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Muscular System Note Slides

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Naming of Names

• The roots…

• Myo- and Mys- = muscle

– Epimysium: connective tissue that wraps the muscle

• Sarco = flesh

– Sarcolemma: muscle plasma membrane

• Skeletal and Smooth Muscle cells are called fibers

Characteristics of Muscle Tissue • Excitability: responsiveness/irritablility; the ability

to receive and respond to stimuli

– Stimuli from the internal and external environments

• Contractility: ability to shorten forcibly when adequately stimulated

• Extensibility: ability to be stretched or extended

• Elasticity: ability of a muscle cell to recoil and resume its resting length after being stretched

• Conductivity: ability to propagate electrical signals over membrane

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Side Note • Muscles contract, they do NOT flex

– Flexion is a description of the movement of a joint

– Contraction is the shortening of a muscle

• Extensibility does NOT equal Extension

– Extensibility is one of the special characteristics of muscle; stretch

– Extension is a description of a joint movement

Functions of Muscle • Produce Movement: just about all movements

in the human body and its parts result from muscle contraction

– All three types: skeletal, cardiac and smooth

• Maintain Posture and Body Position: skeletal muscle continuously adjusts posture and positioning in relation to gravity

• Stabilize Joints: skeletal muscles stabilize and strengthen the joints of the skeleton

Functions Continued • Generate Heat: muscles generate heat as they

contract – Maintains normal body temperature

– Skeletal muscle accounts for ̴40% of body mass (most responsible for heat production)

• Miscellaneous – Skeletal muscle: protects organs by encasing them

– Smooth muscle: maintain blood pressure, passage of internal substances, dilate and constrict pupils of the eye, form arrector pili muscles attached to hair follicles

– Cardiac Muscle: propels blood through the body

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Smooth Muscle Tissue • Location: walls of hollow visceral organs

– i.e., stomach, urinary bladder, respiratory passages, walls of arteries

• Cell Description: elongated fibers without striations

• Contraction: slow, sustained, involuntary – Lacks neuromuscular junctions

– Under autonomic control – sypathetic, parasympathetic

– Peristalsis: wave of contraction followed by relaxation

Cardiac Muscle Tissue

• Location: the heart

• Cell Description: short, striated, quadrangle shaped

• Contraction: in syncytium (all-at-once)

– NOT voluntary

– Autorhythmic: heart has its own pacemaker

– Under Autonomic control – sympathetics and parasympathetics will speed up and slow down rate

Skeletal Muscle Tissue • Skeletal Muscle = Voluntary Muscles • Cell Description: long, cylindrical,

multinuecleated • Contraction: rapid, but tires easily

– Some predetermined by brain stem and can be over-ridden

• Skeletal Muscle Tissue has obvious stripes called striations

• Each muscle is served by one nerve, an artery and one or two veins

• Each motor neuron supplies multiple muscle cells – Neuromuscular junction: each muscle fiber is supplied

with a nerve ending that controls contraction

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Skeletal Muscle Tissue 2 • Requires huge amounts of Oxygen and

nutrients via blood supply, and give off large amounts of metabolic wastes

• Individual Muscle Fibers are held together by several connective tissue sheaths

– Support to each cell and reinforce the whole muscle (prevent muscles from bursting during strong contraction

– Continuous with one another and tendons

– Contribute to elasticity, entry and exit for blood vessels and nerve fibers

Connective Tissue Sheaths • Epimysium

– “Outside the muscle”: dense irregular connective tissue that surrounds the whole muscle

– Sometimes blends with deep fascia between muscles or superficial fascia deep to skin

• Perimysium – “Around the muscle”: fibrous connective tissue surrounds

fascicles – Groups of muscle fibers are grouped into fascicles that

resemble bundles of sticks

• Endomysium – “Within the muscle”: areolar connective tissue surrounds

individual muscle fiber – Not to be confused with the sarcolemma (coming up)

Attachments

• Most skeletal muscles span joints and are attached to bone in at least two place

– Dierect: Epimysium of muscle is fused the periosteum of bone

– Indirect: Connective tissue wrappings extend beyond the muscle as

• Tendon

• Aponeurosis

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Microscopic Anatomy 1 • Sarcolemma: plasma membrane surface of a

muscle fiber

• Sarcoplasm: cytoplasm of a muscle cell – Large amounts of glycosomes (stored glycogen)

and myoglobin (oxygen binding protein)

• Myofibrils: rodlike bundles of contractile filaments found in muscle fibers ̴80% cell volume – Contractile organelle

– Striations: repeating series of dark (A) and light (I) bands along the length of the myofibril

Microscopic Anatomy 2 • Sarcomere: smallest contractile unit

– Region between two Z discs; functional unit of skeletal muscle

• Myofilaments: molecular level of the sarcomere – Made of contractile proteins

– Thick filaments: protein myosin ( ̴ 300 molecules) • Ea. Molecule has a rod-like tail and two globular heads

called cross bridges ( ATP and Actin binding sites)

• Held in place by M Line proteins

– Thin filaments: contractile protein actin (contains binding sites for myosin • Regulatory proteins tropomyosin (covers binding site) and

troponin (moves tropomyosin)

The Bands, Zones, Discs and Lines… • A Bands: Actin and Myosin overlap

• I Bands: no Myosin (lighter)

• H Zone: Myosin only; thin (actin) and thick (myosin) filaments do not overlap

• M Line: appear darker due to the presence of desmin (aka myomesmin); helps anchor thick filaments – M Line of the H Zone of the A Band

• Z Disc: coin shaped sheet of proteins (titins = connectins) that anchor thin filaments and connects myofibrils to one another

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Microscopic Anatomy 3 • Sarcoplasmic Reticulum: elaborate smooth ER

– endoplasmic reticulum:“network within the cytoplasm”)

– Runs longitudinally and surrounds each myofibril communicating at the H Zone • Extensive system of interconnected tubes and parallel

membranes enclosing fluid filled cavities (cisternae)

• Terminal cisternae: “end sacs” at the A Band – I Band junction – Calcium Ion storage

• T Tubules: elongated tubes that penetrate into the cell at A Band – I Band Junction – Invaginations of the sarcolemma that conduct

impulses deep into the cell

Sliding Filament Model of Contraction • Think of it as Myosin ratcheting itself along Actin

which shortens the Sarcomere

– Calcium is released→ Calcium binds to troponin→ Troponin moves tropomyosin→ Myosin binds to Actin → Actin is pulled toward center of Sarcomere→ Sarcomere shortens→ muscle fiber shortens→ muscle shortens

• Thick and thin filaments do NOT change length

• Requires ATP

Muscle Metabolism and ATP • ATP: Adenosine Triphosphate

– Adenine + ribose + 3 phosphates

– Stores and releases chemical energy

– Hydrolysis reactions: uses water to break bonds • ADP +P

• Muscle only stores 4-6 seconds of ATP – After hydrolized into ADP + P, quickly regenerated to ATP

• Muscle fatigue: state of physiologic inability to contract even though muscle is still receiving stimuli

• Contractures: state of continuous contraction due to lack of ATP, cross bridges are unable to detach

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Sliding Filament Model of Contraction • 6 Steps

– 1)Calcium released from terminal cisternae • Action potential in the Sarcolemma causes Calcium release

• Change in the conformation of the Troponin-Tropomyosin complex, tropomyosin slides over

– 2) Myosin binds to Actin • When binding site is exposed, energized Myosin head can

bind and form a cross bridge

– 3) Power stroke of the myosin head causes sliding of thin filaments • The chemical energy of ATP is changed to mechanical energy

of contraction

• Myosin head flexes pulling Actin toward the center of the Sarcomere

Sliding Filament Model of Contraction • 6 Steps

– 4) ATP binds to Myosin head resulting in disconnect from Actin

• ATP molecule must bind to its site on the myosin head for Actin to be released – If ATP in muscle is depleted, muscle will stay in a state of “rigor”

– 5) Hydrolysis of ATP leads to re-energizing and repositioning of myosin heads

• Energy is transferred to the myosin head returning is to its high energy conformation

– 6) Transport of Calcium back into terminal cisternae

• Active transport – requires ATP

Innervation of a Muscle: Vocabulary • Motor Neuron: nerve cell that activates a muscle

fiber – Motor neurons reside in the brain, but their long axons

travel to the muscle fiber

• Neuromuscular junction: region where motor neuron comes into close contact with fiber – Synaptic Cleft: functional space between muscle fiber and

the motor neuron

– Acetycholine (Ach): neurotransmitter released within synaptic cleft, stimulates electrical impulse creating action potential

– Acetylcholinesterase (AChE): binds with and breaks down Ach; terminates action potential

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For Contraction to Occur • For a Skeletal Muscle to contract

– it must be activated (stimulated by a nerve ending) causing a change in the membrane potential

– It must generate and propagate an electrical current (action potential) along its Sarcolemma • A resting Sarcolemma is polarized – it has a potential

difference or voltage across the membrane with the inside being relatively negative

– A short-lived rise in intracellular Calcium ion levels occurs to trigger contraction • This is caused by the action potential traveling down

the T Tubules

Excitation-Contraction Coupling

• Sequence of Events by which transmission of an action potential leads to the sliding of filaments

• Depolarization: loss or reduction of negative membrane potential

• Repolarization: movement of the membrane potential to the initial negative resting (polarized state)

• Refractory Period: period during the repolarization phase when the cell cannot be stimulated again until repolarization is complete

At the Neuromuscular Junction • Action Potential arrives at axon terminal • Voltage-gated calcium ion channels open

– calcium enters axon terminal

• Calcium Ion release causes release of Ach • Ach diffuses across cleft and binds to receptors • Ach binding opens ion channels that allows Sodium

into and Potassium out of the muscle fiber – More Sodium goes in than Potassium out, this causes

depolarization – Action potential is propogated as the local depolariztion

spreads to adjacent areas

• Ach effects are terminated by enzymatic breakdown by AChE

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Muscular System Images

Skeletal Muscle System

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3 Types of Muscle

Smooth Muscle

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Cardiac Muscle

Skeletal Muscle

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Nerve and Blood Supply

Connective Tissue Sheaths

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From Gross to Microscopic Anatomy

Thin and Thick Filaments

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The Bands, Zones, Discs and Lines…

Sarcoplasmic Reticulum and T Tubule

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Sliding Filament Model of Contraction

Sliding Filament Model of Contraction

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At the Neuromuscular Junction