# Anatomy and Physiology of Animals/Muscles

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## Objectives

After completing this section, you should know:

-   The structure of smooth, cardiac and skeletal muscle and where they
    are found.
-   What the insertion and origin of a muscle is.
-   What flexion and extension of a muscle means.
-   That muscles usually operate as antagonistic pairs.
-   What tendons attach muscles to bones.

## Muscles

Muscles make up the bulk of an animal's body and account for about half
its weight. The meat on the chop or roast is muscle and is composed
mainly of protein. The cells that make up muscle tissue are elongated
and able to contract to a half or even a third of their length when at
rest. There are three different kinds of muscle based on appearance and
function: smooth, cardiac and skeletal muscle.

### Types of Muscle

-   Smooth muscle

Smooth or Involuntary muscle carries out the unconscious routine tasks
of the body such as moving food down the digestive system, keeping the
eyes in focus and adjusting the diameter of blood vessels. The
individual cells are spindle-shaped, being fatter in the middle and
tapering off towards the ends with a nucleus in the centre of the cell.
They are usually found in sheets and are stimulated by the non-conscious
or autonomic nervous system as well as by hormones (see Chapter 3).

-   Cardiac muscle

Cardiac muscle is only found in the wall of the heart. It is composed of
branching fibres that form a three-dimensional network. When examined
under the microscope, a central nucleus and faint stripes or striations
can be seen in the cells. Cardiac muscle cells contract spontaneously
and rhythmically without outside stimulation, but the sinoatrial node
(natural pacemaker) coordinates the heart beat. Nerves and hormones
modify this rhythm (see Chapter 3).

-   Skeletal muscle

Skeletal muscle is the muscle that is attached to and moves the
skeleton, and is under voluntary control. It is composed of elongated
cells or fibres lying parallel to each other. Each cell is unusual in
that it has several nuclei and when examined under the microscope
appears striped or striated. This appearance gives the muscle its names
of striped or striated muscle. Each cell of striated muscle contains
hundreds, or even thousands, of microscopic fibres each one with its own
striped appearance. The stripes are formed by two different sorts of
protein that slide over each other making the cell contract (see diagram
7.1).

![](Anatomy_and_physiology_of_animals_Striped_muscle_cell.jpg "Anatomy_and_physiology_of_animals_Striped_muscle_cell.jpg")

Diagram 7.1 - A striped muscle cell

### Muscle contraction

Muscle contraction requires energy and muscle cells have numerous
mitochondria. However, only about 15% of the energy released by the
mitochondria is used to fuel muscle contraction. The rest is released as
heat. This is why exercise increases body temperature and makes animals
sweat or pant to rid themselves of this heat as part of
thermoregulation.

What we refer to as a muscle is made up of groups of muscle fibers
surrounded by connective tissue. The connective tissue sheaths join
together at the ends of the muscle to form tough white bands of fiber
called **tendons**. These attach the muscles to the bones. Tendons are
similar in structure to the **ligaments** that attach bones together
across a joint (see diagrams 7.2a and b).

![](_Anatomy_and_physiology_of_animals_Structure_of_a_muscle.jpg "_Anatomy_and_physiology_of_animals_Structure_of_a_muscle.jpg")

Diagram 7.2 a and b - The structure of a muscle

Remember:

**Tendons Tie** muscles to bones

:   

    :   and

**Ligaments Link** bones at joints

### Structure of a muscle

A single muscle is fat in the middle and tapers towards the ends. The
middle part, which gets fatter when the muscle contracts, is called the
**belly** of the muscle. If you contract your biceps muscle in your
upper arm you may feel it getting fatter in the middle. You may also
notice that the biceps is attached at its top end to bones in your
shoulder while at the bottom it is attached to bones in your lower arm.
Notice that the bones at only one end move when you contract the biceps.
This end of the muscle is called the **insertion**. The other end of the
muscle, the **origin**, is attached to the bone that moves the least
(see diagram 7.3).

![](Anatomy_and_physiology_of_animals_Antagonistic_muscles,_flexion&tension.jpg "Anatomy_and_physiology_of_animals_Antagonistic_muscles,_flexion&tension.jpg")

Diagram 7.3 - Antagonistic muscles, flexion and extension

### Antagonistic muscles

Skeletal muscles usually work in pairs. When one contracts the other
relaxes and vice versa. Pairs of muscles that work like this are called
**antagonistic muscles**. For example the muscles in the upper forearm
are the biceps and triceps (see diagram 7.3). Together they bend the
elbow. When the biceps contracts (and the triceps relaxes) the lower
forearm is raised and the angle of the joint is reduced. This kind of
movement is called **flexion**. When the triceps is contracted (and the
biceps relaxes), the angle of the elbow increases. The term for this
movement is **extension**.

When you or animals contract skeletal muscle it is a voluntary action.
For example, you make a conscious decision to walk across the room,
raise the spoon to your mouth, or smile. There is however, another way
in which contraction of muscles attached to the skeleton happens that is
not under voluntary control. This is during a **reflex action**, such as
jerking your hand away from the hot stove you have touched by accident.
This is called a **reflex arc** and will be described in detail in
chapter 14-15.

## Summary

-   There are three different kinds of muscle tissue: **smooth muscle**
    in the walls of the gut and blood

:   vessels; **cardiac muscle** in the heart and **skeletal muscle**
    attached to the skeleton.

-   **Tendons** attach skeletal muscles to the skeleton.
-   **Ligaments** link bones together at a joint.
-   Skeletal muscles work in pairs known as **antagonistic pairs.** As
    one contracts the other in the

:   pair relaxes.

-   **Flexion** is the movement that reduces the angle of a joint.
    **Extension** increases the angle

:   of a joint.

## Test Yourself

1\. What kind of muscle tissue:

:   a\) moves bones
:   b\) makes the heart pump blood:
:   c\) pushes food along the intestine:
:   d\) makes your mouth form a smile:
:   e\) makes the hair stand up when cold:
:   f\) makes the diaphragm contract for breathing in:

2\. What structure connects a muscle to a bone?

3\. What is the insertion of a muscle?

4\. Which muscle is antagonistic to the biceps?

5\. Name 3 other antagonistic pairs and tell what they do.

6\. When you bend your knee what movement are you making?

7\. When you straighten your ankle joint what movement happens?

8\. What organelles provide the energy that muscles need?

9\. State the difference between a tendon and a ligament.

10.In the section \"Skeletal Muscle\" there are 2 proteins mentioned.
Name these proteins, state their size difference, and tell what they
actually do to help produce movement.

## Website

-   <http://health.howstuffworks.com/muscle.htm> How muscles work

Description of the three types of muscles and how skeletal muscles work.

## Glossary

-   Link to
    Glossary

