BIO259 - General Principles of Muscle Contraction (+ cardiac muscle from last unit)

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cardiac muscle: voluntary or involuntary?

involuntary

cardiac muscle: defining features

shorter fibers than striated that are fused together for added length; intercoalated discs at the point of attachment; branched; centrally located nucleus

cardiac muscle: contraction speed

slow

cardiac muscle: contraction

needs no stimulation; all fibers beat on their own at different frequencies, but because neighboring contractions pass it down, the fasted fiber dictates speed of all others; nervous system can override this natural beating

cardiac muscle: purkinje fibers

specialized fibers to control the rate of contraction among the 4 cardiac chambers

motor unit

a motor neuron and all of the muscle fibers it innervates

the basic unit of skeletal muscle contraction

motor unit

smallest unit of skeletal muscle contraction

motor unit

low innervation ratio motor units

fewer muscle fibers per motor unit, increasing amount of control

high innervation ratio motor units

more muscle fibers per motor unit, increasing amount of power

types of contraction

isotonic, isometric

isotonic contraction

same force; muscle fibers shorten against the resistance of a movable force/object

isometric contraction

same length; muscle fibers do not shorten against the resistance of an immovable force/object

types of contraction: which is more common in our daily lives?

a combination

muscle twitch

the mechanical response of muscle fibers to the received impulse

recording contractions: time 1 (first)

brain tells the fiber to contract, but nothing happens

recording contractions: time 2

electrical activity occurs at the nerve electrode, and then it dissipates

recording contractions: time 3

electrical activity at the muscle electrode along the sarcolemma and t-tubules, then dissipates

recording contractions: time 4

contraction in muscle fiber (muscle twitch)

recording contractions: time 5 (last)

everything is over, all is relaxed again

recording contractions: what accounts for the delay before step 4?

the time it takes to change the permeability of the SER to ca ions

recording contractions: what accounts for the delay before step 3?

the time it takes the electrical signal to travel from one cell to another

recording contractions: what accounts for the delay before step 2?

the time it takes for the signal to go from the brain to the distal end of the neuron

threshold

the minimum amount of strength required of the stimulus to generate a response from the motor unit

suprathreshold

a stimulus with enough strength to generate a response

subthreshold

a stimulus that lacks the strength to generate a response

organ level contraction

variable contraction of whole muscles based on recruitment of motor units taking place in the CNS

how is the contraction strength controlled?

the CNS fires more units at once for a stronger response; fewer at once for a weaker response

how is the contraction duration controlled?

twitch fusion and tonus

twitch fusion

the sending of multiple stimuli, staggered, to create a longer contraction

twitch fusion subtypes

single twitch, partial twitch fusion, complete twitch fusion

partial twitch fusion subtypes

summation, incomplete tetanus

complete twitch fusion subtypes

complete tetanus

summation

applying a stimulus and then a second (and third, fourth, etc.) before the first twitch has completely ended

incomplete tetanus

applying stimuli evenly spaced so that the amplitude remains the same at each jump up; never fully relaxes

complete tetanus

the muscle doesn't relax at all because the stimuli come so frequently

what is the drawback of twitch fusion?

it stops after the last stimulus ends, or muscles fatigue - which is after only 20 seconds

tonus

staggering the stimuli to different motor units so that some can be contracted while others are resting, allowing for a longer contraction

energetics of contraction

contraction requires ATP to be synthesized as quickly as it is being used (aka very fast), so while ATP is going to the relaxed muscles to contract them, ADP is going to getting a high energy P from phosphocreatine (ultimate energy source in the background is still glucose)

what happens if the PCr process doesn't provide enough energy/quickly enough?

the process finally goes anaerobic

muscle fiber types

red and white fibers

other names for muscle fiber types

slow and fast twitch fibers

red fibers

slow twitches with lots of mitochondria and a high consumption of oxygen

white fibers

fast and powerful twitches with fewer mitochondria, low consumption of oxygen (glycolytic)

the ratio of red:white fibers in a muscle is determined by

function and genetics

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