Free Nerve Endings
A sensory receptor is the first component of a sensory system, which is activated by a specific stimulus from the internal or external environment or an organism. Well-known and well-studied sensory receptors include those that detect pressure (mechanoreceptors in the skin), light (photosensitive receptors in the retina), chemicals (chemoreceptors in the nose and tongue), and temperature (thermoreceptors in the skin). Once the receptor is activated it begins sensory transduction, which is a conversion of a sensory stimulus from one form (e.g., chemical) to another (e.g., electrical). Activated sensory receptors create action potentials or graded potentials in the same neuron or in an adjacent cell. Receptors involved in taste and smell have specific receptor molecules that respond and bind to different chemicals. For example, in the olfactory system, odor receptors found in the olfactory sensory neurons are activated by the interaction of molecular structures on the odor molecule. Free nerve endings, however, are commonly used to detect pain and temperature. This is because these sensations need to be sent to the brain quickly so that a reaction can be processed.
Anatomy and Physiology
A free nerve ending is an unspecialized afferent neuron and is used by vertebrates to detect pain or temperatures of the external and internal environments. They have no complex sensory structures and are the most common type of nerve ending found mostly in the skin. Free nerve endings resemble the small roots of plants. They go through the epidermis and end in the stratum granulosum, which is a thin layer of cells in the epidermis. Free nerve endings are classified based on their rate of adaptation, stimulus modalities, and their fiber types.
In the sensation of taste, free nerve endings are used to sense the temperature of the tongue and the food placed in the mouth. These free nerve endings also sense pain that is caused by spicy foods, particularly capsaicin. Capsaicin comes from plants and vegetables that are in the genus Capsicum. These plants contain molecules that are collectively called capsaicinoids, which are the molecules responsible for hot (not temperature) and spicy flavors. The only plant in the Capsicum genus that does not contain capsaicinoids is Capsicum annum, which is also known as the bell pepper. Scientists have been researching capsaicin activation in taste receptors and discovered the transient receptor potential (TRP) vanilloid subfamily member 1 (TRPV1). It is this receptor that is activated by capsaicin and is part of the superfamily of TRP receptors, which sense stimuli from external events.
Foods and beverages provide calories and nutrients that are essential for humans to survive. There are five basic tastes—bitter, salty, sour, sweet, and umami—that help humans find the foods that their bodies need, particularly those required for nutrition. In general, the bitter sensation detects possibly poisonous foods. Most people will spit out bitter foods, which is a survival mechanism. Salty sensation is important to control salt (NaCl or KCl) balance within the body. Sodium (Na+), potassium (K+), and chloride (Cl−) are necessary chemicals for most cellular functions, such as propagating action potentials in neurons. Similar to salty sensation, sour detection helps maintain acid balance of the body. The sweet sensation helps a person find calorie-rich foods. These are needed when energy is depleted and needs to be replenished quickly. Finally, umami—which is a Japanese word for “savory”—detects protein-rich foods. These are needed to keep muscle health as well as the desire to eat essential amino acids that the human body cannot make.
Spicy and minty foods are often mistaken for additional taste sensations. These, however, are sensations that stimulate pain or temperature receptors on free nerve endings within the mouth. For pain sensations, these nerve endings are stimulated by capsaicin found in spicy food, while menthol—found in mint oils—activates the transient receptor potential cation channel subfamily M member 8 (TRPM8) for temperature sensation.
1—2 mint candies (like Tic-Tac®, Lifesavers®, or Altoids®)
1 small paper cup
2 to 4 tablespoons of soda (such as a cola type)
Pour the soda into the paper cup. As a control for this experiment, take a sip of the soda to determine its temperature. Next, for about 1 to 2 minutes, chew/dissolve, swish, and coat your mouth with the mint. You want to ensure that the spearmint oils are bound to your free nerve endings, which are located on the tongue, cheeks, and the roof of the mouth. For best results, ensure you take your time with this step. Swallow the chewed candy with your saliva.
Next, purse your lips and suck in. Your mouth should instantly feel very cool or cold. This is because the spearmint oils are stimulating the free nerve endings even though the temperature in your mouth did not significantly drop. Next, drink some of the soda. The spearmint should make the soda feel much colder than it did during the control taste.
Jennifer L. Hellier
The Scoville scale measures the “heat” of capsaicin, which has a range of 0 (bell peppers) to 15 million Scoville units (pure capsaicin). In the United States, pepper sprays have a range of 2 to 5.5 million Scoville units. Capsaicin is not water soluble, thus drinking water after eating a hot pepper will not alleviate the pain. Capsaicin is lipid soluble and can be unbound from the TRPV1 receptor by dairy-based products like milk, sour cream, yogurt, or ice cream. This is why many Mexican foods are served with sour cream.
Types of Free Nerve Endings
Free nerve endings can be rapidly adapting, intermediately adapting, or slowly adapting. Rapidly adapting nerve endings react and adapt quickly to a stimulus. This means after a relatively short amount of time, a person will no longer feel the stimulus. These are generally called Meissner’s corpuscles. There is no response to continued pressure, unless there is a change in pressure. Conversely, slowly adapting nerve endings react and adapt slowly to a stimulus. Thus, it will take longer for the feeling to go away. These receptor types are also called Ruffini corpuscles. There will continue to be a response to pressure for as long as it is sustained. Intermediate adapting free nerve endings fall somewhere in between.
Free nerve endings are also classified based on the type of modality they can detect. Types of modalities include but are not limited to temperature (thermoreceptors), mechanical stimuli (mechanoreceptors), and pain (nociceptors). Because of this, free nerve endings have polymodality, meaning they can respond to different types of stimuli.
The final classification of free nerve endings is based on their fiber types. The majority of free nerve endings contain (1) A delta fibers, also known as group III fibers, and (2) C-fibers, also known as group IV fibers. A delta fibers are sensory nerve fibers that carry cold, pressure, and some pain signals. They are myelinated and therefore carry signals much faster than C-fibers. C-fibers are unmyelinated with a small diameter and slow conduction velocity. These fibers include but are not limited to postganglionic fibers in the autonomic nervous system and nerve roots in the dorsal root ganglia. They are also sensory nerve fibers that respond mostly to pain.
See also: Taste System; Thermoreceptors
O’Neill, Jessica, Christina Brock, Anne Estrup Olesen, Trine Andresen, Matias Nilsson, & Anthony H. Dickenson. (2012). Unravelling the mystery of capsaicin: A tool to understand and treat pain. Pharmacological Reviews, 64(4), 939—971.
Schepers, Raf J., & Matthias Ringkamp. (2010). Thermoreceptors and thermosensitive afferents. Neuroscience and Biobehavioral Reviews, 34, 177—184.