ANATOMY OF MUSCLES

Muscle tissue properties:

• Irritability

• Ability to respond to stimulus

• Contractility

• Ability of muscle to contract against resistance

• When stimulated

• Extensibility

• Ability of a muscle to stretch beyond its resting length

• Elasticity

• Ability of muscle to return to its resting length after it has been stretched

Connective tissue coverings:

• Epimysium

• Surrounds the entirety of the muscle

• Perimysium

• Surrounds bundles of fibers

• Fasciculus

• Endomysium

• Surrounds individual muscle fibers

Muscle Fibers

• Muscle fibers are individual muscle cells

• Have the same organelles as a typically shaped cell

• They are multinucleated

• The cell membrane surrounding a muscle fiber is called sarcolemma

• Myofibrils : contractile units of muscles

• Hundreds to thousands of myofibrils/fibers

• Made of sarcomeres

• Functional units of the cells

• Sarcoplasm is the cytoplasm in muscle fibers

Myofibrils

• Made of thick and thin filaments

• Thin filaments -> Primarily actin

• Also contain :

• Troponin - binds CA+2 and causes movement of the tropomyosin

• Tropomyosin - covers actin-myosin binding site

• Arrangement of filaments explains striations in skeletal muscle

THE MECHANICS OF MUSCLE CONTRACTION

Sliding filament model →

During contraction, actin slides past myosin which results in the myofilaments moving towards each other. The Z-discs are brought together and the sarcomere shortens

The length of myofilaments do not change

In fully contracted muscle, actin and myosin overlap

Excitation-Contraction coupling

• At rest - myosin cross bridges in weak binding states

• CA+2 binds to troponin

• Tropomyosin uncovers active sites

• Cross bridge forms strong binding state

• Pi is released from myosin

• Cross bridge movements occur

• ADP released from myosin

• ATP attaches to the myosin and breaks the cross bridge

• Forming weak binding state

• Energizes myosin

• Continues as long as CA+2 and ATP are present

PROPRIOCEPTION

Sensory information

We rely on our senses to help us respond to our environment

• During exercise proprioceptors help us respond to the challenges facing our bodies

• Proprioceptors are found in the skin, muscles, joints, and tendons

• Respond to stretch, tension, length, and contraction

GTOS

• Golgi tendon organs are located with the tendons

• Sense tension and contraction

• Overall function is protection

Action Potentials

• Electric signals that tell muscles when to contract

• A contraction will only occur if the signal from the CNS is strong enough

• Subthreshold stimulus - stimulus isn't strong enough

• Threshold stimulus - stimulus is strong enough to cause contraction

• Maximal stimulus - stimulus is strong enough to make all motor functions fire

Motor Units

• Consist of the motor neurons and fibers

• Action potential arrives at dendrites than travel down axon to fibers

The neuromuscular junction

The site where a motor neuron communicates with a muscle fiber

• Axon terminal releases neurotransmitter

• Causes action potential

• Muscle fiber contracts

MUSCLE CONTRACTION FROM START TO FINISH

Nine steps

Step One - Step Four

1. Signal from motor neuron

2. ACh released from vesicles at the axon terminal

3. ACh triggers AP at the ends of the motor end plate

4. AP travels down T-Tubules

Step five and Step Six

5.  Calcium binds to troponin

6.   Shifts tropomyosin

Step seven - Step nine

7.   Shifted tropomyosin exposed myosin binding site

8.  Pulling of Actin toward the middle of the sarcomere then releases

ATP’s around

9.  Repeats process rapidly