Muscle Fiber Contraction and Whole Muscle Physiology

Muscle Fiber Contraction

Ion Channels

Ion channels play a crucial role in changing membrane potentials, which is essential for muscle fiber contraction. There are two main types of ion channels:

• Chemically gated ion channels: Opened by chemical messengers such as neurotransmitters (e.g., acetylcholine receptors on muscle cells).

• Voltage-gated ion channels: Open or close in response to voltage changes in the membrane potential.

Example: Acetylcholine (ACh) binds to chemically gated channels on the sarcolemma, initiating muscle contraction.

Anatomy of Motor Neurons and the Neuromuscular Junction

Skeletal muscles are stimulated by motor neurons, which transmit signals from the central nervous system to muscle fibers.

• Axon branches end on muscle fibers, forming the neuromuscular junction (NMJ) or motor end plate.

• Each muscle fiber has one NMJ with one motor neuron.

• The NMJ consists of the axon terminal, synaptic cleft, and junctional folds.

• Synaptic vesicles in the axon terminal contain the neurotransmitter acetylcholine (ACh).

• Junctional folds in the sarcolemma increase surface area for ACh receptors.

The Big Picture: Steps for Skeletal Muscle Contraction

Four major steps must occur for skeletal muscle contraction:

1. Events at the neuromuscular junction

2. Generation of an action potential across the sarcolemma

3. Excitation-contraction (E-C) coupling

4. Cross bridge cycling

Events at the Neuromuscular Junction

The neuromuscular junction is where the motor neuron communicates with the muscle fiber to initiate contraction.

1. An action potential (AP) arrives at the axon terminal.

2. Voltage-gated calcium channels open, allowing Ca2+ to enter the axon terminal.

3. Calcium entry causes the release of ACh into the synaptic cleft.

4. ACh diffuses across the synaptic cleft and binds to ACh receptors on the sarcolemma.

5. ACh binding opens chemically gated ion channels, allowing Na+ to enter, resulting in an end plate potential.

6. ACh effects are terminated by acetylcholinesterase, which breaks down ACh in the synaptic cleft.

Generation of an Action Potential Across the Sarcolemma

The action potential is generated and propagated along the muscle fiber membrane (sarcolemma), leading to contraction.

• Resting sarcolemma is polarized: There is a voltage difference across the membrane, with the inside of the cell being more negative compared to the outside.

Occurs in three steps:

1. End plate potential: ACh binds to receptors, opening chemically gated ion channels. Na+ diffuses in, making the interior less negative (depolarization).

2. Generation and propagation of the action potential (AP): If the end plate potential reaches threshold, voltage-gated Na+ channels open, and AP is generated and propagated along the sarcolemma.

3. Restoration of resting conditions: Na+ channels close, K+ channels open, and K+ efflux restores the resting membrane potential. The Na+/K+ pump helps re-establish ion gradients.

Refractory period: Muscle fiber cannot be stimulated again until repolarization is complete.

Excitation-Contraction (E-C) Coupling

Excitation-contraction coupling refers to the events that transmit the action potential along the sarcolemma, leading to the sliding of myofilaments and muscle contraction.

• AP is propagated along the sarcolemma and down T tubules.

• Voltage-sensitive proteins in T tubules stimulate Ca2+ release from the sarcoplasmic reticulum (SR).

• Ca2+ release leads to contraction.

Muscle Fiber Contraction: Cross Bridge Cycling

Cross bridge cycling is the process by which myosin heads bind to actin filaments, resulting in muscle contraction.

• At low intracellular Ca2+ concentration, tropomyosin blocks myosin-binding sites on actin.

• At high Ca2+ concentration, Ca2+ binds to troponin, causing tropomyosin to move and expose binding sites.

• Myosin heads bind to actin, initiating contraction.

• When nervous stimulation ceases, Ca2+ is pumped back into the SR, ending contraction.

Four steps of the cross bridge cycle:

1. Cross bridge formation: Myosin head attaches to actin filament active site.

2. Working (power) stroke: Myosin head pivots, pulling actin filament toward the center of the sarcomere.

3. Cross bridge detachment: ATP binds to myosin head, causing it to detach from actin.

4. Cocking of myosin head: Energy from ATP hydrolysis "cocks" the myosin head into a high-energy state, ready for another cycle.

Whole Muscle Contraction

Whole muscle contraction involves the coordinated activity of multiple muscle fibers and motor units.

• Contraction may or may not shorten the muscle, depending on the type of contraction and load.

• Isotonic contraction: Muscle shortens because tension exceeds load.

• Isometric contraction: Muscle tension increases but does not exceed load, so muscle does not shorten.

• Each muscle is served by at least one motor nerve, which contains axons of hundreds of motor neurons.

• The motor unit is the functional unit, consisting of a motor neuron and all the muscle fibers it innervates.

• Smaller motor units allow for finer control of muscle contraction.

The Muscle Twitch

A muscle twitch is the simplest contraction resulting from a muscle fiber's response to a single action potential from a motor neuron.

• Muscle fiber contracts quickly, then relaxes.

• Three phases of a muscle twitch:

  1. Latent period: Events of excitation-contraction coupling; no muscle tension seen.

  2. Period of contraction: Cross bridge formation; tension increases.

  3. Period of relaxation: Ca2+ reentry into SR; tension declines to zero.

Graded Muscle Responses

Graded muscle responses allow for variations in muscle contraction strength and duration.

1. Changing Frequency of Stimulation

• Increased frequency results in temporal (wave) summation if two stimuli are received in rapid succession.

• Muscle does not have time to relax completely, so twitches increase in force with each stimulus.

• Further increases in frequency lead to unfused (incomplete) tetanus and then fused (complete) tetanus as muscle tension plateaus.

2. Changing Strength of Stimulation

• Recruitment (multiple motor unit summation): Increasing stimulus strength recruits more motor units, increasing contraction force.

• Types of stimulus involved:

  • Subthreshold stimulus: Not strong enough to cause contraction.

  • Threshold stimulus: Strong enough to cause first observable contraction.

  • Maximal stimulus: Strongest stimulus, all motor units recruited.

Isotonic and Isometric Contractions

Muscle contractions can be classified based on whether the muscle changes length or not.

• Isotonic contractions: Muscle changes length and moves load.

• Concentric contractions: Muscle shortens and does work (e.g., lifting a weight).

• Eccentric contractions: Muscle lengthens and generates force (e.g., lowering a weight).

• Isometric contractions: Muscle tension increases but muscle neither shortens nor lengthens (e.g., holding a weight steady).

Factors of Muscle Contraction

The force of contraction depends on the number of cross bridges attached, which is affected by several factors:

• Number of motor units recruited: More units recruited, greater force.

• Relative size of fibers: Larger muscle fibers produce more tension; muscle cells can increase in size (hypertrophy) with exercise.

• Frequency of stimulation: Higher frequency, greater force.

• Degree of muscle stretch: Muscle fibers with sarcomeres at optimal length generate more force (80–120% of resting length).

Velocity and Duration of Contraction

How fast a muscle contracts and how long it can stay contracted is influenced by:

1. Muscle Fiber Type

• Speed of contraction: Determined by how fast myosin ATPases split ATP and the pattern of electrical activity of motor neurons.

• Metabolic pathways: Fibers using aerobic pathways are more fatigue-resistant.

Based on these criteria, skeletal muscle fibers are classified into three types: slow oxidative, fast oxidative, and fast glycolytic fibers. Additional info: Classification details inferred from standard A&P textbooks.

2. Load

• Muscles contract fastest when no load is added.

• The greater the load, the slower the contraction and the shorter its duration.

3. Recruitment

• The more motor units contracting, the faster and more prolonged the contraction.