1) Differentiate the concepts of perception, discrimination, and haptics
2) Differentiate the neuroanatomical structures that support sensory perception, discrimination, and higher-order sensory function
3) Introduce methods of testing sensation
4) Discuss how sensory information contributes to reflexes and function
Human somatosensation is supported by complex networks, allowing the perception of touch, joint pressure and motion, muscle stretch and tension, pain, pressure, temperature, vibration, and itch. From these simple modalities, the human sensory system can characterize the location of touch, the magnitude of the stimulus, the orientation of the body or limb in space, and the identification of object properties such as texture, roughness, weight, shape, and identity. This chapter will focus on the neural processing of somatosensory sensations and how that processed information is used. We will also address how common tests of sensation can be used to elucidate different sensory abilities.
To say that sensation is complex is at best an incredible understatement. Think of some of the sensations that you have experienced such as the tickle of hair as the hairdresser cuts your bangs or the feel of an ice cube melting in your hand. Imagine the feel of cool crisp sheets as you climb into bed at night or the rumpled warmth of those same sheets the next morning. Feel the golf club in your hand as you take a long back swing and then swing forward to hit the ball from the tee, immediately sensing whether it is a good or a bad shot. There are an endless number of sensations that can be described, but basically, they fall into two general categories: cutaneous sensation and proprioception.
Cutaneous sensation allows the perception of touch (pressure, vibration, tickle, texture), temperature (hot/cold), pain (extreme temperatures, tissue damage, mechanical force), and itch via the activation of specific sensory receptors that respond to unique stimulus characteristics. Cutaneous receptors can collectively be referred to as mechanoreceptors, encompassing Pacinian corpuscles, Meissner corpuscles, Merkel disks, and Ruffini endings (Table 3-1, Figure 3-1); these receptors, as their name implies, convert mechanical forces to the skin, muscles, and other tissues into neural signals. Mechanoreceptors are also designated by their mode of adaptation (rapid or slow) and their location in the skin (superficial = 1, deep = 2). Slow-adapting receptors are activated by a maintained stimulus and continue firing for the duration of the stimulus; conversely, rapidly adapting receptors respond quickly to stimulus change, including the initiation or cessation of a stimulus, but cease activation during a maintained stimulation. Mechanoreceptors project centrally via large diameter, moderately myelinated, A beta (Aβ) neurons, which conduct slightly slower than the A alpha neurons that are discussed later in relation to muscle spindles and Golgi tendon organs.1,2 Mechanoreceptors collectively allow humans to discriminate ...