Muscles and Muscle TissuePart B

Prepared by Janice Meeking & W. Rose.

Figures from Marieb & Hoehn 8th ed.

Portions copyright Pearson Education

Motor Unit: The Nerve-Muscle Functional Unit• Motor neuron and all (four to several hundred) muscle fibers

it supplies

• Small motor units in muscles that control fine movements (fingers, eyes)

• Large motor units in large weight-bearing muscles (thighs, hips)

• Muscle fibers from a motor unit are spread throughout muscle so that a single motor unit causes weak contraction of entire muscle

• Motor units in a muscle usually contract asynchronously; helps prevent fatigue

Copyright © 2010 Pearson Education, Inc. Figure 9.13a

Spinal cord

Motor neuroncell body

Muscle

Nerve

Motorunit 1

Motorunit 2

Musclefibers

Motorneuronaxon

Axon terminals atneuromuscular junctions

Axons of motor neurons extend from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibers scattered throughout the muscle.

Muscle Twitch• Response of a muscle to a single, brief threshold

stimulus

• Simplest contraction observable in the lab

• Three phases: latent period, contraction, relaxation

• Different muscles have different strengths and duration of twitches, due to variations in metabolic properties and enzymes

Copyright © 2010 Pearson Education, Inc. Figure 9.14a

Latentperiod

Singlestimulus

Period ofcontraction

Period ofrelaxation

(a) Myogram showing the three phases of an isometric twitch

Copyright © 2010 Pearson Education, Inc. Figure 9.14b

Latent period

Extraocular muscle (lateral rectus)

Gastrocnemius

Soleus

Singlestimulus

(b) Comparison of the relative duration of twitch responses of three muscles

Regulation of Muscle ForceRequired for proper control of skeletal movement

Responses are graded by:

1.Changing the frequency of stimulation

2.Changing the number of motor units activated

Response to Change in Stimulus Frequency• One stimulus results in a muscle twitch

• With rapid stimuli, muscle can’t completely relax between each (Ca accumulates in cytoplasm: temporal summation)

• Further increase in stimulus frequency unfused (incomplete) tetanus

• Very fast stimuli : fused (complete) tetanus

Copyright © 2010 Pearson Education, Inc. Figure 9.15a

Contraction

Relaxation

Stimulus

Single stimulus single twitch

A single stimulus is delivered. The muscle contracts and relaxes

Copyright © 2010 Pearson Education, Inc. Figure 9.15b

Stimuli

Partial relaxation

Low stimulation frequencyunfused (incomplete) tetanus

(b) If another stimulus is applied before the muscle relaxes completely, then more tension results. This is temporal (or wave) summation and results in unfused (or incomplete) tetanus.

Copyright © 2010 Pearson Education, Inc. Figure 9.15c

Stimuli

High stimulation frequencyfused (complete) tetanus

(c) At higher stimulus frequencies, there is no relaxation at all between stimuli. This is fused (complete) tetanus.

Response to Change in # of Active Motor Units• Size principle: motor units with larger and larger

fibers are recruited as stimulus intensity increases

• In the lab: Stimulus strength = amount of voltage or current applied to nerve

• Threshold stimulus: stimulus strength at which the first observable muscle contraction occurs

• Stronger stimulus activates more nerve fibers and motor units

Copyright © 2010 Pearson Education, Inc. Figure 9.16

Stimulus strength

Proportion of motor units excited

Strength of muscle contraction

Maximal contraction

Maximalstimulus

Thresholdstimulus

Copyright © 2010 Pearson Education, Inc. Figure 9.17

Motorunit 1Recruited(smallfibers)

Motorunit 2recruited(mediumfibers)

Motorunit 3recruited(largefibers)

Muscle Tone• Constant, slightly contracted state of all muscles

• Due to spinal reflexes that activate groups of motor units alternately in response to input from stretch receptors in muscles

Copyright © 2010 Pearson Education, Inc. Figure 9.18a

Copyright © 2010 Pearson Education, Inc. Figure 9.18b