© 2014 Pearson Education, Inc. Human Biology Concepts and Current Issues Seventh Edition Michael D. Johnson Lecture Presentations by Robert J. Sullivan

© 2014 Pearson Education, Inc.

Human Biology Concepts and Current Issues Seventh Edition

Michael D. Johnson

Lecture Presentations byRobert J. Sullivan

Marist College

6The Muscular

System


Introduction to Muscles

Muscle tissue is found in every organ Muscles participate in every activity that requires

movement Large proportion of body weight is muscle

– 40% of body weight in males

– 32% of body weight in females Skeletal muscle: attaches to skeleton and provides

strength and mobility Cardiac muscle: exclusively in the heart Smooth muscle: walls of digestive tract, blood

vessels, uterus, ureters


Muscles Produce Movement or Generate Tension

Muscles may produce movement– Voluntary: conscious control over movement (picking

up a pen)

– Involuntary: unconscious control over movement (beating of heart)

Many muscles resist movement– Maintenance of posture

– Maintenance of blood pressure Muscles generate heat


The Fundamental Activity of Muscle Is Contraction

Excitable: contract in response to electrical or chemical stimuli

All muscle cells have one mechanism of action:– They contract (shorten), then relax (lengthen)


Figure 6.1Pectoralis major•Draws arm forwardand toward the body

Serratus anterior•Helps raise arm•Contributes to pushes•Draws shoulder blade forward

Biceps brachii•Bends forearm at elbow

Rectus abdominus•Compresses abdomen•Bends backbone•Compresses chest cavity

External oblique•Lateral rotation of trunk•Compresses abdomen

Adductor longus•Flexes thigh•Rotates thigh laterally•Draws thigh toward body

Sartorius•Bends thigh at hip•Bends lower leg at knee•Rotates thigh outward

Quadriceps group•Flexes thigh at hips•Extends leg at knee

Tibialis anterior•Flexes foot toward knee

Trapezius•Lifts shoulder blade•Braces shoulder•Draws head back

Deltoid•Raises arm

Triceps brachi•Straightens forearmat elbow

Latissimus dorsi•Rotates and drawsarm backward andtoward body

Gluteus maximus•Extends thigh•Rotates thigh laterally

Hamstring group•Draws thigh backward•Bends knee

Gastrocnemius•Bends lower leg atknee

•Bends foot away fromknee

Achilles tendon•Connectsgastrocnemiusmuscle to heel


Skeletal Muscles Cause Bones to Move

600 skeletal muscles Synergistic muscles: work together to created the

same movement Antagonistic muscles: muscles that oppose each

other Many muscles attach to bones via tendons Origin: end of muscle that attaches to relatively

stationary bone Insertion: end of muscle attached to another bone

across a joint, action pulls insertion toward origin


Figure 6.2

Scapula

Shoulderjoint

Origins fromscapula andhumerusTriceps muscle

Tendon

Elbow joint

Ulna Radius

Tendon

Biceps muscle

Humerus

Tendons

Tricepsrelaxes

Tricepscontracts, pulling forearm down

Biceps relaxes

Biceps contracts, pulling forearm up

Origins fromscapula

Insertion on ulna

Insertion on radius

Origin and insertion. The point of attachment of a muscle to the stationary bone is its origin; the point of attachment to the movable bone is its insertion.

Movement. Antagonistic muscles produce opposite movements. The forearm bends when the biceps contracts and the triceps relaxes. The forearm straightens when the biceps relaxes and the triceps contracts.


A Muscle Is Composed of Many Muscle Cells

Muscles– Group of muscle cells with same origin, insertion, and

function Fasicles

– Bundles of muscle fibers (cells) wrapped with connective tissue (fascia)

Muscle fibers (muscle cells)– Long, tube shaped– Vary in length from few mm to 30 cm– Multinucleate– Packed with myofibrils, which are long cylindrical

structures which contain proteins actin and myosin


Figure 6.3Muscle bundle (fascicle)surrounded by connectivetissue (fascia)

Whole muscle

Single musclecell (fiber)

Tendon

Bone


Figure 6.4

Musclecell

Myofibril

Nuclei

Musclecell

A single muscle cell contains many individual myofibrils and has more than one nucleus.

A photograph of portions of several skeletal muscle cells.

© 2014 Pearson Education, Inc. Animation: Muscle Structure and Function Right-click and select Play


The Muscle Contractile Unit Is the Sarcomere

Sarcomere: contractile unit– Myosin: forms thick filaments

– Actin: forms thin filaments Z Lines: attachment points for sarcomeres A sarcomere is a segment of myofibril extending

from one Z line to the next Arrangement of filaments gives rise to striated

appearance of skeletal muscle


Figure 6.5Myofibril

Z-lineZ-line

Sarcomere

Myosin

Actin

Thin filament(actin)

Thick filament(myosin)

A closer view of a section of a myofibril showing that it is composed of sarcomeres joined end to end at theZ-line. An electron micrograph

cross section of a sarcomerein a region that contains bothactin and myosin.

Sarcomeres contain thin filaments of actin that attach to the Z-lines and thicker filaments of myosin that span the gap between actin molecules.

A transmission electron micrograph ( 11,300) of a longitudinal section of a sarcomere. The rounded red objects are mitochondria.


Individual Muscle Cells Contract and Relax

Muscle contraction: each sarcomere shortens a little Basic process of contraction:

1. Skeletal muscle must be activated by a nerve

2. Nerve activation increases the concentration of calcium ions in the vicinity of the contractile proteins

3. Presence of calcium permits contractions

4. When nerve stimulation stops, contraction stops


Nerves Activate Skeletal Muscles

Acetylcholine is released from motor neuron at neuromuscular junction

Electrical impulse transmitted along T tubules Calcium (Ca) is released from sarcoplasmic

reticulum (modified smooth endoplasmic reticulum) Ca initiates chain of events that cause contraction

when it contacts the myofibrils


Figure 6.6

Motor neuron

Electricalimpulse

T tubule

Sarcoplasmicreticulum

Muscle cellplasmamembrane

Z-line

Myofibrils

Ca2

Acetylcholine

The release of acetylcholine at the neuromuscular junction causes anelectrical impulse to be generated in the muscle cell plasma membrane

The electrical impulse triggers the release of Ca2 from the sarcoplasmic reticulum

The electrical impulse ( ) is carriedto the cell’s interior by the T tubules

1

2

3


Calcium Initiates the Sliding Filament Mechanism

Thick filaments: myosin Thin filaments: actin Contraction: formation of cross-bridges between thin

and thick filaments Ca must be present for cross-bridges to form

– In absence of Ca, protein complex of troponin-tropomyosin covers myosin binding sites on actin molecules

– Presence of Ca, binding sites available


Figure 6.7

Myofibril

Thickfilament

Thinfilament

Myosin molecule head

Myosinmolecule

ActinmoleculeRelaxed state. The myosin heads do

not make contact with actin.

Contraction. The myosin heads form cross-bridges with actin andthen bend, pulling the actin filaments toward the center of the sarcomere.


When Nerve Activation Ends, Contraction Ends

1. Calcium is released from sarcoplasmic reticulum

2. Calcium binds to troponin

3. Troponin–tropomyosin complex shifts position

4. Myosin binding site is exposed

5. Myosin heads form cross-bridges with actin

6. Actin filaments are pulled toward center of sarcomere

7. Sarcomere shortens


Figure 6.8

Myofibril


Thickand thinfilaments

Tropomyosin

Actin filament

Troponin

Myosin head

Myosinfilament


Electricalimpulse

Calcium release

Myosinbindingsites

Cross-bridge

Ca2 Ca2

Resting sarcomere. In the absence of calcium the muscle is relaxed because the myosin heads cannot form cross-bridges with actin.

Cross-bridge attachment. The binding of calcium to troponin causes a shift in the troponin-tropomyosin complex, allowing cross-bridges to form.


Muscles Require Energy to Contract and to Relax

Nerve activation ends, contraction ends Ca pumped back into sarcoplasmic reticulum

(requires ATP) Ca no longer bound to troponin Myosin binding site covered ATP must bind to myosin before myosin heads can

detach from actin No calcium no cross-bridges Muscle relaxes


Muscles Require Energy to Contract and to Relax

Principle source of energy: ATP ATP required for contraction ATP required for relaxation ATP is replenished by a variety of means

– Creatine phosphate

– Stored glycogen

– Aerobic metabolism of glucose, fatty acids, and other high-energy molecules


Table 6.1


The Activity of Muscles Can Vary

Isotonic contractions: muscle shortens, while maintaining a constant force, movement occurs

Isometric contractions: force generated, muscle doesn’t shorten, no movement

Degree of nerve activation influences force Terms to know:

– Motor unit

– Muscle tension

– All-or-none principle

– Muscle tone

– Recruitment


The Degree of Nerve Activation Influences Force

Motor unit – Motor neuron and all the muscle cells it controls

– Smallest functional unit of muscle contraction Muscle tension

– Mechanical force that muscles generate when they contract

– Determined by– Motor unit size– Number of active motor units– Frequency of stimulation of motor units



All-or-none principle – Individual muscle cells are completely contracting or

are relaxed Muscle tone

– Whole muscles—maintain intermediate level of force known as muscle tone

Recruitment– Activation of additional motor units increases muscle

tone


Figure 6.9

Muscle

Muscle cells

Neuromuscular junctions

Two motorneurons

A motor unit consists of a motor neuron and all of the muscle cells it controls. Any one muscle cell is controlled by only one motor neuron, but a motor neuron controls more than one muscle cell.

Photograph of the muscle cells in a motor unit, showing branches of the motor neuron and neuromuscular junctions.



Complete cycle of contraction-relaxation in response to stimulus

Can be observed using a myogram (laboratory recording of muscle activity)– Latent period

– Contraction

– Relaxation

– Summation

– Tetanic contraction


Figure 6.10

Tetanus

SummationLatent period

ContractionRelaxation

500

Stimulus

0 Time (msec)

Mu

scle

fo

rce


Slow Twitch versus Fast Twitch Fibers

Slow Twitch Contract slowly Make ATP as needed by

aerobic metabolism Many mitochondria Well-supplied with blood

vessels Store very little glycogen “Red” muscle Used for endurance

activities

Fast Twitch Contract quickly Rapidly break down ATP Fewer mitochondria Little or no mitochondria Store a lot of glycogen “White” muscle Capable of anaerobic

metabolism Used for brief high-intensity

activities


Exercise Training Improves Muscle Mass, Strength, and Endurance

Strength training– Resistance training

– Short, intense

– Builds more myofibrils, particularly in fast-twitch fibers Aerobic training

– Builds endurance

– Increases blood supply to muscle cells

– Increase in mitochondria and myoglobin

– Reach target heart rate for at least 20 minutes, three times a week


Cardiac and Smooth Muscles Have Special Features

Involuntary Able to contract entirely on their own in absence of

nerve stimulation Cardiac muscle cells are joined by intercalated disks

– Have gap junctions allowing cells to electrically stimulate the next one

Smooth muscle cells joined by gap junctions allowing cells to activate each other

Cardiac and smooth muscle cells respond to stimulation from autonomic nervous system, which can modify the degree of contraction


Figure 6.12

Intercalated discCardiac muscle cell

Adhesion junction

Protein channel

Gap junction

Cell membranes of adjacent cells

A closer view showing that intercalated discs arebridged by gap junctions that permit direct electrical connections between cells.

A view of several adjacent cardiac muscle cells showing their blunt shape and the intercalated discs that join them together.


Speed and Sustainability of Contraction

Skeletal muscle: fastest Cardiac muscle: moderate Smooth muscle:

– Very slow

– Partially contracted all of the time

– Almost never fatigues


Arrangement of Myosin and Actin Filaments

Cardiac muscle– Sarcomere arrangement of thick and thin filaments

– Striated appearance Smooth muscle

– Filaments arranged in criss-crossed bundles, not sarcomeres

– No striations


Figure 6.13

Filament bundles

Thinfilament

Thickfilament

Cellmembraneprotein

Relaxed state.

Contracted state. Thecrisscross arrangementof bundles of contractilefilaments causes the cellto become shorter andfatter during contraction.

A closer view showing how actin filaments are attached to cell membrane proteins.


Diseases and Disorders of the Muscular System

Muscular Dystrophy– Genetic disease: Duchenne Muscular Dystrophy

– Modified dystrophin protein enables leakage of Ca into cells

– Extra Ca activates enzymes that destroy muscle proteins

– Muscle weakening and wasting

– Muscle mass is replaced with fibrous connective tissue

– Life expectance: approx. 30 years



Tetanus– Infection of deep wound by bacteria, Clostridium tetani

– Bacteria produce tetanus toxin which causes muscles to contract forcefully

– Death due to respiratory failure

– Preventable by tetanus vaccine



Muscle cramps: often caused by dehydration and ion imbalances

Pulled muscles: result from overstretching of a muscle, fibers tear apart

Fasciitis: inflammation of fascia– Plantar fasciitis: sole of foot

Documents

© 2014 Pearson Education, Inc. Human Biology Concepts and Current Issues Seventh Edition Michael D. Johnson Lecture Presentations by Robert J. Sullivan