Ch. 6
The Muscular
System
Case Study Investigation
You are selected for an internship at the National
Space Biomedical Research Institute in Houston,
Texas. Part of your job is to monitor the health of the
astronauts for the National Aeronautics and Space
Administration (NASA) before, during, and after a
space mission. By chance, your time in the lab
corresponds with the return of an American astronaut
from the International Space Station. She just spent
171 days in the space station and is now getting back
to her research studies on earth. Unfortunately, it was
discovered that she would have to go through at least 3
weeks of physical therapy.
You learned from reading her physical health report that
she lost 30% of her skeletal muscle mass. Most of it
occurred within the last 2 months of the mission in space.
The normal muscle loss during a space mission is less
than 20%. A majority of muscle atrophy results from
disuse, yet she followed all the exercise programs NASA
requires. Your job is to assist the research team in
investigating her problem.
By the end of this chapter, you
will be asked to determine the
most likely cause of her
accelerated loss of muscle mass.
Applied Learning Outcomes
Use the terminology associated with the
musculature system
Learn about the following:
• Different types of muscle cells
• Muscle tissue development
• Gross and fine muscle structure
• Gross muscle function
• Muscle cell physiology
• Muscle types and actions
• Muscle development and growth
Understand the aging and pathology of the
musculature
Overview
Muscle cells change their shape by
shortening or contracting. They are
composed of specialized contractile
cells.
Over half the body’s mass is
composed of muscle tissue, and over
90% of this muscle tissue is involved
in skeletal movement.
Muscles require glucose, oxygen,
calcium and electrolytes and
release metabolic wastes.
Chapter 6 – The Muscular System
Muscle
Three types of muscle are found in the human body:
a) Cardiac muscle
(slightly striated
& involuntary;
b) Skeletal muscle
(striated &
voluntary)
c) Smooth muscle
(non-striated &
involuntary)
Skeletal Muscle Function
1. Skeletal Movements = pulling on bone (relaxation / contraction)
Prime mover =muscle
responsible for the movement
Synergist = muscles that
help the prime mover
Antagonist = muscle that produces the opposite movement as prime mover
2. Posture / Muscle Tone: balance weight distribution and
hold muscles in position
a. tonic contraction:
only a few muscles
contract at a time;
NO shortening and
NO movement
3. Regulate Organ Volume
a. sphincters: bands of smooth muscle that prevent outflow of fluids
from hollow organs
4. Move substances within the body:
a. cardiac muscle – pumps blood
b. smooth muscle – moves food through digestive sys. (peristalsis)
5. Heat Production: when muscles contract they produce
heat
a. Hypothermia – decrease in
body temp below normal
b. Hyperthermia – increase in
body temp above normal
raises eyebrow
closes jaw
closes eyes
smile muscle
closes jaw
extends head / neck
closes lips
yes/no muscle
Muscles of the Trunk
adducts
arm
deltoid
abducts
arm
adducts
upper
arm
flex trunk
flexes
elbow
MUSCLES of the
ARM
extends
elbow
flexes forearm
pronates & supinates
flexes thigh + lower leg
(middle quadricep)
Quadriceps
extend the leg
(lateral quadricep)
(medial quadricep)
Vastus intermedius
(inner quadricep)
plantar-flexes ankles
dorsi-flexes ankles
plantar-flexes ankles
extends thigh
Hamstrings
flex the leg
adducts thigh
(middle hamstring)
(medial hamistring)
(lateral hamstring
Musculature:
Gross Anatomy
2. Body: rest of the muscle
(masseter)
3. Insertion:
moveable bone the
muscle attaches to
(mandible)
1. Origin: stationary bone the
muscle attaches to
(zygomatic arch)
Origin:
Scapula /
Humerus
Sacs of synovial fluid btwn
tendon & bone for lubrication
Body
Biceps
Brachii
Insertion:
Radius
Tendon
attachs muscle
to bone
Concept Check #1
1. What are muscles composed of that allow them to do their jobs?
specialized contractile cells
2. What are 5 functions of the muscular system.
1- movement 2- posture
3- regulate organ volume
4 – move substances w/I body 4- heat production
3. How are the prime mover, synergist, and antagonist involved in
movement? If I were to flex my elbow what muscles would represent
the PM, S, and A?
Prime mover – main muscle responsible for movement (bicep)
Synergist – other muscles that help PM (brachioradialis)
Anatogonist – produces opposite movement (tricep)
4. What is a tonic contraction and give an example of one.
No movement and no shortening of a muscle
Ex. pulling up on something heavy
Concept Check #2
5. How do sphincters regulate organ volume?
Smooth muscle prevents fluid from flowing out of a hollow cavity
6. Give an example of how cardiac and smooth muscle move
substances within the body.
Cardiac muscle – pumps blood throughout the body
Smooth muscle – moves food throughout the body (peristalsis)
7. How does the origin and insertion of a muscle differ? How are
they involved in muscle movement?
Origin – immovable bone that muscle attaches to
Insertion – movable bone that muscle attaches to
The insertion bone moves towards the origin bone
8. Research the origin and insertion for the following muscles:
Gastrocnemius: Origin: femur
Insertion: achilles tendon
Pectoralis major: Origin: clavicle /sternum Insertion: humerus
Skeletal Muscle Structure
surrounds fassicles
surrounds each
surrounds entire
muscle
bundle of muscle cells
http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP13904 Muscle Structure Animation
Microcopic Muscle Cell Structure
 skeletal muscle cells are LONG, CYLINDRICAL cells covered
with an excitable membrane called the sarcolemma
 sarcolemma contains proteins that respond to signals from other
cells/environment and transmits the information to the muscle
cells causing them contract
 contractile unit of muscle is called a sarcomere (thousands run the
length of a muscle cell)
 chains of sarcomeres form myofibrils
 sarcomere contains
2 types of myofilaments
1. thick m.f. contain
protein myosin
2. thin m.f.  contain
proteins actin,
tropomyosin
& troponin
 sarcoplasmic reticulum
surrounds sarcomeres &
stores Ca for contractions
Sarcomere
marks
boundaries
between
sarcomeres
Sarcomere Chain
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter42/animations.html#
Myosin (thick)
Actin (thin)
Muscle Cell Function
 contraction occurs when sarcomeres shorten simultaneously
1. Nerve Stimulation – takes place at the neuromuscular
junction (motor neuron meets muscle fiber)
 Ca causes nerve cell to release acetylcholine, a neurotransmitter,
which binds to sarcolemma and allows Na+ /K to cross
membrane
http://www.galaxygoo.org/biochem/neuro/nmj_flash.html
(basic neuromuscular junction animation)
2. Muscle Contraction – takes place inside muscle cell
 ion movement causes SR releases Ca which travels to
sarcomere to start contraction
 Ca binds to troponin & removes tropomyosin ( blocks
myosin heads from attaching to actin)
 Myosin attaches & pulls on actin  shortens sarcomere 
http://science.howstuffworks.com/environmental/life/human-biology/muscle2.htm
shortens muscle
Sliding filament theory animation
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter10/animation__action_po
tentials_and_muscle_contraction.html
Rigor Mortis  muscle tension caused by Ca leakage out of SR into
sarcomere after death
Creatine phosphate, glycogen, & myoglobin  all serve as energy or oxygen
reserves for muscle contraction
3. Muscle Relaxation: when neural stimulations stop exciting
the sarcolemma
 Na+/K+ levels are completely recovered; SR regains most Ca
 troponin/tropomyosin covers binding site  myosin releases
actin  sarcomere relaxes  muscle elongates
Sliding
Filament
Model
Types of Skeletal Muscle Fibers
unique muscle composition is genetically determined
https://www.23andme.com/health/Muscle-Performance/
Feature
Slow Red Twitch Fibers
(Type I)
Fast White Twitch Fibers
(Type IIb)
Size
Smallest
largest
Mitochondria
large amount
(get ATP through aerobic
processes)
small amount
(get ATP through anaerobic
processes)
Color
red
(high myoglobin content)
white
(low myoglobin content)
Speed of
Contractions
Slow
Fast
Resistance to
Fatigue
High (very fatigue resistant)
Low (not fatigue resistant)
Activities
Maintaining posture,
endurance activities (i.e.
marathon runners)
Rapid, intense movements of short
duraction (i.e. sprinters, throwing a
ball or weight lifting)
Muscle: soleus
Muscle: gastrocnemius / vastus lateralis
Concept Check #3
9. List the three membranes of muscle in order from superficial to
deep then explain how they each differ.
10. What is the name given to the membrane that surrounds a muscle
cell and why is it important?
11. What is a sarcomere? And how do the 2 myofilaments that make
up a sarcomere differ?
12. Draw a picture of a sarcomere and label the two different
myofilaments.
Concept Check #4
13. Why is the sarcoplasmic reticulum that surrounds the sarcolemma
so important?
14. Describe the steps that must occur for a muscle to contract and
then relax.?
15. What is the neuromuscular junction and what occurs here?
16. Why is Ca necessary for starting a muscle contraction?
Concept Check #5
17. Explain the sliding filament theory (shortening of a sarcomere).
19. Why would a leak of Ca from the sarcoplasmic reticulum cause
rigor mortis.
20. When does a muscle relax?
21. Why might marathon runners have more red twitch fibers and
sprinters have more white twitch fibers? Include speed of contraction
and fatigue resistance in your explanation.
Skeletal Muscle Action
 skeletal muscle structure responds to the amount of work it must do
 irregular use or lack of neural stimulation
 cells lose sarcomere proteins
 contraction strength decreases
 muscle size decreases
(hypotrophy/atrophy)
 regular use & increased blood flow
 muscle strength increases
 muscle size increases (hypertrophy)
http://visionals.info/MuscleHypertrophy
.php
 Does your muscle diameter increase OR does your sarcomere density
& strength increase w/o a significant increase in overall muscle size ???
(determined by genetic differences & different blood flow patterns)
Skeletal Muscle Action
 shortening of the muscle brings the insertion closer to the origin
 threshold stimulus - minimal level of stimulus required to cause a
fiber to contract
 muscle cells are controlled by different motor units with different
threshold levels so not all muscles contract at the same time
 All or None Theory – a fiber contracts completely or not at all;
It’s the difference in picking up 1 textbook or 25 textbooks.
http://entochem.tamu.edu/VertInvertContractswf/index.html
Strength = contracting more fascicles
Endurance = producing more
contracting and relaxing groups
of fascicles.
Skeletal Muscle Action
 muscles can be categorized by the effect it has on joint
motion
• abductor – muscles that move a bone away from the
midline (deltoid, gluteus maximus)
• adductor – muscles that move a bone closer to the
midline (gracilis, pectoralis major)
• extensor – muscles that increase the angle of a joint
(tricep, quadriceps)
• flexor – muscles that decrease the angle of a joint
(bicep, hamstrings)
• sphincter – muscles that decrease the size of an
opening (esophogeal sphincter, rectal spincter)
Skeletal Muscle Action
 isotonic contractions: when a muscle is actively shortening
or lengthening
ex. lifting weights
 isometric contractions: when a muscle is not shortening or
lengthening
ex. pushing against an immovable object
Fatigue
 during muscular exercise:
 b.v. in muscles dilate & blood flow and O2 delivery
increases
 muscle fatigue occurs after extended or strong
muscle contractions and O2 & ATP can’t supply
muscle fibers fast enough
 oxygen debt – amount of O2 taken in to “pay back”
resting metabolic conditions
 if O2 is unavailable, glucose is converted into lactate
which helps break down glucose for energy
 production of lactic acid in the body causes soreness
Concept Check #6
21. List 2 things that can happen when muscles are used irregularly.
22. List 2 things that can happen when muscles are used regularly.
23. How do hypertrophy and hypotrophy (atrophy) differ?
24. Do all the muscle fibers (cells) in my arm contract when I pick up
a pencil? Use the terms threshold stimulus and all or none theory
in your explanation.
Concept Check #7
25. Explain the difference between isotonic and isometric
contractions. Give an example of each.
26. What is the relationship between muscle fatigue and oxygen
debt?
Aging and Pathology of the Skeletal System
• many disorders of the musculature are due to interactions with the
skeletal and nervous systems
Strains vs. Sprains
strains – overworking the muscle’s force on joints and tendons
 pain and swelling of fascia, joints, ligaments, and tendons
 nerves signal pain when stretched or swollen (stiff)
sprains – more severe; sudden or violent stress
on a joint/muscle
 tearing of a ligament, muscle, or tendon and
damage to nearby blood vessels
 require time for tissue and protein replacement
 To keep swelling down apply cold 1st
followed by continuous warmth to speed
healing
• contusions – related to sprains; direct hit(s) to a muscle
Myopathy / Neuromuscular disorders

inability of the nervous system to communicate properly to muscles
1. mitochondrial myopathies – genetic abnormalities of the mitochondria;
muscles can’t produce energy from food; muscles become easily cramped
2. myosistis ossificans – caused by damage to soft tissues @ muscle;
bone growing within muscle tissue; pain during contraction
a. Nonhereditary
traumatica
b. Herditary (A.D.)
progressiva
3. muscular dystrophy- involves progressive weakness in the
voluntary muscles; inability of nervous system to stimulate muscle
action; results in atrophy and wasting
Steroids on Muscles
 boost body’s ability to produce muscle
 prevent muscle breakdown &
decreases recovery time
 resemble chemical structure of
testosterone
 {T} directs the body to produce
more or enhance male
characteristics (like increased
muscle mass, etc)
Aging of the Muscular System
 natural causes  sarcopenia
 cachexia  muscle loss usually associated with AIDS, cancer, starvation,
anorexia, bulimia
 decrease in neural stimulation -> atrophy
 malnutrition / undernutrition  decrease in carbs and proteins 
less energy and nutrients for repair and maintenance
 decline in sex hormones and insulin-like growth factor –1 (IGF1); needed for muscle cell growth, maintenance, & repair
 physical therapies 
1) electrical stimulation pulses (causes muscles to retain protein)
2) muscle massages
Concept Check #8
27. What are the differences between strains, sprains, and contusions?
28. Name and describe 2 different types of myopathies.
29. What is cachexia in terms of muscle aging.
30. Describe 4 factors that contribute to muscle aging.
Case Study Investigation
 Selected for an internship at National Space Biomedical Research Ins.
 Required to monitor astronauts for NASA before, during, and after a
space mission.
 American astronaut spent 171 days in the space stations has to go
through 3 weeks of physical therapy.
 She lost 30% of her skeletal muscle even after following all the
exercise programs NASA required. The average is less than 20%.
Questions:
1. Why did the American lose 30% of her skeletal muscle after her
mission to space? Be specific.
2. She lost 10% more than the average astronaut. What else associated
with her mission could have contributed to the American’s muscle
loss?
CSI - ANSWER
 much of the muscle’s actions go into counteracting the affects of
gravity on the body; they have to work harder in gravity environments
 regular resistance produces muscle mass and strength
 weightlessness of space flight reduces the muscles need to work
against gravity; muscles will lose their mass and strength and they
atophy
 protein metabolism  astronauts don’t receive large amounts of
amino acids they need
 sex hormones  decline in sex hormones from the stress of working
in space for 6 mo