Overview of Sensation

Our senses can be broadly grouped into exteroception, for the detection of stimuli that occur outside of our body, and interoception, for stimuli occurring inside of our bodies. However, what constitutes a sense is a matter of great debate, leading to difficulties in precisely defining what it is. Traditionally, human beings are considered to have five main senses: sight, hearing, taste, smell, and touch.

The peripheral nervous system (PNS) consists of sensory receptors that extend from the central nervous system (CNS) to communicate with other parts of the body. These receptors respond to changes and stimuli in the environment. Sense organs (made up of sensory receptors and other cells) operate the senses of vision, hearing, equilibrium, smell, and taste.

Sight

Sight or vision (ophthalmoception) is the ability of the eye(s) to focus and detect images of visible light on photoreceptors in the retina that generate electrical nerve impulses for varying colors, hues, and brightness. There are two types of photoreceptors: rods and cones. Rods are very sensitive to light, but do not distinguish colors. Cones distinguish colors, but are less sensitive to dim light. The inability to see is called blindness.

Hearing

Hearing or audition (audioception) is the sense of sound perception. Mechanoreceptors in the inner ear turn vibration motion into electrical nerve pulses. The vibrations are mechanically conducted from the eardrum through a series of tiny bones to hair-like fibers in the inner ear that detect the mechanical motion of the fibers. 

Sound can also be detected as vibrations conducted through the body by tactition. The inability to hear is called deafness or hearing impairment.

Taste

Taste (gustaoception) refers to the ability to detect substances such as food, certain minerals, poisons, etc. The sense of taste is often confused with the concept of flavor, which is a combination of taste and smell perception. Flavor depends on odor, texture, and temperature as well as on taste. 

Humans receive tastes through sensory organs called taste buds, or gustatory calyculi, concentrated on the upper surface of the tongue. Five basic tastes exist: sweet, bitter, sour, salty, and umami. The inability to taste is called ageusia.

Smell

The olfactory system is the sensory system used for the sense of smell (olfaction). This sense is mediated by specialized sensory cells of the nasal cavity.  In humans, olfaction occurs when odorant molecules bind to specific sites on the olfactory receptors in the nasal cavity. These receptors are used to detect the presence of smell. They come together at a structure (the glomerulus) that transmits signals to the olfactory bulb in the brain. The inability to smell is called anosmia.

Touch

Touch or somatosensation (tactioception, tactition, or mechanoreception), is a perception resulting from the activation of neural receptors in the skin, including hair follicles, tongue, throat, and mucosa. A variety of pressure receptors respond to variations in pressure (firm, brushing, sustained, etc.).

The touch sense of itching is caused by insect bites or allergies that involve special itch-specific neurons in the skin and spinal cord. The loss or impairment of the ability to feel anything touched is called tactile anesthesia.

Paresthesia is a sensation of tingling, pricking, or numbness of the skin that may result from nerve damage and may be permanent or temporary.

Additional Senses

Many scientists and philosophers argue that humans have additional senses including:

• Pain or nociception (physiological pain): Signals nerve and other tissue damage.
• Balance or equilibrioception: Allows the sensing of body movement, direction, and acceleration, and to attain and maintain postural equilibrium and balance. 
• Body awareness or proprioception: Provides the parietal cortex of the brain with information on the relative positions of the parts of the body.
• Sense of time or chronoception: Refers to how the passage of time is perceived and experienced but is not associated with a specific sensory system. According to psychologists and neuroscientists, however, human brains have a system governing the perception of time.
• Temperature sensing or thermoception: The sensation of heat and the absence of heat (cold).

If interoceptive senses are also considered, sensation can be expanded to include stretch (as in muscles or organs like the lungs), oxygen and carbon dioxide sensing, pH sensing, and more. 

While the exact definition of sensation is still controversial, most scientists agree that all senses rely on four fundamental sensory capacities: 

1. Chemical detection (chemoreception). 
2. Light detection (photoreception).
3. Force detection (mechanoreception).
4. Temperature detection (thermoreception). 

Our nervous system has sensory systems and organs that mediate each sense and these systems rely on chemoreceptors, photoreceptors, mechanoreceptors, or thermoreceptors to detect the state of the internal or external environment.

The five senses

Photographic depiction of the five senses.

These are five closeup, black and white photographs of an ear, eye, tongue, nose, and hand.