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MUSCULAR SYSTEM
Bio 221
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: connective tissue around &
between muscles
ATTACHMENTS OF SKELETAL
MUSCLE
Connective tissue attachments join
muscles to bones, to cartilages, or to CT
coverings of other muscles
- Tendons - cordlike bundles of
collagen fibers
- Aponeuroses (sing. -sis) - sheetlike
arrangements of collagen fibers
Skeletal Muscles Cells are Different
Fibers (skeletal muscle cells):
- Long, cylindrical, multinucleate
Sarcolemma: cell membrane
Sarcoplasm: cytoplasm
Numerous mitochondria
Sarcoplasmic Reticulum (SR): Smooth
E.R., stores Ca2+
MICROSCOPIC ANATOMY
OF A MUSCLE CELL
Myofibrils are Contractile
Organelles
Myofibrils
- Contractile organelles
- Lie parallel to one another
- Run entire length of cell
- Composed of Myofilaments
(Protein)
*Actin – Thin myofilament
*Myosin – Thick myofilament
Myofilaments
Thick myofilament
- Myosin heads free, project out from ends
- Myosin tails attached, central
- Myosin heads can attach to actin, forming
crossbridges
Myofilaments
Thin myofilament
- Actin & regulatory proteins
* Tropomyosin
Covers sections of actin
* Troponin
Attaches to actin & tropomyosin
Binding site for Ca2+
Myofibrils are composed of
Sarcomeres
Sarcomeres:
- Contractile units of myofibrils
- Source of fiber’s striations
- Banding caused by arrangement of
myofilaments (Actin & Myosin)
Thin & Thick Myofilaments
Sarcomere Anatomy
A (Dark) bands: correspond to length of
myosin filaments
I (Light) bands: actin (no myosin)
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
Stimulation of Fibers
Fibers must be stimulated to contract
Motor Neurons deliver the stimulus
Point of communication between a motor
neuron and a fiber = Neuromuscular
Junction (NMJ)
Fibers & Motors Neurons do not touch
Neurotransmitter molecules directly
stimulate muscle fibers
Structure of Neuromuscular
Junction (NMJ)
Axon Terminal – end of motor neuron
Synaptic Cleft (Gap) – space between
axon terminal & sarcolemma
Motor End Plate:
*Sarcolemma at NMJ
*Invaginated
*High SA (surface area)
*ACh (neurotransmitter) Receptors
STEPS IN CONTRACTION
Sliding Filament Theory
Nerve Impulse arrives at axon terminal
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
Myosin heads attach to actin, form crossbridges & pivot
STEPS IN CONTRACTION
Sliding Filament Theory
Actin slides towards center of
sarcomere
ATP provides energy to release & recock myosin heads
Relaxation occurs from:
- Cholinesterase breaks down ACh at
NMJ
- Ca2+ actively pumped back into SR
ACTIVITY OF SINGLE
FIBERS (CELLS)
“All-or-None” Law: At threshold, a fiber
will contract to its maximum extent
- No “partial” contractions of individual
fibers
- 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 (2-2000)
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 the number of motor units
activated
- Changing frequency of stimulation
Different Types of Muscle Fibers
Red Slow (Slow Oxidative)
White Fast (Fast Glycolytic)
Intermediate (Fast Oxidative-Glycolytic)
Types of Muscle Fibers
Red Slow
-
Fewer myofibrils, weaker
Lots of myoglobin, mitochondria, capillaries
Needs oxygen to make ATP
Contracts slowly; fatigues slowly
Endurance
Types of Muscle Fibers
Types of Muscle Fibers
White Fast
- Most myofibrils, strongest
- Low myoglobin, fewer mitochondria &
capillaries
- Makes ATP without oxygen
- High glycogen stores
- Contracts rapidly, fatigues rapidly
- Short-term powerful movements
- May hypertrophy in response to training
Types of Muscle Fibers
Intermediate
-
Intermediate diameter/number of myofibrils
Lots of myoglobin, mitochondria, capillaries
Can make some ATP without oxygen
Contracts rapidly, moderately resistant to
fatigue
- Walking, jogging, biking over short to
moderate distances
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
 Adduction – moves appendage toward 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
BODY MOVEMENTS:
MUSCLE GROUPS
Prime mover: Muscle primarily
responsible for an action
Antagonist: Muscle(s) that resist prime
mover, or move opposite to it
Synergist: Muscle(s) that assist(s) prime
mover
NAMING SKELETAL
MUSCLES: CRITERIA
Muscle attachments: Origin and/or insertion
(e.g. sternocleidomastoid)
Muscle action (Adductor magnus)
Direction of muscle fibers (Rectus abdominis)
Location of muscle (Temporalis)
Size of muscle (Gluteus maximus)
Number of origins/heads (Biceps brachii)
Shape of muscle (Deltoid)