Source: APK2100C Study Guide SLOs, University of Florida
Difficulty: Intermediate | Prerequisites: Chapter 6 (bone tissue, periosteum), Chapter 9 (joint movements)
Tags: muscular system, skeletal muscle, muscle tissue, sarcomere, myofibril, myofilament, actin, myosin, titin, tropomyosin, troponin, neuromuscular junction, NMJ, motor unit, muscle contraction, sliding filament, sarcoplasmic reticulum, T-tubule, triad, muscle fiber types, Type 1, Type 2A, Type 2B, epimysium, perimysium, endomysium, tendon, origin, insertion
This chapter shifts from the passive structures (bones, joints) to the active ones: muscles. You need to understand skeletal muscle from the macro level (the whole muscle and its connective tissue wrappings) right down to the molecular level (individual protein filaments sliding past each other). The neuromuscular junction is the bridge between the nervous system and movement, and understanding the step-by-step sequence from nerve impulse to muscle contraction is one of the highest-yield topics for this exam. If you are not comfortable with basic cell biology (membrane potentials, ion channels, exocytosis), review those concepts, as they underpin the NMJ mechanism.
Skeletal muscle is organised from whole muscle down to individual protein filaments (muscle → fascicle → muscle fibre → myofibril → myofilament). Contraction happens at the sarcomere level via the sliding filament mechanism, triggered by calcium release from the sarcoplasmic reticulum after a signal arrives through the neuromuscular junction. The three skeletal muscle fibre types differ in speed, fatigue resistance, and metabolic strategy.
Contractility
The ability of muscle to shorten and generate force. Muscles can only pull (contract), never push.
Excitability
The ability of muscle to respond to stimuli by producing electrical signals.
Extensibility
The ability of muscle to stretch without being damaged.
Elasticity
The ability of muscle to return to its original length and shape after being stretched.
Epimysium
The connective tissue sheath covering an entire muscle. Think of it as the outermost wrapper.
Perimysium
The connective tissue wrapping a bundle of muscle fibres called a fascicle.
Endomysium
The connective tissue wrapping each individual muscle fibre. Capillary networks within it are wavy at rest, allowing for extensibility.
Fascicle
A bundle of muscle fibres wrapped by perimysium within a skeletal muscle.
Tendon
A cord-like connective tissue structure attaching skeletal muscle to a bone's periosteum, continuous with all three CT sheaths (epimysium, perimysium, endomysium).
Aponeurosis
A broad, flat tendon. In simple terms, a sheet-like version of a tendon.
Origin
The attachment site on the stationary bone during contraction.
Insertion
The attachment site on the moving bone during contraction.
Strain
Damage to muscle and/or tendon, often involving bruising.
Sprain
Damage to ligaments.
Muscle fibre (muscle cell / myocyte / myofiber)
A long, cylindrical, striated cell with multiple peripherally located nuclei. Contains myofibrils.
Myofibril
A specialised contractile organelle within a muscle fibre, running its entire length, composed of a series of sarcomeres.
Myofilament
The protein filaments within myofibrils: actin (thin), myosin (thick), and titin (elastic).
Sarcomere
The basic contractile unit of a myofibril, composed of myofilaments arranged between two Z-discs. During contraction, filaments do not change length – only the amount of overlap changes.
Z-disc (Z-line)
The boundary of a sarcomere, appearing like a zipper. Actin filaments attach here.
M-line
Proteins running down the middle of the sarcomere, anchoring myosin filaments.
A-band
The region of the sarcomere spanning the length of the thick (myosin) filament, including overlapping areas with actin. The A-band stays the same size during contraction.
I-band
The region of the sarcomere containing only thin (actin) filaments with no myosin overlap. Decreases during contraction.
H-zone
The region of the sarcomere containing only thick (myosin) filaments with no actin overlap. Decreases during contraction.
Actin (thin filament)
A contractile protein consisting of two strands wound in a double helix, projecting toward the middle of the sarcomere from the Z-disc.
Myosin (thick filament)
A contractile protein composed of hundreds of myosin molecules with protruding heads (shaped like golf club knobs) that pull on actin during the power stroke.
Titin (elastic filament)
A very large structural protein that coils like a spring, allowing the sarcomere to stretch and recoil, and anchoring myosin filaments to the Z-disc.
Dystrophin
A structural protein at the ends of myofibrils that anchors myofilaments to the sarcolemma, keeping the myofibril in place.
Troponin
A regulatory protein that binds to tropomyosin and holds it in position on actin. When calcium binds to troponin, it causes a conformational change that moves tropomyosin off the binding sites.
Tropomyosin
A regulatory protein that covers the myosin-binding sites on actin when the muscle is at rest. Think of it as the gatekeeper of contraction.
Sarcolemma
The plasma membrane of a muscle fibre.
Sarcoplasmic reticulum (SR)
A specialised smooth endoplasmic reticulum in muscle cells that stores and releases calcium.
T-tubule (transverse tubule)
An invagination of the sarcolemma that tunnels into the muscle fibre, carrying electrical signals deep into the cell.
Terminal cisternae
The enlarged ends of the sarcoplasmic reticulum flanking each T-tubule. The main storage site for calcium.
Triad
A structural unit consisting of one T-tubule flanked by two terminal cisternae of the SR (terminal cisterna // T-tubule // terminal cisterna). There are two triads per sarcomere.
Motor unit
A single motor neuron plus all the muscle fibres it innervates. The neuron extends from the spinal cord to the muscle via axon terminals that form neuromuscular junctions.
Neuromuscular junction (NMJ)
The synapse between a motor neuron's axon terminal and a muscle fibre's motor end plate. Components: axon terminal, synaptic cleft (the gap), and junctional folds of the sarcolemma (which increase surface area for receptors).
Acetylcholine (ACh)
The neurotransmitter released at the NMJ that triggers muscle fibre excitation.
Contractility – shorten and generate force (muscles only pull)
Excitability – respond to stimuli by producing electrical signals
Extensibility – stretch without damage
Elasticity – return to original length after stretching
Movement, posture and joint stabilisation
Opening/closing body passageways
Thermogenesis (heat production during contraction)
From largest to smallest:
Muscle (wrapped by epimysium)
Fascicle (wrapped by perimysium)
Muscle fibre / muscle cell / myocyte / myofiber (wrapped by endomysium)
Myofibril (contractile organelle within the fibre)
Myofilaments (actin, myosin, titin)
The CT sheaths hold skeletal muscle cells together in parallel so they can generate more force as a whole. They also provide elasticity.
Tendons are continuous with all three CT sheaths and attach muscle to bone's periosteum
Aponeurosis – a broad, flat tendon
Origin – stationary bone attachment during contraction
Insertion – moving bone attachment during contraction
Direct attachment – CT strands so short the muscle appears to attach directly to bone
Indirect attachment – CT fibres are longer, forming an obvious tendon or aponeurosis
Strain – damage to muscle and/or tendon
Sprain – damage to ligaments
Long, cylindrical, striated, multinucleated (nuclei at the periphery)
Contains myofibrils (contractile organelles running the full length of the fibre)
Nuclei sit on top of the myofibrils, covered by the sarcolemma
Each myofibril is a chain of sarcomeres end to end. The sarcomere is the basic contractile unit.
Key sarcomere landmarks:
Z-disc – sarcomere boundary, actin attaches here
M-line – midline proteins anchoring myosin
A-band – full length of thick (myosin) filament, including overlap zones. Stays the same during contraction.
I-band – region of thin (actin) filament only, no myosin overlap. Gets smaller during contraction.
H-zone – region of thick (myosin) filament only, no actin overlap. Gets smaller during contraction.
During contraction, filaments do not change length. Only the degree of overlap changes. The H-zone and I-band shrink; the A-band remains constant.
Contractile proteins – generate force
Actin (thin filaments) – two strands in a double helix
Myosin (thick filaments) – hundreds of molecules, heads pull in the power stroke
Structural proteins – maintain organisation
Titin – elastic recoil and anchoring of myosin to Z-disc
Dystrophin – anchors myofilaments to the sarcolemma at myofibril ends
Regulatory proteins – control whether contraction can occur
Troponin – holds tropomyosin in place; calcium binding causes conformational change
Tropomyosin – covers myosin-binding sites on actin at rest
Each triad = terminal cisterna of SR // T-tubule // terminal cisterna of SR. There are two triads per sarcomere. The sarcoplasmic reticulum is the main calcium storage site in muscle cells.
Each muscle is innervated by a single nerve that branches and synapses with multiple muscle cells
A motor unit = one motor neuron + all the muscle fibres it innervates
Motor neurons extend from the spinal cord to the muscle
Each muscle is supplied by a single artery that branches
Capillaries within the endomysium are wavy at rest, allowing tissue extensibility
The NMJ has three structural components: the axon terminal, the synaptic cleft (gap), and the junctional folds of the sarcolemma (increasing receptor surface area).
The full sequence from nerve impulse to contraction:
Action potential travels down the motor neuron to the axon terminal at the NMJ
Electrical signal opens voltage-gated calcium channels at the terminal
Ca²⁺ from extracellular fluid enters the axon terminal, signalling synaptic vesicles containing ACh
ACh is released into the synaptic cleft via exocytosis
ACh crosses the cleft and binds to ACh receptors (ligand-gated ion channels) on the motor end plate
Binding initiates a new electrical signal at the sarcolemma of the muscle cell
The signal propagates across the sarcolemma until it reaches a T-tubule
Signal travels down the T-tubule and reaches the terminal cisternae of the SR
Ca²⁺ is released from the SR into the cytosol, binds to troponin, causing a conformational change
Tropomyosin shifts off the actin binding sites, allowing myosin heads to bind actin and produce contraction via the sliding filament mechanism
Enzymes in the synaptic cleft break down ACh, limiting the signal to a single muscle twitch.
Slow Oxidative (Type 1)
High myoglobin, slow contraction, aerobic metabolism
High fatigue resistance, red colour, small diameter
Primary function: maintaining posture and endurance activities
Fast Oxidative-Glycolytic (Type 2A)
High myoglobin, fast contraction, aerobic + anaerobic metabolism
Intermediate fatigue resistance, pink colour, intermediate diameter
Primary function: walking, sprinting
Fast Glycolytic (Type 2B/2X)
Low myoglobin, fast contraction, anaerobic metabolism
Low fatigue resistance, white colour, large diameter
Primary function: rapid, intense movements of very short duration
Students often think the A-band shrinks during contraction. It does not. The A-band spans the full length of the myosin filament, which does not change. The I-band and H-zone shrink because actin and myosin overlap more.
"Muscle fibre," "muscle cell," "myocyte," and "myofiber" all refer to the same thing. Myofibril is a different structure (an organelle within the cell). Confusing fibre with fibril is one of the most common errors.
Tropomyosin and troponin are both regulatory proteins, but they have different jobs. Tropomyosin physically covers the binding sites on actin. Troponin holds tropomyosin in place and is the protein that calcium binds to.
A strain is not the same as a sprain. Strain = muscle/tendon damage. Sprain = ligament damage.
⚠️ Know the organisational hierarchy from whole muscle down to myofilaments, including which CT sheath wraps each level.
⚠️ Be able to label all sarcomere landmarks (Z-disc, M-line, A-band, I-band, H-zone) and state what happens to each during contraction.
⚠️ The 10-step NMJ-to-contraction sequence is heavily tested. Know it in order.
⚠️ Classify proteins as contractile, structural, or regulatory and give examples and functions of each.
⚠️ Know the triad structure and that there are two triads per sarcomere.
⚠️ Be able to compare the three muscle fibre types across all characteristics (myoglobin, speed, metabolism, fatigue resistance, colour, diameter, function).
True or false: The A-band decreases in size when a muscle contracts.
Fill in the blank: The CT sheath wrapping an individual muscle fibre is the __________.
True or false: Troponin covers the myosin-binding sites on actin.
Fill in the blank: There are __________ triads per sarcomere.
True or false: Type 2B muscle fibres have high fatigue resistance and are suited for endurance activities.
Answers: 1. False (the A-band stays the same; the I-band and H-zone decrease). 2. Endomysium. 3. False (tropomyosin covers the binding sites; troponin holds tropomyosin in place). 4. Two. 5. False (Type 2B has low fatigue resistance and is suited for rapid, intense, short-duration movements; Type 1 is the endurance fibre).
Q: List the organisational hierarchy of skeletal muscle from largest to smallest.
A: Muscle → fascicle → muscle fibre (muscle cell/myocyte/myofiber) → myofibril → myofilaments.
Q: What are the three components of a triad, and how many triads are present per sarcomere?
A: A triad consists of a T-tubule flanked by two terminal cisternae of the sarcoplasmic reticulum. There are two triads per sarcomere.
Q: Describe what happens to the H-zone, I-band, and A-band when a sarcomere contracts.
A: The H-zone decreases (myosin-only region shrinks as actin slides inward). The I-band decreases (actin-only region shrinks as overlap increases). The A-band stays the same (it spans the full, unchanged length of the myosin filament).
Q: A sprinter relies heavily on which muscle fibre type(s) for short bursts, and a marathon runner relies on which?
A: A sprinter relies heavily on Type 2A (fast oxidative-glycolytic) and Type 2B/2X (fast glycolytic) fibres. A marathon runner relies primarily on Type 1 (slow oxidative) fibres for sustained endurance.
Q: Outline the role of calcium at two different points in the sequence from nerve impulse to muscle contraction.
A: First, Ca²⁺ enters the axon terminal through voltage-gated calcium channels and triggers the release of ACh vesicles into the synaptic cleft. Second, Ca²⁺ is released from the sarcoplasmic reticulum into the cytosol, binds to troponin, and causes tropomyosin to shift off the actin binding sites, enabling contraction.
The muscle terminology and contraction mechanism here are essential for Chapter 11, where you will apply this knowledge to specific muscles, their origins, insertions, and actions. The NMJ sequence ties into neurophysiology (action potentials, neurotransmitters) and is revisited in pharmacology contexts (e.g. how nerve agents or anaesthetics affect ACh). The fibre-type distinction connects to exercise physiology and explains why endurance training and strength training produce different adaptations.
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