Muscle 1 notes PDF

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08.STRUCTURE AND FUNCTION OFMUSCLE 11. TYPES OF MUSCLE TISSUEA. Skeletal muscle i. Voluntary muscles – attachments to bones - Movement of bones - Maintains posture - Stabilises joints - Generates heat B. Cardiac muscle i. Involuntary muscle only presents in the heart - Circulation of blood C. Smooth...

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08.11.18

STRUCTURE AND FUNCTION OF MUSCLE 1 1. TYPES OF MUSCLE TISSUE A. Skeletal muscle i.

B.

Voluntary muscles – attachments to bones -

Movement of bones

-

Maintains posture

-

Stabilises joints

-

Generates heat

Cardiac muscle i.

Involuntary muscle only presents in the heart -

C.

Circulation of blood

Smooth muscle i.

Involuntary muscle found in walls of hollow organs E.g.: -

Moves chime through GI tract

-

Involved in control of BP (constriction and dilation of arterioles)

-

Autoregulation e.g. in the kidney

2. SKELETAL MUSCLE STRUCTURE A. Whole muscle B. Fascicle (bundle of muscle cells) C. Muscle fibre (muscle cell) D. Myofibril (contractile structures within muscle fibres) E. Myofilaments (contractile protein filaments within myofibrils)

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3. STRUCTURE OF MYOFIBRIL

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4. MUSCLE PROTEINS A. Contractile i.

Myosin

ii.

Actin

B.

Regulatory i.

Troponin

ii.

Tropomyosin

C.

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Structural i.

Titin

ii.

Nebulin

iii.

Alpha-actin

iv.

Myomesin

v.

Dystrophin

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5. HOW A MUSCLE FIBRE CONTRACT

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i.

Action potentials (nerve impulses) are sent from the brain to the muscle

ii.

The action potentials are transferred to the muscle across a synapse

iii.

This then initiates an action potential in the muscle sarcolemma

iv.

This causes interaction between the carefully arranged thick and thin filaments to make the muscle shorten

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6. EXCITATION-CONTRACTION COUPLING i.

Action potentials (nerve impulses) are sent along motor neurons to the muscle fibres

ii.

The action potentials travel along the sarcolemma and down the T-tubules

iii.

The action potentials cause calcium channels to open in the membrane of the sarcoplasmic reticulum, and calcium to be released

iv.

As the sarcoplasmic reticulum surrounds the myofibrils, a lot of calcium is released throughout the muscle fibre all at the same time

v.

The calcium ions combine with troponin and change its chemical configuration

vi.

This causes tropomyosin to move, revealing the myosin binding sites on the actin filaments

vii.

Myosin heads bind to the actin filaments

viii.

The process of contraction (cross-bridge cycle) can now begin

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7. THE CONTRACTION CYCLE

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8. LIST OF WHAT HAPPENS WHEN A MUSCLE CONTRACTS

A. What happens to muscle after death, and why? i.

Muscle cell membranes stop functioning correctly and calcium ions leak into the cells, allowing the binding of myosin head to the actin filaments; as ATP synthesis stops, the cross bridges cannot detach, and the muscle remains contracted. Eventually, enzymes digest the protein cross bridges and the muscles relax.

9. MOTOR UNITS i. 11

Number of muscle fibers in a motor unit vary in different muscles from 2 or 3 to more than 1000.

ii.

Average: 80-100 muscle fibers to a motor unit.

iii.

Muscles which have to perform fine grade, intricate movements have motor units with as few as 3-5 muscle fibers to a unit. e.g. hand, eye

iv.

Muscles with relatively crude movements, number of muscle fibers is quite large. E.g. muscles of lower limbs

v.

In one whole muscle, different motor units overlap

10.

ALL OR NONE LAW A. In a single muscle fibre exactly the same as in the single nerve fibre. i. B.

A sub-threshold stimulus does not produce a response while a threshold or suprathreshold stimulus produces a maximal response. In whole muscle the response is different

i.

A gradual ↑ in stimulus strength causes a gradual ↑ in muscle contraction till a maximum is obtained. This is because with each ↑ in stimulus strength more & more motor units are stimulated.

ii.

When all motor units are activated---all muscle fibers are contracted, then a further ↑ in the strength of the stimulus is without any additional contractile effect.

11.

CARDIAC MUSCLE Cardiac muscle has the same arrangement as skeletal muscle, but also has intercalated discs. i.

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The intercalated discs allow rapid transport of ions and therefore electrical activity between the cells, resulting in almost simultaneous contraction throughout the muscle.

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SMOOTH MUSCLE i.

Smooth muscle contractions start more slowly and last longer than skeletal and cardiac muscle contractions

ii.

Smooth muscle can shorten and stretch to a greater extent

iii.

Ca2+ ions enter the smooth muscle cells slowly as there are no t-tubules (unlike skeletal muscle)

iv.

Ca2+ ions also leave the cells very slowly, resulting in prolonged, smooth contraction

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