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About Senses Senses are the physiological capacities within organisms that provide inputs for perception. Human beings have a multitude of senses, the five major are the senses of sight (ophthalmoception), hearing (audioception), taste (gustaoception), smell (olfacoception or olfacception), and touch (tactioception) other senses include temperature (thermoception), kinesthetic sense (proprioception), pain (nociception), balance (equilibrioception) and acceleration (kinesthesioception).
General senses: General senses are those with receptors distributed over a large part of the body. The general senses are widely distributed throughout the body and include the senses of touch, pressure, pain, temperature, vibration, itch, and kinesthetic sense (proprioception). They are divided into 2 groups, the somatic senses and the visceral senses. Many of the general senses are associated with the skin and others are associated with the deeper structures.
Somatic senses Somatic senses provide sensory information about the body and the environment.
Visceral senses Visceral senses provide information about the various internal organs. They consist primarily of pain and pressure.
Receptors These are sensory nerve endings or specialized cells that are capable of responding to stimuli by developing action potentials. These are associated with both special and general senses. The different types of receptor cells are the mechanoreceptors , chemoreceptors, photoreceptors, photoreceptors, thermoreceptors, and nociceptors.
About the receptor cells: Mechanoreceptors- respond to mechanical stimuli such as bending or stretching of the receptors. Chemoreceptors- respond to chemicals such as odor molecules. Photoreceptors- respond to light. Thermoreceptors- respond to changes in temperature. Nociceptors- respond to stimuli that result in the sensation of pain.
Receptors in the Skin Free nerve endings- relatively unspecialized neuronal branches similar to dendrites. Free nerve endings are distributed throughout the body .
There are different type of free nerve endings and each responds to different stimuli such as painful stimuli, itch, temperature and movement.
Hot and Cold receptors Hot receptors respond to increasing temperatures but stop responding at temperatures above 47°C. Cold receptors respond to decreasing temperatures but stop responding at temperatures below 12°C.
Receptors found in the skin Touch receptors- structurally more complex then free nerve endings and many of them are enclosed in capsules
Receptors found in the skin Merkel’s disks- superficial nerve endings in detecting light touch and superficial pressure. Hair follicle receptors- also involved detecting light touch. - Light touch receptors are very sensitive although they are not very discriminative making it difficult to locate the point of touch.
Receptors found in the skin Meissner’s corpuscles- located deep in the epidermis, they are very specific in localizing tactile sensations.
Pacinian corpuscles – the deepest receptors associated with tendons and joints, relays information concerning deep vibration, pressure, and position (proprioception).
Receptors found in the skin Ruffini’s end organs- deeper tactile receptors, plays an important role in detecting continuous pressure in the skin
Pain Pain is a sensation characterized by a group of unpleasant perceptual and emotional experiences. The two type of pain are: Sharp, well localized, pricking or cutting pain resulting from rapidly conducted action potentials. Diffused, burning, or aching pain resulting from action potentials that are propagated more slowly.
Pain Superficial pain- are sensations in the skin that are highly localized as a result of simultaneous stimulation of pain receptors and the tactile receptors. Deep or visceral pain- sensations not highly localized because of the absence of tactile receptors in the deeper structures so it is normally perceived as diffused pain. - Tactile receptors- helps localize the source of pain stimuli.
Suppressing Pain Local anesthesia- suppressing pain by injecting chemical anesthetics near a sensory receptor or nerve. This results in reduced pain sensation. General anesthesia- suppressing pain by producing a loss of consciousness and can be accomplished by chemical anesthetics affecting the reticular formation.
Referred Pain Referred pain is a painful sensation perceived to originate in a region of the body that is not the source of the pain. Referred pain is sensed in the skin or other superficial structures when deeper structures such as internal organs are damaged or inflamed. This occurs because sensory neurons from the superficial area to which pain is referred and the neurons from the deeper visceral area where the pain originates converge onto the same ascending neurons in the spinal chord.
Sense of smell (Olfaction) Occurs in response to airborne molecules called odorants that enter the nasal cavity. Olfactory neurons are bipolar neurons within the olfactory epithelium lining the superior part of the nasal cavity.
The dendrites of the olfactory neurons extend to the epithelial surface of the nasal cavity and their ends are modified to bulbous enlargements that posses long specialized cilia.
Sense of smell (Olfaction) The cilia of the olfactory cells lie in a thin mucus film on the epithelial surface. The mucus plays an important role in olfaction.
Mucus- keeps the nasal epithelium moist, traps and dissolves airborne molecules and facilitates removal of molecules and particles from the nasal epithelium.
The process of Olfaction
Process of Olfaction Airborne odorants come in contact with the mucus on the surface of the epithelium, dissolve and bind to the receptor molecules on the membranes of the specialized. Once the dissolved odorant comes in contact receptors, it causes olfactory neurons to depolarize.
Process of Olfaction The threshold for detection of odors is very low so very few odorants bound to an olfactory neuron can initiate an action potential. Once an odor molecule is bound to the receptor, the receptor does not respond to another odor molecule for some time.
Pathway for Olfaction Odorants come in contact with the cilia and pass through the olfactory receptor cells through the cribriform plate, to olfactory bulbs, then the signal travels through olfactory tracts and finally, it terminates in the olfactory cortex.
Facts about olfaction: It stated once a odorant molecule is bound to a receptor cell, it does no function so prolonged exposure to a given odorant will disable the receptor cells receiving stimuli from the smell resulting in a person adapting to a specific odor. Once a person adapts to an odor he or she no longer smells the odor or the effect of the odor is lessened