muscle anatomy and mechanics

What are the four key properties of muscle tissue?

Contractibility, excitability, extensibility, and elasticity.

What percentage of body weight is skeletal muscle?

About 40%.

Which type of muscle is voluntary and attached to the skeleton?

Skeletal muscle.

Where is cardiac muscle found?

In the walls of the heart.

Is cardiac muscle voluntary or involuntary?

Involuntary.

Where is smooth muscle located?

In the walls of organs.

What type of control does smooth muscle have?

Involuntary.

What is the primary role of skeletal muscle in movement?

Moving bones to produce body movement.

How do muscles contribute to stabilization?

By maintaining posture and joint stability (e.g., sitting and standing).

How do muscles help regulate body temperature?

Through heat generation from muscle contractions.

What structural components make up muscle tissue?

Muscle fibers, connective tissues, blood vessels, and nerves.

Which proteins form the myofilaments in muscle fibers?

Actin and myosin.

How do muscles attach to bones?

Through direct and indirect connections.

What percentage of muscle cell cytoplasm is made up of myofibrils?

Approximately 80%.

What are myofibrils?

Specialized contractile organelles in muscle cells.

What is the functional unit of skeletal muscle tissue?

The sarcomere.

What marks the boundaries of a sarcomere?

Z discs.

Which filaments extend from Z discs?

Thin (actin) filaments.

Where are thick (myosin) filaments located in the sarcomere?

In the center of the sarcomere.

What is the difference between A bands and I bands?

They refract polarized light differently due to filament arrangement.

What are the two primary types of muscle contraction?

Concentric (muscle shortening) and eccentric (muscle lengthening while generating force).

How do myosin heads contribute to contraction?

They pull along the entire length of thin filaments during contraction.

What triggers the sliding filament mechanism?

Calcium ions.

What is the sarcoplasmic reticulum?

A specialized smooth endoplasmic reticulum in muscle cells.

What are T-tubules?

Deep invaginations of the sarcolemma that help transmit action potentials.

What is a triad in muscle fibers?

A T-tubule flanked by two terminal cisterns of the sarcoplasmic reticulum.

When is the greatest force produced in a muscle fiber?

When the muscle fiber starts slightly stretched.

How can muscles act like a “brake”?

By resisting gravity during lengthening contractions (eccentric action).

What is the neuromuscular junction?

The critical point where nerve endings meet muscle fibers to initiate contraction.

What are terminal boutons?

Axon endings that store neurotransmitters.

What is the synaptic cleft?

The space between the axon terminal and the muscle cell membrane.

What neurotransmitter is released at the neuromuscular junction?

Acetylcholine (ACh).

How does ACh stimulate muscle fibers?

By triggering changes in the sarcolemma, causing muscle fiber excitation.

What limits the action of ACh to a single muscle twitch?

Enzymatic breakdown of ACh in the synaptic cleft.

What are motor units?

A motor neuron and all the muscle fibers it innervates.

How do axons contribute to neuromuscular junctions?

Axons branch into terminal boutons, forming multiple neuromuscular junctions.

What are the three primary types of muscle fibers?

Slow oxidative (Type I), fast oxidative (Type IIa), and fast glycolytic (Type IIx).

What are the characteristics of slow oxidative fibers (Type I)?

Red, aerobic metabolism, slow contraction, fatigue-resistant.

What are the characteristics of fast oxidative fibers (Type IIa)?

Intermediate diameter, oxygen-dependent, rapid contraction.

What are the characteristics of fast glycolytic fibers (Type IIx)?

Few mitochondria, anaerobic metabolism, powerful and fast contractions.

From what cells does muscle tissue develop?

Myoblasts.

How do skeletal muscle fibers form?

Myoblasts fuse together to form skeletal muscle fibers.

What is the approximate muscle mass percentage in females?

36–42% of body mass.

How does male muscle mass generally compare to female muscle mass?

Males typically have a higher percentage of muscle mass.

How does muscle change with aging?

Connective tissue increases, muscle fiber number decreases, and strength declines.

What is sarcopenia?

The process of age-related muscle wasting.

What is a circular fascicle arrangement? Give examples.

Fascicles arranged in concentric rings around body openings; e.g., orbicularis oris, orbicularis oculi.

What is a convergent fascicle arrangement? Give an example.

Broad origin with fascicles converging to a narrow insertion; e.g., pectoralis major.

What is a parallel fascicle arrangement? Give examples.

Fascicles run parallel to the muscle’s long axis; e.g., sternocleidomastoid, biceps brachii.

What are the types of pennate fascicle arrangements?

Unipennate (fascicles insert on one side of tendon) and bipennate (fascicles insert on both sides of tendon).

How does fascicle arrangement affect muscle function?

It determines the direction of force and type of movement the muscle can produce.

Where do skeletal muscles attach, and what do they produce?

They attach to the bony skeleton and produce body movements.

What are the four components of a lever system in the human body?

Rigid bar (bone), fulcrum (fixed point), applied force (effort), and resistance (load).

What defines a first-class lever? Give an example.

Fulcrum is between the load and effort; e.g., lifting the head off the chest.

What defines a second-class lever? Give an example.

Load is between fulcrum and effort; e.g., standing on tiptoe.

What defines a third-class lever? Give an example.

Effort is applied closer to the fulcrum than the load; e.g., flexing the forearm with the biceps brachii.

What is a power lever?

A lever where effort is farther from the fulcrum, giving a mechanical advantage for lifting heavy loads.

What is a speed lever?

A lever where effort is closer to the fulcrum, sacrificing power for increased speed.

What is the role of a prime mover (agonist) in muscle action?

The primary muscle responsible for producing a movement.

What is the role of an antagonist muscle?

To oppose or reverse the action of the prime mover.

What is the role of a synergist muscle?

To assist the prime mover by adding force or reducing undesirable movements.

Which muscle exemplifies third-class leverage in forearm flexion?

The biceps brachii, with the elbow joint as the fulcrum.

How is muscle action determined?

By the muscle’s location relative to the joint it crosses.

What are muscle compartments?

Distinct groups of muscles in a limb that share similar actions, innervation, and often work as agonist/antagonist pairs.

How are muscles in opposing compartments related?

They function as agonist and antagonist pairs.

How is each muscle compartment innervated?

By a single nerve.

What muscles are in the anterior compartment of the upper limb?

Flexor muscles, e.g., biceps brachii.

What muscles are in the posterior compartment of the upper limb?

Extensor muscles, e.g., triceps brachii.

What is the role of the anterior compartment in the lower limb?

Muscles flex the hip and extend the knee.

What is the role of the posterior compartment in the lower limb?

Muscles extend the hip and flex the knee.

What is the role of the medial compartment in the lower limb?

Muscles adduct the thigh.

What factors influence muscle naming?

Location, shape, relative size, fiber direction, attachment points, and action.

Give examples of action-based muscle names.

Flexor, extensor, adductor, abductor.