Muscular System Notes
d6-1
Two purposes of the muscular system:
Movement
Heat production
Pectorals
Zygomaticus
Trapezius
Masseter
Deltoids
Biceps
Triceps
Obliques
Laterals
Abdominals
Gluteus
Maximus
Adductors
Hamstrings
Quadriceps
Calves
Muscular System Notes
6-2
Many muscles can be at the same location in the body. Fascias are layers of connective tissue that
separate muscles to prevent them from interfering with each other.
The anatomy of a muscle consists of smaller and smaller bundles of different things.
Fascicle-bundles of muscle fibers that make up a muscle
Muscle fiber-the basic unit of a muscle made of myofibrils
Myofibril-bundles of protein fibers that make up a muscle fiber
Fascicle
Muscle
Myofibril
Muscle
fiber
Myofibril Anatomy:
A myofibril is made of alternating strands of thick and thin protein that don’t go all of the way down the
myofibril.
Sarcomere: the repeating unit of a myofibril that goes from Z line to Z line
Thick filaments: Myosin
Thin Filaments: Actin
Z line: area where only the Actin is.
Actin
(thin)
Myosin
(thick)
Z-line
Muscular System Notes
6-3
Muscle Contraction:
1. Motor neuron receives an impulse
2. Motor neuron releases the neurotransmitter acetylcholine into the gap between the neuron and
the muscle
3. The acetylcholine attaches to receptors on the muscle fiber membrane.
4. The muscle responds by releasing calcium ions into the myofibrils
5. The calcium ions attach to the Actin and unlock the binding sites
6. The cross-bridges from the myosin attach to the binding sites
7. The cross-bridges fire and pull the actin which shortens the sarcomere
8. All of the shortening sarcomeres lead to a shortening (contracting) of the muscle
6-4
Strengthening muscles:
How to strengthen a muscle depends on how you want to improve it:
If you do weight-bearing exercises like running or cardio, then you will work on slow fibers. These fibers
are in change of sustained activities and will get better are working a long time and less prone to fatigue,
but will not get stronger.
If you lift weights or do short-term, high-stress exercise, then you will work on fast fibers. These fibers are
in charge of strength and power and will get stronger and be able to do more.
When you build up muscles, your muscles are adding more Actin and Myosin. You don’t gain more
muscles or more muscle fibers, you gain more protein fibers which makes the muscles bigger and
stronger.
When you use your muscles for a long time, they get tired. Your muscles start by using glucose and
oxygen. They get their oxygen from a special type of blood called myoglobin which only works in muscles
and makes it so more oxygen is available to the muscles. When they run out of oxygen, they switch to
anaerobic metabolism which makes less energy and produces lactic acid. As the lactic acid builds up,
your muscles are not able to work as well and get tired.
Muscular System Notes
If you don’t use muscles for an extended period of time, then you will lose them. They will lose actin and
myosin and become weaker, turn into fat, and degrade in a process called atrophy.
Steroids work because they look a lot like testosterone and stimulate the growth of actin and myosin in
the muscles. More action and myosin means stronger muscles. However, it comes with risks: kidney
problems, liver problems, baldness, sterility, and much more.
6-5
Muscles and bones work together to create movement.
Origin- where a muscle attaches to the bone that doesn’t move
Insertion-where a muscle attached to the bone that does move
When the muscle contracts, it shortens and thus pulls the insertion closer to the origin. If this
increases the angle of the joint, we call is extension. If it decreases the angle of the joint, we
call is flexion.
Muscles often work in groups. The main muscle that moves something is called the prime
mover. If other muscles contract to assist the main muscles in doing its work, we call them
synergists. Often one muscle will move a bone one way and another muscle will move the bone
the other way. When muscles work in opposite directions, we call them antagonists (like the
bicep and tricep).
6-6
There are three types of muscles, each with a different location and function:
Location
Function
Voluntary/Invol
untary?
Slow or fast
contractions?
Rhythmic?
Skeletal
On bones
Movement
body, generate
heat
Smooth
Around visceral
organs
Move
substances
through viscera
Cardiac
Heart
Move blood around the
body
Involuntary
Involuntary
Voluntary
Fast
Slow
Fast
No
Yes
Yes