Within the glabrous skin of the hand there are four principal mechanoreceptors, each serving different perceptual function and the sum of activity of these systems gives tactile perception. These four systems include: slowly adapting type I (SAI) afferents ending in Merkel cells, rapidly adapting (RA) afferents that end in Meissner’s corpuscles, rapidly adapting afferents ending in Pacinian corpuscles (PC) and slowly adapting type II (SAII) afferents ending in Rufinni endings. They are all thought to respond to mechanical stimulation by producing a depolarising receptor potential.
The Pacinian corpuscle is composed of concentric layers of cellular membranes alternating with fluid filled spaces. They are widely distrinuted in muscle, bone and abdomen, but especially abundant in the finger and palm. They are thought to play a role in the perception of events though an object in the hand. Meissner’s corpuscles have relatively large receptive fields and respond best to low frequency vibration. Ruffini endings are thought to play a role in providing information about finger position and hand shape. Experiments involving spatial event plots have shown that SAI fibres (attached to Merkel cells) are involved in fine discrimination, although our overall perception of hand held objects is determined by the pattern of activity of all of the receptors mentioned.
The rapidly responding receptors respond only to the onset of the stimulus, whereas the slowly adapting receptors give a tonic response to a steady stimulus. The rapidly adapting receptor responds to low frequency sinusoidal mechanical stimuli with a single action potential for each phase of the stimulus, effectively treating each period of the waveform as a new stimulus. This is known as phase locking.
Warmth and cold are believed to exist as two modalities because of the existence of warm and cold spots throughout the body, although there are usually more cold spots than warm and the proportion of each varies throughout the body. It is thought there are many more warm receptors than there are spots and it requires the simultaneous activation of many receptors to elicit the sensation of warmth. This is known as spatial summation in the sensory system. Cooling stimulates the Aδ fibres and warmth stimulates a sub-population of C fibres. There is maximum firing where the two fibre types overlap (around body temperature) enhancing small changes in temperature away from body temperature.
Evidence of sensory stimuli being unequally perceived is shown in the two-point discriminatory test. This shows that tactile acuity is better in certain parts of the body e.g. hands and fingers than others e.g. the back. Experiments have shown that the smallest discriminable distance between two points of contact, the two-point limen, improves twenty fold from the shoulder to the fingers. In general, tactile acuity increases with mobility of the body parts. Areas with high tactile acuity have small receptive fields. If two points contacting the skin stimulate only one receptive field it will only be processed as one point. RAI and SAI fibres have small receptive fields and the highest density is on the fingertips.
Pain is different from the other sensations of the skin and is difficult to understand for a variety of reasons. It is not only a feeling, but demands both a motor and emotional reaction. Pain varies greatly between individuals and more importantly its magnitude varies depending on the emotional state of the person at the time. Pain is detected by nociceptors that do not possess any specialised endings so are often referred to as free nerve endings. This means that the receptor is particularly sensitive to chemicals released or produced during injury. Pain is often separated into a first, sharp pain and a second dull pain. The first, sharp pain is due to the myelinated Aδ fibres and the second, dull pain is due to the unmyelinated C fibres.
A dermatome is an area of skin innervated by a single dorsal root. The dermatomal boundaries overlap by mixing of fibres from several dorsal roots in the peripheral nerve. This is important because pain in a particular area will not pinpoint the dorsal root affected due to this overlapping.
One of the keys to understanding how the sensory system works is by considering the way in which sensory receptors are relayed to the central nervous system. The main pathway for touch and proprioception is through the dorsal column-medial lemniscal (DC-ML) system, consisting of large diameter myelinated fibres. These fibres ascend ipsilaterally to reach the dorsal column nuclei and the decussate to reach the thalamus. This system is oligosynaptic, meaning there is little modulation. The nociceptive pathway is referred to as the spinothalmic tract or anterolateral system. The axons ascend in the contalateral, anterolateral white matter. This system is polysynaptic and partly explains the variable nature of pain and temperature sensations. Within the spinal cord, the dorsal horn is the sensory processing area. Sensory innervation of most of the head comes from the trigeminal nerve, which has three branches (ophthalmic, maxillary and mandibular).
The thalamus is an essential link between sensory receptors and the cerebral cortex for all modalities except olfaction. It does more than just this; it acts as a gatekeeper for information to the cerebral cortex, filtering out unimportant information and enhancing useful information depending on the behavioural state of the animal. The thalamus is composed of many well-defined nuclei, which are often classed into four groups: anterior, medial, ventrolateral and posterior. Sensory stimuli travel from the thalamus to the primary somatosensory cortex (SI) located in the post central gyrus (immediately posterior to the central sulcus).
The homunculus is evidence of an unequal perception of sensory stimuli over the body. It maps out the somatosensory input to the cortex showing that the area of cortex devoted to processing information from a particular part of the body is not proportional to the topography of the skin, but reflects the degree of innervation in that area. Each part of the body is represented according to its importance to the organism. In humans there is a proportionally large area devoted to the hands and face, whereas in rodents whiskers are more dominant.
Sensory stimuli are not equally perceived all over the body, but different stimuli are detected more strongly in some areas than others. Our perception of touch is best in our fingers, which makes sense because this is the most mobile and dextrous part of our body. Proprioceptors are essential in our ability to sense the position and location of various different parts of the body. They are located within the muscles, joints and deep connective tissue, where movement takes place. The statement is more true of pain and temperature sensations, which can be felt through most of the body but these sensations are modulated in a different way.
