Grade 12 Exercise Science
Muscle Structure and Function Note
Properties of Muscle Fibre
- Excitability/ Irritability
- ability to receive and respond to stimuli
- Contractibility
- ability to shorten and thicken, or contract
- Extensibility
- ability to stretch, or extend
- Elasticity
- ability return to its original shape after contraction or extension
- Conductivity
- ability to transmit nerve impulses
Muscle Types
- Skeletal
- named for its location (attached to bones and moves the skeleton)
- it is striated (has striations, or alternating bands of light and dark bands visible under microscope)
- it is voluntary because it can be made to contract and relax by conscious control
- Cardiac
- forms the bulk of the wall of the heart
- it is striated and involuntary
- Smooth
- involved with internal processes
- located in the internal structures (e.g. blood vessels, the stomach, and the intestines)
- it is non-striated and involuntary
Muscle Function
- Motion
- walking, beating of the heart, churning food in stomach, etc.
- Maintenance of posture
- contraction of skeletal muscles holds the body in stationary positions
- Heat production
- skeletal muscle contractions produce heat which helps to maintain normal body temperature
Connective Tissue Components
- Fascia/ Epimysium
- a sheet or broad band of fibrous connective tissue beneath the skin or around muscles and other organs of the body
- Fasiculi or fasicules
- bundles of muscle fibres
- Perimysium
- a fibrous connective tissue that covers the fasicules
- Endomysium
- the fibrous connective tissue that wraps each individual muscle cell
- Tendon
- a cord of connective tissue that attaches a muscle to a bone
- made of epimysium, perimysium, and endomysium
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
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
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)