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Biology 121 Anatomy and physiology. Lecture 5 Chapter 7 The Muscular System. VIII. The Muscular System A. Introduction 1. Kinds of Muscle (3 types). a. Skeletal- striated, voluntary

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biology 121 anatomy and physiology

Biology 121Anatomy and physiology

Lecture 5

Chapter 7

The Muscular System


VIII. The Muscular System

A. Introduction

1. Kinds of Muscle (3 types).

a. Skeletal-striated, voluntary

b. Smooth-nonstriated, involuntary

c. Cardiac-striated, involuntary

2. Number of skeletal muscles.

a. Approximately 700

3. Purpose of skeletal muscle.

a. Movement-by shortening (contraction) usually in pairs

b. Maintain posture

c. Support soft tissue and organs

d. Maintain body temperature

e. Guard entrances and exits


4. Properties of muscle tissue

a. Excitability (irritability).-responds to stimulation

b. Contractility-shortens or contracts upon stimulation

c. Elasticity – can be stretched.

C. Structure of Skeletal muscle

-Skeletal muscle is an organ composed of

a. muscle tissue (muscle fibers or cells)

b. connective- binds muscle forms tendons

c. Blood vessels and nerves

-provide oxygen and nutrients

-nerves transmit signals from brain/ sp. cord


C. Structure of a Skeletal Muscle continued (bundles of bundles) 1. Muscle –covered with connective tissue: epimysium.

epimysium=collagen separates muscle from surrounding organs

2. perimysium-divides muscle into compartments of muscle fibers (facscicles)- this layer has blood vessels and nerves

3. Each individual muscle fiber (cell) within a fascicle are bound to adjacent fibers with endomysium

Epimysium, perimysium, endomysium

Come together to form tendons

special structures of muscle fiber cell
Special Structures of muscle fiber (cell)
  • Sarcolemma=cell membrane, conduct impulses from a nerve
  • Transverse tubules=openings in the cell membrane, conduct nervous impulses into cell interior
  • Sarcoplasmic reticulum=ER, does job of ER in regular cell but in muscle stores calcium.
    • Releases calcium into sarcoplasm when stimulated from nervous impulse
    • Pumps calcium back into SR when muscle relaxes
  • Myofibrils=bundles of myofilaments (thick and thin) that are responsible for muscle contraction
muscle cell ultrastructure


Muscle cell ultrastructure


= cell membrane

  • Transverse tubules (T tubules)
    • Transmit nervous stimulus (action potential)from sarcolemma through cell
    • Allows entire muscle fiber to contract simultaneously

Sarcoplasmic reticulum

Calcium store: releases calcium when stimulated causes myofilaments to interact

muscle myofibrils
Muscle myofibrils
  • Myofibrils
    • Made up of bundles of protein filaments (myofilaments):
      • Myofilaments are responsible for muscle contraction
    • Types of myofilaments:
      • Thin filaments:
        • made of the proteins actin +troponin/tropomyosin
      • Thick filaments:
        • made of the protein myosin

Thin and Thick Filaments

  • Thin filaments:
    • actin (filamentous actin):
      • two twisted rows has a myosin binding site
    • Tropomyosin;
      • prevents actin–myosin interaction
    • Troponin:
      • controlled by Ca2+
  • Thick filaments:
    • Contain twisted myosin subunits
    • Tail:
      • binds to other myosin molecules
    • Head:
      • made of two globular protein subunits
      • reaches the nearest thin filament

Myosin can bind and pull on actin


Overlapping arrangement of myofilaments is called a sarcomere=the unit of contraction

    • Hundreds of sarcomeres make up a single myofibril
    • Sarcomeres shorten upon contraction causing the muscle cell to shorten



Pattern of light and dark is called??

myofibril contraction
Myofibril Contraction
  • In the absence of calcium myosin and actin cannot interact
  • In the presence of calcium Myosin binds to Actin and begins contraction
  • How?
    • Ca2+ binds to receptor on troponin-tropomyosin complex which lies on actin
    • Troponin–tropomyosin complexchanges position
    • Exposes a myosin binding (interaction) site
    • Myosin interacting with actin causes the sarcomere to shorten (contract)
the contraction cycle myosin interacting with actin
The Contraction Cycle=myosin interacting with actin
  • Five Steps of the Contraction Cycle
    • Exposure of active sites
    • Formation of cross-bridges
    • Pivoting of myosin heads
    • Detachment of cross-bridges
    • Reactivation of myosin
molecular events of the contraction process
Molecular Events of the Contraction Process

Calcium binds troponin and causing tropomyosin to move aside

This exposes the myosin binding site on actin

molecular events of the contraction process2
Molecular Events of the Contraction Process

Figure 7-5

Myosin-actin releases ADP causes myosin head to pull on actin

molecular events of the contraction process3
Molecular Events of the Contraction Process

Figure 7-5

ATP binds to Myosin enabling myosin to release actin


molecular events of the contraction process4
Molecular Events of the Contraction Process

Figure 7-5

ATP is split causing myosin to “cock”

sarcomere shortening
Sarcomere Shortening

Figure 7-3 A band stays the same length, H and I bands become smaller

the contraction cycle
The Contraction Cycle
  • Relaxation
    • Ca2+ concentrations decrease (by SR calcium pump moving calcium back into the Sarcoplasmic reticulum)
    • Ca2+ detaches from troponin
    • Active sites are re-covered by tropomyosin

D. Muscle Activity by nervous control

1. Motor unit – A nerve fiber and all the muscle fibers

it stimulates.

2. Neuromuscular junction – location where the

nerves motor end plate

contacts the muscle


3. Neurotransmitter and action potential (impulses)

a. Charges at membrane

1.) Resting potential -70 mV (millivolts)

a.) More Na+ outside cell than within

b.) More K+ inside than out

b. The sodium-potassium exchange pump uses ATP

to move Na+outand K+ in to repolarize the cell


c. Depolarization-action potential-repolarization

(-70mV +30) (+30mV -70)

-Action potentials are electrical currents (Ions) that flow across a membrane through ion channels

-The channels are normally closed and Na,K pump establishes resting potential (-70mV)

-An action potential causes ion channels to open allowing ions to flood into the cell . Result: the membrane to become temporarily +30mV

-The flip in potential causes nearby channels to also open = spread of the action potential down the cell membrane

After a period of time the Na K pump restores the resting potential


1 nerve controls a few muscle cells

Fine control

Common in eye

1 motor nerve controls 100’s of muscle cells

No fine control ut lots of force

Common in muscles of the back

Nerve fiber

Motor endplate

the neuromuscular junction
The Neuromuscular Junction
  • Action potential (electrical signal) nervous impulse
    • Travels along nerve axon
    • Ends at synaptic terminal:
      • Synaptic terminal:
        • releases neurotransmitter (acetylcholine or ACh)
        • into the synaptic cleft (gap between synaptic terminal and motor end plate)

4. Mechanism of contraction

a. Impulse and T-tubules

b. Calcium release and its role

c. ATP and its role with actin and myosin

d. Shortening of sarcomere

2. Impulse enters t tubule

1. Impulse spreads from nerve across sarcolemma

3. Impulse spreads from t tubule into SR causing Ca release


5. Chemistry of contraction

a. Source of ATP

1.) The liver stores glucose as animal starch

or glycogen

2.) The Kreb’s cycle and ETS may provide sustained

energy as ATPif sufficient oxygen is available.

3.) Glycolysis may anaerobically supply small

bursts of ATP energy quickly

4.) Phosphocreatine (PC) may recharge ATP for

5-10 second bursts of energy

PC + ADP + P ATP + creatine (C)


b. Lactic acid and oxygen debt

1.) If O2 is lacking, pyruvate lactic acid

causing muscle fatigue, burning, soreness

2.) To recover you stop activity, deliver O2to cells

to make ATP. Lactic acid leaves muscles and

may be converted back to pyruvate, glucose or

glycogen in the liver, (reverse glycolysis).


E. Types of Muscle Contraction

1. Tonic – partially contracted muscle, different motor

groups contracting at different times.

2. Isotonic – shortening of cells, no increase in tension.

3. Isometric – no shortening, but increase in tension.

4. Tetanic – maximal, sustained contraction due to

rapid-firing stimuli. Muscle never gets to



F. Origin, Insertion, and action of muscle

1. Origin – attached end of muscle that does not move.

2. Insertion – attached end of muscle that moves.

3. Action – movement produced by the muscle.

a. Prime mover (agonist) – muscle mostly responsible

for movement.

b. Antagonist - muscle whose action opposes


c. Synergist – muscle that helps a prime mover.

May add pull to prime mover near


1.) Fixator – a synergist that stabilizes the origin of a

prime mover.



a. Prime Mover – biceps brachii

b. Antagonist – triceps brachii

c. Syngergist – brachialis


a. Prime Mover – triceps brachii

b. Antagonist – biceps brachii

c. Fixator - brachialis


G. Naming Muscles

1. Action – adductor, abductor, extensor, flexor.

2. Shape – Deltoid, orbicularis, serratus.

3. Location – anterior, external, rectus.

4. Origin or Insertion points – biceps, triceps,


5. Number of units makeup – (same as above).

6. Size – maximus, minimus.

7. Direction of fibers – oblique, rectus, transverse.