JL

Psych 5.5 The Other Senses

Introduction to Sensory Systems

Learning Objectives

By the end of this section, you will be able to:

  • Describe the basic functions of the chemical senses (taste and smell).

  • Explain the basic functions of the somatosensory, nociceptive, and thermoceptive sensory systems.

  • Describe the basic functions of the vestibular, proprioceptive, and kinesthetic sensory systems.

While vision and hearing are extensively studied, we possess other crucial sensory modalities including chemical senses and body senses, which are explored in this section.

The Chemical Senses (Taste and Smell)

Both taste (gustation) and smell (olfaction) are categorized as chemical senses because their sensory receptors are designed to respond to molecules present in food or air.

Interaction of Chemical Senses

  • There is a significant interaction between taste and smell.

  • The perception of a food's "flavor" is a combined experience of its gustatory and olfactory properties.

Taste (Gustation)

  • Basic Taste Groupings: Historically taught as four (sweet, salty, sour, bitter), current research indicates at least six distinct taste groupings.

    • Umami: The fifth taste, a Japanese word translating to "yummy," associated with the taste of monosodium glutamate (Kinnamon & Vandenbeuch, 2009).

    • Fatty Content: Growing experimental evidence suggests a distinct taste for the fatty content of food (Mizushige, Inoue, & Fushiki, 2007).

  • Mechanism of Taste:

    • Molecules from food and beverages dissolve in saliva.

    • These dissolved molecules interact with taste receptors located on the tongue, mouth, and throat.

  • Taste Buds:

    • Formed by groupings of individual taste receptor cells.

    • Possess hair-like extensions that project into a central pore of the taste bud.

    • Have a lifecycle of approximately 10 days to two weeks, ensuring regeneration even after damage (e.g., burning the tongue).

  • Neural Transmission:

    • Taste molecules bind to receptors on the hair-like extensions.

    • This binding triggers chemical changes within the sensory cell, leading to neural impulses.

    • These impulses are transmitted to the brain via different nerves depending on the receptor's location.

    • Taste information travels to the medulla, thalamus, and limbic system.

    • Finally, it reaches the gustatory cortex, which is tucked underneath the overlap of the frontal and temporal lobes (Maffei, Haley, & Fontanini, 2012; Roper, 2013).

Smell (Olfaction)

  • Olfactory Receptor Cells: Located in a mucous membrane at the top of the nose.

  • Mechanism of Smell:

    • Small hair-like extensions from these receptors serve as interaction sites for odor molecules.

    • Odor molecules dissolve in the mucus and bind to chemical receptors on these extensions.

  • Neural Transmission:

    • Once an odor molecule binds to a receptor, chemical changes within the cell send signals to the olfactory bulb.

    • The olfactory bulb is a bulb-like structure at the tip of the frontal lobe where olfactory nerves originate.

    • From the olfactory bulb, information is relayed to regions of the limbic system and the primary olfactory cortex, which is situated in close proximity to the gustatory cortex (Lodovichi & Belluscio, 2012; Spors et al., 2013).

  • Species Variation in Olfactory Sensitivity:

    • Dogs possess significantly superior olfactory systems compared to humans, capable of detecting dangerous drops in blood glucose levels and cancerous tumors (Wells, 2010).

    • This enhanced ability is attributed to a much greater number of functional genes for olfactory receptors in dogs (between 800 and 1200) compared to humans and other primates (fewer than 400) (Niimura & Nei, 2007).

  • Pheromones:

    • Chemical messages sent by one individual to another, to which many species respond (Wysocki & Preti, 2004).

    • Often convey information about the reproductive status of a potential mate (e.g., female rats using pheromonal signals to attract males for sexual behavior) (Furlow, 1996, 2012; Purvis & Haynes, 1972; Sachs, 1997).

    • Research and controversy exist regarding the presence and influence of pheromones in humans (Comfort, 1971; Russell, 1976; Wolfgang-Kimball, 1992; Weller, 1998).

Somatosensory, Nociceptive, and Thermoceptive Sensory Systems

These systems involve a network of receptors distributed throughout the skin and body to perceive touch, temperature, and pain.

Touch (Somatosensation)

  • Skin Receptors: The skin contains various specialized receptors attuned to different touch-related stimuli (Abraira & Ginty, 2013):

    • Meissner’s corpuscles: Respond to pressure and lower frequency vibrations.

    • Pacinian corpuscles: Detect transient pressure and higher frequency vibrations.

    • Merkel’s disks: Respond to light pressure.

    • Ruffini corpuscles: Detect stretch.

  • Free Nerve Endings: In addition to specialized corpuscles, free nerve endings in the skin serve sensory functions, responding to a variety of touch stimuli, as well as temperature (thermoception) and potential harm/pain (nociception) (Garland, 2012; Petho & Reeh, 2012; Spray, 1986).

  • Neural Pathway for Somatosensory Information: Sensory data from receptors and free nerve endings travels up the spinal cord and is transmitted to the medulla, thalamus, and ultimately to the somatosensory cortex, located in the postcentral gyrus of the parietal lobe.

Temperature Perception (Thermoception)

  • Perception of temperature is primarily mediated by free nerve endings.

Pain Perception (Nociception)

  • Nature of Pain: An unpleasant experience combining both physical and psychological components.

  • Adaptive Function: Pain is highly adaptive as it:

    • Alerts us to injury.

    • Motivates us to withdraw from the source of injury.

    • Reduces the likelihood of further injury by encouraging carefulness with injured body parts.

  • Types of Pain:

    • Inflammatory Pain: Signals tissue damage.

    • Neuropathic Pain: Results from damage to neurons in either the peripheral or central nervous system, leading to exaggerated pain signals sent to the brain.

  • Treatment Options: Diverse approaches ranging from relaxation therapy and analgesic medications to deep brain stimulation, with the most effective option depending on severity, persistence, and individual medical/psychological conditions.

  • Congenital Insensitivity to Pain (Congenital Analgesia):

    • A very rare genetic disorder where individuals are born without the ability to feel pain.

    • While they can detect temperature and pressure differences, they cannot experience pain.

    • Consequences: Suffer significant injuries (e.g., young children with serious mouth and tongue injuries from repeated biting), leading to much shorter life expectancies due to injuries and secondary infections (U.S. National Library of Medicine, 2013).

Vestibular Sense, Proprioception, and Kinesthesia

These body senses contribute to our spatial awareness, balance, and body movement.

Vestibular Sense

  • Function: Crucial for maintaining balance and body posture.

  • Major Sensory Organs: Located adjacent to the cochlea in the inner ear, these fluid-filled organs include:

    • The utricle.

    • The saccule.

    • The three semicircular canals (posterior, superior, and horizontal).

  • Mechanism: These organs contain hair cells, similar to those in the auditory system, which respond to movements of the head and gravitational forces.

  • Neural Transmission: Stimulated hair cells send signals to the brain via the vestibular nerve.

  • Conscious Awareness: While not typically consciously aware of its information under normal circumstances, its importance becomes evident during experiences like motion sickness or dizziness caused by inner ear infections (Khan & Chang, 2013).

  • Role in Movement Control: Beyond balance, the vestibular system collects vital information for controlling movement and initiating reflexes that adjust body parts to compensate for changes in body position.

Proprioception (Perception of Body Position)

  • Definition: The perception of the position of our body parts; our sense of where our body is in space.

  • Interaction: Works in conjunction with the vestibular system.

  • Information Source: Gathers information from receptors that respond to stretch and tension in muscles, joints, skin, and tendons (Lackner & DiZio, 2005; Proske, 2006; Proske & Gandevia, 2012).

Kinesthesia (Perception of Body Movement)

  • Definition: The perception of the body's movement through space; our sense of movement.

  • Interaction: Interacts with information provided by the vestibular system.

  • Information Source: Similar to proprioception, it gathers information from receptors responsive to stretch and tension in muscles, joints, skin, and tendons (Lackner & DiZio, 2005; Proske, 2006; Proske & Gandevia, 2012).

Neural Pathway for Proprioceptive and Kinesthetic Information

  • Information travels to the brain via the spinal column.

  • The cerebellum, along with several cortical regions, receives and exchanges information with the sensory organs of the proprioceptive and kinesthetic systems.