Transcript Neuoromuscular System (continued)

Neuoromuscular System
(continued)
EXS 558
Lecture #2
Wednesday September 7, 2005
Review Question #1
 Which muscle structure surround the
cellular contents of each muscle fiber?
a.) perimysium
b.) fasciculus
c.) sarcolemma
d.) endomysium
Review Question #2
 The sarcomere, functional unit of the
muscle cell, is measured from:
a.) Z-disk to Z-disk
b.) A-band to A-band
c.) actin to myosin
d.) H-zone to Z-disk
Review Question #3, 4
 What type of activity would a motor unit
that contains only a few muscle fibers be
best suited for?
 True/False: All fibers associated with a
motor unit are contracted when
innervated.
Review Question #5
 What is released from the presynaptic
side of the neuromuscular junction (NMJ)
to trigger a muscular contraction?
a.) adenosine triphosphate (ATP)
b.) calcium (Ca2+)
c.) sodium
d.) acetylcholine (Ach)
Review Question #6
 Explain the role of calcium in the
production of a muscular contraction.
Regulation of Tension Production
 Motor Unit (MU) = motor neuron and all
myofibers in innervates
 When motor neuron delivers impulse ALL fibers
maximally contract
 2 mechanisms by which CNS controls muscle
tension
1.) RATE CODING - ↑ or ↓ rate of firing of neuron
2.) RECRUITMENT - ↑ or ↓ # of MU’s activated
Which dominates?
 With small homogenous muscle, initially
rely upon recruitment (at 50% MVC, all
MU’s recruited), additional force
production caused by ↑ rate coding
 With heterogeneous larger muscle, first
see ↑ rate coding of low threshold MU’s,
then see ↑ recruitment until reach 90%
MVC, then ↑ rate coding to reach 100%
MVC.
Size Principle of MU Recruitment
 Principle of orderly recruitment states that
motor units are activated in a fixed order,
based on their ranking in the muscle.
 Size principle states that the order of
recruitment is directly related to their motor
neuron size
 Slow-twitch fibers, which have smaller motor
neurons, are recruited before fast-twitch fibers.
Size Principle of MU Recruitment
(continued)
Why do FT MU develop more force?
 FT develop electrical impulses to
myofibers more quickly
 FT MUs have myofibers with larger CSAs
 FT MUs have greater # of associated
myofibers than ST MUs
Functional Classification of Muscles
 Agonists—prime movers; responsible
for the movement
 Antagonists—oppose the agonists to
prevent overstretching of them
 Synergists—assist the agonists and
sometimes fine-tune the direction of
movement
Muscle Action
Neuromuscular Adaptations
 “An understanding of the type of
alterations seen with a given training
program will help the coach or athlete
develop the most appropriate training
program and set the most realistic
training goals”
- Hoffman
Mechanisms of Gains in Muscle Strength
 Neural Adaptations
 Synchronization and recruitment of additional motor units
 Coactivation of agonist and antagonist muscles
 Rate coding—the firing frequency of motor units
 Muscle Hypertrophy
 Fiber hypertrophy
 Fiber hyperplasia
Muscle Fiber Hypertrophy
Neural Effects of Muscle Performance
 Bilateral deficit: maximum force produced
by simultaneous contraction of both limbs
less than total force produced with each
limb acting singly
 EMG activity ↓ when activating both limbs
concurrently as compared to singly
Neural Effects of Muscle Performance
(continued)
 Why?
 May be limit of neural drive from higher
centers in CNS (central drive)
 Bilateral deficit associated with greater EMG
activity to antagonist
 Pre-contraction of antagonist can ↓ recruitment of
antagonists
Effects of Resistance Training
 Early ↑ in strength not accompanied by
muscle hypertrophy
 Caused by neural adaptations ( ↑ EMG activity)
 See ↑ in strength in untrained contralateral limb
(↑ EMG activity) [cross training effect]
 ↑ EMG activity indicated improved MU activation
(MUA)
Effects of Resistance Training
 Maximal force development requires complete MUA
 For many max effort does not induce complete MUA
 Training ↑ ability to reach full MUA with max voluntary effort
 Training ↑ ability to keep threshold MUs activated
 Training ↑ duration at which high and low threshold MUs can
sustain optimal firing rates
 Training ↓ decrement between tension seen with MVC and
tension induced with supramaximal stimulation of nerve
 Training ↑ motor neuron excitability during voluntary effort
 ↑ ability to recruit MUs and disharge them at higher frequencies
 Training ↑ synchronization in activation/firing rate of MUs
 Does NOT ↑ max force production, but may ↑ rate of max force
production
Resistance Training Fact
 Once your goals for strength
development have been achieved, you
can reduce training frequency, intensity,
or duration and still prevent losses in
strength gained for at least 12 weeks.
However, you must continue training with
a resistance maintenance program that
still provides sufficient stress to the
muscles.
Effects of Endurance Training
 Training causes ↑ in MU activation occurs in
early stages, ↑ skill acquistion/coordination
 Delays fatigue
 Training causes rotation of activity among
synergists and among MU of prime mover
 Delays fatigue
 Training ↑ consistency of firing rates of motor
neurons
 Finer control of muscle
 Delays fatigue
Are muscle fiber type conversions possible?
 Early studies showed no change in fiber type but
changes in characteristics of muscle fibers
 Cross-innervation studies and chronic stimulation
studies demonstrate changes
 Possible change from FTb to FTa, and from FTa to ST
with endurance training, and FTb to FTa with
resistance training
 A combination of high intensity resistance training and
short-interval speed work can lead to a conversion of
ST to FTa fibers