ENGINE FOR SALE
Large Force (200kN/m2)
Very Efficient (>50%)
Doesn’t Overheat (38°C)
Uses a Variety of Fuels
Lasts a Lifetime
Good to Eat
£10-00 per kg at your Supermarket
Muscle is indeed a remarkable tissue. In engineering terms it far superior to anything we have been able to invent, and it is responsible for almost all the movements in animals. There are three types of muscle:
This is always attached to the skeleton, and is under voluntary control via the motor neurones of the somatic nervous system. It is the most abundant & best understood type of muscle. It can be subdivided into red (slow) muscle and white (fast) muscle (see module 3).
This is special type of red skeletal muscle. It looks and works much like skeletal muscle, but is not attached to skeleton, and is not under voluntary control (see module 3 for details).
This is found in internal body organs such as the wall of the gut, the uterus, blood arteries, the iris, and glandular ducts. It is under involuntary control via the autonomic nervous system or hormones. Smooth muscle usually forms a ring, which tightens when it contracts, so there is no need of a skeleton to pull against.
Unless mentioned otherwise, the rest of this section is about skeletal muscle.
Muscles and the Skeleton
Skeletal muscles cause the skeleton to move (or articulate) at joints. They are attached to the skeleton by tendons, which transmit the muscle force to the bone and can also change the direction of the force. Tendons are made of collagen fibres and are very strong and stiff (i.e. not elastic). The non-moving attachment point (nearest to the trunk) is called the origin, and moving end (furthest from the trunk) is called the insertion. The skeleton provides leverage, magnifying either the movement or the force.
Muscles are either relaxed or contracted. In the relaxed state muscle is compliant (can be stretched), while in the contracted state muscle exerts a pulling force, causing it to shorten or generate force. Since muscles can only pull (not push), they work in pairs called antagonistic muscles. The muscle that bends (flexes) the joint is called the flexor muscle, and the muscle that straightens (extends) the joint is called the extensor muscle. The best-known example of antagonistic muscles are the biceps and triceps muscles, which articulate the elbow joint:
The "relaxed" muscle is actually never completely relaxed. It is always slightly contracted to provide resistance to the antagonistic muscle and so cause a smoother movement. This slightly contracted condition is called tonus, or muscle tone. Most movements also involve many muscles working together, e.g. to bend a finger or to smile. These groups of muscles are called synergistic muscles.
A single muscle (such as the biceps) contains around 1000 muscle fibres running the whole length of the muscle and joined together at the tendons.
Each muscle fibre is actually a single muscle cell about 100µm in diameter and a few cm long. These giant cells have many nuclei, as they were formed from the fusion of many smaller cells. Their cytoplasm is packed full of myofibrils, bundles of proteins filaments that cause contraction, and mitochondria to provide energy for contraction.
The electron microscope shows that each myofibril is made up of repeating dark and light bands. In the middle of the dark band is a line called the M line and in the middle of the light band is a line called the Z line. The repeating unit from one Z line to the next is called a sarcomere.
A very high resolution electron micrograph shows that each myofibril is made of parallel filaments. There are two kinds of alternating filaments, called the thick and thin filaments. These two filaments are linked at intervals by blobs called cross bridges, which actually stick out from the thick filaments.
The thick filament is made of a protein called myosin. A myosin molecule is shaped a bit like a golf club, but with two heads. Many of these molecules stick together to form the thick filament, with the "handles" lying together to form the backbone and the "heads" sticking out in all directions to form the cross bridges.
The thin filament is made of a protein called actin. Actin is a globular molecule, but it polymerises to form a long double helix chain. The thin filament also contains troponin and tropomyosin, two proteins involved in the control of muscle contraction.
The thick and thin filaments are arranged in a precise lattice to form a sarcomere. The thick filaments are joined together at the M line, and the thin filaments are joined together at the Z line, but the two kinds of filaments are not joined to each other. The position of the filaments in the sarcomere explains the banding pattern seen by the electron microscope:
Mechanism Of Muscle Contraction- the Sliding Filament Theory
Knowing the structure of the sarcomere enables us to understand what happens when a muscle contracts. The mechanism of muscle contraction can be deduced by comparing electron micrographs of relaxed and contracted muscle:
These show that each sarcomere gets shorter when the muscle contracts, so the whole muscle gets shorter. But the dark band, which represents the thick filament, does not change in length. This shows that the filaments don’t contract themselves, but instead they slide past each other. This sliding filament theory was first proposed by Huxley and Hanson in 1954, and has been confirmed by many experiments since.
The Cross Bridge Cycle
What makes the filaments slide past each other? Energy is provided by the splitting of ATP, and the ATPase that does this splitting is located in the myosin cross bridge head. These cross bridges can also attach to actin, so they are able to cause the filament sliding by "walking" along the thin filament. This cross bridge walking is called the cross bridge cycle, and it has 4 steps. One step actually causes the sliding, while the other 3 simply reset the cross bridge back to its starting state. It is analogous to the 4 steps involved in rowing a boat:
1. The cross bridge swings out from the thick filament and attaches to the thin filament. [Put oars in water.]
2. The cross bridge changes shape and rotates through 45°, causing the filaments to slide. The energy from ATP splitting is used for this "power stroke" step, and the products (ADP + Pi) are released. [Pull oars to drive boat through water.]
3. A new ATP molecule binds to myosin and the cross bridge detaches from the thin filament. [push oars out of water.]
4. The cross bridge changes back to its original shape, while detached (so as not to push the filaments back again). It is now ready to start a new cycle, but further along the thin filament. [push oars into starting position.]
One ATP molecule is split by each cross bridge in each cycle, which takes a few milliseconds. During a contraction, thousands of cross bridges in each sarcomere go through this cycle thousands of times, like a millipede running along the ground. Fortunately the cross bridges are all out of synch, so there are always many cross bridges attached at any time to maintain the force.
Control Of Muscle Contraction
How is the cross bridge cycle switched off in a relaxed muscle? This is where the regulatory proteins on the thin filament, troponin and tropomyosin, are involved. Tropomyosin is a long thin molecule, and it can change its position on the thin filament. In a relaxed muscle is it on the outside of the filament, covering the actin molecules so that myosin cross bridges can’t attach. This is why relaxed muscle is compliant: there are no connections between the thick and thin filaments. In a contracting muscle the tropomyosin has moved into the groove of the double helix, revealing the actin molecules and allowing the cross bridges to attach.
Contraction of skeletal muscle is initiated by a nerve impulse, and we can now look at the sequence of events from impulse to contraction (sometimes called excitation contraction coupling).
1. An action potential arrives at the end of a motor neurone, at the neuromuscular junction.
2. This causes the release of the neurotransmitter acetylcholine.
3 This initiates an action potential in the muscle cell membrane.
4. This action potential is carried quickly throughout the large muscle cell by invaginations in the cell membrane called T-tubules.
5. The action potential causes the sarcoplasmic reticulum (large membrane vesicles) to release its store of calcium into the myofibrils.
6. The calcium binds to troponin on the thin filament, which changes shape, moving tropomyosin into the groove in the process.
7. Myosin cross bridges can now attach and the cross bridge cycle can take place.
Relaxation is the reverse of these steps. This process may seem complicated, but it allows for very fast responses so that we can escape from predators and play the piano.