Muscle Structure and Function

STRUCTURE AND FUNCTION OF MUSCLE

MUSCLE TYPES

·  Skeletal (striated)

·  Smooth (non striated)

·  Cardiac

MAJOR MUSCLE GROUPS – learn and become familiar with.

Biceps, Triceps, Deltoid, Latissimus Dorsi, Trapezius, Gluteus Maximus, Hamstrings, Gastrocnemius, Achilles Tendon, Pectoralis Major, Rectus Abdominus, Sartorius, Quadriceps, Tibialis Anterior.

STRUCTURAL FEATURES OF SKELETAL MUSCLES

Draw a diagram which shows a typical muscle – include tendons, belly, bundles and fibres.

Skeletal muscles are composed of 1000’s of contractile muscles fibres bound together by sheaths.

SHAPES (FIBRE ARRANGEMENTS) OF MUSCLES

The muscle fibres arranged according to the type of work the particular muscle has to perform.

·  FUSIFORM – Used for mobility

·  PENNATE – UNIPENNATE + BI PENNATE + MULTIPENNATE – Used for strength and power activities.

THE MICROSCOPIC STRUCTURE OF SKELETAL MUSCLES

The muscle is made of bundles of muscle fibres – the fibres are made up of myofibrils. The myofibrils are made up of myofilaments (actin and myosin).

Myofibrils are characterized by alternating light and dark areas which gives the striated effect – light areas (I bands) dark areas (A bands). Z line is the dark line in middle of I band.

When a muscle contracts one set of filaments slides over the other thus shortening the muscle. Actin and myosin length do not change. ATP is needed for the process to take place.

THE MOTOR UNIT

The number of muscle fibres greatly exceeds the number of nerve fibres, thus nerve fibres must branch repeatedly. All muscle fibres served by the same motor nerve contract and relax at the same time thus act like a unit. Therefore SINGLE MOTOR NERVE + THE MUSCLE FIBRES IT SUPPLIES = MOTOR UNIT.

The ratio of muscle fibres innervated by a single motor nerve is not determined by the size of the muscle, but by the precision, accuracy and co-ordination needed by the muscle.

SLIDING FILAMENT THEORY

It is thought that the actin filaments slide across the myosin filaments, shortening the muscle and producing movement.

With energy in the form of ATP the myosin heads make contact with cross bridges with the actin attachment sites.

This repeats many times, and at both ends; therefore each time the myosin heads attach further down, the actin strand; causing filaments to slide inwards. As long as ATP is present the crossbridges can ‘make and break’ repeatedly and cause movement.

THE ALL OR NONE LAW

All fibres of a motor unit contract at the same time, and they contract to the maximum possible strength.

DIFFERENT KINDS OF MOTOR UNITS

Fast – twitch (FT)

Slow – twitch (ST) fibres.

In humans, some muscle fibres function more efficiently aerobically than others and some are better equipped to work anaerobically. The majority of our muscles contain an approximately equal mixture of FT and ST fibres, although specific muscles are predominantly ST or FT.

TYPES OF MUSCULAR CONTRACTION

·  ISOMETRIC CONTRACTION

Tension developed, but no change in length because the external force is greater than the internal contractile force of a muscle e.g. Doorway

·  ISOTONIC ECCENTRIC CONTRACTION

Muscle lengthens during development of active tension. E.g. Lowering weight in bicep curl.

·  ISOTONIC CONCENTRICCONTRACTION

Muscle shortens and the tension remains the same throughout.

RECIPRICOL INHIBITION

When a muscle contracts its antagonist needs to relax. Reciprocal inhibition is the circuit of nerves which inhibits the antagonist muscle.

AGONIST / ANTAGONIST RELATIONSHIP

Throughout the body there are sets of opposing muscles which form agonist / antagonist relationships. E.g. biceps / triceps.