Human Physiology: Muscle Types & Functions

• Human Physiology BIOL 2052 A is taught by Dr. Cristina V Dieni.
• Dr. Dieni's email is [email protected].
• The phone number for the class is 942-4291, extension 2248.
• The office for the class is BGSC213.

Types of Muscle

• Three muscle types exist in the body.
• Skeletal muscles are striated, voluntary, and attached to bone.
• Cardiac muscles are striated, involuntary, and found in the heart.
• Smooth muscles are not striated, involuntary, and found in hollow organs.

Muscle Functions

• Producing body movements include walking, running, and localized movements.
• Stabilizing body positions involves skeletal muscle contractions to stabilize joints.
• Skeletal muscle contractions maintain body positions, such as standing or sitting.
• Postural muscles contract continuously when awake to hold the head upright.

Additional Muscle Functions

• Storing and moving substances is achieved by sustained contractions of ring-like bands of smooth muscle called sphincters.
• Sphincters prevent outflow from hollow organs.
• Sphincters facilitate temporary storage of food in the stomach.
• Cardiac muscle contractions of the heart pump blood through the blood vessels.
• Muscle contractions generate heat, a process known as thermogenesis.

Muscle Properties

• Electrical excitability is a property of both neurons and muscle cells.
• Electrical excitability describes the ability to respond to stimuli by producing action potentials.
• Contractility describes the ability of muscle to contract forcefully when stimulated.
• Muscle contraction generates tension and possibly movement.
• Extensibility is the ability of muscle to stretch without being damaged.

Organization of Skeletal Muscle

• Skeletal muscles are connected to bones by tendons.
• Connective tissue surrounds the muscle fascicles and fibers.
• Muscle fibers are individual muscle cells.

Components of a Skeletal Muscle Fiber

• Myoblasts fuse to form skeletal muscle fibers.
• Satellite cells are associated with skeletal muscle fibers.
• Muscle fibers have nuclei, a sarcolemma, and sarcoplasm.
• Myofibrils, mitochondria, and sarcoplasmic reticulum are present in muscle fibers.
• Transverse tubules and terminal cisternae form triads within muscle fibers.
• Thick and thin filaments are found in sarcomeres in muscle fibers.

Intracellular Muscle Structure

• The sarcolemma surrounds the sarcoplasm.
• The sarcoplasmic reticulum (SR) stores calcium ions (Ca2+).
• Transverse tubules aid in the transmission of signals.
• Sarcomeres contain thick and thin filaments made of proteins.
• Myofibrils contain sarcomeres.
• Mitochondria and glycogen granules are present for energy.
• Myoglobin stores oxygen.
• Dystrophin is a structural protein.

Sarcomere Arrangement

• A sarcomere is the basic functional unit of a muscle fiber and is delineated by Z discs.
• Myofibrils consist of repeating sarcomeres.
• Thick and thin filaments overlap within the sarcomere.
• The I band contains only thin filaments.
• The H zone contains only thick filaments.
• The A band spans the length of the thick filaments.
• The M line is in the middle of the H zone.
• Titin filaments help stabilize the thick filaments.

Types of Muscle Proteins

• Three types of muscle proteins exist: contractile, regulatory, and structural.

Contractile Proteins

• Two main contractile proteins are Myosin and Actin.
• Myosin has heads that bind to actin.

Regulatory Proteins

• Two primary regulatory proteins include Tropomyosin and Troponin.

Structural Proteins

• Structural proteins include titin, alpha-actinin, myomesin, nebulin, and dystrophin.
• Strutural proteins contribute to alignment, stability, extensibility, and elasticity.

Sliding Filament Mechanism

• During muscle contraction, thick and thin filaments slide past each other.
• The H zone and I band narrow during contraction.
• The A band remains constant during contraction.
• The Z discs move closer together during contraction.

Contraction Cycle

• The contraction cycle involves four major steps.
• ATP hydrolysis is the first step in the cycle.
• Myosin attaches to actin, forming a crossbridge.
• The power stroke occurs as the myosin head pivots.
• Myosin detaches from actin with ATP binding.

The Neuromuscular Junction

• The neuromuscular junction is the synapse between a somatic motor neuron and a skeletal muscle fiber.
• The motor end plate is the region of the muscle membrane.
• An end plate potential (EPP) occurs at the muscle membrane.
• An action potential is generated at the muscle membrane.

Muscle Action Potential

• The muscle action potential includes a depolarizing phase and a repolarizing phase.
• Depolarization involves voltage-gated Na+ channels opening.
• Repolarization involves voltage-gated Na+ channels closing and voltage-gated K+ channels opening.

Excitation-Contraction Coupling (Relaxed)

• In a relaxed muscle, calcium ions (Ca2+) are stored in the sarcoplasmic reticulum.
• Tropomyosin blocks the myosin-binding sites on actin.
• Troponin holds tropomyosin in position.

Excitation-Contraction Coupling (Contraction)

• A muscle action potential leads to calcium release from the sarcoplasmic reticulum.
• Ca2+ binds to troponin, causing a conformational change.
• Tropomyosin moves away from the myosin-binding sites on actin.

Skeletal Muscle Contraction Summary

• A nerve action potential triggers acetylcholine (ACh) release from a somatic motor neuron.
• ACh binds to receptors on the motor end plate, generating an end plate potential.
• The end plate potential generates a muscle action potential.
• The resulting muscle action potential causes the release of calcium from the sarcoplasmic reticulum.
• Ca2+ binds to troponin on the thin filament, exposing myosin-binding sites.
• Myosin heads bind to actin, and thin filaments are pulled toward the sarcomere center.

Answers to questions

• Q1: C) Flexibility is NOT a characteristic property of muscle tissue.
• Q2: D) Myomesin contributes to the stability of the muscle.
• Q3: C) The motor neuron releases acetylcholine into the synaptic cleft first at the neuromuscular junction.
• Q4: b) False, calcium ions bind to troponin during muscle contraction and NOT actine.
• Q5: The steps of the muscle contraction cycle are: ATP hydrolysis, Attachment of myosin to actin, Power stroke and Detachment of myosin from actin