what is the simple structure of a skeletal muscle
skeletal muscle –> fascicles –> muscle fibres –> myofibrils –> myofiliaments
detailed version of structure of skeletal muscle (5)
- skeletal muscle is surrounded by epimysium
- made up of bundles of muscle fibres (fasicles) surrounded by epimysium
- each fasicle contains individual muscle fibres
- fibres arranged into myofibrils, running parallel to each other and the length of muscle fibre
- myofibrils contain chain sarcomeres, composed of actin and myosin filaments responsible for creating movement
difference between epimysium and perimysium
epimysium is connective tissue that surrounds the entire muscle (each muscle)
perimysium is the connective tissue that surrounds the fasicles
what is a muscle fibre
a muscle fibre forms part of the fasicles and are comprised of myofibrils.
what is a sarcomere
a sarcomere is a segment of a myofibril which is the functional unit of a muscle contraction and is made up of myofilaments
what is included in a sarcomere
- z line
- A band
- M line
- H zone
- I band
- Myosin
- actin
- cross-bridges
Draw picture
What is a motor unit
a motor unit consists of the motor neuron and specific muscle fibres it innovates
- motor unit consists of only 1 fibre type
Motor neuron - what is consists of
motor neuron consists of cell body, axon and dendrites. allows it to transmit electrochemical impulses from spinal cord to muscle. this impulse travels towards the axon terminal branches at the motor end plate
sliding filament theory
- sensory receptors detects stimulus and send message through sensory neuron to CNS
- CNS sends message from brain to spinal cord to peripheral nerves
- the motor neuron picks up the message via dendrites and the nerve impulse travels through cell body down to axon, to axon terminal to the motor end plate
- this stimulates the release of acetylcholine over the synaptic cleft
- key electrolytes such as Ca are required for a muscle contraction otherwise the muscle will weaken and may cause minor tears
- calcium is released from the sarcoplasmic reticulum
- presence of Ca makes the binding sites on actin available
- ATP molecule is required to activate mysoin heads
- myosin cross bridges can now attach to actin
- actin slides over myosin and the muscle contracts by shortening. As this occurs the H-zone and I-band decrease/disappear and the Z lines move closer together
- as the muscle relaxes Ca returns to sarcoplasmic reticulum, myosin cross bridges detach via ATP and sarcomere returns to relaxed state
All or nothing principle
when an electrical impulse reaches a certain threshold all of the muscle fibres of that motor unit will contract at the same time and as forcefully as possible. Until the threshold is reached none of the fibres will contract
fibre recruitment (how increase contraction force of muscle)
- increased number of motor units recruited: more motor units activated and more muscle fibres contract
- Increased frequency of impulse to motor unit: greater frequency of arrival of nerve impulse greater force is produced by muscle
characteristics of type 1 muscle fibre
- large capillary network: increase the delivery of oxygen and removal of co2 to working muscle
- high number mitochondria: maximises utilisation of aerobic fuel stores (increase storage of triglycerides) and oxygen to produce large amounts of energy (ATP)
- Large number of myoglobin enhances O2 extraction from the blood stream, increases delivery and storage of O2 for utilisation by the mitochondria
characteristics of type 2b muscle fibre
- large amount of PCr stores: preferred fuel source for anaerobic activity and is readily available as the presence of o2 isn’t required. This will increase the capacity of this system
- high number anaerobic enzymes: facilitates faster reactions and breakdown of anaerobic fuels
- stimulated by large motor neuron: more forceful contraction
force velocity
the force a muscle can create decreases with increasing velocity of shortening. while the force a muscle can resist increases with increasing velocity of lenghtening
force length
force length relationship states the amount of force a muscle can produce will vary at different lengths
muscle at resting length
there is optimal overlap of actin and myosin myofilaments resulting in optimal cross-bridge formation. this formation increases the overall force production
contracted beyond resting length (shorted)
There is too much over lap of actin and myosin this disrupts cross-bridge attachment as there is less space available for optimal binding (there is little cross-bridge formation). this results in less force being produced
stretched (lengthened) beyond resting length
myofilaments have minimal overlap resulting in less available binding sites and inefficient cross-bridge attachment. therefore less force is being produced
