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Chapter 6: The Muscular System
Three Types of Muscle Tissue
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Tissue is just a mass of cells that are similar in function and form.
Muscle tissue refers to a collection of cells that shorten during contraction and, in doing so, create tension that
results in bodily movement of one kind or another.
In humans (and other mammals) muscle tissue can be classified into three main groups:
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Smooth Muscles:
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Cardiac Muscles:
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Skeletal Muscles:
These surround the body’s internal organs, including the blood vessels, hair
follicles, and the urinary, genital, and digestive tracts. This type of muscle tissue contracts more slowly
than skeletal muscles, but can remain contracted for longer periods of time. Smooth muscles are also
involuntary, and their spindle-shaped fibres are usually arranged in dense sheets.
It is only found in the heart. Cardiac muscles are responsible for creating the
action that pumps blood from the heart to the rest of the body. They are involuntary muscles because
they are not controlled consciously, and are instead directed to act by the autonomic nervous system.
Like skeletal muscles, cardiac tissue is also striated (striped).
These are the muscles that are attached to the bones (by tendons and other
tissues). They are the most prevalent muscle type in the human body – they comprise 30-40% of human
body weight. Skeletal muscles are voluntary since human have conscious control over their skeletal
muscles; that is, the brain can tell them what to do. Skeletal muscles are also referred to as striated, or
striped, because of their appearance of alternating light and dark stripes.
The Components and Functions of the Musculoskeletal System
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The musculoskeletal system (also called the “locomotor” system) is the body system that allows humans
to move; it involves the bones, joints, and muscles that provide form, support, and stability to a body, and
therefore gives us the ability to move.
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Skeletal muscle fibre generally connects to bone directly through tough tissue fibres, called tendons. The
bones themselves are bound tightly together with other bones through ligaments. The articulation points
where bones come together are called joints (the articular system). These joints act as levers in the human body
and serve to facilitate human movement.
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The primary functions of the musculoskeletal system are to support the body and keep it upright, to allow
movement to occur, and to protect the body’s vital organs. Remember that the skeletal portion of the
musculoskeletal system also serves as the main storage system for calcium, phosphorus, and other critical
components of the blood.
How Skeletal Muscles are Named
Action of the Muscle
Direction of the Fibres
Location of the Muscle
Number of Divisions/heads
Shape of the Muscle
Muscle’s Point of Attachment
Flexion, extension
Example: Flexor carpi ulnaris, extensor carpi ulnaris
Rectus, transversus
Example: Rectus abdominus, transversus abdominus
Anterior, posterior
Example: Tibialis anterior, tibialis posterior
Number of heads (2 or 3)
Example: biceps brachii, triceps brachii
Example: Deltoid – resembles the Greek letter Delta
Example: Trapezius – resembles a trapezoid
Sternum, clavicle, mastoid process
Example: Sternocleidomastoid
Agonist and Antagonist Muscle Pairs
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Skeletal muscles are typically arranged as opposing pairs. Since muscles pull on bones, another muscle (on the
opposite side) is required to move the bone in the opposite direction. As one muscle contracts, the other muscle
relaxes. For example, when your biceps contract to bend your elbow, your triceps relax.
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The muscle primarily responsible for movement of a body part is referred to as the agonist
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For example: The tibialis anterior muscle dorsiflexes the ankle, and the gastrocnemius muscle extends the ankle.
Complex movements, such as running, involve many muscles acting as agonist and antagonist muscle pairs as
well as stabilizers at various points during the movement.
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Stabilizers are muscles that provide support and hold a joint in place so that desired movements can occur at
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another joint. For example, in running or kicking a ball, the hip and torso muscles act as stabilizers while the
quadriceps, hamstrings, and lower leg muscles act as agonist and antagonist muscles during various stages of the
movement.
Examples of opposing muscles and muscle groups:
muscle. The
muscle that counteracts the agonist, lengthening when the agonist muscle contracts, is called the antagonist
muscle.
Shoulder
abduction
Shoulder
adduction
Hip extension
Hip flexion
Knee extension
Knee flexion
Agonist (prime mover)
Antagonist
Deltoid
Latissimus dorsi
Latissimus dorsi
Deltoid
Gluteus maximus
Iliacus
Iliacus
Gluteus maximus
Quadriceps
Hamstrings
Hamstrings
Quadriceps
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Origin, Insertion, and Function of Major Muscles and Muscle Groups
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Skeletal muscle is attached to the bone either indirectly (via tendons) or directly (when the outer membrane of the
muscle attaches to the outer membrane of the bone).
The more common of the two ways in which muscles attach to bones is the indirect method (via tendons).
When skeletal muscles contract, it causes movement of the attached bone.
The point where the muscle attaches to the more stationary of the bones of the axial skeleton is known as the
origin. The other end, the point where the muscle attaches to the bone that is moved most, is known as the
insertion.
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When a muscle contracts, it usually moves its insertion point toward its point of origin. For example, the short
head of the biceps brachii originates from the coracoid process of the scapula. When you contract your biceps,
you pull your forearm towards your shoulder, so you are pulling towards the origin, while the origin remains in a
relatively fixed position. The insertion is on the bones of the forearm (the radius), called the radial tuberosity, and
it is the forearm that moves during a contraction.
The Anatomy of Skeletal Muscle
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The basic unit of skeletal muscle is the individual skeletal muscle fibre or muscle cell.
Looking outward from the surface of the individual muscle fibre is a sheath of connective tissue called the
perimyseium, which binds groups of muscle fibres together.
These bundles are called fasciculi and they are
bound together by a larger and stronger sheath of tissue called the epimysium which envelopes the entire
muscle. The epimysium then extends beyond the muscle and changes its properties as it becomes one with the
tendon. The tendon extends itself and becomes one with the bone’s periosteum. This connection happens at both
attachment sites – the muscles origin and insertion.
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Each muscle fibre is surrounded by a sheath of connective tissue called the endomysium. Beneath the
endomysium lies a plasma membrane called the sarcolemma, which contains the muscle cell’s cytoplasm
(which is known as the sarcoplasm). The sarcoplasm contains large amounts of stored glycogen and the
protein myoglobin, as well as higher concentrations of calcium and other cellular organelles such as mitochondria.
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Running along the muscle fibre’s length are thread-like structures known as myofibrils. Within these
myofibrils are finer “thick” filaments known as myosin and “thin” filaments known as actin. Myosin and
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actin are contained within repeating structural components called sarcomeres.
Myosin is comprised of a “head” and a “tail” and looks similar to a golf club. The myosin head has an attachment
site for actin, and actin has a binding site for the myosin head. Actin has two other proteins:
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troponin – which
has a binding site for calcium, and tropomyosin – which is the “stringy looking” cord-like structure that
covers the binding site on actin.
Together, these two proteins (troponin and tropomyosin) behave like a swivel-lock mechanism – they will not
allow the myosin head to attach until calcium is released by the sarcoplasmic reticulum (a network of
membranous channels associated with each muscle fibre – these channels transport the electrochemical substances
involved in muscle activation). During muscle contraction, these protein filaments slide across one another (ie:
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the sarcomere shortens). This sliding action is synchronized across the muscle, and what we see as a muscle
contraction occurs.
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This interlocking mechanism is commonly referred to as the “sliding
filament theory of muscle
contraction”.
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When an entire muscle contracts, the mechanism is achieved by the overlapping of the actin and myosin
filaments. This causes the sarcomere (and therefore the entire muscle fibre) to contract (ie: to shorten).
Ove and over, myosin crossbridges (small bridges on the thick filaments that extend to the thin
filaments) attach, rotate, detach, and reattach in rapid succession. This process results in the sliding or
overlapping of the filaments, a shortening of the sarcomere, and what we see is a muscle contraction.
At the molecular level, the “trigger mechanism” for the sliding filament process is the release of calcium
ions when the nerve impulse is transmitted through the muscle fibre.
The release of calcium from the sarcoplasmic reticulum, in the presence of the proteins troponin and
tropomyosin facilitates the interaction of myosin and actin molecules.
Specifically, the calcium ions attach to the troponin proteins. This changes the shape of the troponin.
In response to this change of shape, the tropomyosin proteins will now move out of the way (as they
were blocking the binding sites on actin for the myosin heads).
Now, the binding sites on the actin for the myosin heads is revealed and myosin heads will attach, swivel
and reattach. This is how the myosin and actin filaments slide over one another without actually
shortening themselves.
Muscle relaxation caused by the re-uptake of calcium ions requires ATP
(adenosine
triphosphate) – the energy carrying molecule that results from food metabolism.
ATP is also used
to detach myosin from the actin molecule. As the work of the muscle increases, more and more ATP is
used up and must be replaced through food metabolism for the process to continue.
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Excitation-Contraction Coupling
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Muscles work essentially by converting chemical energy into mechanical energy.
The process of muscle contraction as a whole is often referred to as “excitation-contraction coupling”.
The electrical signal that begins the process originates in the spinal cord and moves along the nerve axon to the
neuromuscular junction (the contact point of the motor nerve and the muscle fibre). Once there, the signal is then
transmitted by chemical means across the synapse to the muscle fibre through the release of acetylcholine (ACh)
at the nerve terminal.
The signal is then transmitted down into the muscle fibre through the tubular membranes. The transverse tubulae
system is a network of interconnecting rings, each surrounds a myofibril, and serves as a link between the outside
of the muscle fibre and the actin and myosin deeper inside.
A change in the tubulae causes a rapid release of calcium ions, which in turn sets off a series of other chemical
reactions, leading to contraction of the muscle fibre.
The release of calcium ions is the critical “trigger mechanism” in this complex process. Calcium ions are released
into the sarcoplasm by the terminal cisternae. These cisternae sacs form part of the sarcoplasmic reticulum.
On the actin filament, there is one troponin and one tropomyosin molecule for every seven actin units. These
proteins serve to “inhibit” or regulate the interaction of actin and myosin. If calcium is not present, the actin and
myosin proteins do not interact.
The interaction of calcium with troponin and tropomyosin removes this obstacle to actin-myosin interaction. The
“coupling” effect is then allowed to unfold, and muscle contraction occurs. The signal for the contraction to
begin is synchronized over the entire muscle fibre so that all the myofibrils (which together make up the
sarcomere) shorten simultaneously.
The Neuromuscular System
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The neuromuscular system is a general term referring to the complex linkages between the muscular
system and the nervous system. These linkages involve two sophisticated bodily systems “linking up” and
working together in a complex interface.
The nerves that transmit the message directing the muscle to move come into contact with the muscles at points
called neuromuscular junctions. The electrical impulse travels along nerve pathways to the contact point between
the nerve and the muscle (the junction).
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At this junction, a chemical neurotransmitter is released (the chemical acetylcholine – ACh). This
chemical is detected by receptors on the surface of the muscle fibre which initiates the sarcoplasmic reticulum to
release calcium ions.
The calcium ions then attach to troponin which ultimately moves tropomyosin and therefore, exposes the binding
sites on actin so that the myosin head can attach and the actin and myosin filaments can slide over one another to
create what is known as a muscle contraction.
Nerves transmit impulses in “waves” that ensure smooth movements. A single nervous impulse and the resulting
contraction are called a muscle twitch.
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One neuron or nerve (called a “motor
fibres.
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The motor neuron, its axon (pathway), and the muscle fibres it stimulates are together referred to as the motor
neuron”) may be responsible for stimulating a number of muscle
unit.
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Motor units can be categorized into small or large units – small motor units have a few muscle fibres that it
stimulates which produce fine motor (muscle) movement (ie: your eyelid); whereas large motor units have lots of
muscle fibres that stimulate gross (large) motor movements (ie: your quadriceps).
A single motor unit within the quadriceps may stimulate 300 to 800 muscle fibres. In order for maximal muscle
force to be produced, all motor units within that muscle or muscle group must be recruited at the same time.
Generally, slow-twitch muscle motor units are smaller because they have fewer muscle fibres than fast-twitch
motor units.
Motor units also comply with a rule known as the all-or-none principle. This principle stipulates that,
when a motor unit is stimulated to contract, it will do so to its fullest potential – either all fibres will contract, or
none will contract.
Types of Muscle Contractions
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There are three types of muscle contractions:
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Concentric contraction:
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Eccentric contraction:
These are shortening contractions that occur when muscle fibres shorten. For
example, when the biceps shorten to lift an object.
These are lengthening contractions that occur when the muscle fibres lengthen.
For example, when the biceps lengthen as a weight is placed back on the ground.
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Isometric contraction:
These are static contractions that occur when the muscle fibres do not change in
length. For example, when you try to lift or move an immovable object.
The Nervous System and the Control of Movement
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There are two major components to the human nervous system: the central nervous system (CNS) and the
peripheral nervous system (PNS).
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The brain and spinal cord make up what is known as the central nervous system (CNS). The central
nervous system accepts and coordinates information from all parts of the body. The CNS has nerves going to and
from it.
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The peripheral nervous system (PNS) is responsible for the beating of the heart and the digestive
system, and all other voluntary and involuntary neuromuscular controls.
The PNS can be thought of as a kind of massive road network carrying traffic (information) in out and out the
CNS.
The peripheral nervous system contains both the autonomic and the somatic divisions.
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The Autonomic Nervous System (ANS):
 The autonomic nervous system is responsible for activities such as the involuntary contraction of the
cardiac muscles and the smooth muscles of our internal organs.
 There are two branches within the ANS: The sympathetic nervous system and the parasympathetic
nervous system.
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The sympathetic nervous system causes localized bodily adjustments to occur (ie: sweating or
cardiovascular changes) and it prepares the body for emergencies. This involves the release of
adrenaline from the adrenal gland, an increase in heart rate, a widening of blood vessels, and similar
“flight or fight” responses to deal with imminent danger.
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The parasympathetic nervous system helps to return the body to normal after it has been
altered by the sympathetic nervous system. For example, the sympathetic nervous system increases the
heart rate, and the parasympathetic nervous system lowers the heart rate.
The autonomic nervous system is not under conscious control (involuntary).
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The Somatic Nervous System (SNS):
 The somatic nervous system contains both afferent nerves (nerves that send information to the CNS) and
efferent nerves (nerves that send instructions to skeletal muscles).
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Through this system, the PNS receives and processes information from receptors in the skin, in
voluntary muscles, tendons, and joints, and gives us the sensations of touch, pain, heath, cold, balance,
body position, and muscle action.
The somatic nervous system handles the muscles in our extremities, and allows us to move around.
The somatic nervous system is under conscious control (voluntary).
Muscle and Tendon Injuries
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The most common injuries sustained to the body during sport and physical activity are to the muscles and
tendons.
Muscle Strains and Tears
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Muscle strains are caused by excessive twisting or pulling on a muscle or tendon.
If strains remain untreated, tears in the muscle or tendon fibres may worsen.
Strains can be either acute or chronic.
Chronic strains are the result of prolonged overuse and repetitive movement.
Strains and tears fall into 3 categories of severity: 1st, 2nd, and 3rd degree:
 1st degree injuries are the least severe. There is slight swelling and bruising, and pain is felt only at the
end of the full range of movement or upon stretching or contraction of the muscle.
 2nd degree injuries are moderate, but more severe than a 1st degree injury. They will require
physiotherapy treatment once diagnosed by a doctor.
 3rd degree injuries are the most severe and may require surgery and rehabilitation. They may take six to
twelve months to fully repair.
Delayed Onset Muscle Soreness (DOMS)
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Delayed onset muscle soreness (DOMS) is the result of microscopic tearing deep within the muscle
fibres.
It is frequently felt when you begin a new exercise program, change your exercise routine, or dramatically
increase the duration or intensity of your exercise routine.
It may last from several hours to several days after the exercise session.
Any movement can cause DOMS but movements that cause your muscles to forcefully contract while they
lengthen (ie: running downhill) seem to cause the most soreness.
The soreness is usually felt in the first 24 hours, peaks from 24-72 hours then disappears after 5-7 days.
The pain of DOMS is not the same as the immediate acute pain of a pulled or strained muscle, nor is it the same
as the muscle pain or fatigue you experience during exercise.
In addition to tearing, swelling can occur in and around a muscle.
DOMS can be minimized by performing proper warm –up and cool-down exercises, as well as gradually
increasing the intensity of a new exercise program.
If you experience soreness from DOMS, try icing the area and gently stretching, but most importantly, take the
time to rest and recover.
Tendonitis
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Tendonitis is inflammation of a tendon caused by irritation due to prolonged or abnormal use.
Tendonitis is typically an overuse injury, often occurring when a new activity or exercise is begun that causes the
tendon to become irritated.
It is usually named after the affected tendon or joint. For example, tendonitis of the Achilles tendon is known as
“Achilles tendonitis”.
Tendonitis of the elbow is known as “tennis elbow” (lateral epicondylitis) or “golfer’s elbow” (medial
epicondylitis).
Symptoms of tendonitis include:
 Pain or tenderness on the tendon near or around a joint.
 Stiffness and pain in the tendon, which restricts movement.
 A strong pulling or sharp pain when moving a joint.
 Occasionally, mild swelling, numbness, or a tingling sensation at the joint.
Treatment depends on the specific type of tendonitis, but most often includes:
 Resting and avoiding movement that aggravates the area.
 Protecting the area with splints, slings, or casts.
 Applying an ice pack.
 Taking prescribed oral medication for inflammation and pain.
 Participating in physical therapy.
Reflexes, Proprioception, and Movement
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Reflexes are an important part of all physical movement. They are an automatic and rapid response to a particular
stimulus.
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If the command centre for the reflex is located in the brain, it is commonly referred to as a cerebral
the command centre for the reflex is located in the spinal cord, it is called a spinal
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reflex; if
reflex.
Autonomic reflexes are mediated by the autonomic division of the nervous system, and usually involve the
activation of smooth muscle, cardiac muscle, and glands. These reflexes regulate such bodily functions as
digestion, elimination, blood pressure, salivation, and sweating.
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Somatic reflexes involve stimulation of skeletal muscles by the somatic division of the nervous system, and
include such reflexes as the stretch reflex and the withdrawal reflex. Reflex contraction of skeletal muscle is not
dependent on conscious intervention by higher centres of the brain, but is a way in which the body responds to an
unexpected stimulus.
The Reflex Arc
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There are three types of neurons in the human body: sensory neurons, motor neurons, and interneurons.
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Sensory neurons detect or sense information from the outside world, such as light, sound, touch, and heat.
Motor neurons send signals away from the CNS and elicit a response – for example, moving your leg or arm
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away from your body.
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Interneurons form interconnections between other neurons in the CNS.
A reflex arc is a simple neural pathway (or circuit) in the body along with the initial sensory stimulus and the
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corresponding responding message travel. A reflex arc allows an organism to respond rapidly to inputs from
sensory neurons, and consists of only a few neurons. The stimulus from sensory neurons is sent to the CNS and a
signal is transmitted to motor neurons which will elicit a response (ie: the knee jerk reaction).
The basic arrangement of the reflex arc involves the following 5 parts:
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The receptor, which receives the initial stimulus (ie: a pinprick to the skin or a loud noise)
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The sensory
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(or afferent) nerve, which carries the impulse to the spinal column or brain.
The intermediate nerve fibre (known as the adjustor or interneuron) which
interprets the signal and issues an appropriate response.
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The motor (or efferent)
the muscle or organ.
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The effector organ itself (ie: the muscle) which carries out the response (such as moving the hand
or leg away from danger).
nerve, which then carries the response message from the spinal cord to
Proprioceptors and the Control of Movement
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The ability of a muscle to relax and coordinate contraction with other muscles occurs in specialized receptors
located within tendons, muscles, and joints. These receptors are called proprioceptors which are sensory
receptors found in muscles, tendons, joints, and the inner ear and provide sensory information about the state of
muscle contraction, the position of body limbs, and body posture and balance.
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Balance, or equilibrium, is a part of a broader sense called proprioception, which is a person’s ability to
sense the position, orientation, and movement of the body.
The proprioceptor system plays a crucial role in physical movement. Tendon organs and muscle spindles
continuously monitor muscle actions and are essential components of the neuromuscular system. They “tell” the
nervous system about the state of muscle contraction and act as a type of safety devise, allowing the nervous
system to respond accordingly.
Muscle Spindles and the Stretch Reflex
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Muscle spindles are sensory receptors within a muscle fibre that primarily detect changes in the length of the
muscle.
They are the means by which muscles constantly and automatically adjust to the demands placed on them.
They lie parallel to the main muscle fibre and send constant signals to the spinal cord.
The spindle fibres are thinner and shorter than regular skeletal muscle fibres, although they behave and look more
or less the same.
Muscle spindles help to maintain muscle tension and are sensitive to changes in muscle length (rather than
tension).
Muscle spindles contain two afferent (sensory) nerve fibres and one efferent (motor) nerve fibre.
The spindle will detect changes in the muscle fibre length and responds to it by sending a message to the spinal
cord, leading to the appropriate motor responses. The resulting contraction allows the muscle to maintain proper
muscle tension or tone (ie: an erect posture).
Muscle spindles are involved in the reflex contraction of muscles, known as the stretch reflex. A typical example
of this is the knee-jerk reflex as well as how quickly our bodies respond to additional weight suddenly placed on
them.
Golgi Tendon Organs (GTO) and the Tension Reflex
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Golgi tendon organs (GTOs) are sensory receptors found at the end of muscle fibres that merge into the
tendon itself and detect changes in muscle tension.
GTOs are aligned in series with the muscle, such that any muscle stretching also stretches the GTOs.
The GTOs project to the motor neurons located within the spinal cord. When changes in tension are detected, an
impulse is sent along afferent (sensory) neurons to the central nervous system (CNS), where they synapse with
motor neurons of the same muscle. The efferent (motor) neurons instantly transmit an impulse, causing the
muscle to relax, thereby preventing injury.
The sequence of steps is the same as in the muscle spindle (stretch reflex).
Essentially, GTOs serve as a kind of tension detection device for the muscle system – they help protect the muscle
from excessive tension that would otherwise result in damage to the muscle or the joint, or both.
GTOs provide feedback to the CNS and play an important role in the development of strength and power, since in
order to be able to exert greater force, it is necessary to overcome obstacles presented by the GTOs themselves.
Muscles of the Neck – lateral view, and Deep Muscles of the Back (erector spinae group)
Muscles of the Anterior Thoracic Wall – posterior view and Muscles of the Abdominal
Wall – lateral view
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Muscles of the Chest and the Back
Muscles of the Shoulder, Scapula and Rotator Cuff
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Muscles of the Upper and Lower Arm
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Muscles of the Hip, Adductors, Gluteus, Quadriceps, and Hamstrings
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Muscles of the Lower Leg and Foot
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