Physiology - Chapter 8: Muscle Physiology

Muscle fiber is excited

Ca+ pulls troponin tropomyosin complex from blocking binding site

The link between excitation and contraction

Calcium

What pulls the thin filaments inward?

Cycles of cross-bridge binding and bending, sliding filament mechanism

Is a motor protein, similar to kinesin

Myosin

Extrafusal vs intrafusal fibers

*Extrafusal* --> 99% of fibers -alpha motor neurons -contractile strength *Intrafusal* --> very few -gamma motor neurons -do NOT contribute to contractile strength

gradation accomplished in skeletal muscle

- # of motor units contracting - frequency of stimulation (twitch summation)

Three structures that play important roles in linking excitation and contraction

- Calcium - Sarcoplasmic reticulum - Transverse Tubules

Contractile activity at lengths lower than Lo

- actin-myosin overlap = no cross bridge - ends of myosin pushed up against discs - no calcium release (80% lower than Lo)

Where can single-unit smooth muscle be found?

- digestive - reproductive - urinary (tracts) - small blood vessels

How do muscles respond to demands and adapt?

- improve oxidative capacity (changing the # of mitochondria or capillaries) - muscle hypertrophy (increase actin/myosin production)

gradation accomplished in multiunit smooth muscle

- number of muscle fibers contracting - cytosolic calcium concentration in each fiber

polypeptide units that make up troponin

- one binds to tropomyosin - one binds to actin - one binds to Ca

smooth muscle contraction

- phasic - tonic

How are skeletal muscles stimulated to contract

- release of ACh at neuromuscular junctions between motor neuron terminal buttons and muscle fibers. - ACh binds with the motor end plate - permeability changes as a result - action p. conduction

how to override withdraw pain reflex (ex. vaccine shot)

- send epsps to motor neurons of triceps (to activate) 2 send ipsps to biceps (to inhibit)

categories of fibers

- slow oxidative (1) - fat oxidative (2a) - fast glycolytic (2x)

three kinds of motor activity

- somatic reflex (reflex arc: postural or protective) - voluntary movement (can become a reflex) - rhythmic (walking, chewing...etc)

How do muscles shorten?

- the individual sarcomeres shorten which pulls the z discs closer together - by moving specialized intracellular components, causing them to develop tension and shorten.

gradation accomplished in cardiac muscle

- varying length of fibers - cytosolic calcium concentration (through autonomic, hormonal, and local metabolite influences.

multiunit smooth muscle can be found in

- walls of large blood vessels - in small airways of lungs - ciliary muscle - muscle of iris - the base of hair follicles (goosebumps)

Functions of titin

1) It stabilizes the thick filaments and keeps them in place 2) It acts like a spring attached to myosin returning the muscle to its normal position after stretch 3) Participates in signal transduction (stimulus to activity)

Controlled contraction of muscle allows:

1) Purposeful movement of the whole body or parts of the body 2) Manipulation of external objects 3) Propulsion of contents through various hollow internal organs 4) Emptying contents of certain organs to the external environment

Adjustable factors to produce graded contractions?

1) number of muscle fibers contracting 2) tension developed by each contracting fiber

sites crucial for contractile processes in each cross bridge

1- actin binding site 2- myosin ATPase site

Influences each muscle fiber and the tension developed

1- frequency of stimulation 2- contraction onset 3- fatigue 4- fiber thickness

4 steps in the excitation, contraction, and relaxation processes that require ATP:

1- splitting of ATP into ADP and Pi by ATPase (provides E for powers stroke) 2- Bind ATP to myosin 3- Active transport of Ca2+ back into lateral sacs of the SR during relaxation 4- Activity of Na+-K+ pump during action potential production

More ATPase

2a and 2x fibers (more cycling=faster)

describe alpha muscle spindle reflex

1. change in muscle length 2. recognized by muscle spindles 3. alpha output to extrafusal fibers 4. contraction

types of muscle atrophy

1. disuse atrophy (cast/brace) 2. innervation atrophy (nerve supply to muscle is lost) 3. sarcopenia (age related; decrease in motor neurons, protein synthesis and hormone levels = 1/4 pound of muscle lost a year) 4. limited repair: muscle cannot divide mitotically to repair itself.

ways to increase size of muscle (hypertrophy) / increase the production of actin and myosin

1. influence of testosterone (increases synthesis) 2. interconversion of fast muscle types

proprioceptors send information about length and tension through

1. muscle spindles (length) 2. golgi tendon organs (tension)

gamma neuron output

1. output to edges of fibers 2. initiate contraction of intrafusal fibers

postural somatic reflexes input comes from...?

1. proprioceptors (muscles and joints) 2. touch receptors in the skin 3. vestibular apparatus 4. eyes (horizon)

why is there an increase in oxygen consumption after exercise?

1. reset medical disturbances 2. restore nutrients 3. repair O2 debt (resynthesize phosphate, pyruvate glucose...)

Power stroke

Actual motion (single power stroke) pulling the thin filaments toward the m-line, cross bridge bends pulling myofilament inward

T/F: only one myosin head can attach to the actin at a time

True

T/F: contractile activity outlasts electrical activity that initiates it

True, but why? - Ca+ takes more time to be reabsorbed as lateral sacs have to reabsorb all of it, action p. is over before binding of actin and myosin occurs

T/F neither thick nor thin filaments change length during shortening

True, contraction is accomplished by the movement of thin filaments from opposite sides, not shortening of the filaments themselves

more actin or myosin in a single muscle fiber?

actin (32 billion) > myosin (16 billion)

proteins of thin filaments

actin (spherical) tropomyosin, troponin

Myofibrils are composed of

actin and myosin myofilaments / highly organized cytoskeletal microfilaments

motor neurons in the spinal cord receive direct input from

afferent neurons, brain stem, primary motor cortex

influence motor output

afferent neurons, primary motor cortex, brainstem(multineural), cerebellum

detachment (3rd step of cross bridge activity)

after power stroke, cross bridge detaches and returns to its original conformation

maximum contraction

all muscle fibers activated

Why is contraction during muscle twitch not maximal?

Not all cross bridges find a binding site as cytosolic Ca declines.

phasic vs tonic

Phasic- change, short-lived things (hollow tubes) tonic - contracted all the time ex. arterioles

multineuronal motor system

Regulates body posture and involuntary movements of larger muscle groups

_________ allows for the troponin tropomyosin complex to slip back into its blocking position

Removing cytosolic Ca+ by SERCA pump

What splits ATP into ADP and Pi (inorganic phosphate)?

The enzyme ATPase on the myosin Inorganic phosphate is released during the power stroke

fulcrum

The fixed point around which a lever pivots

When do actin and myosin detach?

Until ATP attaches to myosin, ending the power stroke as it reduces binding affinity between myosin head and actin. ATP is then split by ATPase.

contraction does not take place in...

a relaxed muscle fiber

why can thin filaments be pulled along the thick filaments for further distance in smooth muscle?

because cross bridges are present along the entire smooth muscle filament length

Why is single-unit smooth muscle also called visceral smooth muscle?

because it can be found in the walls of hollow organs.

Why does tone exist in tonic smooth muscle?

because of its low resting potential

flexion and extension

bending and extension of a limb biceps and triceps

the formation and breaking apart of ATP is only needed for release not for...

binding

how are smooth muscle cells turned on?

by calcium dependent phosphorylation of myosin

chemical change in smooth muscle

calcium phosphorylation chain

force > load

can move

the two types of muscle that physically reposition troponin-tropomyosin complex to uncover actin cross bridge binding sites

cardiac and skeletal muscle (smooth muscle = chemically)

autonomic / involuntary muscle

cardiac and smooth muscle

Isometric contraction

constant length (ex. standing, holding beverage in between drinks)

isokinetic contraction

constant motion (ex. machines at a gym)

Isotonic contraction

constant tension (ex. lift object)

Basal nuclei receive input from

cortical motor and sensory areas

First source of supplying ATP for skeletal muscle contraction (that is, during the first few seconds, at the very beginning of contraction)

creatine phosphate

most rapid energy pathway

creatine phosphate (one equation)

creatine kinase equation

creatine phosphate + ADP (forward-reverse arrows) creatine + ATP

most energy is stored in muscle as

creatine phosphate pools

Availability of calcium in the cytosol allows for...?

cross bridges to participate in the cycling process for a longer time

load and velocity

directly related = eccentric contractions inversely related = concentric contractions

SERCA pump (Sarcoplasmic, endoplasmic reticulum, calcium ATPase).

energy consuming carrier in the SR, which actively transports Ca+ from cytosol to lateral sacs to relax the skeletal muscles

terminal cisternae

enlarged areas of the sarcoplasmic reticulum surrounding the transverse tubules; lateral sacs

the rise in cytosolic calcium is mostly from __________ in smooth muscle.

extacellular fluid (ECF)

The number of fibers contracting within a muscle depends on...?

extent of motor unit recruitment

motor neurons in the spinal cord serve as

final common pathway for efferent component for somatic spinal reflexes, voluntary motor activities, and rhythmic activities.

The brainstem serves as

final link in multineuronal motor system

muscle fiber more resistant to fatigue

has higher capacity for ATP production

asynchronous recruitment

helps avoid fatigue during sustained muscle contractions, refers to different motor units taking turns maintaining muscle tension, and makes an entire muscle seem to contract smoothly.

fatigue

inability to maintain a given level of muscle tension or intensity during activity

increase in tension

increase afferent firing

spastic paralysis

increased muscle tone (rigidity) and augmented limb reflexes (lowered inhibitory responses)

alpha motor neurons

innervate extrafusal muscle fibers

functional sycytium

interconnected muscle cells that function electrically and mechanically propagate action potentials in single unit smooth muscle, contract as a single unit

Where is tension produced?

internally within the sarcomeres

Muscle tension

is transferred to bone as titin

Three types of contraction

isotonic, isometric, isokinetic, concentric, eccentric

lowering object --> biceps ______

lengthen

muscle is attached to bone

limitation on muscle lengths

flaccid paralysis

low muscle tone; a state in which the muscles are limp and cannot contract, caused by nerve lesion

Tetanus

maximum number of cross bridge binding sites uncovered

more or less actin in smooth muscle

more (15-20 actin per myosin) skeletal m. = 2 to 1

the stronger the contraction

more motor units recruited

motor units _____ resistant to fatigue are recruited first

most

The brainstem receives input from:

motor cortex, cerebellum, basal nuclei, afferent neurons

Troponin when Ca+ is present

moves tropomyosin away from blocking the binding site

type of smooth muscle that initiates contraction

multiunit smooth muscle

compromises largest group of tissues in the body:

muscle

eccentric contraction

muscle lengthens (ex. lower object)

concentric contraction

muscle shortens

Tendons

muscle to bone attachment non-contractile elastic tissue

muscle fibers are composed of...?

myofibrils

can excite or inhibit to modify contraction

myogenic single-unit smooth muscle

oxidative fibers have more...?

myoglobin, mitochondria, capillaries

what protein does calmodulin activate?

myosin light chain kinase (MLC kinase, which phosphorylates myosin light chain)

approximately at Lo

naturally relaxed, vary 30% on average throughout the day

myogenic activity in single-unit smooth muscle

nerve independent in which muscle initiates contraction itself

neurogenic

nerve produced contractile activity (multiunit smooth muscle and skeletal muscle)

10% larger than Lo

no contraction

tetanus contractions

no relaxation; spasm and much more frequent

is threshold always reached in slow wave potentials?

no, depends on starting point of membrane potential at onset

I - band

non-projecting portion of thin filaments into A-band

areas without myosin in smooth muscle

none

troponin and tropomyosin in smooth muscle

none

Muscle twitch

one action potential in one muscle fiber

muscle tone

ongoing state of muscle tension even when at rest

where do slow wave potentials occur?

only in smooth muscle of the digestive tract

red fibers

oxidative fibers

used to form energy for light exercise such as walking or jogging

oxidative phosphorylation

pacemaker potential

pacemaker potentials: shifts in passive ionic fluxes accompany automatic changes in ion channel permeability resulting in the membrane potential gradually depolarizing on its own

series elastic component

passive elastic property of muscle derived from the tendons

All multiunit smooth muscle is

phasic

all musltiunit smooth muscle is _________

phasic

which type of smooth muscle contracts in bursts?

phasic smooth muscle

neuromuscular junction

point of contact between a motor neuron and a skeletal muscle cell releases ACh which diffuses and triggers action p.

examples of skeletal muscles being subject to involuntary regulation

posture, balance, walking

corticospinal tract

precise, coordinated limb movements

connective tissue role in muscle

prevents over-stretching

issues voluntary command to appropriate motor neurons

primary motor cortex descending system

send information about length and tension

proprioceptors

muscle atrophy decreases the number of ______________.

protein molecules (actin/myosin) NOT MUSCLE FIBERS

Muscles can only _____ never _____

pull, push

How is it impossible to achieve summation of action potentials?

refractory period occurs

calmodulin

regulatory protein that facilitates contraction in smooth muscles, calcium binding

functional unit of muscle

sarcomere (smallest component that can perform all functions of the organ)

person becomes aware of pain once impulse reaches __________.

sensory area of the cortex

Dihydropiridine Receptors in smooth muscles

serve as calcium channels

Foot proteins (ryanodine receptors)

serve as calcium release channels, in sarcoplasmic reticulum

how is tension transmitted from sarcomere to bone?

shortening of sarcomere ---> tightens series elastic component ---> force applied to bone moving it against a load

precise motor activity

smaller amount of motor fibers

clear cut length tension relationship is not present in

smooth muscle

has no T-tubules

smooth muscle

myosin molecules are arranged in a way so that cross bridges are present along the entire filament length in what muscle?

smooth muscle

only tropomyosin can be found in

smooth muscle

slowest speed of contraction

smooth muscle

to allow actin/myosin cross bridge in skeletal vs. smooth muscle

smooth muscle = chemical change skeletal muscle = physical change

what initiates slow wave potentials?

specialized clusters of nonmuscle pacemaker cells within the digestive tract and spread to adjacent smooth muscle via gap junctions

slow wave potentials

spontaneous; membrane potential undergoes gradually alternating depolarizing and hyperpolarizing swings

cells that repair minor injury in muscle

stallate cells (through giving rise of myoblasts)

maximum tension cannot be reached

start anywhere other than Lo

Rigor Mortis

stiffness of death; Ca+ released in excessive amounts as it leaks out of lateral sacs causing the troponin-tropomyosin complex to move aside and the actin and myosin contract. Because ATP is needed to detach them, and it cannot be produced after death, actin and myosin are attach forever.

Important for maintaining posture

submaximal isometric contractions

How does Ca+ get into the cytosol?

surface --> T-tubules --> SER --> cytosol

striations

t-tubules present (cardiac and skeletal)

Contraction ends when...?

the cross bridges become inactive and the reuptake of Ca+2 is finished

End plate potential

the depolarizations of skeletal muscle fibers caused by neurotransmitters binding to the postsynaptic membrane in the neuromuscular junction

while the rise in cytosolic calcium is what triggers contractions in skeletal and smooth muscle, the difference is in...?

the type of change induced (chemical/physical)

A- band

thick filaments + overlapping thin

H zone

thick filaments only, near M-line

Contractile activity at lengths greater than Lo

thin filaments pulled out of thick filaments

latent period

time between application of a stimulus and the beginning of a response in a muscle fiber

coiled portion at the end of the sacromere that connects the m-line to the z-disc

titin

involved in the complex pathway of muscle enlargement during weight lifting (signal transduction)

titin

Largest protein in the body?

titin (30,000 A.A)

examples of skeletal muscles that are not attached to 2 ends but produce movement

tongue external eye circular sphincters

can maintain low level tension even without action potentials, therefore are too myogenic

tonic single unit smooth muscle, why? - have sufficient cytosolic calcium

type of smooth muscle that maintains tone?

tonic smooth muscle

_________ covers actin sites that bind to cross bridges

tropomyosin

Troponin when not bound to Ca2+

troponin stabilizes tropomyosin in blocking position over actin's cross-bridge binding sites

How is tension generated by sarcomeres transmitted to bones when they do not come in contact?

via tendons

single unit

visceral smooth muscle ex: uterus gap junctions during birth

rhythmic activity examples

walking or chewing

example of tonic smooth muscle?

walls of arterioles

where is phasic smooth muscle most abundant?

walls of hollow digestive organs

Ca+ binds to troponin causing tropomyosin to change shape

what happens after calcium diffuses into cytosol?

Why may someone not be able to support a heavy object?

when all motor fibers participate, since it is a maximal contraction, it is impossible to alternate motor unit activity and prevent fatigue

how is a light chain phosphorylated in smooth muscle?

when inorganic phosphate attaches to it

when is energy released during creatine phosphate?

when the bond between creatine and phosphate is broken

when can myosin attach to actin in smooth muscle?

when the light chain is phosphorylated

levels of organization of muscle:

whole muscle muscle fiber (cell) myofibril (intracellular structure) cytoskeletal elements (thick and thin filaments) protein molecules (myosin and actin)

How do we get ATP?

- creatine phosphate (high energy phosphate from creatine phosphate to ADP) - oxidative phosphorylation (electron transport system +chemiosmosis) - glycolysis

gradation accomplished in single-unit smooth muscle

- cytosolic calcium concentration (through myogenic activity) - influences of autonomic nervous system, mechanical stretch, local metabolites, hormones

pain reflexes

1. withdraw (can override it with cortical influence) 2. cross extensor (require activation of both sides of the body)

skeletal muscle body weight

40% in males, 32% in females

Glycolysis

A metabolic process that breaks down carbohydrates and sugars through a series of reactions producing pyruvate which is then converted to lactate when there is a lack of oxygen

sarcoplasmic reticulum

A modified form of endoplasmic reticulum; stores calcium that is used to trigger contraction when muscle is stimulated

motor unit

A motor neuron and all of the muscle fibers it innervates

What leads to contraction of muscle fiber

Binding of myosin and actin at cross bridges

T/F: actin and myosin can only be found in muscles

F, they are only more abundant and organized in muscles

______ becomes smaller as thin filaments move inward, ______ narrows.

H-zone, I-band

what causes striations?

Highly organized internal arrangements, muscle fibers lying parallel to one another then bundled together by connective tissue.

Consists of thin filaments that do not overlap

I - band

Why does the width of the A-band remain unchanged as the thin filaments are pulled inward?

It is determined by the thick filaments, which do not change length during contraction

how is fatigue a protective mechanism?

It prevents reaching a point with no ATP (rigor)

genetic endowment of muscle fiber types

Largely determined by the type of activity for which the muscle is specialized

Multineural

Neural integration of muscle action Communicates with subcortical areas: - Basal ganglia - Thalamus - Hypothalamus - Reticular formation

serves as source of cytosolic calcium in all muscle types

SR

Dihydropiridine Receptors

Serve as voltage sensors in Transverse Tubules, activated when local depolarization of T tubules occurs. They trigger the opening of foot proteins

presence of gap junctions

Skeletal - no Cardiac - yes at intercalated discs Smooth - yes in unitary muscle (mostly single unit)

How does the muscle relax?

Through the reabsorption of Ca+

monitors motor commands from motor cortex and sensory feedback from muscles

cerebellum

how does smooth muscle permit the binding of cross-bridges

chemically brings about the phosphorylation of myosin cross bridges in thick filaments so that they can bind with actin

work

force x distance

dense bodies

found in smooth muscle, since they do not contain sarcomeres, they do not contain z-lines, so the dense bodies contain the same proteins the z-line contains.

what links fibers of single-unit smooth muscle?

gap junctions

What connects cardiac muscle cells?

gap junctions in intercalated discs

when oxidative phosphorylation cannot keep up

glycolysis

slower relaxation

slower removal of calcium

all fibers in a motor unit are

same type

The T tubules dip deep into

sarcolemma, between the A and I bands of myofibrils, allowing action potentials to reach deep into the muscle

reciprocal innervation

simultaneous stimulation of nerve supple and inhibition of nerves to antagonist muscle

What forms a functional syncytia?

single unit smooth muscle or visceral smooth muscle

single-unit vs multiunit smooth muscle

single unit: gap junctions so contractions occur as one unit myogenic (modulate by stretch, neural, and hormonal control) Ex. GI, uterus, small diameter blood vessels -> typically move contents within hollow organs multiunit: few to no gap junctions neurogenic-faster change in force via recruitment ex. hair and feather erectors, lens, iris **DO NOT have APs, cells are small enough Ca entry depolarization and graded potentials to spread

may be phasic or tonic

single-unit smooth muscle

How do each of the types of muscles maintain homeostasis

skeletal : generating heat, chewing, breathing، swallowing smooth: regulates the movement of blood, food, air, urine cardiac: pumps life sustaining blood

striated muscle

skeletal and cardiac muscle

troponin and tropomyosin

skeletal and cardiac muscle

ECF does not serve as source of cytosolic calcium in

skeletal muscle

Most abundant muscle?

skeletal muscle

no gap junctions

skeletal muscle

which type of muscle cannot be modified by hormones?

skeletal muscle

sarcoplasmic reticulum is best developed in _________ and most poorly developed in ____________.

skeletal muscle, smooth muscle

all types of muscle use the same mechanism of contraction which is

sliding filament mechanism

which type of fiber has the highest resistance to fatigue and why?

slow oxidative, has most capacity for ATP formation

muscle memory

the process by which the brain memorizes muscle coordination (motor skills) through repetition and stores the info. in the cerebellum

Sustained elevation in cytosolic calcium and more time to stretch tendons results in...?

twitch summation

What happens when muscle fiber is stimulated a second time before it has relaxed?

twitch summation: second contractile response or piggyback on top of the first twitch add together.

Myosin made up of

two identical heads

the sarcomere is the area between

two z-lines

The shortening of the muscle is in reality...

z lines being brought closer together as thin filaments attached to them slide inward

cytoskeletal disc that ,m,connects the thin filaments or two adjoining sarcomeres

z-line