muscle contractions

motor unit

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

• AP in neuron of motor neuron = contraction of all muscle fibers in the unit

• unique → vary in size and sensitivity

muscle twitch

• response of a muscle fiber to a single AP along its motor neuron

• 3 phases

  • lag: between stimulus and muscle response

  • contraction

  • relaxation

stimulus frequency

higher frequency = muscle will contract with greater force with each successive stimulus

subthreshold stimulus

not strong enough to cause an AP in any of the axons in a nerve → no contraction

threshold stimulus

strong enough to produce an AP in a single motor unit axon → contraction

submaximal stimulus

progressively stronger → produce AP in axons of additional motor units

maximal stimulus

produce AP in axons of all motor units of the muscle

multiple-motor unit recruitment

• each motor unit is in an all-or-none fashion

• more motor units activated = greater force of muscle contraction

  • increased tension

treppe

tension produced increases for the first few contractions

• Ca²⁺ released during first stimulus not completely removed before second stimulus

• Ca²⁺ accumulates → more cross-bridge formation → more efficient contraction

  • tension for each contraction increases

• can also result from

  • maximal stimulus at low frequency

  • increasing rate of enzyme activity

adenylate kinase

transfers one P from an ADP to a second ADP

• result: one ATP and one AMP

• produces a few seconds of ATP

• no oxygen required

• 1 ATP yield per ADP

• type of work supported: moderate/extreme exercise

creatine kinase

• during periods of rest → muscle fibers accumulate extra ATP

• extra ATP utilized to transfer P from one ATP to small protein called creatine

  • makes creatine phosphate: bank for high energy phosphate

• creatine kinase transfer P from creatine phosphate to ADP → make ATP when levels are low

• no oxygen required

• 1 ATP yield per creatine phosphate

• type of work supported: moderate/extreme exercise

anaerobic respiration

• no O₂ needed

• breakdown of glucose → make ATP and lactate

• only produce enough for 30-40 seconds of contraction

• steps

  • glycolysis → produce 2 pyruvate and 2 ATP

  • pyruvate turned into lactate → can be broken down or used to make glucose

aerobic respiration

• O₂ required

• breaks down glucose → produce 36 ATP, CO₂ and H₂0

• hours of contraction result

• occurs in mitochondria

• more efficient

• Steps

  • Glycolysis → glucose broken down into two pyruvate molecules

  • Pyruvate molecules taken to mitochondria → processed through citric acid cycle

  • Electron transport chain occurs

levers

machines that consist of a rigid beam that can pivot/move at a hinge

• fulcrum (F): hinge → joints

• weight (W): part that force is applied to → body part

• pull (P): force → provided by muscles

class 1 lever system

fulcrum between pull and weight

• seesaw

• looking up

  • F: atlanto-occipital joint

  • P: neck muscles inferior to joint

  • W: back of the head

class 2 lever system

weight located between fulcrum and pull

• Ex: wheelbarrow

  • F: wheel

  • P: person lifting on the handles

  • W: the load carried in the wheelbarrow

class 3 lever system

pull between fulcrum and weight

• Most common

• Ex: person using a shovel

  • P: hand placed on part of handle closest to blade

  • F: hand placed near the end of the handle

  • W: the shovelful of dirt

• Not a lot of weight can be lifted → but greater distance

parallel (longitudinal) muscle

fascicles lie parallel to one another and to the long axis of the muscle

• examples

  • trapezium

  • rhomboidal

  • quadrate

convergent (triangular) muscle

broadly distributed fascicles converge at a single tendon

• example: pectoralis major muscle

pennate

fascicles originate from a tendon that runs the length of the entire muscle

• unipennate: fascicles are only one side of the tendon

• bipennate: fascicles are on both sides of the tendon

• Examples 

  • Unipennate → extensor digitorum longus in the leg

  • Bipennate → rectus femoris in the thigh

  • Multipennate → deltoid in the shoulder