Memmler Ch 8 Notes – The Muscular System
There are three kinds of muscle tissue:
1. Smooth
2. Cardiac
3. Skeletal
Smooth muscle
Smooth muscle makes up most of the walls of the hollow body organs as well as those of the blood
vessels and respiratory passageways.
Smooth muscle contracts involuntarily. Produces wavelike motions of peristalsis that move substance
through the system.
Smooth muscle can also regular the diameter of an opening such as the central opening of blood vessels,
or produce contractions of hollow organs, such as the uterus.
Smooth muscle fibers (cells) are tapered at each end and have a single, central nucleus. The cells appear
smooth under the microscope because they do not contain the visible bands (striations) that are seen in
other types of muscle cells.
Smooth muscle may contract in response to a nerve impulse, hormonal stimulation, stretching and
other stimuli. The muscle contracts and relaxes slowly and can remain contracted for a long time.
Cardiac Muscle
Also involuntary.
Makes up heart’s walls and creates the pulsing action of that organ.
Cell of cardiac muscle are striated, like those of skeletal muscle.
Cardiac muscle differs in having one nucleus per cell and branching interconnections. The membranes
between the cells are specialized to allow electric impulses that travel rapidly through them, so that
contractions can be better coordinated. These special membrane regions appear as dark lines between
the cells and are called intercalated disks, because they’re “inserted between” the cells.
The electric impulses that produce cardiac muscle contractions are generated within the muscle itself
but can be modified by nervous stimuli and hormones.
Skeletal Muscle
Skeletal muscle appears heavily striated when viewed under the microscope.
The arrangement of protein threads within the cell that produces these striations is described later {??}
The cells are very long and cylindrical, and because of their great length compared to other cells, they
are often described as muscle fibers.
They have multiple nuclei per cell because during development, groups of precursor cells called
myoblasts fuse to form large multinucleated cells.
Skeletal muscle is under the control of the nervous system division known as the voluntary, or somatic,
nervous system – it is under the conscious control.
This muscle tissue contracts and relaxes rapidly.
This muscle tissue is named skeletal because most of these muscles are attached to bones and produce
movement at the joints. There are few exceptions. Muscles of the abdominal wall, for example, are
partly attached to other muscles, and the muscles of facial expression are attached to the skin.
Skeletal muscle constitutes the largest amount of the body’s muscle tissue – making up 40% of the total
body weight.
Composed of more than 600 individual skeletal muscles – although each one is a distinct structure,
muscles usually act in groups to execute body movements.
The three primary functions of skeletal muscles:
1. Movement of the skeleton
2. Maintenance of posture
3. Generation of heat
Muscle structure
In forming whole muscles, individual muscle fibers (cells) are arranged in bundles, or fascicles – held
together by dense connective tissue. These layers are:
1. Endomysium – deepest layer of this connective tissue and surrounds the individual fibers with
fascicles.
2. Perimysium – a connective tissue layer around each fascicle.
3. Epimysium – a connective tissue sheath that encases the entire muscle. The epimysium forms
the innermost layer of the deep fascia, the tough, fibrous connective tissue membrane that
encloses and defines a muscle.
Muscle fiber’s cytoplasm Is called sarcoplasm.
Its plasma membrane is the sarcolemma. Extensions of the sarcolemma tunnel deep in the interior of
the muscle fiber as a network of T-tubules, which are important in muscle cell stimulation.
Muscle fibers contain large amounts of smooth endoplasmic reticulum, known as the sarcoplasmic
reticulum (SR). This organelle stores calcium, an important element in muscle contraction that will be
discussed later. {??}
Most of the muscle’s fiber volume is taken up by myofibrils, which are bundles of protein filaments. It is
these myofibrils that accomplish the work of muscle contraction.
Satellite cells: stem cells that can produce new myoblasts. These myoblasts can then fuse with an
existing muscle fiber, making it (and thus the muscle) larger and stronger.
Communication
Communication is the transmission of signals between cells or even within a single cell – is critical to all
aspects of body function, ranging from muscle contraction to the development of an entire individual
from a single cell. Defects in communication are responsible for many diseases.
Two types of signals:
1. Electrical: travel down cells much like electricity travels down a wire
2. Chemical: chemical signals alter cell activity by binding to specific proteins called receptors. The
signal that binds to the receptor is known as the ligand. A ligand fits into its receptor like a key in
a lock. Once the ligand binds, the receptor initiates events that change the target cell’s activity.
A cell’s response may vary depending on the receptors it contains.
Muscle Cells in Action
Signals coming from the brain and the spinal cord stimulate skeletal muscle fiber contractions. Because
these signals often stimulate movement, the neurons that carry these signals are described as motor
neurons. In contrast, sensory signals travel in sensory neurons from the periphery towards the central
nervous system. Each neuron has a long extension called an axon. The axons of motor neurons branch
to supple from a few to hundreds of individual muscle cells or in some cases more than 1,000.
A single neuron and all the muscle fibers it stimulates constitute a motor unit. Stimulation of the neuron
activates all of the associated muscle fibers, so stronger or weaker contractions use more or fewer
motor units (respectively).
Muscles containing small motor units (with few muscle fibers per neuron) provide more control because
they can change contraction strength in small increments. Ie. Hand / eye movement. Precise.
The Membrane Potential
As a result of unpaired charges, the plasma membrane of a living cell carries a difference in electric
charge (voltage) on either side that is know as a membrane (or trans-membrane) potential. Membrane
potential is measured inside the cell, so in resting cells, it is negative (about -70 millivolts or mV).
Muscle cells and neurons show the property of excitability, because their membrane potential can
change. ie. The membrane potential becomes more negative if negative ions enter the cell. It becomes
less negative if positive ions enter the cell to neutralize the unpaired negative ions.
This spreading wave of electric current is called the action potential because it calls the cell into action.
The Neuromuscular Junction
The point at which a nerve fiber contacts a muscle cell is called the neuromuscular junction (NMJ).
It is here that a chemical signal classified as a neurotransmitter is released from the neuron to stimulate
the muscle fiber. The specific neurotransmitter released here is acetylcholine (ACh).
The NMJ is an example of a synapse, a point of communication between a neuron and another cell.
At every synapse, there is a tiny space, the synaptic cleft, across which the neurotransmitter must travel.
Until its release, the neurotransmitter is stored in tiny membranous sacs, called vesicles, in the axon
ending. Once released, the neurotransmitter crosses the synaptic cleft and attaches to a receptor, which
is a protein embedded in the muscle cell membrane. The muscle cell membrane forms multiple folds at
this point, and these serve to increase surface area and hold a maximum number of receptors.
The muscle cell’s receiving membrane is known as the motor end plate.
Contraction
Another important property of muscle tissue is contractility. This is a muscle fiber’s capacity to undergo
shortening, becoming thicker. Studies of muscle chemistry and observation of cells under the powerful
electron microscope have increased our understanding of how muscle cells work.
Each myofibril contains many threads, or myofilaments, made primarily of two kinds of proteins, called
actin and myosin. Filaments made of actin are thin and light; those made of myosin are thick and dark.
The filaments are present in alternating bundles within the myofibril. It is the alternating bands of light
actin and heavy myosin filaments that give skeletal muscle its striated appearance. They also give a view
of what occurs when muscles contract.
Note: when actin and myosin filaments overlap where they meet – it is just like how fingers overlap
when you fold your hands together.
A contracting subunit of skeletal muscle is called a sarcomere. It consists of a band of myosin filaments
and the actin filaments on each side.
The Role of Calcium
In addition to actin, myosin, and ATP, calcium is needed for muscle contraction. IT enables cross-bridges
to form between actin and myosin so the sliding filament action can begin.
When muscles are at rest, two additional proteins called troponin and tropomyosin block the sites on
actin filaments where cross-bridges can form.
When calcium attaches to the troponin, these proteins move aside, uncovering the binding sites. IN
resting muscles, the calcium is not available because it is stored within the cell’s SR.
Calcium is released into the sarcoplasm in response to the action potential along the sarcolemma and Ttubules. Muscles relax when nervous stimulation stops, and the calcium is pumped back into the SR,
ready for the next contraction.
Storage Compounds:
-
Myoglobin: stores oxygen
Glycogen: the storage form of glucose
Fatty acids: stored as triglycerides – formed into fat droplets.
Anaerobic metabolism
Anaerobic metabolism is the creation of energy through the combustion of carbohydrates in
the absence of oxygen. This occurs when your lungs cannot put enough oxygen into the
bloodstream to keep up with the demands of your muscles for energy.
Aerobic metabolism is the way your body creates energy through the combustion of
carbohydrates, amino acids, and fats in the presence of oxygen. Combustion means burning,
which is why this is called burning sugars, fats, and proteins for energy.
Breakdown of creatine phosphate: creatine phosphate is a compound similar to ATP in that is
has a high-energy bond that breaks down to release energy. This energy is used to make ATP
for muscle contraction. It generates ATP very rapidly, but its supply is limited.
Anaerobic glycolysis: The process breaks glucose down incompletely without using oxygen. A
few ATPs are generated in these reactions, as is a byproduct called lactate, which is later
oxidized for energy when oxygen is available.
Types of muscle contractions:
Muscle tone refers to a muscle’s partially contracted state that is normal even when the muscle
is not in use.
Tone / tonus is due to the action of the nervous system in keeping the muscle in a constant
state of readiness for action.
Isotonic contractions: the tone or tension within the muscle remains the same, but muscle length
changes, and the muscle bulges as it accomplishes work.
Concentric contractions: muscle as a whole shortens to produce a movement.
Eccentric contractions: the muscle lengthens as it exerts force.
Isometric contractions: there is no change in muscle length, but there is a great increase in
muscle tension.
The Mechanics of Muscle Movement:
Aponeurosis: a broad sheet that may attach muscles to bones or to other muscles.
In moving the bones, one end of a muscle is attached to a more freely movable part of the
skeleton, and the other end is attached to a relatively stable part.
The less moveable (more fixed) attachment is called the origin; the attachment to the body part
that moves is called the insertion.
Muscles Work Together
The main muscle that performs a given movement is the prime mover. ie. The brachialis is the
prime mover for flexion of the arm at the elbow.
Because any muscle that performs a given action is technically called an agonist, the muscle
that produces an opposite action is termed the antagonist.
For any given movement, the antagonist must relax when the agonist contracts.
These “helping” muscles are called synergists, because they work with the prime mover to
accomplish a movement. ie. The biceps brachii and the brachioradialis are synergists to the
brachialis in flexing the arm.
Levers and Body Mechanics
There are three classes of levers, which differ only in location of the fulcrum (F); the effort (E), or
force; and the resistance (R), the weight or load
Most lever systems in the body are of the third-class type {??}
Skeletal Muscle Groups
A number of different characteristics are used in naming muscles, including the following:
-
Location
Size
Shape
-
Direction of fibers
Number of heads
Action
Muscles of the head:
-
The muscles of facial expression include ring-shaped ones around the eyes and the lips,
called the orbicularis muscles.
The muscle surrounding the eye is called the orbicularis oculi, whereas the lip muscle is
the orbicularis oris. These muscles all have antagonists. For example, the levator
palpbrae superioris, or lifter of the upper eyelid, is the antagonist for the orbicularis oculi.
One of the largest muscles of expression forms the fleshy part of the cheek and is called the
buccinator. Used in whistling or blowing, it is sometimes referred to as the trumpeter’s
muscle.
There are four pairs of mastication (chewing) muscles, all of which insert on and move the
mandible. The largest are the temporalis, which is superior to the ear, and the masseter at
the angle of the jaw.
The tongue has two muscle groups. The first group called the intrinsic muscles, is located
entirely within the tongue. The second group, the extrinsic muscles, originates outside the
tongue.
Muscles That Move:
The position of the shoulder depends to a large extent on the degree of contraction of the
trapezius, a triangular muscle that covers the posterior neck and extends across the
posterior shoulder to insert on the clavicle and scapula.
The latissimus dorsi is the wide muscle of the back and lateral trunk.
A large pectoralis major is located on either side of the superior chest.
The serratus anterior is below the axilla, on the lateral chest.
The deltoid covers the shoulder joint and is responsible for the roundness of the upper arm
just inferior to the shoulder.
The biceps brachii, located at the anterior arm along the humerus, is the muscle you usually
display when you want to “flex your muscles.”
The brachialis lies deep to the biceps brachii and inserts distally over the anterior elbow
joint.
Another forearm flexor at the elbow is the brachioradialis, a prominent forearm muscle that
originates at the distal humerus and inserts on the distal radius.
The triceps brachii, located on the posterior arm, inserts on the olecranon of the ulna.
The flexors in the anterior forearm and the extensors in the posterior forearm act on the
hand (carpi muscles) and fingers (digitorum muscles).
The most important muscle involved in the act of breathing is the diaphragm. This domeshaped muscle forms the partition between the thoracic cavity above and the abdominal
cavity below.
The intercostal muscles, which also act to change thoracic volume, are attached to the ribs
and fill the spaces between them.
The external oblique on the exterior, the internal oblique in the middle, and the transversus
abdominis, the innermost. The connective tissue from these muscles extends anteriorly and
encloses the vertical rectus abdomis.
The midline meeting of the aponeuroses forms a whitish area called the linea alba, which is
an important abdominal landmark.
These four pairs of abdominal muscles act together to protect the internal organs and
compress the abdominal cavity, as in forcefully exhaling, coughing, emptying the bladder
and bowel, sneezing, vomiting, and childbirth.
The pelvic floor or perineum has its own form of diaphragm, shaped somewhat like a
shallow dish. One of the principal muscles of this pelvic diaphragm is the levator ani, which
acts on the rectum and thus aids in defecation.
The erector spinae muscles make up a large group located between the sacrum and the
skull.