Notes on Somatosensation

By and large, somatosenses or bodily sensations are mostly concerned with sensations that arise from stimulation of the skin of the body. Primarily referred to as touch sensation, this was later known to encompass more properties, e.g. pressure, vibration, warmth, and cold. When we interact with an object, we are usually aware of its many different attributes such as the shape, texture, plasticity, hardness, and temperature. One distinctive feature of touch is that it arises from specialized receptors distributed throughout the whole skin known as the mechanoreceptors. There are also specialized receptors within the muscles and joints that provide sensory feedback known as proprioception and kinesthesia. These sensors convey muscle length, change in length, and force (tension), allowing us to be aware of the limb location and movement without vision in space. Receptors encapsulating nerve endings are stimulus-specific and have their unique physiological properties. 

Historically, researchers have viewed the senses as generally subserving a primarily discriminative role (Mountcastle, 2005). Touch and proprioception are naturally related to motor control. Touch is also related to affective and social neuroscience, that is, how one feels and interacts with each other.

1. Peripheral receptors

Mechanoreceptors beneath the skin surface give rise to the perception of touch, pain, and temperature. Free nerve endings are known to provide information of pain and temperature to the brain. There are several properties worth reporting, i.e. receptor size, myelination, location, conduction speed, and response adaptation. More information can be found in Table 3.1 and 3.2 below and also Fig-1.

1. Mechanoreceptors with small fields can distinguish two closely spaced objects better (higher spatial resolution) than receptors that have a large field size (low spatial resolution).
2. Some receptors are located superficially near the epidermis and are called Type-I receptors. Conversely, receptors located in deep skin are of Type-II receptors. 
3. Fast conduction is important to provide afferent signals that build the withdrawal response or reflex arc, e.g. to detect adverse event. 
4. Response adaptation, in this case, refers to how the fibers respond to continuous touch stimulation: slow-adapting fibers (SA) and fast-adapting (FA) fibers. FA-fibers are only sensitive to the onset and offset of the stimulus. As such, their response property is called phasic, dependent on on/off time. SA-fibers, however, respond continuously but gradually to a stimulus, that is, with a tonic response. See below!

Fig-1: Properties of mechanoreceptors and free nerve endings in the peripheral system. Take note the bottom left figure. Rapidly adapting receptors exhibit phasic response, while slow-adapting receptors show a more tonic response.

All somatosensory information is sent to the central nervous system through spinal nerves. In the case of the head, neck, and face region, information is sent through a set of cranial nerves out of the brain stem. The entire body surface can therefore be divided into discrete areas that are represented by a dermatome, and the map representing the skin surface devoted to all spinal nerves is called the dermatomal map. Although it appears that the boundaries are exact in this representation, there is actually some overlap in innervation between adjacent spinal nerves. Dermatomal maps are valuable as a clinical tool in the event of injury or infection to a particular dorsal root.

Fig-2: Graphical illustration of a dermatomal map in relation to the spinal cord.

As mentioned on top, an incoming stimulus makes contact with receptor organ on the skin. An action potential is generated in the immediate vicinity of the receptor organ itself, unlike conventional multipolar neurons where the depolarizing signal must first reach the cell body to produce an action potential. With the mechanoreceptor, action potentials then flow along the peripheral to reach the spinal cord. The cell body of a mechanoreceptor neuron is located in the dorsal root ganglion next to the spinal cord. There, the neuron carries synapse with interneurons and cortical neurons, some of which send information to the cerebral cortex. 

The topic on ascending pathways and somatosensory cortices have been discussed in the earlier blog post. To add on, the primary somatosensory cortex or S1 in the postcentral gyrus is arranged in a colunnar organization (Mountcastle, 1997). Somatosensory inputs from different parts of the body are arranged as columns of neurons that run from the surface to the white matter and encompass all six layers of the cortex. An example is shown here for a part of Area 3b that represents the digits. The expanded view at the bottom shows that each digit is represented by a different column, which in turn is subdivided into inputs from FA- and SA-type afferents.

2. Perceptual aspects of tactile sensation

Among the different somatosenses, the perception of touch was the first to be studied and remains the most widely studied topic. The perception of touch became the main interest of Fechner and Weber in their pioneering work in psychophysics, and was later developed further by Weinstein in 1960s. Tactile sensation can be examined based on the intensity, spatial, and temporal aspects. 

• Tactile intensity. To measure the absolute detection threshold of touch on the various parts of the body, a device called aesthesiometer was invented using nylon filaments. In practice, the filaments will produce skin deformity that in turn gives rise to a perception of touch. See the figure below. The facial regions have the lowest absolute threshold (most sensitive), whereas the foot area has the highest (least sensitive). Another measure of psychophysical properties of touch is the difference threshold but it warrants careful and systematic experiments as it can be influenced by two main factors: intensity of the stimulus and the location of the contact.
• Place of contact. According to Weber, the two-point limen is the smallest separation of two points applied simultaneously to the skin that can still be discriminated, i.e. they evoke the sensation of two separate points. Traditionally, the two-point limen was seen to improve steadily from the shoulder to the fingertip, resulting in a near twice reduction of the threshold. What is the physiological basis for this? The size and density of the mechanoreceptors. The smaller the size of the receptive field, the greater the ability to discriminate two different contact points. Such property is apparent for FA-I and SA-I type afferents that are more superficial on the skin. Differences in packing density also play an important role in determining tactile acuity (Vallbo & Johansson). The superiority in acuity (discriminative ability) of the fingertips allow visually-impaired people to read Braille alphabets.

Fig-3: Although the facial regions are most sensitive to touch (*), it is the fingers that show the highest spatial discrimination ability. Similarly, much of the upper torso is quite sensitive to touch but shows poor spatial resolution. The feet display just the opposite pattern: relatively good acuity or discrimination ability (#) but poor sensitivity.

• Temporal aspect. Much of the research done with the temporal aspects made use of either a prolonged stimulation or a short but repetitive vibrotactile stimulation. Prolonged application of vibrotactile stimulation is shown to cause adaptation by reducing the sensitivity of vibration at the skin stimulation site (Gescheider and Wright, 1969). In contrast, vibratory stimuli produce a different threshold value than static/prolonged stimuli do. Temporal changes in touch can best be detected by fast-adapting mechanoreceptors. The relation between vibration frequency and skin displacement is therefore a U-shaped recruitment curve. Our perception is determined by the property of the mechanoreceptors active: 
• Very small vibrations between 200–300 Hz are captured by the FA-II receptors (Pacinian) that respond to vibrations > 50 Hz, 
• Whereas FA-I receptors (Meissner) respond to vibrations between 20–40 Hz. 
• For stronger vibrations well above these threshold levels, more than one type of mechanoreceptor typically responds to movements of the skin. 

Lastly, vibrotactile stimulation has been widely used to examine sensory processing and memory in primates (e.g. Romo et al.). Touch is ... an intermediary sensory system in that its spatial resolving power is poorer than that of vision but superior to that of audition, and its temporal resolving capacity is better than vision but inferior to audition (Lynette Jones, MIT).

3. Perceptual aspects of limb position

Proprioception and kinesthesia are critically important to the motor system in guiding our movement through the environment. Unlike tactile sensation, proprioceptive stimuli are primarily internal that are generated by the position or movement of a body part. Static forces on the joints, muscles, and tendons, which maintain limb position against the force of gravity, indicate the position of a limb. The movement of a limb is indicated by dynamic changes in the forces applied to muscles, tendons, and joints. More recent studies have looked into how proprioception and kinesthesia are also responsible for sensing effort and force exertion through tension placed on the muscles. 

Receptors associated with the limb position sense generally can be divided into 4 groups depending on the discharge properties, type of stimulus, and the neurons innervating the organ. Interestingly, some mechanoreceptors are found to give rise to the sensation of limb position and movement, e.g. presumably due to skin stretch, vibration. For historical reason, the naming convention is different from the receptors innervating the skin. Compare the table above and below. Both Group I and II afferents are analogous to the Aα and Aβ fibers respectively, that have large diameters and heavily myelinated. Muscle spindles are arranged in parallel with the extrafusal fibers that make up the main body of the muscle. This parallel arrangement is the best for detecting a change in fiber length, and the speed of that change. Consequently, these fibers are among the fastest in terms of signal transmission to the spinal cord.

From a clinical and sports science perspective, it is common to associate proprioception to primarily balance and postural control; and kinesthesia to the sensation of movement. In one of the clinical tests, for example, we look into how the proprioceptive components are working properly when the visual cues are missing and proprioceptive cues are the major sources of information.

The roles of proprioception and kinesthesia in motor control and learning cannot be denied. Every time we consciously move one limb, a command is issued from the motor centers of our brain to the appropriate muscles. If the perceptual centers in our brain could somehow get a copy of that command, then it would have the means to know what movement is taking place. This information would be independent of that being generated by the proprioceptors in the muscles and joints. Such is the principle of corollary discharge. Not only that these receptors sense movement and position, but they also allow us to perceive the sensation of effort and force. A comprehensive review of this system has been written by Proske & Gandevia, 2012. 

The sensitivity of the limb position and movement in space is quite remarkable. Among the joints, the hip joint appears to be the most sensitive to detecting a movement as little as 0.2°. Among the major limb joints, the following order has been reported in terms of decreasing sensitivity: shoulder, knee, ankle, elbow, wrist, and finger base. Unfortunately, the data from kinesthetic experiments is complicated by several factors such as the direction of the joint movement, the degree to which the limb is stretched, and the precise way in which the measurements are performed. From psychophysics, the Weber fraction for weight discrimination is about 0.02. On the other hand, the perceived effort (force generation) can be fitted to a power function with an exponent value of 1.7.

One popular paradigm to test proprioception is the joint position matching test. Experimental factors that affect matching errors (Goble, 2010): (1) Ipsilateral matching, (2) Left arm advantage - right hemisphere damage people are more prone to proprioceptive deficits, (3) Tau effect - longer time to complete reference movement by experimenter leading to targets being perceived as further from the starting point, (4) Age of participants (matching errors increase in older population) (5) Left workspace bias in joint position matching task. To elaborate on the second factor: a study by Naito and colleagues (2005, 2007) used a tendon vibration paradigm in combination with neuroimaging to map regions of the brain responsible for processing input from key proprioceptors—the muscle spindles. Hence, they concluded proprioceptive performance lies more within the right hemisphere.