Muscles Flashcards
Functions of muscle
Motion of the body but also moves substances through the body (e.g. alimentary canal, blood vessels, sphincters, the heart)
Maintenance of posture – some skeletal muscles act ‘automatically’ to maintain posture e.g. abdominal muscles
Heat production – heat is generated by muscle movement but also by shivering (mechanism invoked when we are too cold)
Properties of muscle
Excitability
Contractility
Extensibility
Elasticity
Types of muscle
Skeletal muscle (also striated / striped - so called because of striped appearance under microscope)
Called skeletal because it attaches to and moves the skeleton i.e. anywhere where an individual can consciously control the muscle
The diaphragm is skeletal muscle because it is under voluntary and involuntary control
Cardiac muscle – found only in the heart
Also has a striated appearance under the microscope
Under involuntary control
Beats around 72 times per minute and does not fatigue (limit of endurance – marathon running – 2 hours 20 minutes)
Does not require an external stimulus to initiate muscle contraction
Smooth muscle – lining the alimentary canal, ureters and bladder
Does not have a striated appearance under the microscope
i.e. anywhere where the body controls the contractions and relaxations automatically
What makes up a sarcomere?
Z disc / zone – separates each functional unit
H zone – thick filaments only
A band – thick filaments and overlap with thin filaments
I band – thin filaments only
Sliding filament theory
ATP binds to the binding sites on the myosin head
Calcium ions are released by the sarcoplasmic reticulum and bind to the troponin
This causes the troponin to ‘move’ out of the way freeing up the binding sites on the actin heads
The ATP breaks down into ADP
The actin and myosin molecules bind together
Power stroke – the myosin head rotates and releases the ADP - this causes the filaments to move relative to each
Another ATP molecule binds to the ATP which releases the actin
The cycle starts again and continues until either maximal muscle contraction occurs or for as long as ATP is available and there are sufficient calcium levels etc
This process requires large amounts of energy, and is dependent on an adequate supply of calcium ions
Initiation of muscle contraction
Motor neurone – nerve that supplies muscle i.e. initiates movement
The neuromuscular junction is the connection between the motor neurone and the muscle fibre
The synapse is the region where the communication between the nerve and the muscle must take place
In order to bridge the gap at the synapse – a neurotransmitter is released which crosses the synaptic cleft
Acetylcholine is the neurotransmitter in this case – indirect cell communication i.e. motor neuron releases acetylcholine into synaptic cleft, the acetylcholine interacts with the muscle fibre and contraction is stimulated
The effect of the production of the muscle action potential is to cause calcium ions to be released which then bind with the troponin thereby initiating the muscle contraction
Phosphocreatine
This is a very high-energy molecule found in muscle fibres, in concentrations greater than ATP
Phosphocreatine breaks down to form creatine and phosphate and releases large amounts of energy in the process
This released energy is used to convert ADP back into ATP
Phosphocreatine and ATP can provide enough energy for muscle to contract at full strength for about 15 seconds
Glycogen
Once the supplies of phosphocreatine have been depleted, energy is obtained from the breakdown of glycogen
Glycogen is the stored form of glucose
Glycogen is stored in muscle and the liver
When glycogen is broken down, enough ATP is liberated for muscle to contract maximally for several minutes
Fats
If muscle contraction continues so that all the energy resources of glycogen have been exhausted, muscle fibres can break down fat
