The Muscular System Chapter 8

All movement occurs because muscles use energy to contract.

There are three types of muscle tissue, cardiac, smooth, and skeletal. This chapter will concentrate on skeletal.

Each skeletal muscle is a complicated organ composed of skeletal muscle tissue, nerves, connective tissue, and blood.

Sometimes connective tissue holds muscle into broad, fibrous sheets called aponeuroses.

Each myofibril is surrounded by a system of channels called the sarcoplasmic reticulum. Transverse tubules (the T system) extends inward into the fiber connecting the channels with enlarged cisternae where the actin and myosin meet.

Each muscle fiber is connected to a motor neuron from the spinal cord at a neuromuscular junction. At this motor end plate, vesicles at the nerve ends store neurotransmitters that cause the muscle to contract when they are secreted.

Mechanics of the Muscular Contraction The actin and myosin filaments are made up of globular parts that are twisted together. The actin fibers contain troponin and tropomysin which are important in the contraction. When acetylcholine is released at the motor end plate, Ca ions are released into the T-system from the sarcolemma. In the presence of Ca, troponin and tropomysin open binding sites that allow the myosin to form cross bridges to the actin. Globular parts of the myosin release ATPase which causes the breakdown of ATP which gives the crossbridges the energy to curl and shorten the sarcomere. The muscle contracts When production of acetylcholine stops, the Ca is reabsorbed by the sarcolemma, the cross bridges disconnect, acetylcholinesterase decomposes the acetylcholine at the end plate and the muscle relaxes.

Fig. 8.07

Energy for the Muscle Contraction ATP must be supplied to cause the changes in the cross bridges. ATP→ADP + P + ENERGY The ATP supply is quickly used up and the energy to rebuild the ATP is supplied by creatine phosphate which muscles store. CP →C + P + ENERGY As the CP is used up, cellular respiration oxidizes glucose to supply energy. As oxygen runs low in the muscle, anaerobic respiration changes glucose to pyruvic acid for energy. The pyruvic acid is changed to lactic acid. The lactic acid is converted by to glucose in the liver at the expense of energy. An oxygen debt occurs and muscular fatigue and possibly cramps follow. About half of the energy produced in these processes is lost as heat.

Muscle fibers donot contract until a certain threshold stimulus is reached at the end plate. At this time, the fiber will contract to it’s full potential. This is called an “All-or-None” response.

Muscle contractions may be studied on a myogram as a muscle twitch.

Study of the twitch has led to the discovery of two kinds of muscle fibers: Slow twitch or fast twitch.

Sometimes a muscle may be restimulated before it has completed relaxation. This brings about a sustained contraction called a tetanic contraction.

A high intensity stimulation may cause a muscle fiber to stimulate fibers around it to contract. This is called recruitment.

Summation and recruitment together brings about a sustained contraction of increasing strength.

The body maintains a certain amount of sustained contraction at all times. This is called muscle tone and is very important in maintaining posture.

The long, tapered filaments of smooth muscle are not as well organized as those of skeletal muscle thus there are no striations.

Smooth muscle may be of two types: Multiunit -- The fibers are separate as in the iris of the eyes. Multiunit -- The fibers are separate as in the iris of the eyes. Visceral -- The fibers are organized into sheets. They may stimulate each other with great rhythmicity to generate peristalsis. Visceral -- The fibers are organized into sheets. They may stimulate each other with great rhythmicity to generate peristalsis.

Smooth muscle contracts slower than striated muscle but can maintain the contraction longer with a given amount of ATP.

Cardiac muscle is arranged into striated networks with intercalated disks which help to transfer stimuli from cell to cell. This makes the contractions rhythmic and self-exciting.

A number of terms are used to describe the action of muscles: origin - the attachment of the muscle that doesn't move origin - the attachment of the muscle that doesn't move insertion - the attachment of the muscle that moves in the action insertion - the attachment of the muscle that moves in the action flexion - bends a joint flexion - bends a joint extension - straightens a joint extension - straightens a joint prime mover or agonist - muscle that causes an action prime mover or agonist - muscle that causes an action synergist - muscle that aids the prime mover synergist - muscle that aids the prime mover antagonist - muscle that resists the prime mover and causes an opposite action. antagonist - muscle that resists the prime mover and causes an opposite action.

Clinical Terms Related to the Muscular System: convulsion - series of involuntary contractions of a group of muscles fibrillation - spontaneous, rapid, uncontrolled activity of a muscle fibrositis - inflammation of the muscle connective tissue muscular dystrophy - group of inherited disorders in which the lack of certain glycoproteins causes the collapse of muscle cells.

myalgia - any muscle pain myasthenia gravis - chronic disease of the neuromuscular junction where the muscles are weak and easily fatigued myoma - muscle tumor myositis - muscle inflammation paralysis - loss of the ability to move a body part shin splints - inflammation of the anterior muscles of the lower leg