Molecular Biology of the Cell - Volume 3 - Alberts B., Bray D., Lewis J., Raff M., Roberts K., Watson J. 1994

From Cells to Multicellular Organisms
The Nervous System
Sensory Information Processing

We have previously seen how Nerve Cells conduct, process, and register electrical signals, and then send them to Muscles to trigger contraction. But where do these signals come from? There are two primary types of sources: spontaneous excitation and sensory stimuli. Some Neurons are spontaneously active, such as the brain neurons that set the breathing rhythm; a remarkably complex pattern of spontaneous activity can be generated in a single Cell through appropriate combinations of ion channel types, similar to those we encountered when discussing the mechanisms of information Processing by neurons. The reception of sensory information is likewise based on principles already familiar to us, though it involves cells of highly diverse and striking types.

Sense Organs must meet exceptionally rigorous demands: they must distinguish between Different types of stimuli with high precision, perceive changes in stimulus intensity across a phenomenally wide range, and exhibit a sensitivity as high as the laws of physics permit. The olfactory receptor cell of a male moth (*Lymantria dispar*) can detect a single molecule of a specific sex attractant (known as a pheromone) released by a female miles away from the male's Location. The human eye operates effectively in both bright sunlight and on a starlit night when illumination is 10¹² times weaker, with as few as five photons absorbed by the retina being perceived as a flash of light.

We will focus our attention on two sense organs in which the cellular mechanisms of sensory Transduction are beginning to become clear: the vertebrate auditory and visual systems. Each of these two gateways to The Nervous system contains highly specialized sensory cells that are very different from one another, yet both exhibit extraordinary selectivity and sensitivity across a broad spectrum of stimuli. Before diving into the details, however, it is helpful to review a few General Principles.

19.6.1. The Magnitude of the Receptor Potential Reflects Stimulus Intensity [38, 39]

Any signal received by the nervous system must first be converted into an electrical one. The conversion of a signal from one form into another is called transduction, which is why all sensory cells function as transducers. In a broader sense, nearly every neuron acts as a transducer: upon receiving chemical signals at synapses, it converts them into electrical ones. Although some sensory cells respond to light, others to Temperature, still others to specific chemical substances, and others to mechanical force or displacement, The process of transduction in all these cells is grounded on a set of fundamental principles that we already examined when discussing neurotransmitter-mediated synaptic transmission. In some sense organs, the transducer forms part of an impulse-conducting sensory neuron, whereas in others it is part of a specialized sensory cell that is adapted for signal transduction but does not participate in long-distance communication; instead, such a cell transmits its signals to an associated neuron via a synapse (Fig. 19-44). In both cases, however, the application of an external stimulus induces an electrical shift in the transducer cell—known as a receptor potential—which is analogous to a postsynaptic potential and ultimately serves to regulate neurotransmitter release from another region of The Cell.

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Fig. 19-44. Various modes of transmitting sensory stimulation to the nervous system. In some cases, the sensory transducer is part of a nerve cell (top diagram); in other cases, it consists of separate sensory cells (bottom two diagrams). In all three instances, the graded receptor potential generated in the sensory transducer is converted into a frequency of impulses that rapidly convey the signal to the CNS.

Just as at a synapse, external stimuli can influence the electrical state of a cell both directly—by acting on Ion Channels—and indirectly—via receptor molecules that trigger the synthesis of an intracellular messenger, which in turn acts upon ion channels. It is believed (though not yet definitively proven) that auditory sensory cells utilize a direct mechanism involving channel-linked receptors, whereas visual sensory cells employ an indirect pathway through G-protein-linked receptors.

19.6.2. Hair Cells of the Inner ear Respond to Stereocilia Deflection [40]

The ear is designed for more than just Hearing: the inner ear also perceives information about the direction of gravity and accelerated motion, making it essential for maintaining balance and motor coordination. Mechanoreception underlies all sensory Functions of the ear, specifically the detection of minute displacements in the medium surrounding the sensory cells. In the case of sound waves, these displacements take the form of rapid vibrations, whereas the perception of acceleration or gravity involves slower, smoother movements. All inner ear cells responsible for Various Forms of mechanoreception share a characteristic architecture: the apical surface of such a cell bears a bundle of giant microvilli known as stereocilia (Fig. 19-45; see also Section 11.6.10). For this reason, these cells are referred to as Hair cells.

In higher vertebrates, all hair cells reside within the epithelium of the membranous labyrinth of the inner ear, where they form discrete clusters, or patches. The hair cells in each cluster are anchored in place by intervening supporting cells, and are capped by a layer of gelatinous Extracellular matrix connected to the tips of the stereocilia (Fig. 19-46). When this matrix layer is displaced, the stereocilia tilt, and the resulting mechanical deformation of the hair cells generates a receptor potential (Fig. 19-47). The specific functions of different hair cell groups are determined primarily by The Nature of the surrounding structures through which mechanical forces are transmitted. In the case of hair cells that respond to linear acceleration and gravity, the overlying matrix is pulled downward by dense calcium carbonate crystals: during HEAD movement or tilting, the matrix shifts relative to the hair cells, causing the stereocilia to bend. By contrast, hair cells sensitive to rotational acceleration are arranged such that when the head turns, the overlying matrix is subjected to a shearing force caused by fluid flow within the semicircular canals of the inner ear.

Fig. 19-45. (A) Micrograph of a sensory hair cell isolated from the inner ear of a bullfrog, showing the bundle of stereocilia atop the cell. (B) Low-magnification transmission electron micrograph of a hair cell in its natural environment among supporting cells. (A.J. Hudspeth, Science, 230:745-752, 1985. Copyright 1985 by the AAAS.)

Fig. 19-46. Schematic cross-section of the auditory organ (organ of Corti) within the mammalian inner ear. Auditory hair cells are housed within complex structures formed by supporting cells, with the tectorial membrane (an extracellular matrix layer) arching over them. Inner hair cells are thought to be primarily responsible for hearing, owing to the specialized transduction mechanism described in the text. These cells form synapses with neurons that transmit auditory signals from the ear to the brain. By contrast, outer hair cells are densely innervated by a separate set of efferent axons conveying signals from the brain; the function of these cells remains enigmatic. Evidence suggests that outer hair cells can somehow act as reverse transducers, forming part of a feedback system that regulates the mechanical stimulation of inner hair cells.

The hair cells that detect sound in The Mammalian Ear constitute the most complexly organized system (Fig. 19-46). Auditory hair cells are arranged in rows along the basilar membrane—a narrow, long, elastic partition separating two fluid-filled spiral channels running parallel in a specialized region of the inner ear known as the cochlea. Sound waves cause the tympanic membrane to vibrate, and via the tiny BONES OF THE Middle ear, this vibration is transmitted to the fluid within the cochlear canals and subsequently to the basilar membrane, whose oscillations cause the stereocilia of the auditory hair cells to tilt. Thanks to the specialized Organization OF THE cochlea—whose different regions resonate preferentially depending on the frequency of the sound waves—the pattern of activated hair cells provides information about pitch.

Fig. 19-47. Diagram illustrating how the movement of the overlying extracellular matrix layer (tectorial membrane) deflects the stereocilia of auditory hair cells in the mammalian inner ear. The stereocilia behave as rigid rods with a "hinged" attachment at their base. The tips of the stereociliary bundles mechanically interact with the overlying matrix, either through direct contact or via the viscous fluid filling the gap between them.

19.6.3. Deflection of Stereocilia Bundles Opens Mechanically Gated Cation Channels at Their Tips [40, 41]

When the matrix layer overlying a cluster of hair cells is abruptly displaced, the stereocilia deflect by a few degrees: this alters the permeability of The Plasma Membrane and generates an inward current known as the receptor current (Fig. 19-48). The response reaches a plateau within 100–500 microseconds, which is comparable to the time required to open acetylcholine-gated cation channels at the Neuromuscular Junction, but much faster than the electrical changes produced by any known non-channel-linked receptors. It therefore seems highly probable that the mechanical stimulus directly opens an ion channel. As experiments involving altered extracellular ion concentrations have demonstrated, the mechanically gated ion channel—much like the Acetylcholine Receptor—is nearly equally permeable to all small cations, and the current passing through it is carried predominantly by potassium ions. (The ionic environment inside the ear is somewhat unusual, establishing a large K⁺ electrochemical gradient across the hair cell membrane.) But in which part of the hair cell are these channels located, and how does signal transduction relate to the cell's intricate Structure?

On each hair cell, the stereocilia are arranged in dense rows of graduated height (resembling the pipes of an organ) (see Figs. 19-45 and 19-49). Positioned just behind the midpoint of the tallest row is often a true cilium, or kinocilium, which is invariably present during development even though it does not participate in signal transduction and sometimes disappears later (as in mammalian auditory hair cells). When stereocilia are deflected using a microprobe, they behave as elastic rods bundled together; each rod pivots around its attachment point at the hair cell surface and slides relative to its neighbors, causing the tips of the stereocilia to shift relative to one another. As revealed by Electron Microscopy, In addition to the lateral links that bind the stereocilia into a bundle, fine filaments extend more or less vertically from the tip of each shorter stereocilium to a higher attachment point on the adjacent taller stereocilium (Fig. 19-49). Microelectrode experiments have shown that maximal depolarization of the hair cell membrane occurs when the stereocilia are tilted in the direction that maximally stretches these fine vertical filaments. It appears that the transmembrane current generated by stereocilia deflection—which leads to the receptor potential—enters the hair cell near the tips of the stereocilia. Thus, the entire structure is engineered so that the ion channel at the tip of the stereocilium is opened by mechanical tension resulting from the deflection of the stereociliary bundle (Fig. 19-50).

Fig. 19-48. Recordings of receptor currents (left) entering the hair cells of the bullfrog inner ear in response to sudden deflection of the stereocilia bundles. The greater the deflection of the stereocilia, the larger the current. (Based on D.P. Corey and A.J. Hudspeth, J. Neurosci. 3:962-976, 1983.)

Fig. 19-49. (A) Scanning electron micrograph of a mammalian auditory hair cell, showing fine filaments extending from the tips of shorter stereocilia to taller stereocilia of the next row. (B) Transmission electron micrograph showing the same structures, with arrows indicating the filaments. (B, micrograph by M.P. Osborne; A and B reproduced with permission from J.D. Pickles, Prog. Neurobiol. 24:1-42, 1985. Copyright 1985, Pergamon Press plc.)

The Mechanism of hair cells is remarkably sensitive: the weakest sounds we can hear stretch the vertical filaments attached to the tops of the stereocilia by an average of 0,04 nm, which is half the diameter of a hydrogen atom. An Analysis of the receptor current indicates that there are probably one to five mechanically gated channels located within the stereocilia. Each human auditory hair cell bears about a hundred stereocilia, and there are approximately 3500 such cells in a single ear, meaning that our ability to hear is mediated by fewer than 4 million transducer molecules.

19.6.4. Photoreceptors have high sensitivity and are capable of adaptation, but respond relatively slowly [42]

The sensitivity of photoreceptors in vertebrate eyes approaches the ultimate limit imposed by the quantum nature of light. Moreover, the perceptual range is extraordinarily wide, spanning from the maximum tolerable brightness down to barely perceptible illumination. Yet, compared to auditory transducers, the response speed of photoreceptors is very low. Under optimal conditions, the "fastest" photoreceptor in the human eye takes about 25 ms to reach the peak of its electrical response to a flash of light, which is more than 100 times the response time of a typical hair cell. This relative sluggishness of the photoreceptor response is likely due to fundamental constraints inherent in the mechanism of visual transduction.

Fig. 19-50. Diagram of the hypothesized mechanism by which tilting a hair cell's stereocilia opens an ion channel. Fine mechanical measurements—supported by electrical recordings from a single hair cell when its stereociliary bundle is deflected by a flexible Glass probe—demonstrate that mechanically gated channels indeed respond to an applied force. This method shows that the force required to open a single hypothetical channel is roughly equal to 2∙10^-13 newtons, causing its "gate" to displace by a distance of about 4 nm.

19.6.5. The receptor potential generated in a rod results from the closure of sodium channels [43]

As discussed in Chapter 17 (Section 17.2.2), vertebrate eyes contain Two Types of photoreceptor cells. Cones are responsible for Color Vision and the perception of fine detail, requiring relatively high light levels. Rods mediate monochromatic vision in dim light and are capable of producing a measurable electrical response to a single photon (Fig. 19-51). Although the Mechanisms of action in rods and cones appear similar, rods have been studied in much greater detail.

A rod (Fig. 19-52) consists of an outer segment containing the light-capturing apparatus, an inner segment packed with Mitochondria, a nuclear region, and (at the Base of the cell) a synaptic body that forms contacts with retinal nerve cells (see Fig. 17-6). Surprisingly, in the dark, the cell is heavily depolarized; this depolarization keeps the voltage-gated calcium channels of the synaptic body open, and the inward flux of Ca²⁺ ions drives the continuous release of neurotransmitter. The depolarization is maintained by open sodium channels in the plasma membrane of the outer segment. When light strikes the cell, these channels close, so the receptor potential manifests as a hyperpolarization that reduces the influx of Ca²⁺ and decreases The rate of neurotransmitter release (Fig. 19-52). Because the neurotransmitter exerts an inhibitory effect on many postsynaptic neurons, illumination disinhibits these neurons, thereby exciting them. The rate of neurotransmitter release by photoreceptors varies in proportion to light intensity: the brighter the light, the greater the hyperpolarization and the more strongly transmitter release is suppressed. Under very dim Background illumination, when the cell is in its most sensitive, "dark-adapted" state, the absorption of a single photon reduces the sodium influx by a million or more Na⁺ ions, producing a hyperpolarization of ~1 mV.

Fig. 19-51. Electrical response of a rod to single photons. A. Photomicrograph illustrating the experimental setup. A small piece of toad retina is dissected, and the outer segment of a single rod is drawn into the tip of a glass micropipette, which then serves as an electrode to record the current flowing across the rod membrane. B. Current traces recorded during a series of weak light flashes; the number of photons absorbed by the cell during each flash varies stochastically, but is always an integer. The tall peaks on the graph most frequently correspond to the absorption of one or two photons, whereas many flashes elicit no response because no photons were absorbed. (D. A. Baylor et al., J. Physiol., 288, 589–611, 1979.)

Fig. 19-52. Response of a rod to illumination. Photons are absorbed stochastically by rhodopsin molecules located in the outer segment. This leads to the closure of sodium channels in the plasma membrane and a decrease in the rate of neurotransmitter release from the synaptic body.

19.6.6. Photons alter the conformation of rhodopsin molecules [43, 44]

How is light initially perceived by the cell, and What is the chain of events leading to the closure of sodium channels? The outer segment, where the key steps of light transduction take place, is a cylinder containing about a thousand densely packed discs (see Fig. 17-7). Each disc is formed by a closed, vesicle-like membrane containing light-sensitive rhodopsin molecules at a packing density of roughly 10⁵ per 1 µm². A rhodopsin molecule consists of the transmembrane glycoprotein opsin (348 amino acid residues long) covalently linked to its prosthetic group, 11-cis-retinal, which absorbs the light. Upon photon absorption, 11-cis-retinal isomerizes almost instantaneously to all-trans-retinal, changing its shape and triggering a slower conformational change in the opsin protein. All these events take place within about 1 ms. Then, after roughly 1 min, all-trans-retinal dissociates from opsin via the Hydrolysis of their linking bond and diffuses into the Cytosol, where it is eventually converted back to the 11-cis form; the latter recombines with opsin, thus regenerating the light-sensitive rhodopsin molecule. It is this early light-induced conformational change in rhodopsin that ultimately drives the closure of sodium channels. However, because rhodopsin resides within the discs—some distance away from the channels—an intracellular "messenger" is required to couple these events.

19.6.7. Light-induced reduction of cyclic GMP levels in the Cytoplasm of photoreceptor cells leads to the closure of plasma membrane sodium channels [43, 44, 45]

When a rod captures light, changes occur in the intracellular concentrations of both Ca²⁺ and cGMP, making either molecule a potential intracellular messenger. Using patch-clamp techniques (Section 4.2.3), it has been established that the primary signal is a drop in the cytosolic concentration of cyclic GMP. In a decisive experiment, a small patch of outer segment membrane was drawn into a microelectrode, exposing its intracellular face to the bathing medium (Fig. 19-53). When cyclic GMP was introduced into the solution perfusing the membrane patch while an Electrochemical Potential difference was applied across the membrane, an Na⁺ current appeared; upon the removal of cyclic GMP, the current ceased despite the high concentration of Ca²⁺. Thus, cGMP opens sodium channels, whereas light, by causing a decrease in cGMP concentration, closes them. Typically, the effects of Cyclic NUCLEOTIDES are mediated through the phosphorylation of specific Proteins by activated protein Kinases (Section 12.3.14); in rods, however, cGMP acts directly on sodium channels to keep them open. But how does the light-induced conformational change in rhodopsin lower the cGMP concentration in the rod cytosol?

Fig. 19-53. Diagram of the experiment demonstrating that cytosolic cGMP directly controls the opening and closing of ion channels in the rod plasma membrane. The relatively slow increase and decrease in current upon the addition and removal of cGMP are due to the time required to change the COMPOSITION OF THE bathing solution. The intrinsic response speed of the channels to cGMP is too fast to be resolved by this method. (After E. E. Fesenko, S. S. Kolesnikov, and A. L. Lyubarsky, Nature, 313, 310–313, 1985.)

Fig. 19-54. The enzymatic cascade leading to the generation of a receptor potential following the absorption of a single photon by a dark-adapted rod. Diverging arrows indicate the Amplification steps.

The absorption of a single photon by a single rhodopsin molecule triggers the hydrolysis of numerous cGMP molecules. This signal amplification is achieved through an enzymatic cascade. A single activated rhodopsin molecule catalyzes the Activation of a G protein called Transducin at a very high rate—approximately 1000 molecules per second. Transducin is homologous to the Gs protein (Section 12.3.4), which functionally couples receptors to adenylyl cyclase (and rhodopsin itself is homologous to such receptors; see Section 12.3.12). However, activated transducin does not interact with adenylyl cyclase; instead, it activates cGMP phosphodiesterase, which specifically hydrolyzes cGMP at a rate of about 4000 molecules per second, rapidly depleting cGMP levels. As a result of this entire cascade, which takes about a second, more than 10⁵ cGMP molecules are hydrolyzed per absorbed quantum of light, leading to the transient closure of 250 sodium channels (Fig. 19-54).

19.6.8. Photoreceptors adapt to light intensity [46]

To restore a photoreceptor to its resting state following light excitation, each reaction in the light-initiated enzymatic cascade must be "counterbalanced" by a corresponding inactivation step. Light appears to accelerate both the activating and inactivating reactions, but the latter effect is slightly delayed; consequently, light elicits an immediate positive response that subsequently decays very rapidly. This delayed inactivation not only helps ensure a brief response to a short flash of light, but also enables photoreceptor adaptation: continuous light, rather than simply driving the cell into saturation with a near-zero cGMP concentration, produces two opposing effects that largely cancel each other out, allowing the cell to remain responsive to subsequent changes in illumination.

The light-induced drop in intracellular Ca²⁺ concentration appears to play a critical role in both terminating the flash response and driving adaptation. If The change in Ca²⁺ concentration is artificially delayed by introducing a Ca²⁺ buffer into the photoreceptor, the electrical response to a light flash is prolonged, and the cell adapts too slowly to steady illumination. If the photoreceptor is placed in a solution that completely blocks Ca²⁺ movement across the plasma membrane, these effects become even more pronounced, and adaptation fails entirely. Under normal conditions, the channels that permit sodium ions to enter the photoreceptor outer segment are also permeable to other cations, including Ca²⁺ to some degree. By closing these channels, light halts the calcium influx, while Ca²⁺ extrusion (mediated by a Na⁺/Ca²⁺ exchanger in the rod plasma membrane) continues, causing the intracellular calcium concentration to fall.

This decrease is thought to accelerate the enzymatic reactions (particularly the synthesis of cGMP by guanylyl cyclase) that counteract the light-induced drop in cGMP, thereby facilitating cellular adaptation.

19.6.9. Neurons process the initial information provided by sensory receptor cells [47]

A massive stream of information enters the nervous system via sensory receptor cells. The brain must process this information and isolate meaningful elements: picking words out of a chaotic wall of sound, distinguishing a face amid a mosaic of light and dark patches, and so on. This constitutes the Second Stage of sensory information processing—a much more subtle and complex level of processing than that which occurs within receptor cells. This second stage involves "computations" performed by an intricately interwoven network of neurons, where each neuron typically receives multiple signals, comprising both excitatory and inhibitory inputs. Each neuron generates an output signal that carries information about the presence or absence of specific features in the raw data supplied by the receptor cells. For example, distinct groups of cells within the visual centers of the brain generate action potentials when the eye perceives a straight line oriented in a specific way within space. The output signals from one group are then received by other neurons carrying out the next step of the process, and so on, progressing to increasingly higher levels of perception—all the way up to the recognition of such subtle and complex entities as intelligible words and facial expressions.

This type of information processing requires a remarkably sophisticated organization of anatomical connections among nerve cells. The details of the relationship between the Anatomy of the Nervous System and Higher Nervous Activity lie beyond The Scope of a book on cell biology. Nevertheless, What are the fundamental mechanisms responsible for creating such complex yet orderly anatomical structures? The next section of this chapter will be devoted to exploring this question.

Conclusion

Specialized transducers convert sensory stimuli into electrical signals. For instance, in vertebrates, the hair cells of the inner ear function as mechanoreceptors: the apical surface of each hair cell bears a bundle of stereocilia (giant microvilli), and the deflection of these bundles opens ion channels, leading to a shift in Membrane Potential. The membrane potential of photoreceptor cells in the vertebrate eye changes upon the absorption of light by rhodopsin molecules contained within these cells. In both cases, the electrical signal generated in the sensory cell—initially in the form of a receptor potential—is transmitted to neighboring neurons via chemical synapses. However, these two classes of receptor cells employ different "strategies" to generate receptor potentials: one relies on receptor molecules directly coupled to ion channels, whereas the other depends on receptor molecules that are not directly coupled to channels. In hair cells, physical tension among the stereocilia generates mechanical forces that act directly on ion channels in the plasma membrane, causing them to open or close rapidly. In retinal rods, light-activated rhodopsin molecules trigger an enzymatic cascade that hydrolyzes cyclic GMP in the cytosol, which in turn leads to the closure of sodium channels in the plasma membrane. Although a mechanism based on catalytic reactions cannot be exceptionally fast, it enables detection of a single photon.