Week 04 Lect. 2 - Skeletal Muscle + NMJ Flashcards
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.
- One AP of an alpha motor neuron always results in an AP of the skeletal muscle (and thus contraction)
- There is no contraction of skeletal muscle without a motor neuron AP
What is a motor unit?
An alpha motor neuron and all skeletal muscle fibers it innervates.
What is the relationship between muscle control precision and muscle fiber-to-neuron ratio in motor units?
Higher precision requires fewer muscle fibers innervated per neuron
What special synaptic cleft structure houses enzymes for neurotransmitter “recycling”?
What enzymes?
- a protein matrix in the synaptic cleft holds acetylcholinesterase enzymes for breakdown of ACh
After ACh transmits a neuronal AP to skeletal muscle, how is it “recycled”?
- ACh is released from Nicotinic ACh Receptors on the skeletal muscle membrane
- ACh-esterase breaks down ACh into Acetate and Choline
- Choline is reuptaken into the neuron via an Na+-Choline Cotransporter (acetate diffuses away)
- ACh is reformed by transfering an acetyl group from Acetyl-CoA to choline via Choline Acetyltransferase (ChAT)
- Reformed ACh is secondary-active transported into synaptic vesicles via an H+-ACh Antiporter driven by a proton gradient from an H+-ATPase
How high of a concentration of ACh can be found in synaptic vesicles?
150 mM
What happens to acetate that has been removed from choline by ACh-esterase in the synaptic cleft?
it diffuses away
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?
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+
What inhibits the nACh receptor?
Medical aspects?
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
How is the nACh receptor selective for only positive ions?
- the pore formed by its subunits has a narrow, negatively charged area which repels anions
What inhibitor affects release of ACh from alpha-motor neurons?
Medical aspect?
Botulinum toxin
used in cosmetics (“botox”)
What are some inhibitors of ACh-esterase?
Medical aspects?
-
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
What inhibits choline reuptake?
Hemicholine
Describe the structure of skeletal muscle cless
- 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)
What are the elements of the “triad” which allows for propagation of APs along skeletal muscle membranes?
- 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
What two receptors regulate calcium release in skeletal muscle?
And where are they located?
- DHP (dihydropiridine) receptor - a special voltage-dependent Ca++ channel, located on the T-tubule membrane
- Ryanodine Receptor - releases Ca++ from SR to cytoplasm, located on the terminal cistern membrane
Describe the sequence of receptor-related events at a triad that results in increase [Ca++]IC?
- AP depolarization in the T-tubule changes DHP receptor conformation
- Ryanodine + DHP receptors mechanically couple
- Calcium is released from the SR into the cytoplasm via the Ryanodine receptor
Which mechanisms prevents a Ca2+ depletion of the SR?
SERCA (Ca2+ ATPase) actively pumps Ca2+ back into SR
Where can calsequestrin be found in a skeletal muscle cell?
What is its function?
Ca2+ binding protein in terminal cisterns of SR
⇒ Ca2+ bound to calsequestrin does not effect the free [Ca2+] gradient, hence easier Ca2+ recycling back into SR by SERCA
How does extracellular calcium concentration effect skeletal muscle contraction and why?
It does not effect it…
…because of high sarcoplasmic reticulum concentration of Ca++…
…skeletal muscle can contract independent of EC calcium.
Describe the structure of a sarcomere.
What are their names and what makes them up?
-
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
Describe the structure of the thick filament
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
What are the proteins related to the thin filament of skeletal muscle?
- 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
How does calcium regulate actin-myosin binding?
- Calcium binds to Troponin C, inducing a conformation change
- troponin T allows Tropomyosin to move from myosin binding sites on actin
- Myosin binding can now occur
What are the steps of the cross-bridge cycle starting from the “resting state”?
Myosin head is bound to ADP and Phosphate in the resting/cocked state
- Cross-bridge forms + myosin head binds to new binding site on actin
- Phosphate is released and myosin head changes conformation creating a power stroke in which filaments slide past each other
- ADP is released and the myosin head remains attached
- ATP rebinds myosin, releasing it from actin
- ATP is hydrolzyed, and ADP + P bound myosin head is again resting/cocked
How do you call the state where the myosin head remains attached to the actin filament?
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.)
How do twitch durations differ between skeletal and smooth muscle?
Skeletal: 20-200 ms
Smooth: 200 ms - sustained
How do excitation mechanisms in smooth vs. skeletal muscle differ?
Skeletal: Neuromuscular Transmission
Smooth:
- Synaptic Transmission
- Hormone receptors
- Electrical Coupling
- Pacemaker Potentials (ex: interstitial cells of Cajal)
How does the electrical activity of skeletal vs. smooth muscle differ?
Skeletal: AP spikes
Smooth:
- Action Potential Spikes
- Plateaus
- Graded Membrane Potentials
- Slow Waves
What are the different “Ca2+ sensor” molecules in skeletal vs. smooth muscle?
Skeletal: Troponin C
Smooth: Calmodulin
What are the different excitation-contraction coupling mechanisms in skeletal vs. smooth muscle?
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
How does force regulation differ between skeletal and smooth muscle?
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.)
How does the metabolism of smooth vs. skeletal muscle differ?
Skeletal: Oxidative or glycolytic
Smooth: Oxidative only
What determines the force/smoothness of contraction?
AP frequency
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?
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
What happens when APs arrive at very high (25-50 Hz) frequency at neuromuscular junctions and do not allow relaxation of muscle fibers?
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)
What is the cause of tetanus?
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
What are the 3 types of skeletal muscle contraction with respect to changes in tension and muscle length?
- Isometric - muscle contraction w/o changing length (ex: holding a weight steady with your arm)
- Isotonic - muscle length changes, but force is steady (ex: lifting a weight repeatedly as in bicep curls)
- Auxotonic - both length and load vary during contraction
Draw the graph for the length-tension relationship of skeletal muscle.
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
What is the reason for the length-tension relationship of skeletal muscle?
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
What is a synonym for fast and slow twitch muscles?
- slow twitch = type I
- fast twitch = type IIa, IIb
Describe the features of type I skeletal muscle fibers w/r/t
- fatigue
- color
- metabolism
- mitochondria
- glyocogen
- motor unit
Describe the features of type IIa skeletal muscle fibers w/r/t
- fatigue
- color
- metabolism
- mitochondria
- glyocogen
- motor unit
FR = Fast twitch Resistant type. Chicken leg meat, has hemoglobin so it’s darker. Needs more oxidative capacity because chickens walk a lot.
Describe the features of type IIb skeletal muscle fibers w/r/t
- fatigue
- color
- metabolism
- mitochondria
- glyocogen
- motor unit
FF = Fast twitch Fatigable type, as in chicken breast meat (white). Chicken can’t fly (fatigable) but can powerfully flap its wings briefly
Which mechanisms cause fatigue in skeletal muscle fibers?
NOT from ATP depletion, but may be from:
- depletion of acetylcholine
- depletion of creatine phosphate and glycogen
- accumulation of metabolic waste products
