CHAPTER 8 • BASICS IN EXERCISE PHYSIOLOGY 35MUSCULAR CONTRACTION
MOTORNEURONS
•Two specific motor neurons for initiating and regulat-
ing muscle contraction are the alpha and gamma neu-
rons. Alpha motor neurons are responsible for
initiating contractions in the contractile (extrafusal)
fibers of the muscles, whereas the gamma neurons
innervate muscle spindle (intrafusal) fibers of the
muscle (see below).
MOTORUNIT
- The motor unit, the basic functional unit of move-
ment, comprises an alpha motor neuron, a synaptic
junction (motor endplate), and the muscle fibers it
innervates. - One motor neuron can innervate 10 to several thou-
sand muscle fibers. Release of acetylcholine by the
neurons initiates contraction and degradation of
acetylcholine by cholinesterase terminates the action
potential.
EXCITATION-CONTRACTIONCOUPLING
(EC COUPLING)
•Muscle contraction is triggered by an electrical
impulse involving acetylcholine release, arrival at the
sarcolemma (muscle cell membrane) after crossing
the synaptic junction, and the subsequent entry of cal-
cium into the myofibril. The electrical impulse travels
quickly into the interior of the muscle cell, down the
T-tubules with release of calcium from the SR to the
myofibril.- Once calcium stores are released from the SR, a non-
stoppable contraction is initiated: calcium entry acti-
vates crossbridge linkages and contraction of the
sarcomere. - Sliding filament theory:The sliding filament theory
states that muscle contraction occurs when the two
major myofilaments (actin and myosin) slide past one
another through a series of crossbridge linkages.
Myosin crossbridge linkages combine, detach, and
recombine in an oscillatory pattern such that at any
point in time about 50% of the myosin heads are
attached to actin binding sites. None of the myofila-
ments actually change length. - All or none law:If a motor neuron initiates an action
potential and releases acetylcholine at the neuromuscular
junction, then all muscle fibers innervated by that motor
neuron will contract simultaneously. Muscle fibers
achieve gradation of contraction strength by recruiting
fewer or more motor neurons to initiate contraction
and/or by changing the frequency of action potentials to
sustain contractions.
FIG. 8-1 Basic anatomy and struc-
ture of skeletal muscle. (Reprinted with
permission from http://www.medicdirect.com
and http://www.medicdirect.co.uk.))