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 classes of ion channels:

• Chemical-gated ion channels: Opened by chemical messengers such as neurotransmitters. Example: Acetylcholine (ACh) receptors on muscle cells.

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

Anatomy of Motor Neurons and the Neuromuscular Junction

Skeletal muscles are stimulated by somatic motor neurons. The neuromuscular junction is the site where a motor neuron communicates with a muscle fiber.

• Axons: Long, threadlike extensions of motor neurons that travel from the central nervous system to skeletal muscle.

• Each axon divides into many branches as it enters muscle.

• Axon branches end on muscle fiber, forming the neuromuscular junction or motor end plate.

• Each muscle fiber has one neuromuscular junction with one motor neuron.

• Axon terminal: The end of the axon, separated from the muscle fiber by a gel-filled space called the synaptic cleft.

• Axon terminals contain membrane-bound synaptic vesicles filled with the neurotransmitter acetylcholine (ACh).

• The sarcolemma at the junction contains junctional folds with ACh receptors.

The Big Picture: Steps for Skeletal Muscle Contraction

1. Events at the neuromuscular junction

2. Muscle fiber excitation

3. Excitation-contraction coupling

4. Cross bridge cycling

Events at the Neuromuscular Junction

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

2. Voltage-gated Ca2+ channels open, Ca2+ enters the axon terminal, causing synaptic vesicles to release ACh into the synaptic cleft.

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

4. ACh binding opens ion channels, allowing Na+ to enter and K+ to exit, leading to a change in membrane potential (end plate potential).

5. ACh effects are terminated by its breakdown in the synaptic cleft by acetylcholinesterase and diffusion away from the junction.

Generation of an Action Potential Across the Sarcolemma

The resting sarcolemma is polarized, meaning a voltage exists across the membrane. Action potentials are caused by changes in electrical charges and occur in three steps:

1. Generation of end plate potential: ACh binds to receptors, opening chemically gated ion channels. Na+ diffuses in, K+ diffuses out, leading to local depolarization (end plate potential).

2. Depolarization: If the end plate potential reaches threshold, voltage-gated Na+ channels open, causing an action potential that spreads across the sarcolemma.

3. Repolarization: Na+ channels close, K+ channels open, and K+ efflux restores the resting membrane potential.

Refractory period: The 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.

• The action potential is propagated along the sarcolemma and down T tubules.

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

Muscle Fiber Contraction: Cross Bridge Cycling

Cross bridge cycling is the process by which myosin heads pull actin filaments, resulting in muscle contraction. This process is regulated by Ca2+ concentration.

• At low Ca2+ concentration:> Troponin blocks active sites on actin, and myosin heads cannot attach, so the muscle fiber remains relaxed.

• At high Ca2+ concentration: Ca2+ binds to troponin, causing it to change shape and move tropomyosin away from myosin-binding sites. Myosin heads then bind to actin, forming cross bridges.

The four steps of the cross bridge cycle are:

1. Cross bridge formation: Myosin head attaches to actin.

2. Power (working) stroke: Myosin head pivots, pulling actin filament toward the M line.

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

4. Cocking of myosin head: Energy from ATP hydrolysis "cocks" the myosin head, preparing it for the next cycle.

When neural stimulation ceases, Ca2+ is pumped back into the SR, and contraction ends.

Whole Muscle Contraction

Muscle Tension

Muscle contraction produces muscle tension, the force exerted on a load or object. Contraction may or may not shorten the muscle:

• Isometric contraction: Muscle tension increases but does not exceed the load; no shortening occurs.

• Isotonic contraction: Muscle tension exceeds the load, and the muscle shortens.

Motor Units

A motor unit consists of a motor neuron and all the muscle fibers it supplies. The number of muscle fibers per motor unit determines the degree of fine control.

• Smaller motor units allow for greater fine control.

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. It consists of three phases:

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 varying strength of contraction for different demands, which is essential for proper skeletal movement. Responses are graded by:

1. Changing Frequency of Stimulation

• Wave (temporal) summation: Occurs if two stimuli are received in rapid succession; muscle fibers do not have time to relax completely, resulting in increased force.

• If frequency increases further, muscle tension reaches maximum, resulting in fused (complete) tetanus, a smooth, sustained contraction.

2. Changing Strength of Stimulation

• Recruitment: Increasing the number of motor units stimulated increases muscle force.

• Subthreshold stimulus: Not strong enough to cause a contraction.

• Threshold stimulus: The minimum stimulus required to cause the first observable contraction.

• Maximal stimulus: The strongest stimulus that increases contractile force.

Isotonic and Isometric Contractions

• Isotonic contractions: Muscle changes length and moves a load. Can be concentric (muscle shortens) or eccentric (muscle lengthens).

• Isometric contractions: Muscle does not change length; tension increases but does not exceed the load.

Examples:

• Concentric: Biceps contracting to pick up a book.

• Eccentric: Lowering a book back down.

Factors Affecting Muscle Contraction

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

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

• Relative size of fibers: Larger fibers generate more force; regular exercise increases fiber size (hypertrophy).

• Frequency of stimulation: Higher frequency, greater force.

• Degree of muscle stretch: Muscle fibers with sarcomeres at 80-120% of their normal resting length generate more force.

Velocity and Duration of Contraction

The speed and duration of muscle contraction are influenced by:

• Muscle fiber type: Fibers can be slow or fast, depending on their speed of contraction and metabolic pathways used for ATP synthesis.

Additional info: Other factors such as load and recruitment also affect contraction velocity and duration.