11 Contraction

11.1 Introduction

Within each myofibre are one or two thousand myofibrils - each capable of contraction by filament sliding.  Thick myofilaments move past thin myofilaments and sarcomeres shorten - their Z-lines move closer together.  To explain a little of how this happens we need to introduce the following.
This information will be essential later for understanding meat toughness.  If you cannot agree this is important - please drop this course and find another career. We already have enough people producing tough meat.

11.2 Sarcoplasm

11.3 Sarcoplasmic reticulum and transverse tubules 

The sarcoplasmic reticulum initiates muscle contraction by releasing calcium ions when prompted to do so by the transverse tubular or T system.

Each T tubule is a finger‑like inpushing from the surface membrane of the myofibre,  coming into very close contact with elements of the sarcoplasmic reticulum.

11.4 Steps of contraction 

(1) The sequence of events leading to contraction is initiated somewhere in the central nervous system, either as voluntary activity from the brain or as reflex activity from the spinal cord.
(2) A motor neuron in the ventral horn of the spinal cord is activated, and an action potential passes outwards in a ventral root of the spinal cord.
(3) The axon branches to supply a number of myofibres  called a motor unit, and the action potential is conveyed to a motor end plate on each myofibre.
(4) At the motor end plate, the action potential causes the release of quanta (packets) of acetylcholine into the synaptic clefts on the surface of the myofibre.
(5) Acetylcholine causes the electrical resting potential under the motor end plate to change, and this then initiates an action potential that passes in both directions along the surface of the myofibre.
(6) At the opening of each transverse tubule onto the myofibre surface, the action potential spreads inside the myofibre.

(7) At every point where a transverse tubule touches part of the sarcoplasmic reticulum, it causes the sarcoplasmic reticulum to release calcium ions.
(8) The calcium ions are detected by troponin and movement  of tropomyosin on thin myofilaments  enables myosin molecule heads to "grab and swivel" their way along  thin myofilaments.  This is the driving force of muscle contraction.
Contraction is turned off by the following sequence of events.
(9) Acetylcholine at the neuromuscular junction is broken down by acetylcholinesterase, and this terminates the stream of action potentials along the myofibre  surface.
(10) The sarcoplasmic reticulum ceases to release calcium ions, and immediately starts to resequester all the calcium ions released.
(11) In the absence of calcium ions, there is a change in the configuration of troponin, and tropomyosin then blocks the action of the myosin molecule heads, and contraction ceases.
(12) In the living animal, an external stretching force, such as gravity or an antagonistic muscle, pulls the muscle back to its original length. In other words, there is no special system to increase the length of a muscle.
Muscle contraction may take either of two forms, or a combination of both. In isotonic contraction, the muscle shortens against a constant load which is moved. In isometric contraction, the load is too great to be moved, and the muscle generates tension as it attempts to shorten.

In slow‑contracting myofibres the accumulation of calcium ions by mitochondria may contribute to muscle relaxation.

11.5 Thick & thin myofilament structure

On the left of the diagram below we see how globular actin molecules are arranged in a double helix to form thin myofilaments.  On the right is a single myosin molecule.  In the middle is a thick myofilament showing protruding myosin molecule heads.

Troponin and tropomyosin on the thin myofilament control the access of the myosin molecules to the active sites on the actin molecules. Contraction is turned ON when the tropmyosin is IN the groove of the thin myofilament.  Contraction is turned OFF when tropomyosin is OUT of the groove.

11.6  Myosin

The way myosin molecules are arranged in a thick filament is shown below. Half of the molecules in a filament face towards one Z‑line of their sarcomere, while the remainder face towards the opposite Z‑line. Although the heads protrude at regular intervals along the thick filament, there is a short headless zone at the midlength of the filament where the light meromyosins of opposite sides overlap by their tails. There are several hundred myosin molecules in a thick filament, and the length of a whole filament is about 1.5 microns.


11.7 Actin

Further information

Structure and Development of Meat Animals and Poultry.  Chapter 5.