PNB Quiz Prep - Skeletal Muscle Physiology

What are the 4 things muscles specialize in?

excitability, contractility, elasticity, extensibility

What is excitability?

ability of muscle to respond to stimuli, usually from a motor neuron

What is contractility?

ability of muscle to shorten forcefully when stimulated

What is elasticity?

ability of muscle to return to its original length after beinig stretched or contracted

What is extensibility?

ability of muscle to be stretched without damage 

What are the functions of muscle tissue?

heat production, movement, posture and stability

What is heat production?

muscles generate heat as a byproduct of contraction, helps maintain body temp.

What is movement?

muscles produce movement by contracting and pulling on bones via tendons, allows for voluntary and involuntary actions 

What is posture and stability?

muscles maintain body posture and balance by continously making small contraction, stabalizes joint movement

What muscle works when standing still?

erector spinae; prevents back from collapsing

What do muscle fibers derive form?

myoblasts in embryo

What do multiple myoblasts fuse into?

single cell with multiple nuclei

What are satellite cells?

myoblasts that didn’t fuse; assist in repair if muscle is injured

What are myosatellite cells?

stem cells located on muscle fibers; activate during growth or repair

What are myoblasts?

proliferating cells that fuse together to form muscle fibers 

What are muscle fibers (myocytes)?

mature, multinucleated cells responsible for muscle contraction

What are the types of muscle tissue?

skeletal, smooth, cardiac

How do muscle tissues vary in morphology and internal structure?

cell density, striations, number and position of nuclei, cell shape

What is muscle tissue composed of?

network of bundled fascicles 

What are fascicles?

small bundle of fibers

What does each fascicle contain?

multiple muscle fibers (myocytes)

What individual muscle cells are responsible for contraction?

myocytes

What is Type I skeletal muscle?

slow twitch fibers; aerobic oxidative

What is Type IIa skeletal muscle?

intermediate fast twitch fibers; anaerobic (glycolytic) and aerobic (oxidative)

What is Type IIb skeletal muscle?

fast twitch fibers; anearobic CP (creatine phosphate) 

What are key features of Type I skeletal muscle?

high endurance, fatigue-resistant 

Whar are key features of Type IIa skeletal muscle?

intermediate fatigue resistance

What are key features of Type IIb skeletal muscle?

powerful, quick fatigue

How does the nucleus of a myocyte differ from a typical cell?

myocyte is multinucleated

What is the shape of a myocyte?

elongated, cylindrical shape 

What organelles are present in myocytes?

myofibrils (acting & myosin)

What do myocytes have that typical cells don’t?

striations (in skeletal & cardiac muscle)

What is the sarcoplasm?

equivalent to the cytoplpasm of a regular cell

What is the sarcolemma?

equivalent to the plasma membrane 

What is the sarcoplasmic reticulum (SR)?

equivalent to the endoplasmic reticulum (ER)

What are muscle morphologies and structures specialized for?

muscle contraction and calcium storage

What is the most abundant structure within the cardiac myocyte?

myofibrils

What does the myofibril comprise?

the contractile apparatus responsible for muscle contraction 

What are myofibrils made up of?

repeating units called sarcomeres

What are myofibrils?

complex organells composed of bundles of myofilaments in a muscle fiber (appear banded in skeletal muscle)

What are thin filaments?

actin slide across thick filaments: myosin toward the center of the sarcomere 

What are proteins of the sarcomere?

myosin, actin, troponin, tropomyosin

What is myosin?

thick filament, generates force

What is actin?

thin filament, interacts with myosin for contraction

What are contractile elements?

myosin and actin 

What are regulatory elements?

troponin and tropomyosin

What is the troponin complex?

Tnl, TnT, TnC

What is Tnl?

inhibits acting-myosin interaction

What is TnT?

binds troponin to tropomyosin 

What is TnC?

binds calcium to initiate contraction

What is tropomyosin?

blocks myosin-binding sites on actin at rest

What is calcium release during the sliding filament theory?

sarcplasmic reticulum releases calcium; calcium diffuses through muscle fiber

What happens to calcium at rest?

low Ca²⁺

What does tropomyosin do at rest?

blocks the myosin binding site on actin

What happens to calcium during depolarization?

high Ca²⁺ 

What does Ca²⁺ bind to during depolarization?

TnC, causing tropomysoin to move 

What happens during depolarization when tropomysoin moves?

myosin binds to actin, contraction occurs

What is the mitochondria?

powerhouse of the cell, provides ATP

What is ATP essential for?

myosin-actin cross bridge cycling, contractile protein assembly & proper function 

Why is energy important?

muscle contraction is energetically expensive 

What is G-actin?

globular, single actin monomers floating in the cytoplasm

What is F-actin?

filamentous, long chains of actin monomers (polymers)

What does F-actin form?

part of the thin filament in muscle or the cytoskeleton in other cells

What is the first step of polymerization (building filaments)?

G-acting binds ATP before it joins a growing F-actin filament 

What is the second step of polymerization (building filaments)?

ATP-actin adds mainly to the “plus” end of the filament

What is the third step of polymerization (building filaments)?

energy-dependent process because of ATP-actin has higher affinity for the filament and provides stability

What is ATP hydrolysis and depolymerization?

once incorporated into F-actin, the ATP on actin is eventually hydrolyzed to ADP

What is ADP-actin?

less stable and more likely to fall off the filament at the “minus” end 

What is ATP exchange?

the ADP-actin that falls off is recharged by exchanging ADP for ATP

What happens when ADP is exchanged for ATP?

converts it back to G-actin ready for another round of polymerization

What does ATP drive?

actin filament dynamics; without it, filaments cannot grow, shrink or turnover efficiently

Why is turnover important in muscle cells?

allows reorganization of actin filaments, supports contraction, and maintains structural integrity 

What is the first step of myosin ATP hydrolysis & cross-bridge formation?

ATP binding

What does ATP bind to during cross-bridge formation?

myosin head

What does myosin ATPase hydrolyze?

to ADP + P_i

What happens to the myosin head during ATP binding?

“cocks” into high-energy position

What is the second step of mysoin ATP hydrolysis & cross-bridge formation?

cross-bridge formation 

What rises during cross-bridge formation?

Ca²⁺ in cytosol

What does Ca²⁺ bind to during cross-bridge formation?

troponin, moves tropomyosin off actin;s myosin binding site

What does the myosin head bind to during cross-bridge formation?

actin, cross-bridge forms

What is the third step of myosin ATP hydrolysis & cross-bridge formation?

power stroke, P_i is released

What is released during power stroke?

P_i from myosin

What happens to the myosin head during a power stroke?

pivots, pulling actin toward M-line

What is released during a power stroke?

ADP

What is the fourth step of myosin hydrolysis & cross-bridge formation?

myosin release & reset 

What does new ATP bind to during myosin relase & reset?

myosin; myosin detaches from actin

What happens to ATP during myosin release & rest?

hydrolyzed, myosin head “cocks” again

What allows the myosin ATP hydrolysis & cross-bridge formation cycle to repeat?

as long as Ca²⁺ and ATP are present

What is rigor mortis?

stiffening of muscles shortly after death 

What happens to muscle proteins during rigor mortis?

gradually break down, muscles relax

What is the key point regarding rigor mortis?

caused by a lack of ATP, which prevents myosin from detaching from actin

What is cold shortening of meat?

myocytes contract in response to cold temperatures after slaughter

What is the problem regarding cold shortening of meat?

leads to tough meat if muscles contract before rigor mortis 

How is cold shortening of meat prevented?

apply eletrical pulses to muscles, prevents cold-induced contraction

What is the key point regarding cold shortening of meat?

controlled postmortem handling preserves meat tenderness

What is the neuromuscular junction (NMJ)?

synapse between a motor neuron and a muscle fiber

What is the function of the NMJ?

initiates muscle contraction when the motor neuron stimulates the muscle fiber 

What is the NMJ essential for?

transmitting the electrical signal that triggers contraction

What are motor units?

a single motor neuron and all the muscle fibers it controls

What principle does a motor unit follow?

all-or-none response; a motor unit contracts fully or not at all