Muscular System Flashcards
Draw and describe the structure of a skeletal muscle.
Skeletal muscle cells are multinucleate, resulting from the fusion of many myoblasts. They have a diameter of 10-100um and their length can be many centimetres. The myofibre consists of many myofibrils which are long ‘columns’ of sarcomeres. Sarcomeres are the contractile unit of muscle cells which consists of many myofilaments (contractile proteins) which are composed of thick filaments (myosin) and thin filaments (actin).
Actin is a globular protein composed of many actin monomers arranged into a two strand helix. Myosin’s main subunit is a globular and filamentous protein, it consists of 2 heavy chains and 2x2 light chains.
Draw the structure of a sarcomere
Describe the events of muscle excitation and contraction and how they are coupled.
The sliding filament model explains muscle contraction in the context of the sarcomere. Myosin head move actin filaments, shortening the sarcomere and therefore the muscle fibres. This process requires chemical energy which generated mechanical energy.
Motor neuron cell bodies in the central nervous system (spinal cord and brainstem) stimulate muscle contraction. Motor neuron axons are in the spinal and cranial nerves, and the synapse is on the skeletal muscle cells at neuromuscular junction (NMJ). An action potential in a neuron leads to an action potential and contraction in the muscle cell it innervates.
The muscle cell membrane is excitable. Motor neurons release acetylcholine (Ach) at the synapse which binds to Ach receptors (ligand gated ion channel) on muscle cells. This causes sodium and calcium ions to enter and some potassium to exit, causing depolarisation of the muscle cell membrane. Voltage gated potassium channels then open to repolarise the cell.
Muscle cell depolarisation causes calcium channels in the ER membrane to open. This causes a efflux of calcium from the ER to the cytoplasm. Calcium binds to a protein (troponin) causing a shape change and exposes the myosin binding sites to actin. Myosin and actin now interact (cross bridge cycling) causing shortening in the sarcomere length.
Rapid, repeated stimulation of a muscle leads to partial relaxation or no relaxation between stimuli, and/or additional muscle cells can be made to contract.
