The Muscular System

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The Muscular System – an overview There are three types of muscular tissue:

Skeletal muscle Smooth muscle Cardiac muscle

Muscular tissue is contractive tissue. What is the function of the muscular system?

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The Muscular System Has Many Functions Allows movement through the environment Stabilizes movement at the joints Aids in the flow of lymph and blood through

the body Protects our internal organs Maintains homeostasis by producing heat

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Skeletal Muscles are Contractile Organs All human skeletal muscles have a similar

function and structure. Function:

They contract to produce movement. They relax to their original length.

Structure: The origin is the end that remains stationary when

the organ shortens. The insertion is the end that moves during

contraction.

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Figure 6.1 Muscle origin and insertion

Body movements To coordinate and control body movements,

most human skeletal muscles function as a member of an antagonistic or synergistic pair. Antagonistic (synergistic) – muscles with opposing

actions working together to provide smooth and controlled movements. Example: moving your hand to your shoulder requires:

1. simultaneous contraction of the prime movers (the

brachialis and biceps brachii muscles), and

2. relaxation of the antagonist (the triceps brachii).

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Figure 6.2 Anterior view of the superficial muscles of the body

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Figure 6.3 Posterior view of the superficial muscles of the body

Anatomy of a muscle

Skeletal muscle is composed of numerous elongated structures, one nested inside the other.

Individual muscle cells are long (sometimes 30 cm), slender and fragile.

There are essentially 3 layers of the muscle: Epimysium – separates muscles Perimysium –covers and supports muscle cells Endomysium – covers muscle cells on top of

membrane

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Figure 6.4 Anatomy of a muscle

Organization of skeletal muscles Myofiber – a single muscle cell Sarcolemma – cell membrane covering

myofiber T tubules – specialized areas in the sarcolemma

that conduct the contraction messages Myofibrils – linearly arranged groups of the

contractile proteins actin and myosin Sarcomeres - contractile units which hold the

proteins in regular arrangements Sarcomeres are banded (striated)

6-11Copyright 2008 John Wiley & Sons, Inc.

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Figure 6.5 Organization of skeletal muscles from gross to molecular

Copyright 2008 John Wiley & Sons, Inc.

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Figure 6.5 Organization of skeletal muscles from gross to molecular

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Figure 6.5 Organization of skeletal muscles from gross to molecular

Actin and myosin are microscopic proteins that interact and cause the entire muscle tissue to shorten and, thus, move the skeletal tissue. Actin is a thin, globular protein Myosin is a larger, heavier protein

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Figure 6.6 Structure of thick and thin filaments Figure 6.7 Myosin filament

Muscle Contraction Occurs as Filaments Slide Past One Another The contraction of a skeletal muscle begins

when an impulse reaches the neuromuscular junction.

At this junction, the motor neuron ends very close to muscle cells separated by the synapse.

Nerves send a contraction impulse across the synapse with neurotransmitters.

Acetylcholine (ACh) is the most common.

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Muscle Contraction Occurs as Filaments Slide Past One Another ACh is released from the axon terminal and

diffuses across the synaptic cleft and binds to receptors on the muscle cell membrane.

The skeletal muscle will contract in response to ACh binding.

The impulse to contract is then passed through the entire muscle cell via T tubules.

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Figure 6.8 Neuromuscular junction

More details… Inside the muscle cell, sarcoplasmic reticulum (SR) stores

calcium ions and releases them when the ACh binds to the surface of the cell.

Calcium is held in the SR by the enzyme calcium sequestrin.

This enzyme on the surface of the SR stores and releases calcium from the cytoplasm into the SR. The enzyme works by converting ATP to ADP.

Since free calcium in the cell is toxic, this ensures the survival of the cell.

Neither actin nor myosin undergo chemical transformations.

Actin merely slides over the myosin filament.

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Figure 6.9 Contraction cycle

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Figure 6.10 Summary of events in contraction and relaxation of skeletal muscle

Whole-Muscle Contractions Emerge from Tiny Impulses How does an entire large muscle like that of

your thigh contract and generate movement? Muscle cells are grouped in motor units

1 motor neuron and the set of muscle cells it controls The entire motor unit contracts when it receives a

signal from the motor nerve. This causes calcium ions to be released which

triggers the sliding action.

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Whole-Muscle Contractions Emerge from Tiny Impulses

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Figure 6.12 Motor unit

Muscle cells contract on an all-or-nothing basis If the nerve stimulus is too weak, nothing

happens. When the threshold stimulus is reached,

calcium is released and the entire muscle cell contracts.

Single twitches are not effective in producing body movement, because they last a fraction of a second.

The motor unit requires multiple stimuli.

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Each contraction builds on the last until the muscle cell is continuously contracted.

The buildup of contractions is called summation.

Increased strength for similar stimuli is known as treppe. Example – warming up for an athletic event takes

advantage of treppe Once continuous contraction is achieved, the

muscle is in tetanus. Example – the neck muscles of an adult are in

tetanus most of the day

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Muscles Require Energy to Work Smoothly and Powerfully What is the general source of energy inside

cells? ATP

The body can make ATP for muscular contractions through either aerobic or anaerobic pathways.

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Figure 6.16 Krebs cycle

ATP production Aerobic pathway (highly efficient)

Burns glucose, forming water, carbon dioxide and ATP

Aerobic ATP is generated in the mitochondria Produces the largest amount of ATP Is the dominant method of energy production

During heavy muscle activity, oxygen cannot keep up with the energy demands. ATP production shifts to anaerobic pathways.

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ATP production

Anaerobic pathway (less efficient) Produce fewer ATP molecules per glucose molecule Produces lactic acid which is eventually removed

from the tissue by conversion to pyruvic acid This requires oxygen (which is one reason we breathe

heavily after exertion). Oxygen is carried through the bloodstream to the lactic

acid-laden tissue, reacts with the acid converting it to pyruvic acid and then to coenzyme A.

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Figure 6.17 Conversion of lactic acid to pyruvate and then on to ATP

Creatine phosphate is important in the anaerobic phase of muscle energy production because it stores energy much like ATP, in a phosphate bond.

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Figure 6.18 Creatine phosphate reaction

Muscle twitches can be fast or slow There are two types of muscle cells:

Slow twitch (slow oxidative; aerobic) Appear red, have a large blood supply, have many

mitochondria, and store myoglobin. Purpose is to supply oxygen to the mitochondria of the

cells and sustain the supply of ATP within sarcomeres. Fast twitch (fast oxidative; anaerobic)

Provide short bursts of incredible energy and contraction power, but fatigue quickly.

These cells are thicker and usually contain larger glycogen reserves and less developed blood supply.

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Figure 6.19 Fast twitch and slow twitch cells

Toned muscles work better, look better Muscle tone is the constant partial contraction of

muscle when the body is “in shape”. Toned muscles are more effective at burning

energy (using more ATP per gram). Therefore, people who are in shape can eat more

without gaining weight because the low-level contractions of their toned muscles constantly burn ATP.

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Exercise or chemical compounds can change the size of a muscle. Scientists think that the total number of muscle fibers is

inherited; therefore, we alter the muscle by enlarging cells (hypertrophy).

Hypertrophy is caused by the addition of new myofibrils within the endomysium of muscle cells, which thickens the myofibers.

Hypertrophic muscles should have thicker muscle cells, packed with more sarcomeres.

Exercise that requires muscle to contract to at least 75% of maximum tension will cause hypertrophy.

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Summary

Our muscles are designed to: Provide movement Manipulate the environment Maintain homeostasis Protect our organs Maintain our upright position

All movement requires the production of ATP, either stored in the cell or produced via metabolic pathways.

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