A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. There are four main types in the glabrous skin of humans: Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini corpuscles. There are also mechanoreceptors in the hairy skin, and the hair cells in the cochlea are the most sensitive mechanoreceptors, transducing air pressure waves into sound.
Mechanoreceptors are primary neurons that respond to mechanical stimuli by firing action potentials. Peripheral transduction is believed to occur in the end-organs.
In somatosensory transduction, the afferent neurons transmit the message through synapses in the dorsal column nuclei, where the second order neurons send the signal to the thalamus and synapse with the third order neurons in the ventrobasal complex. The third order neurons then send the signal to the somatosensory cortex.
More recent work has expanded the role of the cutaneous mechanoreceptors for feedback in fine motor control . Single action potentials from RAI and PC afferents are directly linked to activation of related hand muscles, whereas SAI activation does not trigger muscle activity.
The human work stemmed from Vallbo and Johansson's percutaneous recordings from human volunteers in the late 1970s, . Work in rhesus monkeys has found virtually identical mechanoreceptors with the exception of Ruffini corpuscles which are not found in the monkey.
Mechanoreceptors are mainly cutaneous ones, but there are also other types, e.g. hair cells.
Cutaneous mechanoreceptors are located in the skin, like other cutaneous receptors. They are all innervated by Aβ fibers, except the mechanorecepting free nerve endings, which are innervated by Aδ fibers. They can be categorized both by morphology, by what kind of sensation they perceive and by the rate of adaptation. Furthermore, they have different receptive field.
Cutaneous mechanoreceptors provide the senses of touch, pressure, vibration, proprioception and others.
Cutaneous mechanoreceptors can also be separated into categories based on their rates of adaptivity. When a mechanoreceptor receives a stimulus it begins to fire impulses or action potentials at an elevated frequency (the stronger the stimulus the higher the frequency). The cell, however, will soon “adapt” to a constant or static stimulus and the pulses will subside to a normal rate. Receptors that adapt quickly (i.e. quickly return to a normal pulse rate) are referred to as ‘’phasic’’. Those receptors that are slow to return to their normal firing rate are called ‘’tonic’’. Phasic mechanoreceptors are useful in sensing such things as texture, vibrations, etc; whereas tonic receptors are useful for temperature and proprioception among others.
Some free nerve endings are intermediate adapting.
Cutaneous mechanoreceptors with small, accurate receptive fields are found in areas needing accurate taction (e.g. the fingertips). In the fingertips and lips, innervation density of slowly adapting type I and rapidly adapting type I mechanoreceptors are greatly increased. These two types of mechanoreceptors have small discrete receptive fields and are thought to underlie most low threshold use of the fingers in assessing texture, surface slip, and flutter. Mechanoreceptors found in areas of the body with less tactile acuity tend to have larger receptive fields.
Other mechanoreceptors than cutaneous ones include the hair cells, which are sensory receptors in the vestibular system in the inner ear, where they contribute to the auditory system and equilibrioception.
Pacinian corpuscles are pressure receptors. They are located in the skin and also in various internal organs. Each is connected to a sensory neuron. Because of its relatively large size, a single Pacinian corpuscle can be isolated and its properties studied. Mechanical pressure of varying strength and frequency is applied to the corpuscle by the stylus. The electrical activity is detected by electrodes attached to the preparation.
Deforming the corpuscle creates a generator potential in the sensory neuron arising within it. This is a graded response: the greater the deformation, the greater the generator potential. If the generator potential reaches threshold, a volley of action potentials (also called nerve impulses) are triggered at the first node of Ranvier of the sensory neuron.
Once threshold is reached, the magnitude of the stimulus is encoded in the frequency of impulses generated in the neuron. So the more massive or rapid the deformation of a single corpuscle, the higher the frequency of nerve impulses generated in its neuron.
The knee jerk is a stretch reflex. Your physician taps you just below the knee with a rubber-headed hammer. You respond with an involuntary kick of the lower leg. The hammer strikes a tendon that inserts an extensor muscle in the front of the thigh into the lower leg. Tapping the tendon stretches the thigh muscle. This activates stretch receptors within the muscle called muscle spindles. Each muscle spindle consists of sensory nerve endings wrapped around special muscle fibers called spindle fibers (also called intrafusal fibers) Stretching a spindle fiber initiates a volley of impulses in the sensory neuron (a I-a neuron) attached to it. The impulses travel along the sensory axon to the spinal cord where they form several kinds of synapses:
Some of the branches of the I-a axons synapse directly with alpha motor neurons
(1). These carry impulses back to the same muscle causing it to contract. The leg straightens. Some of the branches of the I-a axons synapse with inhibitory interneurons in the spinal cord
(2). These, in turn, synapse with motor neurons leading back to the antagonistic muscle, a flexor in the back of the thigh. By inhibiting the flexor, these interneurons aid contraction of the extensor.
(3). Still other branches of the I-a axons synapse with interneurons leading to brain centers, e.g., the cerebellum, that coordinate body movements.