Types of Muscle Tissue
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Smooth muscle: Found in hollow organs,
involuntary.
Cardiac muscle: Striated, involuntary,
found in the heart.
Skeletal muscle: Striated, voluntary,
attached to bones.
Structure of Skeletal Muscles
1. Myofibril – parallel threads that are
squished together to form a muscle.
a. Divided lengthwise into segments
called sarcomeres that contains two
strands of protein or myofilaments.
 Actin – thin strands
 Myosin – thick strands,
shaped like a golf club head.
b. Z-lines mark the ends; contraction
brings them closer and where actin is
attached.
c. M-Line – accessory proteins that
holds the myosin.
2. Muscle Fiber – actual muscle cells with
mitochondria, multiple nuclei, and a cellular
membrane called sarcolemma.
a. Sarcolemma - the protective outer
shell of a skeletal muscle cell
- Sarcoplasm muscle cell’s
cytoplasm inside the
sarcolemma.
b. Surrounded and reinforced every
muscle fiber by the endomysium
(within the muscle).
3. Fascicles – group of muscle fibers that form
larger, string-like bundles
a. Surrounded by a layer of fibrous
connective tissue called perimysium
(around the muscle).
4. Muscle Organ - combined fascicles.
a. Wrapped in a layer of dense
connective tissue called epimysium
(upon the muscle).
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Built like a rope: myofibrils → muscle
fibers → fascicles → muscle organ.
Each muscle is an organ with connective
tissue, blood vessels, and nerves.
Special Functional properties of
skeletal muscles
1. Irritability (responsiveness) ability to receive and respond to a
stimulus.
2. Contractility - ability to forcibly
shorten when an adequate stimulus is
received.
3. Extensibility - ability of muscle
cells to be stretched.
4. Elasticity - ability to recoil and
resume resting length after
stretching.
Skeletal muscles must be stimulated by a motor
neuron (nerve cell) to contract.
Protein Rules
1. Proteins change shape when stuff binds to
them.
2. Shape changes allow proteins to bind/unbind
with other things.
Sarcomeres: Actin & Myosin
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Sarcomeres contain thin actin and thick
myosin filaments.
Z-lines mark the ends; contraction brings
them closer.
Sliding Filament Model
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At rest, actin and myosin don’t touch—but
they want to.
Tropomyosin and troponin block binding
sites or the actin from the myosin.
Calcium and ATP remove the blockers.
Myosin binds to actin, pulls it, and contracts
the muscle.
ATP resets myosin for another cycle.
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Sarcoplasmic Reticulum – its walls are
loaded with calcium pumps which uses a
bunch if ATP to save up a bunch of calcium
ions.
- Studded with calcium channels that
are linked to voltage-sensitive
proteins in the muscle membrane.
Muscle Attachments
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Intrinsic and extrinsic muscles
Nervous System Role
Action potential travels from brain to
muscle.
 Triggers calcium release from sarcoplasmic
reticulum.
 Calcium binds to troponin → tropomyosin
moves → myosin binds to actin.
 Muscle movement = contraction +
relaxation.
 Powered by actin, myosin, calcium, and
ATP.
 Sarcomeres shorten during contraction.
In Action:
1. Your brain sends an electrical impulse.
2. That impulse travels through the
sarcolemma.
3. T-tubules carry it to the inner muscle fiber.
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4. This triggers calcium release → actin &
myosin interact → muscle contracts.
Functional groups of muscles
1. Prime Mover or Agonist – muscle whose
contraction is responsible for a particular
movement. Main muscle doing the work.
2. Antagonists – working in reverse of that
particular movement by staying relaxed, or
stretching, or contracting just enough to
keep those prime movers from overextending
3. Synergists – muscles that assist the
movement of the prime mover or agonists,
lending a little extra oomph stabilizing joints
against dislocation.
4. Fixators – stabilize the origin of the prime
mover so it can move more efficiently.
Origin – point of attachment where least
movement occurs.
Insertion – point of attachment with
greatest movement.
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Muscles are at times grouped as to the
regions where they are found and function.
Intrinsic muscles are localized to a specific
are (i.e hand, tongue, back). They function
and are contained only within that area.
Extrinsic muscles extend into other areas
and are responsible for movements of areas
other than where they are attached.
Action Potential
electrical signal that triggers muscle
contraction. It’s a rapid change in
membrane voltage that travels along
the muscle fiber, initiating the steps
that lead to contraction.
Neuromuscular Junction – association site of axon
terminal of the motor neuron and sarcolemma of a
muscle.
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Neurotransmitter – chemical released by a nerve
upon arrival of nerve impulse in the axon terminal.
- Acetylcholine (Ach) is the
neurotransmitter that stimulates
skeletal muscle.
Synaptic Cleft – gap between nerve and the muscle
filled with interstitial fluid.
- Although very close, the nerve and
muscle do not make contact.
Muscle Contraction Sequence (Action Potential)
When a nerve impulse reaches the axon terminal
of the motor neuron:
1. Calcium channels open, allowing calcium
ions to enter the axon terminal.
2. Calcium entry causes synaptic vesicles to
release acetylcholine (ACh).
3. ACh diffuses across the synaptic cleft and
binds to receptors on the sarcolemma.
4. If enough Ach is released, the sarcolemma
becomes temporarily permeable to Na⁺ ions.
o K⁺ ions diffuse out.
o More Na⁺ enters than K⁺ leaves →
depolarization occurs.
5. Depolarization opens more Na⁺ channels →
action potential is created and travels
across the muscle cell.
6. Acetylcholinesterase (AChE) breaks down
ACh into acetic acid and choline, ending
the contraction.
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Cell Returns to Resting State When:
 Potassium ions diffuse out.
 Na⁺/K⁺ pump restores ion positions.
What Causes Filaments to Slide?
 Calcium ions bind to regulatory proteins on
thin filaments.
 This exposes myosin-binding sites →
myosin heads attach.
 Each cross-bridge pivots, sliding thin
filaments inward.
 Contraction occurs and muscle fiber
shortens.
 Cross bridges attach and detach
repeatedly during contraction.
Neuromuscular Control
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Skeletal muscle contraction is controlled by
a nerve impulse (action potential)
transmitted by the motor nerve from the
brain or spinal cord.
Motor Unit consists of all the muscle fibers
controlled by a single motor neuron.
Fine control muscles (i.e eyelid muscles)
have fewer muscle fibers/nerve (2:1).
A contraction is initiated by an action
potential (nerve impulse) and followed by
the release a chemical neurotransmitter at
the neuromuscular junction (NMJ).
Neurotransmitter for skeletal muscle is
acetylcholine.
Each muscle fiber is innervated by a single
motor neuron.
Contractions may be graded or full due to
the number of muscle fibers that respond to
the stimulus. The more fibers, the greater the
muscle contraction.
Synapse – functional connection between a
nerve fiber and its target cell.
Neuromuscular Junction – synapse
between a motor nerve and a muscle fiber.
Every muscle contraction is preceded by a
nerve impulse from the CNS.
Muscle Twitch
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Single, brief, jerky contraction
Not a normal muscle function
Twitching happens in individual motor units
When stimulations become more frequent,
muscle contractions get stronger and
smoother
Tetanus – when all those little twitches blend until
they feel like one gigantic contraction, where any
person will hit a ceiling maximum tension.
 Tension – when myosin and the calcium
pumps are burning up the muscle cells’ ATP
and the finite supply of the ATP is what
makes it impossible to maintain vigorous
muscle activity indefinitely.
 Muscle Fatigue – prolonged contraction of
muscle and all the tension crashes to zero.
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Unfused (incomplete) tetanus - where the
muscle fibers are stimulated repeatedly, but
not so rapidly that they completely fuse
together.
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Fused (complete) tetanus -when the muscle
is stimulated so rapidly that no evidence of
relaxation is seen. Most intense and
sustained form of muscle contraction.
Relaxation Period of a twitch is when all the
calcium is being pumped back into the
sarcoplasmic reticulum.
Frequency is the only way to create a grade of
force.
Grade of Force - refers to how strongly a
muscle can contract.
- The strength of the stimulus can be
increased by sending action
potentials to more motor units.
Recruitment (Motor Unit Summation) – when
contractions intensify as your motor neurons
stimulate more muscle fibers.
- Example: Picking up a light grocery
bag, your brain activates small motor
units first which control fewer
muscle fibers and generate low force.
If the bag is heavier than expected,
your body recruits more motor units
to suffice the needed force to lift a
grocery.
- The recruitment follows the size
principle.
The Size Principle of Recruitment
- When the smallest motor units with
the smallest fibers are activated by
your most excited neurons.
- Then some larger motor units with
larger fibers are enlisted, increasing
the strength of contraction
Muscles of the Axial Skeleton
- Muscles of the axial skeleton are
essential for respiration, speech,
facial expression, posture, and
chewing.
Muscle of the Appendicular Skeleton
- Includes the muscles of the shoulder,
abdominopelvic cavity, upper and
lower extremities.
- Muscles of the upper extremities are
classified according to the bones they
move.
Tendons – connect muscle to bone, made of
collagen fibers.
Ligaments – connect bone to bone, composed of
dense connective tissues, mainly collagen.
Fascia – connects, surrounds, and separates muscle
to muscle.
a. Superficial fascia: Lies just under the skin
and connects it to underlying muscles
b. Deep fascia: Surrounds individual muscles
and groups them into compartments
Isotonic Contractions
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Myofilaments can slide past each other
during contractions.
Muscle contractions where the tension
remains constant while the muscle
changes length. Walking, nodding.
Two Types of Isotonic Contractions
1. Concentric – muscle
shortens while generating
force. (ex. Curling dumbbell
upward)
2. Eccentric – muscle lengthens
while still generating force.
(ex. Lowering the dumbbell
slowly)
Isometric Contractions
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When a muscle generates tension
without changing its length.
That means the muscle is working, but
there's no visible movement at the joint.
It's like flexing your biceps and holding
the position—you're exerting force, but
nothing's moving.
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Adenosine triphosphate (ATP)
Only energy source that can be used to
directly power muscle contraction.
Stored in muscle fibers in small amounts
that are quickly used up.
After this initial time, other pathways must
be utilized to produce ATP.
Muscle Growth
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Muscle develops when existing muscle
fibers hypertrophy.
Muscle strength and size differ among
individuals because of such factors as
exercise, nutrition, gender, age, and
genetic constitution.
Changes nutrition or exercise affect
muscle strength and size in an
individual.
Three pathways to generate ATP
1. DIRECT PHOSPHORYLATION OF
ADP BY CREATINE PHOSPHATE.
- The fastest way muscles regenerate
ATP during short bursts of intense
activity like sprinting or lifting a
heavy object. This process is part of
the phosphagen system, also known
as the ATP–CP system.
Molecule
Role in the Reaction
Creatine Phosphate
(CP)
Donates a phosphate
group to ADP
ADP (Adenosine
Diphosphate)
Accepts the phosphate to
become ATP
ATP (Adenosine
Triphosphate)
Powers muscle
contraction
Creatine Kinase
Enzyme that catalyzes
the reaction
Reaction:
Creatine Phosphate + ADP → ATP + Creatine
This is a one-step, anaerobic reaction meaning it
doesn’t require oxygen, and it happens almost
instantly.
Why It’s Important
 Provides immediate energy for up to 5–10
seconds of maximal effort.
 Supports activities like sprinting, jumping,
or heavy lifting.
 Acts as a buffer system, maintaining ATP
levels during sudden energy demands.
Recovery & Replenishment
 After exertion, ATP produced via aerobic
respiration rephosphorylates creatine back
into creatine phosphate.
 This recovery takes 1–2 minutes, depending
on fitness level.
2. AEROBIC PATHWAY
- Cellular process that uses oxygen to
convert nutrients—primarily
glucose—into ATP, the energy
currency of the cell. It’s the most
efficient way for your muscles to
produce energy during sustained,
moderate-intensity activities like
walking, jogging, or cycling.
- Oxygen acts as the final electron
acceptor in electron transport chain.
- Oxygen allows for a complete
oxidation of glucose.
Stages of the Aerobic Pathway
Location
Key
Outputs
Glycolysis
Cytoplasm
2 ATP, 2
NADH, 2
Pyruvate
Pyruvate
Oxidation
Mitochondrial
Matrix
Acetyl-CoA,
NADH, CO₂
Stage
Stage
Location
Key
Outputs
Mitochondrial
Citric Acid Cycle
Matrix
2 ATP, 6
NADH, 2
FADH₂, CO₂
Inner
Mitochondrial
Membrane
~34 ATP,
H₂O, NAD⁺,
FAD
Oxidative
Phosphorylation
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Total ATP Yield: Up to 38 ATP per glucose
molecule, though realistically it's often
around 30–32 ATP due to energy costs and
transport inefficiencies.
What Happens Next
 Lactic acid accumulates, lowering pH and
causing muscle fatigue.
 Once oxygen is available again, lactic acid
is:
o Converted back to pyruvate for
aerobic metabolism.
o Or sent to the liver for
gluconeogenesis via the Cori cycle.
Range of Motion
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3. ANAEROBIC GLYCOLYSIS AND
LACTIC ACID FORMATION
- Rapid energy pathway your body
taps into when oxygen is limited—
like during intense exercise or
sprinting. It allows muscle cells to
keep producing ATP without
oxygen, but it comes with a side
effect: lactic acid formation.
How It Works
Step
Process
Glycolysis
Glucose → 2 Pyruvate + 2 ATP + 2
NADH (in cytoplasm, no oxygen
needed)
Lactic Acid
Formation
Pyruvate + NADH → Lactic Acid
+ NAD⁺ (via lactate
dehydrogenase)
This conversion regenerates NAD⁺,
allowing glycolysis to continue.
 The lactic acid (or lactate) builds up in
muscle cells, contributing to that burning
sensation during strenuous activity.
Why Muscles Use This Pathway
 Oxygen delivery can’t keep up with demand
during high-intensity efforts.
 Anaerobic glycolysis provides fast ATP,
keeping muscles working for short bursts.
 It’s a survival mechanism—quick energy
when you need it most.
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Flexion – bending, decrease in the angle of
joint
Extension – straight, increase in in the angle
of joints
Abduction – when a bone moves away from
the midline of the body
Adduction – when a bone moves toward the
midline of the body
Elevation – lifting upward
Depression – downward movement
Supination – movement of the radius
around the ulna to turn palm up
Pronation – turning the palm downward
Rotation – moving a bone around its long
axis.
Protraction – moving forward.
Retraction - moving backwards
Internal Rotation – turning towards
midline
External Rotation – turning away from
midline
Circumduction – moving in a circle
Naming of Muscles
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