BSC2085 Final: Lesson 21

Four major phases of contraction and relaxation:

Excitation, Excitation-contraction coupling, Contraction, Relaxation

Excitation definition

action potentials in the motor nerve fiber lead to action potentials in the muscle fiber

What is excitation-contraction coupling?

The events that link the action potentials on the sarcolemma to activation of the myofilaments.

What is the first step in excitation-contraction coupling?

The release of Ca2+ from the sarcoplasmic reticulum (SR).

Contraction definition

the step in which the muscle fiber develops tension and may shorten

Relaxation definition

when stimulation ends, a muscle fiber relaxes and returns to its resting length

Steps of excitation

AP reaches motor neuron axon terminal.

Ca²⁺ enters axon terminal.

Ca²⁺ triggers ACh exocytosis into NMJ.

ACh diffuses across synaptic cleft.

ACh binds receptors on motor end plate.

Ligand-gated Na⁺ channels open → Na⁺ influx.

Muscle fiber action potential is generated on sarcolemma.

Steps of excitation-contraction coupling

AP travels along sarcolemma and down T-tubules.

AP reaches terminal cisterns of SR.

Voltage-gated Ca²⁺ channels in SR open.

Ca²⁺ is released into the sarcoplasm.

Ca²⁺ binds to troponin.

Troponin changes shape.

Tropomyosin shifts, exposing actin's myosin-binding sites.

Steps of contraction

Myosin (high-energy state, with ADP + Pi) binds to exposed actin site → cross-bridge forms.

ADP + Pi are released from myosin.

Myosin head pivots → power stroke pulls actin toward the M line.

ATP binds to myosin → myosin detaches from actin.

ATP is hydrolyzed to ADP + Pi → myosin resets to high-energy state.

What is required for the contraction cycle to continue?

Ca2+ in the sarcoplasm and ATP available to bind onto the ATP binding site on the myosin head.

What do Ca2+ pumps do during muscle relaxation and contraction?

They use ATP to move Ca2+ into the sarcoplasmic reticulum (SR) to keep Ca2+ levels low in the sarcoplasm.

Relaxation steps

AChE breaks down ACh in the synaptic cleft.

Less ACh → ligand-gated Na⁺ channels close.

No Na⁺ influx → no muscle action potential.

No AP in T-tubules → SR voltage-gated Ca²⁺ channels close.

SR Ca²⁺ pumps lower Ca²⁺ levels in the sarcoplasm.

Low Ca²⁺ → Ca²⁺ cannot bind troponin.

Troponin allows tropomyosin to shift back.

Tropomyosin blocks actin binding sites → myosin cannot attach.

What happens to the sarcomere during muscle contraction?

it shortens and the Z lines at each end get closer to the M line.

What is produced as sarcomeres shorten?

Tension

Where can muscle shortening occur in a muscle?

at both ends or at only one end of the muscle.

What determines how a muscle shortens?

the way the muscle is attached at the ends.

How is a muscle typically held at during contraction?

at the origin with the insertion end moving towards the fixed end.

How does muscle length return to resting length?

passively, with titin helping the sarcomere return to normal size.

What is rigor mortis?

hardening of muscles and stiffening of the body after death.

When does rigor mortis begin after death?

3-4 hours after death.

What causes the onset of rigor mortis?

The deteriorating sarcoplasmic reticulum releases Ca2+ and the deteriorating sarcolemma allows Ca2+ to enter the sarcoplasm.

Why do muscles stay contracted during rigor mortis?

Muscle relaxation requires ATP, and since the cell is dead and cannot produce new ATP, the muscles stay contracted.

When does rigor mortis peak?

about 12 hours after death.

How long does rigor mortis last?

diminishes over the next 48-60 hours after peaking.

As a whole, what are the two states of a muscle fiber?

either contracted or relaxed.

Contraction/relaxation depends on 3 factors:

• The number of pivoting cross-bridges that are formed

• The fiber's resting length at the time of stimulation

• The frequency of stimulation of the muscle cell

What does the length-tension relationship describe?

amount of tension generated by a muscle depends on how stretched or shortened it was before it was stimulated.

What happens if a muscle is overly shortened before stimulation?

The contraction is weak due to thick filaments butt up against Z discs and some thin filaments overlapping.

What occurs if a muscle is too stretched before stimulation?

The contraction is weak due to minimal overlap between thick and thin filaments, resulting in minimal cross-bridge formation.

What produces the greatest force when a muscle contracts?

Optimum resting length.

How does the nervous system maintain muscle tone?

By ensuring that resting muscles are near the optimum length for contraction.

What does a single neural stimulation produce?

A single contraction or twitch (7-100 msec.)

What is required for sustained muscular contractions?

Many repeated stimuli from the motor neuron.

Threshold

minimum voltage that causes a muscle twitch

different stimuli cause twitches varying in ______.

strength

What influences tension generation in muscles?

The muscle's starting length

What happens to muscles after continual use?

muscle fatigue

How does temperature affect muscle function?

Warmer muscles' enzymes work more quickly.

What factor influences cross-bridge formation in muscle cells?

The muscle cell's hydration level

What effect does increasing the frequency of stimulus delivery have on muscles?

increases tension output.

What is Treppe?

A stair-step increase in twitch tension.

What causes Treppe?

Repeated stimulations immediately after the relaxation phase.

What is the stimulus frequency for Treppe?

Less than 50 stimuli per second.

What is the effect of Treppe on muscle contractions?

It causes a series of contractions with increasing tension.

What causes the increase in tension during Treppe?

A gradual increase in Ca2+ concentration in the sarcoplasm.

Why does Ca2+ concentration increase during Treppe?

Ca2+ pumps are not fast enough to pump all of the previously released Ca2+ back into the SR.

What is wave summation?

increasing tension or summation of twitches in muscle contractions.

What causes wave summation?

repeated stimulations before the end of the relaxation phase.

What is the stimulus frequency required for wave summation?

greater than 50 per second.

What do low frequency stimuli produce?

identical twitches.

What happens with higher frequency stimuli?

temporal (wave) summation.

Temporal (wave) summation

where each new twitch 'rides piggyback' on the previous one, generating higher tension

What is the result of only partial relaxation between stimuli?

fluttering, incomplete tetanus, where the maximum tension fluctuates.

What is complete (fused) tetanus?

occurs with unnaturally high stimulus frequencies, causing a steady contraction (maximum tension is a flat line)

What is tetanus commonly known as?

Lockjaw

What bacterium causes tetanus?

Clostridium tetani

What effect does the tetanus toxin have on skeletal muscle motor neurons?

overactivity by blocking inhibitory interneurons in the spinal cord.

What are some symptoms of tetanus?

Muscle stiffness, headaches, and difficulty swallowing.

What vaccines are used to prevent tetanus?

Tdap (tetanus, diphtheria, and pertussis) and Td (tetanus and diphtheria).

At what age are Tdap or Td vaccines recommended for children?

children over 7 years old and adults.

What do motor units in skeletal muscle contain?

Hundreds of muscle fibers

How do muscle fibers in a motor unit contract?

at the same time

How many motor neurons control the muscle fibers in a motor unit?

A single motor neuron

What must muscles do to perform different tasks?

contract with variable strength.

What happens when the nerve is stimulated with higher voltages?

stronger contractions.

What is recruited when higher voltages excite more nerve fibers?

More motor units are stimulated to contract.

What is the process of bringing more motor units into play with stronger stimuli called?

Recruitment or multiple motor unit (MMU) summation.

What principle explains the recruitment of motor units?

The size principle.

Size principle

weak stimuli (low voltage) recruit small units

strong stimuli recruit small and large units for powerful movements

How is maximum tension generated?

when all motor units reach tetanus.

How long can maximum tension be sustained?

only a very brief amount of time.

What is sustained tension?

Less than maximum tension

Why is sustained tension important for motor units?

It allows motor units to rest in rotation

What is muscle tone?

The normal tension and firmness of a muscle at rest.

What do muscle units do?

actively maintain body position, without motion.

What happens when muscle tone increases?

increases metabolic energy used, even at rest.

What is an isometric contraction?

A contraction without a change in length.

What happens during an isometric contraction?

Muscle produces internal tension but external resistance causes it to stay the same length.

How can isometric contraction be a prelude to movement?

When tension is absorbed by the elastic component of the muscle.

What are isometric contractions important for?

postural muscle function and antagonistic muscle joint stabilization.

What is an isotonic contraction?

A contraction with a change in length but no change in tension.

What happens during a concentric contraction?

The muscle shortens while maintaining tension.

What is an example of a concentric contraction?

Lifting a weight.

What happens during an eccentric contraction?

The muscle lengthens while maintaining tension.

What is an example of an eccentric contraction?

Slowly lowering a weight.

Muscle cannot ______ return to their resting length. Other forces are involved.

actively

What type of forces pull elastic elements like tendons and ligaments?

Elastic forces

What do elastic forces do during muscle relaxation?

They expand the sarcomeres to resting length.

What is the role of opposing muscle contractions during muscle relaxation?

They reverse the direction of the original motion at opposing skeletal muscle pairs.

What can take the place of opposing muscle contraction to return a muscle to its resting state?

Gravity