chapter 40: Overview of Muscle Types and Their Functions

Skeletal Muscle

Voluntary, Conscious control

Skeletal Muscle Structure

Striated, long multinucleated (cells fuse during embryonic development)

Fibers, cells, bound together by connective tissue (fascia)

Cardiac Muscle

Involuntary, autoregulatory

cardiac muscle structure

Striated, branched, uni-or binucleated

Cells smaller than skeletal

Smooth Muscle

Involuntary, ANS control

smooth muscle structure

Nonstriated, spindle shaped cells, central nucleus

Sheets of cells

Muscle Basics

Presence of contractile proteins and muscle tissue (phylogeny).

Animals with Muscle Tissue

All metazoans except cnidarians.

Animals with Contractile Proteins

All metazoans (porocytes in sponges regulate water flow).

Actin

Thin filaments

tropomyosin and troponin

calcium binding site

Myosin

Thick filaments

two interwoven protein chains, each head can hydrolyze ATP and can bind an actin monomer

Muscle Function

Convert chemical energy, ATP, into mechanical energy.

movement (body and internally) and manipulation

Muscle Activity: Contraction

Shortens when stimulated - electrical impulse, hormones.

Muscle Activity: Relaxation

Elongates when stimulation ceases; actin and myosin fibers return to resting position.

Agonist muscle

contract together producing a motion - biceps

antagonist muscle

contraction opposes motion of agonists - triceps

connective tissue: skeletal muscle

surround and hold groups of muscle fibers (cells) together

Endomysium

Around individual muscle fiber

Perimysium:

Around bundle of fibers

Epimysium:

Around entire muscle.

Sarcolemma

Plasma membrane

Sarcoplasm:

Cytoplasm

Sarcoplasmic reticulum:

Endoplasmic reticulum

T tubule:

Transverse tubules.

Triad

SR T-tubule, SR.

Myofibril

Threadlike structures that run lengthwise through sarcoplasm and contain myofilaments.

Myofilaments

Contractile proteins: produce tension; regulatory proteins: control when contraction occurs; structural proteins: produce myofilament structural stability.

Skeletal Muscle Unit

A contractile unit with a structure with repeating sarcomeres.

Z Line

Defining point of a sarcomere; beginning and end of sarcomere

sarcomeres connect and actin is anchored here

I Band

Z line and actin only; part of 2 sarcomeres.

goes from the end of myosin in one to end of myosin in the other

A Band

Runs length of myosin; includes actin overlap with myosin.

H Band

Zone within A band that contains myosin only.

M Line

Proteins that provide sarcomere stability; hold myosin in place and anchor elastic filaments.

Skeletal Muscle Contraction steps

1. Action potential and stimulation of contraction; nerve impulse travels along sarcolemma and down T-tubule - stimulates release of Ca from S, Ca enters cytoplasm

2. Ca bind to troponin, changes shape and causes tropomyosin to roll and exposed myosin - binding sites for actin

3. myosin head energized by ATP, forms cross bridge with exposed binding sight

4. power stroke is initiated, contraction (shortening) begins, flexion

5. ATP binds myosin site and cross bridge is broken (at end of contraction cycle)

Muscle Tone

Muscles are in a state of partial contraction - such as sitting or standing.

sarcomere shortens

happens during a contraction cycle

active and myosin filaments move past one another - sliding filament

Overlap

Increases; I band and H zone decrease in length

actin and myosin do not change in length

Power for muscle contraction

Immediate use of ATP; preformed ATP is rapidly depleted.

Maintaining ATP availability

Creatine phosphate: Phosphate transferred to ADP making ATP - rapidly depleted.

Without sufficient O2 pyruvate and lactic acid formed - fermentation

Glycogen

Stored in muscle and liver.

Oxygen debt

Metabolism of build up lactic acid.

Sarcomere length

Influences strength of contraction; mouth of actin and myosin overlap determines the number of cross bridges that can form

the longer = more strength

shorter it gets = less strength

Motor unit

Defined as the nerve and muscle fibers (cells) it innervates.

Recruitment of motor units

motor units and muscle strength

Single stimulus

Brief electrical stimulus produces a twitch; muscle contraction followed by relaxation - single quick simple twitch.

time for contraction is longer than

action potential

Action potential

Lasts 1-2 seconds; finished before contraction begins.

Summation of multiple stimuli

Separate stimuli arrive very close together - twitches can fuse together.

Unfused tetanus

Occurs when twitches do not completely fuse.

Fused (complete) tetanus

Occurs when twitches completely fuse.

Smooth muscle

Surrounds tubular (hollow) organs; regulates diameter of bronchioles and arterioles.

Skeletal muscle

Attached to bones; responsible for whole body movement.

Regulation of contraction: smooth

myosin linked - contraction initiated by change in myosin

Regulation of contraction: skeletal

actin linked - contraction initiated by change in actin

Initiation of contraction: smooth

autonomic nerve impulse (neurogenic) or self-generated action potential (myogenic)

Initiation of contraction: skeletal

motor nerve impulse.

Strength of contraction

Smooth muscle is stronger than skeletal muscle.

amount of contraction

Smooth shortens more than striated muscle

Myosin head along length

Type of contraction

Smooth muscle contraction is slow and sustained, cycles Ca slowly, use less ATP

gap junctions

Pass electrical signals rapidly between fibers

Gap junctions present: smooth

yes, contract as a sheet

Calcium source: smooth

extracellular and from the sarcoplasmic reticulum, Initiates cascade leading to activation of myosin ATPase

Gap junctions present: skeletal

individual cells stimulated

Calcium source: skeletal

sarcoplasmic reticulum binds troponin - none in smooth

Sarcomeres

Present only in skeletal and cardiac muscle.

contractile myofilaments: smooth

myosin to actin ratio of 1:10 to 1:15

contractile myofilaments: skeletal

myosin to actin ratio of 1:2 to 1:4

Smooth muscle contraction

Absence of striated does not mean no actin or myosin; there is just a different arrangement.

Cnidarians

no muscle fibers and contractile proteins in bundles as a part of non muscle cell layers

epithelia muscular cells

nematodes

single layer of muscle cells running longitudinally

Mollusks

Good example of long, sustained contraction in smooth muscle; keeps the shell tightly closed.

Annelids

Coordinated circular and longitudinal contraction in individual segments for movement.

more and thicker myosin fibers

Arthropods

Use skeletal muscle for fast movements and along the digestive tract.