General Senses

what's covered

In this lesson, you will learn about the types of sensory modalities and general senses. Specifically, this lesson will cover:

Table of Contents 1. Sensory Modalities 2. Somatosensation (Touch)

1. Sensory Modalities

think about it

Ask anyone what the senses are and they are likely to list the five major senses—taste, smell, touch, hearing, and sight. However, these are not all of the senses. The most obvious omission from this list is balance. Other overlooked senses include temperature perception by thermoreceptors and pain perception by nociceptors.

Within the realm of physiology, senses can be classified as either general or specific:

• A general sense is one that is distributed throughout the body and has receptor cells within the structures of other organs. Mechanoreceptors in the skin, muscles, or the walls of blood vessels are examples of this type. General senses often contribute to the sense of touch, as described above, or to proprioception (body movement) and kinesthesia (body movement), or to a visceral sense, which is most important to autonomic functions.
• A special sense is one that has a specific organ devoted to it, namely the eye, inner ear, tongue, or nose.

Each of the senses is referred to as a sensory modality. Modality refers to the way that information is encoded, which is similar to the idea of transduction. The main sensory modalities can be described on the basis of how each is transduced. The chemical senses are taste and smell. The general sense that is usually referred to as touch includes chemical sensation in the form of nociception, or pain. Pressure, vibration, muscle stretch, and the movement of hair by an external stimulus, are all sensed by mechanoreceptors. Mechanoreceptors also sense hearing and balance. Finally, vision involves the activation of photoreceptors.

Listing all the different sensory modalities, which can number as many as 17, involves separating the five major senses into more specific categories, or submodalities, of the larger sense. An individual sensory modality represents the sensation of a specific type of stimulus.

EXAMPLE

The general sense of touch, which is known as somatosensation, can be separated into light pressure, deep pressure, vibration, itch, pain, temperature, or hair movement.

terms to know

General Sense

A sensory system that is widely distributed throughout the body and in the organs of the body.

Special Sense

A sensory system that has a specific organ devoted to it.

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2. Somatosensation (Touch)

Somatosensation is the group of sensory modalities that are associated with touch, proprioception, and interoception. These modalities are listed below:

• Pressure
• Vibration
• Light touch
• Tickle
• Itch
• Temperature
• Pain
• Proprioception
• Kinesthesia

Somatosensation is considered a general sense, meaning that its receptors are not associated with a specialized organ but are instead spread throughout the body in a variety of organs. Many of the somatosensory receptors are located in the skin, but receptors are also found in muscles, tendons, joint capsules, ligaments, and in the walls of visceral organs.

Two types of somatosensory signals that are transduced by free nerve endings are pain and temperature. These two modalities use thermoreceptors and nociceptors to transduce temperature and pain stimuli, respectively. Temperature receptors are stimulated when local temperatures differ from body temperature. Some thermoreceptors are sensitive to just cold and others to just heat. Nociception is the sensation of potentially damaging stimuli. Mechanical, chemical, or thermal stimuli beyond a set threshold will elicit painful sensations. Stressed or damaged tissues release chemicals that activate receptor proteins in the nociceptors.

EXAMPLE

The sensation of heat associated with spicy foods involves capsaicin, the active molecule in hot peppers.

Capsaicin molecules bind to a transmembrane ion channel in nociceptors that is sensitive to temperatures above 37°C. The dynamics of capsaicin binding with this transmembrane ion channel is unusual in that the molecule remains bound for a long time. Because of this, it will decrease the ability of other stimuli to elicit pain sensations through the activated nociceptor. For this reason, capsaicin can be used as a topical analgesic, such as in products such as Icy Hot™.

The skin (integument) contains a variety of sensory receptors specific to the detection of pressure, vibration, and light touch. For the benefit of review, each of these receptors is summarized again below. If you drag your finger across a textured surface, the skin of your finger will vibrate. Such low-frequency vibrations are sensed by mechanoreceptors called tactile discs, also known as Merkel discs or type I cutaneous mechanoreceptors. Merkel cells are located in the epidermis's stratum basale (deepest layer). Deep pressure and vibration are transduced by lamellar corpuscles, also known as Pacinian corpuscles, which are receptors with encapsulated endings found deep in the dermis, or subcutaneous tissue. A light touch is transduced by the encapsulated endings known as tactile corpuscles, also known as Meissner corpuscles. Follicles are also wrapped in a plexus of nerve endings known as the hair follicle plexus. These nerve endings detect the movement of hair at the surface of the skin, such as when an insect may be walking along the skin. Stretching of the skin is transduced by stretch receptors known as bulbous corpuscles, also known as Ruffini corpuscles or type II cutaneous mechanoreceptors.

Other somatosensory receptors are found in the joints and muscles. Stretch receptors monitor the stretching of tendons, muscles, and the components of joints in order to determine the position of the body in space (proprioception) and the current movement of the body (kinesthesia). These sensory signals help provide information about how the body is currently moving (kinesthesia), where the body is in space (proprioception), and if the muscle or joint is reaching the end of its range of motion, hoping to avoid overstretching it. In skeletal muscle tissue, these stretch receptors are called muscle spindles. Golgi tendon organs similarly transduce the stretch levels of tendons. Bulbous corpuscles are also present in joint capsules, where they measure stretch in the components of the skeletal system within the joint. The types of nerve endings, their locations, and the stimuli they transduce are presented in the table below.

Name	Historical (eponymous) name	Location(s)	Stimuli
Free nerve endings	n/a	Dermis, cornea, tongue, joint capsules, visceral organs	Pain, temperature, mechanical deformation
Tactile discs	Merkel’s discs	Epidermal-dermal junction, mucosal membranes	Low frequency vibration (5–15 Hz)
Bulbous corpuscle	Ruffini’s corpuscle	Dermis, joint capsules	Stretch
Tactile corpuscle	Meissner’s corpuscle	Papillary dermis, especially in the fingertips and lips	Light touch, vibrations below 50 Hz
Lamellar corpuscle	Pacinian corpuscle	Deep dermis, subcutaneous tissue	Deep pressure, high-frequency vibration (around 250 Hz)
Hair follicle plexus	n/a	Wrapped around hair follicles in the dermis	Movement of hair
Muscle spindle	n/a	In line with skeletal muscle fibers	Muscle contraction and stretch
Tendon stretch organ	Golgi tendon organ	In line with tendons	Stretch of tendons

terms to know

Capsaicin

The active molecule in hot peppers which creates the sensation of heat from spicy foods.

Muscle Spindles

A mechanoreceptor in skeletal muscles that detects stretch.

Golgi Tendon Organ

A mechanoreceptor in tendons that detects stretch.