The document discusses the mechanism of muscle contraction, which involves the sliding of actin and myosin filaments past each other, shortening the sarcomere. Muscle contraction is triggered by acetylcholine releasing calcium ions from the sarcoplasmic reticulum. Calcium ions bind to troponin, exposing binding sites on actin for myosin to form cross-bridges. Myosin hydrolyzes ATP to generate power strokes that slide actin inward, contracting muscles. Muscles relax as acetylcholinesterase breaks down acetylcholine, calcium ions are pumped back into the sarcoplasmic reticulum, and cross-bridges detach.

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“Discussing the mechanism of muscle contraction and
how ATP is expended in the process.”
BY: MUUNDA MUDENDA, MSc Molecular Biology and Biotechnology
Email: muundamudenda@gmail.com
INTRODUCTION
Muscle contraction is the result of the shortening of the sarcomeres which are the
contractile units in the process. The mechanism of muscle contraction is a characteristic feature
of animals that allows them to carry out voluntary actions like movement or involuntary actions
like peristalsis and heartbeats. Muscle contraction can be the result of nervous impulses
coordinated by the brain (voluntary muscle actions) or the spinal cord (involuntary muscle
actions). The mechanism is best explained through the sliding filament theory. In this theory,
muscle contraction is a cyclic cascade of molecular processes involving two proteins, myosin
(thick) and actin (thin), which slide past each other thereby shortening the sarcomeres and
contracting the muscles (Brenner & Eisenberg, 1987). Myosin binds to actin forming cross-
bridges that lead to movement of about 10nm per stroke of the myosin head. The sliding of
these filaments happens in the presence of two cofactors namely, calcium ions (Ca2+
) and
Adenosine Triphosphate (ATP), an energy-carrying molecule. Other important players include
troponin and tropomyosin. This assignment discusses steps involved in the mechanism of
muscle contraction. The discussion will also highlight the structure of muscle cells, the
sarcomeres in particular, and the neurotransmitter Acetylcholine (ACh) which triggers the
release of Calcium ions (Ca2+
) from the Sarcoplasmic Reticulum (SR) into the sarcoplasm,
leading to muscle contraction.
THE MUSCLE CELL
Muscles are made of tissues which are a composition of cells called the muscle cells/
fibers with an elongated or tubular shape. Key features of muscle cells are; the nuclei, multiple
mitochondria, the sarcolemma, the sarcoplasm, the sarcoplasmic reticulum (SR), and
myofibrils composed of thick (myosin) and thin (actin) filaments which are organized in a way
that they form contractile units called sarcomeres (striations). According to Hinic-Frlog,
(2019), the sarcomere is organized into discs, bands, and zones which can be made up of both
myosin and actin or anyone of them. Actin filaments are surrounded by troponin and

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tropomyosin (Windhorst & Mommaerts, 1996). Using figure 1.2 below, it can be noted that
muscle contraction is the shortening of the sarcomere (distance between the Z-discs). The
filaments do not shorten but merely slide past each other.
Figure 1.1 Figure 1.2
Source: IvyRose Holistic Source: Saxena, (2020)
MECHANISM OF CONTRACTION
The sequence of events that build up to muscle contraction begins when an action
potential coordinated by the brain or spinal cord arrives at the motor neuron of the muscle
fibers. This action potential triggers the release of a neurotransmitter called acetylcholine
(ACh). Once released, acetylcholine diffuses across the synaptic cleft to its acetylcholine
binding receptors on the motor endplate. According to Saxena, (2020), motor endplates have
junctional folds in the sarcolemma that create a large surface area for the neurotransmitter to
bind to receptors. The receptors are sodium channels and once a neurotransmitter signal is
received, they open to allow the passage of Na+
ions into the cell (Hinic-Frlog, 2019). The
passage of Na+
ions into the cell causes the movement of the action potential down the T-
tubules of the sarcolemma (depolarization process) to the sarcoplasmic reticulum which stores
calcium ions (Ca2+
). The action potential to the sarcoplasmic reticulum triggers the release of
Ca2+
ions into the sarcoplasm. When muscles are in a resting state, the concentration of Ca2+
ions in the sarcoplasm is usually low. Also, the organization of the actin components, troponin,
and tropomyosin, is in such a way that the binding sites for the myosin heads on the thin actin
filament are covered by tropomyosin.
The Ca2+
ions released into the sarcoplasm then bind the troponin complex causing a
change in its conformation (Peter H. Raven et al., 2005). This change causes movement of
tropomyosin leading to exposure of the binding sites as illustrated in Figure 1.3 below. When
the myosin heads bind to the actin sites they form cross-bridges between myosin and actin
filaments. ATP plays a major role in the formation and breaking of the cross-bridges as
explained in the next paragraph. The result of the cross-bridges is power strokes leading to the

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sliding of the thin actin filaments past the thick myosin filaments. The sliding is inwards
towards the H-Zone on both ends of the sarcomere. Each power stroke causes sliding of about
10nm in distance inward towards the H-Zone.
The energy used for power strokes and sliding of actin filaments is the result of the
hydrolysis of ATP by the myosin ATPase enzyme. This hydrolysis happens in the presence of
magnesium ions (Mg2+
) (Brenner & Eisenberg, 1987). ATP attaches to the myosin head and is
hydrolyzed. The product is Adenosine Diphosphate (ADP) and inorganic phosphate (Pi). When
this happens, the myosin head is at a high energy level and once the binding sites on actin are
exposed, the head moves to bind to actin as the phosphate is released. When the stroke happens,
ADP is also released while the myosin head is still binding to actin. What follows then is the
attachment of a new ATP molecule to the myosin head causing its release from the actin site.
The ATP is hydrolyzed again to form ADP and Pi while the myosin head is in a cock position.
The process of binding, stroking, and sliding is repeated as long there is still calcium and ATP
available to repeat the cycle. The result of sliding in of actin filaments is the shortening of the
sarcomere and hence the contraction of the muscles. Muscle contraction can go on until it
reaches anatomical limits. The entire process of muscle contraction is summarized in figures
1.3 and 1.4 below.
Figure 1.3 Figure 1.4
Source: Peter H. Raven et al., (2005) Source: Hinic-Frlog, (2019)
MUSCLE RELAXATION
The muscles relax when there are no more Ca2+
ions to cause exposure of myosin-
binding sites on actin. This means the cross-bridges are terminated and the sarcomeres can
return to their resting state. What essentially happens is that an enzyme Acetylcholinesterase
(AChE) breaks down acetylcholine to acetate and choline. This causes the closing of Na+
ion
channels, the repolarization of the sarcolemma and T-tubules, and consequently, the activation
of the Ca2+
pump on the sarcoplasmic reticulum. Once the Ca2+
pump is open, calcium ions are

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returned to the sarcoplasmic reticulum terminal cisternae. Contraction happens again when a
neurotransmitter signal is received.
REFERENCES
Brenner, B., & Eisenberg, E. (1987). The mechanism of muscle contraction. Biochemical,
mechanical, and structural approaches to elucidate cross-bridge action in muscle. Basic
Research in Cardiology, 82 Suppl 2, 3–16. https://doi.org/10.1007/978-3-662-11289-2_1
Hinic-Frlog, S. (2019). Introductory Animal Physiology. Pressbooks, 171–179.
Peter H. Raven, G. B. J., Kenneth A. Mason, Jonathan B. Loson, &, & Singer, S. R. (2005).
Muscle Physiology. Cell Motility, 874–879.
Saxena, A. (2020). Muscle Fiber Contraction and Relaxation. Anatomy and Physiology,
Figure 1, 1–13. https://opentextbc.ca/anatomyandphysiology/chapter/10-3-muscle-fiber-
contraction-and-relaxation/#navigation
Windhorst, U., & Mommaerts, W. F. H. M. (1996). Physiology of Skeletal Muscle.
Comprehensive Human Physiology, 911–934. https://doi.org/10.1007/978-3-642-60946-
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