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Decks by Naomi Justus (1)
Muscle TissueCharacteristics of ALL muscles
Contractility: Contract and relax to make movement
Extensibility: Extend, or stretch, allow muscles to return to rest
Excitability: Respond to regulatory signals from nerves, hormones & stimuli
Functions of skeletal muscles
Posture, heat production, movement, protection
Types of Muscle Tissue
Skeletal muscle
Structure (anatomy): multinucleated, regular arrangement of
actin and myosin fibers into bands of light and dark (striated)
Function (physiology): move the skeleton, especially the limbs
Location: usually connected to bones or fascia
Cardiac muscle
Structure (anatomy): 1-2 nuclei, branching with intercalated disks
organized as a syncytium to allow for coordinated contraction
Function (physiology): Pump blood through the circulatory system
Location: Heart
Smooth muscle
Structure (anatomy): 1 nucleus, no regular arrangement of
actin and myosin proteins in cytoplasm (non-striated/smooth)
Function (physiology): Goosebumps, moves food
through digestive tract, blood through circulatory system
Location: parts of viscera (organs, ducts) throughout the body
Characteristics of Cardiac Muscle
Highly coordinated contractions of cardiac muscle
Similarity with skeletal muscle: striated, organized into sarcomeres
Differences with skeletal muscle: only 1-2 nuclei, multiple mitochondria
and myoglobin, extensively branched fibers cells, intercalated discs.
Intercalated discs have sarcolemma with gap junctions & desmosomes
(allow heart to work as a pump by coordinating cardiac contraction)
Gap junctions: channels between adjacent cells that allow ions to flow from one cell to another quickly. Depolarization spreads quickly between cells to allow for coordinated contraction of entire heart, creates a syncytium (unit of contraction).
Desmosomes anchor the ends of cardiac muscle fibers together so the cells do not pull apart during the stress of individual fibers contracting
Smooth Muscle
Similarity to skeletal muscle: actin & myosin contractile proteins, thick & thin filaments.
Differences with skeletal muscle: 1 nucleus, spindle-shaped,
no striations, sarcomere, troponin, tropomyosin
Thin filaments are anchored by dense bodies
(similar to Z-discs) attached to sarcolemma.
Ca2+ enters sarcoplasm from SR and ECF and binds to regulatory protein calmodulin
Structure of a skeletal muscle
3 layers of connective tissue enclose a muscle to provides structure
Epimysium
Dense, irregular connective tissue around muscle organ
Allows a muscle to contract/move; structural integrity
separates muscle from other regional tissues; allows movement
Perimysium
Middle layer of connective tissue
Allows nervous system to trigger a movement of a muscle by activating fascicle
Endomysium
Thin layer of collagen and reticular fibers around muscle fiber
Organizes muscle fibers into fascicle (individual bundles)
Has ECF and nutrients supplied by blood
Skeletal Muscle Fibers (Cells)
Skeletal muscle cells (muscle fibers) are long and cylindrical. During early development, embryonic myoblasts, each has own nucleus, with 100s of other myoblasts to form the multinucleated skeletal muscle fibers (myofibrils) with multiple copies of genes to allow bulk production of proteins and enzymes for muscle contraction.
Sarcolemma: plasma membrane of muscle fibers
Sarcoplasm: cytoplasm of muscle fibers
Sarcoplasmic reticulum (SR): specialized smooth ER: stores, releases Ca2+ ions
Sarcomere: Unit of skeletal muscle fiber: highly organized arrangement of contractile proteins (actin, myosin myofilaments) and regulatory proteins (troponin, tropomyosin)
Sarcomere (Unit of Skeletal Muscles)
3D cylinder striations (bands of light & dark due to actin and myosin myofilaments)
Each myofibril can contain 100-1000s sarcomeres connected end to end
All sarcomeres within a myofibril contracts (and relaxes) simultaneously,
contracting (and relaxing) the entire myofibril & muscle cell
Thin filament
Starts from Z-discs and projects partway to the center
Has thinner actin strands and its troponin-tropomyosin complex
Thick filament
Starts from the center and projects partway to the Z-discs
Has thicker strands and their multiple heads
Z-discs (Z-lines)
Forms the boundary of sarcomeres at both ends
Anchored to actin myofilaments
Myofilaments
Each myofibril has 1000s myofilaments
4 different kinds of protein molecules make up myofilaments
Protein molecules
Actin (thin filaments): Has active sites (myosin binding sites) bind to myosin heads
Myosin (thick filament): Has myosin heads that are
attracted to actin and forms cross bridges with actin
Tropomyosin (regulatory protein): at rest, it blocks the myosin binding sites on actin
Troponin (regulatory protein): at rest, it holds tropomyosin in place, bind to Ca2+ ions
The Neuromuscular Junction
Location: Nerve ending meets the muscle fiber
All cells have membrane potential (gradients across membrane): -60 to -90 mV
When the membrane potential is LESS NEGATIVE: Depolarization occurs and AP starts
Membrane potentials change when ions either enter or leave the
cell via ion channels (open and close depending on the stimuli)
This change generates AP. An action potential (AP) in a nerve at the NMJ releases a NT which leads to the start of an AP in the muscle. This AP in the muscle
causes muscle contraction (Excitation-contraction coupling)
Every skeletal muscle fiber is innervated by a motor neuron at the NMJ
A signal from the motor neuron can cause the contraction of skeletal muscle fibers
Each motor neuron can innervate from 10s to 1000s skeletal muscle fibers
Contraction of Skeletal Muscle
AP reaches the end of the motor neuron
NT (acetylcholine or ACh) is released into NMJ
ACh binds to receptors on ligand ion channels for Na+ on the skeletal muscle fiber
Na+ channels open: Na+ enters sarcoplasm of muscle fiber
Membrane potential of muscle fiber changes
AP starts along the sarcolemma of muscle fiber: AP travels into the
interior of the cell via T-tubules (extensions of the sarcolemma)
AP starts with the sarcolemma of muscle fiber: AP travels into the interior
of the skeletal muscle cell via T-tubules (extensions of the sarcolemma)
Action potential depolarizes the cell membrane
Voltage-gated Ca2+ channels in SR
Ca2+ diffuses out of SR into sarcoplasm
Ca2+ binds to troponin on thin filament: Troponin-tropomyosin moves to myosin sites
Myosin binds actin at its myosin-binding site to form cross-bridge: ADP &
inorganic phosphate (Pi) generated in the previous contraction cycle are released
Myosin head pivots toward M-line at center of the sarcomere- power stroke
New ATP attaches to the myosin head
Cross-bridge is detached
ATPase in myosin head hydrolyzes ATP to ADP
Pi, releases energy
Angle of myosin head moves into a cocked position (re-cock),
ready to form another crossbridge with next myosin binding site
Sliding Filament Model of Contraction
Skeletal muscle fiber contracts when thin filaments are pulled
and then slide past the thick filaments within the fiber’s sarcomeres
Requires Ca2+ and ATP
Ca2+ starts contraction by exposing actin-binding site to form myosin crossbridges
ATP sustains contraction: Each cycle in cross-bridge needs energy by hydrolysis of ATP
Without ATP, the myosin head will be still attached to actin: rigor mortis
Myosin is in a high-energy configuration when myosin
head is cocked: this energy is used during the power stroke
Relaxation of a Muscle Fiber
Muscle contraction usually stops when
Nerve signal stops
Muscle runs out of ATP and becomes fatigued
Process
Nerve signal stops
Release of ACh stops
Ligand gated Na+ channels close
Sarcolemma and T-tubules repolarizes
Voltage-gated Ca2+ channels in the SR close
Ca2+ ions are pumped back into SR using ATP
Tropomyosin moves to cover myosin-binding sites
Thick and thin filament interaction relaxes
Sources of ATP
Skeletal muscle stores only small amount of ATP,
requiring rapid replacement to sustain contraction.
Sources of ATP:
Creatine Phosphate: Transfers energy from excess ATP to make ADP and creatine.
It can quickly regenerate ATP but only sustains energy for about 15 seconds.
Glycolysis: An anaerobic process that breaks down glucose to produce ATP at a slower rate than creatine phosphate, providing a burst of energy for up to 1 minute.
Aerobic Respiration: The breakdown of glucose or other nutrients in the presence of oxygen, resulting in carbon dioxide, water, and ATP. This is a more efficient process that produces approximately 95% of the ATP needed by muscles.
Motor Units
Each skeletal muscle fiber is controlled by one motor neuron, but one motor neuron can control multiple muscle fibers, depending on the muscle type.
Motor Unit: Group of muscle fibers innervated by a single motor neuron
Small Motor Units: Control fewer than 10 muscle fibers, allowing for precise movements, like eye movements. They use smaller, more excitable motor neurons.
Larger Motor Units: Control thousands of muscle fibers and are involved in larger movements, like those in the thigh. They use bigger, less excitable motor neurons.
Recruitment Process: Activating smaller motor units before large ones to INCREASE muscle contraction. As more motor units are recruited, the strength of the contraction increases, allowing variations, such as lifting a feather versus a heavy weight.(0)(1)(1)