The Muscular System
Muscles are the “the machines of the body”
Contraction “shortening” – unique characteristic that sets muscle tissue
apart from other tissue

Overview of Muscle Tissue
All muscles …
 Are elongated (muscle fibers)
Contraction depends on microfilaments
 Latin prefix
‘myo’ and ‘mys’ - muscle
‘sarco’
- flesh
Muscle Tissue Characteristics
• Excitability (irritability) – response (electrical
impulse) to stimuli (usually chemical)
producing contraction
• Contractility – shorten forcibly
• Extensibility – able to be stretched
• Elasticity –resume resting length
Classification of Muscle Types
-Skeletal -Cardiac -Smooth
I.
Skeletal Muscle
1. Cover skeleton – attaching to
bone, muscle and/or skin
2. Cell shape - single, very long,
cylindrical, multinucleate,
striated
3. Voluntary – (via nervous system)
4. Can contract rapidly but tires easily
5. Fragile – layered strength
Classification of Muscle Types
II.
Smooth Muscle
1. Found in walls of hollow visceral
organs
2. Cell shape – single, fusiform,
uninucleate, no striations
3. Involuntary – controlled by
nervous system, hormones,
chemicals, response to stretch
4. Very slow contraction
Classification of Muscle Types
III. Cardiac Muscle
1. Found in walls of the heart
2. Cell shape – branching chains
of cells, uninucleate,
striations
3. Involuntary
4. Slow contraction (rhythmic)
Muscle Functions
1.
2.
3.
4.
Produce movement
Maintain posture
Stabilize joints
Generate heat
– Muscle = group of fascicles
– Muscle fibers extend length of muscle from tendon to
tendon
Skeletal Muscle – Gross Anatomy
• Muscle (organ) – muscle fibers, blood vessels, nerve
fibers and connective tissue (CT)
• Each muscle served by one nerve, an artery, one or more
veins, CT sheaths
• Organization (large to small)
Muscle (organ)→ Fascicle (muscle portion)→ Muscle fiber
(cell) → Myofibril (organelle with bundles of
myofilaments) →Sarcomere (myofibril segment) →
Myofilament (thick and thin)
Skeletal Muscle – Gross Anatomy
• Connective Tissue Sheaths
Individual muscle fibers wrapped and held together by CT
sheaths
(Internal → External)
Endomysium [reticular fibers]– surrounds each muscle
fiber → Muscle fibers grouped into Fascicles (bundles)
and surrounded by Perimysium [fibrous
CT]→Epimysium [dense irregular CT] which surrounds
the whole muscle
Components of a muscle fiber
Skeletal Muscle – Gross Anatomy
• Connective Tissue Sheaths
Sheaths are continuous with one another and
with tendons
During contraction, muscle fibers pull on
sheaths which transmit force to bone to be
moved
Contribute to muscle elasticity
Entry and exit for blood vessels and nerves
Muscle fiber components
• Sarcolemma: muscle cell
membrane
• Sarcoplasma: muscle cell
cytoplasm
• Motor end plate: contact
surface with axon terminal
• T tubule: cell membrane
extension into the
sarcoplasm (to reach the
myofibrils)
• Cisternae: areas of the ER
dedicated to Ca++ storage
(located on each side of the
T-tubules)
• Myofibrils: organized into
sarcomeres
Figure 12.2 (2 of 2)
Attachments
• Skeletal muscle spans joints and attaches to bones in two
places
– When a muscle contracts, the movable bone (muscle’s
insertion) moves toward the less movable bone (muscle’s origin)
– In limbs, origin lies proximal to insertion
• Direct Attachments – epimysium fuses to periosteum (bone) or
perichondrium (cartilage)
• Indirect – Muscle’s CT wrappings extend beyond muscle as
ropelike tendon or sheet-like aponeurosis
– Connects to CT (bone/cartilage) or fascia (muscle)
Skeletal Microscopic Anatomy
Sarcolemma … Layer underneath plasma
membrane
Myofibril … long, ribbon-like organelles which
nearly fill cytoplasm; bundles of myofilaments
Striations
Light (I) Bands and Dark (A) Bands create
Banding Pattern
Skeletal Microscopic Anatomy
I band … light
A band …dark
Z line … darkened midline interruption
H zone … lighter central region which
interrupts A band
Sarcomere … tiny contractile units of myofibril
Myofilaments … threadlike proteins
Skeletal Microscopic Anatomy
Myofilaments are made up of the protein molecules
Myosin –make up “thick” filaments
Actin – make “thin” filaments
Myosin has rodlike tail and globular heads
Heads link thick to thin filaments (cross bridges) during
contraction
Arrangement of myofilaments determine banding pattern
The sarcomere
• The myofibrils are
organized into a repetitive
pattern, the sarcomere
• Myosin: thick filament
• Actin: thin filament
• Bands formed by pattern:
A and I and H bands
• Z line: area of attachment
of the actin fibers
• M line: Myosin fiber
centers
The sarcomere
Figure 12.5d
Myosin structure
• Many myosin molecules per
filament, golf club shape
• Long tail topped by a
thickening: the head 
forms crossbridges with the
thin filament
• Presence of the enzyme,
ATPase in the head 
release energy for
contraction
Actin structure
• Formed by 3 different
proteins:
- globular (G) actins: bind to
myosin heads
- tropomyosin: long, fibrous
molecule, extending over
actin, and preventing
interaction between actin
and myosin
- troponin: binds reversibly to
calcium and able to move
tropomyosin away from the
actin active site
Figure 12.4
Contraction of Skeletal Muscle
• “All or None” Law of Muscle – applies to single
muscle cell
• Skeletal Muscle – organ of thousands of muscle
cells
– Graded Responses
• Different degrees of shortening
– Produced by changing speed and changing # cells
Skeletal Muscle Activity
• Muscle Cells … special functional properties
– Irritability – ability to receive and respond to
stimulus
– Contractility – ability to shorten when an
adequate stimulus is received
Action Potential
• Motor neuron may stimulate more than 1 muscle
• Motor Unit - motor neuron and all muscle fibers it
supplies
– Fine control muscles –small motor units
– Large weight-bearing muscles – large motor units
Muscle contraction is investigated in lab using an apparatus
producing a myogram (recording of contractile activity); line
recording is a tracing.
Motor units
•
Motor unit: Composed of one motor
neuron and all the muscle fibers that
it innervates
•
There are many motor units in a
muscle
•
The number of fibers innervated by a
single motor neuron varies (from a
few to thousand)
•
The fewer the number of fibers per
neuron  the finer the movement
(more brain power)
Action Potential
• To contract, a skeletal muscle fiber must be stimulated by
a nerve and propagate an electric current (action
potential) along its sarcolemma
– Action potential causes a short-lived rise in intracellular Ca2+
levels – triggers contraction
– Axon (nerve fiber)… long threadlike extension of neuron
• Axonal Terminals… branches of axon which forms junction with
muscle (Neuromuscular Junctions)
– Separating space – Synaptic Cleft
Action Potential
• Action Potential … “unstoppable”
– Within axonal ending are synaptic vesicles containing
acetylcholine (ACh)
– Motor end plate of sarcolemma has ACh receptors
– Nerve impulse causes Ca2+ to flow into axon from extracellular
fluid and release ACh into synaptic cleft
– ACh fuses onto sarcolemma receptors resulting in change in
membrane potential
– Breakdown of ACh prevents continued contraction
Synaptic events
• The AP reaches the axonal
bulb
• Voltage-gated calcium
channels open
• The influx of calcium in the
bulb activates enzymes the
vesicles containing the
neurotransmitter molecule
dock and release the
neurotransmitter in the
synapse
• The neurotransmitter for
skeletal muscles is always
acetylcholine
• The receptors on the muscle
fiber are cholinergic receptors
• These receptors are nicotinic
(fast) acting receptors
2- The Mechanism of Force Generation in Muscle
Figure 12.7
Muscle relaxation
• Ach is removed from the
receptors by acetylcholinesterase
• Ligand-gated Na+channels close
• Na/K pumps reestablish the RMP
• Ca++ ions leave troponin and are
brought back into the cisternae
(this process needs energy)
• Tropomyosin moves back over
the actin active site
• The myosin heads release their
binding to actin
• The filaments passively move
back into resting position
Applications
• Myasthenia gravis: autoimmune disease where antibodies against the Ach
receptors are produced. Which consequences do you expect?
• Muscular dystrophy: some proteins forming the muscle fibers are
abnormal. Which consequences do you expect?
• Curare binds to the Ach receptor without activating them. What are the
effect of curare on the skeletal muscle?
• The botulism toxin prevents the release of the neurotransmitter into the
synapse. What will be the consequence?
• Nerve gas inhibits acetylcholinerestase present in the synapse. What will
be the consequence?
•
• Rigor mortis: why does the body stiffen
shortly after death?
Sliding Filament Theory
This theory of contraction states that during contraction the thin filaments
slide past the thick ones so that the actin and myosin filaments overlap to
a greater degree
– ATP
– Each cross bridge attaches/detaches several times during
contraction
– Muscle cell shortens
– Attachment of myosin to actin requires Ca2+ (from sarcoplasmic
reticulum)
– Single nerve impulse produces 1 contraction – prevents
continuous contraction
Contraction of Skeletal Muscle
• Tetanus …goal: to produce smooth and prolonged
muscle contraction
– Strength correlates to # cells
– Strongest contraction – when all motor units are active and
stimulated
– Muscle twitch…single, brief, jerky contractions
– Graded muscle responses (response to varying demands):
contractions graded
• By changing the frequency of stimulation
• By changing the strength of the stimulus
Energy For Muscle Contraction
• As muscle contracts, ATP hydrolysed to release
energy
– Only energy source
– Continuous supply:
– Regeneration of ATP (3 ways)
• Direct phosphorylation of ADP by creatine phosphate
(CP) –muscles store more CP than ATP
CP → creatine while ADP → ATP (Coupled Reaction)
[ADP +CP → ATP + creatine] Efficient & Quick
Energy For Muscle Contraction
• Aerobic Respiration
– Takes place in mitochondria
– During exercise, ATP generated by processes that
use O2
[C6H12O6 + 6O2 → 6CO2 + 6H2O +
ATP]
– High yield of ATP (36 per 1 glucose)
– Slow because of steps
– Requires continuous supply O2
Energy For Muscle Contraction
• Anaerobic Glycolysis
– ATP generated by processes that do NOT require O2
– Glucose converted to pyruvic acid releasing ATP (2 ATP per 1
glucose)
– If O2 supplied, pyruvic acid undergoes aerobic respiration
(mitochondria) to produce CO2 and H2O + 36 ATP
– When muscle activity reaches 70% of maximum, O2 is not
supplied (muscles bulge) and pyruvic acid converts to lactic acid
Energy For Muscle Contraction
• Anaerobic Glycolysis (continued)
– Glucose → Pyruvic acid → Lactic acid
– Fast (2.5 times faster than aerobic)
– Less ATP produced (5% of aerobic)
– Accumulating lactic acid results in muscle fatigue
and soreness
Muscle Fatigue/Oxygen Debt
• Strenuous exercise results in muscle fatigue (muscle
unable to contract)
• Muscle fatigue caused by oxygen debt (prolonged muscle
activity)
– Cannot take in oxygen fast enough to keep muscles supplied
– Lactic acid increase
True muscle fatigue – muscle quits entirely
Contractures – no ATP available for cross bridges to detach
(continuous contraction) – writer’s cramp (temporary); rigor
mortis (permanent)
Muscle Fatigue/Oxygen Debt
• Example: If running 100-yard dash in 12 seconds requires
6L O2 for complete aerobic respiration, but VO2 max of
muscles is 1.2L for that 12 seconds, then there is an
oxygen deficit of 4.8L
–
–
–
–
Oxygen Debt must be “paid back”
Repay by breathing deeply (triggered by high H+ in blood)
Rid of lactic acid
Rise in ATP
Muscle Contractions
• When muscle contracts, Tension (force) develops as actin
and myosin slide and interact
– Isotonic contraction – “same tone” myofilaments successfully
slide, muscle shortens, movement occurs
– Isometric contraction – “same measure” muscle does not
shorten, tension increases, myofilaments skid (usually when
trying to move something immovable)
IV- Muscle metabolism
• Muscle fibers use ATP (only first
few seconds) for contraction
• ATP must then be generated by
the muscle cell:
- from creatine phosphate, first
- from glucose and glycogen
- from fatty-acids
ATP formation from the above
compound is possible if oxygen is
present (oxidative
phosphorylation)
Oxygen is delivered to the muscle by
myoglobin, a molecule with high
affinity to oxygen and related to
hemoglobin
If the effort is strong and
sustained, the muscle might
not have enough oxygen
delivered to it by myoglobin
 anaerobic glycolysis with
only 2 ATP formed per
glucose and synthesis of
lactic acid
Figure 12.11
Muscle fatigue
• Muscle fatigue: a decline in the
ability of the muscle to sustain
the strength of contraction
• Causes:
- rapid build-up of lactic acid
- decrease in oxygen supply
- decrease in energy supply (glucose,
glycogen, fatty-acids)
- Decreased neurotransmitter at
the synapse
- psychological causes
Muscle Tone
• State of continuous partial contraction
• Keeps the muscles firm, healthy and ready to
respond
• Skeletal muscle tone helps stabilize joints and
maintain posture
• Nerve damage → can’t stimulate muscle →
reduce tone → flaccid (soft/flabby) → atrophy
Golden Rules of Skeletal Muscle Activity
• All muscles cross at least 1 joint
• The bulk of the muscle lies proximal to the
joint crossed
• All muscles have at least 2 attachments (origin
& insertion)
• Muscles can only pull, they never push
• During contraction, the muscle insertion
moves toward the origin
Force of Muscle Contraction
• Force of contraction is affected by
– Number of muscle fibers stimulated
• More motor units, more force
– Size of muscle fibers stimulated
• Bulkier the muscle, more tension (greater strength)
– Frequency of stimulation
• Internal tension (force generated by myofibrils) transfer tension
(external tension) to load
– Degree of muscle stretch – severely stretched (or severely
contracted ) muscle can’t develop tension (improper overlap of
filaments)
Muscle Fiber Type
• Speed of Contraction - how fast myosin ATPases split ATP
– Slow fibers
– Fast fibers
• Major pathways for forming ATP
– Oxidative fibers (aerobic)
– Glycolytic fibers (anaerobic)
Skeletal muscle cells are slow oxidative fibers; fast oxidative fibers;
or fast glycolytic fibers.
Smooth Muscle
• Microscopic Structures
– Spindle-shaped, one centrally located nucleus, narrower and
shorter than skeletal muscle cells, lack coarse CT sheaths, no
striations, no sarcomeres
– Organized into sheets of closely apposed fibers
• Longitudinal layer: fibers run parallel to long axis of organ
(Contracts- organ dilates and shortens)
• Circular layer: fibers run around the circumference of organ
(Contracts- constricts the lumen (cavity) of organ and cause
elongation)
– Alternating contraction/relaxation - peristalsis
Contraction of Smooth Muscle
• Adjacent muscle fibers exhibit slow, synchronized
contractions
• Whole sheet responds in unison
• Electrical coupling by gap junctions
– Transmit action potentials from fiber to fiber
• Pacemaker cells set contractile pace for entire sheet
• Rate and intensity of contraction may be modified by
neural and chemical stimuli
Contraction of Smooth Muscle
• Takes 30 times longer to contract and relax than skeletal
muscle
• Can maintain same contractile tension for prolonged
period at 1% of energy cost
– Sluggishness of ATPases; myofilaments may latch together
during prolonged contractions
– Important for homeostasis: smooth muscle tone (day in and day
out without fatigue)
– Low energy requirements
Contraction of Smooth Muscle
• Stress-relaxation response
– Allows hollow organ to fill or expand slowly to accommodate
greater volume without promoting contractions (bladder)
• Length and Tension changes
– Stretches more and generates more tension than comparably
stretched skeletal muscle (stretch but not get flabby)
• Hyperplasia (divide to increase numbers) –uterus during
pregnancy
Types of Smooth Muscle
• Muscle in different organs varies in fiber
arrangement, responsiveness, and innervation
– Single-unit (visceral muscle)
•
•
•
•
•
Contract rhythmically and as a unit
Coupled by gap junctions
Exhibit spontaneous action potentials
Arranged in opposing sheets
Exhibit stress-relaxation reponse
Types of Smooth Muscle
• Multiunit – smooth muscles in large airways, large
arteries, arrector pili, and internal eye muscles
– Gap junctions are rare, infrequent spontaneous synchronous
depolarizations
– Like skeletal muscles – multiunit fibers are independent of each
other
– Richly supplied with nerve endings (motor unit)
– Responds with graded contractions
– Innervated by autonomic system
– Responsive to hormonal controls