Muscle Properties
• Irritability - A muscle irritability refers to the ability of the muscle to
respond to a stimulus.
• Contractility - A muscle contractility refers to the muscle’s ability to
shorten in length.
• Elasticity - This refers to the muscles ability to stretch and return to
normal length.
• Extensibility - This refers to the muscle’s ability to extend in length.
• Conductivity - This refers to a muscle’s ability to transmit nerve
impulses.
Sarcomeres
• separated by narrow zones of dense material
called Z lines
• within a sarcomere is a dark area called the A
band (thick myofilaments)
• ends of the A band are darker because of
overlapping thick and thin myofilaments
• the light coloured area is called the I band (thin
myofilaments)
• the combination of alternating dark A bands and
light I bands gives the muscle fibre its striated
appearance
Muscle Contraction
• Muscle structure under a microscope
Muscle fibres
• skeletal muscle viewed under a microscope contains thousands of
these elongated, cylindrical cells
Sarcolemma
• the plasma membrane that covers each muscle fibre
Myofibrils
• found within each skeletal muscle fibre
• cylindrical structures which run longitudinally through the muscle
fibre
• consist of two smaller structures called myofilaments
Myofilaments
• thin myofilaments and thick myofilaments
• do not extend the entire length of a muscle fibre
• they are arranged in compartments called sarcomeres
Myofilaments
Thin myofilaments
• thin myofilaments are anchored to the Z lines
• composed mostly of the protein actin
• actin is arranged in two single strands that entwine like a rope
• each actin molecule contains a myosin- binding site
• thin myofilaments contain two other protein molecules that help
regulate muscle contraction (tropomyosin and troponin)
Thick myofilaments
• composed mostly of the protein myosin which is shaped like a golf
club
• the heads of the golf clubs project outward
• these projecting heads are called cross bridges and contain an
actin- binding site and an ATP binding site
Sliding Filament Theory
• during muscle contraction, thin myofilaments slide inward toward the
centre of a sarcomere
• sarcomere shortens, but the lengths of the thin and thick
myofilaments do not change
• myosin cross bridges of the thick myofilaments connect with portions
of actin on thin myofilaments
• myosin cross bridges move like the oars of a boat on the surface of
the thin myofilaments
• thin and thick myofilaments slide past one another
• as thin myofilaments slide inward, the Z lines are drawn toward each
other and the sarcomere is shortened
• myofilament sliding and sarcomere shortening result in muscle
contraction
• this process can only occur in the presence of sufficient calcium
(Ca++) ions and an adequate supply of energy (ATP)
Contractile Machinery:
Sarcomeres
• Contractile units
• Organized in series (
attached end to end)
• Two types of protein
myofilaments:
- Actin:
thin
filament
- Myosin: thick
filament
• Each myosin is
surrounded by six actin
filaments
• Projecting from each
myosin are tiny contractile
myosin bridges
Longitudinal section of myofibril
a) at rest
High microscope magnification of a single
sarcomere within a single myofibril
Contractile Machinery:
Crossbridge formation and movement
•
Cross bridge formation:
- a signal comes from the motor
nerve activating the fibre
- the heads of the myosin
filaments temporarily attach
themselves to the actin filaments

Cross bridge movement:
- similar to the stroking of the oars and
movement of rowing shell
- movement of myosin filaments in relation
to actin filaments
- shortening of the sarcomere
- shortening of each sarcomere is additive
Longitudinal section of myofibril
b) contraction
Contractile Machinery:
Optimal Crossbridge formation
•
Sarcomeres should be optimal
distance apart
For muscle contraction: optimal
distance is (0.0019-0.0022 mm)
At this distance an optimal
number of cross bridges is
formed
Longitudinal section of myofibril
•
If the sarcomeres are stretched
farther apart than optimal
distance:
- fewer cross bridges can form 
less force produced
c) Powerful stretching
•
If the sarcomeres are too close
together:
- cross bridges interfere with one
another as they form  less force
produced
•
•
d) Powerful contraction
Contractile Machinery:
Optimal muscle length and optimal joint angle
• The distance between sarcomeres is dependent on the
stretch of the muscle and the position of the joint
• Maximal muscle force occurs at optimal muscle length (lo)
• Maximal muscle force occurs at optimal joint angle
• Optimal joint angle occurs at optimal muscle length
Muscle tension during elbow flexion at constant speed