Somatosensory System PDF

Somatosensory System The primary somatosensory cortex is located dorsal of the central sulcus. This cortex contains a representation of the body in separated regions, somatotropic maps Some somatotropic maps are larger compared to the relative size of the body part, e.g. thumb and lower li...

Somatosensory System The primary somatosensory cortex is located dorsal of the central sulcus. This cortex contains a representation of the body in separated regions, somatotropic maps Some somatotropic maps are larger compared to the relative size of the body part, e.g. thumb and lower lip. Reconstructing the body based on the somatotropic maps in the somatosensory and motor cortex give rise to the "little man", homunculus Two types of dorsal root afferents: Touch and pressure: mechanosensory fiber, detect stimulation using mechoreceptors. Stays on the same side Nocireceptive afferent: pain and temperature afferent, detect stimulation in free nerve endings Crosses over straight away PIEZO2 Touch is detected as a change in pressure on the skin. Change of phospholipid pressure in the membrane leaflet operates opening/closing of ion channels. Flow of ions over the membrane: current and change in membrane potential: action potential Receptive field are a central concept of the senses. Large receptive fields have low sensory precision, as stimulations within the field are difficult to discriminate. Large receptive field have generally a high sensitivity for stimulation, as any stimulus in a large area will result in a response.  Pacinian corpuscle Detects general pressure on the skin, e.g. hand grip, very large receptive field Ruffini corpuscle Detects stretching of the skin, otion detection Meissner corpuscle common mechanoreceptor (40% of total in the hand) more sensitive but less precise then merkel cells Merkel cell located at the finger tips small receptive field, high precision Some fibers detect change (rapid adapting) or persistent stimulation (slow adapting).  Both used when rubbing Touch: sensory information from external stimuli Proprioception: sensory information of the body itself Receptors for proprioception are found in muscles and tendons. Muscle spindles: changes in muscle length Golgi tendon: changes in muscle tension Together these continuously map the positioning of the limbs Afferents of proprioreceptors project to the ventral horn (e.g. reflexes) and through Clarke's nucleus to the cerebellum (timing of voluntary movement) as well as the thalamus (stays on the same side of spinal cord) Dermatome: the innervated area of a sensory axons of a single dorsal root ganglion Facial sensory input is mediated through the trigeminal nerve (cranial nerve V) Plasticity Somatotropic maps within individual digits, separate processing of slow and fast adapting input This reassignment, not only in the cortex but also in the thalamus and brainstem, can lead to phantom sensations of lost limbs Training of complex tasks lead to rearrangement of cortical representation  Nocireceptive axons respond only to very strong stimuli Pain perception is mediated by slow fibers TRPV1 Pain A⍺ and Aβ mediate touch, Aẟ fiber is lightly myelinated (fast pain), C fibers are unmyelinated and very thin (slow pain) Pain can be thermal, mechanical or chemical. Transient receptor potential (TRP) channels are located on pain fibers to sense pain and heat. These are adapted cation (Na +, Ca2+) channels. TRPs can be opened in reaction to heat, chemical irritants and substances secreted by damaged tissue. There are many different types responding to selective chemicals (TRPC, TRPV, TRPM, TRPN, TRPP, TRPML) Capsaicin activates the "heat receptor" TRPV1 on C-fibers Capsaicin is a lipophilic molecule like endocannabinoids Fast and slow nocireceptive afferents are spatially organised The anterolateral system: axons of the dorsal horn neurons cross the midline and ascend in the anterolateral white matter Spinal lesion: ipsilateral mechanosensory loss and contralateral pain sensation loss Referred pain is visceral pain projected on the skin Visceral pain neurons also project onto dorsal horn neurons that convey cutaneous pain. Hence pain of the oesophagus or heart are perceived as pain on the chest or arm Long-term effects flight-fight-fright response negative emotion to avoid painful events Pain modulation Descending tracks from e.g. Raphe (serotonin) or Loc Coeruleus (NA) suppress pain. Local mechanosensory (rubbing your arm) input suppresses pain Endocannabionoids and endorphins suppress pain tracks in the spinal cord