Cell surface receptor
Cell surface receptors are receptors that are embedded in the plasma membrane of cells. They act in cell signaling by receiving extracellular molecules, they are specialized integral membrane proteins that allow communication between the cell and the extracellular space. The extracellular molecules may be hormones, neurotransmitters, growth factors, cell adhesion molecules, or nutrients. In the process of signal transduction, ligand binding affects a cascading chemical change through the cell membrane. Many membrane receptors are transmembrane proteins. There are various kinds, including lipoproteins. Hundreds of different receptors are many more have yet to be studied. Transmembrane receptors are classified based on their tertiary structure. If the three-dimensional structure is unknown, they can be classified based on membrane topology. In the simplest receptors, polypeptide chains cross the lipid bilayer once, while others, such as the G-protein coupled receptors, cross as many as seven times.
Each cell membrane can have several kinds of membrane receptors, with varying surface distributions. A single receptor may be differently distributed at different membrane positions, depending on the sort of membrane and cellular function. Receptors are clustered on the membrane surface, rather than evenly distributed. Two models have been proposed to explain transmembrane receptors' mechanism of action. Dimerization: The dimerization model suggests that prior to ligand binding, receptors exist in a monomeric form; when agonist binding occurs, the monomers combine to form an active dimer. Rotation: Ligand binding to the extracellular part of the receptor induces a rotation of part of the receptor's transmembrane helices; the rotation alters which parts of the receptor are exposed on the intracellular side of the membrane, altering how the receptor can interact with other proteins within the cell. Transmembrane receptors in plasma membrane can be divided into three parts; the extracellular domain just externally from the organelle.
If the polypeptide chain crosses the bilayer several times, the external domain comprises loops entwined through the membrane. By definition, a receptor's main function is to respond to a type of ligand. For example, a neurotransmitter, hormone, or atomic ions may each bind to the extracellular domain as a ligand coupled to receptor. Klotho is an enzyme. Two most abundant classes of transmembrane receptors are GPCR and single-pass transmembrane proteins. In some receptors, such as the nicotinic acetylcholine receptor, the transmembrane domain forms a protein pore through the membrane, or around the ion channel. Upon activation of an extracellular domain by binding of the appropriate ligand, the pore becomes accessible to ions, which diffuse. In other receptors, the transmembrane domains undergo a conformational change upon binding, which effects intracellular conditions. In some receptors, such as members of the 7TM superfamily, the transmembrane domain includes a ligand binding pocket; the intracellular domain of the receptor interacts with the interior of the cell or organelle, relaying the signal.
There are two fundamental paths for this interaction: The intracellular domain communicates via protein-protein interactions against effector proteins, which in turn pass a signal to the destination. With enzyme-linked receptors, the intracellular domain has enzymatic activity; this is tyrosine kinase activity. The enzymatic activity can be due to an enzyme associated with the intracellular domain. Signal transduction processes through membrane receptors involve the external reactions, in which the ligand binds to a membrane receptor, the internal reactions, in which intracellular response is triggered. Signal transduction through membrane receptors requires four parts: Extracellular signaling molecule: an extracellular signaling molecule is produced by one cell and is at least capable of traveling to neighboring cells. Receptor protein: cells must have cell surface receptor proteins which bind to the signaling molecule and communicate inward into the cell. Intracellular signaling proteins: these pass the signal to the organelles of the cell.
Binding of the signal molecule to the receptor protein will activate intracellular signaling proteins that initiate a signaling cascade. Target proteins: the conformations or other properties of the target proteins are altered when a signaling pathway is active and changes the behavior of the cell. Membrane receptors are divided by structure and function into 3 classes: The ion channel linked receptor. Ion channel linked receptors have ion channels for anions and cations, constitute a large family of multipass transmembrane proteins, they participate in rapid signaling events found in electrically active cells such as neurons. They are called ligand-gated ion channels. Opening and closing of ion channels is controlled by neurotransmitters. Enzyme-linked receptors directly activate associated enzymes; these are single-pass transmembrane receptors, with the enzymatic component of the receptor kept intracellular. The majority of enzyme-linked receptors are, or associate with, protein kinases. G protein-coupled receptors are integral membrane proteins.
These receptors activate a G protein upon agonist binding, the G-protein mediates receptor effects on intracellular signaling pathways. During t
Sensory neurons known as afferent neurons are neurons that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction; the cell bodies of the sensory neurons are located in the dorsal ganglia of the spinal cord. This sensory information travels along afferent nerve fibers in an afferent or sensory nerve, to the brain via the spinal cord; the stimulus can come from extoreceptors outside the body, for example light and sound, or from interoreceptors inside the body, for example blood pressure or the sense of body position. Different types of sensory neurons have different sensory receptors that respond to different kinds of stimuli; the sensory neurons involved in smell are called olfactory sensory neurons. These neurons contain receptors, called olfactory receptors, that are activated by odor molecules in the air. To Olfactory receptors, taste receptors in taste buds interact with chemicals in food to produce an action potential.
Photoreceptor cells are capable of phototransduction, a process which converts light into electrical signals. These signals are refined and controlled by the interactions with other types of neurons in the retina; the five basic classes of neurons within the retina are photoreceptor cells, bipolar cells, ganglion cells, horizontal cells, amacrine cells. The basic circuitry of the retina incorporates a three-neuron chain consisting of the photoreceptor, bipolar cell, the ganglion cell; the first action potential occurs in the retinal ganglion cell. This pathway is the most direct way for transmitting visual information to the brain. There are three primary types of photoreceptors: Cones are photoreceptors that respond to color. In humans the three different types of cones correspond with a primary response to short wavelength, medium wavelength, long wavelength. Rods are photoreceptors that are sensitive to the intensity of light, allowing for vision in dim lighting; the concentrations and ratio of rods to cones is correlated with whether an animal is diurnal or nocturnal.
In humans, rods outnumber cones by 20:1, while in nocturnal animals, such as the tawny owl, the ratio is closer to 1000:1. Retinal ganglion cells are involved in the sympathetic response. Of the ~1.3 million ganglion cells present in the retina, 1-2% are believed to be photosensitive. Problems and decay of sensory neurons associated with vision lead to disorders such as: Macular degeneration – degeneration of the central visual field due to either cellular debris or blood vessels accumulating between the retina and the choroid, thereby disturbing and/or destroying the complex interplay of neurons that are present there. Glaucoma – loss of retinal ganglion cells which causes some loss of vision to blindness. Diabetic retinopathy – poor blood sugar control due to diabetes damages the tiny blood vessels in the retina; the auditory system is responsible for converting pressure waves generated by vibrating air molecules or sound into signals that can be interpreted by the brain. This mechanoelectrical transduction is mediated with hair cells within the ear.
Depending on the movement, the hair cell can either depolarize. When the movement is towards the tallest stereocilia, the Na+ cation channels open allowing Na+ to flow into cell and the resulting depolarization causes the Ca++ channels to open, thus releasing its neurotransmitter into the afferent auditory nerve. There are two types of hair cells: outer; the inner hair cells are the sensory receptors. Problems with sensory neurons associated with the auditory system leads to disorders such as: Auditory processing disorder – Auditory information in the brain is processed in an abnormal way. Patients with auditory processing disorder can gain the information but their brain cannot process it properly, leading to hearing disability. Auditory verbal agnosia – Comprehension of speech is lost but hearing, speaking and writing ability is retained; this is caused by damage to the posterior superior temporal lobes, again not allowing the brain to process auditory input correctly. Thermoreceptors are sensory receptors.
While the mechanisms through which these receptors operate is unclear, recent discoveries have shown that mammals have at least two distinct types of thermoreceptors. The bulboid corpuscle, is a cutaneous receptor a cold-sensitive receptor, that detects cold temperatures; the other type is a warmth-sensitive receptor. Mechanoreceptors are sensory receptors which respond to mechanical forces, such as pressure or distortion. Specialized sensory receptor cells called mechanoreceptors encapsulate afferent fibers to help tune the afferent fibers to the different types of somatic stimulation. Mechanoreceptors help lower thresholds for action potential generation in afferent fibers and thus make them more to fire in the presence of sensory stimulation; some types of mechanoreceptors fire action potentials. Proprioceptors are another type of mechanoreceptors which means "receptors for self"; these receptors provide spatial information about other body parts. Nociceptors are responsible for processing temperature changes.
The burning pain and irritation experienced after eating a chili pepper, the cold sensation experienced after ingesting a chemical such as menthol or icillin, as well as the common sensation of pain are all a result of neurons with these receptors. Problems with mechanoreceptors lead to disorders such as: Neuropa
In radio communications, a radio receiver known as a receiver, wireless or radio is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna; the antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, recovers the desired information through demodulation; the information produced by the receiver may be in the form of sound, moving data. A radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. Radio receivers are widely used in modern technology, as components of communications, remote control, wireless networking systems. In consumer electronics, the terms radio and radio receiver are used for receivers designed to reproduce sound transmitted by radio broadcasting stations the first mass-market commercial radio application.
The most familiar form of radio receiver is a broadcast receiver just called a radio, which receives audio programs intended for public reception transmitted by local radio stations. The sound is reproduced either by a loudspeaker in the radio or an earphone which plugs into a jack on the radio; the radio requires electric power, provided either by batteries inside the radio or a power cord which plugs into an electric outlet. All radios have a volume control to adjust the loudness of the audio, some type of "tuning" control to select the radio station to be received. Modulation is the process of adding information to a radio carrier wave. Two types of modulation are used in analog radio broadcasting systems. In amplitude modulation the strength of the radio signal is varied by the audio signal. AM broadcasting is allowed in the AM broadcast bands which are between 148 and 283 kHz in the longwave range, between 526 and 1706 kHz in the medium frequency range of the radio spectrum. AM broadcasting is permitted in shortwave bands, between about 2.3 and 26 MHz, which are used for long distance international broadcasting.
In frequency modulation the frequency of the radio signal is varied by the audio signal. FM broadcasting is permitted in the FM broadcast bands between about 65 and 108 MHz in the high frequency range; the exact frequency ranges vary somewhat in different countries. FM stereo radio stations broadcast in stereophonic sound, transmitting two sound channels representing left and right microphones. A stereo receiver contains the additional circuits and parallel signal paths to reproduce the two separate channels. A monaural receiver, in contrast, only receives a single audio channel, a combination of the left and right channels. While AM stereo transmitters and receivers exist, they have not achieved the popularity of FM stereo. Most modern radios are "AM/FM" radios, are able to receive both AM and FM radio stations, have a switch to select which band to receive. Digital audio broadcasting is an advanced radio technology which debuted in some countries in 1998 that transmits audio from terrestrial radio stations as a digital signal rather than an analog signal as AM and FM do.
Its advantages are that DAB has the potential to provide higher quality sound than FM, has greater immunity to radio noise and interference, makes better use of scarce radio spectrum bandwidth, provides advanced user features such as electronic program guide, sports commentaries, image slideshows. Its disadvantage is that it is incompatible with previous radios so that a new DAB receiver must be purchased; as of 2017, 38 countries offer DAB, with 2,100 stations serving listening areas containing 420 million people. Most countries plan an eventual switchover from FM to DAB; the United States and Canada have chosen not to implement DAB. DAB radio stations work differently from AM or FM stations: a single DAB station transmits a wide 1,500 kHz bandwidth signal that carries from 9 to 12 channels from which the listener can choose. Broadcasters can transmit a channel at a range of different bit rates, so different channels can have different audio quality. In different countries DAB stations broadcast in either Band L band.
The signal strength of radio waves decreases the farther they travel from the transmitter, so a radio station can only be received within a limited range of its transmitter. The range depends on the power of the transmitter, the sensitivity of the receiver and internal noise, as well as any geographical obstructions such as hills between transmitter and receiver. AM broadcast band radio waves travel as ground waves which follow the contour of the Earth, so AM radio stations can be reliably received at hundreds of miles distance. Due to their higher frequency, FM band radio signals cannot travel far beyond the visual horizon; however FM radio has higher fidelity. So in many countries serious music is only broadcast by FM stations, AM stations specialize in radio news, talk radio, sports. Like FM, DAB signals travel by line of sight so reception distances are