Muscle belly
A number of long muscle fibres grouped together
Epimysium
The layer of connective tissue that encases the muscle
Endomysium
The connective tissue that surrounds individual muscle fibres
Perimysium
The connective tissue that surrounds a fascicle
Fascicle
A bundle of muscle fibres
Describe the structure of a muscle from muscle down to myosin and actin
- A muscle is actually made up of bunches of fascicles
- A fascicle is a bundle of muscle fibres
- A muscle fibre is made up of lots of myofibrils
- Along the length of a myofibril are contractile units of the myofibril called sarcomeres
- Within each sarcomere we find the myofilaments actin and myosin
Sarcomere
- The contractile unit of a muscle
- Made up of overlapping actin and myosin proteins
- The Z lines form the boundary of the sarcomere
- Actin attaches to Z lines, myosin does not
- The myosin filaments are parallel to the actin and sit in between each actin filament, in the middle of the sarcomere
Actin
The thin protein filament attached to the Z line
Myosin
The thick protein filament that contains cross-bridges
Contraction of a muscle summary
- When there is a neurochemical stimulation, calcium is released into the sarcomere prompting a reaction between the myosin and the actin filaments.
- Calcium causes the actin binding site to be revealed.
- Myosin filaments contain crossbridges at regular intervals. These myosin cross bridges reach out and attach to the actin filament.
- Breakdown of ATP releases energy to stimulate crossbridges which attach and reattach at different times along the actin pulling on them to create movement and maintain tension.
- This causes the actin to move towards the centre of the sarcomere, shortening the I band and H zone as the Z lines are brought closer together. This shortens the length of the myofibril.
- The actin and myosin filaments almost fully overlap when in a fully contracted position
- As the neural impulse ends calcium leaves the sarcomere and the contraction finishes, causing the myosin and actin filaments to return to a relaxed position
3 types of muscle contraction
-Isometric
-Isotonic
-Isokinetic
Isometric muscle contraction
- An isometric force will result in NO CHANGE in muscle length.
- Myosin crossbridges still attach but the actin filaments are not pulled towards the midline of the sarcomere.
- An isometric contraction will produce the most force.
E.g. bridge/prone plank, scrum in rugby
Isotonic muscle contraction
Change in the length of a muscle performed against a CONSTANT load.
Can be:
- Concentric (generally the lifting phase)
Muscle shortens
Occurs when you apply a force against direction of gravity
or
- Eccentric (generally the lowering phase)
Muscular lengthens
Occurs when you apply resist a force with gravity
Isokinetic muscle contraction
- Results in change of length of muscle against a VARYING load.
Define the force-velocity relationship
The force generated by a muscle depends on the total number of cross-bridges attached.
Explain force-length relationship
The force-length relationship explains how muscle tension/force varies at different muscle lengths.
- Maximum tension (force) is created in a muscle fibre at a length slightly greater than resting length.
- This is because the number of cross bridges that can be attached between the myosin and actin is maximised.
- Muscles generate less force as they lengthen or contract beyond optimal length as there are less available binding sites due to the actin filaments overlapping (contracted muscle) or cross bridges not lining up with the binding sites (extended muscle).
Explain force-velocity relationship
- Because of the amount of time it takes for cross-bridges to attach, as the muscle shortens with increasing velocity, the force that muscle can produce decreases due to the lower number of cross-bridges attached.
- As muscle velocity decreases (as the relative filament velocity decreases) more cross-bridges have time to attach and consequently the muscle can generate more force.
-So inverse relationship between force and velocity
Other factors affecting muscle force
- Number of fibres recruited
- Muscle fibre type
- Speed of contraction
- Type of contraction
3 muscle fibre types
- Muscle fibre type 1- slow twitch, endurance
- Muscle fibre type IIa - fast twitch, 800m runner
- Muscle fibre type IIb - fast twitch, power athletes - sprinters, shot putter
Preferential recruitment
Definition: The recruitment of specific muscle fibres for different activities
- Type I fibres have a lower activation level than Type II fibres and are more easily recruited.
- During low intensity exercise, Type I fibres are predominantly recruited.
- At high level intensity, Type IIa fibres are recruited.
- At very high intensity exercise, Type IIb fibres are recruited.
Nervous system muscle contraction path
- Sensory neurons send messages from the sensory receptors (eyes, ears etc) to the brain.
- The brain makes decision and then sends an impulse down the spinal cord.
- The spinal cord relays information from the brain to the body and relays messages from the body back to the brain.
- Motor neurons send messages to the muscles from the brain.
- An impulse must travel down the spinal cord before reaching the motor neuron.
- The muscle fibre is then innervated.
All or none principle
States that if an electrical stimulus reaches the threshold level, then all the muscle fibres associated with that motor unit will contract to their max level, all at the same time.
What is a motor neuron made up of:
- Dendrites - Detect stimulus/impulse from CNS and send it to the cell body
- Cell body - Processes impulse and sends signal to axons
- Axons - Transmits an impulse away from cell body towards the muscle
- Motor end plate - relays stimulus to muscle fibres
What is a motor unit
The motor unit refers to the motor neuron and the muscle fibres which are influenced by that nerve. A motor unit may innervate a few or hundreds to thousands of muscle fibres.
