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

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

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

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

(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

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

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

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

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

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

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

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

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

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

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

Figure 9.18a

Figure 9.18b