We know about the world through our senses. Sensation-awareness resulting from the stimulation of a sense organ and perception interpretation of sensations work together to allow us to experience the world. Vision, hearing, taste, smell, and touch are the popular human senses. Furthermore, there is a kinesthetic sense that helps figure out the position and movements of one’s skeletal joints and a vestibular sense that helps maintain the balance.
Every sensory system consists of a receptor, nerve fibers leading from receptor to the brain or spinal cord, and the various relay stations and processing areas within the brain. For us to make sense of the world, the physical energy must be changed into activity within the nervous system. The process of converting physical energy into nervous-system activity is called transduction.
Transduction occurs at the receptors cells, that convert physical energy into an electric voltage or potential, called the receptor potential. The receptors’ potential either directly triggers the nerve impulses or leads to further electrical events, which in turn trigger nerve impulses. The electrical event that triggers nerve impulses is known as generator potential.
1. Sensory Processes: Vision
Vision is one of the most important sensations of human beings.
At first, the light enters the eye through the pupil, travels through the corona, the lens, and the interior of the eyeball to strike the rod and cons cells of the retina located at the back of the eyeball.
The corona is a transparent protective structure that helps to gather and focus the light. After moving through the cornea, the light enters the eye through the pupil. The pupil is a round opening whose size varies as per the lighting conditions. The pupil is surrounded by the iris, the colored part of the eye that we refer to when we say someone has black, brown, or blue eyes. Iris adjusts the amount of light that enters by constricting or dilating the pupil. The iris widens the pupil in dim light allowing more light to enter, and in bright light, the iris narrows the pupil.
It is in the retina that the electromagnetic energy of light is converted into electrical impulses for transmission to the brain. The name has been given to describe their shapes. Rods are cylindrical whereas cones are cone-shaped receptor cells. The cones are adapted for color vision, daytime vision, and sharp focus. Similarly, the rods are adapted for vision in dim light.
The rods and cons after being stimulated transmit neural information to bipolar cells. The bipolar cells communicate the information to the ganglion cells. “Axons from the ganglion cells converge to form the optic nerve and carry visual information to the brain. (Baron)” One part of the retina lacks rods and cones and is insensitive to light. This part is known as the blind spot.
2. Sensory Process: Hearing
Hearing results when sound waves collected by the ear are converted into neural impulses through the process of transduction and are sent to the brain. Sound waves are funneled into the auditory canal and produce slight movements in the eardrum. The eardrum is aptly named so because it operates as a miniature eardrum, vibrating when sound waves hit it. The movements in the eardrum, in turn, produce movements in the fluid within the cochlea. As the fluid moves, tiny hair cells shift their position, thus generating the nerve impulses we perceive the sound.
The ear consists of the outer ear, the middle ear, and the inner ear. The outer ear consists of the pinna, the ear canal, and the eardrum. The outer ear collects the sound. The pinna is the visible part of the ear. It catches the sound waves and funnels them into the ear canal. The sound wave travels down the ear canal, then bounces into the eardrum. The vibrations are relayed into the middle ear through three tiny bones- the hammer (or the malleus), the anvil (or the incus), and the stirrup (or the stape). The joint action of these bones doubles the amplification of the sound. The innermost bone, the stirrup transmits the amplified vibration to the oval window.
The oval window separates the middle ear from the inner ear. The vibration of the oval window is relayed to the fluid-filled, spiral-shaped structure called the cochlea. This causes movements of the fluid in the cochlea which in turn bend the hair cells of the inner ear. The movements of the hair cells trigger nerve impulses in the attached neurons, which are then transmitted to the brain via the auditory nerve.
3. Sensory Process: Smell
The sense of smell is known as olfaction. As we breathe, we inhale airborne chemical molecules that are detected by the millions of olfactory receptors located on the mucous membrane. When stimulated, these receptors cells send neural messages to the brain where the sense of smell is registered.
4. Sensory Process: Taste
The sense of taste (Gustation), occurs on the surface of the tongue. Our tongue is covered with a taste bud that detects four basic stimulus qualities: sweet, sour, salty, and bitter. The fifth category of taste also exists, a flavor called umami, although there is controversy about whether it qualifies as a fundamental taste. Loosely translated, umami means yummy or delicious in Japanese. Umami is a taste-containing amino acid or food containing meats, cheeses, soy, and monosodium glutamate (MSG).
The receptors cells for taste are located in roughly 10 thousand taste buds that are distributed across the tongue and other parts of the mouth and throat. Most of the taste buds are located in the top outer edges of the tongue, while there are some receptors at the back of the tongue and the walls of the mouth. The food that we eat dissolves and enters the taste buds, triggering nerve impulses that are transmitted to the brain.
5. Sensory Process: Touch
The skin is the largest sense organ of the human body. Skin senses- pain, touch, pressure, and temperature. It plays a crucial role in our survival as it makes us aware of the potential dangers.
There are different kinds of sensory receptors in the skin. Some of these are specialized to respond to just one type of stimuli, such as pressure, warmth, or cold. As the receptors in the skin are not evenly distributed, certain areas of the skin, like the face and fingertips, are more sensitive than others.