsule. So, within that spindle, we find a small number of muscle fibers. Those muscle fibers are called intrafusal muscle fibers. And they're different than the muscle fibers that are found outside the spindle that generate the force when we contract a skeletal muscle. These muscle fibers tend to have a collection of nuclei that are bundled near the center of the spindle shape and then, they have their contractile elements extending away from that central region out to pole of this fusiform form shape. In these two regions of the muscle spindle are supplied by two different kinds of sensory structures. There is a type of sensory axon, called the Group Ia afferent, that innervates this central region of the muscle spindle, this bulbous part, and notice how it does so. These axons just wrap around the central region of these muscle spindles. There's a second type of sensory axon that supplies the muscles spindle. It's called a Group II afferent axon. And rather than wrapping around the central portion of the intrafusal fiber, this axon terminates in a special type of ending that we call a flower spray ending. And it's located on the contractile elements of the intrafusal muscle fiber extending some distance away from this central bulbous region of the spindle. Taken together, these two kinds of spindle afferents, the Group Ia and the Group II, allow this muscle spindle to send signals into the spinal chord that are pertaining to stretch. So, as the muscle is stretched, either actively or passively, basically, this spindle gets stretched out as well. And that spindle ends up putting mechanical pressure on these sensory endings that are found at the terminals of the Group Ia afferents, as well as in these flower spray ending associated with the Group II. So, this makes these sensory signals important for determining the length of a muscle because they're sensitive to the stretch that is being applied. We'll come back and talk more about that later. But let's move now to the last type of sensory receptor that I want to show you. This is also associated with skeletal muscle. But rather than a axon supplying the muscle fiber directly, this is a axon, we call this a Group Ib afferent, and it innervates the junction of the muscle fiber and the tendon. So, there's a capsular region between the tendon and the muscle fiber, which is made of a woven matrix of collagen fibers. And basically what happens, as is detailed down below, is the axon branches and interdigitates within these collagen fibers. And as a result, when there is a force applied to this tendon as these extrafusal muscle fibers contract, this collagen matrix gets stretched. And that matrix, because of the interdigitating axons, puts pressure on those axon terminals. And so, that is the force that opens the ion channels leading to the generation of receptor potentials in these nerve endings. So, this type of receptor, it's called the Golgi tendon organ. This type of receptor is going to be sensitive to muscle force rather than to a stretch being applied to the muscle. We'll talk more about these receptors when we get into the circuitry of the spinal cord and how muscle tone is regulated but for now, just know that there are two kind of receptor systems associated with our musculoskeletal units. There are Golgi tendon organs that are sensitive to the force that's generated by the contraction of the skeletal muscle and there are muscle spindles that are going to report the length of that muscle back to the central nervous system not the force, but the length, and so, the relevant stimulus for the muscle spindle is stretch. Now, lastly, I'll just mention but I won't show you that there are also receptors associated with our joints that inform the nervous system about the movements of those joints or the pressures that are acting upon them. So, taken together with the muscle spindles and the Golgi tendon organs, we can think of these as being the proper proprioceptors that inform the central nervous system about the movements of the musculoskeletal system.
