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Muscle Physiology

BYU PdBio 305 Rhees Muscle Physiology

Motor unit a single motor neuron and the muscle fibers it innervaes
muscle fibers per motor unit 3-6 in hand and eye; 120-500 in large muscles of the back
myoneural junction junction of motor neuron and muscle; it loses its myelin sheath and divides into a number of terminal buttons or end-feet
6 events at a myoneural junction (1-3) 1)Action potential travels over the axon terminal and triggers the entry and release of clcium into the terminal 2)Ca++ triggers the exocytosis of vesicles of Ach 3)Ach diffuses accross the space and binds with receptors in the muscle cell
Events at a myoneural junction (4-6) 4)Binding brings about the opening of sodium channels generating an action potential in the muscle cell 5)Current flow throughout the muscle cell via the transvers (T) tubule system 6)Ach is broken down by acetylcholinesterase
differences between a synapse and a myoneural junction one to one transmission of acion potentials at a myoneural junction; myoneural junction is always excitatory
three types of muscle tissue smooth, cardiac, skeletal
mesoderm all 3 muscle cell types are derived from it
contractile fibers all 3 types of muscle cells are composed of them
smooth muscle elongated, lack cross striations, and under involuntary control, and have one nucleus which is located at the center of the cell
smooth muscel cells control contraction of what internal orans, walls of blood vessels, digestive system, ducts of reproductive glands, the ureters, the baldder, and skin
myogenic spontaneous muscle contraction
neurogenic muscle contraction induced through innervation
cardiac muscel fiber characteristics cross-striations, one centrally-lcated nucleus, involuntary control, bifurcated to form a synctium
skeletal muscle fiber characteristics elongated, multinucleate, voluntary control, well developed cross striations
function of skeletal muscle movement of bones, maintenance of body posture, facial expression, and breathing movements
sarcolemma plasma membrane in muscle cell
fiber muscle cell
sarcoplasm cytoplasm in muscle cell
sarcoplasmic reticulum endoplasmic reticulum in muscle cell
Myofibril a muscle fiber contains many small, round parallel bundles (100-1000s) called myofibril
Myofilament each myofibril is composed of still smaller units called myofilaments
Actin thin contractile protein in the muscle cell
Myosin thick contractile protein in the muscle cell
Sarcomeres the contractile proteins are arranged into compartments
A bands dark bands in myofilaments
I bands lighter bands in myofilaments
Anisotropic a bands
Isotropic I bands
H zone lighter central regions of the A bands contain only myosin (no myosin heads)
Z line where the actin filaments of adjacent sarcomeres join
Actin filaments are composed of actin, tropomyosin, and troponin
G-actin globular individual actin molecules
F-actin douple spherical chains (double helix) called fibrous actin
Troposmyosin covers how many G-actin subunits 7
Troponin I unit that binds to actin
Troponin T portion that binds to tropomyosin
Troponin C component that binds calcium (initiates contraction)
Myosin filaments thick myofilaments
Light meromyosin LMM filaments make up the rodlike backbone of the myosin filaments
Heavy meromyosin HMM filaments form the shorter globular lateral cross bridges (heads) which link
Binding sites on the myosin cross bridge actin binding site and an ATPase binding site
Isometric contraction when a muscle develops tension but does not shorten
Isotonic contraction a contraction in which the muscle shortens
Twitch contractions when an isolated muscle is attached to a device that senses and records changes in muscle length and the muscle then receives a single stimulus, the contraction response is referred to as a twitch
Three phases of a twitch 1)latent period which is the time from actual stimulation until contraction begins 2)contraction period 3)relaxation period
Wave summation or temporal summation first muscle twitch is not completely over when the second one begins and thus the muscle is already in a partially contracted state when the second twitch begins, the degree of muscle shortening in the 2nd contraction is greater than with just a single m
Tetanus occures when the stimuli are applied in a very rapid succession and the contractions fuse together and cannot be distinguished one from the other
Slow-twitch fibers found mostly in the postural muscles such as in the back and legs and have a twitch duration of about 100msec; derive energy from oxidative metabolism; have small diameters, many mitochondria, many capillaries, small amounts of glycogen storage
Red fibers slow-twitch fibers; the red color is due to the high concentration of myoglobin which binds O2
Fast-twicth fibers found mostly in fine, skilled movements and twitch for about 7 msec; fatigue quickly; derive energy from glycolysis; large fibers, high storage of glycogen and few mitochondria
White fibers fast-twitch fibers; they are white because they contain little myoglobin
Why is fast-twitch muscle fast? has more sarcoplasmic reticulum than slow-twitch muscle; also has fast myosin
Causes of muscle fatigue (first 2) 1)ATP use exceeds ATP production 2)lactic acid (H+) accumulation interferes with ATP production and muscle contraction
Causes of muscle fatigue (second 2) 3)glycogen depletion and therefore hypoglycemia may occur 4)inhibition of excitation-contraction coupling. That means inhibition of release of Ca++ from the lateral sacs
Sarcoplasmic reticulum of cardiac muscle is not as extensive
Intercalated disks manner in which cardiac muscle cells are interconnected
2 functions of intercalated disks 1)provide gap junctions that allow impulses to travel from one cell to another 2)provide desmosomes that anchor one cell to another
Cardiac muscle cell refractory period long absolute and relative refractory periods making tetanus impossible
Automaticity factors that increase intracellular Ca++ in the cardiac muscle such as catechoamines and digitalis, which slow the heart down, will increase the force of contraction. Cholinergic agents will decrease Ca++ concentrations and decrease the contraction force
Contraction of cardiac muscle the action potential does not release Ca++ from sarcoplasmic reticulum but is released from the extracellular fluid; the increase of Ca++ releases Ca++ in the sarcoplasmic reticulum (calcium induced calcium release)
What smooth muscle lacks sarcomeres, striations, t-tubule systems
Contraction of smooth muscle slower, requires less energy; innervated by neurons (autonomic nerves); other smooth muscle cells are not innervated and contract in response to hormones or local factors
Smooth muscle mechanism myosin-regulated; actin and myosin only interact when the myosin is phosphorylated
Ca++ involvement in smooth muscle intracellular messenger that sets off a seris of events that result in the phosphorylation of myosin. Most Ca++ comes from the extracellular fluid
Ca++ to phosphorylation in smooth muscle tract increased intracellular Ca++ binds to and activates calmodulin which binds to and activates another myosin kinase which phosphorylates myosin which binds with actin so cross-bridge cycling can begin
Autonomic innervation of smooth muscle cells postganglionic neurons travel across smooth muscle cell’s surface and release neurotransmitters from multiple bulges (varicosities) as an action potential passes along the axon.
Varicosities bulges on the postganglionic axon in smooth muscle cells that release neuro transmitters as an action potential passes along the axon
Adrenergic receptors (alpha and beta)—effects on smooth muscle cells (norepinephrine) Alpha(1) receptors--cause smooth muscles to contract or be stimulated; Beta (2) receptors—cause smooth muscles to relax or be inhibited
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