MUSCLES Flashcards
Types of Muscle Tissue
The three types of cells in muscle tissue are skeletal, cardiac, and smooth muscle (Figure 10.1)
Generating a force called muscle tension is a basic function common to each muscle tissue type
Movement is a fundamental characteristic of all living organisms
Three types of muscular tissue—skeletal, cardiac, and smooth
Important to understand muscle at the molecular, cellular, and tissue levels of organization
Movement is a fundamental characteristic of all living organisms
Three types of muscular tissue—skeletal, cardiac, and smooth
Important to understand muscle at the molecular, cellular, and tissue levels of organization
Both skeletal and cardiac muscle cells have striations, giving both cell types a striped appearance
Both skeletal and cardiac muscle cells have striations, giving both cell types a striped appearance
Striated Muscle Tissue
Skeletal muscle tissue is made up of long muscle cells arranged parallel to one another; some are quite long, extending nearly the entire length of the muscle
Skeletal muscle cells are known as fibers due to their length and appearance; they are multinucleated cells whose contractions arevoluntary (controlled by conscious thought)
Striated Muscle Tissue
Most are found attached by connective tissue to the skeleton, where their contraction can produce movement of a body part
Striated Muscle Tissue
Cardiac muscle cells are found only in the heart
Each cell is short and highly branched, and has one to two nuclei
Intercalated discs join adjacent cells; they contain gap junctions and desmosomes (modified tight junctions) that both unite the cells and permit them to coordinate contraction
Contraction is involuntary, or not controlled by consciousthought
Smooth muscle cells do not have striations, unlike skeletal and cardiac muscle tissue
Smooth muscle cells are long and flat with “spindle-shaped” pointed ends and a single centrally
Skeletal muscle—voluntary, striated muscle attached to one or more bones
Striations—alternating light and dark transverse bands
Results from an overlapping of internal contractile proteins
Voluntary—usually subject to conscious control
Muscle cell, muscle fiber (myofiber)—as long as 30 cm
UNIVERASAL CHARACTERSTIC OF MUSCLE
Responsiveness (excitability)
To chemical signals, stretch, and electrical changes across the plasma membrane
Conductivity
Local electrical change triggers a wave of excitation that travels along the muscle fiber
Contractility
Shortens when stimulated
Extensibility
Capable of being stretched between contractions
Elasticity
Returns to its original resting length after being stretched
STRUCTURE OF MUSCLE
Multiple muscle fibers (surrounded by endomysium) form a fascicle
Each fascicle is surrounded by a layer of connective tissue called the perimysium
Bundles of fascicles make up a skeletal muscle, which is surrounded by the epimysium, a connective tissue layer
STRUCTURE OF SKELETAL MUSCLE
Each individual muscle cell (fiber) is surrounded by thin connective tissue called endomysium (Figure 9.1)
Several (between 10 and 100) muscle cells are bundled together into a fascicle by the connective tissue perimysium
All fascicles that make up a muscle are, in turn, enclosed in an outer fibrous connective tissue wrapping (epimysium)
Interconnected connective tissues taper down and connect to tendons or other connective tissues; attach muscle to bone or other structure to be moved
MUSCLE FIBER
Sarcolemma—plasma membrane of a muscle fiber
Sarcoplasm—cytoplasm of a muscle fiber
Myofibrils—long protein bundles that occupy the main portion of the sarcoplasm
Glycogen: stored in abundance to provide energy with heightened exercise
Myoglobin: red pigment; stores oxygen needed for muscle activity
Multiple nuclei—flattened nuclei pressed against the inside of the sarcolemma
Mitochondria—packed into spaces between myofibrils
Sarcoplasmic reticulum (SR)—smooth ER that forms a network around each myofibril: calcium reservoir
Calcium activates the muscle contraction process
Terminal cisternae—dilated end-sacs of SR which cross the muscle fiber from one side to the other
T tubules—tubular infoldings of the sarcolemma which penetrate through the cell and emerge on the other side
Triad—a T tubule and two terminal cisterns
Contractile proteins—myosin and actin do the work
Regulatory proteins—tropomyosin and troponin
Like a switch that determines when the fiber can contract and when it cannot
Contraction activated by release of calcium into sarcoplasm and its binding to troponin
Troponin changes shape and moves tropomyosin off the active sites on actin
Myofilament Arrangement and the Sarcomere
Striations appear microscopically as alternating:
Light bands, where only thin filaments are found
Dark bands, where both thin and thick filamentsare found
Myosin and actin are proteins that occur in all cells
Function in cellular motility, mitosis, transport of intracellular material
Organized in a precise way in skeletal and cardiac muscle
STRAITIONS
Myosin and actin are proteins that occur in all cells
Function in cellular motility, mitosis, transport of intracellular material
Organized in a precise way in skeletal and cardiac muscle
Sarcomere—segment from Z disc to Z disc
Functional contractile unit of muscle fiber
Muscle cells shorten because their individual sarcomeres shorten Z disc (Z lines) are pulled closer together as thick and thin filaments slide past each other
Neither thick nor thin filaments change length during shortening
Only the amount of overlap changes
During shortening dystrophin and linking proteins also pull on extracellular proteins
Transfers pull to extracellular tissue
Sarcomere—segment from Z disc to Z disc
Functional contractile unit of muscle fiber
Muscle cells shorten because their individual sarcomeres shorten Z disc (Z lines) are pulled closer together as thick and thin filaments slide past each other
Neither thick nor thin filaments change length during shortening
Only the amount of overlap changes
During shortening dystrophin and linking proteins also pull on extracellular proteins
Transfers pull to extracellular tissue
Dystrophin—most clinically important
Links actin in outermost myofilaments to transmembrane proteins and eventually to fibrous endomysium surrounding the entire muscle cell
Transfers forces of muscle contraction to connective tissue around muscle cell
Genetic defects in dystrophin produce disabling disease muscular dystrophy
The Nerve—Muscle Relationship
Skeletal muscle never contracts unless stimulated by a nerve
If nerve connections are severed or poisoned, a muscle is paralyzed
Denervation atrophy: shrinkage of paralyzed muscle when connection not restored
Motor Neurons and Motor Units
Somatic motor neurons—nerve cells whose cell bodies are in the brainstem and spinal cord that serve skeletal muscles
Somatic motor fibers—their axons that lead to the skeletal muscle
Each nerve fiber branches out to a number of muscle fibers
Each muscle fiber is supplied by only one motor neuron
Average motor unit—200 muscle fibers for each motor unit
Small motor units—fine degree of control
Three to six muscle fibers per neuron
Eye and hand muscles
Large motor units—more strength than control
Powerful contractions supplied by large motor units (e.g., gastrocnemius has 1,000 muscle fibers per neuron)
Many muscle fibers per motor unit
The Neuromuscular Junction
Synaptic knob—swollen end of nerve fiber
Contains synaptic vesicles filled with acetylcholine (ACh)
Synaptic vesicles undergo exocytosis releasing ACh into synaptic cleft
Motor Neurons and Motor Units
Motor unit—one nerve fiber and all the muscle fibers innervated by it
Muscle fibers of one motor unit
Dispersed throughout the muscle
Contract in unison
Produce weak contraction over wide area
Provides ability to sustain long-term contraction as motor units take turns contracting (postural control)
Effective contraction usually requires the contraction of several motor units at once
The Neuromuscular Junction
Synapse—point where a nerve fiber meets its target cell Neuromuscular junction (NMJ)—when target cell is a muscle fiber Each terminal branch of the nerve fiber within the NMJ forms separate synapse with the muscle fiber One nerve fiber stimulates the muscle fiber at several points within the NMJ
The Neuromuscular Junction
ACh receptors—proteins incorporated into muscle cell plasma membrane
Junctional folds Serve to Increase surface area holding ACh receptors
Myasthenia gravis- lack of Ach receptors
leads to paralysis in disease myasthenia gravis
Basal lamina—Contains acetylcholinesterase (AChE)
AChE -breaks down ACh after contraction causing relaxation
Electrically Excitable Cells
Muscle fibers and neurons are electrically excitable cells
Their plasma membrane exhibits voltage changes in response to stimulation
Voltage (electrical potential)—a difference in electrical charge from one point to another
Resting membrane potential—about −90 mV
Maintained by sodium–potassium pump
Electrically Excitable Cells
In an unstimulated (resting) cell
There are more anions (negative ions) on the inside of the plasma membrane than on the outside
The plasma membrane is electrically polarized (charged)
There are excess sodium ions (Na+) in the extracellular fluid (ECF)
There are excess potassium ions (K+) in the intracellular fluid (ICF)
Also in the ICF, there are anions such as proteins, nucleic acids, and phosphates that cannot penetrate the plasma membrane
These anions make the inside of the plasma membrane negatively charged by comparison to its outer surface
Stimulated (active) muscle fiber or nerve cell
Ion gates open in the plasma membrane
Na+ instantly diffuses down its concentration gradient into the cell
Depolarization: inside of the plasma membrane becomes briefly positive
Immediately, Na+ gates close and K+ gates open
K+ rushes out of cell (repolarization)
Neuromuscular Toxins and Paralysis
Flaccid paralysis—a state in which the muscles are limp and cannot contract
Curare: compete with ACh for receptor sites, but do not stimulate the muscles
Plant poison used by South American natives to poison blowgun darts
Botulism—type of food poisoning caused by a neuromuscular toxin secreted by the bacterium Clostridium botulinum
Blocks release of ACh causing flaccid paralysis
Botox cosmetic injections for wrinkle removal
Flaccid Paralysis
Myasthenia gravis- lack of Ach receptors
leads to paralysis in disease myasthenia gravis
Neuromuscular Toxins and Paralysis
Spastic paralysis: a state of continual contraction of the muscles
Tetanus (lockjaw) is a form of spastic paralysis caused by toxin Clostridium tetani
Glycine in the spinal cord normally stops motor neurons from producing unwanted muscle contractions
Tetanus toxin blocks glycine release in the spinal cord and causes overstimulation and spastic paralysis of the muscles
Behavior of Skeletal Muscle Fibers
Four major phases of contraction and relaxation
Excitation
The process in which nerve action potentials lead to muscle action potentials
Excitation–contraction coupling
Events that link the action potentials on the sarcolemma to activation of the myofilaments, thereby preparing them to contract
Contraction
Step in which the muscle fiber develops tension and may shorten
Relaxation
When its work is done, a muscle fiber relaxes and returns to its resting length
Four major phases of contraction and relaxation
Excitation
The process in which nerve action potentials lead to muscle action potentials
Excitation–contraction coupling
Events that link the action potentials on the sarcolemma to activation of the myofilaments, thereby preparing them to contract
Contraction
Step in which the muscle fiber develops tension and may shorten
Relaxation
When its work is done, a muscle fiber relaxes and returns to its resting length
