8/20 bioelectricity and muscle excitation

Question 1

what is the ionic basis of resting membane potentials in cells

Answer 1

The Vm is based on the relative conc. of ions inside and outside of the cell, and the permeability of the cell to the ions. The most important ion for Vm by far is potassium. Potassium conc. is high inside the cell, and want to diffuse out. the membrane is permeable to K and allows this diffusion while not allowing any neg. charge ions out. this means that an imbalance develops where the positive and negative charges are separated by the plasma membrane, much like a capacitor.

Question 2

What is the reatonship between ion current, membrane potenial and conductance

Answer 2

Ion current of potassium out of the cell while not allowing the conductance of a negative ion results in the membrane potential of the cell by building up a charge seperation accross the membrane

Question 3

Diagram and explain the passsive forces acting on ions moving through voltage gated channels.

Answer 3

There are two forces: Con. gradient, and Electro Gradient. In both cases the ions want to move down the gradient form high to low or from positive to negative potential.

Question 4

Diagram\explain how both active and passive transport of sodium and potasssium acts tomaintain resing membrane potential.

Answer 4

in a real cell both potassiium and sodium can diffuse accross the membrane through ion channels. Therefore in order to maintain the conc. gradients that result in the normal resting membrane potentials the Na-K pump must actively pump ions accross the membrane other wise inner K would drop inner Na would rise and the Vm would depolarize

Question 5

Diagram and compare the ionic basis of action potntials in heart skeletal muscel and nerve

Answer 5

In the heart pacemaker cells spontaneously fire action potentials. Skeletal muscle action potentials are initiated by an electrical signal from motor neurons. They all start due to an influx of Na!

Question 6

Explain and diagram the response of ventricular action potential to reductions in Na, K and Ca currents.

Answer 6

reduced Na current would lead to a lack of action potential initiation, and a lack of AP; a lack of Ca would lead to a smaller peak and a much smaller refratory period. A smaller K flow would lead to an inability to repolarize the cell.

Question 7

Explain and draw the general mechanism of action potential propagation.

Answer 7

Local circuit currents develop near the leading edge of the AP in a cell due to the difference in Vm. This can happen in a single cell or accross gap junctions. The local circuit currents depolarize the membrane infront of the AP until the Na channal threshold is reached. (the refractory nature of the Na channel prevents the currents from activating AP in the wrong direction

Question 8

list the factors that will decrease action potential conduction veocity.

Answer 8

1. Reduce sodium current
   a. local anesthetics like lidocaine
   b. inactivation of Na channels
2. close gap junstions (heart)
3. Decrease cell diameter (increase electrical resistance)

Question 9

describe the major strutural components of cardiac muscle that are invovlved in excitaion-contraction coupling.

Answer 9

innervation not necessary, the cells are smaller and mechanically/electrically connected (gap junctions, and conneccted tissue weave, intercalated disk). Similar structure to skeletal: Transverse Tublules; sarcoplamic reticulum; sarcolemma; I-band/A-band/Z-line etc.
the sarcolemmal calcium channel, the SR calcium release channel.

Question 10

Predict how individually increasing and deceasing calcium current and SERCA in heart muscle affects the cacium transent in hert muscle.

Answer 10

increasing the Ca current would increase tension to a saturation level. Losing SERCA would cause the cell to loose the ability to relax from contraction

Question 11

Contrast and diagram the mechanism of excitation-contaction coupling in heart and skeletal muscles.

Answer 11

Skeletal muscle: each cell is individually stimulated from the motor neuron. stimulating more and more is recruitment.
Cardiac cells: not individually innervated, they are mechanically and electrically connected to each other. Innervated by the autonomic nervous system (epinephrine and norepinephrine increase the strenght of contractions by up ca rlease from SR.

Question 12

Diagram the temporal relationship between membrane potential, calcium and contraction in heart and skeletal muscle.

Answer 12

Cardiac: AP first, Ca transient soon after, but not as high, muscle tension a little delayed after that. AP curve has a long refractory period due to the calcium flux. then re-pol
skeletal: AP first, then Ca transient soon after and then muscle contraction. The AP is sharper in skeletal muscles.

Question 13

Vm

Answer 13

milli volts – all cells have a neg. potential (higher concentration of positive ions)

Question 14

Nernst Equaiton

Answer 14

shows the relationship between the inner and outer conc. of an ion and the resulting resting potential accross the membrane. this is due largly to potassium in most cells and so can be seen as Vk=60Log[K]o/[K]i. Cells will not quite follow this because of the slight inward leak of Na that slightly depolarizes the cell

Question 15

What is an inward ionic current

Answer 15

the net depolarization of the cell: caused by the in flow of positive charge or the outflow of negative charge. This flow is given a negative sign (convention is possitive out)

Question 16

what is the relationship between the change in Vm and the ionic current

Answer 16

The chage in Vm is dirrectly proportional to the sum of the inward and outward ionic current => net out means polarization/net in is de-polarization.

Question 17

what making the Vm less negative, more negative, and negative beyond the normal Vm

Answer 17

depolarization, repolarizaiton, and hyperpoloization

Question 18

describe the major strutural components of Skeletal muscle that are invovlved in excitaion-contraction coupling.

Answer 18

AP travel via neurons to the muscle (voluntary control). AP travels to the Transverse tubules, then to the sarcoplasmic reticulum which releases Ca that causes the contraction. One muscle fiber and the neuron that inervates it is called a motor unit, and each motor unit is excited individually.

Question 19

What is excitation-contraction coupling?

Answer 19

the sequence of events that leads from action potential to contraction.

Question 20

what are the key events of excitation-contraction coupling in cardiac/skeletal

Answer 20

AP propagates down the cell. The Vm depolarizes to the Na threshold, activates AP to depolarize Vm which then reaches about -40 and the sarcolemmal calcium channel opens (L-type channels) to allow Ca into the cell. The Ca binds to the SR and opens the SR calcium release channel (RyRs). Ca flows into the cytoplasm. This activates the contractile protiens and leads to a contraction. Most Ca now pumped back into SR via SERCA. Cell relaxes.

Question 21

How is calcium transient related to muscle tension

Answer 21

the higher the calcium transient the higher the tension. This relationship saturates however as you approach 100% of the tension

Question 22

how is the excitation-contraction coupling for skeletal muscles.

Answer 22

the T-tubules are mechanically connected to the SR ca release channal (mechanical coupling mechanism). The cardiac has calcium induced calcium release.

Question 23

what is an action potential

Answer 23

a transient change in Vm caused by ion movement across the cell membrane amplitude of about 50mV to 130 mV

Question 24

how is the AP initiated by Na channels?

Answer 24

the channels are voltage gated, when threshold is met they open to allow Na flow; depolarize cell, and then inactivate for a refractory period to prevent AP flowing backward.

Question 25

what is the main determinate of membrane potential

Answer 25

the ratio of potassium in and out of the cell: Raising K outside will depolarize the resting membrane potential while lowering K outside will hyperpolarize the resting membrane potential.

Question 26

Summarize the main purpose of the Na-K pump

Answer 26

To prevent a net loss of K due to ion channel flow, and to prevent the small inward leak of Na ions from producing excessive intracellular accumulations of Na that would depolarize the cell.