Muscle Physiology Lab Flashcards
how are skeletal muscles connected to bones?
either directly or by tendons (strong bundles of collagen fibres)
what are skeletal muscle composed of ?
skeletal muscle are composed of long, multinucleate cells called fibres
what are the fibres of muscle cells grouped into ?
fascicles which are surrounded by perimysium
what is a motor unit ?
a single motor neuron and all the muscle fibres it innervates
explain the steps involved in excitation-contraction coupling?
- release of neurotransmitter at the motor endplate
- binding of the neurotransmitter to postsynaptic receptors
- opening of Na channels
- generation of epp on surface of sarcolemma
- surface propagation of action potential
- passage of action potential down t-tuble system
- depolarisation of sr
- Ca released from sr
- Ca binds to troponin
- conformational change of troponin tugging tropomyosin from actin
- active site on actin uncovered
- myosin heads forming cross bridge attach to the actin
- myosin head tilts pulling actin towards centre of sarcomere
- cross bridge detach and reattach to the next active site
- making and breaking of cross bridges repeats till muscle has shortened enough ( ratchet theory of muscle contraction )
- Ca pumped back to sr
how does action potential in a motor neuron induce action potential into muscle fibre ?
- by releasing ACH into neuromuscular junction
- this increases Ca
- activates contractile molecular machinery inside fibre
- this requires ATP
what are the two ways the size of muscle contraction can be controlled ?
- spatial summation ( motor unit recruitment )
2. temporal summation
what is spatial summation ?
number of motor units active
lots of motor units active more cells shorten and bigger contraction
how can the size of contraction be varied in spatial summation ?
by altering the number of motor units active
what is the difference between muscles with fine degree of control and muscles which require little fine control ?
.muscles with fine degree of motor control will have motor units composed of small number of muscle fibres e.g motor units of larynx only 2-3 muscle
-muscles with little fine control will have motor units comprising many muscle fibres such as Postural muscle - 800+/unit
have around 100-150 muscle fibres -larger muscles
what is temporal summation ?
varying the frequency of stimulation of muscle
what is a muscle twitch ?
response of a muscle to a single stimulus
what are the three phases of a muscle twitch ?
- latent phase - interval between stimulus and beginning of muscle contraction
- contraction phase - when the muscle is shortening
- relaxation phase- during which tension declines
what do we observe muscle twitch ?
when the time between successive stimuli is far apart
e.g. 200 ms
what do you observe in a stimulation intervals between 200 ms and 75 ms ?
time between stimuli has decreased , the 2nd contraction starts before previous one is over - bigger contraction
the additive effect is known as temporal summation
-At stimulation intervals between 200 ms (5Hz) and 75 ms (13.3Hz), [Ca2+] in the muscle is still above baseline levels when the next action potential arrives. The muscle fibre therefore has not completely relaxed and the next contraction is stronger than normal.
why do we observe this additive effect in temporal summation ?
- action potential that triggers stimulus is brief
- action potential is smaller than contraction of cell
- so 2nd contraction starts before first stops contraction
what happens at even higher stimulation frequency
e.g less than 50 ms ?
smooth muscle contraction is very strong which results in tetanus
what can we conclude about temporal summation ?
closer stimuli by decreasing time interval results in bigger contraction
what is a tetanus ?
large smooth muscle contraction in response to a series of closely spaced stimuli
what happened in ex 1 when ulnar nerve stimulated ?
- ulnar nerve we stimulated , ulnar nerve innervates thumb
- action potential is induced into neuron
- action potential is fired up muscle
- change in Ca levels
- leads to muscle contraction
increase amplitude = bigger twitch response
what effect does increasing and decreasing current have on contraction in spatial summation ?
.increase current = recruit more motor units more action potential = more contraction into muscles and fibres
.decreasing current= stimulating fewer neurons = less muscle contraction
what happens at low stimulus current e.g. 1mA? spatial summation
no response because no muscle fibres are stimulated
what is initial threshold result in spatial summation ?
first result
all points before this is zero
this is where first stimulus current occurs resulting in muscle contraction
what happens after maximal response ?
this is the end when all peaks are stationary
why does the graph plateau ?
because no matter stimulus current we cant induce muscle contraction this is because all neurons have been stimuluated
what did we do to measure temporal summation ?
the stimulus current will remain constant however the time frame between two paired stimuli will vary
200 ms - twitch
less than 200 ms - temporal summation
less than 50 ms - tetanus
why is it when you decrease time between stimuli you get greater muscle contraction ?
this because the 2nd muscle contraction will be additive to 1st muscle contraction as the 2nd contraction started before 1st contraction has fully relaxed
what can you observe in temporal summation ?
decreasing stimulus interval bigger force generated ( lager amplitude ) from thumb twitch
What happens when you get an infection example?
from an infection you get overstimulation of muscle contraction – induced to go into tetanus
Infection from bacteria – end up with muscle contraction – associated with lock jaw – affect muscles around jaw they are contracting – cant open mouth
What is the units of the stimulus current?
mA
what is the unit for stimulus interval in temporal summation ?
ms
