Structure and Function of the Muscular, Nervous, and Skeletal Systems

muscle contraction/muscle action

Muscles generate force when they are activated

skeletal muscle

The type of muscle that attaches to bones, causing them to rotate around joints

structure

The function of muscle is dictated by its

epimysium

Each skeletal muscle (e.g., deltoid, pectoralis major, gastrocnemius) is surrounded by a layer of connective tissue referred to as

muscle fibers

From epimysium a muscle is further divided into bundles of

fasciculus/fascicle

A bundle of muscle fibers is called a

perimysium

Each fasciculus is surrounded by connective tissue called

endomysium

Within a fasciculus, each muscle fiber is surrounded and separated from adjacent fibers by a layer of connective tissue referred to as

tendon

Together, these connective tissues help transmit the force of muscle action to the bone via another connective tissue structure, the

sarcolemma

each muscle fiber is surrounded by a plasma membrane that encloses the contents of the cell, regulates the passage of materials such as glucose into and out of the cell, and receives and conducts stimuli in the form of electrical impulses or action potentials

action potentials

a brief electrical signal that travels along a nerve or muscle cell when it is activated. It happens when charged particles (ions) move in and out of the cell, causing a rapid change in voltage.

the nuclei

contain the genetic material, or DNA, of the cell, and are largely responsible for initiating the processes associated with adaptations to exercise.

cytoplasm/sarcoplasm

Within the boundary of the sarcolemma, but outside the nuclei, is the

adenosine triphosphate (ATP)

The only direct source of energy for muscle actions

adenosine triphosphate, phosphocreatine, glycogen, fat droplets

This watery solution (sarcoplasm) contains the cell’s energy sources, such as

mitochondria

the sites of aerobic ATP production within the cell and thus of great importance for aerobic exercise performance.

sarcoplasmic reticulum

This organelle stores calcium and regulates the muscle action process by altering the intracellular calcium concentration

transverse tubules/T-tubules

the sarcoplasmic reticulum releases calcium into the sarcoplasm of the cell when an action potential passes to the interior of the cell via structures called

myofibrils

muscle cell contains columnar protein structures that run parallel to the length of the muscle fiber.

myofilaments

Each myofibril is a bundle of, ,which primarily consist of myosin (thick) and actin (thin) filaments.

myosin and actin

are arranged in a regular pattern along the length of the myofibril, giving it a striated, or striped, appearance.

Myosin filaments

formed from the aggregation of myosin molecules.

head, neck, and tail.

Each myosin molecule consists of a

the head

is capable of attaching to and pulling on the actin filament

power stroke

Energy from the splitting, or hydrolysis, of ATP is used to perform the , , an important step in the process of muscle activation.

middle of the myosin filament

is oriented in a tail-to-tail fashion, such that the head portions project outward from the ends of the filament

titin

The protein maintains the position of the myosin filament relative to actin.

individual globular/G-actin

Each actin filament is formed from

filamentous/F-actin.

Each G-actin has a binding site for a myosin head. The G-actin proteins assemble into strands of

regulatory proteins

Collectively, tropomyosin and troponin are considered, ,because they regulate the interaction of myosin and actin, the contractile proteins

Tropomyosin

a rod-like protein that spans the length of seven G-actin proteins along the length of the actin filament.

the muscle cell is at rest

When, ,tropomyosin lies over the myosin binding sites on actin.

calcium

When bound to, , troponin causes the movement of tropomyosin away from the myosin binding sites on actin.

the myosin head to attach and pull on actin

When bound to calcium, troponin causes the movement of tropomyosin away from the myosin binding sites on actin. This allows, , a critical step in the muscle activation process.

nebulin

The protein acts to ensure the actin filaments are the correct length.

sarcomere

the basic contractile unit of muscle

Z-line

sarcomere extends from one Z-line to an adjacent

A-band

is determined by the width of a myosin filament and provides the dark striation of skeletal muscle.

anchored

Actin filaments are, , at one end to the Z-line.

H-zone

The area of the A-band that contains myosin but not actin is the

M-line

In the middle of the H-zone is a dark line called the

align adjacent

The M-line helps , , myosin filaments.

I-band

spans the distance between the ends of adjacent myosin filaments.

two

As such, each I-band lies partly in each of , , sarcomeres.

light striation

The I-bands are less dense than the A-bands, and thus they are responsible for giving skeletal muscle its

In order to contract

, , muscle fibers must normally receive a stimulus from the nervous system.

neuromuscular junction

This communication between the nervous and muscle systems occurs at a specialized region referred to as the

single neuromuscular junction

Each muscle fiber has a , , located at the approximate center of the length of the cell.

motor endplate

the connection between the motor nerve and the skeletal muscle cell

synaptic cleft/neuromuscular cleft.

the space between the axon terminal and motor endplate, referred to as the

sliding filament theory

This theory states that a muscle shortens or lengthens when the filaments (actin and myosin) slide past each other, without the filaments themselves changing in length.

Sliding Filament Theory step 1 part 1

An action potential passes along the length of a neuron, leading to the release of the excitatory neurotransmitter acetylcholine (ACh) at the neuromuscular junction.

Sliding Filament Theory step 1 part 2

When the neuron is at rest, ACh is stored in the axon terminal of the neuron within structures called synaptic vesicles.

Sliding Filament Theory step 1 part 3

It is the action potential that leads to the release of stored ACh into the synaptic cleft between the axon terminal of the neuron and the muscle fiber.

Sliding Filament Theory step 2

The ACh migrates across the synaptic cleft and binds with ACh receptors on the motor endplate of the muscle fiber

Sliding Filament Theory step 3 part 1

This leads to the generation of an action potential along the sarcolemma of the muscle fiber.

Sliding Filament Theory step 3 part 2

In addition, this action potential will travel to the interior of the muscle fiber via T-tubules.

Sliding Filament Theory step 3 part 3

The movement of the action potential down the T-tubule triggers the release of stored calcium from the sarcoplasmic reticulum

Sliding Filament Theory step 4

Once released into the sarcoplasm, the calcium migrates to, and binds with, troponin molecules located along the length of the actin filaments

Sliding Filament Theory step 5 part 1

The binding of calcium to troponin causes a conformational change in the shape of troponin.

Sliding Filament Theory step 5 part 2

Because tropomyosin is attached to troponin, this moves tropomyosin such that binding sites on actin are exposed to the myosin head.

Sliding Filament Theory step 6 part 1

When a muscle is in a rested state, the myosin head is energized; that is, it is storing the energy released from the breakdown of ATP to adenosine diphosphate (ADP) and inorganic phosphate

Sliding Filament Theory step 6 part 2

When the binding sites on actin are exposed to the myosin head, it is able to attach, forming a crossbridge, and attempt to pull the actin filament toward the center of the sarcomere.

Sliding Filament Theory step 6 part 3

Whether it is successful at pulling, and thus shortening the muscle, depends on the amount of force generated by the crossbridges that are pulling and the external force that opposes the crossbridges.

Sliding Filament Theory step 7 part 1

After pulling on the actin filament, the myosin head is now in a lower energy state. In order to cause detachment from the actin filament, as well as to energize the head, a fresh ATP molecule

Sliding Filament Theory step 7 part 2

In order to cause detachment from the actin filament, as well as to energize the head, a fresh ATP molecule must be bound.

Sliding Filament Theory step 7 part 3

Once it is bound, the myosin head detaches from actin, and the enzyme myosin adenosine triphosphatase (ATPase) causes the splitting of the ATP molecule.This once again energizes the myosin head.

Sliding Filament Theory step 7 part 4

If the binding sites on actin are still exposed, the myosin head may once again form a crossbridge with actin, again attempting to pull toward the center of the sarcomere. This process will continue provided that the muscle fiber is being stimulated to contract by its motor neuron.

shorten

It is important to recognize that when stimulated, muscle fibers always attempt to

actin toward the center

That is, the crossbridges always attempt to pull , , of the sarcomere, which would cause shortening of the sarcomere and thus the muscle.

a concentric muscle action

If the amount of force produced by a muscle is greater than the external resistance acting in the opposite direction, , will result.

less than

If the amount of force produced by a muscle is , , an opposing external resistance, the muscle will lengthen even as it attempts to shorten.

eccentric muscle action

This lengthening muscle action is known as an

isometric (static) muscle action

Lastly, if the muscle force is equal and opposite to that of an external resistance, an

more difficult than

During the performance of resistance training exercises, individuals perceive the concentric phase as, ,the eccentric phase.

delayed-onset muscle soreness (DOMS)

it likely results from some combination of connective and muscle tissue damage followed by an inflammatory reaction that activates pain receptors. This damage is primarily caused by eccentric muscle actions and resulting micro-tears in connective and muscle tissues.

nutritional supplements, massage, ice, and ultrasound

Strategies to combat the pain and performance decrements resulting from DOMS have included

exercise

It appears, however, that , , itself may be the best means of decreasing pain associated with DOMS, although its analgesic effects are temporary

contractile performance and basic physiological characteristics

All muscle fibers are designed to contract and produce force, but not all fibers are alike when it comes to

muscle biopsy

To determine muscle fiber type, a , , must be performed.

oxidative capacity

the ability to produce ATP aerobically, a characteristic called

oxidative fibers

Fibers that have large and numerous mitochondria, and that are surrounded by an ample supply of capillaries to deliver blood and oxygen, are considered

myoglobin

delivers oxygen from the muscle cell membrane to the mitochondria, enhancing aerobic capacity and lessening the reliance on anaerobic ATP production.

high ATPase activity

Fibers with a myosin ATPase form that have, , will have a high rate of shortening because of the rapid availability of energy from ATP to support the muscle action process.

low ATPase activity

Fibers with a myosin ATPase form that have, ,will have a low rate of shortening because of the rapid availability of energy from ATP to support the muscle action process.

maximal force production and fiber efficiency

two other contractile characteristics of muscle are

specific tension

fibers may differ in the amount of force they produce relative to their size (cross-sectional area).

more work

An efficient fiber is able to produce , , with a given expenditure of ATP.

type I

Slow fibers have alternatively been referred to as

type IIa

fast oxidative glycolytic (FOG)

type IIx

fast glycolytic (FG)

moderate

The primary distinction between the two is that FOG fibers have , , oxidative and anaerobic capacity, providing them with some fatigue resistance in comparison with the purely anaerobic and highly fatigable FG fibers

the nervous system

directs and controls the voluntary movement.

central nervous system

consists of the brain and spinal cord.

peripheral nervous system

lies outside the central nervous system and may be further divided into motor (efferent) and sensory (afferent) divisions.

The motor branch

of the peripheral nervous system relays nerve impulses from the central nervous system to the periphery (e.g., to skeletal muscles)

the sensory branch

relays nerve impulses from the periphery back to the central nervous system.

The somatic nervous system

is responsible for activating skeletal muscles

The autonomic nervous system

controls involuntary functions such as contraction of the heart and smooth muscle in blood vessels as well as glands.

The parasympathetic division

of the autonomic nervous system is primarily active during rest. It is responsible for processes such as digestion, urination, and gland secretion.