The Muscular System and Homeostasis - Comprehensive Notes

10.1 Movement and Muscle Tissue

• Three types of muscle tissue:
  • Skeletal muscle
  • Smooth muscle
  • Cardiac muscle
• Skeletal muscle functions:
  • Produces body movement
  • Maintains body temperature
  • Provides body support
• Muscle fibers contain myofibrils:
  • Myofibrils house thin (actin) and thick (myosin) contractile protein myofilaments.
• Muscle contraction:
  • Actin and myosin slide past each other.
• Energy for muscle contractions is provided by:
  • Creatine phosphate
  • Fermentation
  • Aerobic cellular respiration

10.2 Muscles, Health, and Homeostasis

• Three types of skeletal muscle:
  • Slow-twitch
  • Fast-twitch
  • Intermediate type
  • These are found in different parts of the body.
• Muscle atrophy:
  • Occurs with inadequate stimulation.
• Muscle hypertrophy:
  • Occurs with appropriate repeated stimulation.
• The muscular system works with other body systems to maintain homeostasis.

Launch Lab: Working in Pairs

• Muscles attached to bones move the body.
• Contractions of muscles cause bone movements at joints.
• Muscles can only pull, not push, because they shorten when they contract.
• Muscles work in pairs:
  • One muscle moves a bone in one direction.
  • The other muscle moves the bone in the opposite direction.

Section 10.1: Movement and Muscle Tissue

• Muscle tissue:
  • Specialized to convert chemical energy into kinetic energy (energy of movement).
• Muscle contraction:
  • All muscles can contract (shorten).
  • Contraction moves part or all of the body.
• Three types of muscle cells:
  • Smooth muscle
    • Causes food movement through the intestines
  • Cardiac muscle
    • Enables the heart's unceasing movement
  • Skeletal muscle
    • Enables body movement by pulling on bones

Smooth Muscle

• Cell characteristics:
  • Long and tapered with one nucleus
  • Arranged in parallel lines forming sheets
• Location and function:
  • Walls of blood vessels: Regulates blood pressure and flow
  • Iris of the eye: Controls pupil size
  • Walls of hollow internal organs: Causes wall contractions
• Contraction:
  • Involuntary (without conscious control)
  • Slower than skeletal muscle
  • Can sustain prolonged contractions without fatigue

Cardiac Muscle

• Location:
  • Unique to the heart, forming the heart wall.
• Cell characteristics:
  • Tubular, striated, and branched with one nucleus
• Contraction:
  • Involuntary.

Skeletal Muscle

• Cell characteristics:
  • Tubular and striated
  • Multiple nuclei
  • Referred to as fibers rather than cells due to their structural organization
• Contraction:
  • Voluntary (consciously controlled by the nervous system)
• Function:
  • Responsible for a great range and types of movement in the human body (over 600 skeletal muscles).

Functions of Skeletal Muscle

• Body Support:
  • Contraction opposes gravity, enabling standing and upright posture
• Bone Movement:
  • Contraction accounts for movements of limbs, eyes, facial expressions, and breathing
• Maintaining Body Temperature:
  • Contraction causes ATP breakdown, releasing heat distributed throughout the body
• Protecting Internal Organs and Stabilizing Joints:
  • Pads bones protecting organs; tendons hold bones together at joints
• Muscles can only pull; they cannot push.
• Work is done during contraction; relaxation is passive.
• Muscles that permit skeletal movements are present in pairs.
• For each muscle action, there is another muscle with the opposite action.
• Example:
  • Biceps muscle: Flexes (bends) the arm.
  • Triceps muscle: Extends (straightens) the arm and stretches the relaxed biceps muscle.

Investigation 10.A: Observing Muscle Tissue

• Three types of muscle tissue (skeletal, smooth, cardiac) have distinct characteristics.
• Part 2
• Tests length changes of glycerinated muscle fibers in different solutions:
  • Glycerol alone
  • Potassium-magnesium salt solution alone
  • ATP alone
  • Both salt solution and ATP

Skeletal Muscle Structure

• Organization levels:
  • Muscles lie along the length of a bone.
  • Tendons attach muscle ends to different bones.
  • Long muscle fibers (up to 20 cm) organized into larger bundles.
  • Connective tissue wraps around each fiber, bundle of fibers, and the whole muscle.
• Blood vessels and nerves run between muscle fiber bundles:
  • Blood supply provides nutrients and oxygen
  • Nerves trigger and control contractions

Muscle Fiber Components

• Myofibrils:
  • Cylindrical subunits within muscle fiber.
• Myofilaments:
  • Protein structures responsible for muscle contractions.
• Other organelles:
  • Numerous mitochondria (about 300 per muscle fiber)
• Sarcolemma:
  • Cell membrane of a muscle fiber
  • Surrounds fiber, regulates material entry/exit
• Sarcoplasm:
  • Cytoplasm of a muscle fiber
  • Site of metabolic processes, contains myoglobin and glycogen
• Sarcoplasmic Reticulum:
  • Smooth endoplasmic reticulum
  • Stores calcium ions needed for muscle contractions

Muscle Fiber Contraction Mechanism

• Involves coordinated action of actin and myosin myofilaments.

Actin Myofilament

• Structure:
  • Two strands of protein (actin) molecules wrapped around each other.

Myosin Myofilament

• Structure:
  • Two strands of protein molecules wound around each other.
  • About 10 times longer than actin.
  • One end has a long rod, and the other has a double-headed globular region (head).
    • Myosin heads move first.
    • Heads bend backward and inward.
    • This moves them closer to their rod-like “backbone”.
    • Heads are chemically bound to an actin myofilament and pull the actin along with them.
• Sliding filament model: Actin slides past myosin.
  • Requires ATP to reposition the myosin head before each flex.

Sliding Filament Model

• Actin anchored at the Z line in striated muscle tissue.
• Actin movement pulls the Z line.
• Mechanism depends on the structural arrangement of myosin myofilaments in relation to pairs of actin myofilaments.
• Actin molecules drag Z lines toward each other as they slide past the myosin core.
• Plasma membranes, attached to Z lines, move towards each other, causing the entire muscle fiber to contract.

Role of Calcium Ions in Contraction

• Relaxed muscle:
  • Myosin heads are raised and ready but cannot bind to actin because attachment sites are blocked by tropomyosin.
• For contraction:
  • Tropomyosin must be moved by troponin, which binds to tropomyosin.
  • Troponin-tropomyosin complex is regulated by calcium ion concentration of the sarcoplasm.
• Low calcium concentration:
  • Tropomyosin inhibits myosin binding and the muscle is relaxed.
• Raised calcium concentration:
  • Ca^{++} binds to troponin, shifting the troponin-tropomyosin complex away from the myosin head attachment sites on the actin.
  • Myosin heads attach to actin and, with ATP energy, move the actin filament to shorten the myofibril.
  • The source of Ca^{++} is the sarcoplasmic reticulum.
  • When a muscle fiber is stimulated, Ca^{++} is released and diffuses into myofibrils.
  • When nerve impulses stop, Ca^{++} is returned via active transport.

Energy for Muscle Contraction

• ATP produced before exercise lasts only a few seconds.
• Muscles acquire new ATP in three ways, depending on oxygen availability:
  • Creatine phosphate breakdown
    • High-energy compound that builds up when a muscle is resting.
    • Regenerates ATP by transferring a phosphate group to ADP: \text{Creatine phosphate} + ADP \rightarrow \text{Creatine} + ATP
    • Provides enough energy for about eight seconds of intense activity; rebuilt when muscle is resting.
  • Aerobic cellular respiration
    • Takes place in mitochondria and usually provides most of a muscle’s ATP.
    • Muscle fiber uses glucose (from glycogen) and fatty acids (from fats) as fuel:
    • C6H{12}O6 + O2 \rightarrow CO2 + H2O + ATP + \text{Heat}
    • C6H{12}O_6: Glucose
    • O_2: Oxygen
    • CO_2: Carbon Dioxide
    • H_2O: Water
    • Myoglobin stores oxygen for use during muscle contractions
    • Roughly two-thirds to three-quarters of the heat maintaining a constant body temperature comes from the aerobic cellular respiration of skeletal muscle throughout the body.
  • Fermentation
    • Supplies ATP without consuming oxygen. Glucose broken down to produce lactate:
    • C6H{12}O_6 \rightarrow \text{Lactate} + ATP
    • Lactate accumulation makes the sarcoplasm more acidic, eventually causing enzymes to cease functioning well.
    • If fermentation continues longer than two or three minutes, cramping and fatigue can set in.

Oxygen Deficit

• Occurs when a muscle uses fermentation to supply its energy needs.
• Athletes who train:
  • The number of mitochondria in muscle tissue increases.
  • Blood glucose is spared for the activities of the brain.

Section 10.2: Muscles, Health, and Homeostasis

Muscle Tone

• During active use, some muscle fibers are contracting and others are relaxing; muscles rarely fatigue completely.
• Even at rest, some fibers are always contracting, resulting in muscle tone.
• Muscle tone is important for maintaining posture.

Muscle Atrophy

• Lack of use leads to muscle impairment
  • Reduced stimulation reduces fiber size and strength
• Atrophy: reduction in size, tone, and power of a muscle.
• Temporary reduction in muscle use can lead to atrophy.
• Damage to the nervous system or spinal cord injury can lead to gradual loss of muscle tone and size in affected areas.
• Initially, atrophy is reversible, but extreme atrophy can lead to permanent loss of muscle function.

Exercise and Muscle Contraction

• Regular, moderate exercise strengthens the muscular system, enabling muscles to use energy more efficiently.
• Hypertrophy: exercise-induced increase in muscle mass due to an increase in the size of individual skeletal muscle fibers, not an increase in their number.
• Physical fitness:
• Enzymes within a trained runner’s muscle fibers are more active and numerous.
• Additional blood vessels develop.
• More glycogen is stored.

Muscle Twitch

• Isolated skeletal muscles studied by artificial stimulation with electrodes.
• Muscle attached to a movable lever records contraction as a visual pattern.
• Muscle twitch: single contraction that lasts a fraction of a second.
  • Latent period
    • Time between stimulation and start of contraction
  • Contraction period
    • Muscle shortens
  • Relaxation period
    • Muscle returns to former length
• All-or-none contraction: stimulation of individual muscle fiber usually results in a maximal response.
• Summation: a rapid series of threshold stimuli results in successive twitches riding piggyback on each other in a cumulative response.
• Tetanus: maximal sustained contraction achieved through rapid stimuli; twitches are fused and blended completely into a straight line.
• Fatigue: apparent when a muscle relaxes even though stimulation continues due to depletion of energy reserves.

Slow-Twitch and Fast-Twitch Fibers

• All muscle fibers metabolize both aerobically and anaerobically.
• Slow-twitch fibers (Type I): tend to be aerobic and contract slowly but resist fatigue (more endurance).
• Most helpful in activities such as biking, jogging, swimming, and long-distance running.
  • Produce most of their energy aerobically and tire only when fuel is gone.
  • Rich in myoglobin.
  • Surrounded by dense capillary beds.
  • Have a substantial reserve of glycogen and fat.
• Fast-twitch fibers (Type II): tend to be anaerobic and adapted for the rapid generation of power.
  • Most helpful in activities such as sprinting, weight lifting, and swinging a hockey stick or tennis racket.
  • Rich in glycogen.
  • Vulnerable to lactate accumulation, causing them to fatigue quickly.
• Human muscles have a third, intermediate form of fibers:
  • Fast-twitch
  • Also have a high oxidative capacity
  • More resistant to fatigue.
• Heredity also plays a role in the proportion of fast-twitch and slow-twitch fibers.
• Adaptations that spare muscle glycogen and/or efficiently remove lactate improve physical endurance.
• Muscle enlargement is produced by frequent periods of high-intensity exercise in which the muscles work against high resistance, as in weight lifting.
  • Muscle hypertrophy: increase in the size of the muscle fibers. Myofibrils thicken due to the synthesis of actin and myosin, and then an increase in the number of myofibrils within the muscle fibers
  • Decline in physical strength with age is associated with reduced muscle mass.

Homeostasis, Muscles, and Other Body Systems

• Movement is essential for maintaining health and homeostasis throughout the life cycle.
• Muscular system helps to maintain a constant body temperature.
• Shivering (involuntary skeletal muscle contractions) generates body heat through ATP use.
• Muscles are involved in other processes such as:
  • Grinding food
  • Peristalsis
  • Breathing
  • Returning venous blood to the heart