lecture 10 MUSCLE TISSUE

Skeletal Muscle: Structure, Function, and Control

Functions of the muscular system
- Produces body movements by moving the skeleton
- Stabilizes the body position (posture; joint control)
- Protects and supports organs
- Voluntary control of swallowing, urination, and defecation
- Generates heat to maintain body temperature
- Stores and mobilizes substances (e.g., glycogen)
Muscle properties
- Electrical excitability: ability to send electrical signal across the sarcolemma
- Elasticity: ability to return to original length when relaxed
- Contractility: ability to shorten and generate force
- Extensibility: ability to stretch/extend
Three types of muscle tissue
- Skeletal
- Cardiac
- Smooth
Skeletal muscle: organization and arrangement
- Structural hierarchy: epimysium → muscle fascicles → perimysium → muscle fibers (muscle cells) → endomysium → myofibrils → sarcomeres → myofilaments → actin & myosin
- Surrounding connective tissues: Epimysium surrounds the whole muscle; Perimysium surrounds each fascicle; Endomysium surrounds each muscle fiber
- Within muscle fiber: sarcolemma (plasma membrane), T-tubules, sarcoplasmic reticulum, mitochondria, nucleus (multinucleated), myofibrils
- Myofibrils are composed of sarcomeres, the basic contractile units, extending from Z line to Z line
Microscopic anatomy of a muscle cell
- Sarcolemma = plasmalemma (cell membrane)
- T-tubules: invaginations that propagate action potentials into the cell
- Sarcoplasmic reticulum: stores Ca^{2+}; terminal cisternae are part of the SR
- Triad: a T-tubule with two terminal cisternae of the SR at its sides
- Nucleus: multiple per cell in skeletal muscle
- Myofibrils: contractile proteins organized into sarcomeres
- Mitochondria: high density to meet ATP demand during contraction
The sarcomere: contractile unit
- Repeating units from Z line to Z line
- Major filaments
- Thin filament: actin
- Thick filament: myosin
- Bands and lines observed in striated muscle
- I band: region with only thin filaments (actin)
- A band: length of the thick filament (myosin); overlaps with thin filaments
- Z line (Z disc): boundary of a sarcomere
- M line: center of the sarcomere
- H zone: region with only thick filaments (no overlap with actin) in a resting sarcomere
- Important structural proteins
- Troponin; Tropomyosin: regulate actin active sites in concert with Ca^{2+}
- Titin; Nebulin: structural and regulatory roles in sarcomere
- Important molecules
- Ca^{2+}
- ATP
- Filament organization: actin (thin) and myosin (thick) form cross-bridge interactions during contraction
The arrangement within a skeletal muscle
- Epimysium surrounds the whole muscle
- Perimysium surrounds each muscle fascicle
- Endomysium surrounds each muscle fiber
- Myofibrils inside muscle fibers contain sarcomeres, which contain actin and myosin
- Sarcolemma and T-tubules facilitate action potential propagation to reach the interior of the cell
- Sarcoplasmic reticulum stores Ca^{2+} and releases it during excitation-contraction coupling
Key concepts of sarcomere structure and contraction
- Neuromuscular junction (NMJ): the synapse between a motor neuron and a muscle fiber
- Cross-bridge cycle drives sarcomere shortening
- Sliding filament model: filaments slide past one another; filaments do not change length
- Neural control: motor neurons control muscle fibers via motor units
The neuromuscular junction (NMJ)
- An action potential travels along a motor neuron to the NMJ
- Synaptic vesicles release acetylcholine (ACh) into the synaptic cleft
- ACh binds to receptors on the sarcolemma (motor end-plate), opening Na^{+} channels and depolarizing the membrane
- The depolarization triggers a muscle action potential that travels along the sarcolemma and into T-tubules
- The action potential in the T-tubules triggers Ca^{2+} release from the sarcoplasmic reticulum, initiating contraction
Transmission at the NMJ: key steps in order
- Action potential arrives at the axon terminal
- Voltage-gated Ca^{2+} channels open; Ca^{2+} influx triggers exocytosis of ACh-containing vesicles
- ACh diffuses across the synaptic cleft and binds to its receptor on the motor end-plate
- ACh receptor activation opens Na^{+} channels; Na^{+} influx generates the end-plate potential and initiates an action potential in the muscle fiber
- The muscle action potential propagates along the sarcolemma and into T-tubules
- The signal triggers Ca^{2+} release from the sarcoplasmic reticulum, enabling cross-bridge cycling
The cross-bridge cycle (contraction mechanism)
- Resting state: myosin head is in a cocked position after ATP hydrolysis
- Ca^{2+} binds to troponin, causing tropomyosin to uncover actin active sites
- Cross-bridge formation: the myosin head binds to actin, forming a cross-bridge
- Power stroke: the myosin head pivots, pulling actin toward the center of the sarcomere; ADP and Pi are released
- Detachment: a new molecule of ATP binds to the myosin head causing detachment from actin
- Re-cocking: ATP is hydrolyzed, returning the myosin head to the cocked position
- Cycle continues as long as Ca^{2+} remains elevated and ATP is available
- Chemical steps summarized with reactions
- Resting ATP hydrolysis that powers the cocked head: ext{ATP}
  ightarrow ext{ADP} + ext{P}_{i}
- Ca^{2+} binds troponin to expose active sites on actin: ext{Ca}^{2+} + ext{Troponin}
  ightarrow ext{Ca}^{2+}- ext{Troponin}
- Power stroke: release of ADP + Pi from myosin as it pulls actin
- Detachment and re-cocking: ext{Myosin{-}ADP{-}Pi} + ext{ATP} ightarrow ext{Myosin{-}ATP} + ext{ADP} + Pi
  ightarrow ext{Myosin{-}ATP (cocked)}
- The cycle continues while both ATP and Ca^{2+} are present
- The latent period: the interval from motor neuron-stimulated release of ACh to the onset of force generation in the muscle fiber
Sliding filament model: what changes during contraction
- Filaments slide, do not change length
- When contracted:
- The I band shortens
- The H zone disappears or shortens
- The Zone of overlap between actin and myosin grows larger
- The Z lines move closer together
- The A band remains the same width
- Result: overall shortening of the sarcomere and muscle shortening while filaments slide over one another
Neural control of skeletal muscle
- Excitation-contraction coupling: conversion of a neural electrical impulse into a mechanical contraction
- Motor neuron: neuron that communicates with muscle cells
- Motor unit: a single motor neuron and all the fibers it innervates
- A muscle may contain multiple motor neurons; each muscle fiber is innervated by only one motor neuron (one fiber → one motor neuron)
- Size of motor units varies by function
- Small motor units: few fibers per neuron → fine motor control
- Large motor units: many fibers per neuron → gross movements
Motor units and recruitment
- A muscle fiber must be innervated by a motor neuron to contract
- A single motor neuron can innervate many fibers, forming a motor unit
- Recruitment and motor unit activation increase force
- The number of active fibers and the rate of action potentials determine force output
Classification of skeletal muscle fibers (based on contraction speed and metabolism)
- Slow oxidative (SO; Type I)
- Slow contraction; aerobic ATP production; many mitochondria; red due to myoglobin
- Fast oxidative glycolytic (FOG; Type IIa; intermediate)
- Fast contractions; mixed aerobic and anaerobic metabolism; fatigue resistance intermediate
- Fast glycolytic (FG; Type IIx; fast twitch)
- Fast contractions; anaerobic glycolysis dominates; few mitochondria; fatigue quickly; white fibers
- Most muscles contain a mix of all three fiber types in different proportions; training and use can alter relative proportions and fiber size
Force production and muscle twitch
- Force is proportional to the number of cross-bridges formed
- A muscle twitch is the smallest unit of contraction: a single contraction-relaxation cycle in response to one action potential in one motor neuron
- In daily life, motor units typically produce graded forces through recruitment and summation rather than single twitches
- Ways to increase force:
- Recruitment: increase the number of active fibers
- Temporal summation: increase the rate of action potentials
- Tetanic contraction: sustained maximal force produced by active fibers; strength depends on the number of fibers activated
Cardiac muscle: structure and specializations
- Similar organization to skeletal muscle but with key differences
- Intercalated discs connect cardiac muscle cells
- Contain desmosomes and gap junctions
- Desmosomes: anchor cells together and withstand tension during contraction
- Gap junctions: connexon channels that allow ion passage, enabling rapid electrical coupling between cells
- Cardiac muscle cells have more mitochondria
- Contractions last much longer than skeletal muscle (approximately 10–15 times longer)
- Electrical conduction can propagate from cell to cell through gap junctions, promoting synchronized contraction
Cardiac muscle: intercalated discs and conduction details
- Desmosomes provide mechanical stability across cell membranes
- Gap junctions permit direct cytoplasmic communication, enabling wave-like propagation of action potentials
- The combination ensures coordinated, rhythmic contractions of the heart
Smooth muscle: structure and properties
- Appears non-striated due to irregular arrangement of actin and myosin
- Thick in the middle, tapered at ends
- Can be organized as single-unit ormulti-unit fibers
- Found in walls of hollow organs (e.g., GI tract)
- Contractions are slower to start but longer-lasting than skeletal or cardiac muscle
- Smooth muscle can shorten and stretch to a greater extent; fibers can shorten in response to stretch
Summary of muscle types: key distinctions
- Skeletal muscle
- Location: attached to bones
- Control: voluntary
- Appearance: striated; multinucleated
- Contraction: rapid and forceful; fatigue-prone depending on fiber composition
- Cardiac muscle
- Location: heart
- Control: involuntary
- Appearance: striated; single nucleus (often); intercalated discs
- Contraction: rhythmic, long-lasting; highly resistant to fatigue
- Smooth muscle
- Location: walls of hollow organs and some structures (e.g., GI tract, blood vessels)
- Control: involuntary
- Appearance: non-striated
- Contraction: slow, prolonged, can be initiated by stretch
Connections to foundational principles and real-world relevance
- The muscular system integrates electrical excitability, chemical signaling (ACh, Ca^{2+}, ATP), and mechanical work to produce movement and homeostasis
- Proper function depends on intact NMJ signaling, Ca^{2+} handling by the SR, and cross-bridge cycling
- Differences between muscle types explain organ-level physiology: e.g., cardiac rhythm, smooth muscle tone in blood vessels, skeletal muscle voluntary control for locomotion
- Ethical and practical implications include the impact of neuromuscular diseases on motor control, the importance of physical training for muscle fiber composition, and considerations for aging and muscle mass maintenance
Quick reference: key terms to memorize
- Sarcomere, I band, A band, Z line, M line, H zone
- Actin (thin filament), Myosin (thick filament)
- Troponin, Tropomyosin, Titin, Nebulin
- Ca^{2+}, ATP, ADP, P_{i}
- Triad: T-tubule + terminal cisternae
- NMJ, Motor neuron, Motor unit, Recruitment, Summation, Tetany
Equations and concepts to remember
- ATP hydrolysis powering myosin head cocking: ext{ATP}
  ightarrow ext{ADP} + P_i
- Ca^{2+} release and troponin-tropomyosin regulation: ext{Ca}^{2+} + ext{Troponin}
  ightarrow ext{Ca}^{2+} ext{-Troponin complex}
- Cross-bridge detachment and re-cocking requires ATP binding to myosin: an ongoing cycle as long as ATP and Ca^{2+} are available
End of lecture takeaways
- The organization of muscle tissue and its connective tissue sheaths organize force transmission
- The sarcomere is the fundamental contractile unit; its shortening is due to actin-myosin sliding rather than filament shortening
- The NMJ couples neural signals to muscle contraction via ACh and Ca^{2+}-mediated excitation-contraction coupling
- Skeletal, cardiac, and smooth muscles differ in structure, control, and contractile properties, reflecting their distinct functional roles
Practice prompts you might encounter
- Describe the organization of a muscle from epimysium to myofilaments
- Outline the events at the NMJ from an action potential to muscle contraction
- Draw and label a sarcomere, indicating I band, A band, Z line, M line, H zone, and the location of titin and nebulin
- Explain the cross-bridge cycle, including the role of Ca^{2+} and ATP in each step
- Differentiate skeletal, cardiac, and smooth muscle in terms of structure, control, and typical contraction properties
- Compare fast-twitch and slow-twitch fibers, including metabolic pathways and fatigue characteristics
- Define a motor unit and explain recruitment and summation as mechanisms to increase force
Final note
- This set of notes consolidates content from the lecture materials on muscle tissue, emphasizing structure, function, and control across skeletal, cardiac, and smooth muscle. It is designed to serve as a comprehensive replacement for the original source for exam preparation.

Here is information based on your questions, drawing from the provided notes on muscle tissue and physiology:

Organization of a Muscle

Skeletal muscle exhibits a hierarchical organization:

Epimysium: Surrounds the entire muscle.
Muscle Fascicles: Bundles of muscle fibers, each surrounded by a perimysium.
Perimysium: Connective tissue that surrounds each fascicle.
Muscle Fibers (muscle cells): Individual muscle cells, each surrounded by an endomysium.
Endomysium: Connective tissue that surrounds each muscle fiber.
Myofibrils: Contractile organelles within the muscle fiber, composed of repeating units called sarcomeres.
Sarcomeres: The basic contractile units, made up of myofilaments.
Myofilaments: Primarily actin (thin filaments) and myosin (thick filaments).

Within a muscle fiber, the sarcolemma (plasma membrane) has invaginations called T-tubules that propagate action potentials into the cell. The sarcoplasmic reticulum (SR) stores and releases Ca^{2+}, critical for contraction. A triad consists of a T-tubule flanked by two terminal cisternae of the SR.

Events at the Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a muscle fiber, enabling neural control of muscle contraction:

An action potential arrives at the axon terminal of the motor neuron.
Voltage-gated Ca^{2+} channels open, and Ca^{2+} influx triggers the exocytosis of vesicles containing acetylcholine (ACh).
ACh diffuses across the synaptic cleft and binds to receptors on the motor end-plate of the sarcolemma.
ACh receptor activation opens Na^{+} channels, leading to Na^{+} influx, which generates an end-plate potential and initiates a muscle action potential.
The muscle action potential propagates along the sarcolemma and into the T-tubules.
This signal triggers the release of Ca^{2+} from the sarcoplasmic reticulum into the sarcoplasm, initiating muscle contraction.

Arrangement of a Sarcomere

The sarcomere is the fundamental contractile unit of striated muscle, extending from one Z line to another Z line:

Major Filaments:
- Thin filament: Primarily actin.
- Thick filament: Primarily myosin.
Bands and Lines:
- Z line (Z disc): Defines the boundary of a sarcomere, anchoring thin filaments.
- I band: Region containing only thin filaments (actin).
- A band: The full length of the thick filament (myosin), overlapping with thin filaments at its ends.
- H zone: Central region within the A band containing only thick filaments (myosin) in a resting sarcomere.
- M line: The center of the sarcomere, anchoring thick filaments.
Structural Proteins:
- Troponin and Tropomyosin: Regulate actin active sites, interacting with Ca^{2+}.
- Titin and Nebulin: Provide structural support and regulate sarcomere assembly and elasticity.

Sliding Filament Model

The sliding filament model explains how muscle contraction occurs:

During contraction, the thin filaments (actin) slide past the thick filaments (myosin) towards the center of the sarcomere.
Crucially, the filaments themselves do not change length; instead, their overlap increases.
Changes observed during contraction:
- The I band shortens.
- The H zone disappears or shortens.
- The zone of overlap between actin and myosin grows larger.
- The Z lines move closer together.
- The A band remains the same width.
This sliding process is driven by the cross-bridge cycle, where myosin heads bind to actin, pivot (power stroke), and pull the thin filaments.

Different Types of Muscle Fibers (Skeletal)

Skeletal muscle fibers are classified based on their contraction speed and metabolic pathways:

Slow Oxidative (SO; Type I):
- Contraction: Slow.
- ATP Production: Primarily aerobic (high mitochondrial density).
- Fatigue Resistance: High.
- Appearance: Red, due to high myoglobin content.
- Function: Sustained activities, posture.
Fast Oxidative Glycolytic (FOG; Type IIa; Intermediate):
- Contraction: Fast.
- ATP Production: Mixed aerobic and anaerobic metabolism.
- Fatigue Resistance: Intermediate.
- Function: Walking, sprinting.
Fast Glycolytic (FG; Type IIx; Fast Twitch):
- Contraction: Fast.
- ATP Production: Primarily anaerobic glycolysis (few mitochondria).
- Fatigue Resistance: Low (fatigue quickly).
- Appearance: White, due to low myoglobin.
- Function: Rapid, powerful movements.
  Most muscles contain a mix of these fiber types, and their proportions can be altered by training.

Differences in Excitability and Contractions of Skeletal, Cardiac, and Smooth Muscle

All muscle types share electrical excitability and contractility, but they differ significantly in structure, control, and contractile properties:

Skeletal Muscle:
- Excitability: Highly excitable; requires neural input from a motor neuron (voluntary control via NMJ).
- Contraction: Rapid, forceful, and can vary in duration; can fatigue depending on fiber type.
- Structure: Striated; multinucleated cells; no gap junctions.
- Control: Voluntary.
Cardiac Muscle:
- Excitability: Highly excitable; intrinsically rhythmic (pacemaker cells) but modulated by nervous system (involuntary control).
- Contraction: Rhythmic, long-lasting (10-15 times longer than skeletal muscle), and highly resistant to fatigue.
- Structure: Striated; typically single-nucleated cells; connected by intercalated discs containing desmosomes (for mechanical stability) and gap junctions (for electrical coupling, allowing rapid propagation of action potentials).
- Control: Involuntary.
Smooth Muscle:
- Excitability: Excitable; can be influenced by nervous system (autonomic, involuntary), hormones, local factors, or stretch; can sometimes generate its own action potentials.
- Contraction: Slower to start but much longer-lasting than skeletal or cardiac muscle; can shorten and stretch to a greater extent.
- Structure: Non-striated (due to irregular arrangement of actin and myosin); spindle-shaped cells, typically single nucleus; can be single-unit (connected by gap junctions) or multi-unit (independent cells).
- Control: Involuntary.

Note: Information regarding agonists, antagonists, origins, insertions, and criteria for naming skeletal muscles was not available in the provided notes.Muscle Response and Key Terminology - Muscles respond to stimuli by contracting. - Excitability: ability to respond to electrical stimuli. - Sarcolemma: muscle cell plasma membrane. - Sarcoplasm: muscle cell cytoplasm. - Contractility: ability to shorten and generate force. - Extensibility: ability to be stretched. - Elasticity: ability to recoil (fourth property). - Functions: move/stabilize skeleton, maintain posture, protect organs, enable voluntary control, generate heat.### Muscle Types and Their Characteristics - Skeletal muscle:

Voluntary control; striated; multi-nucleated.
Attached to skeleton (e.g., biceps, quadriceps). - Cardiac muscle:
Involuntary control; striated with branching fibers and intercalated discs.
Found in heart, pumps blood. - Smooth muscle:
Involuntary control; non-striated; spindle-shaped cells.
Found in walls of hollow organs (e.g., esophagus, uterus).### Skeletal Muscle Structure: Connective Tissue Organization - Connective Tissue Layers (from outer to inner):
Epimysium: encloses entire muscle.
Perimysium: surrounds fascicles (bundles of muscle fibers).
Endomysium: surrounds individual muscle fibers (cells).### Cellular and Subcellular Muscle Architecture - Muscle Fiber (Skeletal Muscle Cell) Components:
Multiple nuclei and many mitochondria (high ATP demand).
Sarcolemma: plasma membrane; forms T-tubules to propagate action potentials.
Sarcoplasm: cytoplasm.
Sarcoplasmic Reticulum (SR): stores and releases calcium ions (Ca^{2+}) to trigger contraction.
Myofibrils: rod-like bundles composed of repeating sarcomeres. - Sarcomere: the functional contractile unit of muscle, defined from one Z-disc to the next.### Sarcomere: The Contractile Unit and Its Filaments - Sarcomere Boundaries:
Z-disc: marks sarcomere ends.
M-line: center of sarcomere. - Filaments:
Thin filaments: Actin (located peripherally).
Thick filaments: Myosin (located centrally). - Bands and Zones:
I band: contains only actin.
A band: contains all myosin (including overlap with actin).
H zone: central region with only myosin.
Zone of overlap: where actin and myosin filaments interact.### Actin and Myosin: Regulatory Proteins on the Filaments - Actin (thin filament): has binding sites for myosin heads. - Myosin (thick filament): has heads that form cross-bridges with actin. - Regulatory Proteins on Actin:
Tropomyosin: covers myosin-binding sites on actin.
Troponin: binds Ca^{2+}; moves tropomyosin. - Calcium (Ca^{2+}) Role: Released from SR, binds troponin, shifting tropomyosin to uncover binding sites, enabling cross-bridge cycling and contraction Key Structural Relationships and Visualizations - Hierarchical Organization: Epimysium → Fascicles → Perimysium → Muscle fibers → Endomysium → Myofibrils → Sarcomeres → Actin/Myosin filaments. - Visual Cues:
Skeletal muscle: multi-nucleated, clear striations.
Cardiac muscle: branched fibers, intercalated discs.
Smooth muscle: non-striated, spindle-shaped.### Real-World Connections and Implications - Enables posture and movement, contributes to heat generation, and plays a role in energy storage. - Understanding muscle structure is key to explaining effects of exercise, injury, and disease.