Neuromuscular System The complex linkages between the muscular system and the nervous system Nerves transmit impulses in “waves” that ensure smooth movements A single nervous impulse and the resulting contraction is call a muscle twitch One neuron or nerve may be responsible for stimulating a number of muscle fibres  

Neuromuscular System The electrical impulse begins at the spinal cord (CNS) travels along the nerve axon to the contact point between the nerve and muscle (neuromuscular junction)

Neuromuscular System At the junction, the neurotransmitter acetylcholine is released, which is detected by receptors on the surface of the muscle fibre, initiating a muscle contraction

Neuromuscular System The signal is transmitted into the muscle fibre through the transverse tubulae system, which is a network of interconnecting tubes that surround a myofibril and act as a link between the outside of the muscle fibre and the actin & myosin filaments The tubulae then initiates a rapid release of calcium ions, leading to muscle contraction

Neuromuscular System The calcium ions are released in to the sarcoplasm by the terminal cisternae, which form part of the sarcoplasmic reticulum (a network of membranes surrounding the myofibrils) The calcium ions interact with the troponin molecules, causing the tropomyosin to allow the myosin heads to bind with the actin

Neuromuscular Junction Clip

Sliding Filament Theory A contraction is caused by an overlapping of the actin and myosin filaments, which causes the sarcomere to contract  

Sliding Filament Theory Myosin crossbridges attach, rotate, detach and reattach in rapid succession, resulting in the overlapping of the filaments, shortening of the sarcomere and muscle contraction

Sliding Filament Theory The trigger mechanism is the release of calcium ions, by the terminal cisternae, when the nerve impulse is transmitted through the muscle fibre.   The energy source behind the release of calcium and detachment of myosin from actin is adenosine triphosphate (ATP). As the work of the muscles increases, more and more ATP is used up and must be replaced through food metabolism Sliding Filament Theory Clip

The Sarcomere Arrangement of myofibrils in a sarcomere: A contractile unit, or compartment of myofibrils A sarcomere goes from one Z-band to the next Z-band Sarcomeres contain two types of tiny filaments called myofibrils (actin and myosin) Arrangement of myofibrils in a sarcomere: Z disk – attaches actin A band – portion containing both actin and myosin I band – portion containing actin myofilament only H zone – portion containing only myosin

Muscular Contraction The All-or-None Principle Whether or not a motor unit activates upon the arrival of an impulse depends upon the so called all-or-none principle An impulse of a certain magnitude (or strength) is required to cause the innervated fibres to contract Every motor unit has a specific threshold that must be reached for such activation to occur Sliding Filament Theory Clip

Types of Muscle Fibres Slow Twitch (Type I ) More mitochondria Most people have an even mix of fast and slow twitch fibers in their bodies, which is genetically determined Slow Twitch (Type I ) More mitochondria More capillaries More of a red colour (due to increased blood supply) Activated more slowly by motor neurons Slower contraction speed, but contraction can be maintained for a longer period.

Types of Muscle Fibres Fast Twitch (Type II) Higher glycogen stores More of a white colour (less blood) Activated more quickly by their motor neurons Contracts more quickly, but fatigues much more easily