Unit 1: muscle

Skeletal muscle

-aka voluntary muscle -attached to bones and skin -muscle fibers are longest of all muscle and have striations due to actin and myosin -multi-nucleated -contract rapidly, tire easily -the structure allows its function

4 characteristics that all muscle shares

1. excitability: ability to receive and respond to stimuli by change in membrane potential 2. contractility: ability to shorten forcibly when stimulated 3. extensibility: ability to be stretched beyond resting length 4. elasticity: ability to recoil to resting length

What are the functions of muscle?

1. production of movement (ex: walking, digesting, pumping blood) 2. maintain posture and body position 3. joint stabilization 4. generate heat

Muscle Morphology

-the name of a muscle is often based on the shape of the muscle

Force generation in muscle

-the max force potential of a muscle is proportional to the sum of the cross-sectional area of all its muscle fibers -the larger the CSA, the more force that can be generated -pennate muscles can produce more force than fusiform muscles of similar volume (more fibers fit into a given area due to angle of pennation)

Connective tissue sheaths from external to internal

1. Epimysium - dense irregular connective tissue surrounding the entire muscle ; may blend with fascia 2. perimysium- fibrous connective tissue surrounding fascicles (groups of muscle fibers) 3. endomysium- fine connective tissue surrounding each muscle fiber all connective tissue sheaths are continuous with one another and the tendon

muscle stricture hiearchy from largest to smallest

1. muscle 2. muscle fibers 3. myofibrils 4. myofilaments

Muscle fiber (cell) Microanatomy

-skeletal muscle fibers and long, cylindrical, multinucleate cells -consists of sarcolemma, sarcoplasm, and modified organelles (myofibrils, sarcoplasmic reticulum, t tubules)

Sarcolemma

muscle fiber plasma membrane

Sarcoplasm

muscle fiber cytoplasm

Myofibrils

-densely packed, rod -ike elements within muscle fibers -single muscle fiber can contain 1000s of myofibrils -striated appearance due to myofilaments (actin and myosin)

Myofilaments

-create striated appearance of muscle fiber -actin filaments: thin filaments -myosin filaments: thick filaments *myosin and actin are organized into repeated functional structures called sarcomeres

Sarcomere

-smallest, contractile unit (functional unit) of muscle fiber -individual sarcomeres align end to end along myofibril

z disk

start and end of a single sarcomere

A band

overlapping of actin and myosin

I band

only actin filaments

H zone

myosin fibers only

M line

dead center

Active proteins in a sarcomere

1. troponin-hold tropomyosin in place 2. tropomyosin-covers binding sites from myosin 3. contractile proteins (actin and myosin)

Passive proteins in sarcomere

1. noncontractile proteins ( titin, desmin, dystrophin) 2. extracellular connective tissue (collagen and elastic)

Actin

-thin myofilaments made from actin protein -has sites for myosin head attachment during muscle contraction

Myosin

-thick filaments made from myosin protein -cross bridges: during contraction, myosin heads attach to actin and link thick and thin filaments together

tropomyosin

-a myofilament protein that is bound to actin -blocks myosin binding sites on actin filaments in a relaxed muscle

troponin

-myofilament protein that is bound to actin -anchors tropomyosin to active site; binds with calcium during contraction and pulls tropomyosin away from binding sites on actin

titin

-myofilament protein -the core of myosin (anchors z disc and m line; holds myosin in place) -elastic; allows recoil after stretch

Dystrophin

-myofilament protein -bound to actin -links thin filaments to integral proteins in sarcolemma

Sliding Filament Model of Muscle Contraction

1. electrical impulse (action potential) from nervous system 2. calcium binds to troponin and troponin pulls tropomyosin off binding sites on actin 3. myosin binds to actin and forms myosin crossbridges 4. myosin cross bridge attachments form and break several times (pulls actin filaments toward center of sarcomere) During muscle contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more

sarcoplasmic reticulum

-smooth endoplasmic reticulum of muscle fiber -tubules of SR surround each muscle fiber -stores and released Calcium

T tubules

-tubes formed by protrusion of sarcolemma deep into cell interior -encircle each sarcomere and pass from one myofibril to the next -conduct electrical impulses to every sarcomere (electrical nerve impulses signal the release of calcium from SR)

Neuromuscular interaction

-each muscle fiber is innervated and controlled by a somatic neuron -a muscle fiber must be stimulated by a nerve impulse to contract -a motor neuron sends nerve impulses to specific muscle cells

what must stimulate a muscle fiber to cause contraction?

a nerve impulse

Electrical impulse > T tubules > electrical impulse reaches Sarcoplasmic reticulum > release of calcium

Motor Unit

-consists of a motor neuron and all the muscle fibers it innervates

motor neuron

-cells that regulate movement -activation of a motor neuron causes contraction of all associated muscle fibers -precise control needed : motor unit with very few muscle fibers -strength needed: motor unit contains 100s of muscle fibers

how many motor neurons innervate a single muscle fiber?

One single motor neuron

neuromuscular junction

-connection between axon of a somatic motor neuron and sarcolemma -NMJ anatomy to know: terminal bouton, motor end plate, synaptic cleft, synaptic vesicles

Steps of Excitation

1. nerve impulses reach terminal bouton 2. synaptic vesicles in terminal bouton stimulated to release ACh (acetylcholine) into synaptic cleft 3. ACh binds to receptors on sarcolemma causing formation of a muscle impulse (electrical event) 4. muscle impulse spreads over sarcolemma from sarcoplasmic reticulum 6. initiates contraction cycle

Steps of the contraction cycle

1. calcium binds to troponin and moves tropomyosin off the myosin binding site on actin filament 2. myosin binds to actin to form cross bridges 3. uses energy from ATP to ADP to perform a power stroke 4. power stroke shortens the sarcomere by pulling acting toward the middle 5. New ATP binds to myosin and allows it to detach from actin 6. ATP hydrolized to ADP 7. myosin used the energy released to re-attach to myosin *this cycle is repeated rapidly to contract muscle

What 4 things are required in order for a muscle to contract?

1. electrical signal from motor neuron 2. Acetylcholine (ACh) 3. Calcium 4. ATP

What causes the end of a muscle contraction?

1. motor neuron stops stimulating the muscle fiber 2. acetylcholinesterase(enzyme) decomposes the ACh remaining in the synaptic cleft to prevent ongoing muscle fiber stimulation 3. calcium is actively transported back into the SR -calcium unbinds from troponin -tropomyosin moves back over myosin binding site

Threshold stimulus

the minimal stimulus that will cause a muscle fiber to contract

what is the all or none response?

-rule for muscle fibers in an individual motor unit -the fibers will either contract completely, or not at all! -if the threshold stimulus is reached, the fibers are contracting!! *contraction is NOT proportional to stimulus strength

What is a muscle twitch?

-a muscle fiber's response to ONE SINGLE action potential from a motor neuron -muscle fiber contracts quickly, and then relaxes (muscles can contract faster than they can relax)

what are the 3 phases of a muscle twitch?

1. latent period: a pause before the muscle contracts. during this, the events of excitation-contraction coupling are happening 2. period of contraction: cross bridge formation. Muscle tension increases (this is when the twitch occurs) 3. period of relaxation: muscle tension declines to 0 and the calcium re-enters SR

What are muscle responses graded by?

-graded muscle responses vary strength of contraction for different demands -responses are graded by: >changing frequency of stimulation (increases stimulation of a single motor unit) > changing strength of stimulation (recruits additional motor units/muscle fibers)

what are the types seen for change in stimulation frequency?

wave (temporal) summation and fused tetanus -when frequency is changed, that frequency is only changing for one single motor unit

Wave (temporal) summation

• a type of change in frequency -results if stimuli are received by a muscle in rapid succession -muscle fibers do not have time to fully relax, so twitches increase in force with each stimulus -produces a smooth, continuous contraction that adds up

Fused (complete) tetanus

-freuquency of stimulation is so high that muscle has no period of relaxation

Size principle

-motor units with smallest muscle fibers are recruited first, followed by larger fibers as stimulus intensity increases -this is because smaller fibers have lower thresholds for activation -the largest motor units are only activated for very powerful contractions

What types of stimuli are involved in muscle fiber recruitment?

1. subthreshold stimulus- stimulus is not strong enough, so no contraction is seen 2. threshold stimulus- stimulus is strong enough to cause first observable contraction 3. maximal stimulus- strongest stimulus that increases maximum contractile force (all motor units have been recruited)

What are the 3 types of skeletal muscle fibers?

1. slow oxidative 2. fast oxidative 3. fast glycolytic most muscles contain a mixture of fiber types but all fibers in one motor unit are the same type

What two characteristics are muscle fiber type classified by?

1. speed of contraction -fiber may contract slow or fast depending on: -how quickly myosin ATPase split ATP -pattern of electrical activity of motor neurons -duration of contraction dependent on how quickly calcium is pumped back into SR 2. metabolic pathways used for atp synthesis -oxidative fibers: use aerobic pathways -glycolytic fibers use anaerobic pathways

slow oxidative fibers (type 1)

-best for low intensity, endurance activities such as maintaining posture -have slow contraction time -nearly fatigue resistant -recruited first

fast oxidative fibers (type 2a)

-good for medium intensity activities such as walking or sprinting -dont fatigue super quickly

fast glycolytic fibers (type 2b)

-for short term, intense, powerful movements like hitting a baseball -contract quickly -fatigue quickly -recruited last

passive length tension

-generated when non-contractile/passive proteins in a muscle are stretched -once reaching critical length, you begin to feel tension in muscle (ex: stretching hamstrings)

active length tension

-a change in muscle fiber length=change in actin/myosin overlap -ideal resting length allows for the most crossbridges to form -when muscle is fully lengthened, sacomeres are too far apart to form cross bridges, and when muscle is completely shorted, crossbridges are already formed so new ones cant form

total muscle force

-combination of active and passive tension of a muscle -past resting length, passive tension increases and active tension decreases

active insufficiency

-occurs at a multi-joint muscle -when muscle is already active at one joint, so it is unable to effectively perform its action at the other joint. (example: hamstring cant flex knee and extend hip to max capabilites simultaneously)

passive insufficiency

-occurs at a multijoint muscle -when a multi-joint muscle is unable to lengthen sufficiently to allow full ROM at both joints

agonists

-the muscle producing movement

antagonist

-the muscle opposing movement -usually on opposite side of body of agonist

synergist

a muscle that assists the prime mover

force couple

occurs when two or more muscles simultaneously generate forces in different linear directions, but produce torque in the same rotary direction

what are the two types of isotonic muscle contractions?

1. concentric -muscle shortens and does work 2. eccentric- muscle lengthens and generates force

isometric muscle contraction

-muscle neither shortens or lengthens but still develops tension -naturally occurs most often in muscles responsible for maintaining upright postures and in muscles stabalizing certain joints

force-velocity relationship

-relationship between max force output and velocity of contraction -concentric max velocity occurs with smaller loads. force production is inversely proportional to muscle shortening -eccentric max velocity occurs with larger loads. force productin directly proportional to muscle lengthening

power

-power = rate of work (force x velocity) >positive work = concentric contraction >negative work = eccentric contraction

muscle tone

-state of a partial contraction in a relaxed muscle (does not produce movements) -keeps a muscle ready to respond -due to alternating activation of different motor units

muscle fatigue

-the inability to contract a muscle, despite ongoing stimulation -intense, short duration exercise causes rapid fatigue via ionic imbalance that recovery quickly prolonged, long intensity (ex: running a marathon) causes gradual fatigue that takes longer to recovery from

hypertrophy

-increase in muscle fiber SIZE and STRENGTH -increased number of myofibrils -number of muscle fibers (cells) does not change -caused by repetitive stimulation to maximum contraction

effects of aerobic/endurance training

-enhances efficiency of aerobic respiration -increases number of capillaries, mitochondria, ability to obtain oxygen from blood, and increase myoglobin concentration

effects of immobilization

-reduction is muscle fiber size (decrease in number of myofibrils) -nerve damage or lack of use could result in atrophy

aging effects

-sarcopenia-loss of muscle tissue (muscle fibers,cells) -loss is greater in type 2 fibers -reduces strength, power and speed of contraction -decreases neuromuscular efficiency

Describe in detail how a muscle contraction happens

1. nervous system generates an impulse(action potential) that travels down a motor neuron to the neuromuscular junction 2. signal travels down nerve axon into the terminal bouton and stimulates the synaptic vesicles to release acetylcholine (ACh) 3. ACh gets released into the synaptic cleft and travels to the motor end plate of the muscle sarcolemma where it binds with receptors causing a muscle impulse (electrical event) 4 From here the impulse travels over sarcolemma and along t-tubules that stimulate the release of calcium by the sarcoplasmic reticulum, initiating the contraction cycle 5. Calcium binds to troponin on the actin myofilament which pulls tropomyosin off the active site. Myosing forms a cross-bridge with actin filament, and energy is used to turn ATP to ADP and perform a power stroke, shortening the sarcomere as actin is pulled towards myosin. 6. At end of contraction process, the motor neuron stops stimulating the muscle fiber and acetylcholinesterase decomposes the ACh inside synaptic cleft. 7. Calcium is transported back to sarcoplasmic reticulum, and tropomyosin covers up the myosin binding site.

What is the force potential of a muscle proportional to?

-the sum of the cross-sectional area of all its muscle fibers

what is a fascicle?

a group of muscle fibers

what causes the striated appearance of a muscle?

myofilaments (actin and myosin)

what is the smallest contractile unit of a muscle?

sarcomere

Where is calcium stored in a muscle?

sarcoplasmic reticulum

At what position can a muscle generate the most force?

mid range