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Motorische systemen bij vertebraten - Phys Chptr 12: Muscle Physiology

12 important questions on Motorische systemen bij vertebraten - Phys Chptr 12: Muscle Physiology

What does the body of the muscle constist of up to the cells?

The body contains fascicles (budles) containing hundreds-thousands of muscle cells (muscle fibers) running from one end of the muscle to the other and is encased in a sheath of connective tissue (sacrolemma). Because the fibers are made up of several cells during embrionic life, the cells have more then one nucleus.

How is the muscle fiber made up?

The semifluid cytoplasm is called sacroplasm and is packed with mitochondria and myofibrils. Myofibrils are bundles of overlapping thick and thin filaments made of the proteins myosin and actin. They are each surrounded by the sarcoplasmic reticulum and transverse (T) tubules.

What is a sacromere?

A unit that repeats over and over in a myofibril. It is bordered by Z lines which anchor the thin filaments at one end. The thick filaments are anchored at the M line which ar found in the middle of the sacromere. The A band is the dark striation due to the thick filaments. The H zone is the lighter zone where the thin filaments dont overlap the thick filaments. The I band is where only thin filaments are (Z line is in the middle of this).
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How is the thin filament structured?

The backbone consists of two strands of poymerized actin molecules wound together to form a double helix. Myosin-binding sites are on individual actin molecules (G actin). In rest the bindingsites are covered by a tropomyosin strand and troponin.

How is the thick filament structured?

Hundreds of myosin molecules that are shaped like golfclubs, consisting of two intertwined subunits, each having a long tail and a fat head called crossbridges. The part in the middle that does not contain crossbridges is called the barezone. The heads contain a actin-binding site and an ATPase site. Strands of titin (elastic protein) extend along each thick filament from the M line to the Z line, anchoring the thick filaments in their proper positions relative to the thin filaments.

What happens with the bands during muscle cell contraction?

The I and H bands shorten, the A band does not.

What characteristics make the neuromuscular junction special?

Each muscle fiber receives input from only one motor neuron. An action potential in the motor neuron triggers the release of acetycholine from each of its many terminal boutons, which causes many acetylcholine receptors to become activated which make the depolization larger than an ordinary postsynaptic potential. Tha action potential travels through the sacrolemma through the T tubules and then releases calcium from the nearby sacroplasmic reticulum.

What two types of skeletal muscle fiber are there?

Slow-twitch and fast-twitch fibers. Fast-twitch fibers attain peak isometric tension sooner than slow-twitch fibers and also have higher maximum shotening velocities when they contract isotonically. The difference between these types is the type of myosin present in the thick filaments. ATP is hydrolized faster in the fast type of myosin.

What three major classes of muscle fiber are there?

Slow oxidative fibers (smallest), tast glycolytic fibers (largest in diameter) and relatively rare fast oxidative fibers.

What is neuromuscular fatigue?

When motor units are stimulated to contract at high frequencies, as occurs when large forces are generated. When cells are stimulated to contract stronggly over long periods, repeated firing of motor neurons can deplete synaptic terminals of acetylcholine.

What two decending pathways are important in voluntary motor control that provide input to motor neurons?

pyramidal tracts/corticospinal tracts (direct from the primary motor cortex to the spinal cord (ventral horn and are callecd upper motor neurons, control of the finer discrete movements) and the extrapyramidal tracts (include all paths that do not cross the pyramidal system, these are indirect pathways between the brain and spinal cord, they do not form synapses on motor neurons. extrapyrimidal system involved in controlling large groups that contract together to  maintain posture and balance).

Which nuclei are involved in the control of posture and balance and cannot initiate voluntary movements?

Reticular formation, the vestibular nuclei and the red nuclei.

The question on the page originate from the summary of the following study material:

Mens en dierkunde II | Wageningen

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