The Muscular System
Chapters 9 & 10
Muscle Types:
General Characteristics
• There are three types of muscle tissue: skeletal, smooth
& cardiac
• Most muscle cells are elongated and called muscle
fibers (not true of cardiac cells)
• Muscle contraction depends on two kinds of
myofilaments, thin actin and thicker myosin containing
microfilaments.
• Prefixes myo and mys (“muscle”) and sarco (“flesh”)
always refer to muscles.
– Ex) Sarcolemma (“muscle husk”) is the plasma
membrane of muscle fibers
Muscle Types: Skeletal Muscle Tissue
• Attached to bones
• Makes up 40% of body weight
• Responsible for locomotion, facial expressions, posture,
respiratory movements, other types of body movement
• Under conscious (voluntary) control; controlled by somatic
motor neurons
• Microscopically, tissue appears striated; Cells are long,
cylindrical and multi-nucleate
Muscle Types: Smooth Muscle Tissue
• Makes up walls of organs and blood vessels; Propel urine,
mix food in digestive tract, dilate/constrict pupils, regulate
blood flow
• Involuntary control by endocrine and autonomic nervous
system
• Tissue is non-striated; Cells are short, spindle-shaped and
mononucleated
• Extremely extensible but maintains ability to contract
Muscle Types: Cardiac Muscle Tissue
• Makes up myocardium of heart
• Autorhythmic, generates movement of blood
• Unconciously (involuntarily) controlled by endocrine and
autonomic nervous systems
• Microscopically appears striated; cells are short, branching
and mono-nucleated
• Cells connected to each other at intercalated disks
Functional Characteristics of Muscle
1. Excitability – can receive and respond to
stimuli
2. Contractility – can shorten/thicken and
generate a pulling force
3. Extensibility – can be stretched and
lengthen
4. Elasticity – after contracting or
lengthening, will recoil to original resting
length
Muscle Functions
1. Producing Movement - (both voluntary and involuntary)
- Respiration (diaphragm contractions)
- Constriction of organs & vessels (peristalsis, vasoconstriciton,
pupils)
- Heartbeat
- Communication (non-verbal & facial)
2. Maintaining Posture
- Also support soft tissues within body cavities
3. Maintaining body temperature (muscle contractions
generate heat, “thermogenesis”)
4. Stabilizing Joints
Gross Anatomy of a Skeletal Muscle
•Tendon: connects the muscle to
bone
•Endomysium: connective tissue
sheath that wraps each muscle
fiber
•Perimysium: collagenic sheath
surrounding bundles, or fascicles,
of muscle
•Epimysium: Coarse sheath that
wraps and strengthens the entire
muscle
•Normal activity is dependent on
rich supply of nerves and blood
Basic Features of a Skeletal Muscle
•Most skeletal muscles span
joints and are attached to
bones in at least two places
•When a muscle contracts, the
movable bone (insertion)
moves toward the
immovable/less movable bone
(origin)
•Attachments may be direct or
indirect (anchored by tendons
or an aponeurosis; more
common)
Nerve & Blood Supply
• Each muscle is usually served by one nerve, an
artery and one or more veins that enter/exit
near the center of the muscle
• Muscle capillaries are long & winding to
accommodate changes in muscle length.
Organizational Levels of Skeletal Muscles
Microanatomy of a Skeletal Muscle
Microanatomy of a Muscle Fiber
Structure of Myofilaments
Sarcoplasmic Reticulum & T tubules
Sliding Filament Mechanism of Contraction
• Myosin heads attach to actin molecules (at binding, active, site)
• Myosin “pulls” on actin, causing thin myofilaments to slide
across thick myofilaments, towards the center of the sarcomere
• Sarcomere shortens, I bands get smaller, H zone gets smaller, &
zone of overlap increases
Role of Ionic Calcium in Contraction
Sequence of Events in Sliding Filament
The Neuromuscular Junction
Motor Unit:
Nerve/Muscle Functional Unit
•Muscles that control fine
movement (fingers, eyes) have
small motor units
•Large weight bearing muscles
(thighs, hips) have large motor
units
•Muscle fibers from a motor
unit are spread throughout
muscle. Contraction of single
motor unit causes a weak
contraction of the entire muscle
•Stronger and stronger
contraction require more motor
units being recruited
Wave Summation and Tetanus
(a)Twitch: a single stimulus is delivered; the muscle contracts
and relaxes
(b)Wave summation: stimuli are delivered more frequently, so
that the muscle does not have adequate time to relax
completely and contraction force increases
(c)Unfused (incomplete) tetanus: more complete twitch fusion
occurs as stimuli are delivered more rapidly
(d)Fused (complete) tetanus: a smooth continuous contraction
without any evidence of relaxation
Providing Energy for Contraction
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The Cori Cycle
Types of Muscle Fibers
• Muscle fibers can be classified based on speed of
contraction & pathway of ATP formation
– Slow v. Fast (based on efficiency of ATPase)
– Oxidative (rely mainly on aerobic path) v. Glycolytic (rely mainly
on anaerobic pathway)
• Based on these characteristics muscle fibers can
categorized as:
– Slow oxidative
– Fast oxidative
– Fast glycolytic
• Most muscles have a mixture of fiber types, giving a
range of contractile speeds and fatigue resistance
Influence of Exercise
Endurance Exercise
• Increases:
# of capillaries
– # of mitochondria
– Amount of myoglobin
–
• Improves:
–
–
–
–
–
–
Overall metabolism
Efficiency of NM coordination
GI mobility
Skeletal strength
Stroke volume of heart
Fatty acid deposits in blood
vessels (removes them)
Resistance Exercise
• Increases size of muscle
fibers (not number of fibers)
amount of connective tissue
between cells (more
protection from injury)
• Glycogen stores are
increased.
• Important to focus on both
parts of an antagonistic pair
Cross-training yields the benefits of both leading to muscles with more
mitochondria, more myofilaments, more glycogen reserves etc.
Smooth Muscle – Layers & Innervation
Peristalsis & Diffuse Junctions
Contraction of Smooth Muscle
• Still stimulated by release of Ach from
nerve endings which generates an
action potential; T-tubules are absent,
but there are membrane infoldings
called caveoli to allow rapid influx of
calcium ins.
• SR is less developed, although it does
release some calcium ions, most comes
from intracellular space.
• Smooth muscle does not contain
sarcomeres but it does contain
alternating thick and thin filaments.
• The filaments are arranged on a
diagonal, causing smooth muscle to
contract in a cork-screw manner.
• Adjacent cells generally exhibit slow,
synchronized contractions (entire sheet
contracts together)
Skeletal Muscle Interaction in the Body
• Body muscles work either together or in
opposition to achieve wide variety of motion
• Muscles can only pull, never push. Therefore
muscles or muscle groups usually work in pairs.
• As a muscle shortens, the insertion usually moves
toward the origin; There is one muscles or muscle
groups to pull the insertion toward the origin and
a second muscle or muscle group to undo the
action and pull the insertion away.
Four Functional Groups
• Prime movers (agonist): Provides the major force for
producing a specific movement
• Antagonists: Muscles that oppose or reverse a particular
movement; Usually stretched or relaxed when prime
mover is active, can provide resistance to prevent
overshoot or help slow or stop the movement
• Synergists: help prime movers by adding extra force and
reducing undesirable or unnecessary movements
(stabilize joints)
• Fixators: category of synergists that help immobilize a
bone or muscles origin (contribute to maintaining
upright posture)
Naming Skeletal Muscles
Multiple descriptive criteria can be used to name
muscles:
•
•
•
•
•
•
•
Location of the muscle: ex) temporalis, intercostal
Shape of the muscle: ex) deltoid (triangle), trapezius
Relative size of the muscle: ex) maximus, minimus, longus, brevis
Direction of muscle fibers: ex) rectus, transversus, oblique
Number of origins: ex) biccep, tricep, quadricep
Location of attachments: ex) sternocleoidalmastoid
Action: ex) flexor, extensor, adductor
Importance of Fascicle Arrangement
• Fascicle arrangement determines the range of motion
and power of a muscle
• Skeletal muscles shorten up to 70% of resting length
when they contract, the longer and more parallel the
fibers are to the long axis of the muscle, the more the
muscle can shorten (this does not equate to power)
• Power depends on the total number of muscle cells;
Bipennate & multipennate muscles pack in a lot of
cells and are very powerful despite relatively minimal
shortening.
Types of Fascicle Arrangement
• Circular: fascicles arranged in concentric rings;
“sphincters”, close openings when contracting
• Convergent: muscle has a broad origin and converges
toward a single tendon or insertion
• Parallel: long axes of fascicle run parallel to long axis of
muscle; strap-like
• Fusiform: parallel, but spindle shaped rather than straplike
• Pennate: fasicles are short and attach obliquely to a
central tendon that runs length of the muscle
– Unipennate: fascicles insert into only one side of the tendon
– Bipennate: fascicles insert from opposite sides (feather-like)
– Multipennate: multiple bipennate fused into central tendon
Muscle Mechanics: Fascicle Arrangement
(a)
(b)
(g)
(f)
(a) Circular
(orbicularis oris)
(b) Convergent
(pectoralis major)
(c)
(c) Parallel
(sartorius)
(e) Bipennate
(rectus femoris)
(d)
(e)
(d) Unipennate
(extensor
digitorum
longus)
(f) Fusiform
(biceps brachii)
(g) Multipennate
(deltoid)
Lever Systems
• A lever is a rigid bar that moves on a fixed point
(fulcrum) when force is applied to it.
• The applied force (effort) is used to move a resistance
(load)
• In the human body, joints are fulcrums, bones act as
levers and muscle contraction provides the effort which
is applied at it’s insertion point.
• The load that is moved is the insertion bone and
overlying tissues and anything else associated.
Power Levers
• Speed levers operate at a
mechanical advantage. In
this type of system, the lever
allows the given effort to
move a heavier load or move
a load farther and faster
than otherwise possible.
• A mechanical advantage
exists if the load is close to
the fulcrum and the effort is
applied far from the
fulcrum.
• A small effort over a large
distance is used to move a
large load over a small
distance
Speed Levers
•
•
•
Speed levers operate at a
mechanical disadvantage
because the force exerted by
the muscle must be greater
than the load moved or
supported.
A mechanical disadvantage
exists if the load is far from
the fulcrum and the effort is
applied near the fulcrum
These levers allow the load
to move rapidly through a
large distance
Classes of Lever Systems
• Levers are classified based on the relative position of the three
elements: effort, fulcrum & load
• First-Class Levers: effort applied at one end of the lever and
the load is at the other end with the fulcrum somewhere in
the middle (E –F – L ); comparable to see-saws and scissors;
can be power or speed
• Second-Class Levers: effort applied at one end, with fulcrum
at the other and load in the middle (E – L – F); comparable to
a wheelbarrow, uncommon in the body
• Third-Class Levers: Effort is applied between the load and
the fulcrum (F – E – L); comparable to tweezers and forceps;
always speed levers; most skeletal muscles operate this way
1st Class Levers
2nd Class Levers
3rd Class Levers
Smooth Muscle Tissue
• Cells are not striated and are narrower and much
shorter than muscle cells
• Lack coarse connective tissue sheaths, but there is a
thin layer of connective tissue between the smooth
muscle cells
• Fibers smaller than those in skeletal muscle
• Spindle-shaped; single, central nucleus
• Lack highly structured neuromuscular junction;
varicosities at the end of autonomic nerve fibers
release NTs into wide synaptic cleft, “diffuse junction”
• Sarcoplasmic reticulum is less developed & T-tubules
are absent
Smooth Muscle Structure
•Grouped into sheets (2
layers) in walls of hollow
organs.
•Longitudinal layer – muscle
fibers run parallel to organ’s
long axis; contraction
causes organ to shorten
•Circular layer – muscle
fibers run around
circumference of the organ;
contraction causes organ to
elongate
• Both layers involved in
peristalsis by alternating
contraction and relaxation
Additional Differences Between
Smooth & Skeletal Muscle Tissue
• Sarcolemma does have
small infoldings called
caveoli that hold
extracellular fluid & Ca2+
ions
• Lack sarcomeres:
– 13x more actin than mysoin
(v. 2x more in skeletal)
– The myofilaments are
arranged diagnolly
– Tropomysosin is present but
no troponin
– Non-contractile intermediate
filaments resist tension by
attaching to dense bodies at
regular intervals which act as
anchors (correspond to zdiscs)
Contraction of Smooth Muscle
• Slow & synchronous contraction of entire sheet
• Some similarities with skeletal muscular
contraction
– Sliding filament
– Ca2+ trigger
– ATP for energy
• Contraction is slow, sustained & resistant to
fatigue
– Contraction/relaxation cycle is ~30x longer
– Tension can be maintained at 1% the energy costx
Cardiac Muscle
• Found only in heart; forms a thick layer called
the myocardium
• Striated fibers that branch
• Each cell usually has one centrally located
nucleus
• Fibers joined by intercalated disks
• Under ANS (involuntary) and Endocrine
(hormones) control
• Some cells are autorhythmic (pacemaker cells)
Cardiac Muscle Tissue
Developmental Aspects of Muscle Tissue
• Cardiac and smooth muscle becomes amitotic
but can lengthen and thicken
• Myoblast-like satellite cells show very little
regenerative ability
• Cardiac cells lack satellite cells
• Smooth muscle has good regenerative ability
Developmental Aspects: Male v. Female
• Skeletal muscle makes up app. 35% of a woman’s
body mass and 42% of a male’s.
• Difference is primarily due to males hormone
testosterone
• With more muscle mass, men are generally
stronger, however body strength per unit muscle
mass is the same for both sexes
Developmental Aspects: Age
• Connective tissue increases and a muscle
fibers decrease with age
• This results in muscles that become
stringier and more sinewy
• 50% of muscle mass is lost by age 80
(sarcopenia)
• Density of muscle capillaries also decreases,
which reduced stamina and increases
recovery time
• Regular exercise reverses sarcopenia