Cell and muscle function Flashcards
The most important ion for cardiac resting membrane potential (CRMP) is?
A Cl
B K
C Na
D Ca
B
Explanation
As in other excitable tissue, changes in the external K concentration affect the resting membrane potential of cardiac muscle, whereas changes in the external Na concentration affect the magnitude of the action potential. At rest, following the activity of the NA/KATPase pump (to help create a RMP), the voltage gated channel of potassium plays the bigger role in achieving the RMP of cells
Extra:
Sodium and potassium are the most important for establishing the resting membrane in excitable tissues. The resting membrane potential is established by diffusion potential of the ions/electrolytes (see explanation below), membrane permeability to the ions/electrolytes and electrogenic nature of the Na/K+ pump (i.e. that it exchanges 3 sodium ion for 2 K+ ions and thus creating a negative charge inside the cell). Potassium contributes more to the resting membrane potential then does sodium because of its significantly increased diffusion potential (see explanation below) and increased membrane permeability. Calcium only contributes to the development of the action potential in cardiac tissue not to the resting membrane potential (i.e. the membrane potential at rest or between action potentials) An explanation of diffusion potential -suppose you have a cell membrane is permeable to potassium ions but not to any other ions. Potassium would tend to diffuse outward because of the higher concentration inside the cell, however as it diffuses outward it creates an increasing negative charge inside the cell to the point where the charge difference is sufficient to prevent net diffusion out of the cell. The Nernst equation can be used to calculate the membrane potential that will prevent net diffusion in either direction of a particular ion. The membrane potential that will prevent further diffusion across a membrane for potassium is -94mv and +61mv for sodium (these numbers can also be described as Nernst potentials). Note that the “Nernst potential for potassium is closer to resting membrane potential than sodium)
Calmodulin is involved in which of the following?
A Skeletal muscle relaxation
B Skeletal muscle contraction
C Smooth muscle relaxation
D Smooth muscle contraction
D
Explanation
Calmodulin is also involved in synaptic function, protein synthesis, activating calcium channels and t cells, and activating phosphorylase
In smooth muscle, Ca binds to calmodulin, and the resulting complex activates calmodulin-dependent myosin light chain kinase. This enzyme catalyses the phosphorylation of the myosin light chain in serine at position 19, increasing its ATPase activity, leading to smooth muscle contraction.
Relaxation of the smooth muscle presumably occurs when the Ca-calmodulin complex finally dissociates or when some other mechanism comes into play.
Extra: Calmodulin (CaM) (an abbreviation for CALcium-MODULated proteIN) is a calcium-binding messenger protein expressed in all eukaryotic cells. CaM is a multifunctional intermediate messenger protein that transduces calcium signals by binding calcium ions and then modifying its interactions with various target proteins Function CaM mediates many crucial processes such as inflammation, metabolism, apoptosis, smooth muscle contraction
Regarding resting membrane potential (RMP), which of the following statements is correct?
A Hypercalcaemia increases the excitability of nerve and muscle cells
B During the action potential, Na exits the cell and K enters the cell
C Hyperkalemia decreases the resting membrane potential
D A decreased extracellular Na concentration decreases the resting membrane potential
C
Explanation
A membrane potential results from separation of positive and negative ions charges across a cell membrane. There is a potential difference between the excess of positive charges outside the cell and excess negative charges inside the cell. In neurons, the RMP is usually about -70mV inside the cell. 70mv is the voltage; the negative sign only refers to inside of the cell.
With hyperkalaemia, the RMP moves closer to the threshold for eliciting an action potential, thus the neuron becomes more excitable. The RMP will decrease i.e. become less than 70mv and move closer to 0mv. (There is less movement of positive ions moving out of the cell across the membrane).
The negative value confuses the issue. Again, it only reflects the net charge across the membrane and has nothing to do with the voltage (mV). The potential difference across the cell decrease as well
The firing level is -55mv (threshold- the voltage where there will be an action potential-all or nothing), the overshoot is +35mv, and chloride influx will not restore the RMP, potassium efflux restores the RMP
A decrease in extracellular Ca (hypoCa) concentration increases the excitability of nerve and muscle cells by decreasing the amount of depolarization necessary to initiate the changes in the Na and K conductance that produce the action potential. Hyper Ca stabilizes the membrane by decreasing excitability. Too much calcium causes the threshold potential to shift away from the RMP. Normally the is RMP -90mv and threshold -60mV, hyperCa will make the threshold -50mv. Now there is a difference of 40mV that the cell has to account for in order to depolarise it (as opposed to a difference of 30 in normal calcium levels). Calcium concentration in the ECF has a profound effect on the voltage level at which the sodium channels become activated.
With regard to bradykinin, which of the following statements is correct?
A It is not related to pain or pain sensation
B It contracts smooth vascular muscle
C Its name is derived from its action e.g. It decreases heart rate and causes hypotension
D It contracts visceral muscle
D
Explanation
Bradykinin is a potent endothelium-dependent vasodilato which leads to hypotension, causes contraction of non-vascular smooth muscle, increases vascular permeability and also is involved in the mechanism of pain. Bradykinin also causes natriuresis, contributing to an even more drop in blood pressure. There is an acceleration of the heart rate
Regarding smooth muscle, initiation of contraction is due to which of the following mechanisms?
A Ca influx
B K influx
C Na influx
D Cl influx
A
Explanation
Smooth muscle generally has a poorly developed sarcoplasmic reticulum, and the increase in intracellular Ca concentration that initiated contraction is due primarily to Ca influx from the extracellular fluid (ECF) via voltage gated and ligand gated Ca channels
Regarding type A nerve fibres, which of the following is correct?
A Delta is to proprioceptive fibres
B Alpha is to motor muscle spindles
C Gamma is to motor muscle spindles
D Beta is to motor muscle spindles
C
Explanation
Fibre type:
A-Beta fibres: touch, pressure A-Alpha fibres: proprioception, somatic motorA-Gamma fibres: motor to muscle spindles
A-Delta fibres: pain and temperature
Other fibres:
B: preganglionic autonomic
C: post ganglionic sympathetic and dorsal root pain temperature
Regarding the cellular features of cardiac muscle, which of the following statements is correct?
A Gap junctions resist the flow of ions
B Its intercalated discs are loosely attached
C Z bands are present in skeletal muscle but not in cardiac muscle striations
D Cardiac muscle resembles skeletal muscle
D
Explanation
Cardiac muscle striations are similar to those in skeletal muscle and Z lines are present
Along the side of the muscle fibres, next to the disks, the cell membranes of adjacent fibres fuse for considerable distances forming gap junctions. These gap junctions provide low resistance bridges for the spread of excitation from one muscle to another.
At the Z lines there are intercalated discs which provide a strong union between fibres, maintaining cell to cell cohesion.
In contracting skeletal muscle
A Z zone increases
B The A zone increases
C The H zone increases
D The I zone decreases
D
Explanation
I band: the pale band containing the z disk plus the areas actin actin filaments not overlapping with myosin filaments (becomes shorter during contraction as overlap increases) A band: the dark band which extends the length of the myosin filament (i.e. never changes length) H zone: the area of myosin filaments not overlapping with actin filaments (becomes shorter during contraction as overlap increases) M line: dark staining line where myosin filaments are anchored to one another in the centre of the sarcomere Z disk: network of protein fibres forming an attachment site for each actin myofilament
With regard to Na - K ATPase, which of the following statements is correct?
A Lies on the extracellular fluid (ECF) side of the membrane
B Consists of an alpha, beta and gamma sub-unit
C Extrudes 3 Na+ out from the cell for every 2 K+ into the cell
D Hydrolyses ADP to ATP
C
Explanation
Na/K ATPase hydrolyses ATP to ADP and uses the energy to extrude 3 Na from the cell and 2K into the cell for each molecule of ATP hydrolysed. It is an electrogenic pump. It is found in all parts of the body. It activity is inhibited by ouabain and related digitalis used in the treatment of CCF. It consists of only two subunits, an alpha unit and a beta unit. Both subunits extend through the cell membrane. Only the alpha unit transports sodium and potassium. The alpha and beta units are heterogenous, with alpha 1,2 and 3 subunits and beta 1,2 and 3 subunits.
Alpha 1=most cells, alpha 2= muscle, heart, adipose and brain, alpha3= heart and brain
Beta 1= widely distributed but is absent in astrocytes, vestibular cells of inner ear and glycolytic fast twitch muscle, beta2= fast twitch muscles
With regard to the action potential of a neuron with a resting membrane potential (RMP) of -70mV, which of the following statements is correct?
A Increasing the extracellular potassium ion concentration will decrease the RMP
B Chloride influx will restore the membrane potential
C The firing level is likely to be -30mV
D The overshoot action potential will not extend much past 0mV
A
Explanation
A membrane potential results from separation of positive and negative ions charges across a cell membrane. There is a potential difference between the excess of positive charges outside the cell and excess negative charges inside the cell. In neurons, the RMP is usually about -70mV inside the cell. 70mv is the voltage; the negative sign only refers to inside of the cell.
With hyperkalaemia, the RMP moves closer to the threshold for eliciting an action potential, thus the neuron becomes more excitable. The RMP will decrease i.e. become less than 70mv and move closer to 0mv. (There is less movement of positive ions moving out of the cell across the membrane).
The negative value confuses the issue. Again, it only reflects the net charge across the membrane and has nothing to do with the voltage (mV). The potential difference across the cell decrease as well
The firing level is -55mv (threshold- the voltage where there will be an action potential-all or nothing), the overshoot is +35mv, and chloride influx will not restore the RMP, potassium efflux restores the RMP
Extra:
I do not think that the current explanation of hyperkaleamia is correct.
It is correct to say that the RMP moves closer to the threshold and thus more excitable, but to say that it increases RMP, I think, is wrong
According to Hyperkalaemia revisited:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1413606/
It writes: A potassium level increase in the extracellular space, the magnitude of the concentration gradient for potassium across the myocyte decreases diminishes, thus DECREASING the RMP
Which of the following statements in relation to skeletal muscle is correct?
A Tropomysin is made up of 3 sub-units
B Myosin is contained entirely within the A band
C The immediate energy source for contraction is GTP
D Troponin T inhibits the interaction of myosin with actin
B
Explanation
The immediate source of energy is ATP.
Troponin has three subunits: I, C, T. These three subunits have a molecular weight ranging from 18000 to 35000.
I subunit inhibits interaction, C subunit binds caclium, T subunit binds tropomyosin
Troponin I inhibits the interaction of myosin with actin. In the resting state Troponin I covers actin and tropomyosin covers the site where myosin head binds to actin. This troponin-tropomyosin complex constitutes a relaxing protein that inhibits the binding of actin and myosin.
The A band contains thick filaments which are made up of myosin
Which of the following is a special feature of the contraction of smooth muscle?
A The membrane potential is unstable
B ATP is not the energy source
C Actin is not involved
D Myosin is not involved
A
Explanation
Visceral muscle is characterized by the instability of its membrane potential and by the fact that it shows continuous, irregular contractions that are independent of its nerve supply. Calcium, actin and myosin are all involved. ATP is required for energy. In smooth muscle calcium binds to calmodulin and the resulting complex activates calmodulin dependant myosin light chain kinase. The phosphorylation increases ATP activity. This is in contrast to cardiac and skeletal where contraction is triggered by Ca binding to troponin C.
With respect to the cardiac action potential of cardiac muscle
A Action potential duration is 20ms
B The resting membrane potential is -90mV
C Unlike nerve action potential there is no overshoot
D Duration of depolarization is 1ms
B
Explanation
Cardiac muscle cell (non pacemaker) resting membrane potential is -90MV, depolarization lasts about 2ms. The plateau phase and repolarization lasts 200ms or more. The initial rapid depolarization and the overshoot (phase 0) are due to the opening of voltage-gated Na channels. Duration of the cardiac action potential is between 200-400ms.
NOTE: In the current textbook there are two references to cardiac muscle cell resting membrane potential. The first says a RMP of -80mv and the second -90mv. The second is found in the cardiac chapters so I would stick with it. Cardiac pacemaker potential does not have a RMP.
The latest edition of the prescribed textbook reads: RMP of cardiac muscle cell is about -90mV
Which of the following occurs on stretching intestinal smooth muscle?
A Relaxation occurs
B It depolarises
C It hyperpolarises
D The tension is due to elastic forces only
B
Explanation
Visceral smooth muscle is unique in that, unlike other types of muscle, it contracts when stretched in the absence of any extrinsic innervation. Stretch is followed by a decline in membrane potential, an increase in the frequency of spikes and a general increase in tone
Upon skeletal muscle contraction
A The A and I zone increase
B The A zone decreases
C The H zone increases
D The I zone decreases
D
Explanation
The I band is the thin filament only (i.e. doesn’t include anything that is covered by thick filament) The H band is thick filament only (doesn’t include anything covered by thin filament.) There is no name for the area where both are covered. Therefore both the I and H band decrease in contraction. Nothing increases. The A band is all of the thick filament and never changes size.
An old question, but worth keeping
Type II muscle fibres have the following characteristics except?
A Glycolytic capacity= high
B Calcium pumping capacity of sarcoplasmic reticulum=High
C Myosin isoenzyme ATPase rate= fast
D Oxidative capacity= high
D
Explanation
Type II muscle fibres- Fast; glycolytic; white.
Myosin isoenzyme ATPase rate= fast
Calcium pumping capacity of sarcoplasmic reticulum=High
Diameter= large
Glycolytic capacity= high
Oxidative capacity= Low
Extra:
Ganong separates Type II muscle fibers in to 2 groups:
IIa - Fast, Oxidative, Glycolytic (FOG) –> Red muscle –> Moderate oxidative capacity –> Fast, fatigue resistant (FR)
IIb - Fast, Glycolytic (FG) –> White muscle –> Low oxidative capacity –> Fast, fatigable (FF) Other characteristics as above.
Relative to the Slow, Oxidative (SO) Type I fibers they both have a lower oxidative capacity, but not none.
Which type of intercellular communication is not mediated by messengers in the ECF?
A Direct cell to cell communication
B Endocrine communication
C Paracrine communication
D Neural communication
A
Explanation
Three general types of intracellular communication are mediated by messengers in the ECF: neural communication- neurotransmitters are released at synaptic junctions from nerve cells and act across a narrow synaptic cleft on a post synaptic cell, paracrine communication- the products of cells diffuse in the ECF to affect neighbouring cells that may be some distance away and endocrine communication- hormones and growth factors reach cells via circulating blood
Note: Cell to cell -gap junction communication does occur via messengers but without entering the ECF
Upon Skeletal muscle contraction
A The A and I zone increase
B The A zone decreases
C The H zone increases
D The I zone decreases
D
Explanation
An old question The H Zone decreases/shortens The I zone decreases/shortens The A zone remains constant The A and I zone decreases/shortens Extra:The I band is the thin filament only (i.e. doesn’t include anything that is covered by thick filament) The H band is thick filament only (doesn’t include anything covered by thin filament.) There is no name for the area where both are covered. Therefore both the I and H band decrease in contraction. Nothing increases. The A band is all of the thick filament and never changes size.
Which is true regarding smooth muscle contraction?
A Calcium + calmodulin +calmodulin dependant myosin light chain kinase
B Calcium + troponin C + myosin and actin
C Calcium + troponin C + calmodulin dependant myosin light chain kinase
D Calcium + calmodulin+ myosin and actin
A
Explanation
Visceral muscle is characterized by the instability of its membrane potential and by the fact that it shows continuous, irregular contractions that are independent of its nerve supply. Calcium, actin and myosin are all involved. ATP is required for energy. In smooth muscle calcium binds to calmodulin and the resulting complex activates calmodulin dependant myosin light chain kinase. The phosphorylation increases ATP activity. This is in contrast to cardiac and skeletal muscle where contraction is triggered by calcium binding to troponin C.
Skeletal muscle involves: release of calcium from the sarcoplasmic reticulum, calcium binds to troponin C, uncovering myosin-binding sites on actin. This results in formation of cross linkages between actin and myosin and sliding of thick on thin filaments, producing movements.
Which is true regarding resting membrane potential RMP of a neuron?
A Increased sodium influx hyperpolarises the RMP
B The all or nothing rule of an action potential begins at the start of depolarisation
C Increased potassium efflux makes the RMP more negative
D The RMP of a neuron is -90mv
C
Explanation
The RMP of a neuron is -70MV. I.e. there is a voltage of 70mv across the membrane with a greater positive ion concentration outside the cell as compared to inside the cell. At rest the electrical force (like repelling like) and the chemical forces (flow form high concentration to low) are in equilibrium. During a stimulus there is opening of the voltage gated NA ion channels, this lead to an influx of positive ions. The RMP depolarises (becomes more positive) and reaches a threshold (all or nothing) of -55mV at which an action potential occurs. Eventually the influx of Na will stabilize (when the electrical and chemical forces are equal), the channel will close. Then the delayed potassium (K) channel opens and there is an efflux of K out of the cell. This removes excess positive ions inside the cell and brings the membrane potential back to its RMP of -70mv (makes the RMP more negative). It actually overcorrects (hyperpolarises) the RMP first and then returns it to -70mv.
Regarding nerve fibre type and functions, which of the following nerve fibre supplies motor to muscle spindles?
A A gamma
B A delta
C A alpha
D A beta
A
Explanation
This question can be presented as an EMQ.
Fibre type:
A-Beta fibres: touch, pressure A-Alpha fibres: proprioception, somatic motor A-Gamma fibres: motor to muscle spindles
A-Delta fibres: pain and temperature
Other fibres:
B: preganglionic autonomic
C: post ganglionic sympathetic and dorsal root pain temperature
Regarding nerve fibre type and functions, which of the following nerve fibre is most susceptible to hypoxia?
A C
B B
CA
B
Explanation
Most susceptible to least susceptible. This question can present as an EMQ
Hypoxia:
B>A>C
Pressure
A>B>C
Local anaesthetics
C>B>A
Note: Katzung slightly contradicts Ganong on LA block susceptibility, stating B=C fibres
