1. Lindsey Bily Anatomy & Physiology Austin High School

2.  Have certain characteristics that allow them to do their job. ◦ Excitability (aka irritability): the ability to be stimulated ◦ Contractility: they can contract or shorten which allows them to pull on bones and produce movement ◦ Extensibility: they than extend or stretch letting them return back to their original shape after contracting.

3.  Remember a muscle is composed of bundles of muscle fibers (cells).  Can range in length from 1mm to 40 mm long!  They share many of the same structural parts as other types of cells, but of course, have will have different names since they are in a muscle fiber.

4.  Sarcolemma: plasma membrane of a muscle fiber.  Sarcoplasm: cytoplasm of a muscle fiber.  Sarcoplasmic Reticulum (SR): network of tubules and sacs similar to the ER of other cells.  They contain many mitochondria and have multiple nuclei.

5.  Fibers have structures that are only present in muscle fibers.  Myofibrils: very thin fibers within the cell that extend lengthwise and almost fill up the sarcoplasm.  Sarcomere: basic contractile unit of the muscle fiber.  T Tubules (transverse tubules): they are tubes that run perpendicular to the myofibrils. ◦ Formed by extensions of the sarcolemma ◦ Allow nerve impulses (electrical signals) move deeper into the cell.

8.  Each muscle fiber contains about a thousand myofibrils. Shockingly, these myofibrils are composed of even smaller fibers called myofilaments.  Myofilaments can be either thick or thin.  Proteins that make up myofilaments… ◦ Myosin ◦ Actin ◦ Tropomyosin ◦ Troponin

9.  Thin Filaments are made up of actin, tropomyosin, and troponin.  Thick Filaments are up of myosin. They are shaped like golf clubs so that they can “grab” the actin on the thin filament.

10.  Contraction of a muscle requires several processes to happen in a sequence.

11.  Excitation of the Sarcolemma ◦ A motor neuron is a specialized nerve cell that connects to the sarcolemma of a muscle fiber at the motor endplate. However, they don’t touch completely, there is a gap. ◦ This connection is called a neuromuscular junction (also a synapse). ◦ Neurotransmitters are chemicals that transmit signals. ◦ When a nerve impulses reaches the end of a neuron, it releases the neurotransmitter, acetylcholine into the synaptic cleft (gap between the muscle fiber and the neuron).

12.  Acetylcholine diffuses across the synapse and binds to receptors on the sarcolemma on the muscle fiber.  Binding to the receptors causes them to sent an electrical impulse into the sarcolemma.

13.  The electrical impulse travels across the sarcolemma and down the T tubules.  The impulse triggers the sarcoplasmic reticulum (SR) to release stores of Ca 2+.  The Ca 2+ combine with the troponin on the thin filaments and cause the active sites of actin to become exposed.  The myosin heads on the thick filament can now bind to the actin and pull the thin filaments past them.  The myosin then releases itself and binds to another actin active site, there by shortening the muscle.  This is the SLIDING FILAMENT THEORY.

14.  Click here to see an animation.  http://msjensen.cehd.umn.edu/1135/Links/ Animations/Flash/0011- swf_breakdown_of_a.swf http://msjensen.cehd.umn.edu/1135/Links/ Animations/Flash/0011- swf_breakdown_of_a.swf

15.  Almost immediately after the SR releases Ca 2+ into the sarcoplasm, it actively pumps it back into its sacs.  Within a couple of milliseconds, the Ca 2+ has been recovered.  The SR steals the Ca 2+ from the troponin which then causes the active sites of the actin to become blocked again.  The contraction process automatically shuts itself off within a small fraction of a second after initial stimulation.

16.  Muscles use ATP for energy.  The myosin head will bind to the ATP and it is “at rest” waiting for actin to be able to bind. Imagine pulling back on a slingshot.  Once the myosin has bound to the actin, a new ATP molecule comes in and the myosin goes back in the “rest” position therefore pulling the actin towards it.

17.  Remember, in order for ATP to be used, the muscle must have an ample supply of glucose and oxygen.  Some muscle fibers store glycogen so they have enough glucose readily available.  Muscle fibers also store oxygen in a molecule called myoglobin (similar to hemoglobin).  Fibers with lots of myoglobin are called red fibers and those with few myoglobin are called white fibers.  Most muscles have a mixture of the two.

19.  If there is not enough oxygen present, then the body will undergo anaerobic respiration.  This will produce a buildup of lactic acid in the muscles and they might start to cramp or fatigue.  The lactic acid will gradually get back into the blood and travel to the liver where it will be converted back to glucose.  Remember the ULTRAMARATHON MAN?