Transduction, coding and organisation of sensory inputs and stimuli are essential to the functioning of the primary somatosensory cortex. The several principles which are encapsulated under these broad terms will be briefly discussed in relation to their role and function in somatosensation.
Frequency coding of the somatosensory system can be understood as the process in which the intensity of a stimulus is coded according to the frequency of the firing received by the cells responding to the stimulus. As the central nervous system is frequency modulated, rather than amplitude modulated. In order to code the intensity of the stimulus the frequency and number of the action potentials generated by the stimulus will determine the intensity size, rather than the strength (“The somatosensory system”, 2018). The action potential is generated once the depolarization caused by the stimulus is above threshold, this all-or-nothing response is then translated into the multitude and frequency of responses. Within the somatosensory system the greater touch pressure will translate to a greater number of action potentials, thus a higher frequency, than lighter touch pressure creating fewer action potentials and consequently a lower frequency will be coded.
Coarse coding of stimulus input in the primary somatosensory cortex is analogous to the process of coarse coding of simple cells in the primary visual cortex, where cells respond more strongly to preferred shapes, brightness and saturation. Within the primary somatosensory cortex specific neurons have varying peak sensitivity to touch qualities, e.g. direction, size and orientation. Specific cells can be strongly driven by one type of orientation falling on its receptive fields, and as the stimulus changes in orientation, the strength of response will adapt to this change as well. The response strength is greater for preferred orientations, somewhat strong for a variant of the preferred orientation, to a very weak response if not similar at all. As any given stimulus cannot be accurately coded by one single receptor, due to the principle of Univariance, the combination of varied responses by the population of cells across the affected receptive fields can accurately identify the correct orientation of stimulus. This integrated process is called population coding.
Population coding is the idea that nervous systems signal and assess boundaries using large populations of imprecise neurons, rather than one or a few precise neurons. Within the somatosensory system, population coding functions to combine input sensations of touch, pain, temperature from neurons located in the receptive fields of the target location, as well as the sensitivity of the frequency coded to determine with acuity where they occur and the intensity of the input (“The somatosensory system”, 2018). For example, say someone touches you on the shoulder: the cutaneous receptors located at that location on the body are activated and will integrate the strength and touch quality of the input across a number of differently tuned mechanoreceptors with sensitivities to varies stimuli and different aspects of perception. The more receptors that respond, the more intense the sensation, and vice versa. This information is then sent to the central nervous system, where it is decoded to specify with acuity where you were touched, and the type of touch you perceived. In other words, we can determine the duration of the touch from one mechanoreceptor, but to attribute the size of the area touched and the pressure a combination of inputs from multiple mechanoreceptors in the same receptive area are needed. The transduction of these coded signals will be explored through the principles of adaption and somatosensory receptive fields.
Adaptation is another form of coding within the somatosensory system. Adaptation can be understood as the principle in which neurons adapt their response rate over time in relation to the changes in stimuli perceived. A basic example of neural adaptation would be the act of standing and walking barefoot upon a hard floor. If you were to stand barefoot on a hardwood floor, the sensation of the floor underfoot would activate the Messiner’s and Pacinian Corpuscles cutaneous receptors on the foot to respond immediately, and rapidly to the initial sensation. Whereas the Merkel's disk and Ruffini Corpuscle with their tonic adaptation rates would be continuously responding to the constant pressure and stretch on the skin. Unless the foot is lifted off of the floor, the neurons will gradually respond less to the touch sensation until there is no longer a response, this is called rapid phasic adaptation. Once a change is perceived the sensory neurons will respond again to the change. Within the same example, the thermal sense of the cold floor will immediately cause responses from thermal sensory neurons, and after a time the temperature is adapted to.
The mechanoreceptors involved in adaptation of touch sensations vary in their receptive field size and properties. The receptive field size of the Pacinian and Ruffini’s Corpuscles is large and originates deeper in the dermis of the skin. Similarly, to magnocellular cells in the visual system, they integrate information and input from a larger area of skin, but at a slower rate. The Merkel’s disks and Meissner’s corpuscles receptive fields occupy the epidermis of the skin and are much smaller. They only respond to sensations in closer proximity, which is analogous to the Parvocellular neurons, but with a faster rate. These smaller receptive fields provide more touch acuity and are important for fine discriminative touch. Additional similarities to the visual system are that receptive fields in the somatosensory system have center-surround organization, and are inhibited or excited depending on whether the stimulus falls within the on or off center. Areas of the body, such as the fingers, have a larger collection of smaller receptors for fine touch acuity, whereas areas like the back have fewer larger receptors, with less precise perception.
The principle of parallel processing is the idea that contrasting populations of neurons within a sensory cortex have differing characteristics of processing information which vary in speed, reaction time, size, and sensitivities to particular stimuli. Each segment of input is processed in parallel streams which combines and analyses information simultaneously, whilst sending it to the higher levels of the primary somatosensory cortex. The adaptation rates and receptive field qualities of cutaneous receptors on the surface of the skin combine inputs and frequencies to translate the quality of touch perception to the primary somatosensory cortex. For example, within the cutaneous receptors on the skin tasks are divided according to receptive field size, adaptation rates, and perception, so that the sensory quality of the input is accurate.
In a general sense, hierarchical organisation within the central nervous system is the organisation of input or signals that are categorised and coded progressively as the information travels from a simple cellular level of the primary sensory neuron, which receives input from the touch receptors on the skin. Then through the neural pathways and secondary sensory neuron, to the higher levels of the ventral posterior lateral nucleus of the thalamus. This functional segregation allows for efficiency in flow of information to the primary somatosensory cortex. The additional benefit of hierarchical organisation is that higher levels of the sensorimotor system are not overloaded with information, and are free to carry out more complex functions, such as discerning between different stimuli (Pinel & Barnes, 2017). Parallel processing occurs during hierarchical organisation, as it is understood that these communicative processes are not limited to a serial order in relaying information, but that some of the signals bypass others to reach higher levels of processing.
Lateral inhibition is the inhibition of adjacent horizontal neurons of a receptive field to enable a sharper perception of the size and position of the touch stimulus. When a stimulus touches the surface of the skin, for example on the tip of a finger, the larger number of small receptive fields will be stimulated and fire quickly and frequently. The response from this touch will be stronger within the center of the receptive fields, and will diminish around the boarders of the receptive field. This occurs laterally across the neighbouring receptive fields that are within the line of touch. Lateral inhibition between the central area of the stimulation and the overlapping receptive fields increases the contrast between the stronger signals at the center and the weaker at the periphery. The result is a finer and accurate perception of the touch location.
The way in which sensory information is organised within the somatosensory cortex is similar to a map of the body. Sensory information received from the legs, torso, arms, hands and face are sent to their appropriate region of the sensory cortex spanning from the medial portion to the lateral regions. This mapping of the somatosensory system is also referred to as the somatosensory homunculus. This somatotopic organisation map of the body is disproportionate and has more cortical space assigned to parts of the body with more receptors, such as the hands, ears, lips and tongue. For example, there is a larger area devoted to the sensations received from each finger and thumb, whereas the leg or trunk occupies a much smaller area.
The over representation of these areas that are used for more tactile discrimination is a principle called cortical magnification. This being that the cortical representation of each somatosensory field is represented proportionally in the somatosensory cortex, in regard to their level of acuity and sensitivity. This principle applies to other sensory regions, such as the visual system, where the central part of the visual field – where the finer, deliberate and accurate processing of visual stimuli – is proportionally over represented in the equivalent region of the visual cortex. Essentially, in order to process stimuli with finer detail and acuity, there is a need for more cells so that the process is efficient and accurate.