Sensory Receptors, the Eye, Hearing, Balance and Muscles Explained

Sensory Receptors

Sensory receptors capture information from the environment and transmit it to the nervous system. Receptors are activated when changes occur that correspond to a specific type of stimulus.

• Photoreceptors detect bright light stimuli in the eyes.
• Mechanoreceptors detect mechanical stimuli such as pressure changes, contact, and sound waves.
• Chemoreceptors capture chemical information. Changes in chemical composition, such as smell or taste, activate them.
• Thermoreceptors are stimulated by temperature changes, such as those in the skin.

The Eye

The eyeball is a spherical structure housed in the bony cavity of the orbit. It is composed of the cornea, iris, pupil, lens, retina, choroid, sclera, and optic nerve.

Lens

The lens is a transparent, elastic, biconvex body fixed to the inside of the eyeball by means of tiny muscles.

Retina

The retina is the innermost layer of the eyeball. It contains photoreceptor cells of two classes: rods and cones. Rods are excited by low-intensity light and do not distinguish colors. Cones distinguish contrast and colors and require greater light intensity.

Choroid

The choroid is the middle layer of the eyeball. Its pigment gives the iris its color, which is different for each person. At the center of the iris is a hole called the pupil, whose diameter is variable.

Sclera

The sclera is the tough outer layer of the eyeball. It is white, and its front part is transparent and forms the cornea.

Optic Nerve

The optic nerve is a set of prolongations of nerve cells that communicate with the rods and cones. Its function is to transmit the light excitation to the brain.

Light Regulation and Focus

Light intensity is regulated by the iris. Excessive light can damage photoreceptor cells, and if not properly regulated, vision can be impaired. Focus allows light reflected from an object to form a sharp image on the retina.

Hearing and Balance

Sound waves are produced by vibrations of air molecules. Balance is perceived in the utricle and saccule. Inside these structures are sensory cells with cilia embedded in a gelatinous layer with mineral particles. Movement is detected in the semicircular canals. When we move, these movements also affect the head and neck.

The Skin

The skin contains sensitive receptors that capture a variety of stimuli, such as touch. They report the shape, appearance, and texture of objects. They also detect the pressure that causes intense tactile sensations, heat or high temperatures above normal, cold or low temperatures below normal, and pain.

Taste

Taste receptor cells are grouped into buds called taste buds. The taste sensations produced are sweet, salty, sour, and bitter.

Smell

The sense of smell consists of detecting gaseous molecules that reach the nose. Olfactory cells are located in the upper nasal cavity.

Muscles

Muscles are organized into groups of muscle fibers, forming a set of muscle cells. Muscle contraction consists of thickening and shortening. Muscles can preferentially take the following forms:

• Fusiform or elongated muscles, which are usually involved in movement.
• Flat muscles, which generally cover and protect organs.
• Circular or sphincter muscles, which open and close body openings or cavities. When they contract, they preferentially narrow them.

Muscle Tissue

Muscle cells contain proteins that bind calcium, which provides strength. These cells form tissue that can be of two types: spongy bone tissue and compact bone tissue. Its functions include:

• Anchorage: muscles attach to bones, so they act as passive organs of the locomotor system.
• Support: the skeletal system provides support for the body and forms the body frame.
• Protection: bones protect vital organs such as the brain and the spinal cord.