Lecture 12 Notes

Chapter 11: Skeletal Muscle Tissue

Overview

• This lecture focuses predominantly on skeletal muscle tissue.

• It will cover motor units, muscle contraction, and other related concepts.

• Subsequent lectures will tackle exercise physiology and smooth muscle.

Motor Units

• Definition: A motor unit consists of one motor neuron and all the skeletal muscle fibers it controls.

• Structure:

  • Each motor neuron has axons that branch to form axon terminals.

  • An axon terminal forms a synapse with a muscle fiber.

• Location: Motor neurons are located in the brainstem and spinal cord; brainstem neurons control facial muscles, while spinal cord neurons control limbs and trunk.

• Somatic Nervous System: This division of the nervous system controls skeletal muscles.

• Size of Motor Units:

  • Small motor units are composed of only a few muscle fibers (e.g., eye muscles).

  • Large motor units contain many muscle fibers, required for powerful movements (e.g., gastrocnemius muscle).

• Functionality: Muscles with small motor units provide greater precision while those with larger units deliver more power.

• Fatigue Management: During prolonged activity, the body can switch to different, rested motor units to maintain performance.

Muscle Contraction

• Neuromuscular Junction (NMJ):

  • Definition: The synapse between the axon terminal of a motor neuron and a muscle cell.

  • Components:

    • Axon terminal

    • Synaptic cleft (gap)

    • Motor end plate (sarcolemma portion receiving neurotransmitters)

  • Neurotransmitter: Acetylcholine (ACh) released from the axon terminal stimulates muscle contraction.

• Calcium Role: Calcium ions released from the sarcoplasmic reticulum are essential for contraction.

Phases of Muscle Contraction

• Four Big Phases:

  1. Excitation: Involves action potentials from the motor neuron reaching the muscle and releasing acetylcholine.

  2. Excitation-Contraction Coupling:

     • Action potentials propagate down the sarcolemma and T-tubules.

     • Calcium channels in the sarcoplasmic reticulum open, releasing calcium.

     • Calcium binds to troponin, moving tropomyosin to expose actin's active sites.

  3. Contraction Cycle:

     • Steps include:

       • Crossbridge formation (myosin head binds to actin).

       • Power stroke (myosin head pivots and pulls actin).

       • Detachment (ATP binds to myosin head, allowing it to release actin).

       • Recovery stroke (myosin head is re-cocked using energy from ATP).

  4. Relaxation:

     • Occurs when action potentials stop, calcium is reabsorbed, and tropomyosin covers actin's active sites.

Sliding Filament Theory

• Describes how muscle contraction occurs when myosin heads pull actin filaments closer together without altering their length.

Muscle Paralysis

• Spastic Paralysis: Muscles contract excessively, examples include:

  • Organophosphate pesticides: Inhibit acetylcholinesterase, leading to continuous contraction.

  • Sarin gas: An acetylcholinesterase inhibitor that causes similar effects.

  • Clostridium tetani: Toxin inhibits glycine release leading to muscle spasms (e.g., locked jaw).

• Flaccid Paralysis: Muscles cannot contract, examples include:

  • Curare: Blocks acetylcholine receptors, leading to inability to contract muscles.

  • Clostridium botulinum: Prevents acetylcholine release, resulting in weakness or paralysis.

Conclusion

• This lecture summarizes key concepts of skeletal muscle function, including the critical role of motor units, the neuromuscular junction, contraction phases, and paralysis types.

• Understanding these principles is foundational for further study in exercise physiology and muscular control.