Understanding Hearing, Touch, and Sensory Receptors
Hearing: How Does it Work?
Sound waves are produced by vibrations of air molecules or any object. When these waves reach the eardrum, it begins to vibrate and transmit the vibrations to the ossicular chain. The greater the intensity of sound, the greater the vibration. The last ossicle (stapes) transfers the vibration to the oval window, in which it rests. This, in turn, causes vibration of the inner ear perilymph, leading to the excitation of certain cells inside the cochlea, called the organ of Corti. This generates a nerve current that travels through a nerve to the brain, where the information is interpreted.
The ears are sensory organs located in cavities of the temporal bones at the temples. These organs capture two different types of stimuli: sounds and changes in body position (balance). The ear has three parts: outer ear, middle ear, and inner ear.
External Ear
Formed by the pinna (ear) and ear canal, which enters the bone. This canal contains wax-producing glands for protection. The canal ends in an elastic membrane, the eardrum.
Middle Ear
It is a bone cavity that extends from the eardrum to a small membrane called the oval window and round window.
It contains three small bones, called ossicles: the hammer (malleus), anvil (incus), and stirrup (stapes). The hammer rests on the eardrum, and the stapes rests in the oval window.
Internal Ear
The deepest part, consisting of a membranous labyrinth, a set of membranes within a complex series of temporal bone cavities that form the bony labyrinth. Between the two labyrinths is a liquid, the perilymph, and inside the membranous labyrinth is another fluid called endolymph.
The membranous labyrinth consists of several parts. One of them, the cochlea, is responsible for detecting sounds, and another, the vestibular apparatus, is responsible for spatial control and balance. The vestibular apparatus consists of three semicircular canals and two vesicles, the utricle and saccule.
The Skin: The Organ of Touch
With the exception of pain receptors, which are simply free nerve endings, the other receptors are located in dermal corpuscles, consisting of a nerve extension covered by protective cells in the form of a cap, i.e., encapsulated nerve endings.
Skin Structures and Their Functions
- Sweat glands: Excrete sweat to help cool the body surface when necessary and to eliminate excretory substances.
- The distribution of dermal corpuscles in the skin is not uniform; some areas are dominated by certain receptors. For example, light touch receptors are most abundant in the fingertips, while pressure receptors are more numerous in the palms of the hands.
- Hair: Covers the entire body surface except the palms, soles of the feet, and mucous membranes. Its main function is protection against the cold.
- Sebaceous glands: Secrete substances that lubricate skin and hair.
Other Structures in the Skin
In addition to sensory receptors, the skin contains other structures that play multiple roles. These include:
- Melanocytes: These cells contain a dark pigment (melanin) that protects the body from harmful solar radiation.
- Adipose tissue: A layer of cells with localized fat in the deepest part of the skin, which serves to absorb shock, insulate the body from low temperatures, and act as an energy reserve.
Sensations of Touch
The skin contains sensory receptors that capture a wide range of stimuli that we perceive as touch.
Touch is not one-way; it allows us to perceive different types of sensations:
- Touch itself, which reports on the shape, appearance, and texture of objects.
- Pressure, caused by intense tactile sensations.
- Heat (temperatures higher than usual).
- Cold (temperatures lower than usual).
- Pain.