Tuesday, September 25, 2007
Rod cells
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. Since they are more light-sensitive, rods are responsible for night vision. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. There are about 100 million rod cells in the human retina.
A rod cell is sensitive enough to respond to a single photon of light, and is about 100 times more sensitive to a single photon than cones. Since rods require less light to function than cones, they are therefore the primary source of visual information at night . Cone cells, on the other hand, require tens to hundreds of photons to become activated. Additionally, multiple rod cells converge on a single interneuron, collecting and amplifying the signals. However, this convergence comes at a cost to visual acuity since the pooled information from multiple cells is less distinct than it would be if the visual system received information from each rod cell individually. The convergence of rod cells also tends to make peripheral vision very sensitive to movement, and is responsible for the phenomenon of individuals seeing something vague occur out of the corner of his or her eye.Because they have only one type of light sensitive pigment, rather than the three types that human cone cells have, rods have little, if any, role in color vision.
Rod cells also respond more slowly to light than cones do, so stimuli they receive are added over about 100 milliseconds. While this makes rods more sensitive to smaller amounts of light, it also means that their ability to sense temporal changes, such as quickly changing images is less accurate than that of cones However, if multiple flashes of sub-threshold light occurs during the 100 millisecond period, the energy of the flashes of light would summate to produce a light which will reach threshold and send a signal to the brain.
Experiments by George Wald and others showed that rods are more sensitive to the blue area of the spectrum, and are completely insensitive to wavelengths above about 640 nm . This fact is responsible for the Purkinje effect, in which blue colors appear more intense relative to reds in darker light, when rods take over as the cells responsible for vision.
Like cones, rod cells have a synaptic terminal, an inner segment, and an outer segment. The synaptic terminal forms a synapse with another neuron, for example a bipolar cell. The inner and outer segments are connected by a cilium. The inner segment contains organelles and the cell's nucleus, while the outer segment, which is pointed toward the front of the eye, contains the light-absorbing materials.
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. Since they are more light-sensitive, rods are responsible for night vision. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. There are about 100 million rod cells in the human retina.
A rod cell is sensitive enough to respond to a single photon of light, and is about 100 times more sensitive to a single photon than cones. Since rods require less light to function than cones, they are therefore the primary source of visual information at night . Cone cells, on the other hand, require tens to hundreds of photons to become activated. Additionally, multiple rod cells converge on a single interneuron, collecting and amplifying the signals. However, this convergence comes at a cost to visual acuity since the pooled information from multiple cells is less distinct than it would be if the visual system received information from each rod cell individually. The convergence of rod cells also tends to make peripheral vision very sensitive to movement, and is responsible for the phenomenon of individuals seeing something vague occur out of the corner of his or her eye.Because they have only one type of light sensitive pigment, rather than the three types that human cone cells have, rods have little, if any, role in color vision.
Rod cells also respond more slowly to light than cones do, so stimuli they receive are added over about 100 milliseconds. While this makes rods more sensitive to smaller amounts of light, it also means that their ability to sense temporal changes, such as quickly changing images is less accurate than that of cones However, if multiple flashes of sub-threshold light occurs during the 100 millisecond period, the energy of the flashes of light would summate to produce a light which will reach threshold and send a signal to the brain.
Experiments by George Wald and others showed that rods are more sensitive to the blue area of the spectrum, and are completely insensitive to wavelengths above about 640 nm . This fact is responsible for the Purkinje effect, in which blue colors appear more intense relative to reds in darker light, when rods take over as the cells responsible for vision.
Like cones, rod cells have a synaptic terminal, an inner segment, and an outer segment. The synaptic terminal forms a synapse with another neuron, for example a bipolar cell. The inner and outer segments are connected by a cilium. The inner segment contains organelles and the cell's nucleus, while the outer segment, which is pointed toward the front of the eye, contains the light-absorbing materials.
Wednesday, September 19, 2007
Bread
Bread is a staple food of European, Middle Eastern, Mexican and Indian cultures which is prepared by baking, steaming, or frying dough. Bread consists minimally of flour and water; salt is present in most cases; and usually a leavening agent such as yeast is used. Breads may also contain some amounts of sugar, spices, fruit, vegetables, nuts, or seeds. There are a wide variety of breads, with preferences differing from region to region.
Fresh bread is prized for its taste and texture, and retaining its freshness is important to keep it appetizing. Bread that has stiffened or dried past its prime is said to be stale. Modern bread is often wrapped in paper or plastic film, or stored in airtight containers such as a breadbox to keep it fresh longer. Bread that is kept in warm moist environments is prone to the growth of mold. It becomes stale more quickly in the low temperature of a refrigerator, although by keeping it cool, mold is less likely to grow. A way to keep bread longer is to put it in the freezer, preserving it for several weeks.
Bread is a staple food of European, Middle Eastern, Mexican and Indian cultures which is prepared by baking, steaming, or frying dough. Bread consists minimally of flour and water; salt is present in most cases; and usually a leavening agent such as yeast is used. Breads may also contain some amounts of sugar, spices, fruit, vegetables, nuts, or seeds. There are a wide variety of breads, with preferences differing from region to region.
Fresh bread is prized for its taste and texture, and retaining its freshness is important to keep it appetizing. Bread that has stiffened or dried past its prime is said to be stale. Modern bread is often wrapped in paper or plastic film, or stored in airtight containers such as a breadbox to keep it fresh longer. Bread that is kept in warm moist environments is prone to the growth of mold. It becomes stale more quickly in the low temperature of a refrigerator, although by keeping it cool, mold is less likely to grow. A way to keep bread longer is to put it in the freezer, preserving it for several weeks.
Monday, September 10, 2007
Bahrain
Bahrain, officially the Kingdom of Bahrain , is a borderless island nation in the Persian Gulf . Saudi Arabia lies to the west and is connected to Bahrain by the King Fahd Causeway , and Qatar is to the south across thePersian Gulf. The Qatar–Bahrain Friendship Bridge, currently being planned, will link Bahrain to Qatar as the longest fixed link in the world.
Bahrain is the smallest, in terms of population, Arab nation in the world, and the smallest Arab member of the United Nations. It is also the least populous country in mainland Asia .
Bahrain, officially the Kingdom of Bahrain , is a borderless island nation in the Persian Gulf . Saudi Arabia lies to the west and is connected to Bahrain by the King Fahd Causeway , and Qatar is to the south across thePersian Gulf. The Qatar–Bahrain Friendship Bridge, currently being planned, will link Bahrain to Qatar as the longest fixed link in the world.
Bahrain is the smallest, in terms of population, Arab nation in the world, and the smallest Arab member of the United Nations. It is also the least populous country in mainland Asia .
Tuesday, September 04, 2007
British Broadcasting Corporation
The British Broadcasting Corporation, usually known as the BBC is the major broadcasting company in the world, employing 26,000 staff in the UK alone and with a budget of £4 billion. Founded in 1922 as the British Broadcasting Company Ltd, it was subsequently incorporated and made a state-owned but independent corporation in 1927. The corporation produces programmes and information services, broadcasting on television, radio, and the Internet. The declared assignment of the BBC is "to inform, educate and entertain", and the motto of the BBC is Nation Shall Speak Peace Unto Nation.
The BBC is a quasi-autonomous Public Corporation operating as a public service broadcaster. The Corporation is currently run by a board of governors appointed by the Queen on the advice of government ministers; but the BBC is, per its charter, to be "free from both political and commercial influence and answers only to its viewers and listeners".
Its domestic programming and broadcasts are mostly funded by levying television license fees, although money is also raised through business tricks such as sale of merchandise and programming. The BBC World Service, however, is funded by the Foreign and Commonwealth Office. In order to justify the license fee the BBC is expected to produce a number of high-rating shows in addition to programmes that commercial broadcasters would not normally broadcast. Quite often domestic audiences have affectionately referred to the BBC as the Beeb, Kenny Everett or as Auntie; the latter said to originate in the somewhat old fashioned Auntie knows best attitude dating back to the early days when John Reith was in charge. Occasionally the terms are used together as Auntie Beeb.
The British Broadcasting Corporation, usually known as the BBC is the major broadcasting company in the world, employing 26,000 staff in the UK alone and with a budget of £4 billion. Founded in 1922 as the British Broadcasting Company Ltd, it was subsequently incorporated and made a state-owned but independent corporation in 1927. The corporation produces programmes and information services, broadcasting on television, radio, and the Internet. The declared assignment of the BBC is "to inform, educate and entertain", and the motto of the BBC is Nation Shall Speak Peace Unto Nation.
The BBC is a quasi-autonomous Public Corporation operating as a public service broadcaster. The Corporation is currently run by a board of governors appointed by the Queen on the advice of government ministers; but the BBC is, per its charter, to be "free from both political and commercial influence and answers only to its viewers and listeners".
Its domestic programming and broadcasts are mostly funded by levying television license fees, although money is also raised through business tricks such as sale of merchandise and programming. The BBC World Service, however, is funded by the Foreign and Commonwealth Office. In order to justify the license fee the BBC is expected to produce a number of high-rating shows in addition to programmes that commercial broadcasters would not normally broadcast. Quite often domestic audiences have affectionately referred to the BBC as the Beeb, Kenny Everett or as Auntie; the latter said to originate in the somewhat old fashioned Auntie knows best attitude dating back to the early days when John Reith was in charge. Occasionally the terms are used together as Auntie Beeb.
