Sensory receptors are cells or ends of neurons that detect specific changes in the surrounding environment. They are part of the nervous system. Neurons are nerve cells that transmit sensory impulses to the central nervous system, which is the brain and spinal cord. Certain changes stimulate each kind of receptor and sensory impulses are sent to the central nervous system for processing.
The cerebral cortex is the region of the brain that receives sensory impulses. The region of the cerebral cortex to which the impulse is sent dictates the type of sensation experienced. The strength of this electrical impulse dictates the extent to which it is sensed. The cerebral cortex interprets the origin of stimulation for a particular impulse so a person perceives a sensation from where it is coming. This is called projection.
Decreased Perception – Adaptation of Sensory Receptors
Sensory adaptation occurs when nerve receptors are continually stimulated. Eventually, the receptors requires greater stimulation in order to transmit impulses along nerve pathways. This is why one may smell an odor really well when first exposed to a scent, but may become used to it and not notice it as time goes on. Removing or replacing the scent will allow the sensory cell to return to its original sensitivity to that scent.
Types of Sensory Receptors
Sensory receptors can be classified into five categories based on how they are stimulated.
- Chemoreceptors are stimulated by chemical changes. Receptors on the tongue and in the nose are chemoreceptors. Chemoreceptors within organs detect chemical changes in the blood. These changes include blood oxygen levels, glucose levels, and hormone levels.
- Tissue damage is sensed when pain receptors are stimulated by excessive chemical, electrical, heat, or mechanical energy.
- Temperature change stimulates thermoreceptors.
- Mechanical forces are detected by various sensory receptors. Mechanoreceptors detect movement when they are misshapen. Proprioceptors detect changes in muscle and tendon tension and detect body position. Baroreceptors detect changes in pressure within some blood vessels. Stretch receptors detect changes in the lungs during respiration and sense the degree of inspiration.
- Light energy stimulates photoreceptors. Rods and cones are the photoreceptors in the eye.
Special senses are the senses of hearing, smell, taste, and vision. They are the result of specialized organs within the head. The senses of hearing and equilibrium come from the ears. The sense of smell is attributed to olfactory organs in the nose. The taste buds on the tongue allow people to taste their food. The combination of smell and taste gives food its flavor. The eyes give sight.
The five categories of sensory receptors contain many kinds of receptors that are specialized according to their location in the body and their role in sensory perception. These various receptors allow people to go through their daily activities without giving much thought to how to carry out these activities. Not only do sensory receptors allow people to see, taste, and feel, but they communicate with the brain so it can subconsciously make adjustments to hormone, sugar, and acid levels as well as make small adjustments to our muscles to allow us to do something as simple as balancing on two feet.
Reference
Shier, D., Butler, J., Lewis, Ricki. Hole's Human Anatomy & Physiology. New York: McGraw-Hill, 2002.
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