IV. CONTRACTIONS AT THE MUSCLE LEVEL When contracting a Skeletal Muscle, how is strength & speed of contraction varied? How is just one portion of a large muscle contracted? How is contraction sustained over time? It involves how the Nervous System stimulates muscles via their Motor Units. Introduction Overview a) Muscle Tension: Force exerted by a contracting muscle b) Load: Weight of the object to be moved. This opposed Muscle Tension c) A skeletal muscle can contract with varying force and for different amounts of time. Depends on the person’s needs at the time

= a motor neuron and the muscle fibers it contacts A. Introduction … 2. Nervous System Action Potentials of Motor Neurons act as the STIMULUS to Muscle Contraction All or None Principle: a single action potential is either sent or not. So to increase a stimulus, more action potentials are sent We have Conscious Control over the Strength and Frequency of Skeletal Muscles Contraction 3. Motor Units of Muscles = a motor neuron and the muscle fibers it contacts a) There are many Motor Units in a muscle b) The fibers of a single Motor Unit are spread throughout the muscle, not clustered.

A. Introduction … 2. Motor Units of Muscles … c) Motor units vary in the number of muscle fibers that link to the Motor Neuron d) Muscles vary in the particular motor units they have A muscle can have different motor units of different sizes e) The more motor units that Action Potentials reach, the stronger the contraction of the Muscle Even if just one motor unit is activated by a single Action Potential, the muscle will contract very weakly

4. The MUSCLE TWITCH-overview A. Introduction … 4. The MUSCLE TWITCH-overview Discovered by Laboratory Testing of Muscles. An isolated animal muscle is hooked to electrodes to produce a recording of the contraction, a Myogram, when a single motor unit is artificially stimulated. Myogram If the stimulation is adequate (= Threshold Stimulus) the muscle fibers of the motor unit contract. It contracts in a single jerky manner in the Lab. The Myogram shows the steps of a single motor unit contraction. These steps are similar to what the entire muscle does when it contracts. So the experimental data can be used to understand how a muscle’s motor units contract smoothly & naturally via stimulation by Action Potentials.

B. Muscle Twitch = Steps in a single motor units response to a threshold stimulus; is like a muscle contraction caused by single action potential (at threshold) Myogram 3 phases: 1. Latent period – just after Stimulus: Excitation- Contraction coupling taking place & no muscle tension yet 2. Period of contraction – cross bridges are forming & tension rises; then muscle shortens 3. Period of relaxation – Ca2+ reabsorbed into SR; no more cross-bridges and tension declines; muscle lengthens to normal Myograms of different Muscles Figure 9.14a

Conclusions on Muscle Twitch B. Muscle Twitch … Conclusions on Muscle Twitch Twitch contractions vary in different muscles. Cause: differences in enzyme activity and metabolic properties of myofibrils Muscle Twitch Data is important, but in real muscles contraction is smooth & can be sustained, not jerky and fast  need to modify Muscle Twitch Data Myograms of Different Muscles

C. Graded Muscle Responses A living muscle has smooth, and sustained contractions = Graded. Graded Responses occur by Changing two main factors: Brain– Conscious Control Frequency of Stimulation Increasing the activity of an individual motor neuron so that it sends more action potentials per time. Strength of Stimulus Increasing the number of different motor neurons that send action potentials at the same time. Both the above increase the number of Action Potentials– either over time OR at the same time, respectively. Spinal Cord

a) Single stimulus = single contraction = a muscle twitch 1. Varying Stimuli Frequency— Purpose: to produce smooth continuous muscle contractions a) Single stimulus = single contraction = a muscle twitch b) Increasing stimuli over time, increases contractile force (muscle can’t completely relax) = wave (temporal) summation Incomplete Tetanus (unfused): get a sustained but quivering contraction when frequency begins to increase– partial wave summation: can be more or less Complete tetanus (fused): If frequency is fast enough get a smooth maximum contraction– complete summation (Not Common) Figure 9.15

b) Recruitment: Muscle contracts stronger as stimulus Graded Muscle Responses … 2. Variation in Stimulation Strength—Purpose: to precisely control the force of contraction by variation in the number of different motor units that are activated a) Threshold stimulus – minimum stimulus strength for contraction for Muscle Twitch (single Action Potential) b) Recruitment: Muscle contracts stronger as stimulus strength increases via more motor units being activated. This causes more action potentials to be sent through a nerve. So more motor units are ‘recruited’ = Multiple Motor Unit Summation

Muscle Strength-Variation in Stimulation Strength … c) Size Principle: recruitment of motor units is not random. Motor units with the smallest diameter fibers are recruited first. i) Small diameter muscle fibers Stimulated by the smallest and most excitable neurons which have the lowest stimulus threshold Are in the smallest motor units ii) Largest diameter muscle fibers are recruited last Stimulated by largest and least excitable neurons Are largest motor units

C. Graded Muscle Responses 3. Muscle Tone: Muscles are always slightly C. Graded Muscle Responses 3. Muscle Tone: Muscles are always slightly contracted even when at rest Caused by Involuntary Spinal Reflexes: Cause one groups of motor units to alternate with other motor units producing a very slight contraction all the time No movement is produced Purpose: keeps muscles firm and ready to respond 4. Isotonic and Isometric Contractions = the two main categories of contractions Isotonic Contractions –muscle changes length, moves load as tension remains the same “same tone” Two Types: Concentric & Eccentric

i) Concentric contractions –muscle shortens and does work EXAMPLE: Biceps brachii in flexing forearm while picking up something ii) Eccentric contractions – muscle contracts as it lengthens, provides control while stretching; “Elongate = Eccentric” stronger than concentric EXAMPLE: Biceps brachii–pu tting down something while Triceps brachii contracts to extend forearm b) Isometric Contractions – no change in muscle length, insertion does not move “same metric dimensions” i) Tension increases to muscle’s capacity ii) Load greater than tension the muscle can develop

What causes a muscle to increase in size do to exercise What causes a muscle to increase in size do to exercise? More myofibrils are added to muscle cells END