Muscles &Muscle Tissue
Chapter 9
Function of Muscles
•
•
•
•
Movement
Posture
Stabilization of Joints
Thermogenesis (heat production)
Functional Characteristics of
Muscle
• Excitability (irritability)
– Can receive and respond to stimuli.
• Stimuli can include nerve impulses, stretch,
hormones or changes in the chemical
environment.
• Contractility – the ability to shorten with
increasing tension.
• Extensibility – the ability to stretch.
• Elasticity – the ability to snap back (recoil)
to their resting length after being
stretched.
Three types of muscle
Skeletal
Smooth
Cardiac
Characteristics of Skeletal Muscle
•
•
•
•
Striated
Multinucleate (it is a syncytium)
Voluntary
Parallel fibers
Arrangement of connective tissue in
skeletal muscle
Muscle microstructure
Myosin
A thick myofilament
A thin myofilament
Made of actin, troponin and tropomyosin
Arrangement of myofilaments
The sarcomere:
the functional unit of skeletal muscle
Anatomy of a myofibril
Summary of skeletal muscle anatomy:
muscles are made of fascicles
Fascicles are made of fibers,
fibers are made of myofibrils
Fibrils are divided into sarcomeres,
sarcomeres are made of myofilaments
Myofilaments
are made of
protein molecules
Muscle Contraction:
the Sliding Filament Theory
• Muscle contraction requires:
– Stimulus – the generation of an action
potential.
– Crossbridge formation – interaction between
the thick and thin myofilaments. This is
triggered by Ca++ ions released from the
sarcoplasmic reticulum.
– Energy – ATP to energize the myosin
molecules.
T- tubules supply the stimulus,
Sarcoplasmic Reticulum supplies the Ca++,
Mitochondria supply the ATP.
The role of Calcium ions
Muscle contraction
Show the animation
Excitation-Contraction coupling
• Stimulus or excitation is required for
muscles to contract.
• In skeletal muscle, the stimulus is from a
motor neuron.
• The stimulus is in the form of an action
potential.
• This action potential starts at the
neuromuscular junction (NMJ).
A neuromuscular junction (NMJ).
acetylcholine
The actual synapse
Excitation-contraction coupling
Show NMJ animation
Motor
units
Micrograph of an NMJ
A Synapse
Synaptic vesicles
Myogram of a single muscle twitch
Comparative speed of different muscles
Graded Muscle Responses
• The “twitch” describes just one fiber, it
really isn’t how muscles normally
work.
• Muscles contract smoothly and
respond to different levels of demand.
• This is accomplished through “graded
responses”. There are two ways
muscle responses are “graded”
Wave Summation
•Wave summation is accomplished by repeated stimuli.
•As the rate of stimulus delivery increases, there is less and
less time for the fiber to relax between stimuli.
•If the stimuli are rapid enough, the muscle fiber will contract
completely and remain contracted until the stimulus stops of
the muscle fatigues.
•This is called tetanus (or tetany).
Recruitment
• Since action potentials are “all-or-none”
responses, when a fiber is stimulated to
tetany, it exerts maximum tension.
• To respond to stronger stimuli and thus
increase the amount of tension, muscles
will recruit more motor units until they
reach maximal stimulus and all the motor
units are recruited
• This continues until they either accomplish
their task or fatigue.
Treppe
Types of contractions
• Isometric – means “same length”.
Force is developed without
measurable movement.
• Isotonic – means “same tension”
movement is achieved with force or
“tension” remaining constant.
Isotonic contraction: Concentric
Eccentric Contraction
This refers to the tension that is applied on
a muscle as it lengthens.
Example: When you lower the dumbbell
you just lifted, you don’t do so by allowing
the muscle to just relax. Think about the
consequences of that!
Eccentric contractions are about 50% more
forceful than concentric ones.
Isometric Contraction
Energy physiology in skeletal muscle
Anaerobic
Metabolism:
a losing
proposition
Aerobic endurance
Anaerobic
endurance
Fiber Types
• Fast Glycolytic (fast twitch a or type IIB)
– few mitochondria, adapted for fast, powerful
contraction, large diameter, little myoglobin
(white fibers), few capillaries, fatigues rapidly
• Fast Oxidative (fast twitch b or type IIA)
– many mitochondria, fast contraction,
intermediate diameter, lots of myoglobin &
capillaries (pink fibers), moderately fatigue
resistant
• Slow Oxidative (slow twitch or type I)
– many mitochondria, slow contraction,
smallest in diameter, don’t hypertrophy, lots of
myoglobin & capillaries, very fatigue resistant,
red fibers
Stretch and tension
Stretch/
Tension
The Effect of
Load on
Contraction
Smooth
Muscle
Varicosities
Contraction of smooth muscle
Skeletal
Smooth
Diameter
10 - 100 m
3 - 8 m
Connective tissue
Epi-, Peri- & Endomysium only
Endomysium
SR
Yes, complex Barely, simple
T - tubules
yes
no
Sarcomeres
yes
no
Gap Junctions
no
yes
voluntary
yes
no
Neurotransmitters
Acetylcholine Ach, epinephrine,
(Ach)
norepinephrine, et al
Regeneration
Very little
Lots, for muscle
Future governors of Califorina?