muscle notes

The Muscular System

Interesting Facts

• We have more than 600 muscles.

• By weight, the muscle system is the largest.

• Muscles use the majority of the body’s energy

• Muscles produce most of the body’s heat.

• You used more than 100 muscles to take out your notebook and write this.

Muscle Basics

• Muscles are made of muscle fibers (cells) and connective tissue.

• Muscle cells are made of various proteins.

• A muscle cell is as long as the muscle it is part of.

Organization

• Each muscle is separated from everything around it by a connective tissue “sac” called the fascia.

• Just deep to the fascia is a connective tissue layer called the epimysium.

• The epimysium holds together many packets of muscle fibers called fascicles.

Organization

• Each fascicle is separately wrapped in a connective tissue layer called the perimysium.

• Within each fascicle, each individual muscle fiber is wrapped in a connective tissue called endomysium.

• Sattelite cells are found within the layer of endomysium

Muscle Microanatomy

• Each muscle cell is composed of:– multiple nucleii– various contractile proteins– Sarcolemma (cell membrane)– Sarcoplasm (cytoplasm)– many mitochondria– sarcoplasmic reticulum and t-tubules

Micro-organization

• Muscles contain many fascicles

• Fascicles contain many muscle fibers

• Muscle fibers contain many myofibrils

• Myofibrils contain the contractile proteins actin and myosin (myofilaments).

• Actin and myosin form the contractile units called sarcomeres.

Muscle Microanatomy

• This tiny structure (2.25 um) is the moving force behind muscle contraction

The Sarcomere

• Is composed of actin and myosin fibers

• Has distinct areas– M-line-runs in the middle of the sarcomere– Z-disks-mark the borders (ends) of the

individual sarcomere– A-band in the area where myosin is found– I-band is the area where ONLY actin is found– H-zone is the area ONLY myosin is found

MYOSIN

THE NEUROMUSCLULAR

JUNCTION

The motor unit is a nerve and the muscle fibers that it stimulates. There are several muscle fibers connected to each nerve.

Sliding Filament Animation

• lSliding Filament Animation

The Junction

• The neuromuscular junction is where the nerve meets the muscle cell. There is not a direct connection, but rather a small space between the two.

The Synaptic Junction

• This space is called the synaptic cleft.

• The purple circles are called vessicles.

• Vessicles are like bags that hold a substance.

The Impulse

• The brain sends a nerve impulse through the motor neuron.

• This impulse causes little sacs called vesicles to release their contents.

The Neurotransmitters

• The vessicles burst open releasing a neurotransmitter called acetylcholine (Ach)

• The Ach travels across the empty space (synapse) to the special receptors on the other side.

The motor end plate

• There is a structure called a motor end plate across form the nerve.

• The receptors a re found on the motor end plate.

The role of calcium

• The receptors, once stimulated signal the sarcoplasmic reticulm to release calcium ions.

• These calcium ions travel down the T tubules to the sarcomere.

The Sliding Filament Theory

• Thin filament-actin-has active sites• Thick filament-myosin-has a head and a tail• The troponin-tropomyosin complex blocks

the active sites on the actin.• A site on the troponin has a receptor that

can bind a calcium ion, modifying the complex so that it no longer blocks the active site.

Sliding Filament Theory

• Once the active site is available, a myosin head attaches to it.

• The myosin head then pivots on its tail making the sarcomere shorter.

• As this happens in thousands of sarcomeres the entire muscle shortens or CONTRACTS.

All or none response

• A muscle fiber contracts fully or not at all. If the stimulus is not high enough, nothing happens, even if it is very close to the threshold. If it is over the threshold-even by just a little, maximum contraction occurs.