Muscular System

Question 1

3 Types of Muscles in the Body

Answer 1

• Skeletal – voluntary physical activities, attached to bones (movement and posture)
• Smooth – internal organs, involuntary
• Cardiac – heart muscle

Question 2

Skeletal Muscle Properties

Answer 2

• All muscles can CONTRACT (shorten) – which reduces the distance between parts they are connected to.
• Eg. Skeletal - Bringing 2 bones closer together, Eg. Cardiac – reduces space in chambers, pushing blood out.
• EXTENSIBILITY – ability to be stretched
• ELASTICITY – ability to return to original length

Question 3

Macroscopic structure of skeletal muscle

Answer 3

• Muscle cells are held together in BUNDLES that are held together by a layer of connective tissue.
• This CT allows adjacent bundles to slide over each other.
• The CT of each bundle join each other, and taper towards the end of the muscle, which forms the TENDON
• Amount of CT increases with age, which decreases muscle strength

Question 4

Microscopic structure of skeletal muscle

Answer 4

• Each muscle bundle consists of MUSCLE FIBRES that lie parallel to each other.
• Each muscle fibre has many nuclei and is surrounded by a plasma (cell) membrane called the SARCOLEMMA, which contains the cytoplasm - called SARCOPLASM.
• They are 10-100 um in diameter and range in length

Question 5

Sliding Filament Theory

Answer 5

• A nerve impulse reaches the muscle
• This stimulates the release of calcium ions from the T-tubules of the sarcoplasm reticulum
• The calcium binds with the troponin, which exposes the myosin-binding sites on the actin fibres
• Myosin heads are already energized due to the presence of the ADP + P molecule in place
• The myosin heads bind to the exposed active site on the actin: forming cross bridges
• The myosin heads continue to swivel, pulling the actin past the myosin, shortening the muscle (Power Stroke)
• The myosin heads rotation ends with the final expenditure of the energy from the ADP + P molecule
• A new ATP molecule attaches to the myosin head and allows the myosin to release from actin (Recovery Stroke)
• This process repeats as long as calcium is present to keep the binding sites exposed
• Many myosin heads are staggered along the myosin fibres, all at different stages of this process, to allow continual contraction of a muscle

Question 6

Muscles working together - Bicep and Tricep

Answer 6

• The Biceps and Triceps work together to move the Humerus. When the Biceps contracts to bend the arm, the Triceps must relax.
• The Biceps on one end is fixed to the scapula and the other end is attached to the Radius. When the Biceps muscle contracts, the scapula remains stationary while the radius bends.
• The end of the muscle attached to the stationary bone is called the ORIGIN and the end attached to the moveable bone is called the INSERTION
• The muscle that causes the desired action is the AGONIST, while the one with the opposite effect to the agonist is the ANTAGONIST
• SYNERGISTS are muscles that help indirectly in steadying a joint during a particular movement. Allows the agonist to work more efficiently
• FIXATOR – when a synergist immobilises a joint. Eg. Muscles that attach the scapula to the axial skeleton. The scapula must be held steady to efficiently act as a firm origin for muscles that move the arm.

Question 7

Muscle Tone

Answer 7

• Skeletal muscles can maintain partial contraction, referred to as MUSCLE TONE
• When muscles need to contract for a long time (ie. Those involved in upright posture), muscle fibres work in ‘shifts’. At any one time some muscle fibres are contracted while other are relaxed. So as the fibres of one section become fatigued, those of another take over while the first recovers.
• Eg. Our head is held up by partial contraction of neck muscles, when someone falls asleep in a chair their head droops forward.