muscle contraction

stimulation and contraction of single skeletal muscle cells

• excitability

• contractility

• extensibility

• elasticity

excitability

• aka responsiveness or irritability

• ability to receive and respond to a stimulus

• electrical signal gets sent to muscle immediately

contractility

• ability to shorten (contract) when an adequate stimulus is received

• second signal

extensibility

ability of muscle cells to be stretched

elasticity

• ability to recoil and resume resting length after stretching, reset

depolarization

• electrical contractility

• ex.) heartbeat, moving a muscle

the nerve stimulus and action potential

• skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract

• motor unit

• neuromuscular junction

• synaptic cleft

• action potential (electrical impulses) reaches the axon terminal of the motor neuron (efferent)

• calcium channels open and calcium ions enter the axon terminal

• efferent

• nerve and muscles: DO NOT TOUCH

motor unit

one motor neuron and all the skeletal muscles stimulated by that neuron

neuromuscular junction

• association site of axon terminal of the motor neuron and muscle

• entire area involved in transmission of nerve impulse to muscle

axon terminal

• end of nerves

• gold bulb

synaptic cleft

• gap between nerve and muscle

• nerve and muscle do not make contact

• area between nerve and muscle is filled with interstitial fluid

efferent

• fibers that go from spinal cord into muscles (going away from nervous system)

  • nerves that come out of spinal cord that go to muscles come out of anterior/ventral side

transmission of nerve impulse to muscle

1.) action potential (electrical impulse) travels down the axon terminal of motor neuron, jumpstarting mitochondria

2.) calcium channels open up and Ca2+ (as released by sarcoplasmic reticulum) enters the axon terminal

3.) Ca2+ entry causes some synaptic vesicles to release their contents (pops) (acetylcholine, a neurotransmitter) by exocytosis

4.) acetylcholine goes across synaptic cleft and binds to receptors (purple channels) in the sarcolemma (gray membrane)

5.) ACh binds sarcolemma making cellular membrane permeable, so Na+ can go in and K+ rushes out

the sliding filament theory of muscle contraction

• activation by nerve causes myosin heads (cross bridges) to attach to binding sites on the thin filament

  • cross bridges allows to hook on actin

• myosin heads then bind to the next site of the thin filament and pull them toward the center of the sarcomere

  • when calcium comes in, the actin spins, revealing their binding sites and cross bridges form, and ADP gets converted to ATP

• this continued action causes a sliding of the myosin along the actin

• the result is that the muscle is shortened (contracted)

graded responses

different degrees of skeletal muscle shortening

• graded responses can be produced by changing

  • the frequency of muscle stimulation

  • the number of muscle cells being stimulated at one time

types of graded responses

• twitch

• summing of contractions

• unfused (incomplete) tetanus

• fused (complete) tetanus

twitch

• single, brief contraction

• not a normal muscle function

summing of contractions (summation)

• one contraction is immediately followed by another

• the muscle does not completely return to a resting state due to more frequent stimulations

• the effects are added

• treppe: starts to relax, then gets more tense

unfused (incomplete) tetanus

• looks like big stairstep, building upon one another

• some relaxation occurs between contractions but nerve stimuli arrive at an even faster rate than summing on contractions

• unless the muscle contraction is smooth and sustained, it is said to be unfused tetanus

fused (complete) tetanus

• no evidence of relaxation before the following contractions

• frequency of stimulations does not allow for relaxation between contractions

• the result is a smooth and sustained muscle contraction

• spasm that is maintained (does not relax)

muscle response to strong stimuli

• muscle force depends on the number of fibers stimulated

• more fibers contracting results in greater muscle tension

• muscles can continue to contract unless they run out of energy

energy for muscle contraction

• initially, muscles used stored ATP for energy

  • ATP bonds are broken to release energy

  • only 4-6 seconds worth of ATP is stored by muscles

• after this initial time other pathways must be utilized to produced ATP

• direct phosphorylation of ADP by creatine phosphate (CP)

• aerobic respiration

• anaerobic glycolysis and lactic acid formation

direct phosphorylation of ADP by creatine phosphate (CP)

• muscle cells store CP

  • CP is a high-energy molecule

• after ATP is depleted, ADP is left

• when ADP and creatine phosphate join together, they make ATP

  • creatine separates by itself, phosphate joins ATP

    • CP + ADP = creatine + ATP

• makes quick energy

• CP supplies are exhausted in less than 15 seconds

• about 1 ATP is created per CP molecule (1:1 ratio)

aerobic respiration

• most efficient way to create ATP

• uses oxygen

• glucose is broken down to carbon dioxide and water, releasing energy (about 32 ATP)

• a series of metabolic pathways occur in the mitochondria

• this is a slower reaction that required continuous oxygen

• glucose turns into pyruvic acid, and since oxygen is present, allows the use of fatty acids and amino acids (proteins)

anaerobic glycolysis and lactic acid formation

• reaction that breaks down glucose without oxygen

• glucose is broken down to pyruvic acid to produce about 2 ATP

• pyruvic acid is converted to lactic acid via glycolysis

• this reaction is not as efficient, but is fast

  • huge amounts of glucose are needed

  • lactic acid produces muscle fatigue

muscle fatigue and oxygen deficit

• when a muscle is fatigued, it is unable to contract even with a stimulus

• common cause for muscle fatigue is oxygen debt

  • oxygen must be “repaid” to tissue to remove oxygen deficit

  • oxygen is required to get rid of (wash out) accumulated lactic acid

• increasing acidity (from lactic acid) and a lack of ATP causes the muscle to contract less