Chapter 6

The Muscular

System

O.K. Everybody

flex!

How many muscles are there?

• 656 (more or less)

• Comprise about half your body weight

• 3 main functions…

3 major functions

1. Body movement

2. Body form and shape, posture

3. Body heat: maintain body temperature

1. Skeletal muscle, a.k.a. voluntary or striated muscle

2. Smooth muscle, a.k.a. involuntary

3. Cardiac muscle (found only in the heart) also Involuntary.

3 types of muscle

• Skeletal muscle, a.k.a. voluntary or striated muscle– Attached to the

skeleton, moves the limbs

– Contracts quickly, tires easily, can’t stay contracted for long periods of time.

– Multinuclear– May be long: ex

thigh 12-16”

• Smooth muscle, a.k.a. involuntary– Unattached to

bone. – Walls of the

internal organs esp digestive tract, but uterus and blood vessels as well.

– Single nucleus, long spindle shape.

– Act slowly, do not tire easily, can stay contracted for long periods of time

• Cardiac muscle- found only in the heart. – Involuntary. – Membranes fused

together at intercalated discs to form a continuous network.

– No independent cell contraction

• Muscles are hundreds to thousands of muscle cells, called fibers. Each fiber is in turn made up of myofibrils

• Muscle cells are long cylinders- each cell may have 100 or more nuclei located just underneath membrane

Striated

Muscle

4 characteristics of muscle1. Contractibility.

-No other body tissue has this quality-Muscles only contract or relax - they do not push

2. Excitability-Both muscle and nerve cells respond to stimuli by producing electrical signals

called action potentials

3. Extendibility- The ability to be stretched

4. Elasticity- The ability to return to the original length when relaxed

These four qualities; contractibility, excitability, extendibility and elasticity create a mechanical mechanism capable of complex, intricate movement

• Muscles need to be attached to something to exert a pulling force

• Muscles only pull, never push. Attached to bones by non elastic cords of connective tissue called tendons.

• Muscles bridge the joints of bones

• Muscles are attached at both ends; bones, tendons, skin or other muscles.

• Muscles arranged in antagonistic pairs- one works in one direction while a second muscle works in the opposite direction.

Biceps/Triceps and movement of arm

One is prime mover, the other is antagonist

• Muscles are made up of muscle fibers (muscle cells), which in turn are made up of myofibrils.

• Myofibrils have two main components– Actin (thin)– Myosin (thick)

• We will discuss later how muscles move (sliding filament theory)

A review of terminology

• When discussing muscles and what they move, you use the terms origin and insertion

• The point of origin moves the least. The insertion is attached to a moveable part. Ex- flex your arm. Origin of biceps is the humerus; it’s insertion is the radius

• Atrophy- shrink from disuse• Hypertrophy- increase in size.

(Note: an increase in the size of a muscle is caused by an increase in cytoplasm, not by an increase in muscle fibers)