This is a sample document to showcase page-based formatting. It contains a chapter from aWikibook called Sensory Systems. None of the content has been changed in this article, but some content has been remo ved. Anatomy of the Somatosensory System FR OM WIKIBOOKS 1 Our somatosensor ysystem consistsofsensors inthe skin andsensors inourmuscles, tendons, andjoints. The re- cep torsinthe skin, the socalled cutaneous rec ep tors, tell usabouttemperature (thermorec eptors ), pressure andsur- face te xture (mechano rec eptors ), and pain (nocic eptors ). The rec ep torsinmuscles andjointsprovide information about muscle length, muscle tension, and joint angles. Cutaneous receptors Sensor yinformationfromMeissner c orpuscles andrapidly adaptingafferents leads toadjustment ofgripforce when objec ts are lifted. These afferents respond with a brief burstofactionpotentials when objec tsmove asmalldis- tance during the early stages of lifting. In response to Figure 1: R ec eptors in the hu- man skin: Mechanorec eptors c an be free rec eptors orenc apsulated. Ex amples for free rec eptors are the hair rec eptors at the rootsof 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 skinGlabrous 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 following description is based on lecture notes from Laszlo Zaborszky, from Rutgers University. 1 Figure 2: Mammalian muscle spindle showing typic 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 veral 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. rapidlyadaptingafferent activity, muscle force increases refle xively untilthe gripped objec tnolonger moves. Such arapidresponse toatactile stimulusisaclear indication ofthe role played bysomatosensor yneurons inmotorac- tivity. The slowlyadaptingMerk el’s rec eptors are responsible forformandte xture perc ep tion. As wouldbe e xpec ted for rec ep tors mediating form perc ep tion, Merk el’ s rec ep tors are present athighdensit yinthe digitsandaroundthe mouth(50/ mm²ofskinsurface), atlower densit yinoth- er glabroussurfaces, andatver ylowdensit yinhairyskin. This inner vationsdensit yshrinksprogressiv ely withthe passage oftime sothatbythe age of50, the densit yinhu- mandigitsisreduc ed to10/ mm². Unlik e rapidlyadapting axons, slowlyadaptingfibers respond notonlytothe ini- tialindentation ofskin, butalsotosustained indentation up to several seconds in duration. A ctivationofthe rapidlyadapting P acinian c orpuscles gives a feeling of vibration, while the slowly adapting R uffini c orpuscles respond to the lataral movement or stretching of skin. Nociceptors Nocic ep tors have free ner ve endings. Func tionally, skin nocicep tors are either high-threshold mechanorec ep tors F rom Wikibooks 2 Rapidly adaptingSlowly adapting Surfac e rec ep tor/ small receptive field Hair receptor,Meissner’s corpuscle: De- tec t an insect or a very fine vibration. Used for recognizing texture. Merk el’ s receptor: Used for spa- tial details, e.g. a round surface edge or “an X” in brail. Deep receptor / large receptive field P acinian corpuscle: “A diffuse vibra- tion” e.g. tapping with a pencil. R uffini’s corpuscle: “A skin stretch” . Used for 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, like the table and Figure 3, intrude into the outside margins. or polymodal rec eptors . P olymodalrec ep torsrespond not onlytointense mechanical stimuli, butalsotoheat and tonoxiouschemicals. These rec ep torsrespond tominute punctures ofthe epithelium, witharesponse magnitude thatdepends onthe degree oftissue def ormation. The yal- sorespond totemperatures inthe range of40–60°C, and change their response rates asalinear functionofwarm- ing(in contrastwiththe saturatingresponses displayed by non-no xious thermoreceptors at high temperatures). P ainsignalscanbe separated intoindividualcompo- nents, corresponding to different types of ner ve fibers used fortransmittingthese signals. The rapidlytransmit- ted signal, which often has high spatial resolution, is called firstpain orcutaneous pricking pain . It iswell local- ized andeasily tolerated. The muchslower , highlyaffec- tive component iscalled sec ond pain orburning pain ; itis poorly localized and poorly tolerated. The third or deep pain , arisingfromviscera, musculature andjoints, isalso poorly localized, can be chronic and is often associated with referred pain. Muscle Spindles Scat tered throughoutvirtuallye ver ystriated muscle inthe bodyare long, thin, stretch rec ep torscalled muscle spin- dles. The yare quite simple inprinciple, consistingofafe w smallmuscle fibers withacapsule surroundingthe middle thirdofthe fibers. These fibers are called intrafusal fibers , incontrasttothe ordinaryextrafusal fibers . The ends ofthe intrafusalfibers are attached toe xtrafusalfibers, sowhen- e ver the muscle isstretched, the intrafusalfibers 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 move- ments. Arrows indicate excitatory connections; filled circles inhibitory connections. F or more examples of how to use HTML and CSS for paper-based publishing, seecss4.pub. stretched. The central region ofeach intrafusalfiber has fe wmyofilaments andisnon-c ontractile, butitdoes have one ormore sensor yendings applied toit. When the mus- cle isstretched, the central partofthe intrafusalfiber is stretched and each sensory ending fires impulses. Muscle spindles alsorec eiv e amotorinner vation. The large motorneurons thatsupplye xtrafusalmuscle fibers are called alpha motorneurons , while the smaller ones sup- plying the contractile portions of intrafusal fibers are called gamma neurons . Gamma motorneurons canregu- late the sensitivit yofthe muscle spindle sothatthissensi- tivity can be maintained at any given muscle length. Joint receptors The jointrec ep torsare low-threshold mechanorec ep tors andhave been divided intofourgroups. The ysignaldiffer- ent characteristics ofjointfunction(position, movements, direc tionandspeed ofmovements). The free rec ep torsor type 4 joint receptors are nociceptors. F rom Wikibooks 4