Muscular System

The muscular system is composed of muscle tissue that is highly specialized to contract (shorten) when stimulated

myo, sarco = muscle

With rare exceptions, all muscle tissue develop from embryonic mesoderm cells called myoblasts

Human have 650 muscles

• largest – Gluteus maximus

• smallest - Stapedius

Types of Muscles

There are 3 major types of muscles:

1. Smooth muscles

• are found in the walls of blood vessels, where they function to regulate blood pressure

• are found in the walls of most internal organs, where they function to move food through the digestive tract

(often called visceral muscles)

• are found in your eyes, where they function to constrict the pupil when a bright light is flashed in them

• are found in your skin, where they cause goose bumps when you are cold

• spindle shaped cells that are tapered at the ends each with a single nucleus

• occur in sheets rather than bundles

• smooth muscles are under involuntary control, part of the ANS – autonomic nervous system

• cannot consciously control their contraction

Smooth Muscle

2. Cardiac Muscles – found only in the heart

• responsible for pumping blood throughout the body

• involuntary muscles, heart beats automatically (ANS)

• most impressive feature is its tireless, rhythmic beating

• branched cells

3. Skeletal Muscles – usually attached to bones

• also called voluntary muscles and striated muscles

• voluntary – can consciously control the contraction (SNS – somatic nervous system)

• cells are striated and multinucleated

• can contract much more rapidly and forcefully than the other types, but it cannot sustain such contraction without tiring

Characteristics of Muscle Tissue

1. excitability – ability to receive and send a stimulus

2. contractibility – ability to shorten forcibly

3. extensibility – ability to be stretched or extended

4. elasticity – ability of a muscle to recoil after being stretched

Functions of the Skeletal Muscles

1. movement

2. maintaining posture

3. heat production – chemical changes for contraction release heat energy which helps maintain body temperature

• most muscles cross at least one joint so that they are attached to 2 different bones

• skeletal muscles are attached to bones by tendons

Movement

• as muscles contract they pull on tendons, which in turn pull on bones (however the 2 bones usually do not move equally)

• the end of the muscle attached to the stationary (non-moving) bone is called the origin• the end of the muscle attached to the movable bone is called the insertion• skeletal muscles have at least one origin and one insertion

O

I

O = origin I = insertion

Contraction of a Muscle

All skeletal muscles are surrounded by a tough connective tissue covering - perimysium

perimysium

• muscle is composed of several bundles of muscle fibers

• each bundle of muscle fibers is composed of very tiny rod-like fibers - myofibrils

•each myofibril is composed of repeating units that contain high amounts of glycogen - sarcomeres

Muscle fibermyofibrils

Sarcomere

Myofibrils

• sarcomeres lie end to end in long chains

• sarcomeres contain 2 different types of protein filaments: actin and myosin

Actin filaments are very thin and look like 2 strings of beads that have been twisted together

Myosin filaments are very thick and look like a bundle of tiny golf clubs

When a muscle fiber contracts, the actin and myosin filaments slide past each other, causing the muscle fiber to shorten

In a fully relaxed muscle fiber, the actin and myosin filaments do not overlap each other very much

• you can see alternating light and dark bands (striations) under the microscope

As the muscle fiber contracts and the actin filaments begin sliding past the myosin filaments, you see a greater degree of overlap

• the dark bands become thicker and the light bands thinner

In a fully contracted muscle fiber you are unable to see any light bands under the microscope

• the actin and myosin filaments overlap each other completely

Each muscle fiber is made up of hundreds of muscle fibers

• if you pick up a light object only a few muscle fibers contract; a very heavy object requires all fibers to contract

• the force of a contraction is determined by the number of muscle fibers stimulated

All-or-none response – a muscle fiber either contracts completely or does not contract at all

Physiology of Muscle Contraction

• muscles must be stimulated by a neuron to create an action potential

• Ca 2+ ions flow in between nerve synapses

•acetylcholine (neurotransmitter) then is released

• the combination causes an electrical event (depolarization) to occur

• this depolarization results in muscle stimulus

•occurs 3 to 4 hours after death, with peak rigidity at 12 hours after death

Rigor Mortis

• dying cells cannot keep out Ca2+ ions

• the more Ca2+ ions in the cell, the higher the action potential – making the muscle contract

• the muscles will relax 48 to 60 hours after death because the muscle tissue begins to decompose

Actions of Skeletal Muscles

1. Flexor – decreases the angle of a joint

ex. Biceps brachii flexes your forearm

2. Extensor – increases the angle of a joint

ex. Triceps brachii extends your forearm

3. Abductor – moves bone away from the midline

ex. Deltoid raises the arm

deltoid

4. Adductor – moves a bone toward the midline

ex. Latissimus dorsi lowers the arm

Latissimus dorsi

5. Levator – produces an upward movement

ex. Masseter elevates the mandible

masseter

6. Depressor – produces a downward movement

ex. hyoglossus (originates on the hyoid bone and inserts on the tongue) depresses the tongue

hyoglossus

7. Rotator – moves a bone around its long axis

ex. External oblique (abdominal muscles) rotates the trunk of the body

External oblique

Muscles typically work in opposable pairs to move parts of the skeleton

• what one muscle does, another muscle will undo

• flexors oppose extensors

• abductors oppose adductors

• levators oppose depressors

Muscles that oppose each other are often referred to as agonists and antagonists

• an agonist is the muscle that causes the desired action

• an antagonist is the muscle that causes the opposite action

• biceps brachii and triceps brachii are examples of agonist and antagonist

Fuel for Muscle Contraction

• ATP (adenosine triphosphate) is used to fuel muscle contractions

• it is produced in your muscle cells by a process called aerobic respiration

Three pathways for energy creation:

•direct phosphorylation of ADP by creatine phosphate

• aerobic Respiration

• fermentation

Direct Phosphorylation of ADP by creatine phosphate

Creatine phosphate:

•high energy molecule that helps ATP store energy

• stores 4x-5x more energy than ATP

• creatine drinks are available which help increase muscle performance

• CP + ADP Creatine + ATP

Aerobic respiration (glycolysis, Kreb’s cycle, and electron transport chain) breaks down glucose in the presence of oxygen to produce ATP

• ATP is required to break the bonds between myosin and actin filaments

•when you die, ATP is no longer produced. The actin and myosin filaments cannot detach from each other, resulting in rigor mortis (stiffening of the body)

When muscles are working hard, the amount of oxygen being supplied to the muscle cells is not enough for aerobic respiration to continue

Thus, the muscle cells must begin producing the ATP they require anaerobically (in the absence of oxygen) by a process called fermentation

During fermentation, glucose is broken down into lactic acid plus very small amounts of ATP

• lactic acid formed during fermentation causes muscle fatigue

• the amount of ATP formed during fermentation is so small, large amounts of glucose must be broken down

When glucose stores run out, the body begins breaking down glycogen (stored glucose)

When glycogen stores in the muscles have been used up, the muscles will begin breaking down fat to produce ATP

• ATP production is greatly reduced when fat is broken down to release energy for

muscle contraction

• As a result, the performance of muscles drops dramatically

Effects of Exercise on Muscles

You can increase muscle endurance (ability to continue exercising) by aerobic training and exercise

• both will strengthen the heart and chest muscles, allowing you to pull more air into the lungs and deliver a larger amount of air to the muscle cells

• thus the muscle cell can produce ATP aerobically for a longer period of time

• exercise can also increase the size and strength of your muscles

• the muscles of males are generally larger than those of females because they are influenced by the male sex hormone - testosterone

Synthetic drugs called anabolic steroids will also increase muscle size and strength

• steroids trick muscles into growing larger because they resemble testosterone

• the problem with steroids is that they have dangerous side effects:

cancerheart diseasealtered sexual development

You can drink chocolate milk to help muscles rebound after a workout

Contains:

• calcium

• carbohydrates

• natural Proteins

• vitamins A and D

Muscular Disorders

1. muscle strain – a pulled muscle

2. sprain – a torn or overly stretched muscle, tendon, or ligament

4. spasm – a sudden involuntary contraction of a single muscle that is not

painful

5. cramp – a sudden involuntary contraction of a single muscle that is painful

6. muscular dystrophy – an inherited condition in which the muscles of the body degenerate

• due to the lack of the protein dystrophin