This is a sample document to showcase page-based formatting. It contains a chapter from a Wikibook called Sensory Systems . None of the content has been changed in this article, but some content has been remo v ed. Anat omy of the Somat osensor y System F R OM  W IKIBOOKS 1 Our somatosensor y system c onsists of sensors in the skin and sensors in our muscles, tendons, and joints. The re- c ep tors in the skin, the so called cutaneous rec ep tors, tell us about temperature ( thermorec eptors ), pressure and sur- fac e te xture ( mechano rec eptors ), and pain ( nocic eptors ). The rec ep tors in muscles and joints pro vide inf ormation about muscle length, muscle tension, and joint angles. Cutaneous r ecept ors Sensor y inf ormation from Meissner c orpuscles and rapidly adap ting aff erents leads to adjustment of grip f orc e when objec ts are lif ted. These aff erents respond with a brief burst of ac tion po tentials when objec ts mo v e a small dis- tanc e during the early stages of lif ting. In response to Figure 1: R ec eptors in the hu- man skin: Mechanorec eptors c an be free rec eptors or enc apsulated. Ex amples for free rec eptors are the hair rec eptors at the roots of hairs. Enc apsulated rec eptors are the P acinian c orpuscles and the rec eptors in the glabrous (hair- less) skin: Meissner c orpuscles, R uffini c orpuscles and Merk el’s disks. Hairy skin Glabrous skin Epidermis Dermis Pacinian corpuscle Papillary Ridges Septa Ruffini ’ s  corpuscle Hair receptor Meissne r ’ s corpuscle Sebaceous gland Free nerve ending Merkel ’ s receptor 1 The f ollo wing descrip tion is based on lec ture no tes from Laszlo Zaborszky , from Rutgers Univ ersit y . 1


Figure 2: Mammalian muscle spindle showing t ypic al position in a muscle (left), neuronal c on- nections in spinal c ord (middle ) and expanded schematic (right). T he spindle is a stretch rec eptor with its own motor supply c on- sisting of se v eral intrafusal mus- cle fibres. T he sensory endings of a primary ( group Ia) afferent and a sec ondary ( group II) afferent c oil around the non-c ontractile c entral portions of the intrafusal fibres. rapidly adap ting aff erent ac tivit y , muscle f orc e increases refle xiv ely until the gripped objec t no longer mo v es. Such a rapid response to a tac tile stimulus is a clear indication of the role play ed b y somatosensor y neurons in mo tor ac- tivit y . The slo wly adap ting Merk el’s rec eptors are responsible f or f orm and te xture perc ep tion. As w ould be e xpec ted f or rec ep tors mediating f orm perc ep tion, Merk el’ s rec ep tors are present at high densit y in the digits and around the mouth (50/ mm² of skin surfac e), at lo w er densit y in o th- er glabrous surfac es, and at v er y lo w densit y in hair y skin. This inner vations densit y shrinks progressiv ely with the passage of time so that b y the age of 50, the densit y in hu- man digits is reduc ed to 10/ mm². Unlik e rapidly adap ting ax ons, slo wly adap ting fibers respond no t only to the ini- tial indentation of skin, but also to sustained indentation up to se v eral sec onds in duration. A c tivation of the rapidly adap ting P acinian c orpuscles giv es a f eeling of vibration, while the slo wly adap ting R uffini c orpuscles respond to the lataral mo v ement or stretching of skin. Nocicept ors Nocic ep tors hav e free ner v e endings. Func tionally , skin nocic ep tors are either high-threshold mechanorec ep tors F rom Wikibooks 2


Rapidly adap ting Slo wly adap ting Surfac e rec ep tor / small rec ep tiv e field Hair rec eptor , Meissner’s c orpuscle : De- tec t an insec t or a v er y fine vibration. Used f or rec ognizing te xture. Merk el’ s rec ep tor: Used f or spa- tial details, e.g. a round surfac e edge or “ an X” in brail. Deep rec ep tor / large rec ep tiv e field P acinian c orpuscle : “ A diffuse vibra- tion” e.g. tapping with a pencil. R uffini’s c orpuscle : “ A skin stretch” . Used f or joint position in fingers. T able 1 Notice how figure captions and sidenotes are shown in the outside margin ( on the left or right, depending on whether the page is left or right). Also, figures are floated to the top/ bottom of the page. Wide content, lik e the table and Figure 3, intrude into the outside margins. or polymodal rec eptors . P olymodal rec ep tors respond no t only to intense mechanical stimuli, but also to heat and to no xious chemicals. These rec ep tors respond to minute punc tures of the epithelium, with a response magnitude that depends on the degree of tissue def ormation. The y al- so respond to temperatures in the range of 40–60°C, and change their response rates as a linear func tion of warm- ing (in c ontrast with the saturating responses display ed b y non-no xious thermorec ep tors at high temperatures). P ain signals can be separated into individual c ompo- nents, c orresponding to diff erent t ypes of ner v e fibers used f or transmit ting these signals. The rapidly transmit- ted signal, which of ten has high spatial resolution, is called first pain or cutaneous pricking pain . It is w ell local- iz ed and easily tolerated. The much slo w er , highly aff ec- tiv e c omponent is called sec ond pain or burning pain ; it is poorly localiz ed and poorly tolerated. The third or deep pain , arising from visc era, musculature and joints, is also poorly localiz ed, can be chronic and is of ten associated with ref erred pain. Muscle Spindles Scat tered throughout virtually e v er y striated muscle in the body are long, thin, stretch rec ep tors called muscle spin- dles. The y are quite simple in principle, c onsisting of a f e w small muscle fibers with a capsule surrounding the middle third of the fibers. These fibers are called intrafusal fibers , in c ontrast to the ordinar y extrafusal fibers . The ends of the intrafusal fibers are at tached to e xtrafusal fibers, so when- e v er the muscle is stretched, the intrafusal fibers are also Anatom y of the Somatosensory System 3


Force control signal Driving signal Length control signal Load External forces Tendon organs Muscle force Muscle length Force feedback Length & velocity feedback Force (Golgi tendon organ) Spindles Gamma bias Length (secondary muscle-spindel afferents) Length error (primary muscle-spindel afferents) Velocity (primary muscle-spindel afferents) Muscle Inter- neurons Figure 3: Feedback loops for proprioc eptiv e signals for the perc eption and c ontrol of limb mov e- ments. A rrows indic ate ex citatory c onnections; filled circles inhibitory c onnections. F or more e xamples of how to use HTML and CSS for paper-based publishing, see css4.pub . stretched. The c entral region of each intrafusal fiber has f e w my ofilaments and is non-c ontrac tile, but it does hav e one or more sensor y endings applied to it. When the mus- cle is stretched, the c entral part of the intrafusal fiber is stretched and each sensor y ending fires impulses. Muscle spindles also rec eiv e a mo tor inner vation. The large mo tor neurons that supply e xtrafusal muscle fibers are called alpha motor neurons , while the smaller ones sup- plying the c ontrac tile portions of intrafusal fibers are called gamma neurons . Gamma mo tor neurons can regu- late the sensitivit y of the muscle spindle so that this sensi- tivit y can be maintained at any giv en muscle length. Joint r ecept ors The joint rec ep tors are lo w -threshold mechanorec ep tors and hav e been divided into f our groups. The y signal diff er- ent charac teristics of joint func tion (position, mo v ements, direc tion and speed of mo v ements). The free rec ep tors or t ype 4 joint rec ep tors are nocic ep tors. F rom Wikibooks 4