Summary of the IB SEHS Unit 1.2 (the muscular system). You will find detailed yet summarized information on the topic, organized according to the IB syllabus (contains all necessary information to achieve a 7).
Muscular system: 600 muscles → 40-50% of body mass
Main functions of muscles:
they exert forces on tendons that pull on bones → movement
moving substances in the body (smooth miscle helps move food, cardiac muscle helps to move
blood)
stabilize and maintain posture
85% of body heat (thermoregulation)
Types of muscles
Feature Skeletal Cardiac Smooth
Movement Voluntary Involuntary Involuntary
Cells are bound together The structure is in between
into bundles by connective skeletal and smooth. Cells are small and not
Structure
tissue (fascia). Connected Branched fibers are not attached to bones
to the bone by a tendon attached to bone
Striated Yes (dark and light bands) Yes (dark and light bands) No
Cells/fibers have several Cells have a single
Nucleus Cells have a single nucleus
nuclei nucleus
In skeletal muscles (e.g In the lining of hollow
Location Heart
biceps, triceps…) organs (e.g. intestines)
Autonomic nervous
Nervous stimulation Somatic motor neurons Autonomic nervous system
system
Tension sensors Yes No No
Stretch receptors Yes No No
1.2: Muscular system 1
, Characteristics of muscle tissue
Contractility: the ability of a muscle to contract
(shorter)
Extensibility: the ability of a muscle to stretch
Elasticity: ability to return passively to its original
resting length
Muscles are arranged in pairs that act
antagonistically
Antagonist muscle: a muscle that relaxes
Example of contractility and extensibility of an antagonistic
Agonist: a muscle that contracts pair of muscles.
Atrophy: loss of muscle mass
Hypertrophy: gaining muscle mass
Controlled by nerve stimuli
Irrigation by capillaries
Structure of skeletal muscle
Fascia: connective tissue that surrounds and is in
between the muscle tissue. Helps to stabilize the
body and allows muscles to move freely
Fibrous connective tissue that is made up of
collagen fibers, adipose tissue, and fluid.
Reduces friction between muscles and supportive
structure.
Three layers of fascia: epimysium, perimysium, and endomysium.
epimysium: outer layer
perimysium: surrounds each bundle of muscle fibers
endomysium: surrounds each muscle fiber.
Each muscle fiber is composed of smaller units →
myofibrils (surrounded by SR)
Has striated appearance due to actin and myosin
(contractile proteins), which are arranged in
sarcomere (basic unit of the muscle cell)
Sarcomere: what is in between two Z-lines
Actin: thin myofilament that interacts with the
heads of myosin
Myosin: thick myofilament
1.2: Muscular system 2
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