Excitable Tissues: Muscle Physiology

Skeletal Muscles

-Contract and allow body to perform variety of movements

-Striated

Cardiac Muscle

-Striated

-Contracts to generate pressure and move blood through vascular system

Smooth muscle

-Unstriated

-Found in walls of hollow organs and tubes

Regulates: Movement of blood; food; air; and urine

Muscle Fiber

-Single skeletal muscle cell

-Large, elongated, and cylindrical

Fascicle

-Bundle of muscle fibers held together

Perinysium

-Outer layer of connective tissue in muscle tissue that surrounds the bundles of muscle fibers

Endomysium

-Connective tissue surround an individual muscle fiber

Myofibrils

-Cylindrical intracellular structures 1um in diameter that extend the entire muscle fiber length

-Specialized contractile elements that constitute 80% of muscle fiber volume

-A and I bands line parallel and produce striation

Non-Contractile Proteins

-Titin

-Nebulin

A Band

-Stacked set of thick filaments with portions of thin filaments that overlap on both ends of the thick filament

-Thick filaments lie ONLY within this band and extend its entire width

-HAS polarity

H Band

-Lighter area in the middle of the A Band where thin filaments do not reach

-Central portions of the thick filaments are found in this region

M Line

-System of supporting proteins holds the thick filaments

-Extends vertically down the middle of the A band within the center of the H zone

I Band

-Consists of remaining portion of thin filaments that do NOT projects into the A band

-Contains ONLY thin filaments from two adjacent sarcomeres

Z Line

-Middle of each I band

-Dense, vertical

-Flat disc that connects the thin filaments of two adjacent sarcomeres

Sarcomere

-Area between the 2 Z lines

-Functional unit of skeletal muscle

-Smallest component of muscle fiber that can contract

-Consists of a whole A band, and half of each of the 2 I bands

How do muscles grow?

-By adding new sarcomeres

Thick Filament Structure

-Contains many myosin molecules packed together

-Myosin has 2 identical subunits shaped like golf clubs

2 Sites for contractile process in Myosin

-Actin-binding site

-Myosin ATPase site

Corssbridges

-Formed from the heads of thick filaments from myosin protein between thick and thin filaments

Thin FIlament Structure

-Consists of actin, tropomyosin,& troponin

-Actin: spherical, primary structure protein and forms backbone of this filament

-Filament is formed by actin molecules joined into 2 helix strands (String of pearls)

-Each actin molecule has a binding site for attaching with myosin crossbridge

Tropomyosin

-Threadlike proteins that lie end to end alongside the groove of actin spiral

-Covers actin sites that bind with crossbridges, blocking the interaction that leads to contraction

Troponin

-Protein complex made up of 3 polypeptide units:

-One binds to tropomyosin (TnT)

-One binds to Actin (TnI)

-One binds with Ca2+ (TnC)

-When this is NOT bound to Ca2+, this protein stabilizes tropomyosin in it's blocking position

Mechanism of Contraction

-Ca2+ binds to troponin and changes its shape so that tropomyosin slips away from its blocking position

-Actin and Myosin can bind and interact to contract

Regulatory Proteins of Muscle Contraction

-Tropomyosin and Troponin

-Because of their role in covering or exposing the actin binding sites

Sliding Filament

-Thin filament on each side of sarcomere slide inward over stationary thick filaments towards the A band's center

-As they slide inward, the thin filaments pull the z lines closer so the whole sarcomere shortens

Excitation-Contraction Coupling (T Tubule System)

-Between plasma membrane, T-Tubule, & Sarcoplasmic reticulum

-Triggers opening of Ca2+ channels from the sarcoplasmic reticulum into the cytosol

-Ca2+ binds to troponin

2 Factors to Accomplish Gradation

-Tension developed by each muscle fiber: Length, Stimulation frequency (twitch vs tetanus)

-The number of muscle fibers contracting within a muscle

Muscle twitch

-From one AP right before contraction is initiated

-Spreads through muscle rapdily

-

Tetanus

-Muscle is stimulated so rapidly that it does not have time to relax

-Constant contraction

Muscle Motor Unit

-The greater the number of fibers contracting, the greater the total muscle tension

-A single muscle fiber is innervated by ONE motor neuron

-One motor neuron can innervate many muscle fibers

-One motor neuron + all muscle fibers innervated

Motor Unit Recruitment

-Stronger contractions occur as more motor units are recruited

-Contraction strength is dependent on the size of this (the number of muscle fibers innervated by one neuron)

Motor units for Muscles for Precise Movement

-Single motor unit may have as few as 12 muscle fibers

-Small unit allows for this type of movement

Motor units for Muscles of powerful controlled movement

-Single motor unit may have 1500-2000 muscle fibers in it

-Results in large incremental increases in whole-muscle tension

-At the expense of less precisely controlled gradations

How does the body prevent fatigue in gravity resistant muscles?

-It will shift from one motor unit to the next to give other units an opportunity to rest

-Carefully coordinated

-During max contractions, this is not possible to alternate motor unit activity

Asynchronous Recruitment of Motor Units

-Sustained contraction is smooth when shifting motor units in gravity resistant muscles

Slow-Oxidative Fibers

-Most resistant to fatigue

-Slow contraction

Fast-Oxidative Fibers

-Speed: Fast

-Fatigue: Intermediate

Fast-glycolytic Fibers

-Speed: High

-Resistance to fatigue: Low

Energy Sources for Contraction

-Energy in phosphate bonds (ATP)

-ATP storage in Creatine

-Anaerobic pathway

-Aerobic pathway

Filaments in Smooth Muscle

-Thick myosin

-Thin Actin (Has tropomyosin but lacks troponin)

-Intermediate filaments: Do NOT participate in contraction but are part of cytoskeletal framework

Smooth Muscle Mechanism of Contraction

-Crossbridge is similar BUT

-Activation is different: Myosin phosphorylation

-NO SARCOMERES

-SLOW but LONG lasting contractions (Non fatiguing)

E-C Coupling in Smooth Muscle

-Excitation

-Rise in Ca2+ from extracellular matrix

-Biochemical events

-Phosphorylation of myosin cross bridges in thick filament

-Binding of actin & myosin at cross bridge

2 Categories of Smooth Muscle

-Multiunit

-Single Unit (Unitary)

Multiunit Smooth Muscle

-Multiple units that function independently of one another and must be separately stimulated by nerves to contract

-All of this type of smooth muscle is Tonic

-Contracts only when stimulated by ANS in: Walls of large blood vessels; small airways; ciliary muscles; dilator/radial, constrictor/circular muscles; base of hair follicles (Goosebumps)

Smooth Muscle E-C Coupling: Unitary (Visceral)

-Most smooth muscle is this type

-Found in walls of hallow organs and is Phasic

-Contract as a single unit

-Does NOT require nervous stimulation (Myogenic) no true resting membrane potential

-Higher when muscle is active & lower when muscle is inhibited: Stretch, hormones & ANS input alter level of activation

-Always in a constant state of partial contraction or tone