Human Sensory Receptors and Organ Anatomy

The Human Sensory System: Receptors and Organs

Understanding Sensory Receptors

Sensory receptors are specialized structures responsible for detecting changes (stimuli) in the world around us and converting them into nerve impulses. These impulses then travel to a higher nerve center where perception begins. Sensory receptors can be simple groups of nerves or, more often, special cells that join together to form the sensory organs.

Classification of Sensory Receptors

Receptors are typically classified based on the type of stimulus they detect:

• Photoreceptors: Detect light stimuli and are found in the eyes.
• Mechanoreceptors: Stimulated by changes in movement, such as pressure, contact, or sound waves. These include receptors in the skin, muscles, and ears.
• Chemoreceptors: Pick up information from chemical changes. These include taste and smell receptors.
• Thermoreceptors: Detect changes in temperature. These include certain skin receptors.
• Nociceptors: These are several kinds of receptors that detect pain. They respond to any stimulus that potentially can produce damage, either by its nature or intensity.

Location-Based Receptor Classification

Receptors can also be classified based on their location:

• Internal Receptors (Interoceptors): They perceive changes in the internal environment and organs. These receptors are scattered throughout the organism.
• External Receptors (Exteroceptors): They capture stimuli originating from the external environment.

Major Sensory Organs

The Eye: Organ of Sight

The eye is a complex structure responsible for sight. It receives light and carries the stimuli to the area where the photoreceptors are located.

The Ear: Hearing and Balance

The ear is the organ of hearing and balance. Each ear is located within a cavity in the temporal bones of the skull.

Outer Ear Structure

The outer ear is formed by the pinna (auricle) and the ear canal. Inside the ear canal, there are glands that produce cerumen (earwax). Cerumen is a sticky substance that prevents the entrance of foreign particles into the ear. The ear canal ends at the tympanic membrane (eardrum).

Middle Ear Structure

The middle ear starts at the eardrum and extends to two small membranes: the oval window and the round window. Connected among themselves, the eardrum, and the oval window are three tiny bones called the ossicles. They are the hammer (malleus), the anvil (incus), and the stirrup (stapes).

Inner Ear Structure

This is the deepest part of the ear. It is primarily made up of labyrinthine membranes. These membranes form two complicated cavities within the temporal bone, one inside the other. There is a fluid called perilymph between the two labyrinth cavities, and within the inner cavities, there is another liquid, the endolymph.

Taste: Detecting Flavors

The sense of taste enables the body to detect flavors. There are four traditionally recognized basic tastes: sweet, salty, sour, and bitter. Nowadays, a fifth taste is also recognized: umami or savory.

Smell: Detecting Chemical Molecules

The sense of smell enables the body to detect chemical molecules in the air. These molecules are detected by chemoreceptors in the nasal cavity. The sensations they receive are called smells. While there are seven basic smells, they can combine, enormously increasing the variety of smells we can distinguish.

The Skin: Sense of Touch

The skin is the organ responsible for most of our sense of touch. There are different types of receptors on the skin that capture various types of stimuli. Their distribution is not uniform; depending on the area, there are more or fewer receptors, and some types are more abundant than others.