Mechanoreceptors

Adaptation

Adaptation of mechanoreceptors refers to the ability of these receptors to adjust their response to a sustained stimulus over time. Adaptation in mechanoreceptors involves both mechanical and electrochemical mechanisms. 

Mechanical components, like the capsule, act as filters, selectively allowing dynamic components of mechanical stimuli to reach the nerve endings while reducing static components. When the capsule is removed, as proven in dissection experiments, the nerve ending's response to sustained stimuli is notably prolonged. 

Moreover, electrochemical processes related to action potential encoding play a major role in producing adaptation. Depending on the duration, there are two main types of adaptation: rapid and slow.

Tactile receptors (Cutaneous mechanoreceptors)

Tactile receptors are present in both the superficial and deeper layers of skin until near the bone. These receptors are low-threshold mechanoreceptors innervated by relatively large myelinated axons (type Aβ), ensuring the rapid central transmission of tactile information.

There are five major categories of tactile mechanoreceptors: epithelial tactile complexes (Merkel cell-neurite complexes), tactile corpuscles (Meissner corpuscles), bulbous corpuscles (Ruffini endings), lamellar corpuscles (Pacinian corpuscles) and hair follicle receptors.

Proprioceptors

Proprioceptors are mechanoreceptors responsible for continuously providing information about the position, movement, and spatial orientation of our muscles, tendons, and ligaments within a three-dimensional space. This group of mechanoreceptors includes muscle spindles, Golgi tendon organs, and joint receptors.

• Muscle spindles are located in the belly of skeletal muscles and sense the stretching or contraction of the muscle fibers.
• Golgi tendon organs are located at the myotendinous junctions, situated between the extrafusal skeletal muscle fibers and their associated tendons. They sense changes in muscle tension within the tendon caused by muscle contraction.
• Joint receptors sense limb position and joint movement. The function of these joint receptors is not yet well understood.

Internal ear hair cells

These mechanoreceptors are specialized sensory cells responsible for detecting mechanical stimuli, such as sound vibrations and changes in head position and movement, playing critical roles in both hearing and balance. The internal ear contains two main types of mechanoreceptors: cochlear hair cells and vestibular hair cells.

Baroreceptors

Baroreceptors are mechanoreceptors relaying information about blood pressure within the autonomic nervous system (ANS). They are mainly located at the aortic and carotid sinuses. When blood pressure is low, baroreceptors are inactive. But, when blood pressure increases, the aortic and carotid sinuses stretch, leading to the activation of baroreceptors. The frequency of baroreceptor action potentials is equivalent to the measure of blood pressure. The action potentials travel to the nucleus of solitary tract within the medulla oblongata. 

Increased activation of the nucleus of solitary tract leads to inhibition of the vasomotor center and stimulation of the vagal nuclei. This process inhibits the sympathetic system and activates the parasympathetic nervous system.

On the other hand, low blood pressure leads to a reduction in stimulation of the nucleus of solitary tract, causing an increase in sympathetic stimulation. This is the baroreceptor reflex which causes increased cardiac output and vasoconstriction.

Low and high threshold mechanoreceptors

Depending on the range of mechanical stimuli detection, tactile (cutaneous) mechanoreceptors are divided into two groups:

• Low-threshold mechanoreceptors react to light, benign pressure. All the above-described tactile receptors fall into this category and are associated with large myelinated Aβ nerves. They are further classified into two groups based on their adaptation to sustained stimulus: phasic and tonic receptors.
• High-threshold mechanoreceptors are part of the nociceptors and react to stronger, harmful mechanical pressure. They are free nerve endings that extensively innervate the epidermis. Free nerve endings are unspecialized and can function as nocireceptors (respond to pain), mechanoreceptors (respond to displacement), or thermoreceptors (respond to temperature). Nociceptors, for example, have thin myelinated (Aδ) and unmyelinated (C) nerve fibers that mediate mechanical pain.