LECTURE 6 Flashcards
what is the order of skeletal muscle fibres?
muscle-> muscle fibers-> myofibrils-> sarcomere.
What is the sarcomere?
functional unit of muscle, has proteins M line, Z line and titin.
What is the sliding filament model?
Proposes that a muscle shortens or lengthens by the actin and myosin filaments sliding past each other without changing length
Myosin cross-bridges cyclically attach, rotate, and detach from actin filaments utilizing energy from ATP hydrolysis
Causes a relative change in sarcomere size with force produced at the Z bands (sarcomere border)
Describe the sequence of events in muscle contraction
Step 1:
Action potential travelling through axon of motor neuron reaches presynaptic terminal (voltage gated Ca2+ channels open)
Acetylcholine (ACh) is released into synaptic cleft and binds with ACh receptors on muscle membrane (sarcolemma)
Step 2:
Sarcolemma depolarizes
Sarcolemma has projections which extend into the muscle called T (transverse) tubules which allow depolarization within muscle
Step 3:
Depolarization of the T tubule system causes Ca2+ release from the adjacent sarcoplasmic reticulum (storage sacs for Ca2+)
Step 4:
Ca2+ binds to troponin–tropomyosin complex on the actin filaments, causing a structural change and exposing myosin binding site
Step 5:
Myosin head activated through splitting of attached ATP to ADP+P and binds to actin
Myosin head swivels causing release of ADP and sliding of actin to create tension
Step 6:
New ATP binds to the myosin cross-bridge; allowing dissociation of myosin from actin
Step 7:
Cross-bridge activation-dissociation cycle continues as Ca2+ concentration remains high enough to inhibit the troponin–tropomyosin system
Step 8:
Muscle stimulation ceases without motor neuron excitation
Intracellular Ca2+ concentration rapidly moved back into sarcoplasmic reticulum through active transport (requires ATP hydrolysis)
Step 9:
Ca2+ removal restores the inhibitory action of troponin–tropomyosin
How is voluntary movement controlled?
controlled from the top down, brain-spinal cord-muscle
What is the role of lower motor neurons?
Innervate skeletal muscle with excitation causing contraction
Subjected to excitatory and inhibitory input from upper motor neurons
Cell bodies are located in brainstem and ventral horn of spinal cord
What is the role of upper motor neurons?
Provide input to LMNs
Essential for initiation of voluntary movement in humans
Located within the cerebral cortex (motor areas) and areas of the brainstem
Most UMNs synapse with interneurons before synapsing with LMNs
What is the motor unit?
consists of the lower motor neuron and the specific muscle fibres it innervates
Provides nerve supply to muscle
It is the individual and combined actions of motor units which produce muscle action
Each muscle fibre generally receives input from only one neuron, yet a single motor neuron may innervate many muscle fibers
what information exits the ventral horn of spinal cord?
efferent
what are the three functional characteristics of the motor unit?
Twitch (speed) characteristics
Tension (force) characteristics
Fatigability (endurance)
What are the 4 common motor unit categories?
Type IIa: Fast twitch, moderate force, high fatigue resistance
Type IIx: Fast twitch, high force, moderate fatigue resistance
Type IIb: Fast twitch, very high force, low fatigue resistance
Type I: Slow twitch, low force, and very high fatigue resistance
What is the all-or none principle?
A stimulus strong enough to trigger an action potential in the motor neuron will activate ALL of the accompanying muscle fibres in the motor unit to contract synchronously
A single motor unit does not exert a force gradation; either the impulse elicits an action or it does not
Changes in overall muscle tension are generated through motor unit recruitment strategies
What determines the force of overall muscle action?
Increased number (recruitment) of motor units A muscle generates considerable force when activated by all of its motor units
Increased frequency of motor unit discharge
Repetitive stimuli reach a muscle before it relaxes to increase the total tension (temporal summation)
What is Hennemans size principles?
: is the orderly recruitment of specific motor units to produce a smooth muscle actions
Allows the CNS to fine tune skeletal activity to meet demands of the task
Within a single motor pool, the motor neurons are recruited in order of ascending size (small to large)
As muscle force requirements increase, motor neurons are recruited with progressively larger axons
What are the two purposes of hennemans principle?
It minimizes the development of fatigue by using the most fatigue resistant fibers most often (for smaller forces)
It permits equally fine control of force at all levels of force output
What are the three types of temporal summation?
wave, unfused tetanus, fused tetanus.
What is wave summation?
Second stimulus occurs before muscle fibre has completely relaxed
Increasing strength of second contraction
What is unfused tetanus?
Frequency of 20-30 stimuli per second
Sustained but wavering contraction