# Anatomy of the Somatosensory System 1FROM WIKIBOOKS Our somatosensory system consists of sensors in the skinandsensorsinourmuscles,tendons,andjoints.There-ceptors in the skin,the so called cutaneous receptors,tellus about temperature(*thermoreceptors*),pressure and sur-face texture(*mechano receptors*),and pain(The receptors in muscles and joints provide informationabout muscle length, muscle tension, and joint angles. Cutaneous receptors Sensory information from *Meissner corpuscles*adapting afferents leads to adjustment of grip force whenobjects are lifted. These afferents respond with a briefburst of action potentials when objects move a small dis-tance during the early stages of lifting. In response to Glabrous skin Hairy skin Papillary Ridges Septa Free nerve Merkel’sending receptor Sebaceous gland Ruffini’s corpuscle Hair receptor Paciniancorpuscle 1 The following description is based on lecture notes from Laszlo Zaborszky, from Rutgers University. *nociceptors* and rapidly Epidermis Meissne r’scorpuscle Dermis This is a sample document toshowcase page-based formatting. It[contains a chapter from aWikibook](http://en.wikibooks.org/)[calledSensory Systems](http://en.wikibooks.org/wiki/Sensory_Systems)[. None of the](http://en.wikibooks.org/)[content has been changed in this](http://en.wikibooks.org/wiki/Sensory_Systems)article, but some content has beenremoved. *Figure 1: Receptors in the hu-**man skin:Mechanoreceptors can**be free receptors or encapsulated.**Examples for free receptors are**the hair receptors at the roots of**hairs.Encapsulated receptors are**the Pacinian corpuscles and the**receptors in the glabrous (hair-**less) skin: Meissner corpuscles,**Ruffini corpuscles and Merkel’s**disks.* 1 --- From Wikibooks *Figure 2: Mammalian muscle**spindle showing typical position**in a muscle(left),neuronal con-**nections in spinal cord (middle)**andexpandedschematic (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(groupIa)afferent and**a secondary (group II) afferent**coil around the non-contractile**central portions of the intrafusal**fibres.* rapidly adapting afferent activity,muscle force increasesreflexively until the gripped object no longer moves.Sucha rapid response to a tactile stimulus is a clear indicationof the role played by somatosensory neurons in motor ac-tivity. The slowly adapting *Merkel’s receptors*for form and texture perception.As would be expected forreceptors mediating form perception,Merkel’s receptorsare present at high density in the digits and around themouth(50/mm²of skin surface),at lower density in oth-er glabrous surfaces,and at very low density in hairy skin.This innervations density shrinks progressively with thepassage of time so that by the age of50,the density in hu-man digits is reduced to10/mm².Unlike rapidly adaptingaxons,slowly adapting fibers respond not only to the ini-tialindentationofskin,butalsotosustainedindentationup to several seconds in duration. Activation of the rapidly adaptinggives a feeling of vibration, while the*Ruffini corpuscles* respond to the lataralstretching of skin. Nociceptors Nociceptors have free nerve endings.Functionally,skinnociceptors are either high-threshold mechanoreceptors 2 are responsible *Pacinian corpuscles* slowly adaptingmovement or --- Anatomy of the Somatosensory System **Rapidly adapting** Surface receptor/ *Hair receptor Meissner’s corpuscle*: De-small receptive tect an insect or a very fine vibration.field Used for recognizing texture. Deep receptor / *Pacinian corpuscle*: “A diffuse vibra-large receptive tion” e.g. tapping with a pencil.field *Table 1* or *polymodal receptors*.Polymodal receptors respond notonlytointensemechanicalstimuli,butalsotoheatandto noxious chemicals.These receptors respond to minutepunctures of the epithelium,with a response magnitudethat depends on the degree of tissue deformation.They al-sorespondtotemperaturesintherangeof40–60°C,andchange their response rates as a linear function of warm-ing(in contrast with the saturating responses displayed bynon-noxious thermoreceptors at high temperatures). Pain signals can be separated into individual compo-nents, corresponding to different types of nerve fibersused for transmitting these signals.The rapidly transmit-ted signal, which often has high spatial resolution, iscalled *first pain* or *cutaneous pricking pain*.It is well local-ized and easily tolerated.The much slower,highly affec-tive component is called *second pain* or *burning pain*;it ispoorly localized and poorly tolerated.The third or *deep**pain*,arising from viscera,musculature and joints,is alsopoorly localized,can be chronic and is often associatedwith referred pain. Muscle Spindles Scattered throughout virtually every striated muscle in thebody are long,thin,stretch receptors called muscle spin-dles.They are quite simple in principle,consisting of a fewsmallmusclefiberswithacapsulesurroundingthemiddlethird of the fibers.These fibers are called *intrafusal fibers*in contrast to the ordinary *extrafusal fibers*.The ends of theintrafusal fibers are attached to extrafusal fibers,so when-ever the muscle is stretched,the intrafusal fibers are also **Slowly adapting** Merkel’s receptor: Used for spa-tial details, e.g. a round surfaceedge or “an X” in brail. *Ruffini’s corpuscle*: “A skinstretch”. Used for joint positionin fingers. Notice how figure captions andsidenotes are shown in the outsidemargin (on the left or right, dependingon whether the page is left or right).Also, figures are floated to the top/bottom of the page. Wide content, likethe table and Figure 3, intrude into theoutside margins. 3 --- From Wikibooks Force Force (Golgi tendon organ) control Inter- Force feedback signal neurons Externalforces Drivingsignal Length (secondary muscle-spindel afferents)Length error (primary muscle-spindel afferents) Length & Musclelength Muscle force TendonMuscle Loadorgans Velocity (primary muscle-spindel afferents) velocityfeedbackLength Spindlescontrol signal Gamma bias *Figure3: 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 hasfew myofilaments and is non-contractile,but it does haveone or more sensory endings applied to it.When the mus-cle is stretched,the central part of the intrafusal fiber isstretched and each sensory ending fires impulses. Muscle spindles also receive a motor innervation.TheFor more examples of how to use large motor neurons that supply extrafusal muscle fibersHTML and CSS for paper-based are called *alpha motor neurons*,while the smaller ones sup-[publishing, seecss4.pub](http://css4.pub) plying the contractile portions of intrafusal fibers arecalled *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 mechanoreceptorsand have been divided into four groups.They signal differ-ent characteristics of joint function(position,movements,direction and speed of movements).The free receptors ortype 4 joint receptors are nociceptors. 4