Senses Glossary

Accessory olfactory system: The sensory cells in the vomeronasal organ of the nose and their central connections in the brain that receive social and sexual information, in the form of pheromones, from other organisms of the same species. It is separate from the main olfactory system, which is involved in the sense of smell.
Actin: A protein that is important in maintaining cell shape. It also acts together with myosin to produce cell movement.
Amblyopia: A partial or total loss of vision that is not due to abnormalities in the retina of the eye.
Auditory system: The sensory cells in the ear and their central connections in the brain that are involved in the sense of hearing.
Axon: The main, fiberlike extension of a neuron by which it sends information to target cells.
Basilar membrane: A membrane in the cochlea, an organ of the inner ear, containing hair cells that respond to the vibrations produced by sound.
Cerebral cortex: The outermost layer of neurons on the brain's hemispheres, often called simply "the cortex." It is responsible for all forms of conscious experience, including perception, emotion, thought, and planning, as well as coordination of motor activity.
Cerebrum: Part of the forebrain of vertebrates, concerned with sensory processing and coordination of activity. It consists of two nearly symmetrical halves, the hemispheres, whose outermost layer forms the cerebral cortex.
Cilia: Short, hairlike projections from the surface of certain cells, such as hair cells.
Cochlea: A snail-shaped, fluid-filled organ of the inner ear that contains the basilar membrane. It is responsible for translating the motion of the basilar membrane into nerve signals that lead to the perception of sound.
Color blindness: In most cases, the inability to distinguish red from green, or to see red and green in the same way as most people do, because of an abnormality in the red or green photoreceptors. About 7 percent of men are red-green color blind, compared to 0.4 percent of women.
Cone cells: Cone-shaped photoreceptor cells located in the retina, responsible for high-acuity vision and color vision in moderate or bright light. The three types of cone cells, loosely called blue, green, and red, are sensitive to different wavelengths of light; their interaction forms the basis of color vision.
Cortex: The cerebral cortex, the outermost layer of the cerebral hemispheres of the brain, responsible for all forms of conscious experience.
Cyclic GMP (cGMP): Cyclic guanosine monophosphate, a molecule that leads to the opening of channels or pores on the surface of photoreceptor cells in the dark; opening of the channel allows sodium ions to enter the cell. When the cell's rhodopsin absorbs light, the concentration of cGMP drops and the pores close.
DNA library: A collection of DNA from a tissue or organism. It is usually made available by cloning this DNA into bacterial plasmids.
EEG: Electroencephalography, the recording of brain waves by means of electrodes attached to the skull.
Electrode: A thin conductor, insulated except at its tip, that is placed either near or inside a nerve cell. It can pick up signals generated by the cell's electrical activity, or can be used to stimulate this activity.
Epithelium: A sheet of cells that covers the body surface or lines body cavities.
fMRI: Functional Magnetic Resonance Imaging, a new method of scanning the brain's activity that needs no radioactive materials and produces images at a higher resolution than PET. It is based on differences in the magnetic resonance of certain atomic nuclei in areas of neuronal activity.
Fovea: A depression in the central region of the vertebrate retina, containing closely packed cones. It is responsible for high-acuity vision.
Hair cells: The receptor cells found in the inner ear. Hair cells bear hair-like projections, cilia, which vibrate in response to sound or movement of the head. Movement of the cilia leads to the opening of ion channels in the cells and to the production of brief electrical signals.
Hemispheres: The two nearly symmetrical halves of the cerebrum. In most people, the left hemisphere is specialized for speech, writing, language, and calculation; the right hemisphere is specialized for spatial abilities and pattern recognition.
Homunculus: An imaginary "little person" within the brain who was thought to perceive objects and events and to make decisions. Sometimes a representation of the body on the motor or somatosensory cortex.
Ions: Atoms or small molecules that carry an electric charge. The movement of ions across cell membranes leads to electrical signalling in the brain.
Ion channel: Channel in a cell's surface membrane that controls the flow of ions into the cell. Usually specific for certain ions like sodium or potassium.
LGN: Lateral Geniculate Nucleus, a part of the thalamus that relays signals from the eye to the visual cortex. It also receives signals back from the cortex.
MEG: Magnetoencephalography, a technique for recording electrical signals from the brain based on changes in magnetic fields.
Microelectrode: A very small electrode used to pick up electrical signals, often from a single nerve cell.
Molecular biology: The study of molecular structures and events that underlie biological processes. Molecular biology is in a period of explosive development, mostly thanks to recombinant DNA technology.
MRI: Magnetic Resonance Imaging, a method of scanning the brain that produces detailed maps of brain structure without X-rays or other radiation by relying on the difference in the magnetic resonance of certain atomic nuclei.
Myosin: A protein that often acts together with actin to produce cell or organelle movements.
Nerve cell: A specialized cell that transmits information to other nerve cells, muscle, or gland cells. It is the basic working unit of the brain. Also called a "neuron.
Neural network: A computational device made of units that resemble neurons. Such networks are often used to simulate brain activity.
Neuron: A nerve cell, the basic working unit of the nervous system.
Neuroscience: The study of the nervous system, including vision, hearing, smell, and the control of movement and other behaviors.
Neurotransmitter: A chemical released by a neuron at a synapse to relay information to an adjacent cell.
Ocular dominance: The tendency of clusters of nerve cells in the visual system to respond primarily to one eye rather than to the other.
Olfactory bulb: A pea-sized structure on the undersurface of the frontal lobe of the brain that receives inputs from the olfactory neurons and projects to the regions of the brain concerned with the sense of smell.
Olfactory cell: A neuron that senses odorant molecules. Olfactory cells lie in the upper part of the nose. Their outer surfaces bear hair-like cilia. From their lower surfaces the cells extend an axon that passes to the olfactory bulb.
Olfactory system: The sensory cells in the nose and the regions of the brain with which they are connected that collectively are involved in the sense of smell.
Ophthalmoscope: An instrument used to view the interior of the eye, especially the retina.
Optic chiasm: The site on the base of the brain where roughly half the nerve fibers from each eye cross to the opposite side of the brain and half stay on the same side. (From the Greek letter chi, X, whose shape is a cross.)
Pathway: A set of nerve connections through which information can travel from one brain region to another.
PET: Positron Emission Tomography, a type of scan that measures changes in blood flow associated with brain function by detecting positrons, positively charged particles emitted by radioactively labeled substances that have been injected into the body.
Pheromone: A chemical produced by an organism that signals its presence to other members of the same species. Of particular importance in sexual behavior.
Photon: A unit of light.
Photoreceptor: One of the 125 million nerve cells in the retina of each eye that emit electrical signals when activated by light of a particular wavelength. There are two types of photoreceptor cells in vertebrates: rods, used in low light, and cones, which respond in brighter light and to color.
Primary visual cortex: The region of the cerebral cortex at the back of the brain, to which the LGN relays information from the eyes. Also known as the "striate cortex." From there the information is sent to higher visual areas.
Receptor: A specialized sensory cell that responds to a unique type of stimulus such as light, sound, or odorant molecules, and transmits this information to the central nervous system. The term is sometimes used for specialized molecules on the surface of cells that respond to external signals.
Retina: A multilayered sheet of nerve cells at the back of each eye which converts light into electrical signals that are transmitted to the brain through the optic nerves and tracts.
Retinal: A derivative of vitamin A which absorbs light. Retinal is a component of visual receptor proteins.
Retinitis pigmentosa (RP): A genetic disorder that causes the degeneration of cells in the retina. If severe, it may lead to complete blindness.
Rhodopsin: The light-sensitive receptor protein in rod cells of the retina. When rhodopsin absorbs a photon of light, its molecular shape is changed and it releases energy, leading ultimately to an electrical signal.
Rod cell: A rod-shaped photoreceptor in the vertebrate retina that is responsible for vision in dim light. Rods, which are far more numerous than cones, have rhodopsin as their photopigment.
Second messenger: A molecule that conveys the chemical message of a hormone or neurotransmitter, or any other extracellular signal (the first messenger,) to the next stage in the cell's biochemical machinery.
Senses: The five senses: vision, hearing, smell, touch, and taste. Touch encompasses all sensations from the body surface, joints, and viscera, including pain, thermal sensitivity, and position sense.
Somatosensory areas: Areas of the brain involved in sensations such as pain, pressure, temperature, joint position, muscle sense, and movement.
Strabismus: A visual disorder, also called cross-eye or wall-eye, involving the inability to fuse the images in the two eyes. Children with strabismus initially have good vision in each eye, but because they tend to favor one eye, they often lose useful vision in the other eye. Vision can be restored if the strabismus is corrected before the age of four.
Synapse: The site of communication between nerve cells. When a neuron fires, neurotransmitter molecules are released from its axon terminal and diffuse across the small gap between cells (the synaptic cleft). The binding of the neurotransmitter to the appropriate receptor molecules on the surface of the post-synaptic cell causes that cell either to be excited or to be inhibited.
Thalamus: The key relay station for information from the sensory pathways to the cerebral cortex.
Touch: The sense by which we determine the size, shape, and texture of objects, using receptors in the skin.
Transduction: The conversion of environmental stimuli, such as light, heat, or vibration, into electrical signals that can be recognized by the nervous system.
Vomeronasal organ (VNO): A small sac on either side of the nasal septum, containing receptor cells that pick up chemical signals (pheromones) from other organisms of the same species.