Signal transduction

Examples of Signal transduction in the following topics:

• Two-Component Regulatory Systems

  • Two component signaling systems are widely occurring in prokaryotes whereas only a few two-component systems have been identified in eukaryotic organisms.
  • Signal transduction occurs through the transfer of phosphoryl groups from adenosine triphosphate (ATP) to a specific histidine residue in the histidine kinases (HK).
  • Two-component signal transduction systems enable bacteria to sense, respond and adapt to a wide range of environments, stressors and growth conditions.
  • These pathways have been adapted to respond to a wide variety of stimuli, including nutrients, cellular redox state, changes in osmolarity, quorum signals, antibiotics, temperature, chemoattractants, pH and more.
  • Signal transducing histidine kinases are the key elements in two-component signal transduction systems.

• Termination of the Signal Cascade

  • Signal cascades convey signals to the cell through the phosphorylation of molecules by kinases.
  • Ligand binding to the receptor allows for signal transduction through the cell.
  • The chain of events that conveys the signal through the cell is called a signaling pathway or cascade.
  • The aberrant signaling often seen in tumor cells is proof that the termination of a signal at the appropriate time can be just as important as the initiation of a signal.
  • Describe the process by which the signal cascade in cell communication is terminated

• Mechanisms of Hormone Action

  • Cellular recipients of a particular hormonal signal may be one of several cell types that reside within a number of different tissues.
  • Different tissue types may also respond differently to the same hormonal signal.
  • As a result, hormonal signaling is elaborate and hard to dissect.
  • Relay and amplification of the received hormonal signal via a signal transduction process.
  • Water-soluble hormones, such as epinephrine, bind to a cell-surface localized receptor, initiating a signaling cascade using intracellular second messengers.

• Binding Initiates a Signaling Pathway

  • This type of receptor spans the plasma membrane and performs signal transduction in which an extracellular signal is converted into an intracellular signal.
  • Another complicating element is signal integration of the pathways in which signals from two or more different cell-surface receptors merge to activate the same response in the cell.
  • The effects of extracellular signals can also be amplified by enzymatic cascades.
  • At the initiation of the signal, a single ligand binds to a single receptor.
  • However, activation of a receptor-linked enzyme can activate many copies of a component of the signaling cascade, which amplifies the signal.

• Olfaction: The Nasal Cavity and Smell

  • These proteins carry out the transduction of odorants into electrical signals for neural processing.
  • Olfactory transduction is a series of events in which odor molecules are detected by olfactory receptors.
  • These chemical signals are transformed into electrical signals and sent to the brain, where they are perceived as smells.
  • Once ligands (odorant particles) bind to specific receptors on the external surface of cilia, olfactory transduction is initiated.
  • In mammals, olfactory receptors have been shown to signal via G protein.

• Transduction and Perception

  • Transduction is the process that converts a sensory signal to an electrical signal to be processed in a specialized area in the brain.
  • The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system.
  • For example, auditory receptors transmit signals over their own dedicated system.
  • When a sound causes the stereocilia to move, mechanosensitive ion channels transduce the signal to the cochlear nerve.
  • Explain how stimuli are converted to signals that are carried to the central nervous system

• Transduction of Light

  • The rods and cones are the site of transduction of light into a neural signal.
  • Visual signals leave the cones and rods, travel to the bipolar cells, and then to ganglion cells.
  • Thus, the visual system relies on changein retinal activity, rather than the absence or presence of activity, to encode visual signals for the brain.
  • Sometimes horizontal cells carry signals from one rod or cone to other photoreceptors and to several bipolar cells.
  • The signal is passed to a G-protein called transducin, triggering a series of downstream events.

• Sensation to Perception

  • Stimuli from the environment (distal stimuli) are transformed into neural signals, which are then interpreted by the brain through a process called transduction.
  • Transduction can be likened to a bridge connecting sensation to perception.
  • The neural signals are transmitted to the brain and processed.
  • All perception involves signals in the nervous system that result from physical stimulation of the sense organs.
  • The process of perceiving speech begins at the level of the sound within the auditory signal and the process of audition.

• Reception

  • Although the sensory systems associated with these senses are very different, all share a common function: to convert a stimulus (light, sound, or the position of the body) into an electrical signal in the nervous system.
  • This process is called sensory transduction.
  • There are two broad types of cellular systems that perform sensory transduction.
  • In the second type of sensory transduction, a sensory nerve ending responds to a stimulus in the internal or external environment; this neuron constitutes the sensory receptor.

• Transduction of Sound

  • When sound waves reach the ear, the ear transduces this mechanical stimulus (pressure) into a nerve impulse (electrical signal) that the brain perceives as sound.
  • The cochlea is a whorled structure, like the shell of a snail, and it contains receptors for transduction of the mechanical wave into an electrical signal.
  • The site of transduction is in the organ of Corti (spiral organ).
  • As a result, the hair cell membrane is depolarized, and a signal is transmitted to the chochlear nerve.
  • The organ of Corti (bottom image) is the site of sound transduction.