Anatomy of the Somatosensory System FROM W IKIBOOKS 1 Our somatosensory system consists of sensors in the skin and sensors in our muscles, tendons, and joints. The re- Thisisasampledocumentto showcasepage-basedformatting.It ceptors in the skin, the so called cutaneous receptors, tell containsachapterfromaWikibook us about temperature (thermoreceptors), pressure and sur- calledSensorySystems.Noneofthe face texture (mechano receptors), and pain (nociceptors). contenthasbeenchangedinthis The receptors in muscles and joints provide information about muscle length, muscle tension, and joint angles. article,butsomecontenthasbeen removed. Cutaneous receptors Sensory information from Meissner corpuscles and rapidly adapting afferents leads to adjustment of grip force when objects are lifted. These afferents respond with a brief burst of action potentials when objects move a small dis- tance during the early stages of lifting. In response to Hairy skin Glabrous skin Figure 1: Receptors in the hu- man skin: Mechanoreceptors can Papillary Ridges be free receptors or encapsulated. Examples for free receptors are the hair receptors at the roots of Free nerMerkel’s Septa Epidermis endingreceptor hairs. Encapsulated receptors are the Pacinian corpuscles and the receptors in the glabrous (hair- Meissne r’s less) skin: Meissner corpuscles, Sebglands corpuscleDermis Ruffini corpuscles and Merkel’s Ruffini’s disks. corpuscle Hair receptor Pacinian corpuscle 1The following description is based on lecture notes from Laszlo Zaborszky, from Rutgers University. 1 FromWikibooks Figure 2: Mammalian muscle spindle showing typical position in a muscle (left), neuronal con- nections in spinal cord (middle) and expanded schematic (right). The spindle is a stretch receptor with its own motor supply con- sisting of several intrafusal mus- cle fibres. The sensory endings of a primary (group Ia) afferent and a secondary (group II) afferent coil around the non-contractile central portions of the intrafusal fibres. rapidly adapting afferent activity, muscle force increases reflexively until the gripped object no longer moves. Such a rapid response to a tactile stimulus is a clear indication of the role played by somatosensory neurons in motor ac- tivity. The slowly adapting Merkel’s receptors are responsible for form and texture perception. As would be expected for receptors mediating form perception, Merkel’s receptors are present at high density in the digits and around the mouth (50/mm² of skin surface), at lower density in oth- er glabrous surfaces, and at very low density in hairy skin. This innervations density shrinks progressively with the passage of time so that by the age of 50, the density in hu- man digits is reduced to 10/mm². Unlike rapidly adapting axons, slowly adapting fibers respond not only to the ini- tial indentation of skin, but also to sustained indentation up to several seconds in duration. Activation of the rapidly adapting Pacinian corpuscles gives a feeling of vibration, while the slowly adapting Ruffini corpuscles respond to the lataral movement or stretching of skin. Nociceptors Nociceptors have free nerve endings. Functionally, skin nociceptors are either high-threshold mechanoreceptors 2 AnatomyoftheSomatosensorySystem Rapidly adapting Slowly adapting Surface receptor / Hair receptor, Meissner’s corpuscle: De- Merkel’s receptor: Used for spa- small receptive tect an insect or a very fine vibration. tial details, e.g. a round surface field Used for recognizing texture. edge or “an X” in brail. Deep receptor / Pacinian corpuscle: “A diffuse vibra- Ruffini’s corpuscle: “A skin large receptive tion” e.g. tapping with a pencil. stretch”. Used for joint position field in fingers. Table 1 or polymodal receptors. Polymodal receptors respond not only to intense mechanical stimuli, but also to heat and to noxious chemicals. These receptors respond to minute punctures of the epithelium, with a response magnitude that depends on the degree of tissue deformation. They al- so respond to temperatures in the range of 40–60°C, and change their response rates as a linear function of warm- ing (in contrast with the saturating responses displayed by non-noxious thermoreceptors at high temperatures). Pain signals can be separated into individual compo- Noticehowfigurecaptionsand nents, corresponding to different types of nerve fibers sidenotesareshownintheoutside used for transmitting these signals. The rapidly transmit- margin(ontheleftorright,depending ted signal, which often has high spatial resolution, is onwhetherthepageisleftorright). called first pain or cutaneous pricking pain. It is well local- ized and easily tolerated. The much slower, highly affec- Also,figuresarefloatedtothetop/ tive component is called second pain or burning pain; it is bottomofthepage.Widecontent,like thetableandFigure3,intrudeintothe poorly localized and poorly tolerated. The third or deep outsidemargins. pain, arising from viscera, musculature and joints, is also poorly localized, can be chronic and is often associated with referred pain. Muscle Spindles Scattered throughout virtually every striated muscle in the body are long, thin, stretch receptors called muscle spin- dles. They are quite simple in principle, consisting of a few small muscle fibers with a capsule surrounding the middle third of the fibers. These fibers are called intrafusal fibers, in contrast to the ordinary extrafusal fibers. The ends of the intrafusal fibers are attached to extrafusal fibers, so when- ever the muscle is stretched, the intrafusal fibers are also 3 FromWikibooks Force control Force (Golgi tendon organ) signal Inter- Force feedback neurons External forces Muscle Driving length signal Muscle Tendon Muscle force Load organs Length (secondary muscle-spindel afferents) Length error (primary muscle-spindel afferents) Length & Velocity (primary muscle-spindel afferents) velocity feedback Length Spindles control signal Gamma bias Figure 3: Feedback loops for proprioceptive signals for the perception and control of limb move- ments. Arrows indicate excitatory connections; filled circles inhibitory connections. stretched. The central region of each intrafusal fiber has few myofilaments and is non-contractile, but it does have one or more sensory endings applied to it. When the mus- cle is stretched, the central part of the intrafusal fiber is stretched and each sensory ending fires impulses. Formoreexamplesofhowtouse Muscle spindles also receive a motor innervation. The large motor neurons that supply extrafusal muscle fibers HTMLandCSSforpaper-based are called alpha motor neurons, while the smaller ones sup- publishing,seecss4.pub. plying the contractile portions of intrafusal fibers are called gamma neurons. Gamma motor neurons can regu- late the sensitivity of the muscle spindle so that this sensi- tivity can be maintained at any given muscle length. Joint receptors The joint receptors are low-threshold mechanoreceptors and have been divided into four groups. They signal differ- ent characteristics of joint function (position, movements, direction and speed of movements). The free receptors or type 4 joint receptors are nociceptors. 4