BYU PdBio 305 Rhees Muscle Physiology

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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

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