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 Smooth muscle may be of two types: two types:

Multiunit -- The fibers are separate Multiunit -- The fibers are separate as in the iris of the eyes. as in the iris of the eyes.

Visceral -- The fibers are organized Visceral -- The fibers are organized into sheets. They may stimulate each into sheets. They may stimulate each other with great rhythmicity to other with great rhythmicity to generate peristalsis. 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 A number of terms are used to describe the used to describe the action of muscles: action of muscles: origin - the attachment of the muscle that origin - the attachment of the muscle that

doesn't move doesn't move insertion - the attachment of the muscle that insertion - the attachment of the muscle that

moves in the action 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 prime mover or agonist - muscle that causes

an action an action synergist - muscle that aids the prime mover synergist - muscle that aids the prime mover antagonist - muscle that resists the prime antagonist - muscle that resists the prime

mover and causes an opposite action.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