Mechanoreceptors sense stimuli due to physical deformation of their plasma membranes. They contain mechanically-gated ion channels whose gates open or close in response to pressure, touch, stretching, and sound.
A fifth type of mechanoreceptor, Krause end bulbs, are found only in specialized regions. Primary mechanoreceptors : Four of the primary mechanoreceptors in human skin are shown. Ruffini endings detect stretch, deformation within joints, and warmth. Pacinian corpuscles detect transient pressure and high-frequency vibration. Krause end bulbs detect cold. They are slow-adapting, unencapsulated nerve endings, which respond to light touch.
Light touch, also known as discriminative touch, is a light pressure that allows the location of a stimulus to be pinpointed. That makes them very sensitive to edges; they come into use in tasks such as typing on a keyboard. They are found primarily in the glabrous skin on the fingertips and eyelids.
They respond to fine touch and pressure, but they also respond to low-frequency vibration or flutter. They are rapidly- adapting, fluid-filled, encapsulated neurons with small, well-defined borders which are responsive to fine details. Meissner corpuscles : Meissner corpuscles in the fingertips, such as the one viewed here using bright field light microscopy, allow for touch discrimination of fine detail. Deeper in the dermis, near the base, are Ruffini endings, which are also known as bulbous corpuscles.
They are found in both glabrous and hairy skin. These are slow-adapting, encapsulated mechanoreceptors that detect skin stretch and deformations within joints; they provide valuable feedback for gripping objects and controlling finger position and movement. Thus, they also contribute to proprioception and kinesthesia. Ruffini endings also detect warmth.
Note that these warmth detectors are situated deeper in the skin than are the cold detectors. It is not surprising, then, that humans detect cold stimuli before they detect warm stimuli.
They are found in the bone periosteum, joint capsules, pancreas and other viscera, breast, and genitals. They are rapidly-adapting mechanoreceptors that sense deep, transient not prolonged pressure, and high-frequency vibration. Pacinian receptors detect pressure and vibration by being compressed which stimulates their internal dendrites. Pacinian corpuscles : Pacinian corpuscles, such as these visualized using bright field light microscopy, detect pressure touch and high-frequency vibration.
The many types of somatosensory receptors work together to ensure our ability to process the complexity of stimuli that are transmitted. Each ending consists of a Merkel cell in close apposition with an enlarged nerve terminal.
This is sometimes referred to as a Merkel cell—neurite complex, or a Merkel disk receptor. A single afferent nerve fiber branches to innervate up to 90 such endings. They are classified as slowly adapting type I mechanoreceptors. Learning Objectives Describe how touch is sensed by mechanoreceptive neurons responding to pressure. Key Points Our sense of touch, or tactile sensation, is mediated by cutaneous mechanoreceptors located in our skin.
Cutaneous mechanoreceptors are categorized by morphology, by the type of sensation they perceive, and by the rate of adaptation. Furthermore, each has a different receptive field. Key Terms receptive field : The particular region of the sensory space e. In the skin, there are four main types in glabrous hairless skin: Ruffini endings. Below are representative slides showing the tract in the medulla and midbrain.
Notice that by midbrain the spinothalamic tract appears to be continuous with the medial lemniscus. They will enter the VPL of the thalamus together. The spinothalamic system enters the VPL, synapses, and is finally carried to cortex by the thalamocortical neurons. Here is a schematic of the entire pathway:. It has been recognized for centuries that opium and related compounds such as morphine are powerful analgesics.
Several decades ago scientists hunted down the opiate receptor which was responsible for the potent effects. They then reasoned that if there was such a receptor in the body, maybe the body used its own endogenous form of opium to control pain. It has also been recognized for centuries that under certain circumstances, i. This hypothetical compound was named " endorphin ", from endogenous-morphine. Soon after, an entire class of peptide neurotransmitters was discovered that interacted with the opiate receptor, and now includes endorphins, enkephalins, and dynorphins.
Synthetic, exogenous forms of these compounds continue to be discovered, prescribed, and abused, and are classed under the general term, "narcotics". There are opiate receptors throughout the central nervous system. In the dorsal horn, they are located on the terminals of the primary afferents, as well as on the cell bodies of the secondary afferents.
Opiate interneurons in the spinal cord can be activated by descending projections from the brainstem especially the raphe nuclei and periaqueductal grey , and can block pain transmission at two sites.
The proprioceptive system arises from primarily the A a afferents entering the spinal cord. These are the afferents from muscle spindles, Golgi tendon organs, and joint receptors. The axons travel for a little while with the discriminative touch system, in the posterior columns. Within a few segments, however, the proprioceptive information slips out of the dorsal white matter and synapses.
After synapsing it ascends without crossing to the cerebellum. Exactly where the axons synapse depends upon whether they originated in the legs or the arms. Leg fibers enter the cord at sacral or lumbar levels, ascend to the upper lumbar segments, and synapse in a medial nucleus called Clarke's nucleus or nucleus dorsalis.
The secondary afferents then enter the dorsal spinocerebellar tract on the lateral edge of the cord. Fibers from the arm enter at cervical levels and ascend to the caudal medulla. In the center of the corpuscle is the inner bulb, a fluid-filled cavity with a single afferent unmyelinated nerve ending.
Lamellar corpuscles detect gross pressure changes and vibrations and are rapidly adapting phasic receptors.
Any deformation in the corpuscle causes action potentials to be generated by opening pressure-sensitive sodium ion channels in the axon membrane. This allows sodium ions to influx, creating a receptor potential. These corpuscles are especially susceptible to vibrations, which they can sense even centimeters away. Lamellar corpuscles have a large receptive field on the skin's surface with an especially sensitive center.
Lamellar corpuscles sense stimuli due to the deformation of their rings of lamellae, which press on the top of the sensory neuron and causes it to bend. If this potential reaches a certain threshold, nerve impulses or action potentials are formed by pressure-sensitive sodium channels at the first node of Ranvier , the first node of the myelinated sensory neuron. Once the top of the neuron is depolarized, it will depolarize the first node of Ranvier; however, as it is a rapidly adapting fibre, this does not carry on indefinitely, and the signal propagation ceases.
This is a graded response, meaning that the greater the deformation, the greater the generator potential. This information is encoded in the frequency of impulses, since a bigger or faster deformation induces a higher impulse frequency. Action potentials are formed when the skin is rapidly distorted but not when pressure is continuous.
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