Sense of Touch

Sensations of touch arise by the activation of sensory receptors located in the skin that are responsive to mechanical stimuli. Proprioception, the perception of the position and movement of the limbs of the body, arises as a result of neural activity in sensory receptors located in and around the muscles underneath the skin. Advancing age is associated with diminished functioning of sensory systems, and the cutaneous and proprioceptive senses are no exception. For example, the ability to detect mechanical disturbances on the skin, such as those produced by a vibrating probe, and the ability to discriminate changes in the spatial patterns of stimulation of the skin, such as those produced by introducing a spatial gap between two objects pressed against the skin, tend to decline with age. An elderly person who has difficulty with the perception of the position and movement of limbs, or who has problems in moving quickly and accurately, may be experiencing an age-related decline in proprioception. The effects of aging on tactile sensitivity and proprioception can best be understood in light of what is known about the anatomy and physiology of receptors and how they conduct neural information about tactile and proprioceptive stimuli to the brain.

The sensory receptors for touch and proprioception are complex in structure, but the basic organization is that of a neuron that has an ending, endings responsible for mechano-electric transduction. Once the mechanical stimulus is transduced into an electrical impulse, the neuron transmits this information very quickly to the spinal cord and then to the brain. Information arising from the mechanoreceptors of the body and face goes to specific regions within the brain that interpret the signals in terms of tactile perceptions. The cortical regions devoted to this function have many independent representations of the body surface.

Many types of mechanical stimuli are used to understand how the tactile and proprioceptive systems work. For example, mechanical stimuli produced by pins or probes applied perpendicularly or tangentially to the skin have been used to determine the basic properties of the transduction of mechanical stimuli to electrochemical neural responses, as well as the subsequent transmission of these neural responses to the central nervous system. These stimuli indent the skin and can be either of a vibratory nature or of the ramp-and-hold variety. Vibratory stimuli are delivered by a probe that moves the skin at a particular amplitude and frequency of oscillation. Ramp-and-hold stimuli consist of an initial dynamic (ramp) indentation of the skin by the probe, followed by static (hold) indentation of the probe until it is withdrawn. The rate and depth of the initial ramp indentation and the duration of the hold state can be varied widely, as can the rate of withdrawal. Other types of stimuli that have been used to understand taction include periodic and aperiodic gratings moved across the skin surface, airpuffs, embossed letters, and everyday items such as sandpaper, cloth, and steel wool.

The classification of mechanoreceptors both in the periphery and in the central nervous system is based on the receptor’s responses to ramp-and-hold-like stimuli. Mechanoreceptors have been found to be either fast adapting (FA) or slowly adapting (SA). Here, adaptation refers to the rate of decline in neural activity with time in response to ramp-and-hold-like stimuli. There are two subclasses of FA and SA mechanoreceptors: FA I and FA II, and SA I and SA II. It has been fairly well established that the FA Is are the Meissner corpuscles and the hair receptors, the FA IIs are the Pacinian corpuscles, the SA Is are the Merkel cell-neurite complexes and the touch pads and the SA IIs are the Ruffini endings. They are defined historically. One could use the phrasiology-corpuscles first discovered by Meisser, corpuscles described by Pacini and the cell-neurite complexes as shown by Merkel. Ruffini was the first to show the existence of another tactile ending.

In psychophysical tasks involving the detection of vibration on the skin, it is possible, by carefully choosing the frequency of vibration, the size of the stimulus, and the site of stimulation, to examine the effects of aging on each of four information processing channels designated as the P, NP I, NP II, and NP III channels. Each of these channels has, as its input stage, one of the four receptor types described above. Experiments on the effects of aging have revealed that the sensitivity of each channel declines with age, especially for the P channel, a finding that can be explained by understanding the functional and structural characteristics of the channels.

At the level of the peripheral nervous system, the inputs to the P channel are FA II nerve fibers of Pacinian corpuscles. This channel is extremely sensitive at the optimal frequency of vibration of 250 Hz, with psychophysical thresholds in young adults being as low as 0.1 micrometers in the amplitude of vibration required to be detected. The exquisite sensitivity of the P channel is attributed partially to the capacity of this channel for spatial summation, which is the improvement in sensitivity that results as the size of the stimulus is increased, activating an increasing number of sensory receptors. The other three information-processing channels, NP I, NP II, and NP III, with their inputs from FAI, SA II, and SA I peripheral nerve fibers, respectively, are less sensitive than the P channel, mainly due to their inability to exhibit spatial summation. The fact that the deleterious effects of aging are substantially greater in the P channel than in any of the three NP channels is due, in part, to this unique capacity for spatial summation. Specifically, as people age, mechanoreceptors die, resulting in a progressive reduction in receptor density that becomes profound by about sixty-five or seventy years of age. Because one mechanism of spatial summation in the P channel is the integration of neural activity over a large number of receptors, the reduction in the density of Pacinian corpuscles has a particularly severe effect on sensitivity. Reduced neural input to the central nervous system from receptors—resulting from a reduction in the number of Pacinian corpuscles—results in elevated detection thresholds in older individuals. The smaller loss of sensitivity with aging found in the NP channels is thought to be due to the fact that the sensitivities of these channels, which are not dependent on spatial summation, are less affected by the reduction of receptor density.

Other factors associated with aging known to affect tactile sensitivity include changes in the physical properties of skin (such as reduced skin compliance) and changes in the peripheral and central nervous systems, resulting in some cases from a reduced blood supply to neurons, which can be due to a variety of vascular problems, including atherosclerosis. At a practical level, a decreased touch sensitivity in elderly individuals can cause a wide range of problems, including the inability to recognize objects by touch and an impaired ability to detect an object that has come into contact with the skin.

Proprioception is mediated by proprioceptors that are located in muscles and joints. The proprioceptive endings are: (1) the muscle spindles located in the muscles themselves, (2) Golgi tendon organs, which attach the muscles to bone, and (3) joint capsules that contain a group of endings similar in structure to the tactile receptors. The decline in proprioception in older individuals is often manifested in dramatic effects on motor performance, including very long reaction times and inaccurate and highly variable motor responses, such as those involved in walking, picking up objects, and driving a car. Of course, a decline in motor performance may result from factors other than, or in addition to, the loss of sensory feedback provided to the brain by proprioceptors. For example, motor performance may decline as a result of impairment of the brain areas associated with movement, cognition, and balance.