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Introduction to Behavioral Neuroscience

9.1 Somatosensory Receptors

Introduction to Behavioral Neuroscience9.1 Somatosensory Receptors

Learning Objectives

By the end of this section, you should be able to

  • 9.1.1 Describe the anatomical properties of different sensory receptors.
  • 9.1.2 Identify the physiological properties of different sensory receptors.
  • 9.1.3 Describe the anatomy and neurotransmitter usage of primary afferents to the spinal cord.

When you lift a hot pot of boiling water from the stove, not only do you feel the temperature but also vibration due to the boiling content. Further, you may also notice the texture of the handle. How does this happen? How can you sense so many properties at once? Among all five senses, the body sense (somatosensory system) starts with the largest variety of different unique receptors. Each receptor connects via an axon to the central nervous system to transmit information to our brain to enable perception. This section will review how these receptors transduce such a variety of skin signals into neural signals headed to the central nervous system.

Peripheral receptors and functional modalities

Each somatosensation starts with the activation of one or more types of peripheral somatosensory receptors. Peripheral receptors include several major types, such as the Merkel disk, Meissner’s corpuscle , Ruffini endings, Pacinian corpuscle , free nerve endings, and hair cells (Figure 9.2). These different sensory receptors enable us to feel touch, pressure, vibration, pain, and temperature.

top: A diagram of a cube of skin, excised to show hypodermis, dermis and epidermis. Within the layers, major cells types and structures are shown. Bottom: A series of 5 panels, each showing a drawing of a cross-section of skin with a different major touch/pain receptor shown in its anatomical position.
Figure 9.2 Mechanoreceptory types In this example from the skin, there are Merkel disks and Meissner’s corpuscles in the superficial part of the skin. Hair follicles, Ruffini endings and Pacinian corpuscles sit in the deeper part of the skin. All of these use large myelinated Aβ-fibers. Unmyelinated C-fiber Free nerve endings are found throughout the skin.

Touch receptors

Merkel disk, Meissner’s corpuscle, Ruffini endings, and Pacinian corpuscle all serve to transduce features of touch sensation. These disks and corpuscles are specialized structures in the skin that are connected to axons that send touch information to the spinal cord. These terminal structures are the starting points of the somatosensory system, so they are also called "sensory receptors". Here the term "receptor" is different from the neurotransmitter receptor on the cell membrane (much smaller, one protein molecule). The sensory receptor is a much larger structure (composed of many proteins). Within these specialized endings sit mechanosensitive cation channels which activate in response to mechanical pull from sensations like touch, pressure, and vibration. These channels, also called piezo channels, are permeable to Na+, K+, Ca2+, and Mg2+, with a slight preference for Ca2+. Figure 9.3 shows the protein structure of two major piezo channels. They both are made of 3 proteins that come together to form a central pore, through which ions move, with blade-like structures surrounding the pore. The discovery of piezo channels led to a Nobel prize in 2021 for Dr. Ardem Patapoutian, the scientist who first described them (Coste et al., 2010) (Figure 9.3). Figure 9.4 shows how these channels open in response to mechanical force. The influx of ions through these channels leads to the generation of action potentials which propagate via highly myelinated Aβ fibers to the spinal cord.

Left shows two protein structures, both with three fan-like blades extending from a central core. Right shows a photo of Dr. Ardem Patapoutian.
Figure 9.3 Piezo channel structure Signals responsible for mechanosensation are transmitted by piezo channels. Piezo channels are composed of three proteins forming blades around a central pore.
Two part diagram showing cross section of a cell membrane with a piezo channel in it. On the left, the channel is closed. On the right, a mechanical force pushes the membrane down and the piezo channel is pulled open. Cations flow into the cell.
Figure 9.4 PIEZO Channels: How Do They Allow Mechanosensation? PIEZO1 and PIEZO2 are both mechanically-activated cation channels. Based on protein structure, it was predicted that the 'blades' of the PIEZO channels undergo a lever-like flattening motion upon application of mechanical stress. This opens up their central pore, allowing an influx on positive charge. The exact mechanism by which mechanical force leads to the central pore opening is not fully understood.

The four major mechanoreceptors (Merkel disks, Meissner’s corpuscles, Ruffini endings, and Pacinian corpuscles) are each sensitive to different kinds of touch. The location and shape of each receptor structure supports these different sensitivities. Figure 9.5 summarizes these key features of the 4 receptor types. Next, we will review some key structural features of these receptors and how they support the receptor function.

Top: Diagram with 3 parts. Top: 4 side-by-side panels, each showing a drawing of a cross-section of skin with a different major mechanoreceptor shown in its anatomical position. Middle: aligned with each panel above, drawing of a finger showing receptor field of that receptor as a teal haze. Bottom: line drawings to represent stimulus onset/offset with resulting neural firing, aligned with above panels. Bottom: Table of skin location and adaptation rate of the 4 major mechanoreceptors.
Figure 9.5 Mechanorecpetor receptive fields and adaptations
Merkel disk Meissner's Corpuscle Ruffini ending Pacinian corpuscle
Skin location Superficial Superficial Deep Deep
Adaptation Slow Rapid Slow Rapid
Table 9.1

First, their different locations in the skin help determine how large or small their receptive fields are, which for touch receptors means the area of the skin in which touch can activate them. Merkel disks and Meissner’s corpuscles are located more superficially around the epidermis and therefore have a smaller and clearly-defined receptive field. Ruffini endings and Pacinian corpuscles are located more in the deeper part of the skin (with a larger, blurry receptive field), near where hair follicles are located.

Second, the shape and composition of each touch receptor help determine how long they respond to stimuli, dividing them into rapid adapting versus slowly adapting receptors. The slowly adapting receptors include Merkel disk and Ruffini endings. When a force or stimulus is applied in their receptive field, those receptors will be constantly activated and generate electrical activities (e.g., pressure). On the other hand, the rapidly adapting receptors such as Meissner’s corpuscle and Pacinian corpuscle only respond or generate electrical activities when there is a dynamic change of forces or stimuli, such as vibration (Johansson and Vallbo, 1983; Koerber and Mendell, 1988). These adaptation properties combine with the receptive field to give the unique sensitivity of each of these 4 receptor types to specific touch modalities described in Table 9.1.

Nociceptors

The specific receptor for sensing a high-threshold stimulus that is damaging or threatens damage to normal tissues is called a nociceptor. Free nerve endings serve as nociceptors, or receptors that transmit pain signals related to mechanical, thermal, or chemical sources. Free nerve endings are different from the preceding four mechanoreceptors. The role of free nerve endings is related to sensing pain and temperature. Free nerve endings lack any specialized structures around their terminals and are connected to or in continuation with unmyelinated C-fibers and myelinated Aδ fibers. There are separate types of nociceptors responding to each modality (i.e., mechanical, thermal, and chemical), but also polymodal receptors that respond to all. Below we will discuss further how nociceptors sense tissue damage and temperature specifically.

Some free nerve endings transmit pain signals related to tissue damage (Figure 9.6). For example, if someone cut their finger, the direct and indirect damage of the nerve terminals (C- or Aδ fibers) at the primary injury site will excite and generate action potentials, which will not only propagate to the spinal cord (generating pain perception), but also propagate through the axonal branch points to spread to other axonal terminals that connect to the same main axon. The invasion of action potentials to the nearby uninjured axonal terminals will lead to the release of several key neural peptides, CGRP (calcitonin-gene related peptide) from Aδ fiber and SP (Substance P) from C-fiber, which cause vasodilatation (redness and temperature) and plasma extravasation (edema). This phenomenon is called axonal reflex, a mechanism contributing to neurogenic inflammation. The primary injury also breaks blood vessels, leading to the accumulation of white blood cells, especially the mast cell. Mast cells can also release these neuropeptides and stimulate the axonal terminals and capillaries, leading to redness (rubor), heat (color), swelling (tumor), pain ( dolor), and loss of function (functio laesa), cardinal signs of inflammation.

Top: Diagram of a C fiber cell body extending process in to the dorsal spinal cord and a separate process in to the tip of a finger. Bottom: Cross-section of skin diagram, with an injury represented that disrupts the surface layers and blood vessels. Mast cells are shown beneath the blood vessels releasing CGRP and SP in response to bleeding. Activation of free-nerve endings also shown.
Figure 9.6 Local mechanisms of nociception

Other free nerve endings contain transient receptor potential (TRP) channels, which are temperature-sensitive mechanoreceptors. TRP channels open in response to temperature and come in a variety of types, each one sensitive to a different temperature. Figure 9.7 reviews the temperature sensitivities of several kinds of TRPs. Dr. David Julius, also pictured in Figure 9.7, shared the 2021 Nobel Prize in Physiology or Medicine for his description of these channels. Figure 9.8 shows an example of how heat causes a warm-sensing TRP channel to open, allowing in positive ions that activate the free nerve ending, which then relays signals to the brain via the spinal cord.

Left shows 5 different TRP channels in a cell membrane, arranged by sensitivity to temperature (cold to hot: TRPM8, TRPM2, TRPV1, TRPA1, TRPM3). Right shows a monochromatic drawing of Dr. David Julius.
Figure 9.7 The TRP ion Channels Signals responsible for temperature and pain sensation are transmitted by transient receptor potential (TRP) channels that activate across different temperature ranges.
A cube of skin is shown with a free nerve ending being exposed to heat from a flame. The axonal fiber of the free nerve ending goes to a cell body in the dorsal horn of the spinal cord. A synapse is formed by the central branch from the cell body onto a dorsal horn neuron. The dorsal horn neuron sends its axon across the midline then up the spine into the brain. A zoom-in on the spot where the flame meets the nerve ending shows a somatosensory cell membrane with a TRP channel that opens in response to heat, allowing ions to flow into the cell.
Figure 9.8 TRP channels transduce temperature information

One notable example of a TRP channel is the capsaicin receptor. Capsaicin is the key chemical ingredient in hot chili peppers. When it binds to the capsaicin receptors, it activates a TRP channel (Samanta et al., 2018) that allows calcium ions to flow into the cell, leading to depolarization (Caterina et al., 1997; Julius, 2013). This is the same channel activated by hot temperatures, which is why a spicy pepper feels hot . A similar, but separate TRP channel activates in response to cold temperatures, as well as chemicals in foods that feel cold, like mint. Activation of TRP channels, therefore, transmit information about temperature, and also are part of our experience of temperature sensations associated with a number of chemicals (see Chapter 8 The Chemical Senses).

Hair follicle mechanosensation

Throughout our body, the skin can be divided into hairy and glabrous skin (mainly on the palms and soles). In the hairy skin, an axonal terminal wraps around the bottom of the hair, the hair follicle, in the dermis. The reason why we can feel a breeze blowing across our faces is that all those tiny hairs (vellus hair, or peach fuzz) are bent by the airflow. The force is transduced to the hair follicles in the skin, causing the slightest displacement (in terms of nanometers). The follicle displacement causes a stretch of the axonal membrane wrapped around the follicle. There are many mechanosensitive Na+ channels in this wrapping axon, which open in response to this stretch. That is how we can feel the movement of hair. If the hair is pulled too hard, it can generate pain.

Proprioceptors

Imagine that in the middle of the night, you feel thirsty. Without turning on the light, you extend your arm to reach for a bottle of water. Even though there is no visual guidance, you grab the water without knocking it over or missing and grabbing air. Your ability to accurately target your hand to that water bottle with little visual or touch feedback relies on your proprioceptors. These specialized receptors exist in various muscles, tendons, and joint capsules, providing information about body position and movement. The muscle spindle and Golgi tendon organ are two of the most prominent proprioceptors that you use every day (Proske, 1979; Boyd, 1980; Hulliger, 1984). The muscle spindle is a specialized muscle fiber that is activated by muscle stretch; whereas the Golgi tendon organ is located in the tendon to sense the tension during muscle contraction. These receptors are connected to large diameter, A-alpha fibers. Together, they give you constant information about the location and trajectory of your limbs through space. More information on these sensory receptors is in Chapter 10 Motor Control.

Primary afferents

As mentioned throughout the previous section, the different types of somatosensory receptors are connected to specialized axons with different diameters and myelination (Aβ, Aδ, C fibers, for example). Figure 9.9 shows the pathways these fibers take from the receptors (e.g., in the skin) into the spinal cord. These first-order axons are also called primary afferents (Crawford and Caterina, 2020). The cell body of the primary afferent fiber is located in the dorsal root ganglion (DRG), from which a pseudo-unipolar neuron sends out a parent axon that splits off into a peripheral branch and a central branch (Lin and Chen, 2018; Haberberger et al., 2019). The peripheral branch is long and projects to the skin where it receives signals from a specific receptor, whereas the central branch is short and enters the nearby spinal cord through the dorsal root. The short branch ends with synapses onto the spinal cord dorsal horn neurons where neurotransmitters are released, or projects directly up the dorsal column of the spinal cord toward the brain. The spinal cord neurons also project up to various levels in the brain.

Horizontal slice of spinal cord. 2 neurons with cell bodies in the peripheral ganglion of the dorsal root send projections out into to cross-section of skin. In the dorsal horn of the spinal cord, one neuron sends axons up the ipsilateral side while the other synapses on a cell body in the horn. The next neuron projects across the spinal cord then ascends on the contralateral side.
Figure 9.9 Primary sensory afferent anatomy Primary sensory afferents have cell bodies in the dorsal root ganglion. Their unipolar process splits into the peripheral branch, which goes to the sensory receptor and the central branch goes into the dorsal horn to synapse on spinal cord neurons.

While the primary afferents all follow a similar path to the spinal cord, they each propagate electrical signals along that path at different speeds. The A-fibers are a family of myelinated axons with different axonal diameters and extent of myelination. As we learned in Chapter 2 Neurophysiology, the larger the diameter, the faster the conduction of the action potential. Figure 9.10 provides a visualization of how these afferents compare in myelination and conduction speed. From large to small, they follow the order: Aα (12-20µm, 72-120 m/s), Aβ(6-12µm, 36-72 m/s), Aδ (1-6µm, 4-36 m/s), and unmyelinated C-fibers (0.2-1.5µm, 0.4-2 m/s). It is the difference of a racing car (120 m/s = 268 miles/hr) versus a fast walking (2 m/s = 4.5 miles/hr). Due to the difference in electrical signal conduction speed, even though both C-fibers and Aδ-fibers contribute to pain perception, Aδ-fibers are involved in fast and pricking pain, whereas C-fibers are involved in more slow and dull pain (Hunt and Mantyh, 2001; Julius and Basbaum, 2001).

Drawing of 4 major sensory fibers in cross-section, showing extent of myelin wrapping. Beneath each fiber drawing is its conduction speed and an image of a macro-object/organism that moves at that speed (race car, helicopter, car, human walking). Below that the primary sensory modality is listed for each fiber type.
Figure 9.10 Sensory fibers Image credit: Race car image by Morio, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=22169642 Helicopter image by Sebastian Koppehel, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=91350504 Highway image by Rl91, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4591786 Walkers by Powerwalkingclub, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=95373352
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