Week 04 Lect. 2 - Skeletal Muscle + NMJ

Question 1

What are two characteristics which make neuromuscular junctions special?

Think of how neuromuscular AP conduction differs from purely neural APs … and how skeletal muscle differs from other muscle types in how it is stimulated.

Answer 1

1. One AP of an alpha motor neuron always results in an AP of the skeletal muscle (and thus contraction)
2. There is no contraction of skeletal muscle without a motor neuron AP

Question 2

What is a motor unit?

Answer 2

An alpha motor neuron and all skeletal muscle fibers it innervates.

Question 3

What is the relationship between muscle control precision and muscle fiber-to-neuron ratio in motor units?

Answer 3

Higher precision requires fewer muscle fibers innervated per neuron

Question 4

What special synaptic cleft structure houses enzymes for neurotransmitter “recycling”?

What enzymes?

Answer 4

• a protein matrix in the synaptic cleft holds acetylcholinesterase enzymes for breakdown of ACh

Question 5

After ACh transmits a neuronal AP to skeletal muscle, how is it “recycled”?

Answer 5

1. ACh is released from Nicotinic ACh Receptors on the skeletal muscle membrane
2. ACh-esterase breaks down ACh into Acetate and Choline
3. Choline is reuptaken into the neuron via an Na⁺-Choline Cotransporter (acetate diffuses away)
4. ACh is reformed by transfering an acetyl group from Acetyl-CoA to choline via Choline Acetyltransferase (ChAT)
5. Reformed ACh is secondary-active transported into synaptic vesicles via an H⁺-ACh Antiporter driven by a proton gradient from an H⁺-ATPase

Question 6

How high of a concentration of ACh can be found in synaptic vesicles?

Answer 6

150 mM

Question 7

What happens to acetate that has been removed from choline by ACh-esterase in the synaptic cleft?

Answer 7

it diffuses away

Question 8

What receptors are present on skeletal muscle for the transmission of neuronal APs at a neuromuscular junction?

Describe its structure.

How do they effect ion transport to the muscle cell?

Answer 8

Muscle-type Nicotinic ACh Receptors

• pentamers of 2 alpha and 1 beta, gamma and delta sub-units
• alpha units with 4 transmembrane regions
• non-selective ligand-gated ion channels which practically only transport Na⁺

Question 9

What inhibits the nACh receptor?

Medical aspects?

Answer 9

Curare (= d-tubocurarine)
- non-depolarizing muscle relaxant
_{does not depolarize motor-end plate = comp. antagonist for ACh}

• succinylcholine: depolarizing muscle relaxant
  _{depolarizes motor-end plate, not degraded by AChE, first ACh action, then causes desensitization → m. fiber no longer stim. by ACh}
• alpha-bungarotoxin: irreversible binds to nAChR

Question 10

How is the nACh receptor selective for only positive ions?

Answer 10

• the pore formed by its subunits has a narrow, negatively charged area which repels anions

Question 11

What inhibitor affects release of ACh from alpha-motor neurons?

Medical aspect?

Answer 11

Botulinum toxin

used in cosmetics (“botox”)

Question 12

What are some inhibitors of ACh-esterase?

Medical aspects?

Answer 12

• Physostigmine, Neostigmine
  used for treatment of myasthenia gravis _{(blocking antibody against nAChR, causes serious m. weakness)}
• Organophosphorous Insecticides
• DFP
• Chemical weapons (Sarin, Tabun)
  all cause twitches, spasms, paralysis, parasymp. effects in CNS

Question 13

What inhibits choline reuptake?

Answer 13

Hemicholine

Question 14

Describe the structure of skeletal muscle cless

Answer 14

• diameter: 10 - 100μm
• length: - 40cm
• multinucleated
• each sk. m. fiber (= cell) innervated by its own neuron (but one α-motor neuron innervates more than 1 sk. m. fiber)

Question 15

What are the elements of the “triad” which allows for propagation of APs along skeletal muscle membranes?

Answer 15

• one T-tubule - a transverse invagination of the plasma membrane into the muscle fiber
• two L-tubules + their terminal cisternae - longitudinal extensions of the sarcoplasmic reticulum and bulbous terminals adjacent to T-tubules

Question 16

What two receptors regulate calcium release in skeletal muscle?

And where are they located?

Answer 16

1. DHP (dihydropiridine) receptor - a special voltage-dependent Ca⁺⁺ channel, located on the T-tubule membrane
2. Ryanodine Receptor - releases Ca++ from SR to cytoplasm, located on the terminal cistern membrane

Question 17

Describe the sequence of receptor-related events at a triad that results in increase [Ca⁺⁺]_IC?

Answer 17

1. AP depolarization in the T-tubule changes DHP receptor conformation
2. Ryanodine + DHP receptors mechanically couple
3. Calcium is released from the SR into the cytoplasm via the Ryanodine receptor

Question 18

Which mechanisms prevents a Ca²⁺ depletion of the SR?

Answer 18

SERCA (Ca²⁺ ATPase) actively pumps Ca²⁺ back into SR

Question 19

Where can calsequestrin be found in a skeletal muscle cell?

What is its function?

Answer 19

Ca²⁺ binding protein in terminal cisterns of SR

⇒ Ca²⁺ bound to calsequestrin does not effect the free [Ca²⁺] gradient, hence easier C^a2+ recycling back into SR by SERCA

Question 20

How does extracellular calcium concentration effect skeletal muscle contraction and why?

Answer 20

It does not effect it…

…because of high sarcoplasmic reticulum concentration of Ca⁺⁺…

…skeletal muscle can contract independent of EC calcium.

Question 21

Describe the structure of a sarcomere.

What are their names and what makes them up?

Answer 21

• A-band - dark (anisotropic) band, contains entire length of thick filament
  
  • H-band/zone - lighter region in middle of A-band, thick filaments not superimposed by thin filaments
  • M-line - dark region in middle of H-band, cross-connected cytoskeletal elements
• I-band - light (isotropic) band surrounding Z-lines, thin filaments not superimposed by thick filaments
• Z line - dark lines in the middle of I-bands, alpha-actinin fibers anchoring actin filaments
• titin - holds the thick filament to the Z line

Question 22

Describe the structure of the thick filament

Answer 22

myosin

• 2 pairs of light chains, 1 pair of heavy chains, which together form 2 “heads” which bond to actin
• angle between head and neck region changes → movement

Question 23

What are the proteins related to the thin filament of skeletal muscle?

Answer 23

• actin - main component of thin filaments, interacts w/ myosin for contraction
• tropomyosin - at rest, it blocks the myosin­-binding sites on actin → has to move for contraction to occur
• troponin Complex - complex of 3 proteins
  
  • troponin T: attaches the troponin complex to tropomyosin
  • troponin I (I for inhibition) inhibits the interaction of actin and myosin by blocking the binding site
  • Troponin C: if ↑[Ca²⁺]_IC, then calcium bonds here, causing conformational change that moves tropomyosin out of the way so myosin can bond to actin
• nebulin - regulates thin filament length

Question 24

How does calcium regulate actin-myosin binding?

Answer 24

1. Calcium binds to Troponin C, inducing a conformation change
2. troponin T allows Tropomyosin to move from myosin binding sites on actin
3. Myosin binding can now occur

Question 25

What are the steps of the cross-bridge cycle starting from the "resting state"?

Answer 25

Myosin head is _bound to **ADP and Phosphate**_ in the resting/cocked state 1. **Cross-bridge** forms + myosin head binds to new binding site on actin 2. _Phosphate is released_ and _myosin head changes conformation_ creating a **power stroke** in which filaments slide past each other 3. _ADP is released_ and the myosin head remains attached 4. _ATP rebinds myosin_, releasing it from actin 5. _ATP is hydrolzyed_, and ADP + P bound myosin head is again resting/cocked

Question 26

How do you call the state where the myosin head remains attached to the actin filament?

Answer 26

**rigor** - bc no longer ATP provided, hence myosin head cannot dissociate _{(If the muscle is severely lacking ATP, as in the case of recent death, this causes the stiff muscles in rigor mortis.)}

Question 27

How do twitch durations differ between skeletal and smooth muscle?

Answer 27

Skeletal: **20-200 ms** Smooth: **200 ms - sustained**

Question 28

How do _excitation mechanisms_ in smooth vs. skeletal muscle differ?

Answer 28

Skeletal: **Neuromuscular Transmission** Smooth: * **Synaptic Transmission** * **Hormone receptors** * **Electrical Coupling** * **Pacemaker Potentials** (ex: interstitial cells of Cajal)

Question 29

How does the _electrical activity_ of skeletal vs. smooth muscle differ?

Answer 29

Skeletal: **AP spikes** Smooth: * **Action Potential Spikes** * **Plateaus** * **Graded Membrane Potentials** * **Slow Waves**

Question 30

What are the different "Ca²⁺ sensor" molecules in skeletal vs. smooth muscle?

Answer 30

Skeletal: **Troponin C** Smooth: **Calmodulin**

Question 31

What are the different _excitation-contraction coupling_ mechanisms in skeletal vs. smooth muscle?

Answer 31

Skeletal: **DHP receptor** (T-tubule) and **Ryanodine receptor** (SR) Smooth: * **Ca²⁺ entry from VGCCs** * **Normal/IP3-mediated Ca²⁺ release from SR** * **Ca²⁺ entry thru store-operated Ca²⁺ channels**

Question 32

How does _force regulation_ differ between skeletal and smooth muscle?

Answer 32

Skeletal: **AP frequency + Multifiber Summation** Smooth: * **Myosin Light Chain Phosphorylation Balance** * **Latch State** (light force, low ATP smc contraction for maintenance of vessel diamter, etc.)

Question 33

How does the _metabolism_ of smooth vs. skeletal muscle differ?

Answer 33

Skeletal: **Oxidative** _or_ **glycolytic** Smooth: **Oxidative** only

Question 34

What determines the force/smoothness of contraction?

Answer 34

**AP frequency**

Question 35

What happens when APs stimulating skeletal muscle occur at around 10 Hz (or just a little too fast for the muscle to relax between stimulations)? What is it about the time scale of APs, Ca signals + contraction makes this possible?

Answer 35

**Temporal Summation** of action potentials (**twitch**) leading to increased force of contraction _= electromechanical coupling_ - Calcium signals (and thus contraction) last much longer than APs ... so as new high-frequency APs arrive they add to the calcium signal induced by previous APs

Question 36

What happens when APs arrive at very high (25-50 Hz) frequency at neuromuscular junctions and do not allow relaxation of muscle fibers?

Answer 36

**Unfused tetanus** - occurs around 25 Hz and results in sustained contraction with slight oscillation around a maximum force **Fused tetanus** - occurs around 50 Hz and results in steady, sustained contraction (but force decreases over time with fatigue)

Question 37

What is the cause of tetanus?

Answer 37

after an AP triggers a twitch, there should be enough time for the SR to reabsorb the Ca²⁺ repeatedly stimulation w/ insufficient time for the reaccumulation of Ca²⁺ → remaining high sarcoplasmic [Ca²⁺] → continued cross­bridge cycling **= tetanus**

Question 38

What are the 3 types of skeletal muscle contraction with respect to changes in tension and muscle length?

Answer 38

1. **Isometric** - muscle contraction w/o changing length (ex: holding a weight steady with your arm) 2. **Isotonic** - muscle length changes, but force is steady (ex: lifting a weight repeatedly as in bicep curls) 3. **Auxotonic** - both length and load vary during contraction

Question 39

Draw the graph for the length-tension relationship of skeletal muscle.

Answer 39

describes effect of muscle fiber length on the amount of tension the fiber can develop, determined for a muscle undergoing an isometric contraction * **passive tension:** developed by simply stretching a muscle to different lengths * **total tension:** developed when a muscle is stimulated to contract at different lengths, sum of the active tension and the passive tension * **active tension:** determined by subtracting the passive tension from the total tension. It represents the active force developed during cross-bridge cycling

Question 40

What is the reason for the length-tension relationship of skeletal muscle?

Answer 40

**active tension developed is proportional to the number of cross-bridges btw filaments** → maximal when there is maximal overlap of thick and thin filaments * muscle is stretched to longer lengths, overlap decreases * muscle length is decreased, the thin laments collide with each other in the center of the sarcomere

Question 41

What is a synonym for fast and slow twitch muscles?

Answer 41

* **slow twitch** = type I * **fast twitch** = type IIa, IIb

Question 42

Describe the features of type I skeletal muscle fibers w/r/t * fatigue * color * metabolism * mitochondria * glyocogen * motor unit

Answer 42

Question 43

Describe the features of type IIa skeletal muscle fibers w/r/t * fatigue * color * metabolism * mitochondria * glyocogen * motor unit

Answer 43

FR = Fast twitch Resistant type. Chicken leg meat, has hemoglobin so it’s darker. Needs more oxidative capacity because chickens walk a lot.

Question 44

Describe the features of type IIb skeletal muscle fibers w/r/t * fatigue * color * metabolism * mitochondria * glyocogen * motor unit

Answer 44

FF = Fast twitch Fatigable type, as in chicken breast meat (white). Chicken can’t fly (fatigable) but can powerfully flap its wings briefly

Question 45

Which mechanisms cause fatigue in skeletal muscle fibers?

Answer 45

_NOT_ from ATP depletion, but may be from: * depletion of **acetylcholine** * depletion of **creatine phosphate and glycogen** * accumulation of **metabolic waste products**