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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
Stabilizes joints
Maintains posture
Generates 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 (stimulate to contract)
- Blood vessels (supply nutrients, oxygen)
- Connective tissue (re-inforce)
ARRANGEMENT OF SKELETAL
MUSCLE
Connective tissue coverings provide
strength & support
- Endomysium: Around each muscle fiber;
insulates
- Perimysium: Around fascicles (bundles
of cells)
- Epimysium: Around entire muscle
(bundles of fascicles)
- Fascia: collective term for C.T. around &
between muscles
ATTACHMENTS OF SKELETAL
MUSCLE
Connective tissue attachments join
muscles to: bones, 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
Muscle Fibers (skeletal muscle cells):
- Long, cylindrical, multinucleate
Sarcolemma: cell membrane
T-tubules: Cell membrane extensions
deep into the muscle cell
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 overlapping
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 bind to and
stimulate muscle fibers
Structure of Neuromuscular
Junction (NMJ)
Axon Terminal – end of motor neuron
Synaptic Cleft (Gap) – space between
axon terminal & motor end plate
Motor End Plate:
*Sarcolemma at NMJ
*Invaginated
*High SA (surface area)
*ACh (acetylcholine) receptors
STEPS IN CONTRACTION
Sliding Filament Theory
1. Nerve Impulse arrives at axon terminal;
Ca2+ enters terminal
2. Exocytosis of synaptic vesicles
3. Neurotransmitter Acetylcholine (ACh)
diffuses across cleft
4. ACh binds to receptors on sarcolemma
5. Prior to contraction, sarcolemma must be
polarized by Na+/K+ pump (+ outside/within)
STEPS IN CONTRACTION
Sliding Filament Theory
6. Sarcolemma now permeable to Na+ and K+
7. Sarcolemma depolarizes as more Na+
diffuses in than K+ diffuses out
8. Action Potential (muscle impulse) spreads
along sarcolemma and down t-tubules
9. SR release Ca2+ into sarcoplasm
ACh opens Na+/K+ Channel
STEPS IN CONTRACTION
Sliding Filament Theory
10. Ca2+ binds to troponin on actin
11. Tropomyosin on actin moves, exposing
binding site
12. Myosin heads attach to actin, form
cross-bridges & pivot
STEPS IN CONTRACTION
Sliding Filament Theory
13. Actin slides towards center of
sarcomere
14. ATP provides energy to release &
re-cock myosin heads
15. 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
Requires oxygen to make ATP
Contracts slowly; fatigues slowly
Recruited first
Endurance
Types of Muscle Fibers
Types of Muscle Fibers
White Fast
- Most myofibrils, strongest & widest in diam.
- Low myoglobin, fewer mitochondria &
capillaries
- Makes ATP without oxygen
- High glycogen stores
- Contracts rapidly, fatigues rapidly
- Short-term powerful movements
- Recruited last
- 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
- The second fiber type recruited
- 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 results 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
Fixator: Stabilize origin of prime mover or
hold the bone still so all tension is used to
move insertion
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)