1. Muscles and Muscle Tissue Part B
HESC310
4/25/2018
Muscles and Muscle Tissue
Part B
Prepared by Janice Meeking & W. Rose.
Figures from Marieb & Hoehn 8th ed.
Portions copyright Pearson Education
Axial Skeleton

2. 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

3. neuromuscular junctions
Spinal cord
Motor neuron
cell body
Muscle
Nerve
Motor
unit 1
unit 2
fibers
neuron
axon
Axon terminals at
neuromuscular 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.
Figure 9.13a

4. 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

5. (a) Myogram showing the three phases of an isometric twitch
Latent
period
Single
stimulus
Period of
contraction
relaxation
(a) Myogram showing the three phases of an isometric twitch
Figure 9.14a

6. Extraocular muscle (lateral rectus)
Latent period
Extraocular muscle (lateral rectus)
Gastrocnemius
Soleus
Single
stimulus
(b) Comparison of the relative duration of twitch responses of
three muscles
Figure 9.14b

7. Regulation of Muscle Force
Required for proper control of skeletal movement
Responses are graded by:
Changing the frequency of stimulation
Changing the number of motor units activated

8. 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

9. A single stimulus is delivered. The muscle contracts and relaxes
Contraction
Relaxation
Stimulus
Single stimulus
single twitch
A single stimulus is delivered. The muscle
contracts and relaxes
Figure 9.15a

10. Low stimulation frequency unfused (incomplete) tetanus
Stimuli
Partial relaxation
Low stimulation frequency
unfused (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.
Figure 9.15b

11. High stimulation frequency fused (complete) tetanus
Stimuli
High stimulation frequency
fused (complete) tetanus
(c) At higher stimulus frequencies, there is no relaxation
at all between stimuli. This is fused (complete) tetanus.
Figure 9.15c

12. 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

13. Maximal stimulus Threshold
Stimulus strength
Proportion of motor units excited
Strength of muscle contraction
Maximal contraction
Maximal
stimulus
Threshold
Figure 9.16

14. Motor unit 1 Recruited (small fibers) unit 2 recruited (medium unit 3
(large
Figure 9.17

15. 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

16. Figure 9.18a

17. Figure 9.18b