MUSCLE FIBERS AND THEIR CONTRACTILE ELEMENTS
Muscle fibers are encased by a plasma membrane known as the sarcolemma and
contain cell components such as cellular plasma called sarcoplasm (which
contains glycogen, fats, minerals, and oxygen-binding myoglobin), nuclei, and
mitochondria (which transform energy from food into energy for the cell). Unlike
other cells, they also have structures called myofibrils. Myofibrils contain myofi
laments that are the actual contractile components of muscle tissue. These myofi
laments are known as actin (thin string like filaments) and myosin (thick filaments).
The actin (thin) and myosin (thick) filaments form a number of repeating sections
within a myofibril. Each one of these particular sections is known as a sarcomere.
A sarcomere is the functional unit of the muscle, much like the neuron is for the
nervous system. It lies in the space between two Z lines. Each Z line denotes
another sarcomere along the myofibril. Two protein structures that are also
important to muscle contraction are tropomyosin and troponin. Tropomyosin is
located on the actin fi lament and blocks myosin binding sites located on the actin
filament, keeping myosin from attaching to actin when the muscle is in a relaxed
state. Troponin, also located on the actin fi lament, plays a role in muscle
contraction by providing binding sites for both calcium and tropomyosin when a
muscle needs to contract.
NEURAL ACTIVATION
Skeletal muscles will not contract unless they are stimulated to do so by motor
neurons. Neural activation is the communication link between the nervous system
and the muscular system. Motor neurons originating from the CNS communicate
with muscle fibers through a specialized synapse called the neuromuscular
junction. One motor neuron and the muscle fibers it connects (innervates) with is
known as a motor unit. The point at which the motor neuron meets an individual
muscle fiber is called the neuromuscular junction (nerve to muscle). This junction
is actually a small gap between the nerve and muscle fiber often called a synapse.
Electrical impulses (also known as action potentials) are transported from the
central nervous system down the axon of the neuron. When the impulse reaches
the end of the axon (axon terminal), chemicals called neurotransmitters are
released. Neurotransmitters are chemical messengers that cross the synapse
between the neuron and muscle fiber, transporting the electrical impulse from the
nerve to the muscle. Once neurotransmitters are released, they link with receptor
sites on the muscle fiber specifically designed for their attachment. The
neurotransmitter used by the neuromuscular system is acetylcholine (ACh). Once
attached, ACh stimulates the muscle fibers to go through a series of steps that
initiates muscle contractions.
Sliding Filament Theory The sliding filament theory describes how thick and thin
filaments within the sarcomere slide past one another, shortening the entire
length of the sarcomere and thus shortening muscle and producing force.
Excitation-Contraction Coupling: Putting it all Together Excitation-contraction
coupling is the process of neural stimulation creating a muscle contraction. It
involves a series of steps that start with the initiation of a neural message (neural
activation) and end up with a muscle contraction.
