Muscular System

Skeletal muscle

Moves the skeleton

Transverse tubules

tubules in the muscles that allow for the spreading of electrical impulses

myofibrils

elongated strands within muscle fibers that contain the contractile proteins actin and myosin, give skeletal and cardiac muscle their striated appearance

thick filaments

myosin filaments

thin filaments

actin filaments

sarcomere

repeating unit between two z-lines

cross-bridges

basis for attachment between actin and myosin filaments

tropomyosin

regulatory protein deciding whether actin is bound to myosin at any point

troponin

regulatory protein allowing or denying access to binding sites by moving tropomyosin

Sarcolemma

plasma membrane of a muscle cell

sarcoplasm

cytoplasm of a muscle cells

sarcoplasmic reticulum

smooth ER of a muscle cell

Where is smooth muscle found?

lining hollow organs

Sliding-filament mechanism

contraction produces shortening of a skeletal muscle fiber causing thick and thin filaments in each sarcomere to move past each other, propelled by the movements of the cross-bridges

How does troponin do its job?

binds calcium reversibly and changes conformation to pull tropomyosin away from the interaction sites, allowing myosin to interact with actin

Cross bridge cycle

1. cross bridge binds to actin

2. cross-bridge moves

3. ATP binds to myosin, causing cross-bridge to detach

4. hydrolysis of ATP energizes cross bridges

Functions of ATP in the skeletal muscle

Na/K-ATPase pump maintains Na and K gradients to allow action potentials to be produced, Ca-ATPase provides energy for active transport of calcium ions to end the contraction, energizing cross-bridges for force generation, binding to myosin dissociates cross-bridges to repeat the cycle

Function of the sarcoplasmic reticulum

Ca stored and released following membrane excitation

T-tubule protein

modified voltage-sensitive Ca channel DHP receptor

Ryanodine receptor

protein embedded in the SR membrane which forms Ca channel

Motor unit

motor neuron and the skeletal muscle fibers it innervates

Can a motor neuron innervate more than one muscle fiber?

yes

Can a muscle fiber be innervated by more than one motor neuron?

no

What does the number of innervated fibers depend on?

muscle type

Neuromuscular junction

junction of an axon terminal with the motor end plate

How are action potentials initiated in skeletal muscle?

Stimulation of nerve fibers

Motor neurons

nerve cells whose axons innervate skeletal muscle

Motor end plate

region of the muscle fiber plasma membrane directly under the terminal portion of the axon

Tension

force generated by muscle fibers

Load

force exerted on the muscle by an object, opposed by tension

Isometric

tension developed without a change in length

isotonic

lengthening

Concentric

tension > load

Eccentric

tension < load, only happens when external forces dictate

Twitch

mechanical response of a muscle fiber to a single action potential

Phases of a twitch contraction

latent period, contraction phase, relaxation phase

Latent period

time of the action potential to the onset of contraction, happens because of excitation-contraction coupling

Contraction phase

time tension is developing due to cross-bridge cycling

Relaxation phase

time that tension is decreasing due to the amount of time it takes to get all the Ca sequestered

What happens with isotonic twitches at heavier loads?

longer latent period to accumulate cross-bridges sufficient enough, shorter twitches, slower shortening

Frequency-tension relationship

increasing the rate of action potentials fired by a motor neuron increases the tension generated by the muscle fiber

Summation

increase in muscle tension from successive action potentials occurring during the phase of mechanical activity

Tetanus

maintained contraction in response to repetitive stimulation

Length-tension relationship

increased stretch causes passive tension to increaseW

What is responsible for passive elastic properties of relaxed muscle fibers?

spring-like characteristics of titin protein

What is the cause of the length-tension relationship?

elongation of titin filaments, if stretched fiber is released it will return to equilibrium length

How can active tension be altered?

changing the length of the fiber. if a fiber is stretched to various lengths and tetanically stimulated at each length, the magnitude of active tension will vary with length

Optimal length, L₀

length at which the fiber develops the greatest isometric active tension

Ways to form ATP

phosphorylation of ADP by creatine phosphate, oxidative phosphorylation of ADP in the mitochondria (terminal step of the electron transport chain), phosphorylation of ADP by the glycolytic pathway in the cytosol

Muscle fatigue

decreased tension, shortening velocity, and rate of relaxation in the face of continuous stimulation

Molecular causes of muscle fatigue

lower ATP, increased ADP, Pi, Mg, H+, oxygen free radicals decrease calcium release, reuptake, and storage, sensitivity of thin filament proteins to activation by calcium release, and inhibit binding and power-stroke motion of cross bridges

Central command fatigue

fatigue at the level of the motor cortex occurs when it fails to send excitatory signals to the motor neurons which may cause a person to stop exercising even though the muscles are not fatigued

Will to win

ability to inititate central commands to msucles during a period of increasingly distressful sensation

How are muscle fibers classified

maximal velocities of shortening and primary pathway used to form ATP

Oxidative muscle fibers

have a lot of mitochondria, most of the ATP produced is dependent upon blood flow to deliver oxygen and fuel molecules to muscle, contain myoglobin

Glycolytic muscle fibers

lots of glycolytic enzyme and glycogen allows for quick bursts of activity

Slow oxidative fibers (type I)

low myosin-ATPase activity, high oxidative capacity

Fast oxidative glycolytic fibers (Type IIa)

high myosin ATPase activity with high oxidative capacity and intermediate glycolytic capacity

Fast glycolytic fibers (Type IIb)

high myosin ATPase activity and high glycolytic capacity

Recruitment

low neural activity only activates type I fibers, as it increases as it recruits each of the subsequent fiber types

Size principle

smallest neurons require the least amount of input to depolarize, which match with smaller muscle fibers

What does total tension depend on?

amount of tension per fiber, number of fibers contracting at a time

What does shortening velocity of a whole muscle depend on?

load on the muscle, types of motor units in the muscle, and the number of motor units working against the load

How does an increase in contractile activity influence muscles?

increases muscle fiber size and capacity for ATP production

What happens to muscles that are not used?

atrophy

Denervation atrophy

nerve damage leading to a loss of function in the muscle

How are smooth muscle cells arranged?

not myofibrils or sarcomeres

Does smooth muscle have troponin and tropomyosin?

only tropomyosin

How are cross-bridge activations of smooth muscle cells controlled?

myosin light chain kinase (MLCK), a calcium regulated enzyme that phosphorylates myosin, which is the only form that can bind actin

Myosin light-chain phosphatase (MLCP)

dephosphorylates myosin to relax smooth muscles

Sources of cytosolic calcium

sarcoplasmic reticulum, extracellular calcium entering the cell through plasma membrane calcium channels

Can input to smooth muscle be excitatory or inhibitory?

yes, both

Can input to skeletal muscle be excitatory or inhibitory?

no, only excitatory

How are smooth muscles regulated?

nerves and hormones, local factors, spontaneous activity

How can nerves and hormones regulate smooth muscle?

neurotransmitters are released by autonomic neuron endings, no motor end-plate

varicosities

swollen regions containing vesicles filled with neurotransmitters which are released when an action potential passes through

Do neurotransmitters only produce one type of effect?

no, they can produce opposite effects in different muscle tissues, the type of response depends on the receptors the neurotransmitter binds to in the membrane and on the intracellular signaling mechanisms those receptors activate

What local factors influence smooth muscle contraction?

paracrine signals, acidity, oxygen, carbon dioxide, osmolarity, ion composition of extracellular fluid

Pacemaker potential

membrane potential change occuring during spontaneous depolarization to threshold

Slow waves

periodic fluctuations in membrane potential due to variation in ion flux across the membrane

Single-unit smooth muscle

cells are connected by gap junctions and respond to stimuli as a single unit

Multi-unit smooth muscles

cells respond to stimuli independently and contain few gap junctions

Branching cells

have intercalated discs with desmosomes and gap junctions

Why are gap junctions important to cardiac muscle?

ability to be electrically coupled

Automaticity/autorhythmicity

nodal cells have the ability to stimulate their own action potentials