Understanding The Muscular System

The Muscular System

• Muscles facilitate all types of body movement.
• The body contains three basic muscle types: skeletal, cardiac, and smooth.

Muscle Types

• Skeletal and smooth muscle cells have an elongated shape and are known as muscle fibers.
• The movement of microfilaments causes muscles to contract and shorten.
• All muscles use similar terminology, with prefixes "myo-" and "mys-" relating to "muscle" and "sarco-" relating to "flesh".

Skeletal Muscle

• Most skeletal muscle fibers attach to bones via tendons.
• Skeletal muscle cells are large, cigar-shaped, and multinucleated.
• This muscle type is also referred to as striated muscle due to its striped appearance.
• It is also known as voluntary muscle because conscious control affects it.

Connective Tissue Bundling

• Connective tissue surrounds and bundles skeletal muscle cells.
• Endomysium encloses single muscle fibers.
• Perimysium wraps around a fascicle (bundle) of muscle fibers.
• Epimysium covers the entire skeletal muscle.
• Fascia is present on the outside of the epimysium.

Epimysium Attachment

• The epimysium of skeletal muscle connects to connective tissue.
• Tendons, which are cordlike structures mostly made of collagen fibers, connect to the periosteum.
• Tendons often cross joints because they are tough and small.
• Aponeuroses are sheetlike structures that attach muscles indirectly to bones or cartilage.

Smooth Muscle

• Smooth muscle lacks striations, is involuntary, and operates without conscious control.
• It exists mainly in the walls of hollow visceral organs, with spindle-shaped, uninucleate fibers.
• Smooth muscle contractions are slow and sustained.

Cardiac Muscle

• Cardiac muscle exhibits striations and is involuntary.
• It is only found in the heart walls and is uninucleate.
• Intercalated discs join branching cells; also, contraction occurs at a steady rate set by the pacemaker.

Muscle Functions of Skeletal Muscle

• Skeletal muscles produce movement, maintain posture and body position, stabilize joints, and generate heat.

Skeletal Muscle Anatomy

• The sarcolemma serves as a specialized plasma membrane.
• Myofibrils, long organelles inside the muscle cell, have light (I) and dark (A) bands which create a striated appearance.

Myofibril Banding Pattern

• An I band is a light band containing thin filaments and a Z disc midline interruption.
• An A band is a dark band containing the entire length of the thick filaments
• An A band also contains the H zone, which is a lighter central area, and the M line in the H zone center.

Sarcomere

• A sarcomere is the contractile unit of a muscle fiber, and is considered the structural and functional unit of skeletal muscle.

Sarcomere Organization

• Myofilaments produce a banding (striped) pattern.
• Thick filaments are myosin filaments, while thin filaments are actin filaments.

Thick Filaments

• Thick filaments are myosin filaments composed of the protein myosin.
• ATPase enzymes inside the filament split ATP to provide energy for muscle contractions and have myosin heads.
• Myosin heads, also known as cross bridges, link thick and thin filaments during contraction.

Thin Filaments

• Thin filaments consist of the contractile protein actin.
• Actin attaches to the Z disc of the filament.
• The A band contains the H zone, which lacks actin filaments at rest.
• H zones disappear during contraction as actin and myosin filaments overlap.

Sarcoplasmic Reticulum

• Sarcoplasmic reticulum (SR) refers to a specialized smooth endoplasmic reticulum that surrounds the myofibril and stores/releases calcium.

Muscle Contraction

• Skeletal muscles possess special functional properties
• Irritability is the ability to receive and respond to stimulation.
• Contractility is the ability to shorten forcibly when stimulated.
• Extensibility is the ability to stretch
• Elasticity is the ability to resume resting length after stretching.

Nerve Stimulus

• Skeletal muscles contract when stimulated by a motor neuron (nerve cell).
• A motor unit refers to one motor neuron and all the skeletal muscle cells it stimulates.

Neuromuscular Junction

• The neuromuscular junction serves as the site where the motor neuron's axon terminal associates with the sarcolemma of a muscle.
• The neurotransmitter, released by the nerve as a chemical, stimulates skeletal muscle and releases a nerve impulse in the axon terminal, where the neurotransmitter acetylcholine (ACh) does so.

Synaptic Cleft

• The synaptic cleft refers to the gap between nerve and muscle filled with interstitial fluid, but nerve and muscle do not make contact.

Nerve Impulse Steps

• A nerve impulse reaching the motor neuron's axon terminal causes calcium channels to open and subsequently, calcium ions to enter the axon terminal.
• Calcium ion entry triggers some synaptic vesicles to release acetylcholine (ACh).
• Consequently, ACh reaches across the synaptic cleft and binds to receptors on the sarcolemma of the muscle cell.

Depolarization

• The sarcolemma becomes more permeable to sodium ions (Na+) if released.
• Potassium ions (K+) diffuse out of the cell and more sodium ions enter in turn.
• Establishing an ion imbalance causes depolarization, and more Na+ channels open in response.

Action Potential

• Depolarization triggers more sodium channels to open so sodium ions can enter the cell, which creates an action potential.
• Once initiated, the action potential is unstoppable and conducts the electrical impulse from one end of the cell to the other
• Acetylcholinesterase (AChE) breaks down acetylcholine into acetic acid and choline, which ends the muscle contraction and allows only a single nerve impulse.

Resting State

• Cells return to the resting state as potassium ions diffuse out of the cell and the sodium-potassium pump restores the ions to their original positions.

Sliding Filament Theory

• Calcium ions (Ca2+) bind regulatory proteins on thin filaments and expose myosin-binding sites so myosin binds to thick filaments.
• Each cross bridge pivots and causes the thin filaments to slide toward the center of the sarcomere
• Each cross bridge attaches and detaches several times during a contraction
• ATP provides the energy for this sliding process as long as calcium ions remain available.

Graded Responses of Muscle Contraction

• Muscle fiber contraction follows the "all-or-none" law, by contracting to its fullest extent when stimulated adequately.
• During the same interval within a whole skeletal muscle, not all fibers receive stimulation.
• Different fiber combinations give different responses depending on stimulus strength resulting in varied degrees of skeletal muscle shortening, known as graded responses.

Producing Graded Responses

• Alternating the frequency of muscle stimulation, or the number of muscle cells stimulated at one time, can produce graded responses.

Muscle Twitch

• A muscle twitch is a single, brief jerky contraction, and is not a normal muscle function

Nerve Impulses

• Nerve impulses get delivered at a rapid rate in all muscle activity types.
• Contractions "sum" (add) for another contraction directly afterward.

Tetanus

• More frequent stimulations strengthen/smooth out muscle contractions, so the muscle exhibits unfused (incomplete) tetanus.
• Fused (complete) tetanus arises when rapid stimulation prevents any relaxation evidence, causing smooth and sustained contractions.

Contraction Strength

• Muscle force is reliant on the number of stimulated fibers.
• Greater tension results the more fibers contract.
• When all motor units are active and stimulated, muscle contraction will be as strong as it can get.

ATP

• ATP directly powers muscle contraction but stores in small amounts, so cells pathways are needed to produce more ATP.

ATP Regeneration

• There are three pathways to regenerate more ATP

1. Direct phosphorylation of ADP by creatine phosphate
2. Aerobic pathway
3. Anaerobic glycolysis and lactic acid formation

Direct Phosphorylation

• Direct phosphorylation of ADP by creatine phosphate (CP) offers the fastest method via muscle cells.
• CP molecule stores a high-energy molecule, and after ATP depletes, CP transfers a phosphate group to ADP for ATP regeneration.
• CP supplies exhaust in less than 15 seconds, but 1 ATP gets produced per CP molecule.

Aerobic Respiration

• Aerobic respiration supplies ATP during rest and light/moderate exercise via oxidative phosphorylation.
• Metabolic pathways use oxygen and occur in the mitochondria, where glucose gets broken down into carbon dioxide and water, releasing 32 ATP.
• Aerobic respiration is slower and requires consistent oxygen and nutrient delivery.

Anaerobic Glycolysis

• The process of anaerobic glycolysis and lactic acid formation occurs without oxygen.
• Pyruvic acid produces ~2 ATP when glucose breaks down.
• Then, pyruvic acid converts to lactic acid, which causes muscle soreness.
• Anaerobic glycolysis not as efficient, it proceeds rapidly and requires substantial amounts of glucose.

Muscle Fatigue

• Fatigue arises from strenuous and prolonged muscle activity.
• Suspected contributors include ion imbalances (Ca2+, K+), oxygen deficit, lactic acid accumulation, and decreased ATP.
• Rapid and deep breathing replenishes the oxygen deficit after exercise.

Isotonic Contractions

• During isotonic contractions, myofilaments slide past each other, so the muscles shorten, and movement occurs
• Actions such as bending the knee, lifting weights, and smiling are Isotonic contractions.

Isometric Contractions

• Muscle filaments pit against an immovable object during isometric contractions while trying to slide.
• In isometric contractions, tension increases but muscles do not shorten.
• Pushing the palms together is an example of an isometric contraction.

Muscle Tone

• Muscle tone stems from a constant state of partial contractions.
• Stimulating different motor units in a systematic way contributes, which allows muscles to remain firm, healthy, and ready for action.

Effects of Exercise

• Exercise boosts muscle size, strength, and endurance.
• Aerobic exercise (biking, jogging) increases muscle strength and flexibility, improving digestion, coordination, and metabolism of the body.
• Individual muscle fibers enlarge during resistance (isometric) exercise like weight lifting.

Golden Rules

• Follow the Five Golden Rules for understanding skeletal muscle activity

Muscle Points

• Muscles need at least two attachment points.
• The origin is an attachment to an immovable (or more immovable) bone.
• The insertion is an attachment to a movable (or more movable) bone.
• When muscles contract, the insertion moves toward the origin.
• Body movement happens when muscles contract across joints.

Flexion

• Flexion reduces the angle of the joint while bringing two bones closer.
• This commonly occurs in hinge joints (e.g., knee, elbow) and ball-and-socket joints (e.g., hip).

Extension

• Extension opposes flexion and increases the angle between two bones.
• Straightening the elbow or knee exemplifies extension, while extension beyond 180° refers to hyperextension.

Rotation

• Rotation functions as a movement of a bone around its longitudinal axis.
• Ball-and-socket include rotational movement.
• An example includes shaking your head "no" when the atlas is moving around the dens of axis.

Abduction and Adduction

• Abduction is the movement of a limb away from the midline.
• Adduction, conversely, shifts a limb toward the midline, and opposes abduction.

Circumduction

• Circumduction combines flexion, extension, abduction, and adduction.
• This occurs in ball-and-socket joints, when the proximal end of bone remains still as the distal end follows a circular path.

Dorsiflexion and Plantar Flexion

• Movement at the ankle include lifting the foot so its superior surface getting closer to the shin during dorsiflexion.
• Plantar flexion involves pointing the toes away from the head.

Inversion and Eversion

• Special foot movements include turning the sole of the foot medially and laterally during inversion and eversion.

Supination and Pronation

• During rotation of the radius around the ulna, supination turns the forearm laterally so the palm faces anteriorly with parallel radius and ulna.
• Forearm rotates medially during pronation, which causes the palm to faces posteriorly and makes radius and ulna cross each other like an X

Opposition

• Thumb opposition includes thumb movement so it touches the tips of other fingers on the same hand.

Muscle Action

• Muscles pull while contracting, meaning they can not push
• In general, opposite actions produce muscle groups in opposite sides of a joint.

Prime Movers

• Prime movers bear the major responsibility for causing a specific movement.
• Antagonists oppose or reverse prime movers
• Synergists assist prime movers or reduce undesirable movements.
• Fixators include specialized synergists that stabilize the origin of a prime mover by holding bone still.

Muscle Characteristics

• Muscles get named based on criteria such as the direction of their fibers
• Rectus (straight) exemplifies muscle fiber direction.
• Muscle also get named by relative size , with maximus (largest) as an example.
• A muscle's location, number of origins, shape, and action determine its name and identifier.
• Temporalis (temporal bone), triceps (three heads), deltoid (triangular), and flexor and extensor (flexes or extends a bone) further exemplify muscle naming criteria.

Muscular System Development

• Increasing muscular control reflects the maturation of the nervous system
• Muscles get controlled in a specific direction from superior/inferior and proximal/distal.
• To maintain muscle health, one must exercise regularly, otherwise muscles atrophy.
• In contrast, muscles hypertrophy with very vigorous exercise.
• As age increases, muscle mass declines, and exercises maintains both muscle mass and strength.