MUSCLESand

MOVEMENT

General Function of Muscle Tissue

• Movement or locomotion• Heat production – maintain the body’s homeostasis of temperature• Posture – using partial contraction

of the “anati-gravity” muscles

Specific Function of Skeletal Muscle Tissue

•Excitability or irritabilityThe ability to be stimulated or simply stated “responds to stimulus” from nerve signals

•Contractility The ability to contract or shorten

including contraction without movement isometric contraction is muscle tissue contracting, tension increasing but the

there is not enough strength to “move the fixed object”

•Extensibility•The ability to extend or stretch

allowing muscles to return to their resting length after a contraction

They may also extend while still exerting force as when lowering a heavy object

Muscle cells are different than other body cells

• Muscle cells have a long, thin, thread-like shape

• During tissue development cells fuse together to form a

multi-nuclei cell that becomes a muscle cell

Adult muscle cells can even have some of the stem cells hanging out to

produce more muscle cells after an injury.

Muscle cells have similar parts but different names

Sacrolemma – plasma membraneSarcoplasm – cytoplasm contains many

mitochondria and more than one nucleiSarcoplasmic reticulum (SR) – a type of

endoplasmic reticulum however the SR stores and pumps calcium ions

T tubules – transverse tubules that allow electrical impulses traveling along the

sarcolemma to move deep into the cell

The SR butts up against both sides of the T tubule in a

muscle fiber (called a triad)

Impulses travel along the T tubule to stimulate the

membranes of the SR to pump Ca++ into the channels

of the SR causing the contraction to happen

Myofribrils – not found in other cellscytoskeletal filaments that extend lengthwise

along the muscle fiber and fill the sarcoplasm.

Myofribrils are made up of finer fibers called Myofilaments of which there are two types

Thick filamentsThin filaments

Sarcomere – is a contractile unit located between the Z lines.

Many sarcomeres line up end to end to form a myofibrilLook at page 399 box 11-1 - use overhead for picture

The microscopic bands give the muscle the “striated” name

MyofilamentsMyofilaments are made of 4 types of proteins

• myosin make up the thick filaments

actin, tropomyosin and troponin make up the thin filaments

The myosin heads of the thick filaments are chemically attracted to the actin molecules on the thin filaments. They are called “cross bridges” because they bridge the

gap between the think and thin filaments

Muscles are at rest unless they

are stimulated by a motor neuron

• The motor neuron endplate forms a junction called a neuromuscular junction.

• The narrow gap between the nerve and the fiber is called a synapse or synaptic cleft

• The nerve releases a neurotransmitter acetylcholine into the gap to begin the electrical impulses in the sarcolemma

-excitation-

1. The impulse is conducted over the muscles fiber’s sarcolemma

2. When it reaches the t-tubule it spread deeper into the fiber

where it release Ca++ into the SR

3. The calcium ions combine with troponin molecules in the thin filament causing it to shift to

expose the actin molecule

4. The myosin heads are chemically attracted to the actin so they “grab” it and pull, then grab the next actin and pull past it and so on until an 80% contraction is reached

This is called the sliding-filament model

Muscle fibers contract to about 80% or their starting length

If a muscle is pulling against an immovable object it is still in a state of active contraction and uses energy to maintain the contraction.

Types of Muscle Fibers

•Slow twitchContains “red fibers”

Red fibers have a high concentration of myoglobin, a redish pigment used by muscle cells to store oxygen

in the muscle cell itself – gives the muscle its red color

Contains type I myosin in the thick filamentsthat reacts or contracts at a slow rate and is able to

produce ATP at a rate that keeps pace with the muscle’s energy needs, this type of muscle is found in anti-gravity muscles--because they can contract

for a long time and not fatigue—and in the muscles used for endurance activies

•Fast TwitchWhite fibers

They contain very little myoglobin

Contain type II myosin that contracts more rapidly than slow fibers and has t-tubules that are more efficient at delivering Ca++ to the

SR

Rapid contraction depletes ATP rapidly even though they tend to contain a higher concentration of glycogen

they have fewer mitochondria and rely on anaerobic respiration to regenerate more ATP

Fast twitch muscles have great force but cannot sustain activity for very long because of lactic acid production from anaerobic activity

Fingers, eyes, muscles used by sprinters

•Intermediate fibersHave characteristics in between both fast and slow twitch

muscle fibers

They are more fatigue resistant than fast fibers yet can generate more force more quickly than slow fibers

Example would be:Muscles that are anti-gravity yet are needed for other

activities such as jumpingmuscles of the leg would be a good example

Page 406 in the text

Motor UnitConsists of the nerve that stimulates the muscles

fibers and the muscle fibers it is stimulating

Some motor units are only a few muscle fibers

Some motor units have thousands of muscle

fibers

Fine motor movements have smaller motor units that only stimulate a few fibers at time , gross motor movements have large motor units that stimulate many fibers at once

MyographyGraphing muscle contraction

Terms you need to know

Threshold stimulus The amount of stimulus applied to make a muscle contract

Twitch contractionA quick jerk of the muscle

Use the information that you learned about the physiology to explain what is happening on the myograph

Twitch contractions rarely happen the body

•We can’t make our muscles twitch individually•Our nervous system “smooths out” our muscle

movements• Smooth movements are more useful – sustained

movements• Prevents injury to muscle groups

The staircase effect is why we warm up before exercise.Calcium diffused through the sarcoplasm of muscles has not had time to relax and pump all of it back into their SR so the muscle can contract with more force.

The relaxation phase become shorter and finally disappears – the muscle stays in partial contraction

called a contractureAfter a while the muscle becomes fatigued and won’t

respond to even the strongest stimuli

Ever had your muscles shake after over working them?

That is what you are feeeling!

It is rarely the entire muscle but certain motor units that are fatigued causing

the shaking

TetanusThis graphic represents what a

sustained contraction looks like.

In a normal body tetanus results from two factors working together at the same time

• Rapid –fire of nerve fibers that permit this wave summation to

happen in each fiber• Coordinated contractions of

different motor units in a muscle organ in an overlapping sequence or “relay team” effect

This is called muscle Tone

Tonic contraction or “tone”

Muscles organs with less than normal “muscle tone” are called flacid

Muscles organs with more than normal muscle tone are called spastic

Stretch sensors in muscles and tendons detect the degree of stretch and adjust the strength of the tonic contraction. This mechanism is a negative feedback

mechanism located in the spinal cord.

Graded Strength PrincipleMuscle organs can produce varying degrees of

strength

• Influenced by the individual “type” of fibersFast twitch, slow twitch or intermediate•Number of motor units contracting

more units are recruited when more strength is needed

The maximum strength of a muscle is directly related to the initial length of it fibers

• If a muscle contractions begins from a shortened length its sarcomers are already compressed

• If the muscle begins a contraction from an overstretched length it cannot develop much tension because the thick and thin myofilaments are too far

apart to effectively pull and shorten the sarcomeres.

Try flexing you biceps: extended, 90 degrees and completely flexed Page 411 bottom of the 2nd column

Now feel the stretch reflex, page 412

Effects of Exercise on Skeletal MusclesPage 412

Concentric ContractionMovement result in

shortening of muscle

Eccentric ContractionMovement results in

lengthening of muscle

Isometric contraction

Muscle stays the same length as tension

increases“Tightening to resist a force”