Excitiability and neuromuscular junction, Excitation-contraction coupling

Question 1

Describe the basic structure of a neuron

Answer 1

Neurons have a dendrite which is a branch like structure that extends away from the cell body. It recieves messages from neurons and allows them to travel to the cell body.
There is also the cell body.
The axon is a tube like structure that carries electrical impulses from the cell body to the axon terminals that can then pass the impulse onto the neuron next to it.
Neurons are places side by side to each other and between them there is a synapse. This is a chemical junction where chemical interactions occur,

Question 2

Define neuron

Answer 2

a neuron is a cell that sends action potentials that cause muscles to contract

Question 3

what does action potential depend on

Answer 3

depends on voltage difference across membrane of excitable tissue at rest

Question 4

define resting membrane potential

Answer 4

electrical potential difference across the membrane at rest

Question 5

why is there a negative charge inside relative to outside in a resting membrane

Answer 5

1. sodium-potassium ion exchange pump, driven by ATP hydrolysis pumps sodium out of the cell into the extracellular environment
2. This is potassium has a greater permeability which will allow a negative charge to be on the inside of the cell
3. once most of the potassium ions are expelled to the extracellular environment there is a great amount of anions in the intercellular environment causing it to have a negative charge
4. influx of chloride ions

Question 6

What is an action potential

Answer 6

An action potential is a rapid alternation in the cell membrane whereby the polarity across the cell is reversed, so that the inside is positive relative to the outside
- changes from resting state of -70mV to +40mV

Question 7

Explain how action potential occurs

Answer 7

1. Stimulus starts the rapid change in voltage or action potential in order to raise the voltage above a threshold for membrane depolarisation
2. Depolarisation occurs due to the rapid influx of sodium ions from the sodium channels into the intracellular environment. The polarity is changing inside the cell so the voltage increases
3. Repolarisation occurs due to the inactivation of sodium channels and the large efflux of potassium ions into the extracellular environment. This is done through the ATPA pump that allows for greater movement of ions across the membrane to ensure there is a repolarisation to a more negative state
4. Hyperpolarisation occurs due to lowered membrane potential compared to rest as a result of the large efflux of potassium ions
5. closing of potassium channels allows the resting membrane potential to return

Question 8

define Synaptic transmission

Answer 8

Synaptic transmission is the process in which one neuron communicated with another. Electrical impulses are sent down the axon and an action potential occurs

Question 9

Excitatory postsynaptic cells result from;

Answer 9

An excitatory postsynaptic cell results from the release of sufficient neurotransmitter to allow a post-synaptic action potential

• facilitation results from depolarisation
• inhibition results from repolarisation

Question 10

Describe the action potential in terms of wave of depolarisation

Answer 10

1. the action potential is first generated in the Axon of Hillock
2. The cell body becomes depolarised
3. depolarisation spreads down axon. There is first a repolarisation due to the inactivation of sodium channels and activation of potassium channels
4. action potential continues to travel down the axon

Question 11

Describe axonal and muscle membrane communication in terms of APs

Answer 11

Myelin is a sheath that surrounds axons and is high in lipid content. Between each myelin is the nodes of Ranvier. Axonal and muscle membrane communication happens as the actional potential jumps from one node to another node. If these nodes were not there the action potential and the voltage would weaken due to the space between each sheath and axon, causing the threshold to not be met.

Question 12

Define neuromuscular junction

Answer 12

Neuromuscular junction in the junction formed by the motor neuron and a single muscle fibre

Question 13

Describe the process of neurotransmitter communication between excitable cells

Answer 13

1. The motor neuron axon contains synaptic vesicles. The synaptic vesicle contains the neurotransmitter called ACh. The action potential reaches terminal axon, depolarises the plasma membrane of the nerve and opens the calcium channels.
2. the calcium allows the myosin binding sites to be exposed and releases the neurotransmitter (exocytosis). Excess neurotransmitter is always released at neuromuscular junction so that it can reach the threshold.
3. the ACh diffuses across the cleft and binds to its receptor in the muscle fibre membrane.
4. The sodium and potassium will pass through these channels at different rates producing a depolaristion of the motor end plate.
5. depolarisation of plasma membrane of the muscle fibre results in an action potential to each end of fibre and interior of the cel via the T-tubules.
6. ACh drops closing the ion channels and is repolarised.

Question 14

Explain the term threshold (for excitation) in relation to neuron cell bodies

Answer 14

The level that a depolarization must reach for an action potential to occur. When the action potential reaches its peak, voltage-activated Na+ gates close, but K+ ions flow outside of the membrane due to their high concentration inside the neuron as opposed to outside.

Question 15

Describe the structure of the T-tubule system

Answer 15

T-tubule membrane - muscle fibres are filled with tubules that run in a transverse direction to the cell surface. They are sections thats provide a pathway for signals deep into the muscle cell. The interior of the tubules have extracellular fluid that allows for electrical potential to make its way into the muscle cell.

Question 16

Describe the structure of the sarcoplasmic reticulum (SR)

Answer 16

the sarcoplasmic reticulum is a network of membrane bags in the muscle cells. The SR creates two spaces being the sarcoplasm and the SR interior. The SR wraps around the sarcomere and part of the the membrane touches the t-tubules. This is important as when the depolarisation wave happens it touches with the t-tubules

Question 17

Describe the location of voltage-gated Ca2+ channels

Answer 17

They are located in the DHP receptor which is in the exterior part of the t-tubules membrane

Question 18

Describe the operation of the voltage-gated Ca2+ channels

Answer 18

The voltage-gate Ca2+ channels have a role in releasing calcium in order for contractions to occur. This is through the process of t-tubules which carry action potentials. the DHP receptor senses the membrane depolarisation and alters its confirmation, activating the RyR receptor which releases the calcium from the voltage-gated channels. The calcium will then bind to the troponin and expose the myosin binding sites allowing for a contraction to occur.

Question 19

Describe the location of calcium pumps of the SR

Answer 19

Calcium pumps are found in the membrane of the sarcoplasmic reticulum

Question 20

Describe the function of calcium pump of the SR

Answer 20

The calcium pump has a function in storing calcium ions and bringing them back into the SR. For muscle contraction to occur, the calcium released will bind to the troponin. However, for muscle relaxation to occur the calcium pumps will pump the calcium back into the SR to decrease the concentration. When calcium concentration decreases the pump is turned off.

Question 21

Describe and explain the combined roles of the voltage-gated calcium channels and calcium pumps in regulating calcium concentration for force development

Answer 21

When depolarisation occurs, the calcium will be released from the volate-gated channels and the calcium pump will pump the calcium out of the SR where it will bind to tropoonin, causing the myosin binding sites to be uncovered. By the pump pumping the calcium ions out it ensures that there is a high concentration maintained.

Question 22

Describe and explain the combined roles of the voltage-gated calcium channels and calcium pumps in regulating calcium concentration for relaxation

Answer 22

As relaxation occurs the voltage-gates will be closed and the pumps will pump the calcium back into the SR and then into the voltage-gates. This is to ensure that there is a low concentration of calcium held so that no binding with troponin occurs.

Question 23

Explain events of excitation and an alpha motor neuron to muscle fibre force development and return of alpha motor neuron to resting state and muscle fibre relaxation

Answer 23

first stage

1. neuron excitation will lead to depolarisation
2. ACh will be released from the terminal ends of the alpha motor neuron
3. as it is released it will attach to the surface of the sarcolemma which allow the muscle cell to be excited
4. as a result of moving the signal deeper within the muscle it will interact with other structures of the muscle so cross bridge cycling occurs

second stage

1. neuron depolrisation occurs
2. ACh will be broken down, uptaken and resynthesised

Question 24

Describe and explain the effect of tropomyosin on the blockage or exposure of cross-bridge binding sites

Answer 24

blockage
- No calcium is bound to troponin and troponin moves tropomyosin to cover binding sites on thin filament

exposure
- calcium is bound to troponin and the troponin moves tropomyosin to uncover binding sites on thin filament

Question 25

Describe and explain the effect of tropomyosin on active force development

Answer 25

high calcium concentration
- when there is a high concentration in calcium the active force development is increased and reaches a maximun. This is as calcium is binding to troponin, moving tropomyosin allowing the myosin binding sites to be uncovered and crossbridge cycling and force development to occur

low calcium concentration
- when there is a low calcium concentration the active force development will decrease as the myosin binding sites are covered by the tropomyosin. the muscle is therefore relaxed

Question 26

Describe how Ca2+ binding and unbinding from troponin effects (i) the shape of troponin and (ii) the position of tropomyosin

Answer 26

Shape 
binded
- when binded the shape changes as the binding sites become more vacant and the troponin is moved 
unbinded 
- the shape remains the same 

Position of tropomyosin

• calcium binding to troponin causes the tropomyosin to be moved to expose the binding sites so cross-bridge cycling occurs
• the troponin pushes the tropomyosin back to cover the binding sites

Question 27

Explain the relationship between intracellular calcium concentration, {Ca2+}, and muscle contraction and relaxation

Answer 27

Contraction
Relationship: when the concentration of calcium is increased muscle contraction will occur. This is as calcium has been released out of the SR cuasing the concentration to increase and allowing cross bridge cycling to occur. This also results in active force development.
- Increased concentration  contraction

Relaxation
Relationship: When the concentration of calcium is decreased the calcium is moved back into the SR causing cross-bridge cycling to cease. Therefore, the muscle is relaxed as no cross-bridges are occurring
- Decreased concentration  relaxation