Mechanical: Mechanical nociceptors respond to intense stretch or strain, like when you pull a hamstring or strain your Achilles tendon.For instance, if you touch a hot stove, nociceptors signaling pain are activated right away, sometimes before you're even aware of what you've done. Thermal: Thermal nociceptors respond to extreme hot or cold temperatures.Once again, lateral inhibition and spatial opponency exist and form detectors that allow people to estimate sharp pressure features along the surface of the skin. Some neurons also have receptive fields that correspond to local patches on the skin, much in the same way as receptive fields works for vision (recall Figure 5.8 from Section 5.2). Inside of the primary somatosensory cortex, neurons fire in a spatial arrangement that corresponds to their location on the body (topographic mapping ). This enables rapid motor response, for the purpose of withdrawing from painful stimuli quickly, and for the knee-jerk reflex. Long before the thalamus, some of the signals are also routed through the spinal cord to motor neurons that control muscles. The signals are routed through the thalamus, with relevant information eventually arriving at the primary somatosensory cortex in the brain, where the higher-level processing occurs. The neural pathways for the somatosensory system work in a way that is similar to the visual pathways of Section 5.2. Through these receptors, the body is aware of the relative positions, orientations, and velocities of its various moving parts. Joint receptors: These lie at the joints between bones and help coordinate muscle movement while also providing information to the central nervous system regarding relative bone positions.The organs report changes in muscle tension. Golgi tendon organs: These are embedded in tendons, which are each a tough band of fibrous tissue that usually connects a muscle to bone.Muscle spindles: As the name suggests, these are embedded inside of each muscle so that changes in their length can be reported to the central nervous system (which includes the brain).Regarding proprioception (and kinesthesis), there are three kinds of receptors: The temporal resolution is not the same as for hearing, which extends up to 20,000 Hz the Pacinian corpuscles allow vibrations up to a few hundred Hertz to be distinguished from a static pressure. This has implications on touch perception, which will be covered shortly. The density is high at the fingertips, and very low on the back. The spatial resolution or acuity corresponds to the density, or receptors per square area, which varies over the body. Touch has both spatial and temporal resolutions. Yet another class is nocireceptors, which appear in joint tissues and cause a pain sensation from overstretching, injury, or inflammation. In some critical places, such as eyelids, lips, and tongue, thermoreceptors called the end-bulbs of Krause also appear in the skin. Meissner's corpuscles are only in parts where there are no hair follicles ( glabrous skin ), and the hair follicle receptors obviously appear only where there is hair. The first four of these receptors appear in skin all over the body.
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