1. Chapter 6
The Muscular system

2. The Muscular System
The three kinds of muscle are built and function in different ways.
Skeletal muscle, composed of long thin cells called muscle “fibers,” allows the body to move.
Smooth muscle is found in the walls of hollow organs and tubes; the cells are smaller than those of skeletal muscle and are not striated.
The heart is the only place where cardiac muscle is found.

3. TRICEPS BRACHII BICEPS BRACHII PECTORALIS MAJOR DELTOID
TRICEPS BRACHII
BICEPS BRACHII
PECTORALIS MAJOR
DELTOID
SERRATUS ANTERIOR
TRAPIZIUS
EXTERNAL OBLIQUE
LATISSIMUS DORSI
RECTUS ABDOMINUS
GLUTEUS MAXIMUS
ADDUCTOR LONGUS
SARTORIUS
BICEPS FEMORIS
Figure 6.2: Some of the major muscles of the muscular system.
QUADRICEPS FEMORIS
GASTROCNEMIUS
TIBIALIS ANTERIOR

4. The Structure and Function of Skeletal Muscles
A skeletal muscle is built of bundled muscle cells.
Inside each cell are threadlike myofibrils, which are critical to muscle contraction.
The cells are bundled together with connective tissue that extends past the muscle to form tendons, which attach the muscle to bones.

5. cells (each has its own connective tissue sheath)
muscle’s outer sheath
(connective tissue)
two bundles of muscle
cells (each has its own connective tissue sheath)
Figure 6.3: Structure of a skeletal muscle. The muscle’s cells bundled together inside a wrapping of connective tissue.
one muscle cell
one myofibril

6. The Structure and Function of Skeletal Muscles
Bones and skeletal muscles work like a system of levers.
The human body’s skeletal muscles number more than 600.
The origin end of each muscle is designated as being attached to the bone that moves relatively little; whereas the insertion is attached to the bone that moves the most.
Because most muscle attachments are located close to joints, only a small contraction is needed to produce considerable movement of some body parts (leverage advantage).

7. The Structure and Function of Skeletal Muscles
Many muscles are arranged as pairs or in groups.
Some work antagonistically (in opposition) so that one reverses the action of the other.
Others work synergistically, the contraction of one stabilizes the contraction of another.
Reciprocal innervation dictates that only one muscle of an antagonistic pair (e.g. biceps and triceps) can be stimulated at a time.

8. at the same time, biceps relaxes
triceps relaxes
origin
triceps contracts,
pulls the forelimb
down
biceps contracts at
the same time, and
pulls forelimb up
at the same time, biceps relaxes
FigureAnimatedTwoopposingmusclegroupsinhumanarmsaWhenthetricepsrelaxesanditsopposingpartnerbicepscontractstheelbowjointflexesandtheforearmbendsupbWhenthetricepscontractsandthebicepsrelaxestheforearmisextendeddown
insertion

9. The Structure and Function of Skeletal Muscles
Humans have two general types of skeletal muscles:
“Slow” muscle is red in color due to myoglobin and blood capillaries; its contractions are slower but more sustained.
“Fast” or “white” muscle cells contain fewer mitochondria and less myoglobin but can contract rapidly and powerfully for short periods.

10. The Structure and Function of Skeletal Muscles
When athletes train, one goal is to increase the relative size and contractile strength of fast (short duration, anaerobic) and slow (long duration, aerobic) muscle fibers.

11. How Muscles Contract A muscle contracts when its cells shorten.
Muscles are divided into contractile units called sarcomeres.
Each muscle cell contains myofibrils composed of thin (actin) and thick (myosin) filaments.
Each actin filament is actually two beaded strands of protein twisted together.
Each myosin filament is a protein with a double head (projecting outward) and a long tail (which is bound together with others).
The arrangement of actin and myosin filaments gives skeletal muscles their characteristic striped appearance.

12. One myofibril inside cell:
FigureAnimatedZoomingdownthroughskeletalmusclefromabicepstofilamentsoftheproteinsactinandmyosinTheseproteinscancontract
A longitudinal section through a skeletal myofiber greatly magnified.
All bands of its myofibrils are arranged in parallel. The striations are due to the alternating A (dark) and I (light) bands.

13. Only the thick myofilaments extend across the H zone.
sarcomere
sarcomere
Z line
H zone
Z line
Z line
c. Many thick and thin
myofilaments overlap
in the A band.
Only the thick myofilaments
extend across the H zone.
Only the thin myofilaments
extend across the I bands with the Z line in the middle.
FigureAnimatedZoomingdownthroughskeletalmusclefromabicepstofilamentsoftheproteinsactinandmyosinTheseproteinscancontract
I band
A band
I band

14. How Muscles Contract
During contraction, the myosin filaments physically slide along and pull the two sets of actin filaments toward each other at the center of the sarcomere.
This is called the sliding-filament theory of skeletal muscle contraction.
When a myosin head is energized, it forms cross-bridges with an adjacent actin filament and tilts in a power stroke toward the sarcomere’s center.
Energy from ATP drives the power stroke as the heads pull the actin filaments along.
After the power stroke the myosin heads detach and prepare for another attachment.

15. a. b. actin myosin actin Sarcomere relaxed.
FigureAnimatedaActinandmyosinfilamentsinasarcomereInteractionsbetweenthetwokindsoffilamentsshortenthesarcomereb–eSlidingfilamentmodelofcontractioninthesarcomeresofmusclecells b.
Same sarcomere, contracted.

16. How the Nervous System Controls Muscle Contraction
Calcium ions are the key to muscle contraction.
Skeletal muscles contract in response to signals from motor neurons of the nervous system.
Signals arrive at the T tubules of the sarcoplasmic reticulum (SR), which wraps around the myofibrils.
The SR responds by releasing stored calcium ions; calcium binds to the protein troponin, changing the conformation of actin and allowing myosin cross-bridges to form.
Another protein, tropomyosin, is also associated with actin filaments.
When nervous stimulation stops, calcium ions are actively taken up by the sarcoplasmic reticulum and the changes in filament conformation are reversed; the muscle relaxes.

17. a b c d section from spinal cord motor neuron
Signals from the nervous system
Endings of motor neuron b
sarcoplasmic reticulum (calcium in storage)
T tubule
plasma membrane of skeletal muscle fiber
section from a skeletal muscle
FigureAnimatedPathwayforsignalsfromthenervoussystemthatstimulatecontractionofskeletalmuscle
one of the myofibrils inside the muscle fiber
part of one muscle cell
Signals travel along muscle cell’s plasma membrane to sarcoplasmic reticulum around myofibrils. c
Z line
Z line d
Signals trigger the release of calcium ions from sarcoplasmic reticulum threading among the myofibrils.

18. How the Nervous System Controls Muscle Contraction
Neurons act on muscle cells at neuromuscular junctions.
At neuromuscular junctions, impulses from the branched endings (axons) of motor neurons pass to the muscle cell membranes.
Between the axons and the muscle cell is a gap called a synapse.
Signals are transmitted across the gap by a neurotransmitter called acetylcholine (ACh).
When the neuron is stimulated, calcium channels open to allow calcium ions to flow inward, causing a release of acetylcholine into the synapse.

19. Vesicles containing ACh molecules
Axon ending of
motor neuron
Synapse
Muscle cell
FigureHowachemicalmessengercalledaneurotransmittercarriesasignalacrossaneuromuscularjunction
Muscle cell receptor for ACh

20. How Muscle Cells Get Energy
ATP supplies the energy for muscle contraction.
Initiation of muscle contraction requires much ATP; this will initially be provided by creatine phosphate, which gives up a phosphate to ADP to make ATP.
Cellular respiration provides most of the ATP needed for muscle contraction after this, even during the first 5-10 minutes of moderate exercise.

21. How Muscle Cells Get Energy
During prolonged muscle action, glycolysis alone produces low amounts of ATP; lactic acid is also produced, which hinders further contraction.
Muscle fatigue is due to the oxygen debt that results when muscles use more ATP than cellular respiration can deliver.

22. Phosphate Transferred from
ADP + Pi
Pathway 1
Phosphate Transferred from
Creatine Phosphate
Relaxation
Contraction
creatine
ATP
Pathway 2
Aerobic
Respiration
Pathway 3
Glycolysis
Alone
FigureAnimatedThreemetabolicpathwaysbywhichATPformsinmusclesinresponsetothedemandsofphysicalexercise
oxygen
glucose from bloodstream and from glycogen breakdown in cells

23. Properties of Whole Muscles
Several factors determine the characteristics of a muscle contraction.
A motor neuron and the muscle cells under its control are a motor unit; the number of cells in a motor unit depends on the precision of the muscle control needed.
A single, brief stimulus to a motor unit causes a brief contraction called a muscle twitch.
Repeated stimulation makes the twitches run together in a sustained contraction called tetanus (tetany).

24. neuromuscular junction
motor unit
slice from spinal cord
motor neuron leading from spinal cord to muscle fibers
neuromuscular junction
FigureaExampleofmotorunitspresentinmusclesbThemicrographshowstheaxonendingsofamotorneuronthatactsonindividualmusclecellsinthemuscle

25. Properties of Whole Muscles
Muscle tone is the continued steady, low level of contraction that stabilizes joints and maintains general muscle health.
Muscle tension is the force a contracting muscle exerts on an object; to contract, a muscle’s tension must exceed the load opposing it.
An isotonic contraction results in sarcomere shortening.
An isometric contraction results in sarcomere tension but they do not shorten.
Muscles fatigue when strong stimulation keeps a muscle in a state of tetanus too long.
After resting, muscles will be able to contract again; muscles may need to rest for minutes (pay off “oxygen debt”) or up to a day (replenish glycogen stores) to fully recover.

26. Muscle Disorders Strains and tears are common muscle injuries.
Muscle strains come from movement that stretches or tears muscle fibers; ice, rest and anti-inflammatory drugs may help.
If the whole muscle is torn,
scar tissue may develop,
shortening the muscle and
making it function less
effectively.

27. Muscle Disorders Sometimes a skeletal muscle will contract abnormally.
A muscle spasm is a sudden, involuntary contraction that rapidly releases, while cramps are spasms that don’t immediately release; cramps usually occur in calf and thigh muscles.
Tics are minor, involuntary twitches of muscles in the face and eyelids.

28. Muscle Disorders Muscular dystrophies destroy muscle fibers.
Muscular dystrophies are genetic diseases leading to breakdown of muscle fibers over time.
Duchenne muscular dystrophy is
common in children; a single mutant gene interferes with sarcomere contraction.
Myotonic muscular dystrophy is usually found in adults; muscles of the hands and feet contract strongly but fail to relax normally.
In these diseases, muscles progressively weaken and shrivel. Muscles that are damaged or which go unused for prolonged periods of time will atrophy (waste away).

29. Keeping fit
Aerobic exercise improves the capacity of muscles to do work.
Walking, biking, and jogging are examples of exercises that increase endurance.
Regular aerobic exercise increases the number and size of mitochondria, the number of blood capillaries, and the amount of myoglobin in the muscle tissue.

30. Keeping fit
Strength training improves function of fast muscle but does not increase endurance.
Even modest activity slows the loss of muscle strength that comes with aging.