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 microscopeMuscle fibres• skeletal muscle viewed under a microscope contains thousands of

these elongated, cylindrical cells Sarcolemma• the plasma membrane that covers each muscle fibreMyofibrils• found within each skeletal muscle fibre• cylindrical structures which run longitudinally through the muscle

fibre• consist of two smaller structures called myofilamentsMyofilaments• thin myofilaments and thick myofilaments• do not extend the entire length of a muscle fibre• they are arranged in compartments called sarcomeres

MyofilamentsThin 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

b) contraction

Longitudinal section of myofibril

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

• If the sarcomeres are stretched farther apart than optimal distance:

- fewer cross bridges can form less force produced

• If the sarcomeres are too close together:

- cross bridges interfere with one another as they form less force produced

Longitudinal section of myofibril

c) Powerful stretching

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