How does a cell sense its size?

How does a cell sense its size?

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Cells come in all sorts of sizes. How do they regulate their cell size to the point where similar cell types have a fairly mono-disperse size distribution?

Reasked from

This is a question that has been the focus of study for the last century (e.g., Amodel of cell size regulation - Ycas et al, J. Theoret. Biol. (1965) 9, 444-470). Cell size regulation may be in part determined by ribosomal activity (through mTor regulation) and is a critical checkpoint in cell division.

How the cell senses its size, however, is not understood. In 2009, two reports suggested that protein gradients could be responsible for the sensing of cell size. You can read a commentary about those articles in Cell size control: governed by a spatial gradient. - Almeyda and Tyers, Dev. Cell. (2009) 17(1), 3-4:

The phenomenon of cell size homeostasis, whereby cells coordinate growth and division to maintain a uniform cell size, has been an outstanding issue in cell biology for many decades. Two recent studies in Nature in fission yeast demonstrate that a gradient of the polarity factor Pom1 is a sensor of cell length that determines the onset of Cdc2 activation and mitosis.

These articles demonstrate one way in which cells may sense their size, but most probably several other mechanisms are also in place.

What Limits Cell Size?

The primary limitation on the size to which a single cell can grow is a mathematical principle called the surface to volume ratio. As the size of a three-dimensional object grows, its volume increases more rapidly than its surface does, which causes metabolic problems for cells. Additionally, the amount of cytoplasm the nucleus can contain and the structural limitations on the cell prevent them from being larger as well.

Cells are discrete metabolic units. They must be able to take resources in and expel waste and energy. The only place a cell can do this is along the thin, skin-like membrane surrounding it. As the volume of the cell increases in size, it must acquire and expel more substances however, because the volume grows more quickly than the surface area, there is a limit to the amount of diffusion that can take place into or out of a cell.

The nucleus of a cell is essentially a small sphere within a larger sphere. Because the nucleus must become larger to control a larger cell, the nucleus is also susceptible to the problem of surface to volume ratio. This limits the size of the nucleus, which in turn, limits the size of the entire cell.

While the outer membrane of a cell protects the cell well on a microscopic level, large cells would require exceptionally thick membranes. As these membranes thicken enough to hold larger cells, they suffer from decreased permeability.

DATA TABLE: Cell Size Comparison

2. Calculate the total surface area for each cell model by the following formula:

surface area = (Length X Width) X 6 sides

Record the surface areas in the DATA TABLE.

3. Calculate the volumes for each cell model by the following formula:

volume = length X width X height

Record the volumes in the DATA TABLE.

4. Calculate the surface area-to-volume ratio for each cell model by the following formula:

ratio = surface area

Record the ratio values in the DATA TABLE.
These ratios show how many times larger the surface area is as compared to the volume. Notice that it becomes less than one very quickly.

1. Which model has the largest surface area?

2. Which model has the largest volume?

3. Which model has the largest ratio?

4. To maintain life, and carry-out cellular functions, materials must be able to move into and out of the cell. Also, material needs to be able to move within the cell. What might be the advantage of having a large surface area?

How does surface area to volume ratio limit cell size?

The surface area to volume ratio (SA:V) limits cell size because the bigger the cell gets, the less surface area it has for its size.


This is important if you are a cell that depends on diffusion through your cell wall to obtain oxygen, water, and food and get rid of carbon dioxide and waste materials.

As you get bigger, your outside is unable to keep up with needs of the inside.

We can see this with agar cubes that have been soaked in NaOH solution.

The "nutrients" have diffused all the way to the centre of the smallest cube, but the largest cube is mostly "starved" in the centre.

If you are a cell like the largest cube, your SA:V has become so small that your surface area is not large enough to supply nutrients to your insides.

At this point, you must divide into smaller cells or die.

So your size is limited by your SA:V.

How does a cell sense its size? - Biology

Each internal region of the cell has to be served by part of the cell surface. As a cell grows bigger, its internal volume enlarges and the cell membrane expands. Unfortunately, the volume increases more rapidly than does the surface area, and so the relative amount of surface area available to pass materials to a unit volume of the cell steadily decreases.

Finally, at some point, there is just enough surface available to service all the interior if it is to survive, the cell must stop growing.

Surface Area to
Volume Ration The important point is that the surface area to the volume ratio gets smaller as the cell gets larger .

Thus, if the cell grows beyond a certain limit, not enough material will be able to cross the membrane fast enough to accommodate the increased cellular volume. When this happens, the cell must divide into smaller cells with favorable surface area/volume ratios, or cease to function.

Surface Area to Volume Ratio

As the ratio gets smaller, it takes longer for items to diffuse.


When the cell increases in size, the volume increases faster than the surface area, because volume is cubed where surface area is squared.

When there is more volume and less surface area, diffusion takes longer and is less effective. This is because there is a greater area that needs to receive the substance being diffused, but less area for that substance to actually enter the cell.

this is actually why cells divide. When they become too large and it takes too long for them to transport materials across the cell, they lose efficiency and divide in half to raise the surface area to volume ratio.


The surface area to volume ratio (SA:V) limits cell size because the bigger the cell gets, the less surface area it has for its size.


This is important if you are a cell that depends on diffusion through your cell wall to obtain oxygen, water, and food and get rid of carbon dioxide and waste materials.

As you get bigger, your outside is unable to keep up with needs of the inside.

We can see this with agar cubes that have been soaked in NaOH solution.

The "nutrients" have diffused all the way to the centre of the smallest cube, but the largest cube is mostly "starved" in the centre.

If you are a cell like the largest cube, your SA:V has become so small that your surface area is not large enough to supply nutrients to your insides.

At this point, you must divide into smaller cells or die.

So your size is limited by your SA:V.


You could consider the cell to be a sphere and them just calculate it


if you know the radius caluculate it like this
with r the radius of the cell

The next question: How would you measure this radius in the first place?

Most cells are spherical in suspension. That is, when they are freely suspended in a liquid medium, they exert the same forces in all directions, thus making them spherical.

You can take a picture in a camera equipped microscope at a known magnification and use a scale bar to measure cell radius. There are also methods to automate this through image processing

Note: This rule however does not apply to plant cells (rigid cell wall), RBCs (flattened) or many bacterial cells that retain a different shape. In these cases, you can approximate the cell to be a cylinder, disc, cuboid, etc and use known formulae, or if you have access to a confocal microscope, you can get 'slices' very much like a CT scan, and you can build a 3D model of the cell from it. Calculation of surface area and volume shouldn't be difficult after this.

Chapter 06 - A Tour of the Cell

  • The cytoskeleton is a network of fibers extending throughout the cytoplasm.
  • The cytoskeleton organizes the structures and activities of the cell.

The cytoskeleton provides support, motility, and regulation.

  • The cytoskeleton provides mechanical support and maintains cell shape.
  • The cytoskeleton provides anchorage for many organelles and cytosolic enzymes.
  • The cytoskeleton is dynamic and can be dismantled in one part and reassembled in another to change the shape of the cell.
  • The cytoskeleton also plays a major role in cell motility, including changes in cell location and limited movements of parts of the cell.
  • The cytoskeleton interacts with motor proteins to produce motility.
    • Cytoskeleton elements and motor proteins work together with plasma membrane molecules to move the whole cell along fibers outside the cell.
    • Motor proteins bring about movements of cilia and flagella by gripping cytoskeletal components such as microtubules and moving them past each other.
    • The same mechanism causes muscle cells to contract.
    • Microtubule fibers are constructed of the globular protein tubulin.
    • Each tubulin molecule is a dimer consisting of two subunits.
    • A microtubule changes in length by adding or removing tubulin dimers.
    • These microtubules resist compression to the cell.
    • Before a cell divides, the centrioles replicate.
    • Many unicellular eukaryotic organisms are propelled through water by cilia and flagella.
    • Cilia or flagella can extend from cells within a tissue layer, beating to move fluid over the surface of the tissue.
      • For example, cilia lining the windpipe sweep mucus carrying trapped debris out of the lungs.
      • They are about 0.25 microns in diameter and 2–20 microns long.
      • Flagella are the same width as cilia, but 10–200 microns long.
      • A flagellum has an undulatory movement that generates force in the same direction as the flagellum’s axis.
      • Cilia move more like oars with alternating power and recovery strokes that generate force perpendicular to the cilium’s axis.
      • Both have a core of microtubules sheathed by the plasma membrane.
      • Nine doublets of microtubules are arranged in a ring around a pair at the center. This “9 + 2” pattern is found in nearly all eukaryotic cilia and flagella.
      • Flexible “wheels” of proteins connect outer doublets to each other and to the two central microtubules.
      • The outer doublets are also connected by motor proteins.
      • The cilium or flagellum is anchored in the cell by a basal body, whose structure is identical to a centriole.
      • Addition and removal of a phosphate group causes conformation changes in dynein.
      • Dynein arms alternately grab, move, and release the outer microtubules.
      • Protein cross-links limit sliding. As a result, the forces exerted by the dynein arms cause the doublets to curve, bending the cilium or flagellum.
      • Each microfilament is built as a twisted double chain of actin subunits.
      • Microfilaments can form structural networks due to their ability to branch.
      • In muscle cells, thousands of actin filaments are arranged parallel to one another.
      • Thicker filaments composed of myosin interdigitate with the thinner actin fibers.
      • Myosin molecules act as motor proteins, walking along the actin filaments to shorten the cell.
      • A contracting belt of microfilaments divides the cytoplasm of animal cells during cell division.
      • Localized contraction brought about by actin and myosin also drives amoeboid movement.
        • Pseudopodia, cellular extensions, extend and contract through the reversible assembly and contraction of actin subunits into microfilaments.
          • Microfilaments assemble into networks that convert sol to gel.
          • According to a widely accepted model, filaments near the cell’s trailing edge interact with myosin, causing contraction.
          • The contraction forces the interior fluid into the pseudopodium, where the actin network has been weakened.
          • The pseudopodium extends until the actin reassembles into a network.
          • This creates a circular flow of cytoplasm in the cell, speeding the distribution of materials within the cell.
          • Intermediate filaments are specialized for bearing tension.

          Concept 6.7 Extracellular components and connections between cells help coordinate cellular activities

          Plant cells are encased by cell walls.

          • The cell wall, found in prokaryotes, fungi, and some protists, has multiple functions.
          • In plants, the cell wall protects the cell, maintains its shape, and prevents excessive uptake of water.
          • It also supports the plant against the force of gravity.
          • The thickness and chemical composition of cell walls differs from species to species and among cell types within a plant.
          • The basic design consists of microfibrils of cellulose embedded in a matrix of proteins and other polysaccharides. This is the basic design of steel-reinforced concrete or fiberglass.
          • A mature cell wall consists of a primary cell wall, a middle lamella with sticky polysaccharides that holds cells together, and layers of secondary cell wall.
          • Plant cell walls are perforated by channels between adjacent cells called plasmodesmata.

          The extracellular matrix (ECM) of animal cells functions in support, adhesion, movement, and regulation.

          • Though lacking cell walls, animal cells do have an elaborate extracellular matrix (ECM).
          • The primary constituents of the extracellular matrix are glycoproteins, especially collagen fibers, embedded in a network of glycoprotein proteoglycans.
          • In many cells, fibronectins in the ECM connect to integrins, intrinsic membrane proteins that span the membrane and bind on their cytoplasmic side to proteins attached to microfilaments of the cytoskeleton.
            • The interconnections from the ECM to the cytoskeleton via the fibronectin-integrin link permit the integration of changes inside and outside the cell.
            • Embryonic cells migrate along specific pathways by matching the orientation of their microfilaments to the “grain” of fibers in the extracellular matrix.
            • The extracellular matrix can influence the activity of genes in the nucleus via a combination of chemical and mechanical signaling pathways.
              • This may coordinate the behavior of all the cells within a tissue.

              Intercellular junctions help integrate cells into higher levels of structure and function.

              • Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact.
              • Plant cells are perforated with plasmodesmata, channels allowing cytosol to pass between cells.
                • Water and small solutes can pass freely from cell to cell.
                • In certain circumstances, proteins and RNA can be exchanged.
                • This prevents leakage of extracellular fluid.
                • Intermediate filaments of keratin reinforce desmosomes.
                • Special membrane proteins surround these pores.
                • Ions, sugars, amino acids, and other small molecules can pass.
                • In embryos, gap junctions facilitate chemical communication during development.

                A cell is a living unit greater than the sum of its parts.

                • While the cell has many structures with specific functions, all these structures must work together.
                  • For example, macrophages use actin filaments to move and extend pseudopodia to capture their bacterial prey.
                  • Food vacuoles are digested by lysosomes, a product of the endomembrane system of ER and Golgi.

                  Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 6-1

                  Size matters: How cells pack in epithelial tissues

                  Normal cell clone arrangement in which they clump together (top, magenta) and mutant clones comprised of smaller cells aberrantly dispersed within epithelial tissue (bottom). Credit: Gibson Lab, Stowers Institute for Medical Research

                  Small-cell clones in proliferating epithelia—tissues that line all body surfaces—organize very differently than their normal-sized counterparts, according to a recent study from the Stowers Institute for Medical Research. Published online September 5, 2019, in Developmental Cell, these findings from the laboratory of Matthew Gibson, Ph.D., may contribute to a better understanding of how some human diseases progress.

                  "A common feature of many cancer types is the pleomorphic nature—or variability in size and shape—of cells in a tumor," says first author and Postdoctoral Research Associate Subramanian P. Ramanathan, Ph.D. "What's not exactly known is whether differential cell size is the driver for, or the result of, cancer progression."

                  Cells in an epithelial sheet are normally connected by Velcro-like structures called adhesive junctions and have a near-uniform size distribution. As a result, cells related by descent tend to stick together in mosaic patches referred to as cell clones. In the fruit fly Drosophila melanogaster, epithelial cell clones that have smaller cells can lose contact with each other and disperse among their neighbors.

                  The striking dispersal of small cells carrying mutations in a gene called Tor was first observed over a decade ago by Gibson when he was a postdoctoral researcher. "I puzzled over it, but just couldn't make sense of why the mutant cells dispersed within the cell layer," says Gibson. Not until 2015, when Ramanathan joined the lab, did they begin to piece together the science behind this observation.

                  Ramanathan has a background in the biomechanics of cell division. He recalls, "we were very excited about the prospect of applying my interest in single-cell mechanics in the context of how epithelial cells organize."

                  "We initially thought we were going to be studying cell division and its relationship to junction formation," says Gibson. "But based on Subramanian's experiments, we determined that the Tor cells divide and make a junction that later becomes unstable. He was able to rule out cytoskeletal or biophysical explanations, and that led to a mathematical treatment of the problem."

                  "The eureka moment was the soap bubbles experiment," says Ramanathan, citing D'Arcy Wentworth Thompson's, On Growth and Form. "About a hundred years ago, Thompson documented the similarity between how soap foam and biological tissue organize. Soap cells grow and shrink in an extremely predictable fashion. We found that the junctions shared by smaller soap cells frequently collapsed separating the small cells. This was strikingly similar to what we saw in epithelial tissue," says Ramanathan.

                  Ramanathan reasoned that the Tor cell dispersal could be a geometrical consequence. "We had to convince ourselves that this is what was happening," he admits. To explore it in the purely in silico framework, they reached out to co-author Matej Krajnc, Ph.D., from Princeton University, who has experience with these types of models. "He was excited to modulate cell size, to see if we could see similar patterns emerging in silico. And that is indeed what we saw."

                  One of the most astounding feats of developmental biology is the transformation of an epithelial sheet into a functionally specialized three-dimensional structure. To do so, epithelial systems rely on uniformity among cells. Developmental processes shape and sculpt epithelia by creating region-specific properties that are classically understood to be driven by molecular pathways.

                  "In the human body, which is primarily made up of epithelial cell layers, the vast majority of mutations we accumulate are not germline, but rather somatic, and therefore clonal in nature," says Gibson. "In a pathogenesis context, because mutations create clonal effects, we don't have a framework for thinking about how clones of cells behave when they're genetically distinct from the surrounding wild type cells, but it's extremely important." The Gibson Lab continues to build off this study and related work to understand the interaction between physically and genetically heterogeneous epithelial cell populations.

                  Agar Cell Diffusion

                  All biological cells require the transport of materials across the plasma membrane into and out of the cell. By infusing cubes of agar with a pH indicator, and then soaking the treated cubes in vinegar, you can model how diffusion occurs in cells. Then, by observing cubes of different sizes, you can discover why larger cells might need extra help to transport materials.

                  Tools and Materials

                  • Agar-agar powder
                  • Digital scale
                  • Graduated cylinder
                  • Water
                  • Whisk or fork
                  • Microwaveable bowl or container at least 500ml in volume
                  • Microwave (not shown)
                  • Hot pad or oven mitt
                  • Heat-safe surface
                  • pH indicator, such as bromothymol blue or phenolphthalein
                  • Ammonia
                  • Small glass baking pan or cube-shaped silicone ice-cube molds
                  • Clear plastic metric ruler
                  • Sharp knife
                  • Clear container for immersing agar cubes
                  • Vinegar
                  • Calculator
                  • Pencil and notepaper
                  • Spoon
                  • White paper or plate
                  • Timers


                  1. Measure out 1.6 g of agar-agar and 200 ml water. Mix them together with a whisk or fork in a large microwave-safe bowl.
                  2. Heat the solution in the microwave on high for 30 seconds. Remove to a heat-safe surface using a hot pad or oven mitts, stir, and return to the microwave for 30 seconds. Repeat this process until the mixture boils. (Keep your eye on it as it can boil over very easily!) When done, remove the container, and set it on a trivet or other heat-safe surface.
                  3. Choose ONE pH indictor to work with (either bromothymol blue or phenolphthalein) and add a few drops of it to the agar solution. ​If you’re using bromothymol blue, add enough indicator so that the mixture turns blue. If it has a greenish hue, add ammonia a drop at a time until it is blue (see photo below). If you're using phenolphthalein, add enough indicator so that the mixture turns pale pink. Add ammonia drop by drop until the mixture turns (and remains) a bright pink color (see photo below).

                  To Do and Notice

                  Place a few millileters of the pH indicator into a small container (either bromothymol blue or phenolphthalein). Using a dropper, add a few drops of vinegar. What do you notice?

                  As an acid, vinegar has a large number of hydrogen ions. When the hydrogen ions come into contact with the pH indicator, the solution changes color.

                  Fill a clear container with vinegar to a 3-cm depth. Place one agar cube of each size in the vinegar, making sure the blocks are submerged. The untreated blocks (one of each size) will be used for comparison. What do you think will happen to each cube?

                  Determine the surface area and volume of each cube. To find the surface area, multiply the length of a side of the cube by the width of a side of the cube. This will give you the area of one face of the cube. Multiply this number by 6 (the number of faces on a cube) to determine the total surface area. To find the volume, multiply the length of the cube by its width by its height. Then determine the surface-area-to-volume ratios by dividing the surface area by the volume for each cube.

                  How will you know if hydrogen ions are moving into the cube? How long do you think it will take the hydrogen ions to diffuse fully into each of the cubes? Why? How would you be able to tell when the vinegar has fully penetrated the cube?

                  After 5 minutes, remove the cubes from the vinegar with a plastic spoon, and place them on white paper or on a white plate. Compare the treated cubes to the untreated cubes and observe any color changes.

                  How much vinegar has been absorbed by each treated cube? One way to measure this is to calculate the percentage of the volume of the cube that has been penetrated by the vinegar. (Hint: It may be easier to first consider the volume that has not been penetrated by the vinegar—the portion that has not yet changed color.) Do you want to adjust any of your predictions for the diffusion times? What are your new predictions?

                  Carefully return all of the treated cubes to the vinegar. Continue checking the vinegar-soaked cubes every 5 minutes by removing them to determine the percentage of the cube that has been penetrated by the vinegar. Continue this process until the vinegar has fully penetrated the cubes. Make a note of the time when this occurs.

                  What do you notice about the percentage of penetration for each of the cubes at the different time intervals? What relationships do you notice between surface area, volume, surface-area-to-volume ratio, and percentage penetration? What does this say about diffusion as an object gets larger?

                  What’s Going On?

                  Biological cells can only survive if materials can move in and out of them. In this Snack, you used cubes of agar to visualize how diffusion changes depending on the size of the object taking up the material.

                  Diffusion occurs when molecules in an area of higher concentration move to an area of lower concentration. As hydrogen ions from the vinegar move into the agar cube, the color of the cube changes allowing you to see how far they have diffused. While random molecular motion will cause individual molecules and ions to continue moving back and forth between the cube and the vinegar solution, the overall concentrations will remain in equilibrium, with equal concentrations inside and outside the agar cube.

                  How did you find the percentage of the cube that was penetrated by the hydrogen ions at the various time intervals? One way to do this is to start with the volume of the cube that has not been penetrated—in other words, the part in the center that has not yet changed color. To determine the volume of this inner cube, measure the length of this inner cube and multiply it by the width and height. Subtract this from the original volume of the cube and you obtain the volume of the cube that has been penetrated. By dividing this number by the original volume and multiplying by 100%, you can determine the percentage penetration for each cube.

                  You may have noticed that the bigger the vinegar-soaked cube gets, the time it takes for additional vinegar to diffuse into the cube also increases—but not in a linear fashion. In other words, if the cube dimensions are doubled, the time it takes for the hydrogen ions to completely diffuse in more than doubles. When you triple the size, the time to diffuse MUCH more than triples. Why would this happen?

                  As the size of an object increases, the volume also increases, but by more than you might think. For example, when the cube doubles from a length of 1 cm to a length of 2 cm, the surface area increase by a factor of four, going from 6 cm 2 (1 cm x 1 cm x 6 sides) to 24 cm 2 (2 cm x 2 cm x 6 sides). The volume, though, increases by a factor of eight, increasing from 1 cm 3 (1cm x 1 cm x 1 cm) to 8 cm 3 (2 cm x 2 cm x 2 cm).

                  Because the volume is increasing at a greater factor than the surface area, the surface-area-to-volume ratio decreases. As the cube size increases, the surface-area-to-volume ratio decreases (click to enlarge the table below). The vinegar can only enter the cube through its surface, so as that ratio decreases, the time it takes for diffusion to occur throughout the whole volume increases significantly.

                  Anything that comes into a cell (such as oxygen and food) or goes out of it (such as waste) must travel across the cell membrane. As cells grow larger, the ratio of surface area to volume decreases dramatically, just like in your agar cubes. Larger cells must still transport materials across their membranes, but have a larger volume to supply and a proportionately smaller surface area through which to do so.

                  Bacterial cells are fairly small and have a comparatively larger surface-area-to-volume ratio. Eukaryotic cells, such as those in plants and animals, are much larger, but have additional structures to help them conduct the required amount of transport across membranes. A series of membrane-bound structures continuous with the plasma membrane, such as the endoplasmic reticulum, provide additional surface area inside the cell, allowing sufficient transport to occur. Even with these strategies, though, there are upper limits to cell size.

                  Going Further

                  While this Snack investigates how the size of an agar cube impacts diffusion, the shape of each cube remains consistent. Biological cells, however, come in different shapes. To see how different shapes of “cells” affect diffusion rates, try various shapes of agar solids. Ice-cube molds can be found in spherical and rod shapes in addition to cubes. How does the shape impact the surface-area-to-volume ratios?

                  Teaching Tips

                  This Snack fits well into a series of investigations on osmosis and diffusion. The Naked Egg Snack will allow students to explore how concentration gradients power movement of materials into and out of cells. The Cellular Soap Opera Snack will help students consider the types of materials that move through cell membranes.

                  To help students better understand the concepts of surface area, volume, and surface-area-to-volume ratio, have them build models with plastic centimeter cubes. Physical models can help make these ideas more concrete. Students can also graph class data to better understand the mathematical relationships involved.

                  If there’s not enough time within a class period for the largest cubes to be fully penetrated by the hydrogen ions present in the vinegar, students can make note of the percentage of the cube that has been penetrated by the vinegar and use that data to extrapolate a result. Alternatively, students in the following period may be able to note the time for the previous class.

                  Agar-agar comes as a powder and can be purchased online or at markets featuring Asian foods. Unflavored gelatin can be used as a substitute, but is more difficult to handle. To make cubes from gelatin, add boiling water (25% less than the amount recommended on the package) to the gelatin powder, stir, and refrigerate overnight. You may need to experiment with the ratio of water to gelatin to achieve the perfect consistency.

                  Cabbage juice can be used as an inexpensive alternative to commercial pH indicator solutions. To make cabbage juice indicator, pour boiling water over chopped red cabbage and let it sit for 10 minutes. Strain out the cabbage, and use the remaining purple water to mix with the agar powder.

                  3. Merkel Cells

                  Merkel cells are also transducers of light touch, responding to the texture and shape of objects indenting the skin. Unlike Pacinian and Meissner corpuscles, they do not adapt rapidly to a sustained stimulus that is, they continue to generate nerve impulses so long as the stimulus remains.

                  Merkel cells are found in the skin often close to hairs. They mediate excellent two-point discrimination In the rat, light movement of a hair triggers a generator potential in a Merkel cell. If this reaches threshold, an influx of Ca ++ ions through voltage-gated calcium channels generate action potentials in the Merkel cell. These cause the release of neurotransmitters at the synapse with its A&beta sensory neuron. (This neuron may also have its own mechanically-gated ion channels able to directly generate action potentials more rapidly than Merkel cells can.)