THE FIVE SENSES AND THE LIMBIC SYSTEM

The limbic system is composed of a group of brain structures that play a vital role in sensory perception, sensory interpretation and motor function. The amygdala, for example, receives sensory signals from the thalamus and uses the information in the processing of emotions such as fear, anger and pleasure. It also determines what memories are stored and where the memories are stored in the brain. The hippocampus is important in forming new memories and connecting emotions and senses, such as smell and sound, to memories. The hypothalamus helps regulate emotional responses elicited by sensory information through the release of hormonesthat act on the pituitary gland in response to stress. The olfactory cortex receives signals from the olfactory bulb for processing and identifying odors. In all, limbic system structures take information perceived from the five senses, as well as other sensory information (temperature, balance, pain, etc.) to make sense of the world around us.

 

Taste, also known as gustation, is the ability to detect chemicals in food, minerals and dangerous substances such as poisons. This detection is performed by sensory organs on the tongue called taste buds. There are five basic tastes that these organs relay to the brain: sweet, bitter, salty, sour and umami. Receptors for each of our five basic tastes are located in distinct cells and these cells are found in all areas of the tongue. Using these tastes, the body can distinguish harmful substances, usually bitter, from nutritious ones. People often mistake the flavor of food for the taste. The flavor of a particular food is actually a combination of the taste and smell as well as the texture and temperature.

 

The sense of smell, or olfaction, is closely related to the sense of taste. Chemicals from food or floating in the air are sensed by olfactory receptors in the nose. These signals are sent directly to the olfactory bulb in the olfactory cortex of the brain. There are over 300 different receptors that each bind a specific molecule feature. Each odor contains combinations of these features and binds to different receptors with varying strengths. The totality of these signals are what is recognized as a particular smell. Unlike most other receptors, olfactory nerves die and regenerate regularly.

 

Touch or somatosensory perception is perceived by activation in neural receptors in the skin. The main sensation comes from pressure applied to these receptors, called mechanoreceptors. The skin has multiple receptors that sense levels of pressure from gentle brushing to firm as well as the time of application from a brief touch to sustained. There are also receptors for pain, known as nociceptors, and for temperature, called thermoreceptors. Impulses from all three types of receptors travel through the peripheral nervous system to the central nervous system and the brain.

 

Hearing, also called audition, is the perception of sound. Sound is comprised of vibrations that are perceived by organs inside the ear through mechanoreceptors. Sound first travels into the ear canal and vibrates the ear drum. These vibrations are transferred to bones in the middle ear called the hammer, anvil and stirrup which further vibrate the fluid in the inner ear. This fluid-filled structure, known as the cochlea, contains small hair cells that output electrical signals when deformed. The signals travel through the auditory nerve directly to the brain, which interprets these impulses into sound. Humans can normally detect sounds within a range of 20 – 20,000 Hertz. Lower frequencies can be detected solely as vibrations through somatosensory receptors, and frequencies above this range cannot be detected but often can be perceived by animals. The decrease of high frequency hearing often associated with age is known as hearing impairment.

 

Sight, or vision, is the ability of the eyes to perceive images of visible light. Light enters the eye through the pupil and is focused through the lens onto the retina on the back of the eye. Two types of photoreceptors, called cones and rods, detect this light and generate nerve impulses which are sent to the brain via the optic nerve. Rods are sensitive to the brightness of light, while cones detect colors. These receptors vary the duration and intensity of impulses to relate the color, hue and brightness of perceived light. Defects of the photoreceptors can lead to conditions such as color blindness or, in extreme cases, complete blindness.