Hoofdstuk 10 Muscle tissue
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Humane anatomie en fysiologie (AB_1125)
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Vrije Universiteit Amsterdam
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Hoofdstuk 10: Muscle tissue
10.
Common properties of muscle tissue
- Muscle tissue shares these common properties:
● Excitability, the ability to receive and respond to a stimulus.
● Contractibility, refers to the ability of a muscle cel to shorten when its stimulated.
● Extensibility, stretching movement of a muscle.
● Elasticity, the ability of a muscle to spring back to its resting length.
Functions of skeletal muscle
- Our skeletal muscles have the following functions:
● Producing movement
● Maintaining posture and body position
● Supporting soft tissues
● Guarding body entrances and exits.
● Maintaining body temperature
10.
- Skeletal muscles are organs composed mainly of skeletal muscle tissue, but they also
contain connective tissues, blood vessels and nerves.
- each cell in skeletal muscle tissue is a single muscle fiber.
- The muscle tissue in a skeletal muscle is surrounded by three layers of connective tissue:
1. Epimysium, a dense layer of collagen fibers that surround the entire muscle
2. Perimysium, divides the skeletal muscle into a series of compartments. Each
compartment contains a bundle of muscle fibers called a fascicle.
3. Within a fascicle the endomysium surrounds the individual skeletal muscle cells,
called muscle fibers and loosely interconnects adjacent muscle fibers.
> This flexible, elastic connective tissue later contains:
1. Capillary networks that supply blood to the muscle fibers
2. Myosatellite cells, stem cells that help repair damaged muscle tissue.
3. Nerve fibers that control the muscle.
- The muscle, the collagen fobers of the epimysium, perimysium and endomysium come
together to form either a bundle known as a tendon or a broad sheet called an aponeurosis.
- Tendons and aponeuroses usually attach skeletal muscles to bones.
Function of skeletal muscle components
- Axons are nerve fibers extending from neurons.
- Skeletal muscles are often called voluntary muscles because we have voluntary control
over their contractions.
- We also control many skeletal muscles at a subconscious level, such as muscles involved
with breathing.
10.
Differences between other cells and skeletal muscle cells
● Muscle cells are bigger
● Skeletal muscle fibers are multinucleated, which means that each cell contains
hundreds of nuclei just internal to the plasma membrane.
- During development, groups of embryonic cells called myoblasts fuse, forming individual
multinucleated skeletal muscle fibers,
● Muscle cells are banded/striated. Each muscle fiber contains hundreds to thousands
of cylindrical structures called myofibrils.
- The striations are due to the precise arrangements of thin (actin) and thick (myosin)
filaments in these myofibrils.
- The arrangement of these filaments forms the repeating functional unit called a sarcomere.
The sliding of these filaments allows the muscle fiber to contract.
The sarcolemma and transverse tubules
- The sarcolemma of a muscle fiber surrounds the sarcoplasm of the muscle fiber.
- In a skeletal muscle fiber, a sudden change in membrane potential leads to contraction.
- The signal to contact is propagated through the transverse tubules.
- Transverse tubules form passageways through the muscle fiber, like a network of tunnels.
- In skeletal muscle fibers a membrane complex called sarcoplasmic reticulum (SR) forms a
tubular network around each myofibril. The SR is similar to the ER of other cells.
- A myofibril is 1-2 micrometers in diameter and as long as the entire muscle fiber.
- Myofibrils consist of bundles of protein filaments called myofilaments.
● Thin filaments composed of actin
● Thick filaments composed of myosine
- In addition, myofibrils contain titin, elastic myofilaments associated with the thick filaments.
- Scattered among the myofibrils are mitochondria and granules of glycogen, the storage
form of glucose.
- Mitochondrial activity and glucose breakdown by glycolysis provide energy in the form of
ATP for short-duration, maximum-intensity muscular contractions.
- A thick filament contains about 300 myosin molecules, each make up a pair of myosin
subunits twisted around one another.
What happens to a sarcomere when a skeletal muscle fiber contracts
● The H bands and I bands of the sarcomeres narrow
● The zones of overlap widen
● The Z lines move closer together
● THe width of the A band remains constant.
- These observations make sense only if the thin filaments are sliding toward the center of
each sarcomere, alongside the thick filaments.
- This is called the sliding-filament theory.
10.
Myofilaments in a sarcomere change position during a contraction, but how do these
changes occur?
- All the cells of the body maintain a membrane potential due to unequal distribution
of positive and negative charges across their plasma membrane. The unequal
charge distribution means cells are polarized, much like batteries.
- The inner surface of the plasma membrane is slightly negative compared to the
outer surface.
Membrane potential:
A measure of cellulare polarization that compares the cytoplasmic membrane
surface charge to the extracellular membrane surface charge.
● There is an excess of sodium ions outside the cell and an excess of
potassium ions inside the cel. These ions can cross a membrane through
specific membrane channels.
● Depolarization:
An influx (toestroom) of sodium causes the membrane potential to become
more negative.
● Hyperpolarization:
The movement of potassium ions out of a cell causes the membrane potential
to become more negative.
● Repolarization:
A return to the resting potential
- Depolarization or hyperpolarization are graded potentials. It does not continue to
spread over the plasma membrane.
- Neurons and skeletal muscle fibers have electrically excitable membranes, which
means that the membranes permit rapid communication between different parts of
the cell.
- In neurons and skeletal muscle fibers, the depolarization and repolarization events
produce an electrical impulse, called action potential.
The control of skeletal muscle activity
- Skeletal muscles cannot begin contraction until they receive instructions from motor
neurons of the central nervous system. The instructions are the action potentials.
- A contraction begins when the sarcoplasmic reticulum releases stored calcium ions
into the cytosol of the muscle fiber.
- Communication between a neuron and another cell occurs at a synapse. When the
other cell is a skeletal muscle fiber, the synapse is known as a neuromuscular
junction (NMJ).
- The NMJ is made up of an axon terminal of a neuron, a specialized region of the
sarcolemma called the motor end plate and in between a narrow space called the
synaptic cleft.
Neurotransmitter:
A chemical released by a neuron to change the permeability or other properties of
another cell’s plasma membrane
Events at the neuromuscular junction:
1. The cytoplasm of the axon terminal contains vesicles filled with molecules of
acetylcholine.
2. The stimulus for ACh release is the arrival of an electrical impulse or action
potential at the axon terminal.
3. When the action potential reaches the neuron’s axon terminal, permeability
changes in its membrane trigger the exocytosis of ACh into the synaptic cleft.
4. ACh molecules diffuse across the synaptic cleft and bind to ACh receptor
membrane channels. ACh binding opens the membrane channel on the
surface of the motor end plate. Sodium ions rush into the cytosol.
5. The sudden inrush of sodium ions result in the generation of an action
potential in the sarcolemma. ACh is removed in two ways:
● ACh diffuses away from the synapse
● ACh is broken down by the enzyme AChE into acetic acid and choline,
This removal closes the ACh receptor membrane channels.
Excitation-contraction coupling:
The link between the generation of an action potential in the sarcolemma and the
start of a muscle contraction.
Effects of repeated stimulations
● Treppe, an increase in peak tension with each successive stimulus delivered shortly
after the completion of the relaxation phase of the preceding twitch.
● Wave summation, occurs when successive stimuli arrive before the relaxation phase
has been completed.
● Incomplete tetanus, occurs if the stimulus frequency increases further.
● Complete tetanus, the stimulus frequency is so high that the relaxation phase is
eliminated.
10.
- The amount of tension produced by a muscle as a whole is the sum of the tensions
generated by its individual muscle fibers.
● In isotonic contractions the muscle length changes with a constant force.
● In isometric contractions the length of the muscle does not change with a constant
force.
- A motor unit is a motor neuron and all the muscle fibers that it controls. The size of a motor
unit indicates how fine a movement can be.
- An involuntary muscle twitch under the skin is called a fasciculation. Unlike other twitches a
fisculation involves more than one muscle fiber.
Recruitment
- A contraction begins with the activation of the smallest motor units in the stimulated
muscle. As the movement continues larger motor units containing faster and more powerful
muscle fibers are activated and tension rises steeply.
- The increase in muscular tension produced by increasing the number of active motor units
is called recruitment.
- In any skeletal muscle some motor units are always active, even when the entire muscle is
not contracting. Their contractions do not produce enough tension to cause movement, but
they do tense and firm the muscle.
● This resting tension in a skeletal muscle is called muscle tone.
- In the recovery period the conditions in muscle fibers are returned to a normal level. After a
period of moderate activity muscle fibers need several hours to recover.
blz 369
10.
Force:
The maximum amount of tension produced by a particular muscle or muscle group.
Endurance:
The amount of time during which a person can perform a particular activity.
Types of skeletal muscle fibers
● Fast fibers
- Large in diameter and contain densely packed myofibrils, large glycogen reserves
and relatively few mitochondria.
- Powerful contractions
- Fatigue rapidly
● Slow fibers
- Only about half the diameter of fast fibers
- Numerous mitochondria
- Higher oxygen supply, because of capillary network
- Contain the pigment myoglobin. This binds oxygen molecules.
● Intermediate fibers
- In between fast and slow fibers
- Intermediate capillary network and mitochondrial supply
- Fast fibers contain white muscle
- Slow fibers contain red muscles, because they have more extensive blood vessels and
myoglobin.
Hypertrophy:
An enlargement of the stimulated muscle. The number of muscles doesn’t change, but the
muscle as a whole enlarges because each muscle fiber increases in diameter.
- Occurs in muscles that have been repeatedly stimulated to produce near-maximal
tension.
- In contrast a skeletal muscle that is not regularly stimulated by a motor neuron loses
muscle tone and mass.
Atrophy:
Reduction in muscle size, tone and power. The muscle becomes soft and the muscle fibers
become smaller and weaker.
The effects of aging on the muscular system:
● Skeletal muscle fibers become smaller in diameter
● Skeletal muscles become less elastic
● Tolerance for exercise decreases
● The ability to recover from muscular injuries decreases.
- We say an active skeletal muscle is fatigued when it can no longer perform at the required
level of activity.
- Factors involved in muscle fatigue:
● Depletion of metabolic reserves
● Damage to the sarcolemma and SR
● A decline in pH within the muscle fibers and the muscle as a whole →
decreases calcium ion binding to troponin and alters enzyme activity
● Reduction in the desire to continue activity, due to effects of low blood pH and
sensations of pain.
- Muscle fatigue is cumulative, which means that the effects become more pronounced as
more neurons and muscle fibers are affected.
Energy use in training
● Anaerobic endurance is the length of time muscular contraction can continue to be
supported by the existing energy reserves of ATP and CP and by glycolysis.
● Aerobic endurance is the length of time a muscle can continue to contract while
supported by mitochondrial activities.
- During exercise blood vessels in the skeletal muscles dilate, which increases blood flow
and supply of oxygen and nutrients to the active muscle tissue.
Improvements in aerobic endurance results from two factors
1. Alterations in the characteristics of muscle fibers
- A person with more slow fibers will be better able to perform under aerobic
conditions.
- Fast fibers trained for aerobic competition develop characteristics of intermediate
fibers.
2. Improvements in cardiovascular performance
- Cardiovascular activity affects musical performance by delivering oxygen and
nutrients to active muscles.
3. Physical training alters cardiovascular function by accelerating blood flow, thus
improving oxygen and nutrient availability.
Was dit document nuttig?
Hoofdstuk 10 Muscle tissue
Vak: Humane anatomie en fysiologie (AB_1125)
126 Documenten
Studenten deelden 126 documenten in dit vak
Universiteit: Vrije Universiteit Amsterdam
Was dit document nuttig?
Hoofdstuk 10: Muscle tissue
10.1
Common properties of muscle tissue
- Muscle tissue shares these common properties:
● Excitability, the ability to receive and respond to a stimulus.
● Contractibility, refers to the ability of a muscle cel to shorten when its stimulated.
● Extensibility, stretching movement of a muscle.
● Elasticity, the ability of a muscle to spring back to its resting length.
Functions of skeletal muscle
- Our skeletal muscles have the following functions:
● Producing movement
● Maintaining posture and body position
● Supporting soft tissues
● Guarding body entrances and exits.
● Maintaining body temperature
10.2
- Skeletal muscles are organs composed mainly of skeletal muscle tissue, but they also
contain connective tissues, blood vessels and nerves.
- each cell in skeletal muscle tissue is a single muscle fiber.
- The muscle tissue in a skeletal muscle is surrounded by three layers of connective tissue:
1. Epimysium, a dense layer of collagen fibers that surround the entire muscle
2. Perimysium, divides the skeletal muscle into a series of compartments. Each
compartment contains a bundle of muscle fibers called a fascicle.
3. Within a fascicle the endomysium surrounds the individual skeletal muscle cells,
called muscle fibers and loosely interconnects adjacent muscle fibers.
> This flexible, elastic connective tissue later contains:
1. Capillary networks that supply blood to the muscle fibers
2. Myosatellite cells, stem cells that help repair damaged muscle tissue.
3. Nerve fibers that control the muscle.
- The muscle, the collagen fobers of the epimysium, perimysium and endomysium come
together to form either a bundle known as a tendon or a broad sheet called an aponeurosis.
- Tendons and aponeuroses usually attach skeletal muscles to bones.
Function of skeletal muscle components
- Axons are nerve fibers extending from neurons.
- Skeletal muscles are often called voluntary muscles because we have voluntary control
over their contractions.
- We also control many skeletal muscles at a subconscious level, such as muscles involved
with breathing.
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