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MUSCULAR SYSTEM
REVIEW OF MUSCLE
TISSUE
Muscle tissue contracts in response to
stimulation
3 types of muscle tissue:
- Skeletal
- Cardiac
- Smooth
REVIEW OF MUSCLE TISSUE
continued
 Skeletal Muscle:
Characteristics
- Cylindrical cells
- Striated
- Multiple, peripheral
nuclei
- Voluntary
- Attached to skeleton
REVIEW OF MUSCLE
TISSUE continued
Cardiac Muscle:
Characteristics
- Branching cells
- Striated
- One or two central
nuclei
- Involuntary
- Heart
REVIEW OF MUSCLE
TISSUE continued
Smooth Muscle:
Characteristics
-
Spindle-shaped cells
Non-striated
Single, central nucleus
Involuntary
Located in the walls of
hollow organs
FUNCTIONS OF SKELETAL
MUSCLE
Produces voluntary movement
- Locomotion
- Manipulation
- Assists in breathing, eating, speech, support
of organs
- With nervous system, generates reflexes
- Provides facial expressions
Stabilizes joints
Maintains posture
Produces body heat
CHARACTERISTICS OF SKELETAL
MUSCLE
Makes up “flesh” of body (~40% by
weight)
Most “meat” is skeletal muscle
Muscles are organs
- Fibers (muscle cells)
- Motor neurons
- Blood vessels
- Connective tissue
ARRANGEMENT OF SKELETAL
MUSCLE
Connective tissue coverings provide
strength & support
- Endomysium: Around each muscle fiber
- Perimysium: Around fascicles (bundles
of cells)
- Epimysium: Around entire muscle
(bundles of fascicles)
- Fascia: loose connective tissue around
muscle groups and between muscles &
skin
ATTACHMENTS OF SKELETAL
MUSCLE
Connective tissue attachments attach
muscles to bones (blend w/periosteum),
cartilages, or to CT coverings of other
muscles
- Tendons - cordlike bundles of
collagen fibers
- Aponeuroses (sing. -sis) - sheetlike
arrangements of collagen fibers
MICROSCOPIC ANATOMY OF A
MUSCLE CELL
Fibers (skeletal muscle cells):
- Long, cylindrical, multinucleate
- Arranged parallel to one another
Sarcolemma: cell membrane
Sarcoplasm: cytoplasm
Numerous mitochondria
Sarcoplasmic Reticulum (SR): Smooth
E.R., stores Ca2+
MICROSCOPIC ANATOMY
OF A MUSCLE CELL continued
Myofibrils
- Contractile organelles
- Lie parallel to one another
- Run entire length of cell
- Composed of Myofilaments
(Protein)
*Actin – Thin filament
*Myosin – Thick filament
MICROSCOPIC ANATOMY:
MYOFIBRILS
Myofilaments composed of repeating
subunits
- Sarcomeres:
*Contractile subunits
*Source of fiber’s striations
MICROSCOPIC ANATOMY:
MYOFIBRILS
A (Dark) bands: correspond to length of
myosin filaments
I (Light) bands: actin only
Z line: anchor for actin; separates
sarcomeres
H zone: center of A band; no actin
M line: Narrow region at center of H zone;
anchor for myosin
MICROSCOPIC ANATOMY:
Neuromuscular Junction
Neuromuscular Junction (NMJ)
- Definition: Point of communication
between a motor neuron and a fiber
- Fibers contract only when stimulated
- Synaptic Knob – terminal end of motor
neuron
- Synaptic Cleft (Gap) – space between
synaptic knob & sarcolemma
MICROSCOPIC ANATOMY:
Neuromuscular Junction
- Motor End Plate:
*Sarcolemma at NMJ
*Invaginated
*High SA
*Ach Receptors
STEPS IN CONTRACTION
Sliding Filament Theory
Nerve Impulse arrives at synaptic knob
Exocytosis of synaptic vesicles
Neurotransmitter Acetylcholine (Ach)
diffuses across cleft
Ach binds to receptors on sarcolemma
Prior to contraction, sarcolemma must be
polarized (+ outside/- within)
Sarcolemma now permeable to Na+ and K+
(depolarizes)
STEPS IN CONTRACTION
Sliding Filament Theory
Na+ diffuses into fiber
SR release Ca2+ into sarcoplasm
Ca2+ binds to troponin on actin
Tropomyosin on actin moves, exposing
binding site
ATP  ADP + E ; Myosin heads attach to
actin, form cross-bridges
Myosin heads swivel, release ADP
STEPS IN CONTRACTION
Sliding Filament Theory
Actin slides towards center of
sarcomere
ATP binds to Myosin heads; crossbridges detach
Relaxation occurs from:
- Cholinesterase breaks down Ach at
NMJ
- Ca2+ actively pumped back into SR
ACTIVITY OF SINGLE
FIBERS
“All-or-None” Law: At threshold, a fiber
will contract to its maximum extent
- No “partial” contractions
- Increasing stimulus strength has no
additional effect
Single nerve impulse produces one
contraction
ACTIVITY OF MOTOR UNITS
A muscle is composed of motor units
Motor Unit: a motor neuron + all the
fibers it controls
Number of fibers varies
Each motor unit responds independently
All muscle cells in a motor unit respond
maximally, or they don’t respond at all
ACTIVITY OF MOTOR
UNITS
Strength of contraction is determined by
number of motor units stimulated
Recruitment: Process of increasing the
number of motor units responding
Strength increases as number of motor
units increases
ACTIVITY OF WHOLE
MUSCLES
Skeletal muscles are capable of Graded
Responses
Different degrees of shortening occur by:
- Changing frequency of stimulation
- Changing the number of motor units
activated
ACTIVITY OF WHOLE MUSCLES:
EVENTS IN A TWITCH
Threshold Stimulus: Strength of stimulus
required to cause contraction
Latent period: Delay between stimulus &
contraction
Twitch: Single, brief contraction
following single threshold stimulus
- Contraction
- Relaxation – caused by transport of
Ca2+ back into SR
ACTIVITY OF WHOLE MUSCLES:
FREQENCY OF STIMULATION
Refractory Period: Time between initial
and subsequent stimuli that is required for
sarcolemma to repolarize
Summation: Larger contractions with
more frequent stimuli
Tetanic contraction: Smooth, continuous
contraction without relaxation between
stimuli
Fatigue: loss of response due to
insufficient ATP
ACTIVITY OF WHOLE MUSCLES :
TYPES OF CONTRACTION
Tonus: Partial, sustained contraction
Isometric: Contraction in which muscle
contracts but stays the same length, but
resistance is increased
Isotonic: Contraction in which resistance
stays the same, muscle contracts, fibers
shorten, attachment sites move
EFFECTS OF EXERCISE
Skeletal muscle cells do not undergo
mitosis
Exercise does not increase the number of
skeletal muscle cells
Hypertrophy: Enlargement of muscle
cells due to exercise
- The number of actin and myosin
myofilaments increases
- Mitochondria increase
- Blood supply increases
EFFECTS OF LACK OF
EXERCISE
Atrophy: Decrease in the size of muscle
cells due to lack of use
- The number of actin and myosin
myofilaments decreases
- Mitochondria decrease
- Blood supply decreases
BODY MOVEMENTS
Produced by contraction of skeletal muscle
Shortening of a skeletal muscle resulting in
movement of attachments
Movement depends on joint, attachments
Skeletal muscles have at least two
attachments
- One attachment is relatively immobile
- The other attachment is more mobile
BODY MOVEMENTS :
MUSCLE ATTACHMENTS
Origin: Less movable attachment
Insertion: More movable attachment
Action: What the muscle “does”
- Moves insertion toward origin
- The “movement” produced
Types of Ordinary Body
Movements
 Flexion – decreases angle between bones
 Extension – increases angle between bones
 Rotation – movement around an axis
 Abduction – moves appendage away from
midline
 Circumduction – moves appendage in a circle
around joint
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 6.32
Body Movements
Figure 6.13
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 6.33
Special Movements
 Dorsiflexion: toes point “up”
 Plantar flexion: toes point “down”
 Inversion: soles of feet “in”
 Eversion: soles of feet “out”
 Supination: face or palm “up”
 Pronation: face or palm “down”
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 6.34
BODY MOVEMENTS:
MUSCLE GROUPS
Prime mover: Muscle primarily
responsible for an action
Synergist: Muscle(s) that assist(s) prime
mover
Antagonist: Muscle(s) that resist prime
mover, or move opposite to it
NAMING SKELETAL
MUSCLES: CRITERIA
Muscle attachments: Origin and/or
insertion
Muscle action
Direction of muscle fibers
Location of muscle
Size of muscle
Number of origins (heads)
Shape of muscle
NAMING SKELETAL
MUSCLES: EXAMPLES
Muscle attachments
- Sternocleidomastoid
Muscle action
- Flexors and extensors
*Flexor carpi radialis
*Extensor carpi radialis
- Abductors and adductors
*Adductor longus
*Adductor magnus
NAMING SKELETAL
MUSCLES: EXAMPLES
Direction of muscle fibers
- Rectus abdominis
- External oblique
Location of muscle
- Temporalis
- Tibialis anterior
NAMING SKELETAL
MUSCLES: EXAMPLES
Size of muscle
- Gluteus maximus
- Teres major
- Vastus lateralis
Number of origins (heads)
- Triceps brachii
- Biceps femoris
Shape of muscle
- Trapezius
- Deltoideus
- Rhomboideus