MUSCLE TISSUE
A. TYPES OF MUSCLE
Although bones and joints provide leverage and the framework for the body, they cannot move the body.
What causes motion?
Motion results from the alternating contraction (shortening) and relaxation of muscles.
The prime function of muscle is to convert chemical energy (ATP) into mechanical energy that can be used to …?
Generate force, perform work, and produce movements
Name and briefly describe the three types of muscle tissue?
Skeletal muscle -- Skeletal muscle is striated muscle and is voluntary (it
can be controlled consciously). It can only be stimulated by the nervous system.
Cardiac muscle -- Cardiac muscle is striated muscle. It is involuntary (no
conscious control). It can be stimulated or inhibited by either the nervous or the endocrine systems.
Smooth muscle -- Smooth muscle is nonstriated and also involuntary. It
also can be stimulated or inhibited by either the nervous or endocrine function.
B. FUNCTIONS OF MUSCLE
Identify and briefly describe the three functions of muscle tissue.
Motion -- Motion can be obvious body movements or less noticeable
motions such as heartbeat and gut movement.
Stabilize body positions and regulate organ volume -- Sustained
contractions of skeletal muscle maintain body posture without creating noticeable movement. Sustained contractions of smooth muscle prevent outflow from hollow organs and maintain them at an appropriate volume.
Thermogenesis -- A by-product of muscle contraction is heat production
and is therefore important in homeostasis of body temperature.
C. CHARACTERISTICS OF MUSCLE
List and define the four basic characteristics of muscle tissue.
Excitability -- Also known as irritability. This is the ability to receive and
respond to certain stimuli by producing electrical messages.
Contractility -- Contractility is the ability to shorten and thicken (contract),
thus generating force to do work.
Extensibility -- Extensibility is the ability to stretch without damaging the
tissue.
Elasticity -- Elasticity is the ability to return to original shape after
contraction or extension.
D. ANATOMY AND INNERVATION OF SKELETAL MUSCLE TISSUE
1. CONNECTIVE TISSUE COMPONENTS
What is fascia?
Fascia refers to a sheet or broad band of fibrous connective tissue beneath the skin or around muscles and organs of the body. There are two types.
Describe superficial fascia.
Superficial fascia (subcutaneous layer or hypodermis) lies
immediately deep to the skin. It is composed of adipose and areolar tissues. Superficial fascia has four functions:
1. Store fat and therefore water
2. Insulation
3. Mechanical protection
4. Pathway for nerves and blood vessels
Describe deep fascia.
Deep fascia is formed of dense irregular connective tissue. It lines
the body wall and extremities and holds muscles together, separating them into functional groups.
Deep fascia allows free movement of muscles, carries nerves, blood vessels, and lymph vessels, and fills the spaces between muscles.
What is the function of the connective tissue specializations that surround skeletal muscle cells?
These connective tissues protect and strengthen the muscles, as
well as provide attachment of the muscle to surrounding structures. All the layers are continuous with each other and surrounding deep fascia.
Identify the following:
Epimysium -- Epimysium is the outermost layer of deep fascia,
wrapping the entire muscle.
Perimysium -- Perimysium is formed by invaginations of the
epimysium, dividing the muscle into bundles of cells called fascicles (fasciculi).
Endomysium -- Endomysium is formed by invaginations of the
perimysium that penetrate the fascicle and wrap each muscle cell, completely insulating it from the others.
Tendon -- All three layers may extend beyond the muscle as a cord
of dense connective tissue, called a tendon, that attaches the muscle to the periosteum of a bone.
Tendon sheath -- Some tendons, particularly those in high stress
areas like the ankle and wrist, are wrapped in a layer of synovial membrane, forming a tendon (synovial) sheath.
Aponeurosis -- When the three connective tissue components
extend from the muscle as a flat sheet, rather that a round cord, they are called an aponeurosis.
2. THE MOTOR UNIT
What is a motor neuron?
A motor neuron delivers the nervous stimulus that ultimately causes
a muscle tissue to contract.
Define the concept of a motor unit?
One motor neuron plus ALL of the skeletal muscle cells it
stimulates is called a motor unit. On average, a single motor neuron makes contact and thus stimulates about 150 individual skeletal muscle cells. All the cells contract and relax together, as a unit.
How are motor units different for precise and gross movements?
Muscles that control precise body movements may have as few as
2-3 muscle cells per motor neuron (eye muscles), while muscles that control gross body movements may have as many as 2,000 muscle cells per motor neuron (gluteal muscles).
How is the total strength of a muscle determined?
Stimulation by its motor neuron results in the simultaneous
contraction of all the skeletal muscle cells in the motor unit. Accordingly, the total strength of any particular muscle is determined by the total number of motor units being used at any given time.
3. THE NEUROMUSCULAR JUNCTION
Describe the neuromuscular junction.
Excitable cells (muscle and nerve) make contact and communicate
with one another at specialized regions called synapses.
At each synapses a small gap, called the synaptic cleft, separates the two excitable cells.
The first cell, the motor neuron, communicates with the second cell, the skeletal muscle cell, across the synaptic cleft via a chemical messenger called a neurotransmitter.
The type of synapse formed between the motor neuron and the skeletal muscle cell is called the neuromuscular (myoneural) junction.
At the synapse, the motor neuron branches into clusters of bulb-shaped axon terminals (end bulbs), each cluster forming a synapse with a group of muscle cells (motor unit).
The region of the muscle cell membrane that participates in the synapse with the axon terminal is the motor end plate.
Describe the use of acetylcholine.
Within each axon terminal are many membrane-enclosed vesicles called synaptic vesicles containing thousands of neurotransmitter molecules.
The neurotransmitter used exclusively by motor neurons for skeletal muscles is acetylcholine (ACh).
The nerve impulse (1) reaching the axon terminal, triggers exocytosis of the synaptic vesicles (2), releasing the ACh into the synaptic cleft.
ACh diffuses through the extracellular fluid in the synaptic cleft and approaches the motor end plate.
On the motor end plate are ACh receptors (30-40 million). These are integral proteins specific for ACh. They recognize the molecule and bind specifically to it, causing Na+ channels to open (3).
Binding of ACh to its receptors on the motor end plate initiates an electrical message in the motor end plate, and therefore in the muscle cell membrane (4).
In most skeletal muscles there is a single neuromuscular junction per cell, located near the cell’s midpoint.
Stimulation of the membrane in this way spreads from the midpoint of the muscle cell towards its ends and therefore causes almost simultaneous contraction of all parts of the cell.
4. MICROSCOPIC ANATOMY OF MUSCLE
Describe each of the following concerning skeletal muscle cells.
Myofiber -- Within a typical skeletal muscle there are thousands of
individual, very long, cylindrical cells called muscle fibers (myofibers), bundled together as fascicles. They lie in parallel to one another, ranging in size from 10-100 microns in diameter and up to 10 cm in length.
Sarcolemma -- The sarcolemma is the cell membrane of a muscle
cell. It surrounds the sarcoplasm, or muscle fiber cytoplasm.
Mitochondria -- Mitochondria lie in rows throughout the muscle
fiber, located close to the muscle proteins that use ATP for the contraction-relaxation sequence.
Nuclei -- Skeletal muscle cells are multinucleated, due to fusion of
precursor cells during embryogenesis; the nuclei are located along the periphery of the cell, out of the way of the contractile elements within the sarcoplasm.
a. MYOFIBRILS
What are myofibrils? Where are they located? What is their
composition? What is responsible for the alternating light and dark striations (bands) seen in skeletal and cardiac muscles?
At higher magnification the sarcoplasm appears to be stuffed
with small threads called myofibrils.
Myofibrils lie in parallel to each other and extend lengthwise throughout the extent of the myofiber.
The prominent alternating light and dark bands evident in myofibrils give skeletal muscle cells their characteristic striations.
Myofibrils are the contractile elements of skeletal muscle. Each is 1-2 microns in diameter and consists of three even smaller structures called myofilaments.
These myofilaments do not extend the length of the myofiber, but rather are stacked into repeating units (compartments) called sarcomeres
What is a sarcomere?
The sarcomere is the functional (contracting) unit of skeletal
muscle.
What is the Z disc (line)?
A dense material called the Z disc is found at each end of a
sarcomere, separating it from the next sarcomere in line.
Where are thin and thick myofilaments located in the sarcomere?
Extending from each Z disc towards the middle of the sarcomere are the thin myofilaments.
Suspended within the sarcoplasm, between the thin myofilaments, and not attached to the z discs, are the thick myofilaments.
What forms the striations of skeletal muscle?
The alternating areas of thin myofilaments, followed by areas of overlapping thin and thick myofilaments, are responsible for the striations seen in skeletal muscle.
Describe the thin myofilament by describing the following proteins:
Actin -- The main component of a thin myofilament is actin,
each molecule of which looks like a kidney bean. Individual molecules of actin are linked together to form the actin filament that is twisted to form a helical strand. On each molecule of actin within the helical strand is a myosin-binding site upon which the thick myofilaments will attach.
Tropomyosin-troponin complex -- Also present on the thin
myofilament are two regulatory molecules called tropomyosin and troponin. In relaxed muscle, the tropomyosin-troponin complex covers the myosin-binding sites on the actin molecules. This blocks the myosin-binding sites and prevents the attachment of the thick myofilaments, thus preventing contraction of the sarcomere.
Describe the thick myofilament by describing the following:
Myosin -- Each thick myofilament is composed of about 200
molecules of a protein called myosin. A molecule of myosin is shaped like two golf clubs twisted together. The tail of the molecule extends to the center of each sarcomere. The projecting “head,” called a cross bridge, extends out towards the thin myofilaments.
Arrangement of molecules -- Tails of adjacent myosin
molecules lie parallel to each other, forming the “shaft” of the thick myofilament, while the “heads” project around the shaft in a spiraling fashion.
Titan -- A third component of the sarcomere is the elastin
filament (also known as titan). The role of titan is to anchor the thick myofilaments in position and to play a role in recovery of the resting sarcomere length when a muscle cell is stretched or contracted.
b. SARCOPLASMIC RETICULUM AND TRANSVERSE TUBULES
Describe the sarcoplasmic reticulum and calcium flux.
A fluid-filled system of tubules called the sarcoplasmic
reticulum encircles each myofibril. In a relaxed muscle cell, the sarcoplasmic reticulum stores calcium ions by sequestering them from the sarcoplasm. Calcium ions released through calcium channels back into the sarcoplasm around the thin and thick myofilaments trigger muscle contraction.
Describe transverse tubules.
Transverse tubules (T tubules) are tunnel- like enfolding of
the sarcolemma. They penetrate the myofiber at right angles to the sarcoplasmic reticulum and the myofilaments. T tubules are open to the outside of the muscle fiber and are therefore filled with extracellular fluid.
What is a muscle triad?
On both sides of a T-tubule are dilated end sacs of the
sarcoplasmic reticulum called the terminal cisternae. A T-tubule, together with its two terminal cisternae, is called a muscle triad.
E. CONTRACTION OF SKELETAL MUSCLE
1. SLIDING FILAMENT MECHANISM
a. ROLE OF CALCIUM AND REGULATOR PROTEINS
b. THE POWER STROKE AND THE ROLE OF ATP
Describe the sliding filament theory of muscular contraction.
In the 1950's it was proposed that skeletal muscle shortens during
contraction because the thin and thick myofilaments slide past one another. This model is known as the sliding filament mechanism.
During muscle contraction, the thick myofilaments attach to, then pull on, the thin myofilaments, causing them to slide inward towards each other.
As the cross bridges apply force to the thin myofilaments, the thin myofilaments move towards the center of the sarcomere. This may occur so far that their tips overlap each other.
As the thin myofilaments “slide” inward, they pull the Z discs towards each other, and the sarcomere shortens, but the lengths of the myofilaments stay the same.
The shortening of the sarcomeres, all in series, causes shortening of the whole muscle fiber, and ultimately of the entire muscle itself, producing force used for work.
Describe the electrical and chemical events of skeletal muscle contraction.
Acetylcholine released by the motor neuron at the neuromuscular junction diffuses across the synaptic cleft and binds to its receptors on the motor end plate. This binding initiates formation of an electrical message in the sarcolemma that spreads in all directions, passing down the transverse tubules and out into the sarcoplasmic reticulum, causing calcium channels there to open, allowing calcium to diffuse into the sarcoplasm. Calcium does two things:
(1) it binds to troponin, causing the tropomyosin-troponin
complex to move, exposing the myosin binding sites on the actin;
(2) it activates ATP on the myosin heads, causing them to
bind to the actin.
Binding of myosin heads to actin causes the hinge regions to tilt, pulling the thin myofilaments across the thick myofilaments, thus causing the sarcomeres to shorten and the muscle cells to contract.
2. RELAXATION
Describe the electrical and chemical events of skeletal muscle relaxation.
Two changes are necessary to permit a muscle to relax after it has
contracted. ACh is rapidly broken down in the synaptic cleft by the enzyme acetylcholinesterase (AChE) present on the motor end plate; this stops the generation of the muscle membrane electrical message. Secondly, calcium ions are pumped into the sarcoplasmic reticulum, where they are bound to the molecule calsequestrin, thus removing them from the sarcoplasm. Without calcium, the tropomyosin-troponin complex moves back over the actin, covering the myosin-binding sites. This prevents binding of myosin cross bridges to actin. Release of the actin by the cross bridges allows the thin myofilaments to slip back to their resting position so that the sarcomere resumes its resting length. The muscle cell is now relaxed.
3. MUSCLE TONE