Chapter 12: Cutaneous Senses

Introduction

In this chapter, we explore the cutaneous senses, specifically our ability to perceive pressure, temperature, and pain through the skin. Much like our exploration of vision and auditory perception, understanding the mechanisms behind cutaneous sensation involves the study of specialized receptors, neural pathways, and perceptual phenomena. Here, we will discuss the four types of mechanoreceptors responsible for detecting pressure on the skin, as well as the two categories each of thermal receptors and nociceptors (responsible for temperature and pain perception, respectively).

Mechanoreceptors: Detecting Pressure

Our journey begins with mechanoreceptors, specialized sensory cells responsible for detecting pressure on the skin. Much like the rods and cones in the eye and the inner hair cells in auditory perception, mechanoreceptors play a crucial role in transducing physical stimuli into neural signals. Four distinct types of mechanoreceptors have been identified, each with unique characteristics:

1. Merkel Discs:

• Fiber Type: SA1 (Slow Adapting Type 1)
• Receptive Field: Small
• Information Response: Pressure

Figure 12.1

2. Meissner Corpuscles:

• Fiber Type: RA1 (Rapidly Adapting Type 1)
• Receptive Field: Small
• Information Response: Taps on the Skin

Meissner Corpuscles are rapidly adapting receptors with small receptive fields. They are particularly responsive to quick taps on the skin, providing feedback about rapid changes in tactile stimuli.

3. Ruffini Endings (Cylinders):

• Fiber Type: SA2 (Slow Adapting Type 2)
• Receptive Field: Large
• Information Response: Stretching of Skin and Joints

Ruffini Endings are slow-adapting mechanoreceptors with larger receptive fields. They primarily respond to stretching of the skin and joint movement.

4. Pacinian Corpuscles:

• Fiber Type: RA2 (Rapidly Adapting Type 2)
• Receptive Field: Large
• Information Response: Vibration

Pacinian Corpuscles are rapidly adapting receptors with large receptive fields. They are highly sensitive to vibrations and play a role in perceiving texture and rapid changes in tactile stimuli. Lowenstein (1960) conducted a classic experiment, in which pressure was applied to the Pacinian corpuscle (PC) and the results showed that the PC responded when pressure was first applied and when it was removed, but did not respond to continuous pressure. However when Lowenstein dissected away the onion-like layers from the PC it fired to the continuous pressure. Lowenstein concluded from this result that properties of the onion-like layers cause the fiber to respond poorly to continuous stimulation, such as sustained pressure, but to respond well to changes in stimulation that occur at the beginning and end of a pressure stimulus or when stimulation is changing rapidly, as occurs in vibration.

Slow-adapting receptors maintain their firing rate throughout sustained pressure, while rapidly adapting receptors respond to the onset and offset of stimuli.

Sensory Pathways: From Skin to Brain

Once these mechanoreceptors detect tactile stimuli, the information travels through sensory pathways to reach our brains. The primary pathway for cutaneous sensation consists of the following stages:

1. Receptor Activation: When pressure, temperature, or pain is applied to the skin, the corresponding mechanoreceptors, thermoreceptors, or nociceptors generate neural signals.

2. Spinal Cord Transmission: The neural signals are transmitted via the dorsal root of the spinal cord to the brain. The two pathways are the spinothalamic tract, for temperature and pain, and medial lemniscal tract for fine touch, vibration, and pressure on the skin.

3. Thalamic Relay: In the thalamus, specifically the Ventral Posterior Nucleus (VPN), sensory information is relayed and processed.

4. Somatosensory Cortex: The information is then directed to the primary receiving area for somatosensory input, known as S1 (Somatosensory Cortex), in the parietal lobe.

This hierarchical organization of sensory processing is similar to that found in vision and auditory perception.

Figure 12.2

Cortical Organization: Mapping the Body

Much like other sensory modalities, the somatosensory cortex exhibits a well-defined organization that maps different areas of the body to specific regions within the brain. This "somatotopic map" allows us to understand how sensory information from different body parts is represented in the cortex.

Notably, the cortex's magnification factor ensures that regions with heightened sensory acuity, such as the fingertips and lips, receive more significant representation.

Figure 12.3

Nociceptors: Receptors for Pain

Nociceptors are specialized receptors responsible for detecting painful stimuli, such as intense pressure, extreme temperatures, or harmful chemicals. Nociceptors come in two primary types: A-delta fibers and C fibers. A-delta fibers are thinly myelinated, allowing them to transmit pain signals more rapidly than C fibers, which are unmyelinated. However, both types of nociceptors send pain signals to the brain more slowly than mechanoreceptors, which convey non-painful tactile sensations. This difference in transmission speed is why we often feel a non-painful sensation before the subsequent pain when, for example, we stub our toe.

Perceptual Phenomena: Beyond Sensation

Our study of cutaneous senses also extends to perceptual phenomena that highlight the complexities of tactile perception. One such phenomenon is the "thermal grill illusion," where the simultaneous application of warm and cold stimuli results in the perception of painfully hot sensations. This illusion demonstrates the intricate interplay between sensory input and perception within the brain.

Phantom Pain and Mirror Therapy

Phantom Pain: Phantom pain is a perplexing and often excruciating phenomenon experienced by individuals who have lost a limb. Even though the limb is no longer present, they continue to feel pain and discomfort in the missing appendage. For many amputees, phantom pain can be debilitating, affecting their daily lives.

Dr. Ramachandran, a renowned neuroscientist, proposed a novel approach to alleviate phantom pain through mirror therapy. The idea behind this therapy is to trick the brain into believing that the missing limb is still present. By placing a mirror strategically, patients can see a reflection of their remaining limb that appears as though it's the missing one. When they move their intact limb, the visual feedback provided by the mirror creates an illusion of movement in the phantom limb. This can help reduce the perception of pain and discomfort.

Changing Body Image: An interesting aspect of mirror therapy is that it can sometimes lead to a shift in a patient's body image. In some cases, individuals begin to perceive their phantom limb as not existing in the external world. While this might sound counterintuitive, it can be crucial for amputees adjusting to prosthetic limbs, as having a phantom can help someone incorporate the prosthetic limb more seamlessly into their self-concept.

Rubber Hand Illusion

Another fascinating experiment related to cutaneous senses is the "Rubber Hand Illusion." This experiment highlights how our brain can be tricked into accepting a fake hand as part of our own body. In this setup, a participant's real hand is hidden from view, while a rubber hand is placed in front of them. A researcher then simultaneously strokes both the real hand and the rubber hand with a paintbrush. Over time, the participant's brain starts to integrate the rubber hand into their body schema, causing them to perceive sensations on the rubber hand as if it were their own. If the rubber hand is suddenly damaged, it can evoke an emotional reaction akin to pain. This illusion emphasizes the brain's remarkable ability to adapt and incorporate external sensory information into our self-perception.

The Third Hand Illusion

Similarly, researchers have explored the "Third Hand Illusion." In this experiment, participants wear EEG sensors and are told that they can control a third hand. Through synchronized movements, they begin to perceive control over this artificial limb. When the third hand is unexpectedly damaged, participants react as if they are experiencing discomfort, further illustrating the brain's plasticity and its capacity to adapt to novel sensory experiences.

Social Exclusion and Pain Perception

Pain perception is not solely linked to physical stimuli. Research by Naomi Eisenberger at UCLA has shown that social exclusion can also induce feelings of pain. When individuals are socially snubbed or excluded, their brains activate regions associated with physical pain, such as the anterior cingulate cortex. This overlap in neural processing suggests that emotional and physical pain may share common mechanisms.

Figure 12.4

The Gate Control Theory of Pain

The Gate Control Theory of Pain, developed by Melzack and Wall, provides a comprehensive framework for understanding pain perception. According to this theory, pain is modulated by the interaction between nociceptive input and non- nociceptive input in the spinal cord. Imagine a gate in the spinal cord that can either open or close based on the relative strength of these inputs.

Figure 12.5

• Nociceptive Input: This input includes signals from nociceptors, which detect painful stimuli. When these signals reach the spinal cord, they aim to open the gate, allowing pain signals to travel to the brain.

• Tactile Input: This input consists of sensory information from mechanoreceptors, which detect non-painful tactile sensations. When this information reaches the spinal cord, it can inhibit the nociceptive signals, reducing the perception of pain.

• Top-Down Influence: The Gate Control Theory also incorporates top-down processes. If a person shifts their attention away from the painful stimulus, their brain can send signals that inhibit the nociceptive input, further closing the gate.

In practical terms, if you stub your toe, you may instinctively rub the affected area. This rubbing stimulates mechanoreceptors, which, in turn, can inhibit the nociceptive signals, providing temporary relief from pain. The Gate Control Theory helps explain why such actions alleviate pain in various situations.

Endorphins and Pain Control

Endorphins are endogenous opiates produced by the body that serve as natural pain relievers. These chemicals activate opioid receptor sites in the brain and spinal cord to reduce the perception of pain. Naloxone, an opioid antagonist, can block these receptor sites, thereby diminishing the effectiveness of endorphins and other pain killers.

Placebos, have been found to induce the release of endorphins, which may explain why they can effectively reduce the perception of pain. For example, if a person is given a nasal spray that they think will reduce pain, it will have this analgesic effect if it is saline because of the release of endorphins. However, it will not reduce pain if it the nasal spray contains naloxone because the opioid antagonist will block the effects of the endorphins.

Hypnosis and Pain Management

Hypnosis is another powerful tool for managing pain. By inducing a focused state of attention and concentration, hypnotherapy can effectively reduce pain perception. Studies have shown that hypnosis can decrease activity in the anterior cingulate cortex, a brain region associated with the emotional aspects of pain. It is used in various medical settings, including surgery, dentistry, and childbirth, to alleviate pain and discomfort.

Acupuncture and Pain Relief

Acupuncture, an ancient practice, is known for its effectiveness in reducing pain. According to the Gate Control Theory, acupuncture may work by providing mechanical stimulation that blocks nociceptive input, similar to how rubbing an injured area can relieve pain. Recent studies using brain imaging techniques have shown that acupuncture can decrease activity in the anterior cingulate cortex, supporting its effectiveness in pain management.

Figure 12.6

Peppers: The Intersection of Taste and Pain

Lastly, it's worth noting that our perception of taste can sometimes intersect with the perception of pain. Spicy peppers, for example, contain compounds like capsaicin that activate receptors in our mouth and on our skin, leading to a sensation of heat or burning. This sensory experience, while not causing tissue damage, can still be perceived as painful. Thus, the boundaries between taste and pain perception can blur, demonstrating the complexity of our sensory experiences.

In the next few chapters, we will explore the chemical senses of taste and smell.

Conclusion

In this chapter, we explored the cutaneous senses, encompassing our ability to perceive tactile, thermal, and painful sensations. We explored the phenomenon of phantom pain and how mirror therapy can offer relief to amputees by tricking the brain into perceiving a missing limb. The Rubber Hand Illusion and the Third Hand Illusion shed light on the brain's remarkable capacity to incorporate external sensory information into our self- perception. Nociceptors, the receptors responsible for detecting painful stimuli, were examined, highlighting the difference in transmission speed between A-delta and C fibers. Moreover, we learned that pain perception extends beyond physical stimuli, with social exclusion activating pain-related brain regions. The Gate Control Theory of Pain provided a comprehensive framework for understanding how non-painful inputs can modulate pain perception. Endorphins, hypnosis, and acupuncture were discussed as effective methods for managing pain. Lastly, we explored how the intersection of taste and pain occurs with spicy peppers, blurring the lines between our senses.