LEC11

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.

Transverse tubule - connects to the surface membrane of the myofibre.
Sarcoplasm - the cytoplasm around the myofibrils.
Sarcoplasmic reticulum - a membranous system within the sarcoplasm and around the myofibrils.
The sarcoplasmic reticulum is activated by transverse tubules.
The sarcoplasmic reticulum stores calcium ions and releases them to trigger muscle contraction.
The sarcoplasmic reticulum resequesters (grabs back) the calcium ions to allow myofibrils to relax.
Calcium ions are detected by troponin - a protein on the thin myofilament.
Troponin moves tropomyosin - another protein on the thin myofilament.
With tropomyosin out of the way, myosin molecules on the thick myofilament grab and swivel against actin molecules on the thin myofilament.
Thick myofilaments move past thin myofilaments and the myofibril contracts.

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

The cytoplasm within a myofibre is called sarcoplasm.
The aqueous phase of the sarcoplasm is called the cytosol.
The calcium ion concentration of myofibre cytosol is regulated by the sarcoplasmic reticulum.
The sarcoplasmic reticulum surrounds the myofibrils within a myofibre and occupies about 4% of the volume of the myofibre.
In a resting myofibre, the cytosol is kept almost free of calcium ions by the rapid sequestering of calcium ions by the sarcoplasmic reticulum.
The concentration of calcium ions in a resting myofibre is about 5 x 10^-8 M. This is increased to about 5 x 10^-6 M when the myofibre contracts.
The protein calsequestrin binds and stores calcium ions within the sarcoplasmic reticulum.
Calsequestrin concentration is higher in fast‑contracting than in slow-contracting myofibres.

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.

T tubules conduct action potentials initiated at the neuromuscular junction and carry them deep into the interior of the myofibre.
Communication between a transverse tubule and the sarcoplasmic reticulum is via protein‑containing bridges between adjacent membranes.
The bridges are composed of four subunits.
The bridge proteins extend through the membrane of the sarcoplasmic reticulum to form calcium ion channels.
At the ends of muscle fibers where transverse tubules may not reach, the sarcoplasmic reticulum may be directly connected to the plasma membrane.
An extracellular marker, horseradish peroxidase, can move into the transverse tubules, thus proving they open onto the myofibre surface.
In the striated muscles of our meat animals, T tubules occur at the level of each A‑I junction.
After contraction has occurred, it is turned off if the sarcoplasmic reticulum resequesters all the calcium ions it initially released.
Sustained muscle contraction or tetanus is the result of the fusion of individual muscle twitches.
The peak tension generated by a single twitch occurs a few milliseconds after the action potential on the myofibre membrane, when about 60% of the maximum calcium ion release has occurred.

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 proteins of the myofibril account for somewhat more than half of the protein content of most myofibres.
Within the myofibril, myosin accounts for 55 to 60% of the protein content and actin accounts for about 20%.
The general shape of the myosin molecule is shown below.
Experimentally isolated myosin molecules are just large enough to be seen by electron microscopy after they have been sprayed with atoms of metal.
The two active heads of each molecule are difficult to resolve separately, and so the molecule usually has the appearance of a miniature matchstick with only one globular head.
Myosin may be extracted in about 15 minutes with a concentration of 0.5 M NaCl.
Myosin is a long asymmetrical molecule composed of two heavy chains (220 kDa) and four light chains.
Two of the light chains (LC2, 18kDa) have a regulatory function and the other two are essential for ATPase activity.
The digestive enzyme trypsin splits the myosin molecule into two large fragments, (1) light meromyosin (LMM) with a molecular weight of about 140 kDa, and (2) heavy meromyosin (HMM) with a weight of about 340 kDa.
When light meromyosins are placed together under conditions similar to those inside a myofibre, they self-assemble into a bundle that resembles the backbone of a thick myofilament.
Papain splits heavy meromyosin into two subfragments. Papain is an enzyme obtained from tropical melons and is sometimes used to tenderize meat.
Subfragment 1 of heavy meromyosin (HMM-1) is composed of the two active heads of the myosin molecule, and these retain their ability to bind to actin and hydrolyze ATP.

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

Individual molecules or monomers of actin have a globular shape and are called G‑actin.
The polymerization of G‑actin to form filamentous F‑actin requires energy from ATP, and the resulting ADP is bound into the filament.
G‑actin molecules have a diameter of about 5.5 nm.
Two strands of F‑actin are twisted around each other to form the thin filament and the cross‑over points are about 35.5 nm apart.
Each complete thin filament has a length of about one micron and contains 300 to 400 G‑actins.
Thin filament assembly by actin polymerization is controlled by several proteins, binding either to the fast-growing or barbed end (gelsolin, villin, and capZ), or to the slow-growing pointed end of the new then filament (tropomodulin in association with tropomyosin).
The overall length of the thin filament may be controlled by a "ruler" of nebulin.

Although G‑actin is globular, it has an active site on its surface where it reacts with myosin molecule heads during muscle contraction
When G‑actin is polymerized to F‑actin, the active sites all face towards the H zone and away from the Z‑line in which the thin filament is anchored.
Thus, within an individual sarcomere, the thin filaments on either side of the H zone are mirror images and are compatible with the the arrangement of myosin molecules in the thick filament.
If a thick filament should penetrate the Z‑line, the immediate thin filaments in the next sarcomere are facing the wrong way for the myosin molecule heads of the intruding thick filament.