Muscular System

Muscle Contraction

Movement requires muscle

• Three types: skeletal, smooth, cardiac

• Skeletal will be our focus

• Long fibrous tissue• Muscle fibers are

enclosed in layers of connective tissue

From Largest to smallest

• Muscle

• Bundle of fibers called a fascicle

• Muscle fiber (one cell)

• Bundle of myofibrils

• Filaments arranged in sarcomeres

• Actin and myosin are the proteins of the myofilament

Terms

• Cytoplasm is called sarcoplasm

• Inside the muscle fiber (muscle cell) are myofibrils

• They contain 2 proteins: actin and myosin

A sarcomere

• The space from one Z line to the next Z line

• Striations (stripes) are caused by the proteins of the myofilament

• Sarcomere is the contracting unit of the muscle fiber Z lines

Contraction• Contractions means

getting smaller• The myofibril can

contract because actin and myosin move

• This movement is described in the sliding filament theory

Sliding Filament Theory

• Head of myosin attaches to the actin

• This can happen only if calcium is present

• Myosin head pulls the actin filament in

• Makes the sarcomere shorter

Neuromuscular Junction

Connection between a motor neuron and a muscle

Neuromuscular Junction

• A nerve fibers comes from the brain or spinal cord to the muscle.

• The nerve and the muscle do not actually touch

• The gap between them is called the synaptic cleft

Neuromuscular Junction

• At the end of the neuron, many tiny vesicles are present

• These vesicles store chemicals called neurotransmitters

• The neurotransmitter will go across the gap (synaptic cleft)

Neuromuscular Junction

• The muscle fiber on the other side of the gap is stimulated by the neurotransmitter

• The muscle fiber contracts

• The neurotransmitter is called acetylcholine

After contraction, relaxation

• Acetylcholinesterase (an enzyme) destroys the remaining acetylcholine

• Calcium ions are taken away

• Without these actions, the muscle would stay contracted

Related Conditions

• Nerve gas works by inhibiting acetylcholinesterase

• Breathing muscles cannot relax

• Botulism prevents the release of acetylcholine, so muscles cannot contract

Related Conditions

• Rigor mortis • Ca++ pumps run out

of ATP • Ca++ cannot be

removed • continuous contraction • eventually tissues

break down

Contraction Facts

• Threshold stimulus must be reached to cause a contraction

• Enough acetylcholine must reach the receptors

• All or none response

• Either the fiber (and its motor unit) contracts or it doesn’t

Contraction Facts

• A motor neuron comes from the brain or spinal cord and connects to a group of muscle fibers, called a motor unit. When a motor neuron transmits an impulse, all the fibers in the motor unit contract

• (If threshold was reached)

Contraction Facts

• A motor unit exhibits an all or none response

• How, then, can some muscle contractions be more forceful than others?

• Determined by the number of motor units stimulated

Contraction Facts• Where does the energy

come from to power muscle contraction?

• ATP (adenosine tri-phosphate)

• If extra energy is available, it is stored in creatine phosphate

• Creatine phosphate is converted to ATP when needed.

Origin and Insertion

• One end of a skeletal muscle is attached to an immovable end, like an anchor. This is the ORIGIN.

• The other end is attached to a movable part, called the INSERTION.

Biceps Brachii

Two origins

Insertion

Contraction Facts

• Muscle Twitch• cycle of

contraction and relaxation

• Contraction only lasts a fraction of a second

Contraction Facts• Tetany is sustained

contraction• Lacks even partial

relaxation• Tetanus bacteria can

cause this• Lockjaw

• Muscle cramps• ATP depletion • dehydration • ion imbalance • massage or stretching

increase circulation

Contraction Facts

Which are You?

• A sprinter or an endurance performer?

• Why do some people excel at one, but not the other?

• Read the article and find out!

Use It or Lose It!

• Muscle inactivity will lead to weakness and wasting

• Aerobic exercise will result in stronger, more flexible muscles with greater resistance to fatigue.

• Resistance (isometric) exercise will result in increased size of muscles.

Which type should you do?

• Resistance: body building; enlarges individual muscle cells.

• Aerobic: endurance, increases strength in other systems as well.

• YOU NEED BOTH!

 

• Muscle fatigue • lack of carbohydrate

source (glucose)• lack of ATP to restore

membrane potential • lactic acid drops pH

which interferes with protein function

Contraction Facts

Oxygen DebtBody cannot supply muscles with enough oxygen during strenuous exercise

Muscles must switch to anaerobic respiration

This produces lactic acid, which the liver will destroy later

Oxygen debt is the amount of oxygen liver cells need to convert the lactic acid into glucose

Happens later, while you are resting or sleeping