Anatomy of the Somatosensory System 1 FROM WIKIBOOKS | Our somatosensory | | | system | consists | | of sensors | in | the skin | | | | | | | ----------------- | --- | --- | ------ | -------- | --- | ---------- | --- | -------- | --- | --- | --- | --- | --- | This is a sample document to | and sensors | | in our | muscles, | | tendons, | and | joints. | The re- | | | | | | | ----------- | --- | ------ | -------- | --- | -------- | --- | ------- | ------- | --- | --- | --- | --- | --- | showcase page-based formatting. It | ceptors | in the | skin, | the | so called | cutaneous | | receptors, | tell | | | | | | | ------- | ------ | ----- | --- | --------- | --------- | --- | ---------- | ---- | --- | --- | --- | --- | --- | contains a chapter from aWikibook usabouttemperature(thermoreceptors),pressureandsur- calledSensory Systems. None of the | face texture | | (mechano | | receptors), | and | pain | (nociceptors). | | | | | | | | ------------ | --- | -------- | --- | ----------- | --- | ---- | -------------- | --- | --- | --- | --- | --- | --- | content has been changed in this | The receptors | | in | muscles | and | joints | provide | information | | | | | | | | ------------- | --- | --- | ------- | --- | ------ | ------- | ----------- | --- | --- | --- | --- | --- | --- | article, but some content has been about muscle length, muscle tension, and joint angles. 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 | | | | befreereceptorsorencapsulated. | | | | | | | | | | | | | | | Examples | for | free receptors | | are | | | | | | | | | | | the hair | receptors | at | the roots | of | Epidermis | | Free nerve | | Merkel’s | | Septa | | | | hairs.Encapsulatedreceptorsare | | | | | | --- | ---------- | --- | -------- | --- | ----- | --- | --- | --- | ------------------------------ | --- | --- | --- | --- | ending receptor | | | | | | | | | | the Pacinian | corpuscles | | and | the | | --- | --- | --- | --- | --- | --- | --- | --- | --- | ------------ | ---------- | -------- | ----------- | ------ | | | | | | | | | | | receptors | in the | glabrous | | (hair- | | | | | | | | | | | less) skin: | Meissner | | corpuscles, | | Meissner’s | Sebace o u s | | | | | | | | | Ruffini | corpuscles | and | Merkel’s | | | ------------ | --- | --- | --- | --------- | --- | --- | --------- | ------ | ------- | ---------- | --- | -------- | --- | | g l an d | | | | | | | corpuscle | Dermis | | | | | | | | | | | Ruffini’s | | | | | disks. | | | | | corpuscle Hair receptor Pacinian corpuscle 1 The following description is based on lecture notes from Laszlo Zaborszky, from Rutgers University. 1 From Wikibooks | 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 | | | | | | | | | | aprimary(groupIa)afferentand | a secondary | (group | II) afferent | | | | | | | | | | | | ---------------- | ------------------- | ------------ | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | | coil around | the non-contractile | | | | | | | | | | | | | central portions | of the | intrafusal | | | | | | | | | | | fibres. | | | | | rapidly | adapting | afferent | | activity, | muscle | | force increases | | | --- | --- | --- | --- | ------- | -------- | -------- | --- | --------- | ------ | --- | --------------- | --- | reflexivelyuntilthegrippedobjectnolongermoves.Such | | | | | a rapid | response | to | a tactile | stimulus | | is a | clear indication | | | --- | --- | --- | --- | ------- | -------- | --- | --------- | -------- | --- | ---- | ---------------- | --- | oftheroleplayedbysomatosensoryneuronsinmotorac- tivity. | | | | | The | slowly | adapting | Merkel’s | | receptors | | are responsible | | | --- | --- | --- | --- | --- | ------ | -------- | -------- | --- | --------- | --- | --------------- | --- | forformandtextureperception.Aswouldbeexpectedfor | | | | | 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- | erglabroussurfaces,andatverylowdensityinhairyskin. | | | | | This innervations | | | density | shrinks | | progressively | with | the | | --- | --- | --- | --- | ----------------- | --- | --- | ------- | ------- | --- | ------------- | ---- | --- | passageoftimesothatbytheageof50,thedensityinhu- | | | | | 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 Anatomy of the Somatosensory System Rapidly adapting Slowly adapting Surfacereceptor/ 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 / Ruffini’s corpuscle: “A skin Pacinian corpuscle: “A diffuse vibra- large receptive stretch”. Used for joint position tion” e.g. tapping with a pencil. 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 thatdependsonthedegreeoftissuedeformation.Theyal- so respond to temperatures in the range of 40–60°C, and change their response rates as a linear function of warm- ing(incontrastwiththesaturatingresponsesdisplayedby non-noxious thermoreceptors at high temperatures). Pain signals can be separated into individual compo- Notice how figure captions and nents, corresponding to different types of nerve fibers sidenotes are shown in the outside used for transmitting these signals. The rapidly transmit- margin (on the left or right, depending ted signal, which often has high spatial resolution, is on whether the page is left or right). called first pain or cutaneous pricking pain. It is well local- Also, figures are floated to the top/ ized and easily tolerated. The much slower, highly affec- bottom of the page. Wide content, like tive component is called second pain or burning pain; it is the table and Figure 3, intrude into the poorly localized and poorly tolerated. The third or deep outside margins. pain, arising from viscera, musculature and joints, is also poorly localized, can be chronic and is often associated with referred pain. Muscle Spindles Scatteredthroughoutvirtuallyeverystriatedmuscleinthe body are long, thin, stretch receptors called muscle spin- dles.Theyarequitesimpleinprinciple,consistingofafew smallmusclefiberswithacapsulesurroundingthemiddle third of the fibers. These fibers are called intrafusal fibers, incontrasttotheordinaryextrafusalfibers.Theendsofthe intrafusalfibersareattachedtoextrafusalfibers,sowhen- ever the muscle is stretched, the intrafusal fibers are also 3 From Wikibooks Force control Force (Golgi tendon organ) signal Inter- Force feedback neurons External forces Driving Muscle length | signal | | Tendon | | Muscle force | | | | | ------ | ------ | ------ | --- | ------------ | ---- | --- | --- | | | Muscle | | | | 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 | oneormoresensoryendingsappliedtoit.Whenthemus- | cle is stretched, | | the | central | part | of the intrafusal | | fiber is | | ----------------- | --- | --- | ------- | ---- | ----------------- | --- | -------- | stretched and each sensory ending fires impulses. | Muscle | spindles | | also | receive a | motor | innervation. | The | | ------ | -------- | --- | ---- | --------- | ----- | ------------ | --- | For more examples of how to use | large motor | neurons | | that | supply | extrafusal | muscle | fibers | | ----------- | ------- | --- | ---- | ------ | ---------- | ------ | ------ | HTML and CSS for paper-based arecalledalphamotorneurons,whilethesmalleronessup- publishing, seecss4.pub. | plying | the contractile | | portions | | of intrafusal | | fibers are | | ------ | --------------- | -------- | -------- | ----- | ------------- | --- | ---------- | | called | gamma | neurons. | Gamma | motor | neurons | | can regu- | latethesensitivityofthemusclespindlesothatthissensi- tivity can be maintained at any given muscle length. Joint receptors | The joint | receptors | | are low-threshold | | mechanoreceptors | | | | --------- | --------- | --- | ----------------- | --- | ---------------- | --- | --- | andhavebeendividedintofourgroups.Theysignaldiffer- entcharacteristicsofjointfunction(position,movements, | direction | and | speed | of movements). | | The | free receptors | or | | --------- | --- | ----- | -------------- | --- | --- | -------------- | --- | type 4 joint receptors are nociceptors. 4