Download 12: Structure and Function of Muscles

THE HUMAN BODY 1
Microscopic and macroscopic
structure of muscle
The levels of organisation of the human body
(from largest to smallest) are: organism, organ
system, organ, tissue, cell.
The muscular system is an organ system and
muscles are the organs of this system.
Muscles are mostly made up of muscle tissue,
which consists of cells called muscle fibres or
myocytes.
Muscles also consist of other tissues, like
connective tissue (for structural support), and
nervous tissue (for communication).
Muscle tissue itself is specialised
contraction and comes in three varieties:
for
Skeletal muscle: enables voluntary movement
of the limbs and appendages.
Cardiac muscle: enables the automatic
contraction of the heart or ‘heart beat’ that
pumps blood around the cardiovascular
system.
Smooth muscle: found in vessels and tracts,
enables involuntary movements such as
constriction of blood vessels and peristalsis
(a kind of movement) of the gut.
Skeletal muscle
The function of skeletal muscle is to produce
movement and heat, and to maintain posture.
Musculoskeletal
12: Structure and
Function of Muscles
In addition to muscle tissue, skeletal muscle
consists of connective tissue, which can be
divided into three layers.
•Epimysium: a layer of connective tissue that
wraps over and around the entire muscle
(the prefix ‘epi-’ originates from Greek,
meaning ‘over’ or ‘upon’).
•Perimysium: each muscle can be divided
into compartments called fascicles that
each consist of a bundle of muscle fibres.
Fascicles are surrounded by perimysium, a
layer of connective tissue that is made up
of epimysium that has extended into the
muscle itself (the prefix ‘peri-’ originates
from Greek, meaning ‘around’).
•Endomysium: a layer of connective tissue
that surrounds each individual myocyte
(the prefix ‘endo-’ originates from Greek,
meaning ‘within’).
All of the connective tissue layers are
connected to one another, and eventually form
a tendon that connects the skeletal muscle to
the skeleton.
Therefore, the layers of connective tissue
support the muscle and transmit the force of
contraction to the bones.
Myocytes, myofibrils
and myofilaments
Each skeletal muscle is comprised of fascicles,
and each fascicle is made up of muscle cells
called muscle fibres or myocytes.
Myocytes are long, skinny, cylindrical cells and
are multinucleated (have many nuclei).
They are made up of many myofibrils. Myofibrils
comprise the bulk of the myocyte, which is
why the nuclei of myocytes are pushed to the
edges of the cell.
Each myofibril is made up of myofilaments.
Myofilaments come in two varieties: thick
filaments of myosin and thin filaments of actin.
Endomysium,
around each individual
myocyte
Epimysium,
around entire
muscle
Perimysium,
around each
fascicle
© aptitute
Figure 2.10
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THE HUMAN BODY 1
Synovial joints
SYNOVIAL JOINT SHAPE TYPES OF MOVEMENT
Hinge
Uniaxial
Flexion & extension
Pivot
Uniaxial
Rotation (supination &
pronation)
Saddle
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Ellipsoid
(Oval)
Plane
Biaxial
Flexion & extension
Abduction & adduction
(Therefore circumduction)
Biaxial
Flexion & extension
Abduction & adduction
(Therefore circumduction)
Multi-axial*
Sliding & gliding
Ball & Socket
Multiaxial
Spherical head of a bone Flexion & extension
fits into a socket
Abduction & adduction
(Therefore circumduction)
Rotation
Condylar**
Uniaxial (Plus)
Flexion & extension
Limited rotation
EXAMPLE(S)
Ankle joint, elbow joint (humerus
and ulna), interphalangeal joints
Radioulnar joints (both proximal
and distal), atlanto-axial joint
(C1-C2) at the top of the
vertebrae (used in head shake
motion)
Carpometacarpal joint at base of
the thumb
Wrist (radiocarpal) joint,
metacarpophalangeal joints,
metatarsophalangeal joints
Intercarpal & intertarsal joints
(between carpal and tarsal
bones)
Hip joint (socket is deep and fully
engulfs the ball, there is great
bony articulation)
Shoulder joint (socket is shallow
and less bony articulation)
Knee joint, temporomandibular
joint (jaw joint)
*A plane joint is multi-axial because it slides in any direction around no particular axis.
**Two condyles (rounded projections found at the end of a bone) articulate at a condylar joint.
Myocyte, or single muscle cell
Sarcomeres are arranged end on end along
the myofibril, giving the myocyte a stripy
appearance. We therefore say that it is
striated.
The boundaries of the sarcomere are marked
by Z-lines. These are placed at regular intervals
along the myofibril, and anchor the thin actin
filaments together.
which has many myofibrils
These are in turn composed of many
myofilaments of actin and myosin.
At this level we see sarcomeres
Figure 2.11
The myosin and actin proteins are arranged
into contractile units called sarcomeres and
work together to produce muscle contraction.
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The thick myosin filament lies in the middle of
the sarcomere.
Z-lines marking the sarcomere boundary
Myosin (thick filament)
Actin (thin filament)
Figure 2.12
During muscle contraction, the thick myosin
filaments pull on the thin actin filaments,
causing the thin filaments to slide over the
thick. This draws the Z-lines closer together
and shortens the sarcomere (the filaments
themselves do not change in length).
© aptitute
THE HUMAN BODY 1
Single (Parallel)
Unipennate
Bipennate
Multipennate
The process of muscle contraction consumes
energy.
•If they run at two angles they are bipennate.
•If they run at multiple angles they are
multipennate.
Muscle form determines function
Muscle tone and posture
The length of the muscle fibres determines the
range of movement of the muscle.
Muscle provides posture through muscle tone,
which keeps us upright against the constant
force of gravity.
A skeletal muscle can shorten up to 50% of its
original length.
Therefore, the longer the resting length of a
muscle fibre, the greater the distance it can
shorten and the larger its range of movement
(ROM).
The number of muscle fibres (i.e. the cross
sectional area) determines the strength of
contraction of the muscle.
The force with which a muscle contracts is
directly proportional to the cross sectional
area of the muscle.
The more muscle fibres, the greater the cross
sectional area, and therefore the greater the
strength of contraction.
The arrangement of muscle fibres affects both
the range of movement and the strength of
contraction of a muscle.
Musculoskeletal
Figure 2.13
Even when our muscles are relaxed they
receive electrical impulses, and are slightly
active.
These electrical impulses are not sufficient to
produce full muscle contraction, and therefore
will not produce movement. However, they are
enough to keep our muscles firm, and to assist
in stabilising our joints (we are therefore able
to maintain posture).
This activity of relaxed muscles is known as
muscle tone.
As well as helping us to maintain posture,
muscle tone keeps our muscles healthy and
ready for action.
Muscle tone is lost while we are sleeping,
which is why we cannot sleep standing up!
If fibres are arranged parallel to the bone to
which they are attached, their length will be
maximized. Parallel arrangement of muscle
fibres therefore leads to an increased range
of movement, but reduced strength (as cross
sectional area is relatively low).
If fibres are arranged at an angle, more can fit
into the same area. This leads to greater cross
sectional area, and therefore greater strength
of contraction.
Angling of fibres is known as pennation:
•If fibres all run at one angle to the line of
pull (the line that the tendon pulls along),
they are said to be unipennate.
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