Chapter 8
The Muscular System
Structure of a Skeletal Muscle
 Each muscle is an organ comprised of:
 Skeletal muscle tissue
 Connective tissue
 Ex: Tendons and Ligaments
 Nervous Tissue
 Blood
Microscopic Anatomy of
Skeletal Muscle Structure
 Sarcolemma: specialized plasma membrane
 Myofibrils: long organelles inside muscle cell
 Sarcoplasmic reticulum: specialized smooth
endoplasmic reticulum
 Sarcomere: contractile unit of a muscle fiber,
contained within two Z discs
Microscopic Anatomy of
Skeletal Muscle
Connective Tissue Coverings
 Fascia
 Layers of dense connective tissue that surround and
separate each muscle
 Extends beyond the ends of the muscle and give rise
to tendons that are fused to the periosteum of bones.
 Aponeuroses
 Tendons that are broad sheets of connective tissue
Fascia and Aponeuroses
Connective Tissue Coverings
 Epimysium
 Layer of connective tissue around each whole
muscle
 Perimysium
 Surrounds individual muscle bundles (fascicles)
within each muscle
 Endomysium
 Connective tissue layer that covers each muscle cell
Connective Tissue Coverings
Skeletal Muscle Fibers
 Each muscle fiber is a single, long, cylindrical
muscle cell.
 Sarcolemma
 The cell membrane of a muscle fiber
 Sarcoplasm
 The cytoplasm of the muscle fiber that contains many
mitochondria and nuclei
 Also contains myofibrils
 Myofibrils are contractile fibers within muscle cells
 Thick filaments of myofibrils are made up of the protein
myosin
 Thin filaments of myofibrils are made up of the protein
actin
 The organization of these filaments produces striations.
Skeletal Muscle Fibers
Skeletal Muscle Fibers
 Sarcomere
 The structural and functional unit of a myofibril
 Extends from Z line to Z line
 I bands (light bands) are made up of actin filaments and
are anchored to Z lines
 A bands (dark bands) are made up of overlapping thick
(myosin) and thin (actin) filaments.
 In the center of A bands is an H zone, consisting of
myosin filaments only.
Sarcomere Organization
 Myofilaments
 Thick filaments = myosin filaments
 Have heads (extensions, or cross bridges)
 Thin filaments = actin filaments
 Anchored to the Z disc (BOUNDARY OF SARCOMERE)
 H zone: zone with no actin filaments when muscle
is at rest
Sarcomere
Parts of the sarcomere video
 https://www.youtube.com/watch?v=RG_pecbIZKs
Neuromuscular Junction
 Structure: Location where the end of the motor
neuron and the muscle meet
 Function: Initiation point of muscle contraction
 Nerve releases acetylcholine
 Neurotransmitter: chemical released by nerve upon
arrival of nerve impulse
Motor Neuron
 A motor neuron is a fiber from a nerve cell that
connects to a skeletal muscle fiber
 The muscle fiber membrane forms a motor end
plate in which the sarcolemma is tightly folded
and where nuclei and mitochondria are
abundant.
 The cytoplasm of the motor neuron contains
numerous mitochondria and synaptic vesicles
storing neurotransmitters
The nerve stimulus and action
potential
The nerve stimulus and action
potential
 Skeletal muscles must be stimulated by motor
neuron (nerve cell) to contract
Definitions
 Motor unit – one motor neuron and all the skeletal
muscle cells stimulated by that neuron
 Synaptic cleft – gap between nerve and muscle
 Nerve and muscle do not make contact
 Area between nerve and muscle is filled with
interstitial fluid
The nerve stimulus and action
potential
Transmission of Nerve
Impulse and Contraction
1. Acetylcholine (ACh) (neurotransmitter) is released
by nerve into synaptic cleft
2. ACh attaches to receptors on the sarcolemma
3. Sarcolemma (muscle cell membrane) becomes
permeable to sodium ions (Na+)
4. Sodium rushes into the cell generating an “action
potential”
 Once started, muscle contraction cannot be stopped
Transmission of Nerve
Impulse and Contraction
Transmission of Nerve
Impulse and Contraction
Neuromuscular Junction
Steps of muscle contraction
video
 https://www.youtube.com/watch?v=e3Nq-P1ww5E
Skeletal Muscle Contraction
 Muscle contractions involve several components that
result in the shortening of sarcomeres and the
pulling of the muscle against its attachments
 Roles of Myosin and Actin
 Myosin consists of two twisted strands with globular
cross-bridges projected outward along the strand.
 Actin is a globular protein with myosin binding sites.
 Tropomyosin and Troponin are two proteins associated
with the surface of the actin filaments
Skeletal Muscle Contraction
Skeletal Muscle Contraction
Sliding Filament Theory
 The myosin cross-bridge attaches to the binding site
on the actin filament and bends
 This pulls the actin filament
 It then releases and attaches to the next binding
site on the actin, pulling again.
 Energy from the conversion of ATP to ADP is
provided to the cross-bridges causing them to be
in a “cocked” position.
Sliding Filament Theory
Sliding filament video
 https://www.youtube.com/watch?v=Ct8AbZn_A8A
Stimulus for Contraction
 Motor neuron releases neurotransmitter
acetylcholine into the synaptic cleft – initiates a
muscle contraction
 Protein receptors in the motor end plate detect the
neurotransmitters – a muscle impulse spreads over
the surface of the sarcolemma and into the TTubules – impulse reaches the sarcoplasmic
reticulum
 Sarcoplasmic Reticulum releases its stored calcium
into the sarcoplasm of the muscle fiber
 High concentration of calcium in the sarcoplasm
interacts with the troponin/tropomyosin molecules
which move aside – exposes the myosin binding
site on the actin filaments
Contraction
 Myosin cross-bridges now bind and pull on the actin
filaments – causes the sarcomeres to shorten
 After the impulse has been received,
acetylcholinesterase rapidly decomposes the
acetylcholine.
 Calcium is returned to the sarcoplasmic reticulum –
linkages between myosin and actin are broken
Energy Sources for Contraction
 Energy for contractions comes from molecules of ATP.
 This chemical is in limited supply and so must often be
regenerated
 Creatine phosphate, which stores excess energy
released by the mitochondria, is present to regenerate
ATP from ADP and phosphate.
 Whenever the supply of ATP is sufficient, creatine
phosphokinase promotes the synthesis of creatine
phosphate
 As ATP decomposes, the energy from creatine phosphate
can be transferred to ADP molecules, converting them
back to ATP.
Oxygen Supply and Cellular
Respiration
 The early phase of cellular respiration yields few
molecules of ATP, so muscle has a high
requirement for oxygen, which enables the
complete breakdown of glucose in the
mitochondria
 Hemoglobin in red blood cells carries oxygen to
muscle.
 Myoglobin is a pigment that stores oxygen in a
muscle tissue.
Oxygen Debt
 During rest or moderate activity, there is enough oxygen
to support aerobic respiration (breathing)
 Oxygen deficiency may develop during strenuous
exercise, and lactic acid accumulates as an end product
of anaerobic respiration.
 Lactic acid diffuses out of muscle cells and is carried in
the bloodstream to the liver.
 Oxygen debt refers to the amount of oxygen that liver
cells require to convert the accumulated lactic acid into
glucose, plus the amount that muscle cells need to
resynthesize ATP and creatine phosphate to their
original concentrations.
 Repaying oxygen debt may take several hours. After
exercise, the oxygen debt is repaid by deep breathing
Muscle Fatigue
 When a muscle loses its ability to contract during
strenuous exercise.
 Usually arises from the accumulation of lactic acid
in the muscle
 A lowered pH as a result of accumulated lactic acid
prevents the muscle from contracting
 A muscle cramp occurs due to a lack of ATP
required to return calcium ions back to the
sarcoplasmic reticulum so muscle fibers can relax.
Effect of Exercise on Muscles
 Exercise increases muscle size, strength, and
endurance
 Aerobic (endurance) exercise (biking, jogging) results
in stronger, more flexible muscles with greater
resistance to fatigue
 Makes body metabolism more efficient
 Improves digestion, coordination
 Resistance (isometric) exercise (weight lifting)
increases muscle size and strength
Heat Production
 Contraction of skeletal muscle represents an
important source of heat for the body
Muscular Responses
 Threshold Stimulus
 A muscle remains unresponsive to stimulation unless the
stimulus is of a certain strength
 All-or-None Response
 When a muscle fiber contracts, it contracts to its full
extent.
 It cannot contract partially
 Recording a Muscular Contractions
 Myogram
 The recording of an electrically –stimulated muscle
contraction.
 Twitch
 A single, short contraction involving only a few motor units
 Latent Period
 The time delay between when the stimulus is applied and
when the muscle contracts.
 Is Less than 0.01 seconds
Myogram
Summation
 When a muscle fiber receives a series of stimuli of
increasing frequency and reaches a point when it is
unable to relax completely and the force of
individual twitches combine
 Tetanic Contraction
 If the sustained contraction does not get any relaxation.
 Recruitment of Motor Units
 An increase in the number of activated motor units
within a muscle at higher intensities of stimulation
 Summation and recruitment together can produce a
sustained contraction of increasing strength.
Summation Contractions
Muscle Tone
 Muscle tone is achieved by a continuous state of
sustained contraction of motor units within a
muscle
 It is a result of a staggered series of nerve impulses
delivered to different cells within the muscle
 Muscle tone keeps muscles healthy and ready to
react
 If the nerve supply is destroyed, the muscle loses
tone, becomes paralyzed, and atrophies.
Naming Skeletal Muscles
 Based on several criteria:
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Direction of muscle fibers
Relative size of the muscle
Location of the muscle
Number of origins
Location of the muscle’s origin and insertion
Shape of the muscle
Action of the muscle
Naming Skeletal Muscles
 Direction of Muscle Fibers
 Rectus: straight
 Oblique: slanted
 Relative size of muscle
 Maximus: largest
 Minimus: smallest
 Longus: long
Naming Skeletal Muscles
 Location of the muscle
 Temporalsis muscle overlies the temporal bone
 Frontalis muscle overlies the frontal bone
 Number of origins
 Biceps: two origins
 Triceps: three origins
 Quadriceps: four origins
Naming Skeletal Muscles
 Location of the muscle’s origin and insertion
 Named for their attachment sites
 Sternocleidomastoid muscle: origin on the sternum (sterno),
the clavicle (cleido) and the inserts on the mastoid process
(mastoid) of the temporal bone
 Shape of the muscle
 Deltoid muscle is triangular and deltoid means triangular
 Action of the muscle
 Extensor: extensor muscles of the wrist extend the wrist
 Adductor: adductor muscles of the thigh all bring about its
adduction
Facial Muscles
 Frontalis – raises eyebrows
 Orbicularis oculi – closes eyes, squints, blinks,
winks
 Orbicularis oris – closes mouth and protrudes the
lips
 Buccinator – flattens the cheek, chews
 Zygomaticus – raises corners of the mouth
Chewing Muscles
 Masseter – closes the jaw and elevates mandible
 Temporalis – synergist of the masseter, closes jaw
Neck Muscles
 Platysma – pulls the corners of the mouth inferiorly
 Sternocleidomastoid – flexes the neck, rotates the
head
Trunk Muscles
 Pectoralis major: adducts and flexes humerus
 Intercostal muscle: deep muscles found between the
ribs
 Rectus Abdominis: flexes vertebral column
 External Oblique: flexes and rotates vertebral
column
Posterior Muscles
 Trapezius: extends neck and adducts scapula
 Latissimus dorsi: extends and adducts humerus
 Deltoid: abducts humerus
Muscles that Move the Upper
Limb
 Pectoralis major: adducts and flexes humerus
 Lattisiumus Dorsi: extends and adducts humerus
 Deltoid: abducts humerus
Muscles that Act on the
Forearm
 Biceps Brachii: flexes elbow and supinates forearm
 Triceps Brachii: extends elbow
Muscles of the Lower Limb
 Muscles causing movement at the Hip Joint
 Gluteus Maximus: extends hip
 Gluteus Minimus: abducts thigh
 Muscles causing movement at the Knee Joint
 Sartorius: flexes thigh on hip
 Quadricep Group
 Hamstring Group
 Muscles causing movement at the ankle and foot
 Tibialis Anterior: dorsiflexes and inverts foot
 Peroneus longus: plantar flex and evert foot
 Gastrocnemius: plantar flexes foot and flexes knee
Disorders relating to the
Muscular System
 Muscular Dystrophy
 Inherited, muscle enlargement due to increased fat
and connective tissue, but fibers degenerate and
atrophy
 Duchenne MD
 Lacking a protein to maintain the sarcolemma
 Myasthemia Gravis
 Progressive weakness due to a shortage of
acetylcholine receptors