VPP 3211Veterinary Anatomy 1Sliding Filament Theory & Purkinje FibresLecturer: Dr Lokman Hakim Idris

Name: Ng Chin LeonMatric Number: 172614

Sliding Filament TheoryThe theory of skeletal muscle contraction, also known as the sliding filament theory, was founded by two groups of people, A.F. Huxley and Niedergerke (1954), and H.E. Huxley and Hanson (1954).At a very basic level each muscle fiber is made up of smaller fibers called myofibrils. These myofibrils contain even smaller structures called actin and myosin, which are the thin and thick filaments respectively. Myosin Head

The above figures show one sarcomere, which starts from a Z-line and ends at the next. The A band is a region where its darker, consisting of the myosin filament while the I band is the lighter band, which make up the rest of the fibre without the myosin filament. According to the sliding filament theory, when the muscle is at rest, ATP is bound to the myosin head. When a nerve impulse arrives at the neuromuscular junction, the signal is relayed in the form of acetylcholine to the motor end plate. This causes the depolarization of the motor end plate, travelling throughout the muscle tubules of the muscle fibre, inducing the release of Calcium ions from the sarcoplasmic reticulum. The released Calcium ions will bind to the troponin, causing conformational changes in the tropomyosin, thus exposing the binding site on the actin. The myosin head is now able to bind to the binding site of the actin, forming a cross-bridge. However, before this can happen, the ATP bound to the myosin head must be hydrolysed to ADP and inorganic phosphate, realizing energy which causes the myosin head to be cocked. As the ADP and inorganic phosphate were released gradually, the myosin head to pull the actin inwards, thus shortening the muscle. This is known as a power stroke As another ATP molecule binds to the myosin head, the myosin head detaches itself from the actin, and the myosin head is cocked due to the energy released by the hydrolysis of ATP. The cycle repeats until the muscle has reached its maximum contraction, or there is no more ATP or Calcium ions present.When there is no more nerve stimulation, acetylcholine will be broken down by acetylcholinesterase present in the synaptic cleft of the myoneural junction. Calcium ions will be reabsorbed into the sarcoplasmic reticulum, the tropomyosin will regain its original conformation, and the binding site on the actin will no longer be exposed

Purkinje Fibres

Purkinje fibres are specialized muscle fibres consisting of cardiomyocytes that are located in the inner walls of the ventricles, in a space beneath the endocardium, called the subendocardium. The electrical impulse originates from the sinoatrial(SA) node and spreads to the atrioventricular(AV) node, where it is relayed to the His bundle, left and right bundle branch and finally the Purkinje fibres. These fibres are spread all around the ventricular walls. The cardiomyocytes are able to conduct cardiac action potentials at a speed between one to four m/s, causing the contraction of the ventricles to be almost instantaneous. The Purkinje fibres are stained lighter than the cardiac muscle cells, besides being larger. They contain numerous fast voltage-gated sodium channels and mitochondria, and less myofibrils than surrounding cells. All these features enable the rapid transmission of nerve impulses, thus synchronizing the contraction of both the right and left ventricles.