Muscle Contraction

Skeletal muscles are EFFECTORS, they are stimulated to contract in RESPONSE to a nerve impulse. 

3 types of muscles in the body: skeletal, smooth & cardiac: 

1) Skeletal muscle 

• Also called striped, striated or voluntary muscle (we have control over) 
• This is muscle attatched to bone and is concerned with locomotion
• Contracts rapidly and fatigues rapidly
• Skeletal muscle is attached to bone via inelastic tendons
• When the muscle contracts it pulls on the skeleton causing the bone which it is attached to, to move.

2) Smooth muscle (e.g. arterioles,arteries etc)

• Also called unstriated, unstriped and involuntary muscle. 
• This is found in the walls of tubular organs such as arteries and the gut. 
• It contracts slowly and fatigues slowly
• It contracts and relaxes

3) Cardiac muscle - only found in the heart

Features of skeletal muscle contraction

• Skeletal muscle is used to move bones at joints.
• Muscles can only pull; they cannot push. 
• When muscles CONTRACT they usually shorten and when they RELAX they can only be returned to their usual length by the action of an ANTAGONISTIC MUSCLE.
• (diagram pg 3): When the triceps muscle CONTRACTS it SHORTENS, pulling on the forearm leading to extension of the elbow joint.
• An opposing force is needed for flexion of the elbow.
• Now the biceps contracts (triceps relaxes) and the forearm is pulled upwards.
• This is an example of ANTAGONISTIC MUSCLE ACTION.
• When 1 muscle contracts the other relaxes. 

Tendons at each end of the muscle connect the muscle to the bone

The muscle is made up of bundles of muscle fibres up to 2cm across. These are bound together by connective tissue, which is continuous with the tendons. 

Each muscle fibre is a single muscle cell surrounded by a cell surface membrane

Each muscle fibre may be several cm's long, but is less than 0.1mm in diameter

Inside the muscle fibre is the cytoplasm containing mitochondria and the other organelles found in a cell. Within each muscle fibre there are also numerous myofibrils; each is composed of repeated contractile units called sarcomeres. 

Muscle fibres have nuclei spread along the length. Muscle fibre is contained within the muscle.

Sarcomere - repeated units, These reduce in size when a muscle contracts.

Myofibril involved in contraction. 

Diagram pg 4

 Muscles fibres are elongated cells packed with myofibrils. Myofibrils consist of bundles of thick and thin filaments.

THICK filaments are made of a protein called MYOSIN

THIN filaments are made of the protein ACTIN

Viewed under the electron microscope myofibrils are shown to display repeated light and dark bands

Myofibrils consist of functional units called SARCOMERES. The ends of each sarcomere are marked by Z lines.

• Z line - Thin actin filaments are anchored into the Z lines. The distance between 2 Z lines = a sarcomere. 
• M line - Where thick myosin (dark) filaments are anchored 
• A band - The dark regions of the A band are where thin and thick filaments overlap
• I band - Contains THIN filaments only (Actin) I = light 
• H zone - Contains THICK filaments only (Myosin) Within dark, a lighter dark area

Diagram pg 5

The actin filaments are pulled over the myosin filaments to make the sarcomere contract and shorten. As this happens the appearance of the muscle changes. 

(When muscle contracts the actin overlaps the myosin). Diagram pg 6:

RELAXED SARCOMERE:

• Z: Distance between Z lines longer 
• M: Same
• H: Wider
• I: Wider
• A: dark areas of A band narrow (stays the same width because myosin length doesn't change. You only get an overlap of actin and myosin. 

CONTRACTED SARCOMERE:

• Z: Distance between Z lines gets shorter
• M: Same
• H: Narrower/may disappear (gets shorter because more actin overlapping myosin so dark bit of A gets bigger)
• I: Narrower/may disappear
• A: Darker areas of A band wider

THICK MYOSIN FILAMENTS (thicker & denser = darker band). heads attach to actin filaments

• The thick filament is made of many molecules of the protein MYOSIN.
• A myosin molecule is shaped a bit like a golf club and consists of a tail and a globular head.
• The tails of the myosin molecules are specifically shaped to bind to each other and form thick filaments.
• The globular heads protrude in all directions to form the cross bridges (actinomyosin bridges) with the thin actin filaments adjacent to it.
• Again, each protein has a specific shape to bind and form the actinomyosin bridges.

THIN ACTIN FILAMENTS (thinner, less dense = lighter band)

• Each actin filament is made up of 2 helical strands of globular actin molecules which twist round each other to form an actin filament.
• Actin filaments are associated with 2 regulatory proteins: tropomyosin & troponin. 
• Tropomyosin is a fibrous protein which is wrapped around the actin filaments in a longitudinal fashion.

I band = thin filaments only. H = Overlap of thick and thin

Muscle stimulation

• An action potential reaches a neuromuscular junction and triggers release of acetylcholine (ACh) from the terminal of the motor neurone. 
• The ACh causes an action potential in the muscle cell membrane (Sarcolemma) in the same way it does in a postsynaptic neurone. 
• The action potential spreads through the muscle fibre via a system of tubules (T tubules) that branch throughout the sarcoplasm (cytoplasm) of the muscle. 
• The tubules are in contact with the sarcoplasmic reticulum (ER in muscle cells) of the muscle, which has actively absorbed calcium ions from the cytoplasm of the muscle. 
• The action potential opens the calcium ion channels on the sarcoplasmic reticulum and calcium ions flood into the muscle cytoplasm down a diffusion gradient. 
• The calcium ions cause the tropomyosin molecules that were blocking the binding sites on the actin filament to pull away. 

• The myosin heads plus attached ADP + Pi can now bind to the actin filament and form an actinomyosin bridge. Once attached to the actin filament the myosin heads change their angle, pulling the actin filament along (the power stroke) and releasing ADP + Pi.
• A new molecule of ATP now attaches to each myosin head, causing it to become detached from the actin filament. The enzyme ATPase, which is activated by calcium ions and located in the myosin head hydrolyses the ATP molecule to ADP + Pi, providing the energy for the myosin head to return to its original position. 
• The myosin head, once more with an attached ADP + Pi, then reattaches itself further along the actin filament and the cycle is repeated as long as nervous stimulation of the muscle continues.

Muscle relaxtion

• When nervous stimulation ceases, calcium ions are actively transported back into the endoplasmic reticulum.
• This reabsorption of the calcium ions allows tropomyosin to block the actin filament again.
• Myosin heads are now unable to bind to actin filaments and contraction ceases and the muscle relaxes. 

Pg 9 for cross bridge cycle diagram

Roles of following in myofibril contraction:

• Tropomyosin - When the muscle is relaxed the tropomyosin molecules block the myosin binding sites on the actin molecule.
• Calcium ions - Active ATPase & also opening of calcium ion channgels in the sarcoplasmic reticulum allows calcium ions to flood into the sarcoplasm. This causes the tropomyosin molecules that were blocking the binding sites on the actin filament to pull away. 
• Actin and Myosin - Myosin heads bind to sites on the actin filaments forming a cross bridge. Myosin heads change their position/angle, pulling the actin filaments along. The myosin head detaches and then re attaches further along the actin filament. 
• ATP- Binding of ATP to the myosin head allows it to detatch from the actin filament. AT hydrolysis provides energy for 1) Myosin head to return to its original position/angle; 2) Power stroke (energy allows myosin head to change angle and pull action) ; 3) Reabsroption of calcium ions into sarcoplasmic reticulum; 4) formation of actinomyosin bridge

• The sarcoplasmic reticulum - Contains high conc. of calcium ions in relaxed muscles. Upon nerve stimulation calcium channgels in the sarcoplasmic reticulum open allowing calcium ions to flood into the sarcoplasm. 
• After death the muscles of an animal stiffen in a state known as rigor mortis. This occurs because cellular respiration stops after death so ATP is not produced and so myosin head cannot detach and muscles cannot relax and remain contracted. Existing cross bridges therefore stay in place. 

The role of ATP and Phosphocreatine 

Muscle contraction requires considerable energy. This is supplied by the hydrolysis of ATP to ADP + Pi.

Equation to represent the synthesis and hydrolysis of ATP:

The energy released is needed for the myosin heads to change shape and for the reabsorption of calcium ions into the sarcoplasmic reticulum by active transport.

The energy for muscle contraction comes from ATP, but during intense exercise the ATP runs out after about 3 seconds. After this, ATP is quickly resynthesised using the phosphate and the energy released from the spliting of phosphocreating (PC) into creatine and phosphate. 

Phosphocreatine cannot supply energy directly to the muscle, so instead it regenerates ATP which can. Phosphocreatine is stored in muscle and acts as a rserve supply of phosphate ions, which are available immediately to combine with ADP and so reform ATP. The phosphocreatine store is replenished using phosphate from ATP when the muscle is relaxed. 

Equation to represent the hydrolysis and synthesis of phosphocreatine:

The ATP/PC system can provide enough energy for a maximum effort - a sprint for example, for up to 10 seconds.

After this for up to about 60-90 seconds, ATP is supplied from glycolysis. This second system can still provide enough energy for about 1 minute, but it is ANAEROBIC, and lactate build up is a painful problem.

After about a minute, supplies of ATP come from full aerobic respiration with ATP production via oxidative phosphorylation. 

Look at graph pg 11 

The graph shows that 1 system gradually takes over from another allowing us to move our muscles continuously.

A nerve impulse is the trigger for a muscle contraction, but the length of time a contraction lasts depends on how long calcium ions remain in the sarcoplasm.

This time is different in different types of skeletal muscle fibres.

FAST twitch fibres (Type 2 fibres) and SLOW twitch fibres (Type 1 fibres) are classified on the basis of their contraction times.

There are 3 differences between fast & slow fibres:

• Slow fibres has LESS sarcoplasmic reticulum than fast twitch fibres. This means that calcium ions remain in their sarcoplasm for longer.
• Slow twitch fibres have MORE mitochondria which provide ATP, via aerobic respiration, for sustained contraction.
• Slow twitch fibres have significantly MORE myoglobin (stores oxygen) than fast twitch fibres. Myoglobin has a HIGHER affinity for oxygen than haemoglobin in blood and so is particularly efficient at extracting oxygen from blood. 

Different people tend to have different proportions of fast and slow twitch fibres in their muscles which may GENETICALLY PREDISPOSE them to being good at one type of exercise rather than another. 

Exercise will increase the proportion of a particular type of fibre.

POWER ATHLETES such as sprinters possess a high percentage of FAST twitch fibres.

EDURANCE ATHELES such as long distance runners have a high percentage of SLOW twitch fibres.

Older people have fewer fast twitch fibres.

Slow twitch muscle fibre: is aerobic, has steady power & has endurance

Fast twitch muscle fibre: is anaerobic, has explosive power & fatigues easily.

Slow = type 1. Fast = type 2.

Functional feature: Role in body - Long term steady movement eg endurance running

Structural feature:  Diameter of fibres - Small & Number of capillaries - More

• Number of mitochondria - More

Mechanical feature: 

•  Speed of contraction - Slower
• Rate of pumping of calcium ions - Slower

Biochemical feature:

• ATPase activity (in myosin heads) - Low (ATP hydrolysed slowly)
• Source of ATP - Oxidative phosphorylation (aerobic)
• Glycogen content - Low (Glucose arrives via the blood, therefore a longer term supply)
• Myoglobin content - High
• Rate of fatigue - Slow

Location: Back & neck muscles involved in posture

Functional feature: Role in body - Rapid powerful movement - eg weight lifting

Structural feature:  Diameter of fibres - Large & Number of capillaries - Few

• Number of mitochondria - Fewer

Mechanical feature: 

•  Speed of contraction - Fast
• Rate of pumping of calcium ions - Fast

Biochemical feature:

• ATPase activity (in myosin heads) - High (ATP hydrolysed fast)
• Source of ATP - Glycolysis (anaerobic)
• Glycogen content - High (Stored in muscles for immediate use)
• Myoglobin content - Low
• Rate of fatigue - Fast

Location: Arms and legs & bird wings