Neuromuscular Structure and Function

Question 1

what 2 ions are responsible for changes in membrane potential?

Answer 1

• Na+
• K+

Question 2

what 2 processes control resting membrane potential?

Answer 2

• diffusion
• active transport

Question 3

what has the greatest influence on membrane potential

Answer 3

active transport of intra-cellular K+ and extracellular Na+

Question 4

what is an anion?

Answer 4

a negative ion, usually set up in electrodes.

Question 5

Overall, at rest, is the inside of the cell more negative or more postive compared to the outside?

Answer 5

more negative that the outside

Question 6

what is the value of the resting membrane potential of the inside of a neurone cell in mV?

Answer 6

• -70mV

Question 7

what does pumps 3Na+ into the cell and 2K+ out called?

Answer 7

Na+,K+, -ATPase

Question 8

in all cells, what 2 things does Na+ and K+ being constantly pumped across the cell membrane maintain?

Answer 8

maintains:
- high Na+ concentration in the ECF and low Na+ concentration in the ICF

• high K+ concentration in the ICF and low K+ concentration in the ECF

Question 9

what allows for constant diffusion of Na+ into the cells?

Answer 9

Na+ channels that are always open (leaky)

Question 10

what allows for constant diffusion of K+ out of the cell?

Answer 10

K+ channels that are always open (leaky)

Question 11

what is the relationship between pumping of Na+,K+ -ATPase and diffusion of Na+ and K+ when the cell is at rest?

Answer 11

at rest, pumping of Na+,K+ -ATPase exactly equals the diffusion of Na+ and K+

Question 12

outline the ion fluxes that occur during an action potential in 10 steps

Answer 12

1. Na+/K+ -ATPase actively maintains membrane potential of -70mV when no stimulus occurs
2. IF stimulus causes AP TO RESCH THRESHOLD:

• stimulus causes some Na+ ion channels to open allowing some Na+ ions into the axon amnd membrane potential is less negative as Na+ diffuse into axon, around -55mV, but not enough to cause depolarisation

3. IF ACTION POTENTIAL CAUSED BY STIMULUS REACHES THRESHOLD:

• ALL Na+ ion channels open, so there is a rapid influx of Na+ ions

4. when membrane potential reaches +40mV, ALL Na+ channels close
5. at the same membrane potential of +40mV, ALL K+ ion channels open
6. there is rapid efflux of K+ ions, so electrical charge inside axon falls as K+ ions move out of axon, so axon membrane starts to be repolarised back to original -70mV
7. K+ ion channels remain open until membrane potential reaches -80/-90mV, which is called hyperpolarisation
8. this causes a refractory period where the axon membrane cannot be stimulated to propagate another action potential
9. the Na+/K+ -ATPase now pumps 3Na+ in and 2K+ out, allowing the membrane potential to go from hyper-polarised back to resting potential of -70mV

Question 13

what does the refractory period affect in the motor unit?

Answer 13

affects motor unit firing rate

Question 14

how do you propagate an action potential in different directions, and why would we do this?

Answer 14

• propagate action potential in different directions through external electrical stimulation
• do this to investigate if there is nerve disorders and potentially where they are located

Question 15

what is a node of Ranvier?

Answer 15

area of an axon with NO myelin AND HIGH density of Na+/K+ -ATPase

Question 16

what is the advantage/need of a myelin sheath and how does it do this?

Answer 16

increases conduction velocity by acting as an electrical insulator at places that are not nodes of Ranvier

Question 17

what is a disease that can cause a lot of demyelination?

Answer 17

multiple sclerosis

Question 18

what is conduction velocity

Answer 18

the speed of AP propagation

Question 19

what is it called when action potentials “jump” from Node of ranvier to node of ranvier?

Answer 19

saltatory conduction

Question 20

in a sensory neurone, is the direction of Action potential Afferent or efferent and why?

Answer 20

Afferent as action potential travels from efferent (distal to centre) to afferent (close to centre)

Question 21

in a motor neurone, is the direction of action potential Afferent or efferent and why?

Answer 21

in a motor neurone, direction of AP is efferent as action potential travels from afferent (close to centre), to efferent (away from centre)

Question 22

what are 2 mechanism by which motoneuron activation occurs?

Answer 22

• Ionotropic mechanism (Na+ and K+)
• Neuromodulation

Question 23

what is motoneuron activation the same as?

Answer 23

same as motor neurone recruitment

Question 24

what is ionotropic mechanism?

Answer 24

when ions generates synaptic currents to initiate an action potential

Question 25

what is **neuromodulation**?

Answer 25

the use of **G-proteins** in a **2nd messeger model** to **indirectly** open **ion channels** (eg: **Na+** or **Ca2+**)

Question 26

what does **neuronmodulation allow?**

Answer 26

allows for lowering of lowering of **activation threshold**

Question 27

what are the 2 **most important** neuromodulators in the **motor system**?

Answer 27

- **serotonin (5-HT)** - **Norepinephrine/Noradrenaline (NE)**

Question 28

**outline** how an **action potential** can travel from the **pre-synaptic** membrane to the **post-synaptic** membrane in **6 steps**

Answer 28

1. **Action potential** arrives at **axon terminal** and **depolarise** the **presynaptic membrane** 2. this causes voltage-gated **Ca2+** channels to **open**, resulting in an **influx** of **Ca2+** ions 3. the **Ca2+** ions promote **fusion** of **ACh** vesicles to the **presynaptic membrane** 4. **release** of **ACh** into the **synaptic cleft** occurs. 5. **ACh** bind to and activate **nACh** receptors, leading to **depolarisation** of the **muscle fibre** membrane via **net influx of Na+ ions** 6. this propagates an **action potential** into the **post-synaptic** membrane

Question 29

how many **ACh** molecules are required to activate **nACH** receptors?

Answer 29

**2** molecules are required

Question 30

what does the release of a **single vesicle** of **ACh** result in compared to when there is release of **several ACh vesicles**?

Answer 30

- **release of single vesicle:** results in **miniature end-plate potential** - **release of several vesicles:** results in **full end-plate potential**

Question 31

what proteins are found on the nicotinic **ACh receptor?**

Answer 31

- **a** (alpha) - **B** (beta) - 𝛄 (gamma)

Question 32

outline how **Ca2+** is released in the **sarcoplasmic reticulum** in **3 steps**

Answer 32

1. **action potential** propagates down **sarcolemma** 2. **transverse tubules (T-tubules)** conduct **Action potential** into the cell's interior 3. this **depolarises** the **sarcoplasmic reticulum**, causing it to **open** **Ca2+ release** channels

Question 33

what are **t-tubules** and where are they **situated**?

Answer 33

deep **invaginations** of the **sarcolemma** situated to the **junction** of the **A-bands** and **I-bands**

Question 34

what is the 2 function of **T-tubules**?

Answer 34

- to provide a mechanism for changes in **membrane** potential to be **communicated** to the **inners** of the **muscle fibre** and - increase **surface area** for **action potential**

Question 35

in striated muscle, how are the **t-tubules** positioned compared to the **sarcoplasmic reticulum** and what does this mean?

Answer 35

- situated in **close apposition** with **SR** - this means that when the membrane on the **t-tubule** is **depolarised**, it also triggers **Ca2+ release** from the **SR** - this therefore means that **t-tubules** and **SR** can **work together** to allow **Ca2+ concentration** to rise in the **sarcoplasm** in the area it is needed