Histology of the peripheral mechanoreceptors.
Hey everyone! It's Nicole here from Kenhub, and I'm going to start this tutorial by asking you to close your eyes and picturing your arms and legs. So, how is it that you know where they are? You can also feel whether you're sitting down or standing up and whether the place you're in is too hot or cold, but how? The answer lies in a specialized set of nerves known as your peripheral mechanoreceptors.
Today, we'll start by covering what mechanoreceptors are including where they're found and what their functions are and then we'll learn about the various different types of mechanoreceptor and how to identify each type on histology sections in the skin, and then we'll look at some clinical notes relating to the peripheral mechanoreceptors. But, first, let's talk about what a mechanoreceptor is.
So, mechanoreceptors are specialized nerve endings sensitive to physical distortion. This means that they fire or alter their firing rate in response to a charge in their membrane shape. In other words, they work in response to being moved. Mechanoreceptors are found in all sorts of locations around the body including the skin, in muscles and fasciae, around arteries, in the walls of the bladder, and even in your inner ear. There are different types of mechanoreceptors in different parts of the body and we'll touch on these briefly.
In this tutorial, however, we'll be focusing in most detail on the cutaneous or peripheral mechanoreceptors which are the mechanoreceptors in the skin. And mechanoreceptor of any type is triggered through its nerve endings – a type of which is shown just here. The signal is then sent along the peripheral nerve to be processed by the central nervous system which specifically refers to the brain and the spinal cord. So most mechanoreceptor signals are generally processed subconsciously – meaning, we don’t even notice them most of the time. While we may not register them, they're definitely valued by our brain and our spinal cord.
Their function is to give us our sensation of touch which can be conscious or subconscious; to stimulate reflexes, for example, tendon reflexes when upon rapid stretch such as that caused by the tap of a tendon hammer – and you can see how the muscle is involuntarily contracting. The signals – especially those from muscles, ligaments, and fascia – are used for proprioception which is our ability to sense where our joints are positioned in relation to the rest of our body parts. To show this, close your eyes and bend one arm. You know instinctively which arm is bent and which one isn't.
In organs in blood vessels, mechanoreceptors monitor stretch. Have you ever felt bloated? These are the receptors that are giving you that info. And specialized receptors in the inner ear known as stereocilia provide us with our sense of hearing and of balance. As I mentioned earlier, we'll be focusing on the cutaneous receptors that give us our sense of touch but if you want to know more about proprioception in action, don't forget to check out our article on the stretch reflex or if you're interested in the stereocilia of the inner ear, take a look at the article entitled the vestibular system which are both available on the Kenhub website. But let's begin by talking about several different types and subtypes of mechanoreceptors.
First, just a word on the image we'll be using as a basis for this section. So, this is a histological slide at section of skin stained with hematoxylin and eosin also known as an H&E stain. The skin surface is here and the dermis is here and the hypodermis is down here. Mechanoreceptors are scattered throughout this image and we'll be looking to identify these today. Let's talk about some different types of mechanoreceptors.
So, mechanoreceptors come in three main types – proprioceptors which monitor stretch to facilitate proprioception, baroreceptors which monitor stretch to detect pressure within a vessel or other enclosed space, and tactile receptors which give us our sense of touch and will be our main focus for today. But, first, let's take a really quick look at the proprioceptors and the baroreceptors.
So, starting with the proprioceptors, and here we can see an example of skeletal muscle, specifically, the biceps brachii. Proprioceptors detect joints and muscle position. They do this by monitoring stretch. And there are three subtypes of proprioceptor – the first being muscle spindle fibers located in the muscle itself. Let's just take a little bit of time to explain a little bit more about the muscle spindle fiber.
So, muscle spindle fibers are contained inside a capsule and are also known as intrafusal muscle fibers. The capsule separates these from the contractile extrafusal muscle fibers which make up most of the muscle bulk. Muscle spindle fibers do not assist with muscular contraction. Instead, they monitor for changes in stretch in a muscle. For example, if a ball is dropped into your hand with your arm flexed, your biceps will automatically contract to compensate.
Dendrites at the distal end of the sensory peripheral nerves supplying the muscle are wrapped in a spiral around the center of the fiber. The nerves are constantly firing at a steady rate because the cerebellum is a little bit of a control freak and is always demanding what's going on in the muscles, so when the muscle spindle fibers stretch, the coils of the spiral are distorted which changes the firing rate of signals along the sensory neuron. And this lets the cerebellum know that the muscle is being lengthened and by how much.
Our next subtype – the Golgi tendon organs – are found at the junction of muscle and tendon just here. So while muscle spindle fibers monitor tension in the muscle, Golgi tendon organs monitor tension in the tendon. So if our muscle is contracting hard against an immovable force, damage will obviously eventually occur and the muscle will keep contracting and the tendon will continue to be stretched quite a lot.
So, the Golgi tendon organs sense the tension in the tendon and initiate a reflex arc to relax the muscle and to avoid damage to the tendon, and these receptors are similar to the muscle spindle fibers in terms of how they detect stretch. For example, the sensory nerve dendrites are spiraled around the collagen fibers which form the tension.
And our third and final subtype are the free nerve endings located in joint capsules and in ligaments as well as fasciae throughout the body. Free nerve endings are exactly what they say on the tin – nerve endings which are not contained within a capsule or other structure. This makes them non-specific receptors technically termed polymodal. So what this means is that they are stimulated by stretch, temperature, chemicals, and any other way you can think of activating a nerve and they're found in connective tissue throughout the body, for example, in fascia, ligaments, joint capsules, skin and in the coverings of your muscles and your bones. They're particularly concentrated in joint capsules where they primarily respond to stretch and the free nerve endings can, therefore, detect pressure, movement and tension of the joint capsule.
The three types of proprioceptor work together with the balance receptors of the inner ear to give the cerebellum a complete picture of our body's position in space – that is, our sense of proprioception. It's a pretty complex sense as you can tell but it's vital to our everyday functioning.
Next stop, the baroreceptors. The baroreceptors actually interact even less with a conscious mind than the proprioceptors. So, baroreceptors are another form of free nerve endings that detect stretch. They are found in the elastic tissue in the walls of distensible organs such as the bowels, the bladder, and the blood vessels, which is what we can see here. The carotid sinus – a specialized part of the carotid artery – is shown here and it constantly monitors our blood pressure without ever seeking recognition from our conscious mind.
Baroreceptor dendrites wrap in a spiral around elastin fibers, so basically a similar arrangement as proprioceptors except for the type of fiber – elastin – instead of collagen. When the pressure changes within the organ, the elastin fibers are stretched deforming the dendrites and changing the firing rate thus providing information to the relevant center in the CNS. For example, the bladder baroreceptors will send information to the micturition center in the pons while the baroreceptors in the blood vessels provide stretch information to the autonomic nervous system for blood pressure control.
So, what about these highly anticipated tactile receptors? So, these are our main focus for today and they come in various forms which we'll look at in turn. These are the Pacinian corpuscles, sometimes called lamellar corpuscles, Ruffini's corpuscles also known as bulbous corpuscles; Meissner's corpuscles which can be called tactile corpuscles; Merkel cells, more free nerve endings, and hair root plexuses. Alright, let's get talking about the tactile receptors.
So, the tactile receptors enable us to detect touch, pressure and vibration. Each one brings a slightly different dimension to our sense of touch, some detected by abrasion for example while others detect pressure, and some localizes sensation on the skin with literally pinpoint accuracy while others respond to stimuli across a large section of skin.
So, let's now have a chat about which one does what starting with the Pacinian corpuscles, which are the largest cutaneous mechanoreceptor being approximately two millimeters by one millimeter wide. They're also the deepest and are found in the hypodermis, otherwise called the subcutaneous tissue of both hairy and glabrous skin types. Each corpuscle has one nerve ending shown here in the center of about twenty to seventy layers of connective tissue arranged like the layers of an onion.
So, this drawing shows the structure of the Pacinian corpuscle clearly, the nerve ending in the center with the layers of the capsule surrounding it, and the layers are formed by layers of connective tissue which protects the nerve ending from being breached by other potential stimuli such as chemicals released by inflammation. So when stimulated, the capsule initially deforms but then compensates to pressure and this quality is known as rapidly adapting and means that the nerve ending is not only stimulated by a change in pressure but adapts to, hence, stops being stimulated by a continuous pressure. As a result, Pacinian corpuscles are very sensitive to vibration but not constant pressure.
Next, we have Ruffini's corpuscles. Ruffini's corpuscles which can also be called bulbous corpuscles also detect vibration like the Pacinian corpuscles we just saw. So, why do we need another vibration sensor, I hear you ask? Ruffini's corpuscles sense slower frequency vibrations than Pacinian corpuscles and this is because Ruffini's corpuscles are still rapidly adapting mechanoreceptors but are more slowly adapting than Pacinian corpuscles and so they detect different frequencies of vibration.
Ruffini's corpuscles are also theorized to detect slippage of objects over the skin surface. They can be found in the dermis and are also present in both skin types. They have this very elongated appearance which you can see just here and a Ruffini's corpuscle has a connective tissue capsule surrounding collagen fibers which are continuous with the collagen fibrous skeleton of the dermis which we can see coming out of the capsule just here. The dendrites of the nerve attach to these collagen fibers within the capsule and they detect movement or twisting of the fibers similar to how a spider detects vibrations on its web.
Another type of corpuscle to mention is Meissner's corpuscles. So, Meissner's corpuscles can also be called tactile corpuscles, and this image from a different H&E stain section to the ones that we've exploring shows one of these nicely helpfully pointed out by this big arrow. Found almost exclusively in glabrous skin, these nerve endings live in the dermal papillae which is the pale wobbly section in our micrograph just here. Meissner's corpuscles are classically egg-shaped on histology with the narrow end pointing towards the skin surface and they also have a lamellar appearance which means that they look layered like so.
The corpuscle is actually made up of one or two nerve endings which coil and spiral upward towards the tip of the dermal papilla but because we see them in cross-section, they appear layered or lamellar. The nuclei we can see are Schwann cells. The dense coiling makes the nerve ending more sensitive to pressure and so Meissner's corpuscles allow us to perceive light touch. They are slowly adapting receptors so these are much less sensitive to vibration but are much more sensitive to constant pressure.
And just so you don't get caught at, let's take a look at some dermal capillaries as shown here on the right as they can look quite similar to Meissner's corpuscles on histology. But, don't worry, I'm going to help you differentiate the two. So, we can look for a lumen like this with cells forming in the walls of the vessels and we can sometimes spy some red blood cells that have been captured within the lumen showing as bright pink circles and ovals but these are not stained clearly here. The axons of nerves do not stain on H&E but we can note the elongated nuclei of Schwann cells on our left hand of image over here, and finally, the general shape of the Meissner's corpuscle will be more regular and egg-shaped while the arrangement of capillaries is a more random structure.
So, now, let's continue up through the skin layers and talk about the Merkel cells. So, in practice, it's actually impossible to differentiate between melanocytes and Merkel cells on an H&E stain so these highlighted cells here could be melanocytes or Merkel cells although melanocytes are more common so they're probably going to be more likely. But let's assume that they're Merkel cells for the purposes of this tutorial.
So, Merkel cells are oval-shaped with a pale cytoplasm and dark nucleus. They're actually modified epithelial cells in the stratum basale which is the deepest layer of the epidermis, and Merkel cells are concentrated in glabrous skin especially in the fingertips. So, if you’re looking at thin skin elsewhere in the body, it's more like to be a melanocyte than it is likely to be a Merkel cell.
A Merkel cell sits on a small nerve ending disc known as a Merkel disc or a tactile disc, and when the Merkel cell is deformed, it moves the tactile disc. This is a very sensitive system allowing the nerve endings to sense light touch and it also results in each nerve ending having a small receptive field, meaning that they only detect signals that are in very close proximity to the nerve ending. Therefore, Merkel cells are very precise at locating a stimulus. This is in contrast to the corpuscles which have large receptive fields detecting stimuli from a wide area of skin.
Alright, now, let's move on to have a look at some free nerve endings. So, unfortunately, we can't actually visualize the individual free nerve endings through the microscope but our diagram can help us just here. So, free nerve endings are located between the cells throughout the epidermis and the dermis. As with the proprioceptive free nerve endings, the cutaneous free nerve endings are polymodal – meaning that they can detect pressure, temperature and chemicals. And the chemical that we're talking about here include those released during inflammation.
Pain and itchiness are detected through the release of chemicals, therefore, free nerve endings are mechanoreceptors or sensitive to pressure, thermoreceptors or sensitive to temperature, chemoreceptors which means that they fire in response to chemicals, and nociceptors, that is, they're responsive to pain. As their name suggests, free nerve endings are not contained by a capsule and so are very sensitive. And since the nerve endings are tiny, not even attached to an epithelial cell like those of the Merkel discs, free nerve endings have a pinpoint receptive field, thus, they are most precise in locating a stimulus.
Alright, last but not least, let's take a look at hair root plexus. So, hair root plexus is a network of free nerve endings wrapped around a hair follicle – that is, that's the bit of hair below the skin surface. So, this image is stained with Ladewig stain which is a special type of stain to look at our connective tissue, and the hair root plexus is situated here within the dermal sheath which is highlighted in green. And this encapsulates the hair follicle in the center just here which we've cut through in transverse section.
The nerve plexus is deformed by the movement of the hair strand which allows detection of tiny movements such as flies or insects landing on or crawling over the skin or wind passing over your body. On histology, they're relatively easy to spot as we can just look for the hair follicle and observe the plexus around it with the dermal sheath of the follicle.
So as far as mechanoreceptors go, we've covered a lot. There's one other cutaneous receptor which is worth mentioning, however, and I'm talking about the bulboid corpuscle or the end bulbs of Krause which is sometimes grouped with the cutaneous mechanoreceptors because they're found in the skin. They're actually thermoreceptors and the more sensitive to cold temperatures but be aware that these are bulboid and not bulbous corpuscles – bulbous being synonymous with Ruffini's corpuscles.
Bulboid corpuscles are found all over the body. In the skin, they're found in the dermis. Nevertheless, they're not actually confined to the skin so this section, for example, is from the epiglottis in the throat. Bulboid corpuscles are named after their bulb-like shape and, on histology, their shape will depend on how they're caught when the section is cut so they may appear bulb-shaped or circular as is the case here.
Fun fact: Bulboid corpuscles are thought to be responsible for a cool of fresh taste sensation such as that you get when you eat a mint. They also may modulate our sense of taste and contribute to why some food tastes better at different temperatures.
So, that's all of the types of receptor we need to cover today, and before we finish, let's discuss a clinical scenario where this knowledge would be useful.
A thermal burn is damage to a tissue resulting from excessive heat or cold. Since your skin is the barrier between you and the outside world, this is where burns most commonly occur. When you touch something hot, your thermoreceptors will detect the heat and you'll notice that this will detect the chemicals released by cell stress and this will, of course, cause you to feel pain.
Free nerve endings are the major nerve endings responsible for these signals, and if your skin comes into contact with a source of extreme heat, it will also detect pressure from that source through your mechanoreceptors. Burns can be classified based on the depth of the skin affected, and the depth of the burn is classified by the anatomical structures which are involved.
For example, if the epidermis alone is involved as is the case here, it is known as a first-degree burn and the patient should be able to detect all normal sensations including pain because the mechanoreceptors and free nerve ending nociceptors will not be disrupted. Second-degree burns affect the dermis and can be split into subcategories based on whether just the papillary dermis is damaged or whether the burn penetrates down to the reticular dermis. And this create a variable pattern of sensory disruption.
However, deeper burns such as those penetrating to the level of the hypodermis known as third-degree burns will cause anesthesia – in other words, a loss of sensation. This includes sensations of pain, temperature, and tactile sensation. Only Pacinian corpuscles, one of which we can actually see just here, would stand a chance of surviving a third-degree burn since they're located in the hypodermis, so a sense of vague vibration or sensation of crude touch may be retained. We can use the appearance of the burn to help work out the degree of burn but the sensation affected may be more important to the patient in many circumstances and it's really imperative to recognize this.
So that just about covers everything in this tutorial. Before we finish, let's recap.
So, today, we talked about mechanoreceptors which are specialized nerve endings that are sensitive to physical distortion. In other words, they detect pressure and stretch and can be found all over the body. Today, we focused specifically on the peripheral mechanoreceptors which are the proprioceptors which are the detectors of joint position, the baroreceptors to pressure senses, and tactile receptors which provide our sense of touch. We then looked in more detail at the types of tactile receptors which include the Pacinian corpuscles which are also called lamellar corpuscles, Ruffini's corpuscles or bulbous corpuscles, Meissner's corpuscles or tactile corpuscles, Merkel cells, free nerve endings, and hair root plexuses. We even included a bonus – peripheral thermoreceptor, which is the bulboid corpuscle or the end bulb of Kraus.
And that's it! Thanks for watching this Kenhub tutorial, happy studying and have a great day!