Physiology of Skeletal Muscle - Rhode Island If tetanus is maintained, then the muscle can no longer...

Biol 335 LabRhode Island College

Dr. J. Montvilo

Physiology ofSkeletal Muscle

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Skeletal Muscle

• Skeletal Muscle Cell

• Skeletal Muscle

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• A muscle cell twitch contraction consists of a complete stimulus-contraction-relaxation cycle, with an “all-or-none” response

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Twitch Contraction

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• A muscle cell contraction is initiated by a stimulus, usually from a motor neuron

• In this exercise, a voltage will be used as the stimulus

• Assume the entire muscle acts as a single muscle cell — for now

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Contraction Stimulation

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Basic Apparatus

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Twitch:Latency

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LATENCY

STIMULUS

15 msec

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Twitch:Contraction

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CONTRACTION

STIMULUS

80 msec

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Twitch:Relaxation

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RELAXATION

STIMULUS

385 msec

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• A motor unit consists of a motor neuron and all the muscle cells it innervates

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Motor Units

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• To increase the force of an intact muscle contraction, more motor units may be added (recruited) into the contraction …

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Recruitment ofMotor Units (1)

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• To increase the force of an intact muscle contraction, more motor units may be added (recruited) into the contraction …

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Recruitment ofMotor Units (2)

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• To increase the force of an intact muscle contraction, more motor units may be added (recruited) into the contraction …

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Recruitment ofMotor Units (3)

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• To increase the force of an intact muscle contraction, more motor units may be added (recruited) into the contraction … and/or you can increase the frequency with which a contraction is initiated

• The increased frequency causes the strength of each subsequent contraction to add (summate) to the strength of the previous contraction

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Summation of Contractions (1)

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• Each muscle cell has a minimum stimulus (voltage) at which it will respond with a contraction

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Summation of Contractions (2)

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• As you increase the voltage/stimulus to the muscle, more cells will respond — up to a maximal stimulus and a maximum response (all muscle cells are contracting; no more force can be generated)

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Summation of Contractions (3a)

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Summation of Contractions (3b)

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• If the frequency of stimulations is such that the muscle does not have a chance to relax completely before the next stimulus, then the subsequent contraction adds (summates) to the first, resulting in a more forceful contraction; a “staircase” effect called treppe results

• (Do not confuse this with summation of sub-threshold stimuli)

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Summation of Contractions (4a)

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Summation of Contractions (4b)

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• If the frequency of stimulations is such that the muscle does not have a chance to relax AT ALL before the next stimulus, then the contraction is maintained at a high level of force; this is called tetanus

• If tetanus is maintained, then the muscle can no longer maintain a contraction, even if stimulated, and the result is called fatigue

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Summation of Contractions (5a)

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Summation of Contractions (5b)

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Threshold Stimulus/Response

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THRESHOLD VOLTAGE

SUBTHRESHOLD STIMULI THRESHOLD STIMULI

FIRST CONTRACTION

NO CONTRACTIONS

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Maximal Stimulus/Response

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SUB-MAXIMAL STIMULI MAXIMAL STIMULI

MAXIMUM CONTRACTIONSSUB-MAXIMUM CONTRACTIONS

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Contraction with Increased Frequency of Stimulation (1)

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(Note increase in frequency of stimulations.)

TETANUS(FUSION)

TREPPE(due to summation of contractions)

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Contraction with Increased Frequency of Stimulation (2)

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(Note increase in frequency of stimulations.)

FATIGUE

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• One frog per table will be anesthetized and then decapitated

• Remove one leg per group andfollow the directions in the labmanual to set up the apparatus

• Follow the directions in the lab manual to do the experiments; record the results on the worksheet and in the journal

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Procedures

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Any questions?

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But wait …there’s more!

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• A sarcomere is the fundamental unit of a muscle cell that is responsible for contraction

• It consists of thick (myosin) and thin (actin) filaments and ends at a Z-disc attached to the actin filaments

• The myosin filaments pull on the actin filaments and move the Z-discs closer to the center of the sarcomere, thus shortening the sarcomere

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Sarcomere (1)

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Sarcomere (2)

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• The amount of force (tension) generated by a muscle can depend on the starting length of the muscle

• At Lo (Lengthoptimum), the myosin/actin overlap ideally and a maximum force is generated

• If compressed, actin filaments overlap and can not move properly; less force is produced

• If stretched, myosin filaments do not interact with the actin properly; less force is produced

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Length-TensionRelationship (1)

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Length-TensionRelationship (2)

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Length-TensionRelationship (3)

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• When a muscle contracts a force is generated

• If the bone to which the muscle is attached does not or can not move (because not enough force is being generated), then the muscle does not shorten, even though a force is being produced

• This production of force but not shortening of the muscle is called an isometric (“same size” muscle) contraction

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Isometric and IsotonicContractions (1)

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Isometric and IsotonicContractions (2)

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• When a muscle contracts a force is generated

• The force generated will increase until the object to be moved actually moves; at that point, no additional force is needed to move the object

• Therefore, the force stays the same (is “isotonic”) even though the muscle is contracting

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Isometric and IsotonicContractions (3)

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Isometric and IsotonicContractions (4)

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Now, are thereany questions?

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