Muscles and Body Movements

Muscle Tension at Organ Level

  • Motor Unit: A motor unit is a single motor neuron and all the muscle fibers it innervates. This unit is fundamental to muscle contraction.

  • Motor Unit Tension: When a motor neuron fires an action potential, all muscle fibers within that motor unit respond with the same amount of tension. The 'all or none' principle applies here.

  • Fine vs. Powerful Muscles: Muscles that require fine motor control, such as those in the hand or eye, have small motor units with fewer muscle fibers per neuron. Conversely, large, powerful muscles, like those in the legs, can have motor units consisting of 2,000-3,000 muscle fibers per motor unit.

  • Motor Unit Composition: Each motor unit contains only one type of muscle fiber, categorized as either Type I (slow oxidative) for slow motor units, Type IIa (fast oxidative-glycolytic), or Type IIx (fast glycolytic) for fast motor units. This specialization contributes to functional diversity.

  • Recruitment: Recruitment is the process of activating additional motor units to produce greater force. The body activates slower motor units first, then progressively recruits faster motor units as the required force increases. This orderly recruitment helps prevent fatigue.

  • Muscle Tone: Even at rest, muscles exhibit a small amount of tension known as muscle tone. This is due to involuntary activation of motor units by the brain and spinal cord, which is crucial for maintaining posture, stabilizing joints, generating heat, and preparing muscles for immediate contraction.

Muscle Twitch

  • A muscle twitch is the smallest contractile response resulting from a single action potential. It can be recorded using a myogram to measure tension over time.

  • Phases of a Muscle Twitch:

    • Latent Period: This is a brief delay of 1-2 milliseconds between the action potential and the start of contraction. During this time, the action potential spreads along the sarcolemma and triggers the release of Ca^{2+} from the sarcoplasmic reticulum.

    • Contraction Period: This phase involves a rapid increase in tension as cross-bridge cycling occurs repeatedly due to the release of Ca^{2+}. The duration can range from 10-100 ms.

    • Relaxation Period: Tension decreases as Ca^{2+} is actively transported back into the sarcoplasmic reticulum (SR), reducing the number of active cross-bridges.

  • Refractory Period: A short period of 5 ms following the latent period when the muscle cannot respond to another stimulus. Skeletal muscle has a shorter refractory period than cardiac muscle, allowing for sustained contractions such as tetanus.

Tension Production and Frequency

  • Wave Summation: The phenomenon where repeated stimulation of a muscle fiber results in progressively greater tension. This occurs because the effects of successive action potentials are summed.

    • Tension is directly related to the frequency of stimulation. Higher frequencies lead to greater tension up to a maximum point.

    • The sarcoplasmic reticulum (SR) releases Ca^{2+} with each stimulation. If subsequent stimulations occur before the SR can completely reuptake the Ca^{2+}, the cytosolic [Ca^{2+}] increases, leading to more cross-bridges forming and higher tension.

  • [Ca^{2+}] increases with each stimulation because the SR pumps do not have enough time to remove all Ca^{2+} from the cytosol before the next action potential arrives.

  • Types of Wave Summation:

    • Unfused (Incomplete) Tetanus: The muscle fiber is stimulated approximately 50 times per second. There is partial relaxation between contractions, but tension increases with each twitch until a maximal tension is reached. This produces a quivering contraction.

    • Fused (Complete) Tetanus: The muscle fiber is stimulated at a high frequency, around 80-100 times per second. There is no relaxation between stimuli, and tension remains constant at a maximal level. This results in a smooth, sustained contraction due to the short refractory period of skeletal muscle.

Length-Tension Relationship

  • The amount of tension a muscle can generate is critically dependent on the sarcomere length before contraction.

    • Optimal sarcomere length is achieved when there is an ideal overlap between thin (actin) and thick (myosin) filaments, allowing the most cross-bridges to form. This generates 100% of the muscle's potential tension.

  • The degree of overlap between thin and thick filaments is crucial for effective tension development.

  • If a muscle is shortened by contraction, the sarcomeres also shorten, leading to large zones of overlap. This reduces the number of available binding sites and consequently decreases tension.

  • Conversely, if a muscle is stretched, the sarcomeres become longer, resulting in small zones of overlap. This reduces myosin's ability to bind to actin, thereby decreasing tension.

  • A muscle in a neutral extended position has sarcomeres of optimal length with ideal zones of overlap, allowing maximal myosin attachment and actin shortening during contraction.

Myofilament Sliding

  • When sarcomeres contract, they increase tension in the sarcolemma and endomysium. This tension is then transmitted to the fascicle via the perimysium, and finally to the entire muscle, including the tendons. The tendons pull on bones, causing movement at the joints.

Types of Muscle Contractions

  • Isotonic Contraction: Muscle length changes while generating force that is greater than the external load.

    • Concentric Contraction: The muscle shortens as tension is produced, such as lifting a weight during a bicep curl.

    • Eccentric Contraction: The muscle lengthens as tension is produced, such as lowering a weight in a controlled manner during a bicep curl.

  • Isometric Contraction: The muscle length remains constant because the force generated equals the external load. An example is holding a weight in a fixed position.

Muscle Fatigue

  • Muscle fatigue is the inability to maintain a given level of intensity during exercise.

    • Causes of Muscle Fatigue:

    • Depletion of Key Metabolites: Depletion of energy substrates such as creatine phosphate, blood glucose, and oxygen bound to myoglobin.

    • Accumulation of Chemicals: Accumulation of metabolic byproducts like Ca^{2+}, phosphate, and ADP, which interfere with muscle function.

    • Environmental Conditions: Extreme environmental conditions such as excessive heat and humidity, leading to dehydration and electrolyte imbalances.

Muscle Origin and Insertion

  • Origin: The less movable, anchoring point of a muscle on a bone, typically proximal.

  • Insertion: The end of the muscle attached to the bone that will be moved when the muscle contracts, typically distal.

Muscle Roles

  • Agonist (Prime Mover): The primary muscle responsible for producing a specific movement. It provides the major force.

  • Antagonist: A muscle that opposes the action of the agonist, usually located on the opposite side of the joint. It helps control movement.

  • Synergist: Muscles that work together with the agonist to produce a movement, assisting by stabilizing joints or providing additional force.

  • Fixator: Muscles that stabilize a bone or joint in place, providing a stable base for other muscles to act upon. This increases efficiency and reduces the risk of injury.

Lever Systems

  • Components of a Lever System:

    • Load (Resistance): The weight or object to be moved.

    • Force: The effort applied to the lever to move the load.

    • Fulcrum: The pivot or hinge point of the lever; in the body, this is usually a joint.

  • Mechanical Advantage: Occurs when the fulcrum is farther from the applied force, allowing a smaller force to move a large load over a short distance. This reduces the effort required.

  • Mechanical Disadvantage: Occurs when the fulcrum is nearer to the applied force. This moves a smaller load a greater distance with greater speed but requires more effort.

Classes of Levers

  • First Class Lever: The fulcrum is positioned between the force and the load. The force is applied in the opposite direction to the movement. Can operate at mechanical advantage or disadvantage (e.g., seesaw).

  • Second Class Lever: The fulcrum is located farther from the applied force, with the load in between. The load moves in the same direction as the force, providing a mechanical advantage (e.g., wheelbarrow).

  • Third Class Lever: The force is applied closer to the fulcrum than the load. The load moves