M I S

TURING MACHINE: -

Hypothetical computing device introduced in 1936 by the English mathematician and logician Alan M. Turing. Turing originally conceived the machine as a mathematical tool that could infallibly recognize undecidable propositions—i.e., those mathematical statements that, within a given formal axiom system cannot be shown to be either true or false. (The mathematician Kurt Gödel had demonstrated that such undecidable propositions exist in any system powerful enough to contain arithmetic.) Turing instead proved that there can never exist any universal algorithmic method for determining whether a proposition is undecidable.The Turing machine is not a machine in the ordinary sense but rather an idealized mathematical model that reduces the logical structure of any computing device to its essentials. As envisaged by Turing, the machine performs its functions in a sequence of discrete steps and assumes only one of a finite list of internal states at any given moment. The machine itself consists of an infinitely extensible tape, a tape head that is capable of performing various operations on the tape, and a modifiable control mechanism in the head that can store directions from a finite set of instructions. The tape is divided into squares, each of which is either blank or has printed on it one of a finite number of symbols. The tape head has the ability to move to, read, write, and erase any single square and can also change to another internal state at any moment. Any such act is determined by the internal state of the machine and the condition of the scanned square at a given moment. The output of the machine—i.e., the solution to a mathematical query—can be read from the system once the machine has stopped. (However, in the case of Gödel's undecidable propositions, the machine would never stop, and this became known as the “halting problem.”)By incorporating all the essential features of information processing, the Turing machine became the basis for all subsequent digital computers, which share the machine's basic scheme of an input/output device (tape and reader), memory (control mechanism's storage), and central processing unit (control mechanism).

NEURONS SYSTEM, HUMAN: -

The stimulus-specific organization of the peripheral nerve fibres is not continued within the spinal cord, as the various afferent nerve fibres do not transmit their impulses exclusively to neurons of only one kind of sensibility. In the dorsal horns (the spinal region that receives afferent impulses) a few neurons are purely nociceptive, but most neurons reporting noxious events receive both noxious and mechanoreceptive input. These latter are called convergent neurons. The size of the peripheral field (the area of the body from which it receives stimuli) of a dorsal-horn neuron continually varies, depending on the state of excitability of the neuron. Furthermore, events in the peripheral field affect future responses. For example, repeated input along a group of afferent nerve fibres produces a gradually decreasing response in the central nervous system. This is called habituation. Also, the region of decreased response spreads from local neurons that received the input to neighbouring neurons.The state of excitability of a dorsal-horn neuron depends on many variables. If it is very excitable, it will respond to impulses from many afferent peripheral nerve fibres; if it is relatively inexcitable, only those peripheral fibres that are habitually connected to it and located near it will affect it. A neuron excited by many afferent fibres receives input from a larger area than a neuron receiving only from the fibres most nearly related to it. For this reason the area of skin or deep tissue connected to neurons of the dorsal horn varies and changes. In experiments using damaged skin, it has been found that a barrage of nerve impulses from the damaged region increases the excitability of the dorsal-horn neurons. Once this hyperexcitable state has been set up, it continues for a time without further input from peripheral nerves. In this state of local excitability, some dorsal-horn neurons receive an input from the area of damaged skin that they would not receive were the skin in a normal state. The convergent neurons mentioned above can have their activity inhibited by tactile stimulation of a region near their peripheral fields or of a homologous region on the opposite side of the body. Also, their responsiveness to stimuli can be increased by damage to the skin in their peripheral fields. From these convergent neurons and from other neurons of the dorsal horns, there arise tracts of long fibers that cross the midline and lead to the thalamus and other nuclei of the brain. These constitute the spinothalamic tracts. The other main pathway of pain impulses ends in the reticular formation of the medulla oblongata and pons and is known as the spinoreticular tract. It is believed.

PATTERN MATCHING: -

COBOL, FORTRAN, and their descendants, such as Pascal and C, are known as imperative languages, since they specify as a sequence of explicit commands how the machine is to go about solving the problem at hand; this is not very different from what takes place at the machine level. Other languages are functional, in the sense that programming is done by calling (i.e., invoking) functions or procedures, which are sections of code executed within a program. The best-known language of this type is LISP (List Processing), in which all computation is expressed as an application of a function to one or more “objects.” Since LISP objects may be other functions as well as individual data items (variables, in mathematical terminology) or data structures (see the section Data structures and algorithms), a programmer can create functions at the appropriate level of abstraction to solve the problem at hand. This feature has made LISP a popular language for artificial intelligence applications, although it has been somewhat superseded by logic programming languages such as Prolog (Programming in Logic). These are termed nonprocedural, or declarative, languages in the sense that the programmer specifies what goals are to be accomplished but not how specific methods are to be applied to attain those goals. Prolog is based on the concepts of resolution (akin to logical deduction) and unification (similar to pattern matching). Programs in such languages are written as a sequence of goals. A recent extension of logic programming is constraint logic programming, in which pattern matching is replaced by the more general operation of constraint satisfaction. Again, programs are a sequence of goals to be attained, in this case the satisfaction of the specified constraints.

Recent developments

Object-oriented languages

An important trend in programming languages is support for data encapsulation, or object-oriented code. The language Smalltalk, in which all programming is done in terms of so-called objects, best illustrates data encapsulation. An object in Smalltalk or similar object-oriented languages consists of data together with the procedures (program segments) to operate on that data. Encapsulation refers to the fact that an object's data can be accessed only through the methods (procedures) provided. Programming is done by creating objects that send messages to one another so that tasks can be accomplished cooperatively by invoking each others' methods. This object-oriented paradigm has been very influential. For example, the language C, which was popular for engineering applications and systems development, has largely been supplanted by its object-oriented extension C++. An object-oriented version of BASIC, named Visual BASIC, is available for personal computers and allows even novice programmers to create interactive applications with elegant graphical user interfaces (GUIs).

In 1995 Sun Microsystems, Inc., introduced Java, yet another object-oriented language. Applications written in Java are not translated into a particular machine language but into an intermediate language called Java Byte code, which may be executed on any computer (such as those using UNIX, Macintosh, or Windows operating systems) with a Java interpretation program known as a Java virtual machine. (See Program translation below.) Thus Java is ideal for creating distributed applications or Web-based applications. The applications can reside on a server in Byte code form, which is readily downloaded to and executed on any Java virtual machine. In many cases it is not desirable to download an entire application but only an interface through which a client may communicate interactively with the application. Java applets (small chunks of application code) solve this problem. Residing on Web-based servers, they may be downloaded to and run in any standard Web browser to provide, for example, a client interface to a game or database residing on a server.