Muscle Tissue

Excitability (responsivness)

responsive to chemical signals, stretch, and electrical canes across the plasma membrane

Conductivity

local electrical excitation sets off a wave of excitation that travels along the muscle fiber

Contactility

shortens when stimulated

Extensibility

capable of being stretched between contractions

Elasticity

returns to original rest length after being stretched

Skeletal muscle

voluntary, striated muscle usually attached to bones of the skeleton

• striations- alternating light and dark transverse bands result from arrangement of internal contractile proteins

• muscle cell is a fiber (myofiber)

3 connective tissue wrappings of skeletal muscle

1. Endomysium

2. Perimysium

3. Epimysium

Endomysium

connective tissue around muscle cell

Perimysium

connective tissue around muscle fascicle

Epipmysium

connective tissue surrounding entire muscle

Tendons attach _____ to bone

muscle

Collagen

somewhat extensible, and elastic stretches slightly under tension and recoils when released

• resists excessive stretching and protects muscle from injury

• returns to muscle to its resting length and contributes to power output and muscle efficiency

What consists of the muscle fiber?

1. Sarcolemma

2. Sarcoplasm

3. Multiple nuclei

4. Mitochondria

Sarcolemma

plasma membrane of muscle fiber

Sarcoplasm

• Myofibrils

• Glycogen

• Myoglobin

cytoplasm of muscle fiber

• Myofibrils- long protein cords occupying most of sarcoplasm

• Glycogen- carbohydrate stored to provide energy for exercise

• Myoglobin- red pigment; provides some oxygen needed for muscle activity

Multiple nuclei

• Myoblasts

• Satellite cells

flattened nuclei pressed against the inside of the sarcolemma

• Myoblasts- stem cells that fused to form each muscle fiber early in development

• Satellite cells- unspecialized myoblasts remaining between the muscle fiber and endomysium that play a role in regeneration of damaged skeletal muscle tissue

Mitochondria

packed into spaces between myofibrils

Sarcoplasmic reticulum (SR)

smooth ER that forms a network around each myofibril

Terminal cisterns

dilated end-sacs of SR which cross the muscle fiber from one side to the other

• acts as a calcium reservoir; it releases calcium through channels to activate contraction

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 associated with it

Myofilament

1. Thick filament

made of several hundred myosin molecules, each molecule shaped like a golf club

• two chains intertwined to form a shaft-like tail and a double globular head

• heads are directed outward in a helical array around the bundle

• heads one half of the thick filament angle to the left, while heads on the other half angle toward the right

• the bare zone is the area in the middle with no heads

Myofilament

2. Thin filament

Fibrous (F) actin- two intertwined strands made up of string of gobular (G) actin subunits each with an active site that can bind to head of myosin molecule

• Tropomyosin molecules each blocking six or seven active sites on G actin subunits

• Troponin molecule- small, calcium-binding protein on each tropomyosin molecule

Contractile proteins

myosin and actin do the work of contraction

Regulatory proteins

tropomyosin and troponin act like a switch that determines when fiber can (and cannot) contract

• contraction is activated by the release of calcium into sarcoplasm and its binding to troponin

• troponin then changes shape and moves tropomyosin off the active sites on actin

Elastic filaments

• Titin

huge, springy protein that runs through the core of thin filament and anchor it to Z disc and M line

• helps stabilize and position the thick filament

• prevents overstretching and provide recoil

Dystrophin

links actin in outermost myofilaments to membrane proteins that link to endomysium

• transfers forces of muscle contraction to connective tissue ultimately leading to tendon

• muscular dystrophy is a genetic disease caused by a defect in dystrophin

Striations

results from the precise organization of myosin and actin in cardiac and skeletal muscle cells

• alternating A-bands (dark) and I-bands (light)

A (anisotropic) band

dark “A” strands fro darkest part is where thick filaments overlap hexagonal array of thin filaments

H band

not as dark; middle of A band; thick filaments only

M line

middle of H band

I (isotropic band)

light

Z disc

provides anchorage for thin filaments and elastic filaments

Sarcomere

segment from Z disc to Z disc

• functional contractile unit of muscle fiber

Muscle cells shorten because their individual sarcomeres ______

• Z disc (Z lines) are pulled closer together as thick and thin filaments slide past each other

shorten

Neither thick nor thin filaments change length during ________

• only the amount of overlap changes

shortening

During shortening, dystrophin and linking proteins also pull on ________ proteins

• transfer pull to extracellular tissue

extracellular

Skeletal muscle cannot contract unless stimulated by a ______

nerve

If nerve connections are severed or poisoned, a muscle is ________

• Denervation atrophy occurs which is the shrinkage of paralyzed muscle when nerve remains disconnected

paralyzed

Somatic motor neurons

nerve cells whose cell bodies are in the brainstem and spinal cord that synapse and act on skeletal muscle

• the axons of the neurons that lead to the skeletal muscle is the somatic motor fiber

• each muscle fiber is supplied by one motor neuron

• each nerve fiber branches out to multiple muscle fibers

Motor unit

one nerve fiber and all the muscle fibers innervated by it

Muscle fibers of ___ motor unit is dispersed throughout the muscle which allows for contraction in unison once the muscle neuron is stimulated

one

stimulation of the neuron produce _____ contraction over wide area

weak

the ability for motor units to take turns contracting provides the ability to sustain ______ term contraction

long

effective contraction usually requires contraction of _______ motor units at once

several

Average motor until contains _____ muscle fibers

200

Small motor units

have about three to six muscle fibers per neuron which allows for fine degree of control

• ex- eye and hand muscle

Large motor units

have hundreds of fibers allowing for more strength than control

• leads to powerful contractions

• ex- quadricep femoris and gastrocnemius which have about 1000 muscle fibers per neuron

Synapse

point where a nerve fiber meets its target cell

Neuromuscular junction (NMJ)

where a nerve fiber meets a muscle fiber

Each terminal branch of the nerve fiber within the NMJ forms a separate synapse with the muscle fiber consisting of what two things?

1. Axon Terminal

2. Synaptic clef

Axon terminal

swollen end of nerve fiber containing synaptic vesicles with acetylcholine (ACh)

Synaptic cleft

gap between axon terminal and sarcolemma

Nerve impulses causes synaptic vesicles to undergo _______ releasing ACh into synaptic cleft

exocytosis

Muscle cell has ACh receptors incorporated into its membrane made up of ______ folds that increases the surface area

Junctional

Myasthenia Gravis

a disease where an individual lacks Ach receptors leading to weakness

• the basal lamina contains acetylcholinesterase (AChE) that breaks down ACh, allowing for relaxation

Muscle fibers and neurons are electrically ______, their membrane exhibit voltage changes in response to stimulation

excitable

Electrophysiology

the study of the electrical activity of cells

Voltage (electrical potential)

a difference in electrical charge from one point to another

Resting membrane potential

about -90 mV in skeletal muscle cells which is maintained by sodium-potassium pump

In an unstimulated (resting cell):

• there are more anions on the inside of the membrane than on the outside

• these anions made the inside of the plasma membrane negatively charged by comparison to its outer surface

• the plasma membrane is electrically polarized (charged) with a negative resting membrane potential (RMP)

• there are excess sodium ions (Na+) in the extracellular fluid (ECF)

• there are excess potassium ions (K+) in the intraceulluar fluid (ICF)

In a stimulated (active) muscle fiber or nerve cell:

• Na+ ion gates open in the plasma membrane

• Na+ flows into cell down its electrochemical gradient

• These cations override the negative charges in the ICF

• Depolarization: inside of plasma membrane becomes positive

• Immediently, Na+ gates close and K+ gates open

• K+ rushes out of cell partly repelled by positive sodium charged and partly because of its concentration gradient

• loss of positive potassium ions turns the membrane negative again (repolarization)

• this quick up-and-down voltage shift (depolarization and repolarization) is called an action potential

a resting membrane potential (RMP) is seen in a waiting _____ cell, whereas an action potential is a quick event seen in a stimulated excitable cell

excitable

An action potential perpetuates itself down the length of a cell’s ______

membrane

an action potential at one point causes another one to happen immediately in front of it, which triggers another one a little farther along and so forth

• this wave of excitation is called an _____

impulse

Toxins interfearing with synaptic function can lead to muscle _____

paralysis

Spastic paralysis

a state of continual contraction of the muscles which can lead to suffocation

Cause of Spastic paralysis

some pesticides contain cholinesterase inhibitors which bind to acetylcholinesterase and prevent it from degrading Ach leading to spastic paralysis

Tetnus (lockjaw)

form of spastic paralysis caused by toxin Clostridum tetani

• Tetanus toxin blocks the release of glycine which in the spinal cord normally stops motor neurons from producing unwanted muscle contraction and overstimulation

Flaccid paralysis

a state in which the muscles are limp and cannot contract

Causes of Flaccid paralysis

curare, a plant poison used in South American natives to poison blowgun darts competes with ACh for receptor sites but does not stimulate the muscles

• leads to Flaccid paralysis

Botulism

type of food poisoning cause by neuromuscular toxin secreted by the bacterium Clostridium botulinum

• blocks release of ACh causing flaccid paralysis

• botox cosmetic injections used for wrinkle removal

Four major phases of contraction and relaxation

1. Excitation

2. Excitation-contraction coupling

3. Contraction

4. Relaxation

Excitation

a 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

the step in which the muscle fiber develops tension and may shorten

Relaxation

when stimulation ends, a muscle fiber relaxes and returns to its resting length

Excitation of Muscle Fiber

• nerve signal opens voltage-gated calcium channels in the synaptic cleft

• calcium stimulates exocytosis of ACh from synaptic vesicles

• ACh released into synaptic cleft

• Two ACh molecules bind to each receptor protein, opening Na+ and K+ channels

• Na+ enters; shifting RMP goes from -90mV to +75 mV, then K+exists and RMP returns to -90mV; quick voltage shift is called an end-plate potential (EPP)

• voltage change (EPP) in end-plate region opens nearby voltage-gated channels producing an action potential that spreads over muscle surface

Excitation-Contraction Coupling

• action potential spreads down into T tubules

• opens voltage-gated ion channels in T tubules and Ca+2 channels in SR

• Ca+2 enters the cytosol

• Calcium binds to troponin in thin filaments

• Troponin-tropomyosin complex changes shape and exposes active sites on actin

Contraction

• Myosin ATPase enzyme in myosin head hydrolyzes an ATP molecule

• Activates the head “cocking” it in an extended position

  • ADP+ Pi remain attached

• head binds to actin active site forming a myosin-actin cross-bridge

• Myosin head releases ADP and Pi, flexes pulling thin filament past thick-power stroke

• Upon binding more ATP, myosin releases actin and process is repeated

  • each head performs 5 power strokes per second

  • each stroke utilizes one molecule of ATP

Relaxation

• Nerve stimulation and ACh release stop

• AChE breaks down ACh and fragments reabsorbed into synaptic knob

• Stimulation by ACh stops

• Ca+2 pumped back into SR by active transport

• Ca+2 binds to calsequestrin while in storage in SR

• ATP is needed for muscle relaxation as well as muscle contraction

• Ca+2 removed from troponin is pumped back into SR

• Tropomyosin reblocks the active sites

• Muscle fiber ceases to produce or maintain tension

• Muscle fiber return to its resting length

  • due to recoil of elastic components and contraction of antagonistic muscles

Length-tension relationship

the amount of tension generated by a muscle depends on how stretched or shortened it was before it was stimulated

-If overly shortened before stimulated, a ____ contraction results, as thick filaments just butt against Z disc

-If too stretched before stimulated, a ____ contraction results, as minimal overlap between thick and thin filaments results in minimal cross-bridge formation

-Optimum resting length produces ____ force when muscle contractions

• nervous system remains muscle tone (partial contraction) to ensure the resting muscles are near this length

weak, weak, greatest

Rigor mortis

hardening of muscles and stiffening of body beginning 3-4 hour after death

• deteriorating sarcoplasmic reticulum releases Ca+2 and deteriorating sarcolemma allows Ca+2 to enter cytosol

• Ca+2 activates myosin-actin cross-bridging

• muscle contracts, but cannot relaze

Muscle relaxation requires ___, and ATP production is no longer produced after death

• fibers remain contracted until myofilaments begin to decay

ATP

Rigor mortis peaks about ___ hr after death, then diminishes over the next 48-60 hr as the proteins disintegrate

12

Myogram

a chart of the timing and strength of a muscle’s contraction

Threshold

minimum voltage necessary to generate an action potential in the muscle fiber and produce a contraction

Twitch

a quick cycle of contraction and relaxation when stimulus is at threshold or higher

Latent period

very brief delay between stimulus and contraction

• time required for excitation, excitation-contraction coupling, and tensing of elastic components of muscle (generating internal tension)

Contraction phase

time when muscles generates external tension

• force generated can overcome the load and cause movement

Relaxation phase

time when tension declines to baseline

• SR reabsorbs Ca2+, myosin releases actin and tension decreases; takes longer than contraction

• entire duration varies between 7 ms and 100 ms

With subthreshold stimuli, no _____ occurs but at threshold intensity and above——twitch is produced

contraction

Even if the same voltage is delivered, different stimuli cause twitches varying in strength because:

• muscle’s starting length influences tension generation

• muscles fatigue after continual use

• warmers muscle’s enzymes work more quickly

• muscle cell’s hydration level influence cross-bridge formation

• increasing the frequency of stimulus delivery increases tension output

Muscle must _____ with variable strength for different tasks

contract

Stimulating the nerve with ______ voltages produces stronger contractions

higher

-______ voltages excite more nerve fibers which stimulate more motor units to contract

higher

recruitment of multiple motor unit (MMU) summation

is the process of bringing more motor units into play with a stronger stimuli

• occurs according to size principle: weak stimuli (low voltage) recruit small units, while strong stimuli recruit small and large units for powerful movements

Treppe

increase in tension that occurs when each successive stimuli is delivered after the relaxation phase of the preceding twitch

• low-frequency stimuli produce identical twitches

Wave summation

higher frequency stimuli produce temporal (wave) summation

• each new twitch rides on the previous one generation higher tension with only partial relaxation between stimuli