Muscle- Excitation, Contraction, and Control of Muscle Force

What is the neuromuscular junction (NMJ)?

The site where a motor neuron communicates with a muscle fiber.

Motor end plate

region of the sarcolemma that synapses with the motor neuron

What neurotransmitter is released by a motor neuron?

Acetylcholine (ACh).

End-plate potentials (EPPs)

A graded depolarization at the motor end plate caused by ACh binding to its receptors.

excitation at the neuromuscular junction sequence

Motor neuron → ACh released → motor end plate → end-plate potential → threshold → muscle action potential.

Is the EPP an excitatory or inhibitory graded potential?

excitatory - makes the inside of the muscle cell more positive

EPP is ________ and makes action potential _______

depolarization, more likely

What happens if the EPP reaches threshold?

Voltage-gated Na⁺ channels open, causing a muscle action potential that travels along the sarcolemma.

Muscle action potential

all-or-none electrical signal that travels along the muscle membrane.

function of the sarcoplasmic reticulum (SR)

Stores and releases Ca²⁺ for muscle contraction.

When a skeletal muscle cell is at rest, what is the relative concentration of intracellular Ca²⁺?

Very low Ca²⁺ concentration in the sarcoplasm.

Where is Ca²⁺ stored?

In the sarcoplasmic reticulum (SR).

What structure of the sarcolemma transmits the action potential deep within the muscle cell?

T-tubules (transverse tubules).

What limiting factor of diffusion do T-Tubules address?

Diffusion over long distances is slow, so T-tubules carry the electrical signal deep into the muscle fiber quickly.

What receptors along the T-tubule membrane are voltage-sensitive?

Dihydropyridine receptors (DHPRs).

What channels on the SR interact with the receptors on the T-tubule membrane?

Ryanodine receptors (RyRs), which release Ca²⁺ from the SR.

What does it mean for myosin to have high binding affinity to actin?

Myosin strongly attracts and binds to actin.

At rest, what regulatory protein blocks myosin from binding to actin?

Tropomyosin blocks the myosin-binding sites on actin.

What regulatory protein controls the position of tropomyosin?

Troponin

What happens to the concentration of Ca²⁺ in the sarcoplasm when Ca²⁺ is released from the SR?

It increases.

What regulatory protein does Ca²⁺ bind to?

Troponin

Calcium changes troponin's

shape

How does calcium release affect tropomyosin?

Tropomyosin moves away, exposing the myosin-binding sites on actin.

What happens after tropomyosin moves away from binding sites?

Myosin can now bind to actin and form cross-bridges, allowing contraction to begin.

Calcium binds to

troponin and it changes shape

Tropomyosin acts as a

barrier covering binding sites so myosin cant attach at rest

after troponin binds calcium…

it changes shape → moves tropomyosin → exposes actin's binding sites → myosin binds actin.

What is a cross-bridge?

The connection formed when myosin binds to actin.

What energy state is myosin in when it forms a cross-bridge with actin?

High-energy, cocked position, ready to pull actin.

What molecules are bound to myosin head?

ADP and inorganic phosphate (Pi).

What is the power stroke?

When myosin pulls actin toward the center of the sarcomere.

What molecular event triggers the power stroke?

The release of inorganic phosphate (Pi) from myosin.

cross-bridge cycle purpose

turn the electrical signal from the nervous system into mechanical force

Purpose of Ca²⁺ release

serves as the link between the electrical signal and the contraction machinery.

Cross-bridge cycle =

mechanical pulling as myosin pulls actin using energy from ATP

True or false: the muscle can generate force without shortening

True- (an isometric contraction), such as when you hold a heavy object still.

Describe the transfer of energy during the power stroke.

Energy stored when ATP was broken down is converted into mechanical energy as myosin changes shape and pulls actin.

How does the transfer of energy after a powerstroke contribute to muscle force and change in length?

Myosin pulls thin filaments toward the center of the sarcomere, generating force and shortening the sarcomere.

What molecular event occurs at the end of the power stroke?

ADP is released from myosin.

At the end of the power-stroke, what is myosin's binding affinity to actin?

High — myosin is still strongly attached to actin.

What molecule binds to myosin at the end of the power stroke?

ATP.

After myosin detaches, what happens to the ATP bound to its head?

Myosin ATPase breaks ATP into ADP + Pi.

What happens to the energy released during ATP hydrolysis?

It re-energizes the myosin head, returning it to a cocked, high-energy position.

ATP purpose during cross-bridge cycle

resets myosin so it can pull again

What triggers the power stroke in the next cycle?

Release of Pi from the myosin head.

Motor unit

one motor neuron and all the muscle fibers it controls.

What is recruitment?

activating more motor units to increase muscle contraction.

When do we need motor unit recruitment?

when you need to produce more force (ex: lifting a heavy weight)

More motor units recruited =

more muscle fibers stimulated + greater muscle force

Explain what occurs for a stimulus to cause a muscle twitch.

Motor neuron → ACh → muscle action potential → Ca²⁺ released → myosin pulls actin → muscle twitch

Low recruitment=

fewer motor units active- less force/ less muscle fibers contracting

Summation

multiple stimuli occur close together and add together because stimuli arrive before the muscle fully relaxes.

Summation means muscle tension

increases bc muscle has less time to relax

Tetanus

sustained muscle contraction caused by rapid, repeated stimulation, so the muscle has little or no time to relax.

How is summation different from tetanus

Twitches ADD together/ some relaxation may occur between twitches, muscle tension increases

How is tetanus different from summation

Twitches MERGE into a sustained contraction- LITTLE to NO relaxation, muscle tension remains sustained/high

What happens to EPP amplitude as stimulation frequency increases?

it stays the same

Frequency of stimulation increases

action potential (frequency) on sarcolemma

Sarcoplasmic (ICF) Ca²⁺ concentration increases during frequent stimulation because

Ca²⁺ is released repeatedly before it is fully taken back up

What happens as stimulation frequency increases?

More Ca²⁺ binds to troponin → more actin sites are exposed → more cross-bridges form → muscle force increases.

What changes do Action potentials have due to higher stimulation frequency?

Action potential amplitude / stim. freq.

Stays the same- all or nothing

Action potential frequency / stim. freq.

AP frequency increases due to higher stimulation frequency

Incomplete tetanus

Muscle produces sustained force but partially relaxes between stimuli.

Complete tetanus

Muscle produces a smooth, sustained contraction with no relaxation between stimuli.

What are the two mechanisms by which we control muscle force?

Recruitment and frequency stimulation

Everytime muscle gets a signal, it requires

Ca2+ to release, which causes it to contract

HIGHER STIM. FREQ=

GREATER FORCE

Larger muscle fibers=

More muscle fibers / myofibrils can produce more force

More muscle fibers contracting=

More fibers pulling together produce greater force.

How many muscle fibers do we have

generally have about the same number throughout life

How can you modulate the size of muscle fibers?

Weight training mainly increases fiber size through hypertrophy.

______ of muscle fibers dont change, ________ of muscle fibers can be changed

Number, size

Hypertrophy

Existing muscle fibers increase in size.

Hyperplasia

An increase in the number of muscle fibers

Sarcomere at resting length (force) :

Produces the greatest force because actin and myosin overlap optimally.

Sarcomere at stretched length (force) :

Produces less force because actin and myosin overlap less, so fewer cross-bridges can form.

Sarcomere at shortened length (force):

Produces less force because the filaments overlap too much and interfere with one another.

Are muscle size/ length considered control mechanisms?

No. Muscle size and muscle length affect the amount of force a muscle can produce, but they are not control mechanisms.

At resting length, what is more available?

Cross-bridges (greater force)