Muscle Tension and Motor Unit Recruitment

Length-Tension Relationship

• A muscle fiber develops maximum tension when thick and thin filaments have an optimal overlap zone.
• Muscle fibers with a thicker diameter contain more myofibrils.
• Thicker fibers generate more tension compared to thinner ones.

Motor Unit Recruitment

• Motor unit recruitment increases the number of active motor units.
• The size of activated motor units impacts how much tension a muscle produces.
• Muscles for precise movements use small motor units, generating small amount of force.
• Entire muscles are not stimulated to contract in unison; some motor units are relaxed while others contract.
• Asynchronous recruitment delays muscle fatigue and sustains muscle contraction for longer.
• Smallest motor units are recruited first with progressively larger units added as needed.
• Slight changes in muscle tension occur when a motor unit is recruited or turned off.

Muscle Tone

• Muscle tone is a small amount of tension in a muscle from weak, involuntary motor unit contractions.
• Muscle tone is established by alternately active and inactive motor units.
• Blood pressure and posture are maintained by muscle tone.

Movement of Skeletal Muscles

• Muscle attachment sites include the origin (stable portion) and the insertion (moveable portion).
• Movement occurs when the insertion pulls towards the origin.

Agonist and Antagonist Muscles

• Agonist muscles make the movement happen.
• Antagonist muscles produce the opposite action at the same joint.

Lever Systems and Leverage

• Skeletal muscles create movement by pulling on bones.
• Bones act a levers, and joints act as fulcrums.
• Levers are rigid structures (like bone) that move around a fulcrum (joint)
• Effort (E) causes movement via muscular contraction
• Load is the weight/resistance that opposes movement

Muscle Contractions

• Isotonic contraction creates movement.
• Muscle length changes (decreases or increases) with constant tension in an isotonic contraction.
• Isometric contraction does not create movement; muscle length remains constant while tension increases.
• Concentric isotonic contraction exerts enough tension to overcome and shorten.
• Eccentric isotonic contraction lengthens a muscle.
• The tension exerted by myosin crossbridges resists movement during eccentric contraction, slowing the lengthening process.

Muscle Fiber Types

• Slow oxidative (SO) fibers have high fatigue resistance.
• Fast oxidative-glycolytic (FOG) fibers have moderate fatigue resistance.
• Fast glycolytic (FG) fibers have low fatigue resistance.

Muscle Fiber Type Characteristics

• SO fibers are the smallest in diameter, FOG fibers are intermediate, and FG fibers are the largest.
• SO and FOG fibers have a large amount of myoglobin, while FG fibers have a small amount
• SO and FOG fibers have many mitochondria and capillaries, while FG fibers have few.
• SO fibers are red, FOG fibers are red-pink, and FG fibers are white (pale).
• SO fibers have a high capacity for generating ATP via aerobic respiration.
• FOG fibers have intermediate ATP capacity using both aerobic respiration and anaerobic glycolysis.
• FG fibers have a low capacity for generating ATP, relying on anaerobic glycolysis.
• The rate of ATP hydrolysis by myosin ATPase is slow in SO fibers, and fast in FOG fibers and FG fibers.
• SO fibers have slow contraction velocity, FOG and FG fibers have fast contraction velocities.
• SO fibers have high fatigue resistance, FOG fibers have intermediate fatigue resistance and FG have low.
• SO fibers have the lowest amount of creatine kinase, FOG fibers have intermediate amount of creatine kinase and FG have the highest.
• SO fibers have low glycogen stores and FOG and FG have intermediate and high amounts respectivley
• SO fibers are recruited first, FOG second, and FG third.
• SO fibers are abundant in postural muscles, such as those in the neck.
• FOG fibers are abundant in lower limb muscles.
• FG fibers are abundant in upper limb muscles.
• SO fibers are primary function in posture as well as endurance
• FOG fibers are primary function is walking and sprinting.
• FG fibers are primary function is rapid intense movements of short duration.

Smooth Muscle

• Smooth muscle fibers have thick and thin filaments
• They do not have transverse tubules and only a small amount of sarcoplasmic reticulum.
• Contraction and relaxation occur more slowly in smooth muscle than in striated muscle.
• The autonomic nervous system regulates smooth muscle which utilizes variscosities to release neurotransmitters.

Types of Smooth Muscle

• Single unit type muscles contract together as one. The visceral muscle is an example
• Multi-unit type muscles act independently of each other. The pupillary muscles are one example.

Smooth Muscle Potentials

• Smooth muscles have two types of potentials: Pacemaker and Slow Wave.
• Pacemaker potentials are spontenaous and always reach threshold.
• Slow wave potentials have constant depolarizations and repolarizations but do not reach threshold.
• Smooth Muscle can contract via autorhythmicity or other stimuli.

Smooth Muscle Activity

• Regulation in smooth muscle is either excitatory, promoting contraction, or inhibitory, promoting relaxation.
• The stress-relaxation response to smooth muscle increases tension initially when stretched, but will release the tension after a minute.
• Smooth muscle produces ATP aerobically and anaerobically.

Cardiac Muscle

• Characteristics include Intercalated Discs, Functional Syncytium and Autorhythmicity.
• Intercalated discs are found at the junction between two muscle cells
• All cells work together as a single unit via Functional Syncytium.
• Electrical potentials are created within Cardiac cells without the need for nervous stimulation

Muscle Type Summary

• Skeletal and cardiac muscle is striated, while smooth is not.
• Skeletal muscle is attached via tendons, cardiac is in the heart, and smooth can be found in vessel walls, airways as well as the eyes
• Skeletal has a diameter of 10-100 um, Cardiac fibers 10-20 um, Smooth 3-8um
• Skeletal's length is 100um-30cm, Cardiac 50-100um, Smooth 30-200um
• Proteins are placed into sarcomeres in skeletal and cardiac musclse, but not in smooth
• Skeletal have abundant sarcoplasmic reticulum, cardiac some, and smooth only a small amount
• Transverse tubles are on skeletal and cardiac, but not smooth
• Only Cardiac and Smooth have autorhythmicity
• The source of Calcium in skeletal is sarcoplasmic reticulum, on cardiac is the same but also extracelluar fluid, and the same is true of smooth
• Skeletal regulation is via troponin and tropomyosin, the same for cardiac, and via calmodulin and myosin in smooth
• Speed of contraction for skeletal is fast, moderate for cardiac and slow for smooth
• Nervous control is generally voluntary for skeletal (somatic) and involuntary (autonomic) for Cardiac and smooth
• Contraction for skeletal is regulated via acetylcholine, the same but also norepinephrine for cardiac, and for smooth accetylcholine hormones and various local stimuli
• For regeneration, skeletal is limited, cardiac has none, and smooth is considerable, via pericytes

Regeneration

• Hypertrophy: Enlargement of existing cells that occurs in skeletal, cardiac, and smooth muscle It can help repair damaged tissue
• Hyperplasia: Increase in the number of fibers Occurs via cell division Can occur in limited types of smooth muscle