Chapter 9 Human and Anatomy 1

Muscle Contraction

• Tetanus is a sustained contraction of skeletal muscle.
• Muscle tone is a state of partial contraction in which some muscle fibers are contracted, while others are not.

Types of Muscle Contraction

• Isotonic contractions involve a change in muscle length, but no change in tension.
• Isometric contractions involve an increase in tension, but no change in muscle length.

Skeletal Muscle Actions

• The origin of a muscle is the end that does not move during contraction.
• The insertion of a muscle is the end that moves during contraction.

Interaction of Skeletal Muscles

• A prime mover is the muscle whose contraction is responsible for most of the movement.
• Synergists are muscles whose contractions aid the action of the prime mover.
• An antagonist is a muscle that opposes the action of the prime mover.

Comparison of Skeletal, Cardiac, and Smooth Muscle Tissues

• Skeletal muscle tissue is voluntary, striated, and multinucleated.
• Cardiac muscle tissue is involuntary, striated, and has intercalated discs.
• Smooth muscle tissue is involuntary, nonstriated, and is found in the walls of organs.

Skeletal Muscle Tissue

• Skeletal muscle fibers are cylindrical, multinucleated, striated, and voluntary.
• Each muscle fiber is connected to a neuron.

Cardiac Muscle

• Cardiac muscle fibers are cylindrical and branched.
• Each fiber is not connected to a neuron.
• Cardiac muscle is striated and involuntary.
• Intercalated disks allow for communication and coordinated contraction between fibers.
• Cardiac muscle exhibits rhythmicity (self-excitation), meaning it can initiate its own contractions.

Smooth Muscle Tissue

• Smooth muscle fibers are tapered without striations.
• Each fiber is not connected to a neuron.
• Smooth muscle is involuntary.
• Gap junctions facilitate communication and coordinated contraction between fibers.
• Smooth muscle exhibits rhythmicity (self-excitation).
• Smooth muscle contractions produce peristalsis, a wavelike movement that propels substances through tubular organs.

Types of Muscle Cells

• Red muscle fibers (slow-twitch fibers) have a good blood supply, abundant myoglobin, and many mitochondria. They contract slowly and are resistant to fatigue.
• White muscle fibers (fast-twitch fibers) have less blood supply, myoglobin, and mitochondria. They contract quickly but fatigue easily.

Muscular Responses

• A threshold stimulus is the minimum amount of stimulus needed to elicit a contraction in a muscle fiber.
• The all-or-none response states that a muscle fiber will contract fully or not at all in response to a stimulus.
• Subthreshold stimuli result in no contraction. Threshold and suprathreshold stimuli cause full contraction.

Muscle Twitch

• A muscle twitch is a single full contraction of a skeletal muscle fiber.
• A myogram is a recording of muscle responses.
• Periods during a muscle twitch include: latent period (time between stimulus and contraction), period of contraction, period of relaxation, and refractory period (when muscle won’t respond to a stimulus).

Sustained Contractions

• Summation occurs when muscle contractions happen so close together that there is not time to relax between them.
• Troponin is a protein attached to tropomyosin. Calcium ions bind to troponin, changing its shape and altering the position of tropomyosin to expose binding sites on actin.

Neuromuscular Junction

• The neuromuscular junction is the site where a motor nerve fiber connects to a muscle fiber.
• A synaptic knob is the end of a motor neuron and contains vesicles of neurotransmitters, like acetylcholine.
• A motor end plate is a folded area of the sarcolemma where the synaptic knob fits. It has receptors for neurotransmitters.
• The synaptic cleft is the space between the synaptic knob and motor end plate, where neurotransmitters are released and bind to receptors.

Motor Units

• A motor unit consists of one motor neuron and all the muscle fibers it innervates.
• Smaller motor units allow for finer and more precise movements.

Stimulus/Steps for Contraction

• Acetylcholine is released from synaptic vesicles at the neuromuscular junction.
• Acetylcholine binds to receptors at the motor end plate.
• This binding initiates a muscle impulse that travels through the sarcolemma.
• The muscle impulse eventually reaches the sarcoplasmic reticulum and the cisternae.

Excitation Contraction Coupling

• Muscle impulses cause the sarcoplasmic reticulum to release calcium ions into the sarcoplasm.
• Calcium ions bind to troponin, changing its shape.
• The position of tropomyosin is altered, exposing binding sites on actin.
• Myosin heads bind to actin sites, initiating the sliding filament mechanism of muscle contraction.

Sliding Filament Model of Muscle Contraction

• During contraction, thin filaments (actin) slide past thick filaments (myosin).
• The H zone and I band narrow, and the Z lines more closer together.

Steps for Muscle Relaxation

• Acetylcholinesterase breaks down acetylcholine, ending stimulation of the muscle fiber membrane.
• Calcium ions are actively transported back into the sarcoplasmic reticulum.
• Tropomyosin covers the active sites on actin, and cross bridges detach.

3 Types of Muscles

• Skeletal muscles are voluntary.
• Smooth muscles (visceral muscles) are involuntary.
• Cardiac muscles are involuntary.

Skeletal Muscle Structure

• Fascia is a layer of fibrous connective tissue that surrounds muscles and helps hold them in place.
• Tendons are cords of fascia that attach muscles to bones.
• Aponeuroses are broad sheets of fascia that can attach to bones or muscles.
• Epimysium is a connective tissue layer surrounding a whole muscle.
• Perimysium is a connective tissue layer that surrounds bundles of muscle fibers (fascicles).
• Endomysium is a connective tissue layer that surrounds each individual muscle fiber.

Structure of a Muscle Fiber

• Sarcolemma is the cell membrane of a muscle fiber.
• Sarcoplasm is the cytoplasm of a muscle fiber.
• Myofibrils are tightly packed bundles of muscle protein filaments, occupying most of the sarcoplasm.
• Myofilaments are the protein filaments within myofibrils.
  • Actin is a thin filament.
  • Myosin is a thick filament.
• Sarcoplasmic reticulum is the endoplasmic reticulum of a muscle fiber.
• Transverse tubules (T-tubules) are invaginations of the sarcolemma that extend through the muscle fiber, containing extracellular fluid.
• Cisternae are enlarged sacs of the sarcoplasmic reticulum located on either side of T-tubules.

Arrangement of Myofilaments

• The A band represents the length of the myosin filament, including some overlapping actin.
• The I band is made up of actin only and sits between A bands.
• The H zone is the center of the A band containing only myosin.
• The Z line runs down the center of the I band.
• A sarcomere is the area between two Z lines, representing the smallest unit of muscle contraction.

Structure of Myofilaments

• Myosin filaments are composed of the protein myosin and have cross bridges.
• Actin filaments are made up of three proteins: actin, tropomyosin, and troponin.

Energy Sources for Contraction

• ATP is necessary for cross bridge formation between actin and myosin.
• Creatine phosphate can break down to release energy that reforms ATP from ADP and P.
• Glycogen can be broken down into glucose for cellular respiration.

Oxygen Supply

• Hemoglobin in red blood cells stores oxygen, which moves from blood capillaries into muscle fibers.
• Myoglobin in muscle cells binds to oxygen with a higher affinity than hemoglobin and stores it for later use.
• This allows muscles to function even during periods of reduced blood supply.

Oxygen Debt

• During strenuous exercise, oxygen may be used up faster than it can be replenished.
• Anaerobic respiration is used to produce energy, leading to the formation of lactic acid which accumulates in the muscle.
• Lactic acid lowers the pH, contributes to muscle soreness, and requires oxygen to be converted back to glucose by the liver.
• Oxygen debt refers to the amount of oxygen needed to convert lactic acid back to glucose and replenish ATP and creatine phosphate levels.

Muscle Contraction and Energy

• Muscles require energy for contraction, primarily from cellular respiration.
• Much of this energy is lost as heat.

Types of Muscle Tissue

• Skeletal (voluntary): Muscles that move the skeleton.
• Smooth (visceral, involuntary): Found in the walls of organs.
• Cardiac (involuntary): Muscle tissue found in the heart.

Skeletal Muscle Structure

• Connective tissue coverings:
  • Fascia: Layers of fibrous connective tissue surrounding muscles, holding them in place.
  • Tendon: A strong cord of fibrous tissue that connects muscle to bone.
  • Aponeuroses: Broad, flat sheets of connective tissue that may attach to bone or muscles.
• Epimysium: A layer of connective tissue surrounding the entire muscle.
• Perimysium: Connective tissue extending inward from the epimysium, surrounding bundles of muscle fibers called fascicles.
• Endomysium: Connective tissue extending inward from the perimysium, surrounding individual muscle fibers.

Muscle Fiber (Cell) Structure

• Sarcolemma: The cell membrane of a muscle fiber.
• Sarcoplasm: The cytoplasm of a muscle fiber.
• Myofibrils: Tightly packed bundles of protein filaments, primarily responsible for muscle contraction, found in the sarcoplasm.
  • Myofilaments: Protein filaments within myofibrils.
    • Actin: Thin filaments.
    • Myosin: Thick filaments.
• Sarcoplasmic reticulum: The endoplasmic reticulum of a muscle fiber, storing calcium ions.
• Transverse tubules (T-tubules): Indentions of the sarcolemma that pass through the muscle fiber, filled with extracellular fluid.
• Cisterna: Enlarged sacs of the sarcoplasmic reticulum located on either side of T-tubules.

Myofilament Arrangement

• A band: The length of the myosin filament, includes some actin where they overlap.
• I band: The region between A bands, containing only actin filaments.
• H zone: The central region of the A band, containing only myosin filaments.
• Z line: Runs down the center of the I band, anchoring the actin filaments.
• Sarcomere: The smallest functional unit of a muscle fiber, extending from one Z line to the next.

Myofilament Structure

• Myosin: Composed of the protein myosin, containing cross-bridges that bind to actin.
• Actin: Composed of three proteins:
  • Actin: The primary protein forming the thin filaments.
  • Tropomyosin: Covers the active sites on actin, preventing myosin binding.
  • Troponin: Attached to tropomyosin, binding to calcium ions.

Neuromuscular Junction

• Synaptic knob: The end of a motor neuron, containing vesicles filled with neurotransmitters.
• Motor end plate: A specialized region of the sarcolemma where the synaptic knob connects.
• Synaptic cleft: The space between the synaptic knob and motor end plate, containing neurotransmitters.

Motor Units

• A motor unit consists of a single motor neuron and all the muscle fibers it innervates.
• Smaller motor units allow for finer, more precise movements.

Steps for Muscle Contraction

• Acetylcholine (ACh) release: Nerve impulses trigger the release of acetylcholine from synaptic vesicles.
• ACh binding: Acetylcholine binds to receptors on the motor end plate.
• Muscle impulse generation: Binding of ACh generates a muscle impulse that travels through the sarcolemma and T-tubules.
• Calcium release: The muscle impulse triggers the release of calcium ions from the sarcoplasmic reticulum.
• Calcium binding: Calcium binds to troponin, causing it to change shape.
• Tropomyosin movement: The change in troponin shape shifts tropomyosin, exposing the active sites on actin.
• Cross-bridge formation: Myosin cross-bridges bind to the exposed active sites on actin.
• Sliding filaments: Myosin cross-bridges pull the actin filaments towards the center of the sarcomere, shortening the sarcomere.

Steps for Muscle Relaxation

• Acetylcholine breakdown: The enzyme acetylcholinesterase breaks down acetylcholine at the neuromuscular junction, stopping the muscle impulse.
• Calcium reuptake: Calcium ions are actively transported back into the sarcoplasmic reticulum.
• Tropomyosin covers active sites: Tropomyosin returns to covering the active sites on actin, preventing myosin binding.
• Cross-bridges detach: Myosin cross-bridges detach from actin, and the muscle fiber relaxes.

Energy Sources for Muscle Contraction

• ATP: Directly powers the cross-bridge formation and sliding of the filaments.
• Creatine phosphate: Used to quickly regenerate ATP from ADP and phosphate.
• Glycogen: Stored in muscle cells, can be broken down into glucose for cellular respiration.

Oxygen Supply

• Hemoglobin: In red blood cells, stores oxygen for transport to the muscles.
• Myoglobin: In muscle cells, stores oxygen for use when blood flow is reduced.

Oxygen Debt

• Oxygen debt: Occurs during strenuous exercise when oxygen supply is limited, leading to anaerobic respiration.
• Lactic acid production: Anaerobic respiration produces lactic acid, resulting in muscle soreness.
• Repaying oxygen debt: Requires oxygen to convert lactic acid back to glucose and restore normal ATP and creatine phosphate levels.

Types of Muscle Fibers

• Red (slow twitch) fibers: Have abundant blood supply, myoglobin, and mitochondria, allowing them to generate ATP quickly and sustain contractions for longer periods.
• White (fast twitch) fibers: Contract quickly, but fatigue rapidly due to lower blood supply, myoglobin, and mitochondria.

Muscular Responses

• Threshold stimulus: The minimum amount of stimulus needed to elicit a contraction.
• All-or-none response: A muscle fiber either contracts completely or not at all.
• Muscle twitch: A single, brief contraction of a muscle fiber in response to a stimulus.
• Summation: When stimuli occur close together, the muscle does not fully relax before the next contraction, increasing tension.
• Tetanus: When stimuli are so close together that there is no relaxation between contractions, producing a sustained contraction.
• Muscle tone: A state of partial contraction in a skeletal muscle, allowing for smooth movements and maintaining posture.

Types of Muscle Contraction

• Isotonic: Contraction where the muscle changes length, moving a bone.
• Isometric: Contraction where the muscle does not change length, but increases tension.

Skeletal Muscle Actions

• Origin: The end of a muscle that remains stationary during contraction.
• Insertion: The end of a muscle that moves during contraction.

Interaction of Skeletal Muscles

• Prime mover: The muscle primarily responsible for a specific movement.
• Synergists: Muscles that assist the prime mover in its action.
• Antagonist: A muscle that opposes the action of the prime mover.

Comparison of Muscle Types

• Skeletal: Cylindrical, multinucleated, striated, voluntary.
• Cardiac: Cylindrical, branched, single nucleus per fiber, striated, involuntary, interconnected with gap junctions, rhythmicity.
• Smooth: Tapered, single nucleus per fiber, non-striated, involuntary, interconnected with gap junctions, rhythmicity, peristalsis.