I



Chapter 11 Muscular Tissue

Note: this chapter deals mostly with skeletal muscle. Cardiac and smooth muscle tissues are dealt with in other chapters.

1 Types and characteristics of muscular tissue

1 Universal characteristics of muscle

These are special adaptations of muscle tissue. Some of these properties are also found in other cell types.

1 Responsiveness or excitability

Both nerve and muscle tissue exhibit electrical excitability, which is the ability to respond to stimuli by producing an electrical signal. Muscle cells can be stimulated by 1. chemicals such as neurotransmitters, hormones or pH or mechanical stimulus (stretch).

2 Conductivity

The electrical signal produced in neurons and muscle cells is called the action potential. The action potential travels - is conducted - along the plasma membrane of excitable cells.

3 Contractility

When stimulated by an action potential, muscle cells decrease in length: they contract. Contraction increases tension on the attached ends of a muscle cell or entire muscle and can cause movement if enough tension is applied.

4 Extensibility

Muscle cells can stretch without being damaged and allows contraction even when the cell is already extended.

5 Elasticity

Contraction would be useless unless muscle cells can return to their starting length. This is due to the elastic nature of muscle cells.

2 Skeletal muscle

Most skeletal muscles move bones of the skeleton. A few skeletal muscles attach to and move skin or other muscles.

1 Striated

Skeletal muscle has alternating light and dark bands.

2 Voluntary

Skeletal muscle works mainly under conscious control by neurons of the somatic (voluntary) division of the nervous system. There is subconscious control as well, for instance in the skeletal muscles involved in breathing.

3 Connective tissue elements

1 Epimysium

Surrounds the entire muscle.

2 Perimysium

Surrounds bundles of muscle fibers (muscle cells).

1 Fascicles

The term for bundles of muscle fibers. When we see "grain" in a cut of meat, we are seeing the fascicles.

3 Endomysium

Separate individual muscle fibers from one another.

2 Microscopic anatomy of skeletal muscle

1 The muscle fiber

Note: muscle fiber = muscle cell.

1 General characteristics

1 Long and thin

A mature muscle fiber is typically 10 to 100 micrometers in diameter but can extend up to 30 centimeters in length.

2 Multinucleated

A single muscle fiber can have 100 nuclei.

3 Muscle cells do not divide

The number of muscle cells developed in embryonic development is the number that we have for our entire lives (notwithstanding the muscle stem cell issue, which is still controversial).

4 Fibrosis

The replacement of muscle tissue with fibrous scar tissue in response to damage or degeneration. Retains the structural integrity of a muscle but not the contractibility of the replaced tissue.

5 Sarcolemma

The plasma membrane of a muscle cell.

1 Transverse (T) tubules

Tiny tunnels that extend from the sarcolemma toward the center of the muscle fiber. They are extensions of the sarcolemma so that the lipid membrane continues over the surface of the cell directly into the cell. This is essential to the function of the muscle cell as described below.

6 Sarcoplasm

The cytoplasm of the muscle fiber. The following are components of the sarcoplasm.

1 Myofibrils

The myofibrils of a muscle fiber are the contractile organelles of the cell. They extend the length of the muscle fiber. They are the component of the muscle fiber that is striated.

2 Glycogen

The storage carbohydrate of muscle cells. Can be hydrolyzed into glucose and utilized for ATP production.

3 Myoglobin

Similar to hemoglobin, this muscle-specific protein carries oxygen to the mitochondria of the muscle fiber for ATP production.

4 Sarcoplasmic reticulum (SR)

Similar to the smooth endoplasmic reticulum in non-muscular cells, the SR is a fluid-filled network of membrane-bound sacs and tubes. They encircle each myofibril.

The SR stores calcium ions (Ca2+).

1 Terminal cisterns

Swellings at the end of the sarcoplasmic reticulum. When calcium ions are released from the terminal cisterns, they trigger the contraction of a muscle.

2 Myofilaments

Myofibrils are made up of smaller structures called myofilaments or filaments. Both thin and thick filaments are directly involved in muscle contraction.

1 Thick filaments

1 Myosin

Myosin is the component of the thick filament.

1 Myosin tail

Long portions of the myosin molecule, lying parallel to one another

2 Myosin head

The myosin head projects away from the myosin tail. They extend toward the thin filaments.

2 Thin filaments

1 Actin

Actin is the protein that makes up the thin filament.

1 Active site or myosin-binding site

Each actin molecule has a myosin-binding site. The actin molecules combine to form a long filament.

2 Regulatory proteins

Both of these regulatory proteins are part of the thin filament.

1 Tropomyosin

In relaxed muscle, tropomyosin molecules cover the myosin-binding sites of actin. The thin and thick filaments cannot join.

2 Troponin

Troponin holds the tropomyosin molecules in place.

3 Elastic filaments

1 Titin

Attached to the thick filament, titin anchors and stabilizes the thick filament.

3 Striations

1 Sarcomeres

The sarcomere refers to the arrangement of the thick and thin filaments.

The sarcomere is the basic functional unit of the muscle fiber.

The pattern of the overlap of the thick and thin gives the muscle its striated appearance.

1 A band

2 Zone of overlap

3 I band

4 H zone

5 M line

6 Z discs

3 The nerve-muscle relationship

1 Motor neurons

1 Somatic motor fibers (neurons)

Somatic motor neurons are the neurons that stimulate skeletal muscle fibers. Every skeletal muscle fiber must be in contact with a neuron for contraction.

2 The motor unit

One nerve fiber and all of the muscle fibers that it innervates is called a motor unit. The muscle fibers of a single motor unit are dispersed throughout a muscle.

A single motor neuron supplies on average about 200 muscle fibers but this number varies. For example, for the muscles that control eye movement, there are only aobut 3-6 muscle fibers per neuron providing a fine degree of control with little strength. The gastrocnemius muscle of the calf - a large motor unit - has about 1000 muscle fibers per neuron.

3 The neuromuscular junction

The neuromuscular junction is the synapse between the somatic motor neuron and the muscle fiber.

1 Synapse

In general, a synapse is the region of communication between a neuron and another cell. The other cell can be a neuron, or in the case of the neuromuscular junction, a skeletal muscle cell.

1 Synaptic cleft

At most synapses, including the NMJ, there is a small gap between the neuron and the target cell called the synaptic cleft. The synaptic cleft prevents the electrical activity of the neuronal action potential from passing directly to the target cell.

2 Synaptic vesicles

In the end of the motor neuron, there are membrane-bound sacs filled with neurotransmitters.

3 Neurotransmitter

Instead of an electrical signal passing between the neuron and skeletal muscle cell in the synaptic cleft, a signaling chemical called a neurotransmitter is released by the neuron into the synaptic cleft.

1 Acetylcholine

Acetylcholine or ACh is the neurotransmitter released in the NMJ.

2 Acetylcholinesterase

The action potential that results from binding of ACh to the ACh receptors doesn't last long because ACh is rapidly degraded by a specific enzyme found in the synaptic cleft called acetylcholinesterase.

2 Motor end plate

The motor end plate is the region of the skeletal muscle sarcolemma opposite the synaptic end of the neuron.

1 Acetylcholine receptors

The motor end plate membrane contains membrane protein receptors that are specific for acetylcholine.

1 Ligand-gated ion channels

Acetylcholine receptors are ligand-gated channels. This means that when the acetylcholine (the ligand) binds to the receptor, ion channels (

for sodium ions, Na+ in this case) are opened.

4 Electrically excitable cells

Muscle cells and neurons are electrically excitable because their plasma membranes show voltage changes in response to stimuli. In skeletal muscle fibers, the change in voltage results in contraction of the muscle fiber.

1 Polarized

In a resting or unstimulated skeletal muscle fiber, their are more negative charges (anions) on the inside of the membrane compared with outside. In this case, the cell is considered polarized.

2 Resting membrane potential

The voltage difference across the plasma membrane of the resting skeletal muscle due to the charge imbalance described above is about -90 mV and is called the resting membrane potential.

3 Depolarization

When a muscle fiber is stimulated the voltage across the membrane changes momentarily from negative on the inside to positive on the inside. The change is called depolarization.

4 Repolarization

The return of the voltage across the membrane to the resting membrane potential is called repolarization.

5 Action potential

The fast depolarization/repolarization voltage changes that occur across the plasma membrane of the skeletal muscle fiber - and travels throughout the entire plasma membrane - is called an action potential.

4 Behavior of skeletal muscle fibers

1 Excitation

Excitation is the process in which action potentials in the nerve fiber lead to action potentials in the muscle fiber. The basic process is as follows:

1 Nerve signal arrives at synaptic knob

1 Stimulates voltage-gated calcium channels to open

1 Calcium ions enter synaptic knob

2 Calcium stimulates exocytosis of synaptic vesicles

1 Acetylcholine released into synaptic cleft

3 Acetylcholine diffuses across synaptic cleft

1 Acetylcholine binds to receptors on the sarcolemma

4 Acetylcholine receptors are ligand-gated channels

1 Sodium diffuses into muscle fiber

1 End-plate potential generated

5 Action potential initiated in sarcolemma

2 Excitation-contraction coupling

Excitation-contraction refers to the events linking the action potential to activation of the myofilaments.

1 Action potentials spread across sarcolemma

1 Action potentials reach T tubules and enter sarcoplasm.

2 Action potential open voltage-gated calcium channels in T tubules

1 Calcium diffuses out of sarcoplasmic reticulum

3 Calcium binds to troponin-tropomyosin

1 Troponin-tropomyosin changes shape

1 Actin active sites exposed

3 Contraction

Contraction is the step in which the myofilaments slide relative to one another and shorten the muscle fiber by shortening individual sarcomeres.

1 Myosin binds ATP

1 ATP hydrolyzed to ADP and inorganic phosphate

1 Energy released by ATP hydrolysis moves myosin head into high energy position

2 High energy myosin head binds to actin active site

1 Cross-bridges formed between myosin and actin (thick filament and thin filament)

3 Myosin ADP released

1 Myosin head moves

1 Thin filament slides past thick filament in power stroke

4 Myosin binds another ATP

5 Myosin released from actin active site

4 Relaxation

This the phase in which the muscle fiber returns to its resting length.

1 Nerve signals stop signaling

1 Synaptic knob stops releasing acetylcholine

2 Acetylcholine degraded by acetylcholinesterase in synaptic cleft

3 Calcium pumped back into sarcoplasmic reticulum (required ATP)

4 Calcium ions dissociate from troponin and are not replaced

5 Tropomyosin returns to actin active site

1 Blocks binding by myosin head

5 The length-tension relationship and muscle tone

1 Length-tension relationship

The amount of tension generated by a muscle - the force of its contraction - depends on the length of muscle before it was stimulated.

In an overly contracted muscle, the thick filaments are close to the Z discs. The stimulated muscle can contract only a limited distance and the contraction is weak.

If a muscle is overly stretched, there will be little overlap between the thick and thin filaments, again resulting in a weak contraction.

There is an optimum length prior to stimulation that results in the strongest contraction.

1 Muscle tone

The central nervous system provides a constant, low level stimulation of muscles (even when sleeping) that results in maintenance of the optimum contraction state. This is one of the factors in muscle tone.

6 Behavior of whole muscles

Note that the discussion of the behavior of whole muscles is derived from studies of isolated muscles in experimental conditions. The activity of muscles in vivo is not so simple and well-defined.

1 Threshold, latent period, and twitch

1 Threshold

A muscle must be stimulated with an electrical stimulus of a minimal voltage. Below the specific voltage, no contraction will occur. The stimulus must meet the threshold of the muscle.

2 Twitch

A single cycle of contraction and relaxation that results from stimulation by a threshold voltage (or higher) is called a twitch.

3 Latent period

There is a delay between the onset of a threshold stimulus and contraction called the latent period. This is the time required for excitation and excitation-contraction coupling, and the tensing of the elastic components of the muscle.

4 Contraction phases

Self-explanatory. This is the phase of the whole muscle activity where tension is developed and the muscle shortens.

5 Relaxation phases

The lengthening of the muscle due to the reduction of tension after the contraction has been completed.

2 Contraction strength of twitches

Although it appears from the discussion above that a twitch reaches its maximum intensity as long as the threshold stimulus voltages is attained, the strength of twitches can vary. There are several reasons.

1 Recruitment

In the stimulation of a whole muscle, increased intensity of stimulus will increase the contraction of a whole muscle because more motor neurons and thus more muscle fibers are stimulated with increased intensity. See Figure 11.13

2 Twitch strength varies with twitch frequency

Twitches that are closer together produce stronger contractions. At a frequency of about 10-20 twitches per second, the muscle can relax completely between stimuli. More frequent stimulation means that the muscle does not completely relax between pulses and the tension in the muscle increases. See Figure 11.14.

1 Treppe

Increasing tension with repetitive stimulation. The muscle does relax completely between pulses but the tension increases. One reason is that there is not time enough for calcium to be returned to the sarcoplasmic reticulum.

2 Temporal summation or wave summation

At even higher frequencies of stimulation, the muscle does not relax completely between pulses.

1 Incomplete tetanus

The result of temporal summation is a sustained, fluttering contraction known as incomplete tetanus.

2 Complete tetanus

At even higher frequency of temporal summation, there is a prolonged, smooth contraction known as complete tetanus. Not seen except in experimental situations.

3 The concentration of calcium in the sarcoplasm can vary the twitch strength

4 The degree of contraction prior to stimulus alters the twitch strength

5 Temperature

6 pH

7 Hydration

7 Isometric and isotonic contraction

1 Isometric contraction

Contraction without change in length.

2 Isotonic contraction

Contraction with a change in length but not in tension.

8 Muscle metabolism

1 ATP synthesis

1 Aerobic respiration

Highly efficient synthesis of ATP in presence of oxygen occurs in the mitochondria.

2 Anaerobic fermentation

Production of ATP in the absence of oxygen. Inefficient and produces toxic by-products especially lactic acid.

2 ATP sources

1 Immediate energy

1 Oxygen source

Respiratory and circulatory system cannot supply oxygen quickly enough and the major source is muscle myoglobin.

2 Phosphagen system

In a similar manner, for immediate energy, ATP is synthesized by transferring inorganic phosphate to ADP from other molecules.

1 Myokinase

Takes inorganic phosphate from one ADP (which turns it into AMP) and delivers it to ADP to form ATP.

2 Creatine kinase

Takes an inorganic phosphate from a phosphate storage molecule called creatine phosphate.

2 Short-term energy

1 Glycogen-lactic acid system

Once the phosphagen system is depleted, muscles convert to anaerobic fermentation.

3 Long-term energy

Aerobic respiration.

3 Fatigue and endurance

Sections 3-6 for your information only.

4 Oxygen debt

5 Physiological classes of muscle fibers

6 Muscular strength and conditioning

9 Cardiac and smooth muscle

Section I will be covered in detail in conjunction with other chapters.

1 Cardiac muscle

2 Smooth muscle

1 Types of smooth muscle

2 Stimulation of smooth muscle

3 Contraction and relaxation

4 Response to stretch

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