Sarcomeres

Muscle Contraction and Relaxation

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Sliding-Filament Model of Contraction

• The thin and thick myofilaments stack to form the myofibrils; they are arranged in a type of lattice-work to form sarcomeres. A Z disc, or Z line, serves as an anchor point for the thin myofilaments. The section between the Z disks is the sarcomere. 

Multiple Choice

Myofibrils are made of stacked

• sarcomeres

• myofilaments

• z disks

Sliding-Filament Model of Contraction

• In a relaxed muscle, the myosin and actin lie side by side, partially overlapping.

Sliding-Filament Model of Contraction

• When a muscle begins to contract, the myosin heads latch on to the actin myofilaments, forming a cross bridge. The myosin heads propel the actin myofilaments forward, toward the center of the sarcomere. Because the actin myofilaments are attached to the Z disks, they pull the Z disks closer together, shortening the sarcomere.

Multiple Choice

When myosin heads latch on to the actin a ___________ is formed.

• sarcomere

• z disc

• crossbridge

Sliding-Filament Model of Contraction

• As the sarcomere shortens, so does the myofibril and the entire muscle. This is the sliding-filament model of contraction. (The myofilaments don’t shorten; the sarcomere shortens because the filaments slide over the top of one another.)

• Muscle contraction requires adenosine triphosphate (ATP).

Multiple Choice

In the sliding filament model the _________ shortens not the _______

• sarcomere, filament

• filament, sarcomere

• Actin, myosin

• myosin, actin

Muscle Contraction

• The connection between a motor neuron and a muscle fiber is a neuromuscular junction.

  
  

• The synaptic cleft is a narrow space between the end of the motor nerve and the muscle fiber.

Multiple Choice

The connection between a motor neuron and a muscle fiber is a

• synaptic cleft

• neuromuscular junction

Muscle Contraction

• Muscle contraction involves the following sequence of events:

  
  

• 1.When an impulse reaches the end of a motor neuron, it causes small vesicles to fuse with the cell membrane and release a neurotransmitter called acetylcholine (ACh).

Muscle Contraction

• 2.ACh quickly diffuses across the synaptic cleft, where it stimulates receptors in the sarcolemma.

  
  

• 3.This sends an electrical impulse over the sarcolemma and inward along the T tubules. The impulse in the T tubules causes the sacs in the sarcoplasmic reticulum to release calcium.

Muscle Contraction

• 4.The calcium binds with the troponin on the actin filament to expose attachment points. In response, the myosin heads of the thick filaments grab on to the thin filaments and muscle contraction occurs.

Muscle Contraction

• When nerve impulses stop arriving at the neuromuscular junction, ACh is no longer released. The enzyme acetylcholinesterase breaks down any remaining ACh and calcium ions are pumped back into the sarcoplasmic reticulum. With the calcium removed, troponin and tropomyosin again prevent the myosin heads from grasping the thin filament, and the muscle fiber relaxes.

Multiple Choice

This is released from the sarcoplasmic reticulum

• Ach

• Ca

• troponin

• tropomyosin

Multiple Choice

ACh is a

• Neurotransmitter

• synaptic cleft

• filament

Muscle Tone

• This is the continuous state of partial contraction.

• Tone allows you to maintain posture, and react quickly to a dangerous situation.

Muscle Tone

• •The strength of a contraction depends on the length of the fibers before the contraction begins. This is called the length-tension relationship.

• •In an overly contracted fiber, the sarcomeres are shortened. Even after stimulation, the fiber can’t contract very far before the thick filaments bump into the Z disks and contraction is weak.

Muscle Tone

• In overly stretched fibers, only a small portion of the thin filament is accessible for the myosin heads to grab. Again, contraction is weak. 

Muscle Tone

• •In partially overlapped thin and thick filaments, the Z disks are far enough apart to allow for movement during contraction. The thin and thick filaments overlap enough to allow the myosin heads to get a firm grip on the actin filaments, producing the strongest contraction.

Multiple Choice

In overly stretched and overly contracted fibers the contraction is

• strong

• weak

• average

Contraction of a Muscle

• Motor unit: A neuron and all the fibers it stimulates, a single motor unit can consist of a few to a few hundred fibers. 

• Threshold: •The minimum voltage needed to cause a muscle fiber to contract is the threshold. When a fiber receives a stimulus at or above threshold, it responds after a brief lag by quickly contracting and then relaxing.

• This single, brief contraction is a twitch. 

Multiple Choice

A neuron and all the fibers it stimulates is called

• Twitch

• Motor unit

• fiber

Contraction of a Muscle

• For a muscle to perform work, many fibers must contract at the same time. The muscle also needs to stay contracted for longer than a split second. 

• Muscles are called on to contract at different strengths depending on the work.

• The force of contraction is affected by the size of the muscle, the degree of stretch, and the number of muscle fibers contracting. 

Other Factors Affecting Muscle Contraction

• Frequency of stimuli

• Strength of stimulus

• Type of contraction

• The nervous system alters the frequency and intensity of a stimulus to control the force of contraction.

Multiple Choice

The frequency and intensity of a stimulus controls the

• Type of contraction

• Force of contraction

• number of contractions

Stimulus Frequency

• When a muscle contracts several times in a row, the last contraction is stronger than the first.

• Called treppe, or “staircase phenomenon.”

Stimulus Frequency (continued_1)

• When impulses arrive even faster, fibers don’t have a chance to relax completely.

• Subsequent contraction builds on force of previous contraction.

• Called incomplete tetanus.

Stimulus Frequency (continued_2)

• If muscle cannot relax at all between stimuli, twitches merge into one prolonged contraction.

• Called complete tetanus.

• 
  

Multiple Choice

A condition of rapid contraction with only partial relaxation is called:

• incomplete tetanus.

• complete tetanus.

• twitch.

• treppe.

Stimulus Intensity

• A stronger stimulus excites more nerve fibers and more motor units. The more fibers contracting at once, the stronger the contraction. The process by which an increasing number of motor units are called into action is called recruitment

Multiple Choice

The process by which an increasing number of motor units are called into action is called

• Strong stimulus

• Recruitment

• Weak stimulus

Type of Contraction

• In isotonic contractions, the muscle changes length and moves a load, whereas the tension within the muscle remains the same. 

• In isometric contractions, the tension within a muscle increases, whereas its length remains the same.

Multiple Choice

The length of the muscle changes while the tension within the muscle remains the same.

• iosotonic

• isometric

Energy Source for Contraction

• All muscle contraction requires energy in the form of adenosine triphosphate (ATP). Muscles store only enough ATP for a few seconds of activity.

• •At rest, muscles obtain most of their energy by metabolizing fatty acids. Because oxygen is plentiful, aerobic respiration is used to break down the fatty acids for energy.

• •When beginning to exercise, the supply of oxygen drops. Muscles break down the high-energy compound creatine phosphate (CP), which can furnish the muscle with fuel for about 20 seconds of high-energy activity or about 1 minute of moderate activity.

Energy Source for Contraction

• If exercise continues, muscles switch to anaerobic respiration of glucose. Anaerobic respiration can generate energy quickly and is useful for intense bursts of activity. It produces lactic acid, which leads to muscle fatigue. 

• After about 10 minutes of more moderate activity, the heart and lungs have had a chance to increase the supply of oxygen to the muscles. Muscles shift back to aerobic respiration, which produces more ATP than anaerobic respiration.

• By-products of aerobic respiration are carbon dioxide and water, which, unlike lactic acid, are not toxic to the muscle.

Multiple Choice

At rest, muscles obtain most of their energy by metabolizing:

• . glucose.

• . lactic acid.

• creatine phosphate.

• fatty acids.