Exam 3Skeletal Muscle Composition and the NMJ Flashcards
3 types of muscles
Cardiac, Skeletal, Smooth
Characteristic of Skeletal muscle
Multinucleated cells
Cells appear striated at the microscopic level due to orientation of contractile proteins
Contraction provides voluntary movement
Connective tissue laters of Skeletal Muscle
Epimysium, perimysium, endomysium
Epimysium
An external sheath of DENSE connective tissue, surrounds the entire muscle.
Septa of this tissue extend inward, carrying the larger nerves blood vessels and lymphatics of the muscle
Perimysium
A thin connective tissue layer that immediately surrounds each bundle of muscle fibers termed a fascicle. Each fascicle of muscle fibers makes up a functional unit in which the fibers work together. Nerves, blood vessels, and lymphatics penetrate the perimysium to supply each fascicle.
Endomysium
A very thin, delicate later of reticular fibers and scatter fibroblasts within fascicles surrounding the external lamina of individual muscle fibers. In addition to nerve fibers, capillaries form a rich network in the endomysium bringing oxygen to the muscle fibers
Function of collagen in CT layers of muscle
serve to transmit the mechanical forces generates by the contracting muscle cells/fibers
Function of CT
Nourishment via rich capillary and lymphatic network
Protection/organization
Transmit forces along muscle fibers (through collagen)
Muscle components
Myofilaments Myofibrils Muscle Fiber Muscle fasciculus Whole Muscle Myotendinous junction
Myofilaments
Protein polymers responsible for contraction (e.g., actin and myosin)
Myofibrils
grouping of myofilaments within a muscle cell
Muscle fiber
muscle cell, made up of myofibrils
Muscle fasciculus/fascicle
bundle of muscle fibers
Whole muscle
many fascicles bundled together
Cardiac muscle characteristics
composed of irregularly branched cells bound together longitudinally by intercalated discs and shows strong, involuntary contractions
Smooth muscle
Composed of grouped, fusiform cells with weak involuntary contractions
Myotendinous junction
connection of muscle and tendon
Infoldings of cell membrane of muscle fibers
Provides insertion sites for collagen fibers of tendon into muscle fibers
Sarcomere
functional unit of muscle
Components of a sarcomere
Thick (myosin) and thin (actin) myofilaments, Titin
Titin functions
assists with transmission of force during contraction, limits range of motion of sarcomere in tension, provides passive tension in resting muscle, maintains appropriate positions of actin and myosin (binds Z-disc to M-line)
Sarcomere landmarks
Z-line (disk)- boundaries of a sarcomere *actin attaches here
I-band: area of actin molecules only (straddles Z-lines; Z-lines bisect I-band)
A-band: area of myosin and actin overlap
H-band (H-zone): area of myosin only
M-line: anchors central area of myosin within each sarcomere; contains kinase - enzyme that adds high energy phosphates to ADP to make ATP
Myofilaments of skeletal muscle
small proteins responsible for muscle contraction (actin and myosin filaments)
Number of myofilaments increases with training hypertrophy while number of muscle fibers is unaltered
Thick filament
Myosin
Light chain of Myosin
Consists of area of ATPase function (globular head)
Serves as connection sites for actin
Globular head
Heavy chain; just beyond the neck/light chain; Functions as an ATPase - enzyme that energizes the process of muscle contraction by facilitating hydrolysis of ATP to release energy
Heavy chain of myosin
Consist of helixes of filamentous protein polymers
thin filament
Actin
Actin
Globular protein polymer, bound to myosin at cross-bridges (on light chain of myosin)
Globular proteins polymerized into long chains
Tropomyosin
Filamentous protein that exists within the “grooves” of actin filaments
Troponin
Complex of three proteins
TnT - binds tightly to tropomyosin
TnC - contains a calcium binding site
TnI - inhibits tropomyosin from uncovering binding sites on actin
Sarcoplasmic Reticulum
SER of muscle cells is specialized as a storage area for calcium ions; has terminal cisterns adjacent to T-tubules
Transverse tubules
invaginations of the sarcolemma that create a network of membranous tubules at each sarcomere of muscle fibers
Triad
complex of two terminal cisterns of SR and one t-tubule; area where sarcolemma depolarization is transmitted to the membrane of the SR
Sarcoplasmic Reticulum: function
Motor neuron creates depolarization of the sarcolemma at the neuromuscular junction;
Depolarization is transmitted from the sarcolemma to the SR via the T-tubule system of the muscle fiber;
SR depolarization stimulates release of calcium from cisterns;
Released Calcium binds to TnC (binding allows cross-bridging between actin and myosin);
At the end of depolarization, calcium is actively returned from the sarcoplasm to the SR
Muscle spindles
Proprioceptors within striated muscle fibers; used reflexively to maintain appropriate muscle tension for posture and regulation of opposing muscle groups
Spindles consist of
A CT capsule that surrounds intrafusal muscle fibers (modified muscle fibers): bag fibers (dynamic and static stretch) and chain fibers (static stretch)
Spindles monitor…
the velocity and degree of muscle stretch and send that information to the CNS (transmit info using 1A and II afferent fibers)
Muscle stretch information from sensory fibers of the spindles synapses at…
the spinal cord to allow reflexive control of the contractile state of muscles
When stretched (quickly) spindles
Stimulate reflexive contraction of the muscle to reduce stretch (MSRs)
Spindles also stimulate reflexive inhibition…
of antagonist muscles to allow agonist muscles to contract without opposition
Gamma motorneuron controls
the sensitive of the spindles to stretch
Golgi tendon organ
Sensory nerve ending imbedded in the collagen fibers of the myotendinous junction; monitors the force of contraction of the muscle and sends info to CNS via 1b afferent fibers;
Provides input to CNS about contraction forces to aid in producing appropriate, smooth contractions
GTO provides an inhibitory reflex that…
protects muscles from creating too much tension (too much tension in a contracting muscle stimulates reflexive relaxation to prevent muscle damage)
glycogen
polymer of glucose; abundant amount stored within the sarcoplasm and serves as a source of glucose for energy production
Myoglobin
oxygen binding protein in sarcoplasm; serves as storage site for oxygen for aerobic respiration; concentrated in muscles that maintain activity for prolonged periods
Neuromuscular junction
site of connection between the nerve fiber of a motorneuron and the muscle fibers of skeletal muscle (also called “motor end plate”); each nerve contains terminal branches allowing it to synapse with up to several hundred muscle fibers (synapse at midpoint of muscle fiber)
Synaptic characteristics
Synaptic trough: invagination of muscle membrane
Subneural clefts: folds within synaptic trough which increase surface area for NT receptors (Ach)
Mitochondria of motorneuron of NMJ
within axon terminal of motorneuron; create ATP energy for acetylcholine synthesis and vesicle exocytosis
Voltage gated calcium channels of motorneuron on NMJ
Opened by spread of action potential to axon terminal; influx of Ca stimulates vesicle release of Ach
How is Ach of motorneuron released? Then what happens?
exocytosis of synaptic vesicles; Ach moves into synaptic cleft and binds nicotinic receptors located in subneural clefts
The post-synaptic membrane of the NMJ
muscle fiber
characteristics of the post-synaptic NMJ membrane
contains synaptic trough and subneural clefts; Ach binding to nicotinic receptor opens Sodium channels that creates local depolarization (end plate potential); most end plate potentials create and action potential in muscle fibers (for fiber contraction); AP propagates in both directions along muscle fiber (moves along t-tubules within fibers)
Neuromuscular contraction
AP is transmitted along the t-tubules and across each sarcomere within the muscle fiber; along t-tubule stimulates depolarization of SR membrane which results in release of Calcium from the cisterns of the SR; ACh is active at receptors for milliseconds, ACh is released from its receptor and is quickly degraded by acetylcholinesterase; Calcium binds with TnC and initiates attractive forces between actin and myosin of muscle fibers; conformation change in Tn causes tropomyosin to “move” and uncover actin binding sites; Once calcium binds, actin and myosin “bridge” together; actin-myosin then slide along side each other, creaing a muscle fiber contraction; following contraction calcium is released from TnC and then pumped back into the SR in preparation for the next depolarization
Myasthenia gravis
Auto-immune disease in which antibodies "attack" (blocks) ACh receptors and render them non functional; disease creates weakness/paralysis due to inability muscle cell to react to nerve impulses; Treatment: Acetylcholinesterase inhibitors that reduce ACh degradation and increase ACh binding to remain functional receptors. Nerve gas (sarin) - inhibits acetylcholinesterase and creates tonic muscle contraction
Alpha motor neuron
extrafusal muscle fibers (skeletal muscle fibers); creates muscle fiber contraction
Gamma motor neuron
intrafusal muscle fibers (muscle spindle); generates muscle response to sensory input from muscle spindles; causes tightening/relaxation of spindles to alter their sensitivity
C-motor neuron
autonomic nervous system fibers to smooth muscle
