Signaling by Sensory Receptors
Humans have 5 senses: touch, taste, smell, sight, and hearing. The senses are Receptors respond to stimuli and send nerve impulses along sensory neurons. In physiology, a stimulus (plural stimuli) is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity. When a stimulus is applied to a sensory receptor, it normally elicits or . Hair cells in these parts of the ear protrude kinocilia and stereocilia into a. The human sensory system is highly evolved and processes thousands of inco. of sensory neurons with receptors for the special senses (vision, hearing, smell, taste Proprioceptors respond to stimuli occurring in skeletal muscles, tendons, .
The properties of these systems dictate the types of stimuli that can be detected and constrain the ways in which these stimuli are reconstructed, integrated, and interpreted. Here we discuss how sensory signals are received and transduced, focusing on the first steps in the complex process of perceiving an external stimulus. A recurrent theme is the way in which the biochemical and biophysical properties of sensory receptor molecules, and the neurons in which they reside, have been sculpted by evolution to capture those signals that are most salient for the survival and reproduction of the organism.
As a result, some classes of sensory receptors, such as the night vision receptor rhodopsin, show great conservation, whereas others, such as olfactory receptors, show great diversity. Evolutionary comparisons are fascinating at many levels, not least of which is their power to highlight the logic of the stimulus—response relationship.
For example, honeybees can see UV light, enabling them to locate sources of nectar and pollen based on the UV reflectance of flower petals Kevan et al. Star-nosed moles use a specialized mechanoreceptive organ on their snout to locate meals and navigate through lightless subterranean tunnels Cataniaand pit vipers have evolved thermoreceptive organs to detect the infrared radiation emitted by their warm-blooded prey Campbell et al.
For simplicity, we focus on eukaryotic sensory systems, in which G-protein-coupled receptors GPCRs and ion channels predominate as sensory receptors. Nociceptors that have bare nerve endings that detect tissue damage and give the sensation of pain. Tests[ edit ] A common test used to measure the sensitivity of a person to tactile stimuli is measuring their two-point touch threshold.
This is the smallest separation of two points at which two distinct points of contact can be sensed rather than one. Different parts of the body have different degrees of tactile acuity, with extremities such as the fingers, face, and toes being the most sensitive. When two distinct points are perceived, it means that your brain receives two different signals.
The differences of acuity for different parts of the body are the result of differences in the concentration of receptors. Prompting is the use of a set of instructions designed to guide a participant through learning a behavior. A physical prompt involves stimulation in the form of physically guided behavior in the appropriate situation and environment.
The physical stimulus perceived through prompting is similar to the physical stimulus that would be experienced in a real-world situation, and is makes the target behavior more likely in a real situation. All materials constantly shed molecules, which float into the nose or are sucked in through breathing. Inside the nasal chambers is the neuroepitheliuma lining deep within the nostrils that contains the receptors responsible for detecting molecules that are small enough to smell.
These receptor neurons then synapse at the olfactory cranial nerve CN Iwhich sends the information to the olfactory bulbs in the brain for initial processing.
The signal is then sent to the remaining olfactory cortex for more complex processing. For a molecule to trigger olfactory receptor neurons, it must have specific properties. The molecule must be: Our olfactory ability can vary due to different conditions. For example, our olfactory detection thresholds can change due to molecules with differing lengths of carbon chains. A molecule with a longer carbon chain is easier to detect, and has a lower detection threshold.
Additionally, women generally have lower olfactory thresholds than men, and this effect is magnified during a woman's ovulatory period. A concave lens can push the focus back bottom. Here are the four common refractive vision problems: Nearsightedness myopia Astigmatism Presbyopia In nearsightedness myopiathe light from distant objects gets focused in front of the retina rather than on it. Myopia happens usually when the eyeball is too long; however, it is sometimes caused by too much focusing power in the lens system.
The result is that the person can see close-up objects fine, but distant objects are blurry. Myopia can be corrected by using a concave lens to diverge, or spread out, the light so that when it passes through the lens system, it comes to focus on the retina.
A convex lens can bring the focus forward bottom. In farsightedness hyperopiathe light gets focused in back of the retina rather than on it. Hyperopia usually happens when the eyeball is too short or when the focusing power of the lens system is too weak. The result is that a person can see distant objects fine, but close-up objects are blurry. Hyperopia can by corrected by using a convex lens to concentrate or converge the light Figure 3 so that when it passes through the lens system, it comes to focus on the retina.
Normal, near, and farsightedness: In astigmatism, the shape of the cornea or the lens is distorted so that the light comes into two focal points. Imagine that the lens is egg-shaped instead of spherical and that light coming over the top and bottom edges is brought to a different focal point than light coming over the right and left sides.
In presbyopia, the cornea and lens of the eye become less stretchy and, therefore, they cannot change shape as readily to bring light to a focus on the retina; this happens naturally as we grow older and is usually observed when a person reaches their 40s. If you have presbyopia, you have trouble focusing light from both near and far objects on the retina. To correct this problem, you might get a pair of bifocal lenses, with the top part for seeing far objects and the bottom part for near objects.
Hearing The ear is the organ for hearing and balance. In hearing the ear detects vibrations, their frequency pitch and amplitude loudness. These become nerve impulses carried to the brain. For balance the ear detects the direction of motion, acceleration and head position related to gravity. There are 3 sections to the ears: See the diagram below for the parts of the ear. It is very important that you know these parts and their functions.
Is said to collect sound waves and channel them into the external auditory canal. It is made of mainly of cartilage. This is the tube that carries sound vibrations to the eardrum. Earwax and hairs prevent foreign material entering the ear. It is a small, tightly stretched membrane that separates the outer ear from the middle ear. It vibrates with the same frequency as the sound waves beating against it.
Ear Ossicles hammer, anvil, stirrup: These are three tiny bones in the middle ear. They transmit and amplify the vibrations of the eardrum to the inner ear. This is a tube that runs from the middle ear to the pharynx throat. When it opens it equalises the air pressure on both sides of the eardrum allowing the eardrum to vibrate freely so accurate sound sensations are generated. This also prevents damage to the eardrum caused by differences in air pressure between the outer and middle ear.
This is a flexible partition allowing transfer of vibrations from the middle ear to the inner ear. This is a flexible partition to deaden the pressure changes in the inner ear.
It allows the pressure waves to dissipate out of the cochlea into the air of the middle ear. This is responsible for the conversion of sound waves into nerve impulses. See discussion of how we hear below Auditory Nerve: These detect the direction of motion and acceleration.
They are responsible for balance. See discussion of balance below How We Hear When sound waves enter the ear they vibrate the eardrum. The eardrum causes the ear ossicles hammer, anvil, stirrup to move vibrate.
These vibrations arrive at the cochlea. Within the cochlea is a liquid called lymph. The lymph is moved within the cochlea. This movement of liquid stimulates sensory sound receptors in the cochlea. There are about 24, sensory cells within our ears. These receptors create an electrical impulse that travels up the auditory nerve and on to our brain.
Animations of normal hearing: Sound waves are first collected in our outer ear called the auricle or pinnapass through our ear canal and cause our eardrum to vibrate. These vibrations are in turn transmitted to our inner ear by the bones of our middle ear. Our inner ear plays a vital role in the transformation of these mechanical vibrations into electrical impulses, or signals, which can be recognized and decoded by our brain.
When the vibrations reach the cochlea through movement of the bones in the middle ear, the fluid within it begins to move, resulting in back and forth motion of tiny hairs sensory receptors lining the cochlea. This motion results in the hair cells sending a signal along the auditory nerve to the brain.
Our brain receives these impulses in its hearing centres and interprets them as a type of sound. Sound waves are vibrations of air particles. As these vibrations strike the outer ear, the Pinna a funnels and amplifies them, sending them down the ear canal.
At the end of the ear canal bthe vibrations move the eardrum c that in turn vibrates the 3 smallest bones in the body. The Hammer, Anvil and Stirrup d further increase the power of the sound and transmit the energy to the inner ear organ of hearing, the cochlea e which converts the vibrations to nerve impulses which move to the brain along the auditory nerve f.
Balance Balance is controlled by the vestibular apparatus in the inner ear. It consists of the 3 semicircular canals. The vestibular apparatus is filled with the liquid called lymph.
BBC - GCSE Bitesize: Receptors
Receptors within the apparatus detect whether the head is vertical or not. Other receptors detect motion. These receptors send impulses to the brain.