The Muscular System

Functions of Muscle Tissue Movement Facilitation Movement Facilitation Thermogenesis Thermogenesis Postural Support Postural Support Regulation of Organ Volume Regulation of Organ Volume Protects Internal Organs Protects Internal Organs Pumps Blood (HEART) Pumps Blood (HEART)

Characteristics of Muscle Tissue Contractility – able to shorten. Contractility – able to shorten. Extensibility (Flexibility) – able to lengthen. Extensibility (Flexibility) – able to lengthen. Elasticity – able to return to original shape. Elasticity – able to return to original shape. Excitability (Irritability) – able to respond to a stimulus. Excitability (Irritability) – able to respond to a stimulus.

Skeletal Muscle Attached to bones Attached to bones Striated appearance under a microscope Striated appearance under a microscope Voluntary control (conscious control) Voluntary control (conscious control) Multinucleated Multinucleated Myofilaments - contractile elements of each muscle fiber Myofilaments - contractile elements of each muscle fiber

Cardiac Muscle Forms the bulk of heart wall (Myocardium) Forms the bulk of heart wall (Myocardium) Striated Striated Involuntary (typically) Involuntary (typically) Fibers are quadrangular and branching Fibers are quadrangular and branching Cardiac fibers typically have a centrally located nucleus Cardiac fibers typically have a centrally located nucleus Sarcolemmas connected by intercalated discs Sarcolemmas connected by intercalated discs –Strengthens cardiac muscle tissue –Propagates an action potential from cell to cell through specialized structures on the intercalated discs called gap junctions

Smooth (Visceral) Muscle Located in walls of hollow internal surfaces such as: Located in walls of hollow internal surfaces such as: –blood vessels- stomach –urinary bladder- intestines Non-striated in appearance Non-striated in appearance Involuntary (typically) Involuntary (typically) Can be stretched to great lengths Can be stretched to great lengths Allows for tremendous size variability Allows for tremendous size variability

Smooth (Visceral) Muscle

Muscle Tissue Histology Myofilaments - structural components of myofibrils Myofilaments - structural components of myofibrils –Myosin - thick myofilaments –Actin - thin myofilaments

Skeletal Muscle Tissue Structures Epimysium – outermost layer covering muscle Epimysium – outermost layer covering muscle Perimysium – surrounds fascicle of muscle fibers Perimysium – surrounds fascicle of muscle fibers Fascicle – bundles of muscle fibers Fascicle – bundles of muscle fibers Endomysium – surrounds muscle fibers Endomysium – surrounds muscle fibers Sarcolemma – cell membrane of a muscle fiber Sarcolemma – cell membrane of a muscle fiber

Skeletal Muscle Tissue Structures T-tubule – tunnel from surface to center, action potentials move through these T-tubule – tunnel from surface to center, action potentials move through these Sarcoplasm – cytoplasm of muscle cells Sarcoplasm – cytoplasm of muscle cells Myofibrils – a bundle of contractile fibers within muscle cells Myofibrils – a bundle of contractile fibers within muscle cells Sarcoplasmic Reticulum – stores and releases calcium ions – muscle contraction Sarcoplasmic Reticulum – stores and releases calcium ions – muscle contraction

Skeletal Muscle Tissue Structures

Muscle Tissue Structures

Sarcomere

Regions of a Sarcomere

Myosin Thick myofilaments Thick myofilaments Occupy the A Band of the sarcomere Occupy the A Band of the sarcomere Overlap free ends of the actin myofilament Overlap free ends of the actin myofilament Shaped like a golf club Shaped like a golf club –Long, thick protein molecule (tail) –Globular head at the ends

Actin Thin myofilaments Thin myofilaments Anchored to the Z Line Anchored to the Z Line Two stranded protein molecule intertwined around each other Two stranded protein molecule intertwined around each other Associated with two regulatory proteins Associated with two regulatory proteins –Tropomyosin - long stranded protein molecule that follows the contour of actin –Troponin - protein located at regular interval along the tropomyosin that covers the active sites on actin. Has three subunits

Myofilaments

Muscle Action Potential An electrical impulse that originates at the motor end plate, travels along the length of the sarcolemma, down a transverse tubule, and causes the muscle to contract.

Sliding Filament Theory of Muscular Contraction Due to an action potential, the actin and myosin myofilaments slide past one another shortening the sarcomere Due to an action potential, the actin and myosin myofilaments slide past one another shortening the sarcomere No change in length of myofilaments No change in length of myofilaments

Sliding Filament Theory 1. Nerve impulse travels down T-tubule 1. Nerve impulse travels down T-tubule 2. Ca++ releases from sarcoplasmic reticulum 2. Ca++ releases from sarcoplasmic reticulum 3. Ca++ binds to troponin 3. Ca++ binds to troponin 4. Tropomyosin rotates exposing an active site 4. Tropomyosin rotates exposing an active site 5. Myosin head attaches to active site (cross bridge) 5. Myosin head attaches to active site (cross bridge) 6. ATP required for sliding of actin filament (contraction) 6. ATP required for sliding of actin filament (contraction) 7. ATP required for myosin head release (relaxation) 7. ATP required for myosin head release (relaxation)

Muscle Nerve Interaction Neuron - nerve cell Neuron - nerve cell Axon - long, threadlike process that transmits impulse away from cell body (may be up to 1 meter in length) Axon - long, threadlike process that transmits impulse away from cell body (may be up to 1 meter in length) Motor Unit - motor neuron and all the muscle fibers it innervates Motor Unit - motor neuron and all the muscle fibers it innervates Neuromuscular Junction - junction between axon terminal and muscle fiber Neuromuscular Junction - junction between axon terminal and muscle fiber

Muscle Nerve Interaction Motor End Plate - location on the muscle fiber at the end of an axon terminal Motor End Plate - location on the muscle fiber at the end of an axon terminal Synaptic End Bulb - distal end of axon terminal Synaptic End Bulb - distal end of axon terminal Synaptic Vesicles - membrane enclosed sacs within the synaptic end bulbs that store neurotransmitters Synaptic Vesicles - membrane enclosed sacs within the synaptic end bulbs that store neurotransmitters

Muscle Nerve Interaction Synaptic Cleft - space between axon terminal and motor end plate Synaptic Cleft - space between axon terminal and motor end plate Subneural Clefts - folds in sarcolemma along the synaptic gutter Subneural Clefts - folds in sarcolemma along the synaptic gutter Acetylcholine (Ach) - neurotransmitter released from synaptic vesicles that initiates an action potential in a muscle Acetylcholine (Ach) - neurotransmitter released from synaptic vesicles that initiates an action potential in a muscle

Neuromuscular Junction

Muscle Response to Nervous Stimuli All or None Principle All or None Principle –Once a threshold stimulus is applied to a motor unit the muscle fibers innervated by that motor unit will contract to their fullest potential Threshold Stimulus - the weakest stimulus from a neuron that will initiate a muscular contraction Threshold Stimulus - the weakest stimulus from a neuron that will initiate a muscular contraction

Events Leading to Muscular Contraction An action potential travels down the motor neuron. When it arrives at the synaptic knob, the membrane of the nerve at the synaptic cleft is depolarized, thereby increasing the Ca++ permeability of the membrane. An action potential travels down the motor neuron. When it arrives at the synaptic knob, the membrane of the nerve at the synaptic cleft is depolarized, thereby increasing the Ca++ permeability of the membrane. Ca++ diffuses from outside of the synaptic knob to inside the synaptic knob. Ca++ diffuses from outside of the synaptic knob to inside the synaptic knob.

The influx of Ca++ into the nerve causes the release of Ach. The influx of Ca++ into the nerve causes the release of Ach. Ach is ejected into the synaptic cleft, diffuses across the cleft, and depolarizes the muscle membrane. Ach is ejected into the synaptic cleft, diffuses across the cleft, and depolarizes the muscle membrane. This increases the permeability of the muscle membrane to Na+. This increases the permeability of the muscle membrane to Na+. Na+ rushes into the muscle cell, depolarizing the membrane as it travels away from the motor end plate thus initiating an action potential. Na+ rushes into the muscle cell, depolarizing the membrane as it travels away from the motor end plate thus initiating an action potential.

Ach is quickly broken down in the cleft by Ach-ase so that each action potential arriving from the nerve initiates only one action potential within the muscle. Ach is quickly broken down in the cleft by Ach-ase so that each action potential arriving from the nerve initiates only one action potential within the muscle. The action potential spreads across the muscle membrane and down the T- tubules deep into the muscle cell. The action potential spreads across the muscle membrane and down the T- tubules deep into the muscle cell. The action potential of the T-tubules depolarizes the membrane of the nearby sarcoplasmic reticulum which results in the release of Ca++ into the sarcoplasm. The action potential of the T-tubules depolarizes the membrane of the nearby sarcoplasmic reticulum which results in the release of Ca++ into the sarcoplasm.

Ca++ is very quickly removed out of the sarcoplasm by the sarcoplasmic reticulum so the effects of one action potential are very short lived and produce a very small contraction. Ca++ is very quickly removed out of the sarcoplasm by the sarcoplasmic reticulum so the effects of one action potential are very short lived and produce a very small contraction. Many action potentials are necessary to produce enough force to produce a strong or prolonged muscle contraction. Many action potentials are necessary to produce enough force to produce a strong or prolonged muscle contraction. The Ca++ released from the sarcoplasmic reticulum binds with troponin and cause troponin to change shape. The Ca++ released from the sarcoplasmic reticulum binds with troponin and cause troponin to change shape.

When troponin changes shape, it physically moves the other regulatory protein, tropomyosin, out of the way exposing the active sites on the actin myofilament. When troponin changes shape, it physically moves the other regulatory protein, tropomyosin, out of the way exposing the active sites on the actin myofilament. Since the heads or cross-bridges of myosin have a very strong affinity for the active sites on actin, they make contact immediately after the active sites have been exposed. Since the heads or cross-bridges of myosin have a very strong affinity for the active sites on actin, they make contact immediately after the active sites have been exposed. The acto-myosin complex has ATPase activity and ATP is split into ADP + P and energy is released. The acto-myosin complex has ATPase activity and ATP is split into ADP + P and energy is released.

The energy released by the splitting of ATP is used to produce movement of the cross-bridges, sliding the actin and myosin filaments past one another which causes the sarcomere to shorten and the muscle to contract and produce force. The energy released by the splitting of ATP is used to produce movement of the cross-bridges, sliding the actin and myosin filaments past one another which causes the sarcomere to shorten and the muscle to contract and produce force. The myosin cross-bridge has a low affinity for ADP but a very high affinity for ATP. The myosin cross-bridge has a low affinity for ADP but a very high affinity for ATP. It discards the ADP and becomes recharged with a new ATP. It discards the ADP and becomes recharged with a new ATP.

The myosin then releases its hold on the active sites on actin, swivels back to its original position, and is ready to respond to another action potential. The myosin then releases its hold on the active sites on actin, swivels back to its original position, and is ready to respond to another action potential. When another action potential comes along the entire process is repeated. When another action potential comes along the entire process is repeated. It takes many action potentials to produce enough shortening of the sarcomeres to generate enough force to produce movement of a body segment. It takes many action potentials to produce enough shortening of the sarcomeres to generate enough force to produce movement of a body segment.

Muscle Contraction Events

Muscle Contraction Events Muscle Contraction Events

Muscle Origin and Insertion Origin Origin –Body segment with most mass –Usually more proximally located –Usually larger surface area of attachment Insertion Insertion –Body segment with least mass –Usually more distally located –Usually smaller surface area of attachment Gaster (Belly) Gaster (Belly) –Fleshy portion of the muscle between the tendons of the origin and insertion

Roles of Skeletal Muscles Agonist (Prime Mover) Agonist (Prime Mover) –Muscle responsible for the majority of force Antagonist Antagonist –Performs the opposite movement Synergist Synergist –Muscle that assists the agonist provides additional force provides additional force redirects the force of the agonist redirects the force of the agonist Fixator (Stabilizer) Fixator (Stabilizer) –Stabilizes a body segment so the prime mover can act more effectively

Selected Superficial Skeletal Muscles (Anterior View) Pectoralis major Pectoralis major Deltoid Deltoid Biceps brachii Biceps brachii Sternocleidomastoid Sternocleidomastoid Diaphragm Diaphragm Quadriceps Quadriceps –rectus femoris –vastus medialis –vastus lateralis

Anterior Skeletal Muscles

Selected Superficial Skeletal Muscles (Posterior View) Trapezius Trapezius Triceps brachii Triceps brachii Gastrocnemius Gastrocnemius Latissimus dorsi Latissimus dorsi Hamstring Group –semimembranosus –biceps femoris –semitendinosus Gluteus maximus

Posterior Skeletal Muscles

Muscle Diseases and Disorders

Myalgia (Fibromyalgia) Painful disorders of muscles, tendons, and surrounding soft tissue Painful disorders of muscles, tendons, and surrounding soft tissue

Muscular Dystrophies Muscle destroying diseases characterized by the degeneration of individual muscle fibers Muscle destroying diseases characterized by the degeneration of individual muscle fibers Leads to progressive atrophy of skeletal muscles Leads to progressive atrophy of skeletal muscles Due to a genetic defect Due to a genetic defect

Shin Splints Pain in the lower leg Pain in the lower leg Tendonitis of the tibialis posterior muscle Tendonitis of the tibialis posterior muscle Inflammation of the periosteum Inflammation of the periosteum Stress fracture of the tibia Stress fracture of the tibia Exaggerated enlargement of muscles within the epimysium Exaggerated enlargement of muscles within the epimysium Pulling away of the periosteum from the underlying bone Pulling away of the periosteum from the underlying bone Treatment: Treatment: –RICE –strengthen tibialis anterior muscle

Sprains the forcible wrenching or twisting of a joint with partial or complete rupture or injury to joint attachments without dislocation the forcible wrenching or twisting of a joint with partial or complete rupture or injury to joint attachments without dislocation 1st Degree Sprain = stretching of ligaments 1st Degree Sprain = stretching of ligaments 2nd Degree Sprain = partial tearing of ligaments 2nd Degree Sprain = partial tearing of ligaments 3rd Degree Sprain = complete tear of ligaments 3rd Degree Sprain = complete tear of ligaments

Strains pulling or overstretching a muscle pulling or overstretching a muscle soft tissue (Muscle) injury soft tissue (Muscle) injury