BIOLOGY Volume 2 - A Guide to General Biology - 2004

17. ANIMAL COORDINATION AND REGULATION

17.5. Structure and Function of Receptors

17.5.1. Mechanoreceptors

Mechanoreceptors are considered the most primitive type of receptors; they detect various mechanical stimuli, notably Touch, pressure, stretch, and vibration.

Touch, pressure, and vibration receptors

The distinction between touch and pressure is purely quantitative, and the perception of these signals depends on the Location OF THE receptors within the Skin. Touch receptors are also found in other PARTS OF THE body, giving rise to increased sensitivity in those areas. For example, the tip of the Tongue can distinguish two simultaneously applied stimuli even when the distance between the points of application is only 1 mm, whereas in the mid-back region this is only possible if the distance between them is at least 60 mm.

In the skin, touch is detected by specialized Sensory Organs located directly beneath the epidermis, known as Meissner's corpuscles (Fig. 17.32). They consist of a single coiled nerve ending enclosed in a fluid-filled capsule. Mechanoreceptors of another type—Pacinian corpuscles (Fig. 17.30)—are located deep within the skin, in joints, tendons, Muscles, and the wall of the digestive tract. They consist of a neuron ending surrounded by numerous concentric layers of Connective Tissue. Pacinian corpuscles respond to pressure and vibration. Other mechanoreceptors are also shown in Fig. 17.32.

It is believed that in touch and pressure receptors, the potential is generated As a result of the deformation of The Cell membrane, which leads to an increase in its permeability to ions.

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Fig. 17.32. Five types of skin mechanoreceptors.

Muscle spindles

Proprioceptors are receptors of The Musculoskeletal System that register the position and movement of body parts. They respond to the degree of Muscle contraction and participate in its regulation, functioning primarily as stretch receptors. Specialized proprioceptors called muscle spindles are present in the skeletal musculature of vertebrates (from amphibians to mammals). They perform three main Functions—one static and two dynamic:

1) they transmit information to the CNS regarding the state and position of muscles and associated structures (static function);

2) they induce a reflex contraction of the muscle, restoring its original length when the tensile load increases (dynamic function);

3) they alter muscle tension and "tune" it to maintain a new length (dynamic function).

The Structure and function of muscle spindles are described in detail in Section 18.4.4.