Sensory receptor cells convert stimuli into electrical energy.

Transcription

1 Biology 30 The Senses Sensory inputs become sensations and perceptions in the brain. Various sensory receptors are able to respond to changes in our external and internal environment. Such changes may be due to light, sound, touch, taste, or smell. The result of this stimulation of a sensory receptor is generation of an action potential. The action potential is conveyed to the central nervous system where we experience a sensation. The brain integrates the sensation with other information and may form a perception. For example, when you first looked at the picture here, you many have seen nothing more than a bunch of random dark spots. That was a sensation. If you were told, however, that this is a picture of a dalmation and you looked again, you might see the dalmation in the picture. That was a perception. Notice that our perceptions depend on integrating what we see with what we know. Sensory receptor cells convert stimuli into electrical energy. The conversion of a stimulus into an action potential is called sensory transduction. In the case of a taste bud, molecules first enter the taste bud where they bind to receptor sites on transmembrane proteins. This causes an ion channel to open allowing positively charged ions to enter the cell. This alters the membrane potential called the receptor potential. The sensory cell is connected to a sensory neuron by a synapse. Neurotransmitter molecules traveling across the synapse cause an action potential in the sensory neuron. The presence of the stimulating molecule results in an increase in the rate of action potentials sent by the sensory neuron. Each sensory neuron is connected to a different interneuron in the brain allowing the brain to make distinctions between the tastes of salty or sweet. The rate of the action potentials signals the degree or strength of how salty or how sweet a substance is. 1

2 Specialized sensory receptors detect five categories of stimuli. Thermoreceptors detect either heat or cold both in the skin and deep in our body. Receiving action potentials from both these sources, the hypothalamus regulates our body temperature. Mechanoreceptors are stimulated by mechanical energy: touch, pressure, motion, or sound. Stretch receptors in our muscles monitor the position of body parts. The movement of cilia in the cell membranes of hair cells can alter the rate of neurotransmitter secretion. Such cells have a typical rate of transmission. If the cilia are moved in one direction, this may speed up the rate. If moved in the other, this may slow down the rate. Hair cells are found in our ears and in the lateral line of fishes. Chemoreceptors include sensory cells in our nose and taste buds which respond to the varying concentration of chemicals in our environment. Our body uses chemoreceptors to monitor CO 2 levels in our blood and send it to our brain stem to regulate our breathing. The male silkworm moth, Bombys mori, can detect female pheromone when as few as 50 receptor cells on his antennae detect one attractant molecule per second. Electromagnetic receptors are sensitive to energy of various wavelengths including electricity and magnetism. Certain fish discharge electric current into the water and use electroreceptors to locate their prey. Sharks and the platypus have electrodetectors that respond to the weak electric field generated when their prey flexes its muscles. Evidence suggests that many species can detect Earth s magnetic field and use it for navigation or migration. Photoreceptors, including eyes, are probably the most common type of electromagnetic detector. They detect the visible light portion of the electromagnetic spectrum. Many insects, however, can also detect ultraviolet light. Rattlesnakes have pits located beneath their eyes that respond to infrared radiation and use them to locate their warm-blooded prey (rodents and birds). 2

3 Three different types of eyes have evolved among invertebrates. The eye cup of the planarian has light sensitive receptor cells that are partially covered with dark pigment. Having two of these combined with the pigmented and unpigmented sections allows the planarian to detect from which direction the light is coming from and to move away from it. Insects have compound eyes made of many light detecting units called ommatidia. These eyes are very good at detecting movement an excellent defense against predators. The octopus has a single-lens eye, like ours, that operates much like a camera. It has an adjustable iris to change the diameter of the pupil and control how much light enters. Behind the pupil is a lens to focus light onto the retina which consists of many photoreceptor cells. Vertebrates have single-lens eyes. The vertebrate eye, although similar to the squid eye, evolved separately. The outer surface is covered with a tough connective tissue called the schlera. At the front of the eye it becomes the transparent cornea that allows light to enter and does most of the focusing. The schlera surrounds a pigmented layer called the choroid which, at the front of the eye, becomes the iris which gives the eye its characteristic colour. Muscles in the iris regulate the size of the pupil. Behind the pupils is the lens held in place by ligaments and then muscles. Changing the shape of the lens allows us to focus on near or far objects. Photoreceptor cells of the retina convert light energy into action potentials that are conveyed to the vision centres of the brain. Photoreceptors are concentrated at the centre of the retina in what is called the fovea. There are no photoreceptors where the optic nerve exits the eye this is called the blind spot. Two fluid filled cavities maintain the shape of the eye: the jelly like vitreous humor and the liquid aqueous humor. The aqueous humor supplies the lens with oxygen and takes away the wastes. Excess pressure 3

4 is called glaucoma and can cause blindness. Surrounding the eye and connecting to the eyelids is the thin conjunctiva. It is lubricated by our tears. To focus, a lens changes position or shape. The lens focuses light by bending light rays. The squid focuses by moving its lens forward or back like you would move a magnifying glass. The mammal focus by altering the thickness of the lens. The thicker the lens, the more bending (refraction). When the eye focuses on a nearby object, the ciliary muscles contract and pull the choroid closer to the lens. This takes the tension off of the ligaments and allowing the lens to thicken. For distant vision, the muscles relax returning tension to the ligaments and the lens thins. Artificial lenses or surgery can correct focusing problems. A vision test measure your visual acuity the ability of your eyes to distinguish fine detail. Normal vision is 20/20 meaning that you can see from 20 feet what a normal person should see at 20 feet. 20/10 means you see at 20 feet what most people can only see from 10 feet and so you are far sighted. 20/50 means you see at 20 feet what most people can see from 50 feet and so you are nearsighted. 4

5 Nearsightedness or myopia is caused by the eye being a bit too long and so the lens focuses an image short of the retina. It is corrected by glasses or contact lenses that are thiner in the middle and thicker at the edges (biconcave). Farsightedness or hyperopia is caused by the eye being too short and so the lens focuses an image behind the retina. It is corrected by glasses or contact lenses that are thicker in the middle and thiner at the edges. (biconvex). Laser surgery can also be used to alter the shape of the cornea. Astigmatism is blurred vision caused by a misshapen cornea or lens. Lenses that correct astigmatism are asymmetrical in a way that compensates for the asymetry of the eye. As we age, the lens becomes less flexible and does not plump up like it should to focus on near objects. People with presbiopia usually require reading glasses or if they already have glasses, bifocals. Correcting nearsightedness. Correcting farsightedness. Our photoreceptor cells are rods and cones. The human retina has about 125 million rod cells and 6 million cone cells, the two types of photoreceptors name for their shapes. Cones are stimulated by bright light and can distinguish colours. Rods are much more sensitive and allow us to see at night, but they only see in shades of grey. Cones are found concentrated around the fovea with rods found more on the outer areas. Each rod and cone contains an array of membranous disks containing light absorbing visual pigments. Rods contain a pigment called rhodopsin and cones contain photopsin. We have three types of cones that respond to blue, red and green light. Rod and cone cells are stimulus transducers that convert a stimulus into an action potential. When rhodopsin and photopsin absorb light they change shape and this alters the permeability of the cell s membrane. Some integration actually takes place in the complex interweaving of interneurons in the retina itself. This helps sharpen the image and heighten the contrast before being sent to the brain via the optical nerve. 5

6 The ear converts air pressure waves into action potentials that are perceived as sound. The ear is a complicated organ that allows us to hear as well as maintain our sense of balance. It is composed of three regions: the outer ear, middle ear and inner ear. The outer ear consists of the fleshy pinna and the auditory canal. They collect sound waves and channel them to the eardrum (tympanum) a sheet of tissue that separates the inner ear from the middle ear. Sound waves striking the eardrum cause it to vibrate and this motion is passed on to the three smallest bones of our body: the hammer (mallus), anvil (incus) and stirrup (stirpes). The stirrup is connected to the oval window, a flexible membrane, on the coclea (latin: snail). The eustachian tube connects the middle ear to the pharynx and equalizes the air pressure between the middle ear and the outside environment. 6

7 The inner ear consists of fluid-filled channels protected by the bones of the skull. A cross section shows our hearing organ, the organ of Corti in the middle canal of the coclea. It consists of an array of hair cells embedded in a basilar membrane (the floor of the middle canal). Another membrane, the tectonum, projects over the hair cells like a shelf from the wall of the middle canal. These hair cells are the receptor cells of the ear. The movement of the fluid surrounding them as well as vibrations from rubbing up against the tectonum cause the cilia of the hair cells to move. This alters the permeability of the membranes of the hair cells allowing positive ions to enter the cell. As a result, the cell develops a receptor potential and releases more neurotransmitter molecules at its synapse with a sensory neuron. Vibrations from the oval window travel through the upper canal to the tip of the coclea, at its centre. The pressure waves then enter the lower canal until they reach the flexible round window which dampens the vibration. Pressure waves passing through the upper canal cause the basilar membrane of the middle canal to vibrate. The brain senses a sound as an increase in the frequency of action potentials it obtains from the auditory nerve. The higher the amplitude of vibrations in the fluid of the coclea, the louder we hear the sound. The decibel scale for human hearing ranges from 0 to 120 decibels which is painful. The pitch of a sound is determined by which parts of the non-uniform basilar membrane vibrate the most in response to the sound. A healthy ear can hear pitches from 20 to 20,000 Hz (Hertz: vibrations per second). Dogs can hear as high as 40,000 Hz and some bats up to 75,000 Hz which they use for echolocation. Deafness can come from stiffening of the three bones (common in old age), or damage to the receptor cells. Frequent or prolonged exposure to sounds of over 90 db can damage or destroy hair cells. Be careful with those earphones kids! 7

8 The inner ear houses our organs of balance. Three semi-circular canals are attached to the utricle and saccule of the inner ear. They are oriented in the X, Y, and Z axis and allow us to sense motion in each of these three planes. A swelling at the base of each semi-circular canal contains a cluster of hair cells with their hairs embedded in a gel-like mass called a cupula. When the head moves, the thick fluid in the canals moves more slowly due to inertia. This causes the liquid to press against the cupula, bending the hairs. The faster you rotate your head, the higher the frequency of action potentials that are sent to the brain. Clusters of hair cells within the utricle and saccule detect the position of the head with respect to gravity. The hairs of these cells project into a gelatinous material containing many small calcium carbonate particles. When the position of the head changes, this heavy material bends the hair cells emit an action potential and the brain determines the new position of the head. 8

9 Odor and taste receptors detect categories of chemicals. Chemoreceptors in our nose detect airborne molecules while those in our taste buds respond to molecules in our food. Research indicates that the same receptor cells generate different patterns of action potentials in response to different scents. Cinnamon, for example, produces a particular pattern of action potentials then the smell of garlic does from the same receptor cells. This pattern is then integrated by the brain with memory for identification. The olfactory apparatus is located at the top of our nasal cavity. Molecules from the air we breath are dissolved into a thin layer of mucous and then attach to the cilia of the chemoreceptor cells. The binding triggers receptor potentials, which alter the rate of action potentials passing into the brain. Our taste buds seem to be categorized into sweet, sour, salt, bitter and the newly proposed umami (from the Japanese word for meaty or tasty ). Umami receptors detect amino acids, the building blocks of protein which is often found in meat. Each type of taste receptor cells responds to a range of tastes within its category. Sweet detector, for example, can respond to many varieties of sugar molecules. 9