Module 4: section 3 - Skeletal muscle mechanics

what is it called when a single muscle fibre contracts

a twitch

what needs to happen for a muscle to develop tension

more than one skeletal fibres need to twitch, since the entire muscle isn’t likely to respond to a regular twitch

how can more fibres twitch to create a whole muscle contraction

either through motor unit recruitment or by increasing the frequency of stimulation

what is motor unit recruitment

when more fibres are called in to contract to create greater muscle tension

how does motor unit recruitment work

when a motor neuron enters a muscle, it branches into more than one muscle fibre, to innervate multiple fibres at once.

what is a motor unit

the group of muscle fibres a motor neuron causes to contract

location of muscle fibres in a motor unit

not adjacent but spread throughout the whole muscle

why are the muscle fibres in a motor unit spread apart

so if only one motor unit is activated, it only causes a weak contraction.

how does the body prevent fatigue during a long contraction

it selectively rotates the activation of motor units so some can rest while others take over

what does rotating motor unit activate help with

maintain contractions for a longer period of time

what happens if a muscle fibre if restimulated after it has completely relaxed?

the twitch will be the same magnitude as the first

what happens in a muscle fibre is restimulated before it has completely relaxed

the second twitch is added onto the first, resulting in twitch summation

what is twitch summation

analogous to the temporal summation of EPSPs

what does generating tension at tetanus depend on

the length of the muscle at the onset of contraction

what is the length-tension relationship

how much tension can be created when specific lengths of muscles

how can the length-tension relationship be explained

by the sliding filament mechanism; each fibre has an ideal or optimal length where they can generate maximal force

what happens with a less than optimal fibre length

the thin filaments are overlapping the part of the thick filament with no cross-bridges, causing a decrease in efficiency of contraction

what happens in the thin filaments just keep overlapping on the thick filaments

the thin filament from opposite sides of teh sarcomere become so overlapped that the thick filaments come in contact with the z-lines

what happens with an optimal fibre length

maximal number of cross-bridge binding sites are available for binding, no overlapping of thin filaments at central regions of the thick happen

*fibres can reach maximal contraction when needed since at rest they are at the optimal length

what happens with a a great than optimal fibre length

the amount of overlapping thick and thin filaments decrease causing less cross-bridges to be available for binding to create tension

*less tension is made (about half of what it can be)

relationship between bone and contracting muscle

contracting muscles does little unless it is exerting a force on a bone

what do muscles need to do during contraction

be greater than the forces that oppose the movement of the bone

what is muscle soreness

myalgia, caused by many things, overexertion and improper test, etc.

main cause of myaligia

shearing nature: contusion, strain, laceration

what does contusion mean

muscle is prone to sudden heavy extrinsic compressive force

what does strain mean

muscle fibres are exposed to an excessive force due to intrinsic tension

types of strains

mild - delayed onset of muscle soreness

severe - where muscle fibres and connective tissues are ruptured

what does laceration mean

deep cuts/tears in muscles

what else can injuries be

in situ necrosis, part of the muscle if necrotized

types of muscle contractions

• isotonic contractions

• isometric contractions

• concentric dynamic contractions

• eccentric dynamic contractions

what are contractions at the motor unit level

isotonic/isometric

what are contractions at the whole muscle level

concentric dynamic contractions/eccentric dynamic contractions

what are isotonic contraction

the muscle fibre tension stays the same as it changes lengths
*movements

what are isometric contractions

the muscle fibre tension increases as it stays the same length, static muscle contraction
*holding a weight in place in the air

what happens with the level of whole muscle contractions

new terminology is used:

lifting loads = dynamic
dynamic = concentric or eccentric
*both need effort/work

what are concentric dynamic contractions

when tension is made while the muscle shortens
*lifting weights

what are eccentric dynamic contractions

when tension is made while the muscle lengthens, (controlled lowering of weights, etc.)

how to remember isotonic

it means “tone”