Muscle Tissues Part 2 Flashcards
Nervous activation of the muscle cell: what is each skeletal muscle controlled by?
A single nerve cell
What are the steps to stimulation of myofiber by motor neuron?
- Resting neuro-muscular junction
Motor end plate: muscle cell membrane
AChE: acetlycholinesterase, free floating in the synaptic cleft
Synaptic cleft: the gap between neuron and muscle cell
Synaptic vesicle: in the neuron, contains acetylcholine, a chemical message/neurotransmitter
Acetylcholine receptors: on the muscle cell membrane, linked to Na++ channels
- Arrival of an action potential at the neuron
The arrival of the action potential stimulate the release of ACh from the vesicles into the synaptic cleft
- Exocytosis of ACh into the synaptic cleft in response to arriving action potential
Signals vesicles to fuse with axon plasma membrane and release contents
- Diffusion of ACh molecules and their binding to receptors on the motor end plate
ACh binds to the receptors on the motor end plate. These receptors are linked to sodium channels, which open. Na++ rushes into the muscle cell, making it become more positive
- Generation of an action potential by the sudden inrush of sodium ions, and then the breakdown of ACh by AChE
Na++ wave extends along the muscle cell, generates an action potential
- Propagation of the action potential across the entire membrane surface
- Excitation-contraction coupling, the dumping on Ca++ ions onto sarcomeres as a result of the movement of an action potential down T tubule
Change of voltage (positive from Na++) opens Ca++ channels, releasing Ca++ into the sarcoplasm from the Sarcoplasmic reticulum
The contraction cycle then begins, and it will continue as long as ATP is available and action potentials are still produced at the motor end plate. As the thick and thin filaments interact, the sarcomeres shorten, pulling the ends of the muscle fiber closer together. During the contraction, the entire skeletal muscle shortens and produces a pull, or tension, on the tendons at the other end
What is excitation-contraction coupling?
The process by which an electrical stimulus triggers the release of calcium by the sarcoplasmic reticulum, initiating the mechanism of muscle contraction by sarcolemma shortening
When Ca++ is released, it binds troponin, troponin shifts tropomyosin off of myosin binding sites on action, and actin and myosin form cross-bridges and myosin starts power-stroking to bring Z-lines closer together. This causes contraction
Summary of stimulation of a myofiber by a motor neuron?
At the synaptic terminal, the action potential causes the release of ACh into the synaptic cleft. The ACh diffuses to the motor end plate, binds to receptors, and opens sodium ion channels, which leads to the production of an action potential in the sarcolemma.
The action potential in the sarcolemma travels along the T-tubules to the triads, where it triggers the release of calcium ions from the terminal cisternae of the sarcoplasmic reticulum
The contraction cycle then begins, and it will continue as long as ATP is available and the action potentials are still produced at the motor end plate
What is a muscle twitch?
A single stimulus-contraction-relaxation
= response of a muscle to a stimulus (action potential)
Each type of muscle cell has a different speed/length of twitch (ex. eye muscle, gastrocnemius)
3 parts; latent period, contraction period, and relaxation period
Latent period continues until myosin and actin bind -> powerstroking starts contraction (contraction period)
Relaxation period: when electrical impulse is no longer there, no longer generating tension
What is tension?
Active (shortening) force exerted by a muscle when it contracts
To generate useful contractions we must be able to vary the amount of tension generated when a muscle contraction
- a contraction is typically held for longer than a single twitch
What are the factors influencing the amount of tension developed by each muscle cell?
- length of muscle cell at time of contraction
- frequency of stimulation
a. treppe
b. wave summation
c. incomplete/unfused tetanus
d. complete/fused tetanus
What effect does the length of muscle cell at time of contraction have on the tension developed by a single muscle cell?
Degree of stretch when the body is in resting position: degree of overlap between thick and thin filaments and # of cross bridges
We can alter the degree of stretch by holding the muscle so that muscle fibers are stretched to a length that is close to ideal before they are stimulated to contract
effect of length on maximum tension:
effect of overstretching the cell: reduces the zone of overlap and # of cross-bridge interactions
effect of over compressing the cell: reduces the zone of overlap and # of cross-bridge interaction
in both cases: the amount of tension possible is reduced
What effect does the frequency of stimulation have on the tension developed by a single muscle cell?
Increased frequency of stimulation -> increased tension
components of this relationship
1. treppe: muscle cell stimulated again, immediately after relaxation ; stimulus <50/s
result: subsequent twitches have increased tension
cause: gradual increase in Ca++ ions in sarcoplasm, because SR ion pumps don’t have time to recapture the Ca++ ions in between stimuli
- wave summation: muscle cell stimulated a second time before relaxation phase is complete ; stimulus >50/s
result: wave summation; muscle is never allowed to relax completely, and tension rises until it is roughly 4x the maximum produced by treppe
cause: not enough time for Ca++ to be completely recaptured by sarcoplasmic reticulum so there is more move out building up in the sarcoplasm
- incomplete/unfused tetanus: cell stimulated repeatedly and never allowed to relax completely reaches a submax tension
- complete/fused tetanus: cell stimulated at high frequency - no relaxation between stimuli reaches max tension
cause of all these phenomena: ca++ levels rise in sarcoplasm as SR ion pumps do not have time to recapture ca++ between stimulations
see pt 2 pg. 30
What does tension produced by the muscle as a whole depend on?
- tension generated by individual muscle cells activated
which depends on
a) fibers resting length at time of stimulation (i.e. degree of overlap between thin and thick filaments; number of cross-bridges)
b) number of cells in muscle that have been stimulated
c) number of contractile proteins in each stimulated muscle cell (e.g. differences between slow, intermediate and fast twitch muscle fibers)
What is a motor unit?
All the muscle fibres controlled by a single motor neuron
motor neurons in motor units specialize in sending impulses to specific muscle fibres for contractions
variation in number of cells in a motor unit: a few to 1000s
What is recruitment?
The activation of more motor units
What is the sequence of recruitment?
- Smallest motor unit containing fewest and slowest muscle fibers (smallest first)
- Larger motor units containing faster and more powerful muscle fibers
- Peak tension production when all motor units are in a state of complete tetanus
How are motor units activated and why
Motor units are activated on a rotating basis
function: limited energy reserves make it necessary for motor units to rest and recover. This is done on a rotating basis to allow for simultaneous recovery and sustained contraction
-asynchronous motor unit summation
What is muscle tone?
Slight level of muscle contraction in muscles that are “at rest”
cause: in any skeletal muscle, some motor units are always active
function: does not generate active movement, but tenses and firms the muscle to maintain/stabilize bone positions and also absorb sudden bumps/shocks
