Muscle Lecture BIO 1021 The Muscular System rev 2-11 Muscle cells are involved in every movement...

Muscle Lecture BIO 102 1

The Muscular System rev 2-11

Muscle cells are involved in every movement that our bodies perform.

Muscles can:• Shorten or contract to produce movement• Relax or be pulled back to their original length by

gravity or by opposing muscle groups, called antagonistic muscles

• Work with other muscle groups, called synergistic muscles, to produce movement

• Resist movement to maintain our posture• Generate heat to maintain our body temperature


• Muscle tissue is made up of tightly packed cells called muscle fibers. The muscle fiber cytoplasm contains proteins which allow the cell to contract


• There are 3 types of muscles:– Skeletal muscles– Cardiac muscles– Smooth muscles

• Skeletal muscles – attach to the bones of our skeleton and provide strength

and mobility for our body• Cardiac muscle

– found in the heart; they pump blood throughout the body

• Smooth muscles– found in most internal organs


• Muscles may also be classified as– voluntary (muscles over which we have

conscious control)– involuntary (muscles over which we have no

conscious control)


Skeletal Muscles• are multinucleated• cells are arranged in a parallel fashion• will not contract unless stimulated by a neuron• are responsible for all locomotion and

manipulation• enable us to respond quickly to changes in the

external environment• compared to other muscle types, their speed of

contraction is fast


Skeletal Muscle

• Is also called Striated or Voluntary muscle– they have striations (or stripes) which are

caused by alternating dark and light “bands”– bands are composed of tightly packed

contractile proteins called myofilaments which are made up of thicker myosin filaments and thinner actin filaments


Cardiac Muscle

• Cells are striated, short, fat, branched and interconnected

• Have specialized areas called intercalated discs• because of these connections, cardiac muscle

works as a single, coordinated unit• will contract without nervous stimulation• usually contracts at a steady rate set by the heart’s

pacemaker, but neural controls allow for a faster beat

• compared to other muscle types, their speed of contraction is moderate


Smooth Muscle

• Cells are shorter than skeletal and cardiac muscle cells

• Because the cells have fewer contractile proteins they do not have striations

• Cells are spindle shaped; each cell has a centrally located nucleus

• Do not require nervous stimulation for contractions

• Are found in the walls of hollow visceral organs

• role is to force fluids and other substances through body channels

• compared to other muscle types, their speed of contraction is slow and sustained


Muscle ContractionContractile proteins or myofilaments, called actin and myosin,

slide past each other using energy from ATP molecules.

• These myofilaments produce alternating light and dark areas called striations

– Dark areas are called A-bands

– Lighter areas are called I-bands

– The Z-line is a thin, dark line where sets of actin myofilaments are woven together

• many myofilaments bundled together are called myofibrils

– The space between 2 Z-lines is called a sarcomere

• A sarcomere is the smallest contractile unit of a muscle fiber


Mechanism of Muscle Contraction: Nerve Activation of Individual Muscle Cells

• In order for a muscle to contract, its cells must be stimulated by a nerve

• The motor neuron secretes acetylcholine (ACh) at the neuromuscular junction– ACh is a neurotransmitter--a chemical which

can either stimulate or inhibit another “excitable” cell

– The ACh diffuses across the space between the neuron and the muscle cell (called the synaptic cleft) and binds to receptor sites on the muscle cell membrane


• The ACh binding causes the muscle cell membrane to generate an electrical impulse which travels along the cell membrane and along the T-tubules– the function of the T-tubules is to allow the electrical

impulse to travel to all cell parts• The T-tubules are in close contact with the sarcoplasmic

reticulum• The electrical impulse triggers the release of calcium from

the sarcoplasmic reticulum so the muscle can contract.– Sliding Filament Mechanism: muscle contracts when

the sarcomeres shorten. This occurs when the thick and thin filaments form cross bridges and slide past each other


Calcium binds to troponin and causes the• Troponin–tropomyosin protein complex to shift

position– To expose the myosin binding site and allow

the myosin heads and actin filaments to make contact, forming cross-bridges.

– The actin filaments are pulled toward the center of the sarcomere and the muscle contracts.


So, in order to contract, a muscle must:– Be stimulated by a nerve– Cause an electric current along the

sarcolemma– Have a rise in intracellular calcium levels

Muscle Relaxation• Nerve cell stimulation ends, contraction ends• In order to stop the contraction, nerve stops

• Calcium pumped back into sarcoplasmic reticulum• Calcium removed from actin filaments• Myosin-binding site covered• No calcium = no cross-bridges


Energy Use by Muscle Cells

• Muscle contraction requires energy– In the presence of calcium, myosin acts as an

enzyme to split ATPADP + inorganic phosphate to release energy.

• ATP is the muscle’s energy source; typically muscle cells store enough ATP for 10 seconds of heavy activity

• After this, ATP can be replenished by:– Creatine phosphate which makes enough ATP for

~ 20-30 seconds– After this short amount of time, energy must be

obtained from stored glycogen


– For long term energy, ATP can also be obtained via aerobic metabolism of glucose, fatty acids, and other high-energy molecules

– Glycogen is broken down by a process called glycolysis or the Krebs Cycle or the Citric Acid Cycle

• Glucose molecules are removed from the glycogen and the cell uses the glucose to synthesize more ATP.– Part of the glucose breakdown process can be done

anaerobically. This is a fast process but only yields 2 ATP molecules per glucose molecule.

– It also produces lactic acid as a waste product which can make muscles sore.

• During vigorous exercise, typically the blood is unable to carry enough oxygen for complete oxidation of glucose in our muscles. So, the muscles will contract anaerobically.

• Lactic acid normally leaves the muscles and goes into the blood. But, with continued exercise, the amount of lactic acid in the blood increases so the lactic acid levels will accumulate in the muscles.

• This will cause muscle fatigue and cramps.

• After exercise, the person rests and must take in enough oxygen to allow the lactic acid to be changed into glucose.

• This oxygen is called the oxygen debt.

• So, the reason we breathe heavily after exercise is to erase the oxygen debt.

Muscle Lecture 16


– The most efficient, but much slower, process for energy production is aerobic metabolism. This yields 36 ATP molecules from 1 molecule of glucose. Carbon dioxide is produced as a waste product.


Gross Anatomy of Skeletal MuscleIndividual muscle fibers are wrapped and held together by

several different layers of connective tissue• The individual muscle fibers are surrounded by a fine

sheath of connective tissue called the endomysium.• The fibers within the muscle are grouped into fascicles,

bundles of muscle fibers with a connective tissue covering.• The perimysium layer is fibrous connective tissue that

surrounds the fascicles. • The outermost connective tissue layer is called the

epimysium. • At the ends of the muscle, all of the fascia come together

and form the tendon.

• Fascia, connective tissue outside the epimysium, surrounds the entire muscle

• At the ends of the muscle, all of the connective tissues come together and form the tendon that attaches the muscle to bone.

Muscle Lecture 19


Movememt• If the muscle spans a joint, one bone moves while

the other one remains stationary– the muscle’s origin is on the bone which does

not move– the insertion is on the bone which moves


Muscle Contractions

• Isotonic contractions: occur when the muscle shortens and movement occurs

• Isometric contractions occur when muscle tension develops but the muscle doesn’t shorten and no bony movement occurs. These contractions help stabilize the skeleton.

• Degree of nerve activation influences force generated by the muscle


Terms to know:• Motor unit: the motor neuron and all the muscle

fibers it supplies. Is the smallest functional unit of muscle contraction.

• Muscle tension: force generated by a contracting muscle upon an object

• All-or-none principle: muscle cells are completely under the control of their motor neuron.

• Muscle tone: low level of contractile activity in a relaxed muscle.


Muscle Activity

• Muscle twitch: a complete cycle of contraction and relaxation

• Humans have 2 types of skeletal muscle fibers: slow-twitch and fast-twitch fibers. The difference is based on how quickly the fibers can produce a contraction and whether the muscle contracts aerobically or anaerobically.

• Slow twitch fibers: break down ATP slowly and contract slowly

• Fast twitch: break down ATP quickly, contract more quickly


Exercise Training

• Strength training:– Resistance training:

• Aerobic training:– Builds endurance– Increases blood supply to muscle cells


Diseases and Disorders of the Muscular System• Muscular dystrophy –inherited; loss of muscle fibers

resulting in muscle wasting and paralysis; death usually from heart failure or respiratory failure

• Tetanus or “lock jaw” –bacterial infection resulting in overstimulation of nerves and therefore muscles resulting in tetanic contractions; death from exhaustion or respiratory failure

• Muscle cramps – painful, uncontrollable muscle contractions; caused by dehydration and ion imbalances caused by heavy exercise

• Pulled muscles –caused by stretching a muscle too far causing some fibers to tear apart

• Fasciitis –inflammation of the connective tissue fascia that surrounds a muscle

Muscle Lecture BIO 1021 The Muscular System rev 2-11 Muscle cells are involved in every movement...

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