Module 1.2 Force generation by skeletal muscle Flashcards
Explain neuromuscular transmission
Once the neural signal that originates in the motor cortex arrives at the NMJ, this signal needs to be transferred across the NMJ and into the muscle fibre to initiate muscle contraction; known as neuromuscular transmission.
List the steps involved in neuromuscular transmission
Step 1. Action potential arrives at the motor neuron terminal
Step 2. Vesicles containing acetylcholine release into synaptic cleft, which then binds to receptors on the sarcolemma of the muscle fibre. This allows influx of sodium into the muscle
Step 3. Movement of sodium into the muscle generates an action potential, which is transmitted along the t-tubules and deep into the muscle fibre
Step 4. Action potential travels into the muscle down t-tubules, to initiate calcium release from the sarcoplasmic reticulum
Explain excitation-contraction coupling
E-C coupling provides the link between step 1 (activation of muscle fibre through neuromuscular transmission) and step 3 (cross bridge formation). The aim of this step is to use the action potential within the t-tubules to prepare the muscle for the formation of cross-bridges and force generation.
Explain transverse tubules (t-tubules)
Invaginations of the sarcolemma (appear like holes) that penetrate deep into each muscle fibre. They are evident at each A band-I band junction and approach each myofibril and every sarcomere
What stage involves calcium ions interacting with the myofilaments to allow the muscle to change into a force producing state?
Cross-bridge cycling
Explain cross bridge cycling
The process by which muscle generates force through the interaction of two contractile proteins - actin and myosin. At rest, cross-bridges are prevented from forming until the calcium ion (Ca2+) concentration in the muscle rises.
Explain myosin
Myosin is one of the contractile proteins (or filaments) within the myofibril. It is a thick filament and is located centrally within the sarcomere. It comprises of a head segment and a tail segment. The head segment exhibits ATPase activity which means it is an enzyme that uses the energy currency in the cell (ATP). The tail segment of myosin contains a flexible hinge region which allows the head to rotate during cross-bridge cycling.
Explain actin
Actin is a much smaller protein than myosin and is located on the other edges of the sarcomere and is connected to the z-line. It contains the binding site for myosin but it is surrounded by regulatory proteins called tropomyosin and a protein complex called troponin.
What happens when all the sarcomeres across the length of a muscle shorten simultanaeously?
The whole muscle contracts to generate force
Which neurotransmitter is used for neuromuscular transmission of an action potential from an alpha-motoneuron to a skeletal muscle fibre?
Acetylcholine, which is the only neurotransmitter used for neuromuscular transmission within skeletal muscle.
What is the primary role of the sarcoplasmic reticulum in skeletal muscle?
It stores, releases, and re-uptakes calcium ions.
The ‘triad’ within skeletal muscle facilitates which part of the contraction process?
Excitation-contraction coupling
What coincide with the detachment of myosin from actin within the cross-bridge cycle?
Myosin binding to a new molecule of ATP
What drives the eventual relaxation of skeletal muscle when there is no longer an activating stimulus coming from the associated alpha motoneuron?
Actively pumping calcium ions (Ca2+) from the cytosol into the sarcoplasmic reticulum (SR)
