Video: Mechanoreceptive somatic senses

We could all use a vacation sometimes, and like a lot of people, I enjoy the beach – the texture of those sand grains, the warm sun, the cool water, and my favorite – leaving my footprints in the sand. Touch, temperature, pressure – these are all somatic sensations. Somatic means they are related to the body. These are sensations our bodies can perceive and we're consciously aware of them. They could be mechanoreceptive, thermal, or painful sensations.

In this tutorial, we're going to be learning more about the mechanoreceptive somatic senses.

Mechanoreceptive somatic senses are evoked by mechanical stimuli that physically move or distort specific types of sensory structures known as the mechanoreceptors. These sensations can be of two kinds – tactile senses and position senses.

Tactile sensations include the ones we saw earlier, like touch and pressure, but also include vibration. Touch and pressure can be discriminative and easily localized, like a butterfly resting against the back of your hand. Or it can be crude and non-discriminative, which is not easily localized, like the feel of socks on your feet. All of these are tactile sensations which are usually picked up by cutaneous mechanoreceptors located in the skin.

Position sense, like the name suggests, is knowing where different parts of our bodies are in relation to each other. There are two types of position senses. The first sense is the static sense. An example of this is if you were to close your eyes and bend your right elbow, you'd know that your right elbow is bent even though you're not looking at it. This is proprioception.

The second kind of position sense is the dynamic sense where we perceive things like the rate at which our arms and legs are moving. Position senses involve proprioceptors located in muscles, tendons, and joints.

Let's first look at the tactile senses which use cutaneous mechanoreceptors.

These receptors are specialized neuronal endings found in the skin. Some are superficial while some are deep. We have different kinds of cutaneous receptors distributed throughout our bodies. There are five cutaneous receptors that are important for tactile sensations. They are low-threshold mechanoreceptors, sensitive to gentle, non-harmful stimuli.

Although free nerve endings can detect crude non-discriminative touch and pressure, we'll learn more about them with pain and temperature where they work as high-threshold receptors. The neurons forming these receptors may or may not have a capsule around their endings. If they don't have a capsule of connective tissue, they are nonencapsulated receptors. If they do have one, they are encapsulated.

Now let's learn a little more about each receptor. We'll start at the junction between the epidermis and dermis. Some neurons end with expanded disc-like structures called the tactile menisci or Merkel discs near the basal layer of the epidermis. They are associated with specialized cells called tactile epithelial cells or Merkel cells. Together, they form an epithelial tactile complex, also known as the Merkel cell-neurite complex.

There can be multiple branches with these discs forming a cluster. In hairy skin, this group is called a touch dome or tactile dome. Although not fully understood, the function of these receptors is related to touch. They help in determining texture, sustained pressure, and are sensitive to the edges of objects.

Next up, we have the tactile corpuscles, also known as Meissner’s corpuscles. They are oriented perpendicular to the surface of skin in dermal papillae, primarily in hairless or glabrous skin, like our palms and soles. They are cylindrical with a capsule and a core of modified wedge-shaped Schwann cells that look like a stack of pancakes. The unmyelinated neuron ending winds back and forth between the stacked cells.

There are numerous tactile corpuscles at the tips of our fingers, perfectly located for fine discriminatory touch. Their high sensitivity is significant in activities like reading Braille. They can also sense low frequency vibration like flutter or the feel of a textured object moving across skin.

Deeper down in the dermis are the bulbous corpuscles, also called Ruffini corpuscles or Ruffini endings. They are branched nerve endings between collagen fibers surrounded by a capsule. They are located in both hairy and glabrous skin and are sensitive to the stretching of skin. They are also situated in joint capsules and help in detecting position and movement as we'll see later on.

In deeper parts of the dermis and subcutaneous tissue are the largest and perhaps most well-known cutaneous mechanoreceptors – the lamellar or Pacinian corpuscles. They have an onion-like structure which you can see here in a cross-section. They are sensitive to deep pressure and high frequency vibration, being the most sensitive to vibrations between 200 and 300 Hertz.

They are also found internally, in joints and interosseous membranes. Each lamellar corpuscle has a central nerve ending, surrounded by layers and layers of modified Schwann cells forming concentric lamellae around it. These are covered by an external fibrous capsule. The nerve ending is unmyelinated since the afferent neuron loses its myelin sheath as it enters the corpuscle.

Let's move on now to these receptors found around the hair follicle bulbs.

These are nerves that can wrap around it or run alongside it. They are called hair follicle endings, also known as hair end organs. They get stimulated when the hair is mechanically bent and again when released. Thus, they respond to objects brushing the surface of the skin, functioning as tactile receptors.

For most of these mechanoreceptors, physical distortion of their structure from stimuli like touch, pressure, or vibration, opens mechanically-gated ion channels. That allows ions like sodium to enter the cell, causing a depolarization potential called a receptor potential. If the intensity of the stimulus is strong enough to bring the membrane potential up to a threshold, that will open voltage-gated sodium channels triggering an action potential, which gets propagated along the sensory or afferent neuron.

Most cutaneous mechanoreceptors have neuron endings as part of their structure. These neuron endings are unmyelinated; however, the axons for these neurons are heavily myelinated. These are type A-beta fibers, which have a high conduction velocity of nearly 30 to 70 meters/second. The advantage of having these kinds of fibers is that they're good for localizing exactly where the stimulus is coming from and in assessing the changes in intensity of that stimulus.

All these afferent neurons are going to carry impulses to the spinal cord, and through the sensory pathways, they will reach the brain. The brain will then be able to perceive that stimulus. Each cutaneous sensory neuron serves an area of the skin called its receptive field. It picks up stimuli from this particular region and then synapses with a second order neuron which will then carry that information to the central nervous system.

These receptive fields can vary in size and can even overlap. For instance, our fingertips have many sensory neurons and smaller fields while the skin on our back would have fewer neurons and larger fields.

Let's stimulate two points placed close together. If these two are separate receptive fields with their own sensory neurons, they will synapse on second order neurons, and once they reach the brain, the stimuli will be perceived as two separate points. Instead, if the fields were larger, the two points may lie in a single field and thus be sensed as a single point.

Large receptive fields can overlap. Neurons carrying information from these fields could converge and summate on a second order neuron. This is spatial summation, which we've seen in earlier lessons. This makes the receptive field of the second order neuron one large field and thus the two separate stimuli get perceived as a single stimulus, even though two separate points were stimulated. This is two-point discrimination, our ability to tell if two points of stimuli are separate points.

The minimum distance between the two points needed to tell them apart is the two-point discrimination threshold, which is much lower on the fingertips than the back. As the stimulus continues, the response of these receptors steadily declines. This is adaptation, which we learned about in our introductory video on sensory receptors.

Mechanoreceptors also experience adaptation. Some of them adapt rapidly and some take their time, adapting slowly.

Lamellar corpuscles are very rapidly-adapting receptors. They respond as soon as the stimulus, like pressure, is applied and very quickly stop responding. But when pressure is released, they respond again. These kinds of receptors are better suited for quickly changing stimuli like high frequency vibration.

Tactile corpuscles are also rapidly adapting. Bulbous corpuscles and epithelial tactile complexes, however, are slowly adapting. They respond for as long as the stimulus is present, keeping the brain continuously informed of the stimulus.

Now that we've tackled tactile senses, let's take a quick look at position or proprioceptive senses.

As we learned earlier, this usually involves proprioceptors. It's important to remember that cutaneous tactile receptors like the bulbous and lamellar corpuscles are also found in joints, along with receptors resembling tendon organs and free nerve endings. They are involved in position sense, detecting the angles of joints and thus informing the brain about the position of different body parts in space.

Muscles and tendons have important proprioceptors – the muscle spindle and the tendon organ. Muscle spindles are encapsulated receptors distributed in skeletal muscles throughout the body. They are composed of specialized muscle fibers in a capsule. These intrafusal fibers, as they're called, are innervated by heavily myelinated A-alpha neurons. They detect stretch of the muscle fibers.

When the muscle stretches, their firing rate increases. When the muscle shortens, the firing rate reduces. This gives the central nervous system information regarding the length of the muscle, rate of change of that length, and thus the angles of joints.

The tendon organ, commonly known as a Golgi tendon organ, is located in tendons, near the attachment with muscle. They are bundles of tendon fibers with a capsule around them. They have heavily myelinated A-alpha afferent nerves that branch after entering the capsule. These Golgi tendon organs are more sensitive to muscle tension when a muscle contracts. They provide information regarding tension in the muscle and the rate of change of tension.

Together, these proprioceptors are important for position sense and are responsible for keeping the central nervous system continuously informed. Thus, they are slowly adapting receptors responding for as long as the stimulus is present. Thus, our mechanoreceptors shape our sensory experiences by picking up both tactile and position senses.

That concludes this tutorial on the mechanoreceptive somatic senses.

Make sure to check out our other study units and articles on the nervous system.