What Are Mechanoreceptors?

Mechanical stimulation receptors (mechanoreceptors) are located on the skin, visceral walls, tendons, joint capsules, and mesentery roots, etc., and can sense pressure, touch, stretch and vibration.

Mechanical stimulation receptor

From

Including: baroreceptors, tactile sensors, stretch sensors, and vestibular sensors

Baroreceptor

The toroidal body is an oval pressure-sensing body, also known as the Parsini body. It is 3 to 4 mm long and consists of a long central part surrounded by multiple layers of collagen fibers. There is a thin layer of space between each layer, which is filled with gelatinous fluid. The body is dominated by a thick myelinated nerve terminal. After entering the body, the myelin is removed. When the body is squeezed, excitement occurs, causing afferent nerve impulses. The frequency of impulses can increase with increasing pressure. Baroreceptors are widely distributed in all parts of the body, such as the dermis and subcutaneous tissue of the skin of the fingers, palms, toes, and plantar skin, external genitalia, lips, mesentery roots, peritoneum, pleural wall, periosteum, and joint capsule cavity. Synovial membranes are found everywhere. The joint capsule baroreceptors can receive high-frequency stimulation (above 100 Hz) and belong to oscillatory receptors. A baroreceptor is a sensor that is prone to adaptation.

Tactile sensor

There are three types of tactile sensors :

Figure 2 Merkel Small Cap

Meckel's Small Cap (Small Cap):

A single cell receptor. It consists of Meckel's cells and myelinated nerve fiber endings. It is small, located in the epidermis, and only a small part of the process enters the dermal papilla. Such small discs on the surface of human skin are not easy to discern, and the small discs of cat skin are very shallow and can be seen on the surface of the skin. Meckel's cells are a variant of epidermal cells. In the skin of humans and American kangaroos, the cells have a large cell body, a leafy nucleus, a rich glycoprotein in the cytoplasm, a deep stain, and a strong metabolism. activity. These small disks are distributed unevenly in groups on the skin of the fingers, palms, forearm palms, lips, and face. Hairy skin is less abundant, and this type of sensor is a slow-adapting type, which is mainly sensitive to slight tactile stimuli (Figure 2).

Meissner's body (referred to as the body):

Long mulberry-shaped, about 90 to 120 microns in length, is located in the papillary layer of the dermis. The long axis is perpendicular to the skin surface. The body is covered with a layer of connective tissue envelope. Divided into several small units. The sensory cells are long wedge-shaped, and they overlap each other, a total of 5-10. The nerve endings are stubby and distributed between cells. There are about 2 to 4 afferent nerve fibers, all of which are myelinated fibers and enter the body from the bottom of the body. Such bodies are denser in the palms of the fingers and in the crusts of the toes. Lips, face, and eyelids are also more intradermal. Such bodies are also found in the skin on the side of the palm of the forearm, and in the tongue and mucous membranes. Its role is to feel the light pressure stimulation of the skin, and can distinguish the distance between the two contacts.

Two special peripheral structures can also be seen in the primate's skin, one called Ruffini's body . It has a long spindle shape with a loose capsule and is located in the dermis (Figure 1). It belongs to a slow-adaptive sensor and has obvious response to skin irritation. The second is Krause's end ball (Figure 1). It has a layer of membrane, and the inner part is composed of nerve fiber spheres formed by multiple thin nerve fibers. This terminal sphere is more common in the mucous membranes of the lips and tongue, and the eye-binding membrane. In the past, it was also considered a cold sensor, and it has not been confirmed yet. Some people think that this is the Meissner body in bare hairless skin.

(3) Nerve endings that feel touch stimulation:

There are two types: free nerve endings whose branches are perpendicular to the surface of the skin and are sensitive to tactile and pressure stimuli; tactile bodies are not found in the skin of the auricle, only such nerve endings; The nerve endings are not free, but first enter the hair follicle from the skin, and then divide into longer thin nerve fibers and wind around the root of the hair. When the skin touches the object or the wind blows the hair and causes it to fall to one side, it can excite nerve endings and generate action potentials.

Mechanical stimuli

include

The stretch receptors include: Golgi's tendon organs, muscle spindle receptors, stretch receptors in the visceral wall and large blood vessel walls.

Patellar tendon organs:

The tendon organ is also called Golgi's tendon organ. It is located at the junction of skeletal muscle fibers and tendons and has a spindle shape. The long axis is parallel to the fibers of tendons. There are also scattered stretch receptors in the fibrous septum of the muscular abdomen. The entire susceptor is shuttle-shaped, with a dense coating on the outer bread, which separates the internal fluid of the susceptor from the outside of the membrane. The muscle fibers pass through one end of the membrane. There is a collar-shaped sleeve at the opening of the membrane sleeve. There is a small tubular structure on the side near the middle, which is the channel through which nerve fibers penetrate the receptor. Inside the receptor, the nerve ending fibers and the collagen fiber bundles forming the tendon are twisted to form a rope. The susceptor itself is divided into several longitudinal compartments by structural tissue, and the compartments are filled with septal cells. This sensor has a high threshold for stretch stimulation, so it is not a sensitive stretch sensor, but a kind of motion sensor. When the muscle is actively contracting, its impulsive frequency is significantly increased, and it can be transmitted to the sensory area of the cerebral cortex. And sports area.

Diaphragmatic shuttle:

Figure 3 Structure of muscle spindle

Muscle spindle receptors are distributed in the abdominal muscles and are sensitive to stretch. Each muscle spindle consists of 2 to 10 thin and short specialized muscle fibers, which are called intrafusal fibers. Each muscle spindle has a connective tissue capsule outside, and the mid-swelling of each fiber within the shuttle has a large number of cell nuclei and more sarcoplasm, but this part has a weak ability to contract. At both ends of the fiber in the shuttle, the horizontal lines are still obvious and have a contraction function. The two ends of the muscle spindle are connected to the fibrous septum of the tendon or the abdomen by connective tissue fibers. When both ends of the intrafusalis muscle contract, the middle part of the intrafusal muscle fibers can be stretched to the two ends to tighten the cyst. There are several thick and myelinated nerve endings in the enlarged part of the muscle spindle, called spiral nerve endings, and the two synthesize stretch receptors. When the muscle abdomen is stretched, the tension in the middle of the muscle spindle is significantly increased, which can excite the nerve endings, leading to an increase in the frequency of afferent impulses. When the muscle abdomen is shortened, the middle of the muscle spindle relaxes, the afferent impulses of nerve endings decrease, and the excitability decreases. . There is no consensus on whether the afferent impulses sent by the muscle spindle can be uploaded to the cerebral cortex. Some people think that it can only reach the midbrain level, and its role is to regulate muscle tension. In the stretch reflex, the muscle spindle is a sensor, the muscle in which it is located is an effector, and the reflex center is in the corresponding spinal cord segment.

lung distractor:

It is in the alveolar wall and small bronchial wall that there are nerve terminal devices that are sensitive to the pull stimulus. The afferent nerve is the afferent fiber of the vagus nerve and the afferent nucleus of the bulbar medulla. When the inhalation reaches a certain level, the alveoli expand, causing this receptor to be stimulated, leading to an increase in afferent impulses, inhibiting the inhalation center (including some inspiratory neurons in the medulla), and converting the inspiratory action into Exhale.

Cardiac-pressure (or stretch) sensor in the vessel wall:

It is mainly located in the atrial wall and aortic wall. When the blood pressure in the atrium and aorta increases, the sensor can be passively stretched to give out impulses, which can inhibit the constrictive neuron group that maintains the brain stem. Among them, the carotid sinus and aortic arch baroreceptors are the main receptors of the cardio-vascular regulatory reflex activity, and are most sensitive to changes in blood pressure. Other blood vessels and even tissues have such sensors, but the sensitivity is low.

Vestibular sensor

Vestibular receptor

Figure 4 Two different types of hair cell models

Vestibular organs) are part of the labyrinth of the inner ear; they consist of three semicircular canals, ellipsoids, and balloons that are filled with fluid and interconnected. Vestibular organs have special functioning receptors, hair cells, which can provide the central nervous system with information about head movements to facilitate the body's orientation and maintain the body's balance. The sense of balance depends on the activities of other sense organs, such as the visual organs, proprioceptors, and skin receptors, in addition to the balance organs.

Only vertebrates have true vestibular organs. However, specialized balanced organs begin with invertebrate coelenterates; for example, many balancers on the edge of the jellyfish umbrella cover have similar functions to the vestibular organs, and their structure is basically similar to that of the vestibular organs: The cell circle hugs a central otolith. In the resting state, due to the attraction of the earth's center, or when the body moves, the relative movement of the otoliths and ciliated cells can stimulate the ciliated sensory cells, excite the neural network in the body, and cause the jellyfish to regulate the position and Maintaining a balanced exercise directly helps the animal to swim in the water. Such balance organs appear in different forms in other higher invertebrates, such as arthropods, arthropods, and mollusks. The gravity-sensing organs in decapoda squid are more complex, similar to vertebrates.

Vertebrate vestibular organs have more complex structures and functions. Generally related to the auditory organ. The development of hearing is generally later than the sense of balance. In lower vertebrates, some balance organs also have auditory functions. In higher vertebrates, the balance organs are further specialized, such as fish's lateral line organs and other animals' listening pots and other special structures.

Vestibular organs in higher animals include an oval capsule, a balloon, and three semicircular canals. The semicircular canal measures rotational acceleration, while ellipsoids and balloons can sense linear acceleration including gravity (gravity attraction). Due to the fine structure and its unique anatomical shape, these vestibular devices can accurately determine the spatial position and movement direction of the head at any time. When the head moves, the vestibular organs are directly stimulated by changes in rotation and linear acceleration. Force is proportional to acceleration. However, the force of the vestibular organ is fixed under the condition of normal heart suction. In the resting state, gravity exerts gravitational force on the head in the form of linear acceleration. When the head or the body moves, the accompanying linear and rotational acceleration can also stimulate the vestibular organs. Because the central nervous system first receives signals in the form of acceleration, it must perform a mathematically equivalent operation to calculate the current speed and position of the head. In short, after the vestibular organs convert the head acceleration or gravity into biological information, the central nervous system can provide the body with subjective sensations about head movement and the relative position of the head with its surrounding environment and space, and cause appropriate Reflection action. Vestibular organs are therefore considered to be the main organs for determining body balance and orientation.