Download Types of muscle

Types of Muscle
There are three types of muscle:
 voluntary or skeletal
 involuntary or smooth muscle
 cardiac muscle
Their names variously suggest where they are found and the type of control we have over them. All of
the muscle types contain contractile fibres made from actin and myosin, but there are structural and
functional differences between them.
Voluntary or skeletal muscle
Voluntary or skeletal muscle is also known as striated or striped muscle because of its physical
appearance – when looked at under the microscope, it has a striped appearance. Contraction of
voluntary muscle is under conscious control, and moves the bones of the skeleton across joints. Skeletal
muscle consists of fibres surrounded by a membrane or sarcolemma, within which there are several
nuclei, many mitochondria and an extensive sarcoplasmic reticulum. The mitochondria provide the ATP
energy for muscle contraction and the sarcoplasmic reticulum stores Ca2+ ions, essential for contraction
of the muscle myofibrils divided into contractile units (sarcomeres). It is the regular arrangement of the
actin and myosin filaments in the sarcomeres that give voluntary muscle its striped appearance.
Contraction of voluntary muscle is rapid and forceful, though prone to fatigue.
muscle, comprised
of bundles of many
fibres
nucleus
I-Band
tendon, attaching
muscle to bone
part of muscle fibre
A-Band
myofibril
Z-line
Z-line
sarcomere
Involuntary or smooth muscle
Contraction of smooth muscle is controlled by the autonomic nervous system and therefore is not under
voluntary control. It is found in the walls of blood vessels, the gut and the urino-genital system. The
cells are spindle shaped, have a single nucleus and are not arranged into fibres, so are not striated.
Contraction is relatively slow and the muscle does not tire or fatigue. The diagram and light micrograph
show smooth muscle in a transverse section of a trachea.
Nucleus
Myofibril
Spindle-shaped cell
Cardiac muscle
As the name implies, this is found in the heart. It has some structural similarity to striped muscle –
striations are visible in the muscle cells. However adjacent cardiac muscle fibres interconnect, their
plasma membranes make very close contact and are freely permeable, so they allow electrical excitation
(action potentials) to pass easily between cardiac muscle cells. Like smooth muscle, cardiac muscle is
myogenic and will contract rhythmically without nervous stimulation. The net-like arrangement of
cardiac muscle, close interconnections and the high conductivity of the muscle itself mean that
contraction can spread quickly over the heart. Cardiac muscle contracts forcefully, but does not fatigue.
The structure of cardiac muscle is shown in the diagram and micrograph:
Myofibril
Intercalated disc
Striations
Bridge
Nucleus
Space between fibres for blood
vessels and connective tissue
Note: The cells that make up the SAN, AVN and Purkyne tissue are modified cardiac muscle cells. They
do contract, albeit weakly. However, they can generate and conduct impulses that control normal
heartbeat. That said, the output of the heart is modified by the autonomic nervous system to suit the
body’s requirements.
Complete the following summary to compare voluntary, involuntary and cardiac muscle:
Voluntary
Muscle type
Involuntary
Cardiac
Where found
Characteristics of
contraction
(control, speed,
force, if prone to
fatigue)
Structure
Atrial and ventricular
muscle
SAN
Function
AVN
Purkyne tissue