Human Muscles Anatomy
Muscular System Anatomy
Muscle Types
There are three types of muscle tissue: Visceral, cardiac, and skeletal.
- Visceral Muscle. Visceral muscle is found inside of organs like the stomach,
intestines, and blood vessels. The weakest of all muscle tissues,
visceral muscle makes organs contract to move substances through the
organ. Because visceral muscle is controlled by the unconscious part of
the brain, it is known as involuntary muscle—it cannot be directly
controlled by the conscious mind. The term “smooth muscle” is often used
to describe visceral muscle because it has a very smooth, uniform
appearance when viewed under a microscope. This smooth appearance
starkly contrasts with the banded appearance of cardiac and skeletal
muscles.
- Cardiac Muscle. Found only in the heart,
cardiac muscle is responsible for pumping blood throughout the body.
Cardiac muscle tissue cannot be controlled consciously, so it is an
involuntary muscle. While hormones and signals from the brain
adjust the rate of contraction, cardiac muscle stimulates itself to
contract. The natural pacemaker of the heart is made of cardiac muscle
tissue that stimulates other cardiac muscle cells to contract. Because
of its self-stimulation, cardiac muscle is considered to be autorhythmic
or intrinsically controlled.
The cells of cardiac muscle tissue
are striated—that is, they appear to have light and dark stripes when
viewed under a light microscope. The arrangement of protein fibers
inside of the cells causes these light and dark bands. Striations
indicate that a muscle cell is very strong, unlike visceral muscles.
The
cells of cardiac muscle are branched X or Y shaped cells tightly
connected together by special junctions called intercalated disks.
Intercalated disks are made up of fingerlike projections from two
neighboring cells that interlock and provide a strong bond between the
cells. The branched structure and intercalated disks allow the muscle
cells to resist high blood pressures and the strain of pumping blood
throughout a lifetime. These features also help to spread
electrochemical signals quickly from cell to cell so that the heart can
beat as a unit.
- Skeletal Muscle. Skeletal muscle is the only voluntary
muscle tissue in the human body—it is controlled consciously. Every
physical action that a person consciously performs (e.g. speaking,
walking, or writing) requires skeletal muscle. The function of skeletal
muscle is to contract to move parts of the body closer to the bone that
the muscle is attached to. Most skeletal muscles are attached to two
bones across a joint, so the muscle serves to move parts of those bones
closer to each other.
Skeletal muscle cells form when many
smaller progenitor cells lump themselves together to form long,
straight, multinucleated fibers. Striated just like cardiac muscle,
these skeletal muscle fibers are very strong. Skeletal muscle derives
its name from the fact that these muscles always connect to the skeleton
in at least one place.
Gross Anatomy of a Skeletal Muscle
Most skeletal
muscles are attached to two bones through tendons. Tendons are tough
bands of dense regular connective tissue whose strong collagen fibers
firmly attach muscles to bones. Tendons are under extreme stress when
muscles pull on them, so they are very strong and are woven into the
coverings of both muscles and bones.
Muscles move by shortening their length, pulling on tendons, and
moving bones closer to each other. One of the bones is pulled towards
the other bone, which remains stationary. The place on the stationary
bone that is connected via tendons to the muscle is called the origin.
The place on the moving bone that is connected to the muscle via tendons
is called the insertion. The belly of the muscle is the fleshy part of
the muscle in between the tendons that does the actual contraction.
Names of Skeletal Muscles
Skeletal muscles are
named based on many different factors, including their location, origin
and insertion, number of origins, shape, size, direction, and function.
- Location. Many muscles derive their names from their
anatomical region. The rectus abdominis and transverse abdominis, for
example, are found in the abdominal region. Some muscles, like the tibialis anterior, are named after the part of the bone (the anterior portion of the tibia)
that they are attached to. Other muscles use a hybrid of these two,
like the brachioradialis, which is named after a region (brachial) and a
bone (radius).
- Origin and Insertion. Some muscles are named based upon
their connection to a stationary bone (origin) and a moving bone
(insertion). These muscles become very easy to identify once you know
the names of the bones that they are attached to. Examples of this type
of muscle include the sternocleidomastoid (connecting the sternum and clavicle to the mastoid process of the skull) and the occipitofrontalis (connecting the occipital bone to the frontal bone).
- Number of Origins. Some muscles connect to more than one
bone or to more than one place on a bone, and therefore have more than
one origin. A muscle with two origins is called a biceps. A muscle with
three origins is a triceps muscle. Finally, a muscle with four origins
is a quadriceps muscle.
- Shape, Size, and Direction. We also classify muscles by their shapes. For example, the deltoids
have a delta or triangular shape. The serratus muscles feature a
serrated or saw-like shape. The rhomboid major is a rhombus or diamond
shape. The size of the muscle can be used to distinguish between two
muscles found in the same region. The gluteal region contains three
muscles differentiated by size—the gluteus maximus (large), gluteus
medius (medium), and gluteus minimus (smallest). Finally, the direction
in which the muscle fibers run can be used to identify a muscle. In the
abdominal region, there are several sets of wide, flat muscles. The
muscles whose fibers run straight up and down are the rectus abdominis, the ones running transversely (left to right) are the transverse abdominis, and the ones running at an angle are the obliques.
- Function. Muscles are sometimes classified by the type of
function that they perform. Most of the muscles of the forearms are
named based on their function because they are located in the same
region and have similar shapes and sizes. For example, the flexor group
of the forearm flexes the wrist and the fingers. The supinator
is a muscle that supinates the wrist by rolling it over to face palm
up. In the leg, there are muscles called adductors whose role is to
adduct (pull together) the legs.
Groups Action in Skeletal Muscle
Skeletal muscles
rarely work by themselves to achieve movements in the body. More often
they work in groups to produce precise movements. The muscle that
produces any particular movement of the body is known as an agonist or
prime mover. The agonist always pairs with an antagonist muscle that
produces the opposite effect on the same bones. For example, the biceps
brachii muscle flexes the arm at the
elbow.
As the antagonist for this motion, the triceps brachii muscle extends
the arm at the elbow. When the triceps is extending the arm, the biceps
would be considered the antagonist.
In addition to the agonist/antagonist pairing, other muscles work to
support the movements of the agonist. Synergists are muscles that help
to stabilize a movement and reduce extraneous movements. They are
usually found in regions near the agonist and often connect to the same
bones. Because skeletal muscles move the insertion closer to the
immobile origin, fixator muscles assist in movement by holding the
origin stable. If you lift something heavy with your arms, fixators in
the trunk region hold your body upright and immobile so that you
maintain your balance while lifting.
Skeletal Muscle Histology
Skeletal muscle fibers
differ dramatically from other tissues of the body due to their highly
specialized functions. Many of the organelles that make up muscle fibers
are unique to this type of cell.
The sarcolemma is the cell
membrane of muscle fibers. The sarcolemma acts as a conductor for
electrochemical signals that stimulate muscle cells. Connected to the
sarcolemma are transverse tubules (T-tubules) that help carry these
electrochemical signals into the middle of the muscle fiber. The
sarcoplasmic reticulum serves as a storage facility for calcium ions
(Ca2+) that are vital to muscle contraction. Mitochondria, the “power
houses” of the cell, are abundant in muscle cells to break down sugars
and provide energy in the form of ATP to active muscles. Most of the
muscle fiber’s structure is made up of myofibrils, which are the
contractile structures of the cell. Myofibrils are made up of many
proteins fibers arranged into repeating subunits called sarcomeres. The
sarcomere is the functional unit of muscle fibers. (See
Macronutrients for more information about the roles of sugars and proteins.)
Sarcomere Structure
Sarcomeres are made of two types of protein fibers: thick filaments and thin filaments.
- Thick filaments. Thick filaments are made of many bonded units of the protein myosin. Myosin is the protein that causes muscles to contract.
- Thin filaments. Thin filaments are made of three proteins:
- Actin. Actin forms a helical structure that makes up the
bulk of the thin filament mass. Actin contains myosin-binding sites that
allow myosin to connect to and move actin during muscle contraction.
- Tropomyosin. Tropomyosin is a long protein fiber that wraps around actin and covers the myosin binding sites on actin.
- Troponin. Bound very tightly to tropomyosin, troponin moves tropomyosin away from myosin binding sites during muscle contraction.
Muscular System Physiology
Function of Muscle Tissue
The main function of
the muscular system is movement. Muscles are the only tissue in the body
that has the ability to contract and therefore move the other parts of
the body.
Related to the function of movement is the muscular system’s second
function: the maintenance of posture and body position. Muscles often
contract to hold the body still or in a particular position rather than
to cause movement. The muscles responsible for the body’s posture have
the greatest endurance of all muscles in the body—they hold up the body
throughout the day without becoming tired.
Another function related to movement is the movement of substances
inside the body. The cardiac and visceral muscles are primarily
responsible for transporting substances like blood or food from one part
of the body to another.
The final function of muscle tissue is the generation of body heat.
As a result of the high metabolic rate of contracting muscle, our
muscular system produces a great deal of waste heat. Many small muscle
contractions within the body produce our natural body heat. When we
exert ourselves more than normal, the extra muscle contractions lead to a
rise in body temperature and eventually to sweating.
Skeletal Muscles as Levers
Skeletal muscles work
together with bones and joints to form lever systems. The muscle acts as
the effort force; the joint acts as the fulcrum; the bone that the
muscle moves acts as the lever; and the object being moved acts as the
load.
There are three classes of levers, but the vast majority of the
levers in the body are third class levers. A third class lever is a
system in which the fulcrum is at the end of the lever and the effort is
between the fulcrum and the load at the other end of the lever. The
third class levers in the body serve to increase the distance moved by
the load compared to the distance that the muscle contracts.
The tradeoff for this increase in distance is that the force required
to move the load must be greater than the mass of the load. For
example, the biceps brachia of the arm pulls on the radius of the
forearm, causing flexion at the
elbow joint
in a third class lever system. A very slight change in the length of
the biceps causes a much larger movement of the forearm and hand, but
the force applied by the biceps must be higher than the load moved by
the muscle.
Motor Units
Nerve cells called motor neurons
control the skeletal muscles. Each motor neuron controls several muscle
cells in a group known as a motor unit. When a motor neuron receives a
signal from the brain, it stimulates all of the muscles cells in its
motor unit at the same time.
The size of motor units varies throughout the body, depending on the
function of a muscle. Muscles that perform fine movements—like those of
the
eyes
or fingers—have very few muscle fibers in each motor unit to improve
the precision of the brain’s control over these structures. Muscles that
need a lot of strength to perform their function—like leg or arm
muscles—have many muscle cells in each motor unit. One of the ways that
the body can control the strength of each muscle is by determining how
many motor units to activate for a given function. This explains why the
same muscles that are used to pick up a pencil are also used to pick up
a bowling ball.
Contraction Cycle
Muscles contract when
stimulated by signals from their motor neurons. Motor neurons contact
muscle cells at a point called the Neuromuscular Junction (NMJ). Motor
neurons release neurotransmitter chemicals at the NMJ that bond to a
special part of the sarcolemma known as the motor end plate. The motor
end plate contains many ion channels that open in response to
neurotransmitters and allow positive ions to enter the muscle fiber. The
positive ions form an electrochemical gradient to form inside of the
cell, which spreads throughout the sarcolemma and the T-tubules by
opening even more ion channels.
When the positive ions reach the
sarcoplasmic reticulum, Ca2+ ions are released and allowed to flow into
the myofibrils. Ca2+ ions bind to troponin, which causes the troponin
molecule to change shape and move nearby molecules of tropomyosin.
Tropomyosin is moved away from myosin binding sites on actin molecules,
allowing actin and myosin to bind together.
ATP molecules power myosin proteins in the thick filaments to bend
and pull on actin molecules in the thin filaments. Myosin proteins act
like oars on a boat, pulling the thin filaments closer to the center of a
sarcomere. As the thin filaments are pulled together, the sarcomere
shortens and contracts. Myofibrils of muscle fibers are made of many
sarcomeres in a row, so that when all of the sarcomeres contract, the
muscle cells shortens with a great force relative to its size.
Muscles continue contraction as long as they are stimulated by a
neurotransmitter. When a motor neuron stops the release of the
neurotransmitter, the process of contraction reverses itself. Calcium
returns to the sarcoplasmic reticulum; troponin and tropomyosin return
to their resting positions; and actin and myosin are prevented from
binding. Sarcomeres return to their elongated resting state once the
force of myosin pulling on actin has stopped.
Types of Muscle Contraction
- The strength of a muscle’s contraction can be controlled by two
factors: the number of motor units involved in contraction and the
amount of stimulus from the nervous system. A single nerve impulse of a
motor neuron will cause a motor unit to contract briefly before
relaxing. This small contraction is known as a twitch contraction. If
the motor neuron provides several signals within a short period of time,
the strength and duration of the muscle contraction increases. This
phenomenon is known as temporal summation. If the motor neuron provides
many nerve impulses in rapid succession, the muscle may enter the state
of tetanus, or complete and lasting contraction. A muscle will remain in
tetanus until the nerve signal rate slows or until the muscle becomes
too fatigued to maintain the tetanus.
Not all muscle contractions produce movement. Isometric contractions
are light contractions that increase the tension in the muscle without
exerting enough force to move a body part. When people tense their
bodies due to stress, they are performing an isometric contraction.
Holding an object still and maintaining posture are also the result of
isometric contractions. A contraction that does produce movement is an
isotonic contraction. Isotonic contractions are required to develop
muscle mass through weight lifting.
Muscle tone is a natural condition in which a skeletal muscle stays
partially contracted at all times. Muscle tone provides a slight tension
on the muscle to prevent damage to the muscle and joints from sudden
movements, and also helps to maintain the body’s posture. All muscles
maintain some amount of muscle tone at all times, unless the muscle has
been disconnected from the central nervous system due to nerve damage.
Functional Types of Skeletal Muscle Fibers
Skeletal muscle fibers can be divided into two types based on how they produce and use energy: Type I and Type II.
- Type I fibers are very slow and deliberate in their contractions.
They are very resistant to fatigue because they use aerobic respiration
to produce energy from sugar. We find Type I fibers in muscles
throughout the body for stamina and posture. Near the spine and neck regions, very high concentrations of Type I fibers hold the body up throughout the day.
- Type II fibers are broken down into two subgroups: Type II A and Type II B.
- Type II A fibers are faster and stronger than Type I fibers, but do
not have as much endurance. Type II A fibers are found throughout the
body, but especially in the legs where they work to support your body
throughout a long day of walking and standing.
- Type II B fibers are even faster and stronger than Type II A, but
have even less endurance. Type II B fibers are also much lighter in
color than Type I and Type II A due to their lack of myoglobin, an
oxygen-storing pigment. We find Type II B fibers throughout the body,
but particularly in the upper body where they give speed and strength to
the arms and chest at the expense of stamina.
Muscle Metabolism and Fatigue
Muscles get their
energy from different sources depending on the situation that the muscle
is working in. Muscles use aerobic respiration when we call on them to
produce a low to moderate level of force. Aerobic respiration requires
oxygen to produce about 36-38 ATP molecules from a molecule of glucose.
Aerobic respiration is very efficient, and can continue as long as a
muscle receives adequate amounts of oxygen and glucose to keep
contracting. When we use muscles to produce a high level of force, they
become so tightly contracted that oxygen carrying blood cannot enter the
muscle. This condition causes the muscle to create energy using lactic
acid fermentation, a form of anaerobic respiration. Anaerobic
respiration is much less efficient than aerobic respiration—only 2 ATP
are produced for each molecule of glucose. Muscles quickly tire as they
burn through their energy reserves under anaerobic respiration.
To keep muscles working for a longer period of time, muscle fibers
contain several important energy molecules. Myoglobin, a red pigment
found in muscles, contains iron and stores oxygen in a manner similar to
hemoglobin in the blood. The oxygen from myoglobin allows muscles to
continue aerobic respiration in the absence of oxygen. Another chemical
that helps to keep muscles working is creatine phosphate. Muscles use
energy in the form of ATP, converting ATP to ADP to release its energy.
Creatine phosphate donates its phosphate group to ADP to turn it back
into ATP in order to provide extra energy to the muscle. Finally, muscle
fibers contain energy-storing glycogen, a large macromolecule made of
many linked glucoses. Active muscles break glucoses off of glycogen
molecules to provide an internal fuel supply.
When muscles run out of energy during either aerobic or anaerobic
respiration, the muscle quickly tires and loses its ability to contract.
This condition is known as muscle fatigue. A fatigued muscle contains
very little or no oxygen, glucose or ATP, but instead has many waste
products from respiration, like lactic acid and ADP. The body must take
in extra oxygen after exertion to replace the oxygen that was stored in
myoglobin in the muscle fiber as well as to power the aerobic
respiration that will rebuild the energy supplies inside of the cell.
Oxygen debt (or recovery oxygen uptake) is the name for the extra oxygen
that the body must take in to restore the muscle cells to their resting
state. This explains why you feel out of breath for a few minutes after
a strenuous activity—your body is trying to restore itself to its
normal state.
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