BI0005 - Lecture 6 - Muscles - Simplified
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What are the three types of muscles in the body?
1. Cardiac muscle found in the heart
2. Smooth muscle found in the wall of blood vessels and the gut.
3. Skeletal muscle which is attached to bone. It is the only type of muscle under voluntary conscious control
How are muscle fibres innervated?
Skeletal muscle fibres are innervated by the branches of motor neurones, each f which terminates in a specialised cholingeric synapse called the neuromuscular junction (or motor end plate).
What happens when a nerve impulse arrives at the neruomuscular junction?
When a nerve impulse arrives at the neuromuscular junction it causes the release of acetlycholine, stimulating an action potential in the sarcolemma.
What is a motor unit?
One motor neurone may supply a few or several hundred muscle fibres. They act as a single functional unit called a motor unit
What are muscle fibers
Skeletal muscle is made up of large bundles of long cells called muscle fibres
What are myofibrils?
Each muscle fibre cell is a single cell and it contains lots of long cylindrical organelles called myofibrils
What are myofilaments?
Each myofibril is in turn composed of two kinds of myofilaments (thick and thin) which overlap to form light and dark bands.
What is sarcolemma?
The plasma cell membrane of muscle fibre cells is called sarcolemma
What is sarcoplasm?
Each muscle fibre cell is filled with cytoplasm called sarcoplasm. It contains many nuclei
What is the transverse system?
The sarcolemma folds inwards across the muscle fibre forming a series of transverse tubules which run through the sarcoplasm.
The tubules are the components of the transverse system and they help to spread electrical impulses throughout the sarcoplasm so thy reach all parts of the muscle fibre
What is the sarcoplasmic reticulum
The endoplasmic reticulum of a muscle fibre is called the sarcoplasmic reticulum.
It forms a network of membranes that store a high concentration of calcium ions that are needed for muscle contraction
What do the T-system and the sarcoplasmic reticulum surround?
The myofibrils of the muscle fibre
What do muscle fibres have lots of?
muscle fibres have lots of mitochondria to provide the ATP that is needed for muscle contraction
What are myofilaments?
Myofibrils contain longitudinal bundles of thick and thin myofilaments that move past each other to make muscles contract.
Thick myofilaments are made up of..
the protein myosin
Thin myofilaments are made up...
of the protein actin.
Each myofibril is made up of...
alternating light and dark bands
What are A bands?
Dark bands contain the thick myosin filaments and some overlapping thin actin filaments. These are called A bands
What are I bands?
Light bands contain thin actin filaments only. These are called I bands.
What are sarcomeres?
A myofibril is made up of many short units called sarcomeres
What is a z line?
- The ends of each sarcomere are marked with a Z line.
- The actin filaments are held together at the Z-line
What s an M line?
In the middel of each sarcomere is an M line. This is the middle of the myosin.
What does the sarcomere form?
The sarcomere forms one complete contractile unit.
What two types of fibre does verebrate skeletal muscle consist of?
Slow-twitch and fast twitch.
What are slow twitch fibres?
- These contract more slowly and provide less powerful contractions over a longer period of time.
- They are adapted to endurance work and are found in muscle such as the calf muslce.
- The rate of ATP production during aerobic respiration is relatively slow, so the contractions of slow twitch fibres are not very powerful.
How are slow twitch fibres adapted?
They are adapted to aerobic respiration and so avoid a build up of lactic acid.
They have a large store of myoglobin. Myoglobin stores oxygen in the muscle cells and aids oxygen delivery to the mitochondria. Myoglobin gives the muscle its red coloration.
A supply of glycogen to provide a source of metabolic energy.
A rich supply of blood vessels to deliver oxygen and glucose.
Numerous mitochondria to produce ATP
What are fast twitch fibres?
- These contract more rapidly and produce powerful contractions but only for a short period of time.
- They are adapted to intense exercise, eg weight lifting and are found in muscles such as the biceps.
- They are white in colour as they have a low concentration of myoglobin.
How are fast twitch muscles adapted to their role?
Thicker and more numerous myosin filaments.
A lower concentration of mitochondria
A higher concentration of enzymes involved in anaerobic respiration
A store of phosphocreatine. The breakdown of this molecule releases phosphate ions which combine with ADP to make ATP. This can occur rapidly in anaerobic conditions and so it enables fast-twitch muscles to function for short periods to maximum effect
Briefly describe the sliding filament theory.
1. Myosin and actin filaments slide over one another to make the sarcomeres contract. The myofilaments themselves don't contract
2. The simultaneous contraction of lots of sarcomeres means that the myofibrils and muscle fibres contract.
3. The sarcomeres return to their original length as the muscles relax
How does the contraction of myofibril occur?
- Myosin moves actin during the contraction of a muscle.
- The contraction of a myofibril occurs as a result of the movement of actin filaments sliding over filaments of myosin.
- + Myosin filaments have globular heads that are hinged so they can move back and forth
- + Each myosin head has a binding site for actin and a binding site for ATP
- + Actin filaments have binding sites for myosin heads called actin-myosin binding sites.
- + Two other proteins called troponin and tropomyosin are found between actin filaments.
- + These proteins are attached to each other and form a blocking complex
When the muscle is relaxed, the myosin heads are held away from the myosin binding sites on the actin filaments. The actin-myosin binding site is blocked by tropomyosin, which is held in place by troponin.
The myofilaments can't slide past each other because the myosin heads can't bind to the actin-myosin binding sites on the actin filaments.
When muscle is contracted, the myosin heads move out from their resting position and link to the myosin binding sites on the actin filaments, forming actin-myosin cross bridges.
How does the muscle contract in detail?
When an action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma. Depolarisation spreads down the T-system tubules to the sarcoplasmic reticulum
This causes calcium channels in the vesicle membranes of the sarcoplasmic reticulum to open.
Calcium ions stored inside diffuse down their concentration gradient into the sarcoplasm surrounding the myofibrils.
Calcium ions bind to troponin which changes its shape.
This causes troponin and the attached tropomyosin to move away from the myosin binding site.
The myosin head binds to the actin filament and an actin-myosin cross bridge is formed.
The myosin head then changes its angle, pulling the actin filament along as it does so.
This moves the actin filament towards the center of the sarcomere.
What happens after the formation of an actin myosin-cross bridge?
An ATP molecule attaches to the myosin head, which breaks the actin-myosin cross bridge
Calcium ions activate the enzyme ATPase, which hydrolyses ATP, releasing energy and producing ADP and phosphate.
This provides the energy to move the myosin head back to its original position.
The myosin head then reattaches to a different binding site further along the actin filament,
A new actin-myosin cross bridge is formed and cycle is repeated.
Many actin-myosin cross bridges form and break very rapidly, pulling the actin filament along - which shortens the sarcomere.
Contraction of the muscle continues as a result of the repeated cycle of forming and releasing actin-myosin cross bridges
What happens when the muscle stops being stimulated?
Calcium ions are actively transported back into the sarcoplasmic reticulum. Hydrolysis of the ATP provides the energy.
The reabsorption of clacium ions allows the tropomyosin to once again block the actin-myosin binding sites.
Myosisn heads are unable to bind to actin filaments and so muscle contraction ceases
The actin filaments slide back to their relaxed position.
Summary of the sliding filament mechanism?
The heads of the myosin filament attach to the binding sites on the actin filaments to form cross bridges.
The myosin heads then flex in unison and pull the actin filaments along the myosin filaments.
They then become detached and, using ATP as a source of energy, return to their original angle and reattach themselves further along the actin filaments
How is the shape of the myosin molecule adapted to its role in muscle contraction?
Myosin is made of two proteins. The fibrous protein is long and thin in shape, which enables it to combine with others to form a long, thick filament along which the actin filament can move
The globular protein forms two bulbous structures (the head) at the end of a filament (the tail).
This shape allows it to exactly fit binding sites in the actin molecule, to which it can become attached.
Its shape also means that it can be moved at an angle.
this allows it to change its angle when attached to actin and so move it along, causing the muscle to contract.
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