Ch 12 part 2 Flashcards
isometric contraction
If tension tension never overcomes the weight of the load, still generates heat
isotonic contraction
If the tension > load, muscle shortens, allowing it to perform external work (e.g. moving the load).
contraction –> tension
- Can be studied in vitro where one end of the muscle is fixed and the other is movable
- Electrical stimulations are applied, and contractions recorded & displayed as currents
Force vs. Velocity
For muscles to contract : F(muscle) > F(load)
-As F(load) ↑ –>V (Shortening) ↓
Muscle Twitch and Summation
- Single action potential generated by a motor neuron stimulates an all-or-none twitch of a muscle fiber. (single muscle twitch generated)
- Latent period – time between stimulus and contraction (Ca2+ release –> binding to troponin –> crossbridge formation)
- If second stimulus occurs BEFORE the muscle completely relaxes from first twitch, second twitch will summate with the first one.
–> Reason: Ca2+ from first twitch has not all been taken back up by s.r., so it ADDS to the total Ca2+ released by second stimulation = summated twitch/contraction (increased rate/freq of stimulation = increased strength)
Tetany
As increase the frequency of stimulations, will increase the tension up to a peak plateau, beyond which the muscle is UNABLE to respond any further.
–>muscle stuck in a contraction
Length-Tension Relationship
Maximum tension generated when muscle is 100-120% of its resting length—above that range and tension decreases (due to fewer interactions between myosin & actin)- optimal cross bridge formation
—> below that range and tension decreases (due to fiber getting shorter & thicker, generating increased fluid pressure, increased distance between actin and myosin, scrunch muscle up = unable to contract)
Under normal conditions this optimum state is maintained by NEURAL reflexes.
Whole Muscle
Individual muscle fibers respond in an all-or-none fashion. Yet, our muscles are capable of smooth, graded movements, allowing variations in effort and fine motor control
Motor Unit
One motor neuron + All the muscle fibers it innervates
- -> A muscle may have many motor units of different types (“a family”).
- -> one neuron does NOT innervate the entire muscle
- this is how all-or-none twitches of single muscle fibers are integrated into smooth, graded movements of a whole muscle
- strength of contraction = # of motor units recruited
- Single motor neuron makes synaptic contact with a number of muscle fibers— this is the basic unit of motor organization.
- The number varies from one muscle to another and from one motor neuron to another (e.g. a single motor neuron may contact 10-20 muscle fibers or > 1000)
- However, a single muscle fiber normally receives synaptic input from only one motor neuron.
- -> When a motor neuron fires, all the muscle cells in that neuron’s motor unit will contract together—fundamental unit of contraction of the whole muscle is not the contraction of a single muscle fiber, but the contraction produced by all the muscle cells in a motor unit.
Increasing motor units activated…
By variation in total number of motor neurons activated (and hence, the total number of motor units contracting)
–As ↑ number of motor units activated –> ↑ strength of contraction
As action potential frequency increases…
By variation in the frequency of action potentials in the motor neuron of a single motor unit.
–As ↑ rate of firing within a motor unit –> ↑ strength of [up to the point of tetany]
Fine muscle control
Fine muscle control requires smaller motor units (fewer muscle fibers per motor neuron
- -Eye muscles: ~20 muscle fibers/motor unit - -Larger, stronger muscles may have 1000s of myofibers/motor unit
Thus, control vs strength are tradeoffs
Summary: Muscle strength is determined by…
- Frequency of stimulation
- Thickness of each muscle fiber (e.g. via strength training—protein synthesis of contractile/regulatory components for new myofibrils)
- Initial length of the fiber at rest
- Number of fibers recruited to contract (concept of the motor unit)
- -> more units required to lift arm quickly versus lifting it slowly
slow twitch muscles
steady contractions, e.g. for standing upright (postural muscles)–rich capillary supply, more mitochondria (high oxidative capacity), more myoglobin (aka red fibers)
- more active aerobically
- use these primarily with low/moderate exercise
- longer lasting
fast twitch muscles
rapid contraction, e.g. running, jumping; fastest are ocular muscles (control eye movements)—fatigue faster, fewer capillaries, fewer mitochondria (lower oxidative capacity), less myoglobin (aka white fibers), have more glycogen stores
- more active anaerobically
- much faster return to baseline
