Chapter 10: Muscle Tissue and Physiology

basic function of all muscle tissue

generating force (muscle tension)

other functions of muscle tissue

• create movement

• maintain posture

• stabilize joints

• generate heat

• regulate flow of materials through hollow organs

skeletal muscle cells

• elongated →> longest type

• multinucleated

• striated

• conscious control and reflexes

• responsible for overall body movement

• mostly attached to skeleton

cardiac muscle cells

• mononucleated 

• striated 

• found only in heart

• cells connected via intercalated discs 

• not as elongated 

• not under conscious control 

• controls beating of heart

intercalated discs

• contain gap junctions and desmosomes

• unite cells and permit coordination of contraction

smooth muscle cells

• mononucleated 

• not striated 

• spindle shaped

• not under conscious control

• function in forcing fluids and substances through internal passages

• found in walls of hollow organs

Properties of muscle cells 

• contractability

• excitability 

• conductivity

• extensibility

• elasticity

CECEE → Cats Eat Cheese Every Evening 

contractability

• ability to contract

• proteins in cells draw closer together

• does not necessarily involve shortening of cell

excitability

ability to respond to stimulus → chemical, mechanical stretch, or local electrical signals 

conductivity

ability to conduct electrical charges across entire plasma membrane 

extensibility

ability to be stretched without rupturing

elasticity

ability to return to its original length after stretching

→ like a rubber band

myocyte

muscle cell

sarcoplasm

myocyte’s cytoplasm

sarcolemma 

myocyte’s plasma membrane 

sarcoplasmic reticulum

• modified smooth endoplasmic reticulum

• forms web-like network surrounding myofibrils

• stores and releases calcium ions

myofibrils

• cylindrical organelles + most abundant organelle 

• other organelles, such as mitochondria, are packed between myofibrils

• arrangement is different in cardiac and muscle cells 

skeletal muscle fiber formation

formed by fusion of many embryonic myoblasts giving each fiber multiple nuclei 

transverse tubulues (T-tubulues)

• deep inward extensions of sarcolemma

• surround each myofibril

• form tunnel-like network within muscle fibers

• filled with ECF 

terminal cisternae

• enlarged sections of SR
  flank each T-tubule

triad 

two terminal cisternae + corresponding T-tubule 

contractile proteins

generate tension

regulatory proteins

dictate when a fiber may contract

structural proteins

maintain proper myofilament alignment and fiber stability

myofilaments 

make up myofibrils 

types of proteins that make up myofilaments

• contractile proteins

• regulatory proteins

• structural proteins 

types of myofilaments

• thick filaments

• thin filaments

• elastic filaments

thick filaments

bundles of myosin

myosin

contractile protein

thin filaments

• actin proteins

• tropomyosin

• troponin

actin proteins 

• contractile protein

• thin filament

tropomyosin

• rope-like regulatory protein

• twists around actin and covers up active sites

troponin

• globular regulatory protein that holds tropomyosin in place

• assists with turning muscle contractions on and off

elastic filaments 

• contains titin proteins

• stabilizes myofibril structure

• resists excessive stretching

titin

single, massive, spring-like structural protein 

structure of thick filament

• globular heads at each end linked by intertwining tails

• heads are connected to tails by hinge-like neck

• each head has an active site that bonds with actin 

thin filament structure

• multiple actin subunits together 

• form intertwining strands in functional thin filament 

• each bead-shaped actin has an active site which binds with myosin heads

I band

• only in thin filaments

• light band → think “i” in light

• held by direct attachment to Z discs

Z disc

in the middle of the I band

Z disc function

• anchor thin filaments in place to one another

• serves as attachment points for elastic filaments

• attach myofibrils to one another across entire diameter of muscle fiber

A bands

• thick filaments overlapping with thin filaments → zone of overlap

• dark band → “A” in dark

• greater tension during contraction 

H zone

middle of A band where only thick filaments exist

M line

dark line in middle of A band 

M line function

• structural proteins hold thick filaments in place

• serve as anchoring point for elastic filaments

sliding-filament mechanism of contraction

Thin filaments slide past think filaments, which generates tension throughout whole sarcomere

1. I band and H zone narrow → myosin heads of the thick filaments “grab” the thin filaments and pull them toward the M line → brings Z-discs closer together and cause the sarcomere as a whole to shorten

2. A band remains unchanged because myosin heads are doing the pulling

functional unit of contraction 

sarcomere extends from one z disc to the next  

electrophysiology

the field of physiology that studies electrical changes that occur across plasma membranes, and the accompanying physiological processes 

electrical gradient

• unequal distribution of negatively charged and positively charged ions across a membrane 

• electrical potential 

voltage

• difference in electrical potential between two points

• measured in volts 

millivolts

voltage measurement for sarcolemma

membrane potentials

electrical potential across the membrane of a cell

resting membrane potential

the electrical potential across the membrane of a cell that is not being stimulated

polarized

• two sides of opposite charges are present

• membrane 

leak channels

• channel proteins that are always open 

• ions able to fit through the channel can diffuse down their concentration gradients and move into or out of the cell

• type of ion channel

gated channels

• channel proteins that have some kind of gate that closes the channel and stops movement of ions through it 

ligand-gated channels

• open or close their gates when a particular substance binds to the channel protein 

voltage-gated channels 

ones that open or close their gates in response to voltage changes across the membrane 

ion channels

ions cannot diffuse through lipid component of plasma membrane and must rely on specific protein channels

mechanically gated channels

open or close in response to mechanical stimulation (pressure, stretch, or vibration)

concentration gradient 

• the main factor that determines movement of uncharged solutes (carbon dioxide, glucose, and oxygen) across membrane 

• more complicated for ions 

electrochemical gradient

diffusion of an ion across plasma membrane is determined by both concentration gradient and electrical gradient 

action potentials

quick, temporary changes in the membrane potential locally

what happens during the action potential

• membrane potential becomes more positive and then reverts to the more negative resting membrane potential

how an action potential is generated

generated by the opening and/or closing of protein channels in the membrane that control the movement of sodium and potassium ions across 

Action potential stages

1. Depolarization

2. Repolarization

depolarization stage steps

1. stimulus is received by the sarcolemma

2. voltage-gated sodium ion channels 

3. sodium ion flow rapidly into the cell 

4. the rapid influx of positively charged sodium ions causes the membrane potential to depolarize (less negative)

5. membrane potential rises rapidly and the inside of the cell is more positive than the outside of the cell 

Repolarization stage

1. caused by the voltage-gated sodium ion channels closing and the simultaneous opening of voltage-gated potassium ion channels 

2. potassium ions flow rapidly out of the cell

3. rapid exit of positively charged potassium ions causes the membrane potential to repolarize (more negative)

4. membrane potential achieves same resting potential state + voltage-gated potassium ion channels close

action potential purpose

they are efficient and effective at long distance signaling → rapid propagation (ripples in pond) allows a single stimulus to have a nearly instantaneous, far reaching effect across the entire cell, including T-tubules 

neuromuscular junction (NMJ)

site where an axon from a motor neuron and muscle fiber meet → synapse 

synapse 

a communication bridge between a motor neuron axon and a muscle fiber 

motor neuron

• found in brain and spinal cord

• axon goes from cell body to particular muscle fibers in muscles of the body

• stimulate muscle fibers to contract 

nerve impulse

action potential of a neuron

neuromuscular junction parts

• axon terminal

• synaptic cleft

• motor end plate

axon terminal

• ending of an axon

• contains lots of mitochondria and synaptic vesicles with neurotransmitters

• contains synaptic vesicles filled with acetylcholine

neurotransmitters

• chemicals that are secreted at the end of axons

• typically cause target cell to respond in some way → allows for cell to cell communication

acetylcholine (ACh)

neurotransmitter used to stimulate skeletal muscle fibers to contract

synaptic cleft

• narrow gap separating the sarcolemma from the membrane of the axon terminal

• where ACh is released 

motor end plate location

sarcolemma of muscle fiber

motor end plate

• specialized region of sarcolemma

• folded surface has many ligand-gated Na+ channels (ACh receptors) → ACh is ligand that opens gates, allowing Na+ to diffuse into muscle cell 

• muscle fiber typically has only one 

skeletal muscle contraction phases

1. Excitation

2. Excitation-Contraction Coupling

3. Contraction 

energy sources for skeletal muscle

• ATP

• Glycolytic Energy Sources 

• Oxidative energy sources 

ATP in skeletal muscles

required to:

• power Na+/K+ pump that maintains ion gradients (action potentials)

• release myosin heads from actin active sites →recocks head for another power stroke

• pump calcium back into SR (relaxation)

creatine phosphate

• initial source for ATP regeneration → ATP is rapidly consumer during muscle contraction

• phosphorylates ADP into ATP which creates enough ATP to run a muscle fiber for about 10 more seconds

glycolytic energy sources

Glycolysis

• anaerobic catabolism of glucose 

• occurs in sarcoplasm

• 2 ATP molecules produced for every glucose → can provide enough ATP for another 30-40 seconds of sustained muscle contraction

• produces lactic acid 

oxidative energy sources

• aerobic catabolism of glucose/glycogen

• starts in sarcoplasm and finishes in mitochondria

• forms many ATP molecules from degradation of one glucose molecule → allows for longer lasting muscle contractions

• in muscle fibers, oxygen binds to myoglobin, so it can store oxygen temporarily →can provide ATP for hours

• predominant energy source after one minute of contraction

muscle twitch

• single contraction in response to a single action potential in a motor neuron

• only in lab settings 

myogram

generated by the changes of tension within the muscle fiber

three periods on a myogram

1. Latent period

2. Contraction period

3. relaxation period

latent period

time delay between stimulus and respose

contraction period

• myosin head regions active at peak tension

• pulling at attachments

relaxation period

• muscle tension decreases toward zero as calcium is pumped back into SR

• muscle fiber returns to normal length 

refractory period

• time when muscle fiber is unresponsive to another stimulus 

• occurs between the start of the latent period and the start of the contraction period 

wave summation

• waves of contraction added together ‘

• repeated stimulation by motor neurons results in twitches with progressively greater tension

tension during twitch factors

• timing and frequency of stimulation

• length of fiber at rest

• type of muscle fiber 

unfused tetanus

• frequent stimuli delivered in rapid succession

• each twitch will be stronger than the previous and only show partial relaxation between contractions

fused/complete tetanus

• frequent stimuli delivered even faster

• no relaxations seen at all

• waves fully fused, producing a smooth line on the myogram 

length-tension relationship

states that the optimal length of a sarcomere is about 100-120% of the natural length of a sarcomere because:

• length of sarcomere must be short enough to allow for a generous zone of overlap between thick and thin filaments 

• length also must be long enough for for the thick filaments to pull the thin filaments toward the M line without running into Z-discs 

classes of skeletal muscle fibers

• speed of contraction

• pathways for forming ATP

speed of contraction

how fast the ATPase on myosin can degrade ATP (fast or slow)

fast twitch fibers

• high myosin ATPase activity

• proceed rapidly through contraction cycles

• found in muscles that move body parts rapidly