What is the Significance of Senses?

The senses connect an organism to the world. A person can know, both consciously and otherwise, what goes on around him and within him through complex systems that send signals through a maze of brain circuits. These begin with the cells that respond to physical stimuli.

It's a dynamic process. The emotions, feelings, moods, memories, and beliefs shape what people see and hear. Thus, the brain is not just a center for receiving sensory signals.

As far back as Aristotle, a schema refers traditionally to the five senses- sight, hearing, smell, taste, and touch. But this is a simplification. For example, the position and parts of our bodies are informed by sensory systems like visceral sensations, temperature, and pain.

Each sensory system is unique, but both function and structure's basic characteristics and similarities are shared among all. They take a long time to develop from the primitive stage throughout childhood and adolescence.

Basic Neural Plan of the Senses

The thalamus is a switching station over the brain stem, and sensory data pass through it. Physical phenomena turn into electrical impulses, which are carried as impulses to the brain in a bundle of nerve fibers. From here, the sensory information engages the brain circuits.

The Five Senses

1. Hearing

The auditory system generates nerve impulses from the mechanical energy sound waves. When sound enters the ear canal, it terminates at the tympanic membrane (eardrum). The vibrations produced are the response to the complex pressure variations. A series of tiny bones, known as ossicles, in the middle ear, amplify and transmit the vibrations to the cochlea of the inner ear. The cochlea is a fluid-filled tube rolled up like a snail shell.

The receptors of the auditory system are the hair cells within the cochlea. Movement of the membrane lining the cochlea stretches the tips of hair cells to allow an influx of K+ and Ca2+ ions that build up to generate electrical signals in the acoustic nerve fibers. The response depends upon the frequency of each hair cell and its position in the cochlea.

From the intermediate brain regions to the primary auditory cortex, the auditory signals are passed. Direct connections facilitate quick reactions to loud sounds. Another pathway permits processing en route-coordinating signals from the two ears.        

2. Taste

The chemical sensory systems'receptors send electrical signals to the brain. The receptors on taste cells are gathered in taste buds bound by molecules. The taste buds are not only on the tongue but also elsewhere in the mouth and throat. The response to different tastes (sweet, sour, bitter, salty, and "umami"-described as "meaty" or "savory") is due to the presence of individual receptors. Sometimes a single taste bud may hold receptors for several tastes.

The binding of the taste receptors to the molecules in the food starts a series of chemical reactions (or, in the case of salty and sour, simply open ion channels) that stimulate the cranial nerves. On the medulla oblongata, the impulses converge in the brainstem and move on to cortical and limbic areas. The taste signals influence digestive processes, emotions, and food decisions. Taste is a complex phenomenon that also includes smell and qualities like texture.

3. Smell

The chemical sensory systems' receptors send electrical signals to the brain. The smell is the most primitive of the senses. It provides an ability to find food and helps sense danger in the surrounding. The way the olfactory system is organized is somewhat primitive. The receptors that are the dendrites of neurons go directly to the brain. The brain can recognize an odor in a span of two to three synapses.

The nasal passage is lined with millions of olfactory receptors, each designed to attach to specific molecules. These molecules activate a molecular cascade that generates action potentials within the associated neurons. Finally, the neurons terminate in the olfactory bulb that transmits the information associated with the odors to the piriform cortex for further processing.

4. Touch

It operates through the peripheral, spinal, and cranial nerves of the somatosensory system. It also includes proprioception (body position and movement), temperature, and pain.

These sense organs for touch are distributed in the skin and all over the body. The majority of the touch receptors are not specialized cells. Instead, they are axon terminals of neurons connected to spinal or cranial nerve tracts.These are encapsulated in non-neural structures.

The touch receptors are sensitive to light touch, vibration, or pressure. The pathway through which their signal passes is from the spinal cord or cranial nerves to the brain stem and then to the somatosensory cortex in the parietal lobe. Thus, the signals are mapped onto a somewhat distorted projection of the body.

The axon terminals of the proprioceptive receptors in muscles and joints are found in structures responsive to stretch or tension, pressure, or movement.

The bare nerve endings for pain and temperature are called the nociceptors. They are composed of fast and slow pathways. These are the pathways through which localized pain caused by mechanical forces or cold and dull, deep pain generated by chemicals (like substance P) are transmitted. The emotional response to the pain is modulated through the anterior cingulate cortex.

5.    Vision

While all the senses are equally important, humans tend to rely mostly on sight because maximum space in the brain is associated withthe processing and storing visual information than all other senses combined.

The light focuses on the retina (the photosensitive area covering three-fourths of the back) from the cornea and lens (the transparent structures at the front of the eye). According to the near and distant visions, the shape of the lens is adjusted.

The sensory work is done by the photoreceptor cells in the retina. For example, rods do not process colors while cone cells respond to red, green, and blue light wavelengths.

When light strikes the surface of a photoreceptor, rhodopsin is activated, and a chemical cascade of signals is initiated. These signals pass through intricate cellular wiring that ultimately converges in the ganglion cells (neurons whose axons form the optic nerve). Finally, the signals get processed, sorted, and condensed here in the retina.

Two other sense systems help in navigating the day-to-day activities:

1. Vestibular system

This system is essential for normal movement and equilibrium. It helps transmit information to the brain about the various motions, head position, and spatial orientation. It also helps in motor functions and maintains body balance. It aids in stabilizing the head and body during movements.

2. Proprioception system

The conscious or unconscious awareness of joint position is described as the proprioception system. It helps to identify the muscles, joints, and limbs located in 3D space and the direction they move respective to the body.

Few examples of the proprioception system are walking or kicking without looking at the feet, balancing on one leg, touching the nose with eyes closed, and the ability to sense the surface on which an individual is standing.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses                                                                                                   

  • Bachelors of Science in Anatomy and Physiology
  • Master of Science in Anatomy and Physiology
  • Bachelor of Medicine and Bachelor of Surgery (MBBS)

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