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P & A Notes Muscular System

February 07, 2011

Muscular System 

There are 3 types of muscle in the human body: *skeletal, smooth, and cardiac

Skeletal muscle:
Is attached to our bones and is under conscious control. The skeletal muscle fiber is a single cell and is the contractile unit of the muscle.
-In the cytoplasm of these cells are many threadlike myofibrils. The thick ones are composed of the protein myosin. The thin ones are composed of the protein actin. These filaments are arranged in units called sarcomeres. (light and dark striations) (see pg. 179-180)
-Each skeletal muscle fiber is connected by a nerve cell called a motor neuron.
-The point where nerve and muscle meet is the neuromuscular junction. At this junction the muscle’s surface has a motor end plate. The end of the motor neuron is branched and one neuron may connect many muscle fibers. This is called a motor unit.
-In a motor neuron the nerve impulse travels from the brain or spinal cord to the effector - in this case it is the muscle.
Muscle Contraction / Relaxation;
*When the nerve impulse reaches the end of a motor nerve fiber, vesicles found at the end of the fiber will release acetylocholine into the neuromuscular junction. * Acetylocholine diffuses through the motor end plate into the muscle fiber. This causes an organelle in the muscle, sarcoplasmic reteculum to release calcium ions in a muscle fiber which in turn causes the actin and myosin filaments to merge. The muscle fibers shorten and pull on its attachments.
*The acetylcholine, which actually stimulated the muscle fiber, is decomposed by an enzyme called cholinesterase which is released by the membrane of the motor end plate. Once acetylcholine is gone, *active transport will quickly move the calcium ions back into the sarcoplasmic reticulum.
This prevents continued contraction of a muscle fiber by one impulse.

ATP is used in the contraction of the muscles. To produce ATP we must go through the process of cellular respiration. Oxygen which is needed in stage II and III of cellular respiration is delivered to the muscles by the blood.
A pigment, myoglobin, is produced in muscle cells and can store oxygen temporarily... reducing the need for continuous blood supply during muscle contraction.
Creatin phosphate stored in the muscle is an energy storing compound.

Oxygen Debt
When exercising strenuously your muscles aren’t getting enough O2 from the blood, so the muscles use the stored O2 in myoglobin. When the O2 levels become low the muscles rely on lactic acid fermentation to make ATP. The muscles will be using ATP faster in exercise than can be produced by lactic acid fermentation and ADP will start to accumulate. When energy levels get low the muscles will use the stored energy in creatin phosphate to produce more ATP. When at rest... the lactic acid must be converted back to glucose, the ADP converted to ATP, O2 replaced in myoglobin, and creatin phosphate regenerated.

Muscle Fatigue:
When a person exercises strenuously for a long period they may use their ability to contract their muscles fatigue.
-This will usually occur due to lactic acid buildup in the muscle from anaerobic respiration. Lactic acid causes enough change in the Ph of the muscle fibers that they no longer responds to stimulation.
-It can happen by acetylcholine being used up. In this case there is nothing to stimulate the contraction.
-Or by interruption of blood flow. Once again the muscles rely on anaerobic respiration, lactic acid will buildup and the muscle will not respond.
-Cramps sometimes happen along with fatigue. It seems to be caused by lack of ATP needed to move calcium ions back into the sarcoplasmic reticulum, which would allow the actin and myosin filaments to slide apart.

Understand the following terms:
Threshold stimulus, all-or-none response, origin, insertion, prime, mover, synergists, antagonists, recruitment, hypertrophy, atrophy

Smooth muscle fibers:
-Contain actin and myosin fibrils, but are more randomly arranged and do not show striations.
2 types:
1.) multiunit smooth muscle-
-occur as separate fibers rather than as sheets.
-found in iris of eyes and in wall of blood vessels.
-each fiber will contract only after stimulation from a motor nerve impulse.

2.) visceral smooth muscle-
-found in sheets and the fibers are in close contact with each other.
-located in walls of visceral organs: stomach, intestines, bladder, uterus.
-when one fiber is stimulated the impulse may move over it and stimulate the fiber next to it...and the next...next. This is rhythmicity. Rhythmicity is responsible for the wavelike motion called peristalsis.

Cardiac muscle:
-found only in the heart.
-are held together at ends by bonds called intercalated disks
-When one portion of the cardiac muscle is stimulated the impulse will pass to other fibers of the networks and the whole structure will contract as a unit. It is rhythmic and self-exciting .

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