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PHY2011: Exam Revision Questions and Answers - Cell and membrane physiology, Sensory periphery and Muscle periphery

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Physiology
Course
PHY2011
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Various
Semester
Spring

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EXAM PREPARATION CELL AND MEMBRANE PHYSIOLOGY: 1. Describe the structure of a biological membrane. What are the functions of the different components that make up the membrane? The cell membrane is a biological membrane that seperates the interior of all cells from the outside environment. The cell membrane is a selectively permeable membrane to ions and organic molecules and controls the movement of substances into and out of cells. The cell membrane is not rigid, but a fluid structure. The fluidity of the cell is due to the phospholipid molecules and proteins moving within the lateral plane of the bilayer. The cell membrane’s fluidity depends on the lipid composition, amount of cholesterol and temperature. The lipid composition and amount of cholesterol varies across the membrane, and between the two surfaces of the membrane. Therefore, the membrane surface is not uniform. There are no covalent bonds between phospholipids or between phospholipids and proteins. Due to this, the molecules can move independently, in the plane of the membrane. Therefore, there is a lot of lateral movement of the phospholipids. The cell membrane is primarily a lipid bilayer. This lipid bilayer is made up of phospholipids, where each phospholipid consists of a polar part, the head, and two long, non-polar fatty acids, the tails. The hydrophilic head faces outwards, whereas the two hydrophobic tails faces inwards towards the hydrophobic tails of the other half of the bilayer. Cholesterol is scattered throughout the membrane, between the fatty acid tails of phospholipids and positioned towards the polar head groups. Lipids are long chains of carbon and hydrogen that can be attached to another molecule (the head group). Common membrane lipids are phospholipids, sphingolipids and sterols, where they are amphipathic, which means that they consist of polar and non-polar parts. The function of being amphipathic is important, so that the membrane lipids are positioned in a fluid environment. As the polar molecules are hydrophilic and non- polar molecules are hydrophobic, lipids can self assemble in water to form structures such as micelles, which are structures with a hydrophobic interior and a hydrophilic exterior. Carbohydrates are chains of carbon, hydrogen and oxygen, which are found on the outer surface of the cell membrane. Proteins with carbohydrates attached are called glycoproteins, while phospholipids with carbohydrates attached are called glycolipids. Carbohydrates form a coat, glycocalyx, across the cell surface. The glycocalyx are involved in cell recognition and protection, while carbohydrates may hold adjoining cells together. Proteins are a long sequence of amino acids, which are folded in some particular complex ways. There are 3 types of membrane proteins – integral trans-membrane protein, integral inner/outer membrane protein and peripheral protein. Transport proteins are integral proteins, where trans-membrane proteins extend through the entire membrane. While, inner membrane proteins are located on the cytoplasmic side of the membrane, and the outer membrane proteins extends part way through the membrane, with some exposed on the outer surface. Trans- membrane proteins are channel proteins that allow substrates through from one side of the cell membrane to the other. Whereas, inner/outer membrane proteins are carrier proteins which bind a substrate, undergo a conformational change and through that change transport substrate across the membrane. On the other hand, peripheral membrane proteins adhere only temporarily to the biological membrane with which they are associated. These molecules attach to integral membrane proteins, or penetrate the peripheral regions of the lipid bilayer. 2. What structural features of a plasma membrane allow it to function as a selectively permeable barrier? The cell membrane provides a barrier for substances to move from one direction to another. Therefore, the membrane is considered to be ‘selectively permeable’ as it allows certain substances through, but not others. The cell membrane is primarily a lipid bilayer. This lipid bilayer is made up of phospholipids, where each phospholipid consists of a polar part, the head, and two long, non-polar fatty acids, the tails. The hydrophilic head faces outwards, whereas the two hydrophobic tails faces inwards towards the hydrophobic tails of the other half of the bilayer. Substances that are non-polar and lipid soluble can diffuse in and out of a plasma membrane. While, polar substances are not able to pass through the cell membrane because the polar heads will repel them. The cell membrane is also impermeable to substances that are not lipid- soluble, as they are unable to pass through the lipids of the membrane. The plasma membrane also contains transport proteins that may assist certain molecules to cross the plasma membrane. Transport proteins are integral proteins and there are two types: carrier proteins and channel proteins. Carrier proteins extend part way through the membrane, with portions exposed on the outer surface, or on the cytoplasmic side of the membrane. Their function is to bind to a substrate, undergo conformational change and through that, change transport substrate across the membrane. Whereas, channel proteins are extended through the entire membrane, where they are aqueous pores that allows a substrate through from one side of the cell membrane to the other. 3. Discuss in detail the process or processes by which each of the following molecules or ions is/are transported across the cell membrane. a) Water Osmosis is the passive diffusion of water across a membrane towards regions of higher solute concentration. Water movement occurs because of changes in the concentration of water on the two sides of the membrane, or because of changes in the concentration of solutes. As water is not able to dissolve in lipids, thus water movement across the membrane is quite slow. However, it is faster than expected due to the presence of special water channels, aquaporin. Therefore, water movement does not only move through the cell membrane, but it can also move through the special water channels. Aquaporin is an integral membrane protein, where the channel possesses a central pore, which allows water molecules to pass through, without having to dissolve in the lipid bilayer. The diffusion of water across a membrane from a region of higher concentration to a region of lower concentration. The water molecules travel through the pore of the channel in single file. The presence of water channels increases the membrane’s permeability to water. b) Small non-polar molecules such as Oxygen and Carbon Dioxide Diffusion is the process where molecules move down the concentration gradient due to collisions between molecules through random spontaneous thermal motion. Overtime, equilibrium occurs where there are equal amount of molecules on both sides, and there is no net movement in either direction. Small non-polar molecules are able to diffuse through the lipid portions of the membrane much more rapidly than depolarized/ionized molecules because non-polar molecules can dissolve in the lipids in the membrane. Since oxygen and carbon dioxide are small molecules, they are able to move through the membrane quite quickly. c) Large polar or charged molecules such as glucose and amino acids Facilitated diffusion is a type of diffusion that is facilitated/helped by cellular membrane proteins. It allows the diffusion of large, membrane in soluble compounds such as sugars and amino acids. Like other carrier-mediated transport systems, facilitated diffusion shows the properties of specificity, competition and saturation. Facilitated diffusion moves molecules from high concentration to low concentration across a membrane by a transporter without the use of energy, until equilibrium is reached. To transport a molecule, the carrier protein has to bind to it loosely through covalent bonds. The binding site has a specific shape and only molecules with the appropriate part that can fit into this binding site can be transported. And, the binding site recognizes a specific part of the transported molecule. Thus, competition occurs if another molecule has a similar shaped bit, as it can also bind to the site. The movement rate of particles will saturate eventually as the number of carrier proteins in any cell membrane at any one time is fixed. Due to the limited number of facilitating proteins, these proteins will become saturated with the substance and there will be no more proteins to facilitate further diffusion. d) Sodium ions Facilitated diffusion is a type of diffusion that is facilitated/helped by cellular membrane proteins. It allows the diffusion of large, membrane in soluble compounds such as sugars and amino acids. Like other carrier-mediated transport systems, facilitated diffusion shows the properties of specificity, competition and saturation. Facilitated diffusion moves molecules from high concentration to low concentration across a membrane by a transporter without the use of energy, until equilibrium is reached. To transport a molecule, the carrier protein has to bind to it loosely through covalent bonds. The binding site has a specific shape and only molecules with the appropriate part that can fit into this binding site can be transported. And, the binding site recognizes a specific part of the transported molecule. Thus, competition occurs if another molecule has a similar shaped bit, as it can also bind to the site. Active transport uses integral membrane proteins to transport substances against the concentration gradient, thus energy input by the cell is required. It requires energy, as it needs to overcome the concentration and electrochemical gradient when pumping specific compounds in or out of the cell. There are 3 classes of direct or primary active transport mechanisms: ion pumps, ABC system, and group translocation. Ion pumps are used to transport ions by pumping them. There are 3 different ion pumps, called P-, V- and F- ATPase ion pumps. They all have one or more binding sites for ATP on the cytosolic face of the membrane. They use the phosphorylation of the transporter by ATP to drive the transport processes. An example of a P-class ion pump is the Na/K ATPase pump in the cell membrane, which exchanges Na from one side of the membrane for K from the other side. The Na/K ATPase pump is crucial for all organs, but especially for nerve cells, and its function is to maintain intracellular Na low, and K high. The hydrolysis of ATP drives Na out of the cell, and K into the cell; 3 Na is pumped out, while 2 K is pumped in. Secondary active transport uses downhill flow of an ion to pump some other substance against its gradient. The ion that is moved down its concentration gradient using a transporter, a protein molecule. In the process, the transporter moves the other molecule, either in the same direction as the ion, or in the opposite direction. The driving ion is usually Na+ with its gradient establishes by the Na+/K+ ATPase, thus energy is required. There are two types of secondary active transport systems: antiport and symport. In an antiport, the ion and the transported species move in opposite directions. While, in an symport, the ion and the amino acid, or sugar are transported in the same direction across the membrane. 4. Discuss the following aspects of diffusion across cell membranes: a) Definition of diffusion Diffusion is the process where molecules move down their concentration gradient because of collisions between molecules through random spontaneous thermal motion, and energy is not required for this process. Overtime, equilibrium occurs where there are equal amount of molecules on both sides, and there is no net movement in either direction. Even though there is no net change in concentration, the molecules still continue to move. b) Driving forces for net diffusion There are a number of factors that influence the rate of diffusion of molecules. The factors include the concentration gradient, temperature, distance to diffuse, surface area available for diffusion, molecular mass of solute, viscosity of medium through which diffusion occurs, and the lipid solubility of the molecules. The concentration gradient determines the net flux. If there is a larger difference in concentrations between the two compartments, then it will take longer for the concentrations on the two sides to reach equilibrium through diffusion. Temperature affects the rate of diffusion, as the number of collisions depends on how active the molecules are. Temperature increases the thermal agitation of molecules, thus by increasing temperature, the rate of diffusion increases, and the speed in which equilibrium is reached also increases. Diffusion is good for rapid movement over short distances. This places a constraint on the size that cells can attain if they are to use diffusion to move substances as required. Therefore, the distance required to diffuse molecules is important. The smaller the surface, the fewer the molecules that can move from one compartment to the other. Surface area available for diffusion to occur is crucial, as a large surface area enable molecules to move across rapidly, and allows more molecules to move across. Large molecules take a longer amount of time to move due to their molecular mass. Thus, molecular mass of the solute is better if it’s small than large. Viscosity of the medium through which diffusion occurs is important, as if the medium is thick, it will take longer for the molecules to move across. Whereas, if the medium is not thick, like water, the molecules are able to move across much more efficiently. Finally, the lipid solubility of the molecule is a crucial factor. Due to the cell membrane structure, the more soluble in lipid that a substance is, the more easily it can penetrate through the lipid bilayer. Therefore, the permeability of a cell to a molecule is correlated with the lipid solubility of the molecule. c) Routes of diffusion There are two routes of diffusion: facilitated diffusion and passive diffusion. Facilitated diffusion is a type of diffusion that is facilitated/helped by cellular membrane proteins. It allows the diffusion of large, membrane in soluble compounds such as sugars and amino acids. Like other carrier-mediated transport systems, facilitated diffusion shows the properties of specificity, competition and saturation. Facilitated diffusion moves molecules from high concentration to low concentration across a membrane by a transporter without the use of energy, until equilibrium is reached. To transport a molecule, the carrier protein has to bind to it loosely through covalent bonds. The binding site has a specific shape and only molecules with the appropriate part that can fit into this binding site can be transported. And, the binding site recognizes a specific part of the transported molecule. Thus, competition occurs if another molecule has a similar shaped bit, as it can also bind to the site. The movement rate of particles will saturate eventually as the number of carrier proteins in any cell membrane at any one time is fixed. Due to the limited number of facilitating proteins, these proteins will become saturated with the substance and there will be no more proteins to facilitate further diffusion. Diffusion is the process where molecules move down their concentration gradient because of collisions between molecules through random spontaneous thermal motion, and energy is not required for this process. Overtime, equilibrium occurs where there are equal amount of molecules on both sides, and there is no net movement in either direction. Even though there is no net change in concentration, the molecules still continue to move. Osmosis is the passive diffusion of water across a membrane towards regions of higher solute concentration. Water movement occurs because of changes in the concentration of water on the two sides of the membrane, or because of changes in the concentration of solutes. As water is not able to dissolve in lipids, thus water movement across the membrane is quite slow. However, it is faster than expected due to the presence of special water channels, aquaporin. Therefore, water movement does not only move through the cell membrane, but it can also move through the special water channels. Aquaporin is an integral membrane protein, where the channel possesses a central pore, which allows water molecules to pass through, without having to dissolve in the lipid bilayer. The diffusion of water across a membrane from a region of higher concentration to a region of lower concentration. The water molecules travel through the pore of the channel in single file. The presence of water channels increases the membrane’s permeability to water. 5. Cell membranes are described as selectively permeable. a) What structural characteristics of cell membranes make them selectively permeable? The cell membrane provides a barrier for substances to move from one direction to another. Therefore, the membrane is considered to be ‘selectively permeable’ as it allows certain substances through, but not others. The cell membrane is primarily a lipid bilayer. This lipid bilayer is made up of phospholipids, where each phospholipid consists of a polar part, the head, and two long, non-polar fatty acids, the tails. The hydrophilic head faces outwards, whereas the two hydrophobic tails faces inwards towards the hydrophobic tails of the other half of the bilayer. Substances that are non-polar and lipid soluble can diffuse in and out of a plasma membrane. While, polar substances are not able to pass through the cell membrane because the polar heads will repel them. The cell membrane is also impermeable to substances that are not lipid- soluble, as they are unable to pass through the lipids of the membrane. The plasma membrane also contains transport proteins that may assist certain molecules to cross the plasma membrane. Transport proteins are integral proteins and there are two types: carrier proteins and channel proteins. Carrier proteins extend part way through the membrane, with portions exposed on the outer surface, or on the cytoplasmic side of the membrane. Their function is to bind to a substrate, undergo conformational change and through that, change transport substrate across the membrane. Whereas, channel proteins are extended through the entire membrane, where they are aqueous pores that allows a substrate through from one side of the cell membrane to the other. b) Why is it essential for cell survival and function that cell membranes are selectively permeable? The cell membrane being selectively permeable is so that it controls the passage of materials into and out of the membrane. By doing this, it helps regulate internal conditions, and maintain homeostasis. A cell needs a selectively permeable membrane in order to maintain the balance of water in the cell, and the ability to bring in nutrients, and export wastes out of the cell to prevent uptake of toxic substances. It is critical for the cell membrane to be selectively permeable so that it can create an internal environment that is different from the external environment, and so that it can facilitate chemical reactions, and develop an electrical potential. 6. Describe 3 different ways by which sodium crosses the plasma membrane. 1. Facilitated diffusion 2. Active transport – P-type ATPase ion pumps 3. Secondary active transport Facilitated diffusion is a type of diffusion that is facilitated/helped by cellular membrane proteins. It allows the diffusion of large, membrane in soluble compounds such as sugars and amino acids. Like other carrier-mediated transport systems, facilitated diffusion shows the properties of specificity, competition and saturation. Facilitated diffusion moves molecules from high concentration to low concentration across a membrane by a transporter without the use of energy, until equilibrium is reached. To transport a molecule, the carrier protein has to bind to it loosely through covalent bonds. The binding site has a specific shape and only molecules with the appropriate part that can fit into this binding site can be transported. And, the binding site recognizes a specific part of the transported molecule. Thus, competition occurs if another molecule has a similar shaped bit, as it can also bind to the site. Active transport uses integral membrane proteins to transport substances against the concentration gradient, thus energy input by the cell is required. It requires energy, as it needs to overcome the concentration and electrochemical gradient when pumping specific compounds in or out of the cell. There are 3 classes of direct or primary active transport mechanisms: ion pumps, ABC system, and group translocation. Ion pumps are used to transport ions by pumping them. There are 3 different ion pumps, called P-, V- and F- ATPase ion pumps. They all have one or more binding sites for ATP on the cytosolic face of the membrane. They use the phosphorylation of the transporter by ATP to drive the transport processes. An example of a P-class ion pump is the Na/K ATPase pump in the cell membrane, which exchanges Na from one side of the membrane for K from the other side. The Na/K ATPase pump is crucial for all organs, but especially for nerve cells, and its function is to maintain intracellular Na low, and K high. The hydrolysis of ATP drives Na out of the cell, and K into the cell; 3 Na is pumped out, while 2 K is pumped in. Secondary active transport uses downhill flow of an ion to pump some other substance against its gradient. The ion that is moved down its concentration gradient using a transporter, a protein molecule. In the process, the transporter moves the other molecule, either in the same direction as the ion, or in the opposite direction. The driving ion is usually Na+ with its gradient establishes by the Na+/K+ ATPase, thus energy is required. There are two types of secondary active transport systems: antiport and symport. In an antiport, the ion and the transported species move in opposite directions. While, in an symport, the ion and the amino acid, or sugar are transported in the same direction across the membrane. 7. Explain what is meant by the term ‘active transport’. Describe three different active transporters, naming the specific solutes transported and the functional importance of each of the transport systems. Active transport uses integral membrane proteins to transport substances against the concentration gradient, thus energy input by the cell is required. It requires energy, as it needs to overcome the concentration and electrochemical gradient when pumping specific compounds in or out of the cell. There are 3 classes of direct or primary active transport mechanisms: ion pumps, ABC system, and group translocation. Ion pumps are used to transport ions by pumping them. There are 3 different ion pumps, called P-, V- and F- ATPase ion pumps. They all have one or more binding sites for ATP on the cytosolic face of the membrane. They use the phosphorylation of the transporter by ATP to drive the transport processes. An example of a P-class ion pump is the Na/K ATPase pump in the cell membrane, which exchanges Na from one side of the membrane for K from the other side. The Na/K ATPase pump is crucial for all organs, but especially for nerve cells, and its function is to maintain intracellular Na low, and K high. The hydrolysis of ATP drives Na out of the cell, and K into the cell; 3 Na is pumped out, while 2 K is pumped in. The other 2 classes of ion pumps, the V and F classes, are similar in structure to eachother but unrelated to P-class pumps. The V and F-class pumps only transport protons (H+). The F-class pump has at least 3 kinds of trans-membrane proteins, while the V-class pump has at least 2 kinds of proteins. V-class pumps maintain the low pH of lysosomes, endosomes, golgi apparatus, and other vesicles in animal cells. They use energy released by ATP hydrolysis to pump H+ from the cytosol to the exoplasm up the proton electrochemical gradient. Whereas, the F-class proton pumps are found in bacterial plasma membranes and in the mitochondria and chloroplasts. They function to power synthesis of ATP from ADP and Pi by movement of H+ from the exoplasm to the cytoplasm down the proton elcctrochemical gradient. The ABC system involves ATP and 2 proteins to bind, transport and release the transported molecule, the ligand. The 3 proteins consist of one to bind the ligand and bring it to the second, membrane-spanning protein which will allow it to move across the membrane, and a third which hydrolyses ATP to provide energy for the ligand to move across. The ligand-binding domain is usually restricted to one type of molecule. The substrates that can be transported includes ions, amino acids, peptides and sugars. The group translocation system involves a sequence of molecules, and the transported substance is chemically modified. In this process, a substance becomes chemically altered during its transport across a membrane, so that once inside, the cytoplasmic membrane becomes impermeable to that substance and it remains within the cell. Although it is an active transport system, it does not use ATP, as the phosphate group in a molecule called PEP, provides the high energy. A phosphate group is travelled from enzyme I to the transporter, and then to glucose. The ligand then gets modified and is trapped inside the cell. The group translocation system is involved in transporting many sugars into bacteria, such as glucose. 8. In what ways do substances, other than water, cross cell membranes? Briefly describe the main characteristics of each process. Diffusion is the process where molecules move down their concentration gradient because of collisions between molecules through random spontaneous thermal motion, and energy is not required for this process. Overtime, equilibrium occurs where there are equal amount of molecules on both sides, and there is no net movement in either direction. Even though there is no net change in concentration, the molecules still continue to move. Facilitated diffusion is a type of diffusion that is facilitated/helped by cellular membrane proteins. It allows the diffusion of large, membrane in soluble compounds such as sugars and amino acids. Like other carrier-mediated transport systems, facilitated diffusion shows the properties of specificity, competition and saturation. Facilitated diffusion moves molecules from high concentration to low concentration across a membrane by a transporter without the use of energy, until equilibrium is reached. To transport a molecule, the carrier protein has to bind to it loosely through covalent bonds. The binding site has a specific shape and only molecules with the appropriate part that can fit into this binding site can be transported. And, the binding site recognizes a specific part of the transported molecule. Thus, competition occurs if another molecule has a similar shaped bit, as it can also bind to the site. Active transport uses integral membrane proteins to transport substances against the concentration gradient, thus energy input by the cell is required. It requires energy, as it needs to overcome the concentration and electrochemical gradient when pumping specific compounds in or out of the cell. There are 3 classes of direct or primary active transport mechanisms: ion pumps, ABC system, and group translocation. Ion pumps are used to transport ions by pumping them. There are 3 different ion pumps, called P-, V- and F- ATPase ion pumps. They all have one or more binding sites for ATP on the cytosolic face of the membrane. They use the phosphorylation of the transporter by ATP to drive the transport processes. An example of a P-class ion pump is the Na/K ATPase pump in the cell membrane, which exchanges Na from one side of the membrane for K from the other side. The Na/K ATPase pump is crucial for all organs, but especially for nerve cells, and its function is to maintain intracellular Na low, and K high. The hydrolysis of ATP drives Na out of the cell, and K into the cell; 3 Na is pumped out, while 2 K is pumped in. The other 2 classes of ion pumps, the V and F classes, are similar in structure to eachother but unrelated to P-class pumps. The V and F-class pumps only transport protons (H+). The F-class pump has at least 3 kinds of trans-membrane proteins, while the V-class pump has at least 2 kinds of proteins. V-class pumps maintain the low pH of lysosomes, endosomes, golgi apparatus, and other vesicles in animal cells. They use energy released by ATP hydrolysis to pump H+ from the cytosol to the exoplasm up the proton electrochemical gradient. Whereas, the F-class proton pumps are found in bacterial plasma membranes and in the mitochondria and chloroplasts. They function to power synthesis of ATP from ADP and Pi by movement of H+ from the exoplasm to the cytoplasm down the proton elcctrochemical gradient. The ABC system involves ATP and 2 proteins to bind, transport and release the transported molecule, the ligand. The 3 proteins consist of one to bind the ligand and bring it to the second, membrane-spanning protein which will allow it to move across the membrane, and a third which hydrolyses ATP to provide energy for the ligand to move across. The ligand-binding domain is usually restricted to one type of molecule. The substrates that can be transported includes ions, amino acids, peptides and sugars. The group translocation system involves a sequence of molecules, and the transported substance is chemically modified. In this process, a substance becomes chemically altered during its transport across a membrane, so that once inside, the cytoplasmic membrane becomes impermeable to that substance and it remains within the cell. Although it is an active transport system, it does not use ATP, as the phosphate group in a molecule called PEP, provides the high energy. A phosphate group is travelled from enzyme I to the transporter, and then to glucose. The ligand then gets modified and is trapped inside the cell. The group translocation system is involved in transporting many sugars into bacteria, such as glucose. Secondary active transport uses downhill flow of an ion to pump some other substance against its gradient. The ion that is moved down its concentration gradient using a transporter, a protein molecule. In the process, the transporter moves the other molecule, either in the same direction as the ion, or in the opposite direction. The driving ion is usually Na+ with its gradient establishes by the Na+/K+ ATPase, thus energy is required. There are two types of secondary active transport systems: antiport and symport. In an antiport, the ion and the transported species move in opposite directions. While, in an symport, the ion and the amino acid, or sugar are transported in the same direction across the membrane. SENSORY PERIPHERY: 1. Discuss the different types of sensory receptors found in and under the skin and describe experiments which have provided direct evidence for the kinds of sensations generated by particular types of receptors. There are different types of sensory receptors found in and under the skin, such as mechanoreceptors, proprioceptors, thermoreceptors, and nocireceptors. Mechanoreceptors respond to mechanical displacement, such as touch, pressure, sound, and muscular contractions. There are 4 main types: pacinian corpuscles, meissner’s corpuscles, merkel’s discs, and ruffini endings. There are also mechanoreceptors in hairy skin, and the hair cells in the cochlea are the most sensitive mechanoreceptors, transducing air pressure waves into nerve signals sent to the brain. Skin mechanoreceptors that contribute to proprioception are Ruffini endings and Pacinian corpuscles, particularly the former. Both are deep-lying receptors with large RFs, with the former being slowly adapting and the latter rapidly adapting receptors. The neurons appear to be activated only by stretch of the skin and this stretch is naturally associated with movement of a limb/joint. Thus, ruffini endings also contribute to proprioception. Cutaneous mechanoreceptors are located in the skin. They are all innervated by A-beta fibers, except the mechanorecepting free nerve endings, which are innervated by A-delta fibers. Furthermore, they each have a different receptive field. Ruffini’s endings detect tension deep in the skin, meissner’s corpuscles detect changes in texture and adapt rapidly, pacinian corpuscles detect rapid vibrations, merkel’s discs detect sustained touch and pressure, and hair follicle receptors are located in hair follicles and sense position changes of hairs. Proprioception covers the senses of limb position and movement, of posture, of effort, and of tension. It involves somatic receptors, visual input and, with respect to head position and movement, also involves input from the vestibular. Somatic proprioceptors provide information about body and limb position, posture, and movement. The receptors are those in muscle, in joints and in skin especially skin around joints. Normally, vision is useful particularly in determining the position of one’s limbs with respect to the position of the body. However, in people with damage to large-diameter somatosensory afferent nerve fibres because of disease, vision is critical. Without somatic proprioceptive feedback, they are unable to carry out movement with accuracy. Joint proprioceptors in joints are found in the joint capsule and in the skin around joints. These receptors are free nerve endings, plus a few Pacinian corpuscles and ruffini endings. These receptors appear to sign
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