Unit 7C: Physiology of Muscle Contraction

sliding filament mechanism

mechanism of muscle contraction, myofilaments slide past each other and causes the entire sarcomere to shorten

examination of sliding filament mechanism from anatomy perspective

Thin myofilaments sliding past thick filaments towards the M line. The A bands move closer, I bands shorten, distance between Z disc decrease, & H band disappears. Nerve cells signal action potential involving Na, K, and neurotransmitters. The action potential releases Ca+ from the SR and causes an increase in Ca+ in the sarcoplasm.Ca+ binds to troponin/tropomyosin complex to destabilize and shift complex. This reveals the myosin bind site on the actin. Myosin becomes phosphorylated and binds to actin to form the crossbridge formation. ADP and P are released and head returns to starting position this process causes the low energy conformation of myosin. The cross bridge detaches and new ATP binds, myosin unbinds and muscles relax.

nerve cell causes an action potential to occur across the sarcolemma

Nerve cells signal action potential involving Na, K, and neurotransmitter

action potential travels down the T tubules

The action potential releases Ca+ from the SR and causes an increase in Ca+ in the sarcoplasm.

calcium binds to troponin/tropomyosin complex

to destabilize and shift complex, This reveals the myosin bind site on the actin

myosin becomes phosphorylated

myosin ATPase as the enzyme in ATP hydrolysis, once phosphorylated myosin has PE to do work

myosin ATPase

enzyme on the head of myosin, performs rxn of ATP hydrolysis

ATP hydrolysis

ADP + P

when myosin has the ability to do work so what does it move

move its head, head becomes perpendicular to actin, the energy from phosphorylation is used to achieve the perpendicular orientation (high energy conformation of myosin)

crossbridge formation

myosin binds to actin

powerstroke

ADP & P are released from head, since P gave myosin the PE to change to the high energy position, losing the P removes the PE so the head must change back to the original parallel position (low energy conformation of myosin) changing conformation of myosin head pulls the actin filament towards M line

crossbridge detachement

new molecule of ATP binds, myosin unbinds to actin, muscle relaxes

synapse

place where a nerve cell meets either another nerve cell or effector organ

effector organ

organ the nerve controls, can be a muscle (smooth, cardiac, or skeletal) or a gland (such as the pancreas). In this chapter, we are focusing on a skeletal muscle being the effector organ

neuromuscular junction (NMJ)

a special type of synapse where the axon of a nerve meets a muscle cell.

synaptic cleft

the small space between effector organ & axon of the nerve cell. The organ and axon do not physically touch, there is a small space between them called the synaptic cleft

What happens in nerve cell at NMJ?

The nerve cell releases a neurotransmitter (NT) by exocytosis called acetylcholine (ACh) from its axon,

What does ACh do after it is released?

Travels across synaptic cleft. Binds ACh receptors on sarcolemma. Triggers action potential in muscle cell

We have seen the nerve cell starts muscle contraction by releasing ACh...but how does nerve cell stop contraction?

1) Nerve cell stops releasing ACh
2) ACh still remaining in the synaptic cleft is degraded

Definition of Acetylcholine esterase: an enzyme in the synaptic

cleft that destroys ACh
3) Ca+ pumped backed into the SR. Since calcium is not present, the myosin bind sites on actin are covered up. Since myosin is unable to bind, the muscle cannot contract.

motor unit

a nerve cell and all the muscle cells it controls. The average motor unit involves about 200 muscle cells.

small motor units

that produce fine movements like finger & eye movements.

large motor units

that produce powerful movements

Muscle cells have 3 mechanisms to produce ATP which are

direct phosphorylation by creatine phosphate, glycolysis, aerobic respiration

Direct phosphorylation by creatine phosphate (CP)

phosphate storage molecule that transfers phosphate to ADP to form ATP, Creatine phosphate + ADP → Creatine + ATP, Creatine phosphate supply is then replenished inside the cell, no oxygen required, 1 ATP per phosphate, 15s long, energy source is creatine phosphate

glycolysis

no oxygen required (anaerobic), 2 atp per glucose, 30-60s long, energy source is glucose. advantageous because it produces ATP very fast. Produces it over 2x faster than aerobic respiration. However, there is a disadvantage of anaerobic respiration. The disadvantage is it harvests only 5% of the total energy in the glucose molecule. When some muscles are forced to use anaerobic mechanisms (such as happens when their oxygen supply decreases), muscle cramps result from the build up of lactic acid

What happens to lactic acid produced from anaerobic respiration?

It diffuses out of the muscle cell and enters blood. The body can use lactic acid for many processes: a) Liver can convert it to pyruvic acid or glucose. The pyruvic acid and glucose can then used for energy b) Organs like brain, kidney , & heart can make ATP directly from lactic acid

Aerobic cellular respiration

oxygen required, glycolysis —> Krebs cycle —> electron transport chain, 36 atp per glucose, hours long, energy course is glucose

muscle tone

when relaxed, muscles are in state of slight contraction This slight state of contraction is not enough to produce movement (such as whole body movement) this is because the myofilaments are not completely pulled towards the M line

Functions of muscle tone

Maintains posture such as holding head upright when walking, Stabilizes joints such as keeping bones of legs aligned to stabilize knee joint while walking, Keep muscles ready to respond if called upon to contract.

muscle tension

force a contracting muscle exerts on an object

load

opposing force exerted on muscle by object being moved. Load can be thought of as the weight of the object because load is proportional to weight.

What happens if tension > load?

Tension overcomes load and the load is moved. Example: muscles of your arm can generate enough tension to overcome load of your A&P text and you can pick up the textWhat happens if tension is < load?

What happens if tension is < load?

Tension does not overcome load so the load is not moved. Example: muscles of your arm cannot generate enough tension to overcome the load of your car so you cannot pick up your car

factors influencing amount of tension produced

muscle size, arrangement of fascicles, size of the motor unit, frequency of nerve stimulation, number of motor units activated, length of fiber before contraction,

Muscle size influencing amount of tension produced

a larger muscle has more muscle fibers (cells); more fibers means more tension can be produced

Arrangement of fascicles influencing amount of tension produced

some arrangement allow for more fibers to be contained within the fascicle; more fibers means more tension

Size of the motor unit influencing amount of tension produced

larger motor units control more muscle fibers than small motor units; more fibers means more tension

Frequency of nerve stimulation influencing amount of tension produced

a muscle contracts when it is stimulated by a nerve cell; more frequent stimulation means the muscle is stimulated to contract more; more contraction means more tension

Number of motor units activated influencing amount of tension produced

more motor units stimulated, means more fibers contracting; more fibers means more tension

overly stretched

If fibers are in the overly stretched position before contraction, how does it influence muscle contraction?

It causes less tension to be produced

medium stretched

If fibers are the medium stretch position before contraction, how does it influence muscle contraction?

It will result in optimum tension being generated

under stretched

If fibers the overly contracted state before contraction, how does that influence muscle contraction?

It will result in less force being produced

Exercise can also influence the amount of tension produced

resistance exercises and aerobic exercises

Resistance exercises

result in enlargement of muscle cells over time. are ones where contraction is done against a high load force. Example lifting weights

Aerobic exercises

results in more efficient metabolism inside the muscle cell because it increases the number of capillaries, the number of mitochondria, and the amount of myoglobin. are endurance activities. Example running

muscle twitch

motor unit response to 1 action potential Duration: 20 to 200 milliseconds

myogram

a graphic representation of muscle twitch, 3 phases latent, contraction, relaxation

Latent

this is a period before contraction actually begins and corresponds to the time it takes for action potential to cause muscle contraction. During this time, Ca is being released from the SR and myosin begins forming crossbridges

Contraction

this is the period when contraction is happening

Relaxation

this is period after muscle contraction when the muscle relaxes and corresponds to the time when Ca is going back into the SR and the filaments return to their original position

Whether it is fast or slow is determined by

the speed at which the myosin ATPase hydrolyzes ATP.

fast twitch muscle

has a fast response because the myosin ATPase hydrolyzes ATP fast

slow twitch muscle

has a slow response because the myosin ATPas hydrolyzes ATP slower

muscle fatigue

physiological inability to contract even though stimuli is still being sent from the nerve cell

Causes of muscle fatigue

Build up of K+: interferes with ability to generate action potential across sarcolemma, Build up of ADP & P: this slows down the function of the myosin ATPase, Lactic acid accumulation: lowers pH of sarcoplasm which then interferes with the role of Ca+, Depletion of glycogen & glucose

types of skeletal muscle fiber

slow oxidative, fast oxidative, fast glycolytic

slow oxidative

slow contraction speed, slow ATPase speed, aerobic, high myoglobin content, low glycogen storage, slow fatigue (resistant), red, small fiber diameter, many mitochondria, many capillaries, good for endurance (running and maintaining posture)

fast oxidative

fast contraction speed, fast ATPase speed, primarily aerobic, high myoglobin content, intermediate glycogen storage, intermediate fatigue (moderate resistance), red to pink, intermediate fiber diameter, many mitochondria, many capillaries, sprinting, walking

fast glycolytic

fast contraction speed, fast ATPase speed, anaerobic, low myoglobin content, high glycogen storage, fast fatigue (fatigable), white, larger fiber diameter, few mitochondria, few capillaries, short term powerful releases like hitting baseball stop and go activities