Rod cell

Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than the other type of visual photoreceptor, cone cells. Rods are concentrated at the outer edges of the retina and are used in peripheral vision. On average, there are approximately 90 million rod cells in the human retina. More sensitive than cone cells, rod cells are almost entirely responsible for night vision. However, 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 (which is why colors are much less apparent in darkness).

Structure

Rods are a little longer and leaner than cones but have the same structural basis. The opsin or pigment is on the outer side, lying on the Retinal pigment epithelium, completing the cell's homeostasis. This epithelium end contains many stacked disks. Rods have a high area for visual pigment and thus substantial efficiency of light absorption.

Cell (biology)

The cell (from Latin cella, meaning "small room") is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life that can replicate independently, and cells are often called the "building blocks of life". The study of cells is called cell biology.

Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Organisms can be classified as unicellular (consisting of a single cell; including bacteria) or multicellular (including plants and animals). While the number of cells in plants and animals varies from species to species, humans contain more than 10 trillion (10¹³) cells. Most plant and animal cells are visible only under the microscope, with dimensions between 1 and 100 micrometres.

The cell was discovered by Robert Hooke in 1665, who named the biological unit for its resemblance to cells inhabited by Christian monks in a monastery.Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, that all cells come from preexisting cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells. Cells emerged on Earth at least 3.5 billion years ago.

Beta cell

Beta cells (β cells) are a type of cell found in the pancreatic islets of the pancreas. They make up 65-80% of the cells in the islets.

Function

The primary function of a beta cell is to store and release insulin. Insulin is a hormone that brings about effects which reduce blood glucose concentration. Beta cells can respond quickly to spikes in blood glucose concentrations by secreting some of their stored insulin while simultaneously producing more.

Control of insulin secretion

Voltage-gated calcium channels and ATP-sensitive potassium ion channels are embedded in the cell surface membrane of beta cells. These ATP-sensitive potassium ion channels are normally open and the calcium ion channels are normally closed. Potassium ions diffuse out of the cell, down their concentration gradient, making the inside of the cell more negative with respect to the outside (as potassium ions carry a positive charge). At rest, this creates a potential difference across the cell surface membrane of -70mV.

Alpha cell

Alpha cells (more commonly alpha-cells or α-cells) are endocrine cells in the pancreatic islets of the pancreas. They make up to 20% of the human islet cells synthesizing and secreting the peptide hormone glucagon, which elevates the glucose levels in the blood.

Functions

To elevate glucose levels, glucagon binds to receptors on hepatocytes (liver cells) and some other cells (e.g. kidney cells). This activates an enzyme, glycogen phosphorylase, inside the hepatocyte to hydrolyse glycogen to glucose. This process is called glycogenolysis. In rodents, alpha cells are located in the periphery of the islets, however in humans the islet architecture is generally less organized and alpha cells are frequently observed inside the islets as well. When being viewed by an electron microscope, alpha cells can be identified by their characteristic granules with a large dense core and a small white halo.

See also

External links

Rod

Rod, Ród, Rőd, Rød, Röd, ROD, or R.O.D. may refer to:

Devices

Media

Rod (optics)

"Rods" (sometimes known as "skyfish", "air rods", or "solar entities") is a term used in cryptozoology, ufology, and outdoor photography to refer to elongated artifacts in the form of light-rods produced by cameras. Videos of rod-shaped objects moving quickly through the air were claimed by some ufologists and cryptozoologists to be alien life forms, "extradimensional" creatures, or very small UFOs. Subsequent experiments showed that these rods appear in film because of an optical illusion/collusion (especially in interlaced video recording), and are typically traces of a flying insect's wingbeats.

Optical analysis

Various paranormal interpretations appeared in the popular culture, and one of the more outspoken proponents of rods as alien life forms is Jose Escamilla, who claims to have been the first to film them on March 19, 1994 at Roswell, New Mexico, while attempting to film a UFO. Since then, Escamilla has made additional videos and embarked on lecture tours to promote his claims.

Corporal punishment

Corporal punishment or physical punishment is punishment intended to cause physical pain on a person.

Common methods include spanking, paddling, caning and also bastinado. It can occur on minors but also on other individuals subjected to a right of corporal punishment such as prisoners in several countries or slaves in former times.

Official punishment for crime by inflicting pain or injury, including flogging, branding and even mutilation, was practised in most civilisations since ancient times. However, with the growth of humanitarian ideals since the Enlightenment, such punishments were increasingly viewed as inhumane. By the late 20th century, corporal punishment had been eliminated from the legal systems of most developed countries.

The legality in the 21st century of corporal punishment in various settings differs by jurisdiction. Internationally, the late 20th century and early 21st century saw the application of human rights law to the question of corporal punishment in a number of contexts: