Skeletal Muscle Physiology: Part 2, and Skeletal Muscle Gross Anatomy

What is a motor unit?

A motor unit consists of a somatic motor neuron and all the muscle fibers it innervates.

How do motor units differ in small vs. large muscles?

Small muscles have fewer fibers per motor unit, while large muscles have many fibers per motor unit.

What is asynchronous recruitment in muscle contraction?

Asynchronous recruitment allows some muscle fibers to relax while others contract, enabling stronger contractions.

How does the body recruit muscle fibers during contraction?

Weaker muscle fibers are recruited first, followed by stronger fibers as the stimulus increases.

What determines the difference in muscle strength between individuals?

The difference is based on the number of myofibrils within each muscle fiber.

Do the number of muscle fibers change throughout a person's life?

No, the number of muscle fibers remains constant; motor units are set at birth.

What can change within muscle fibers over time?

The proteins found within each muscle fiber can change, especially with resistance training.

What is an isometric contraction?

An isometric contraction occurs when the length of the muscle doesn't change, but tension increases.

Give an example of an isometric contraction.

Holding a weight steady without moving it.

What are isotonic contractions?

Isotonic contractions occur when the tension produced is constant but the length of the muscle changes.

What is a concentric contraction?

A concentric contraction occurs when the muscle shortens as tension overcomes resistance.

What is an eccentric contraction?

An eccentric contraction occurs when the muscle lengthens because resistance is greater than the tension.

What role do motor neurons play in muscle contraction?

Motor neurons stimulate muscle fibers to contract, coordinating the strength and timing of the contraction.

What happens to muscle fibers during resistance training?

Resistance training builds more proteins within each muscle fiber, increasing strength.

What is the relationship between motor units and muscle fibers?

Each motor unit innervates multiple muscle fibers, but the number varies based on the muscle's size and function.

What are Type 1 muscle fibers known for?

They are slow twitch, high oxidative, fatigue resistant, and suited for endurance.

What is the primary energy metabolism utilized by Type 1 muscle fibers?

Oxidative metabolism.

How do Type 1 muscle fibers compare in size to Type 2 fibers?

Type 1 fibers are smaller in diameter than Type 2 fibers.

What is a key characteristic of Type 2a muscle fibers?

They are fast twitch, utilize both oxidative and glycolytic metabolism, and are also called fast oxidative-glycolytic fibers.

What distinguishes Type 2x muscle fibers from Type 1 and Type 2a fibers?

Type 2x fibers are the largest, have the most myofibrils, and are the strongest but rely on non-oxidative metabolism.

What is the primary fuel source for Type 2x muscle fibers?

Carbohydrates, as they cannot utilize oxidative metabolism effectively.

What are the characteristics of Type 2 muscle fibers?

They are fast twitch, produce more force, have less mitochondria, and lower aerobic capacity.

What happens to muscle fiber types with training?

Type 2a fibers can convert to Type 2x with resistance training, but Type 1 fibers cannot change to Type 2.

What is the ATP breakdown rate in Type 1 muscle fibers?

Slow ATP breakdown due to slow myosin ATPase.

What is the ATP breakdown rate in Type 2x muscle fibers?

Fast ATP breakdown due to fast myosin ATPase.

What type of muscles are primarily composed of Type 1 fibers?

Postural muscles that require endurance.

What is the blood supply like in Type 1 muscle fibers?

They have a better blood supply compared to Type 2 fibers.

What is the fatigue resistance of Type 2x muscle fibers?

They are fatigable and can only use carbohydrates as fuel.

What is the aerobic capacity of Type 1 muscle fibers?

They have a high aerobic capacity.

What is the aerobic capacity of Type 2x muscle fibers?

They have the lowest aerobic capacity.

Lateral Flexion

Muscles deep within the neck along the anterior margins of the vertebral bodies contracting together.

Forward Flexion of Head

Muscles contracting at a time.

Extension

Posterior neck muscles attached to the occipital bone and mastoid process.

Rotation & Lateral Flexion

Lateral and posterior groups.

Prime Movers

Sternocleidomastoid and Trapezius.

Muscles that Move the Vertebral Column

Used to produce erect posture.

Deep Group

From vertebra to vertebra.

Superficial Group

Extend from vertebra to ribs.

Erector Spinae

Prime movers of back extension.

Slow-Twitch Fibers

Easy to damage.

Scalenes

Elevate first two ribs during inspiration.

External Intercostals

Elevate the ribs as they contract.

Internal Intercostals

Depress ribs during expiration.

Transversus Thoracis

Depresses ribs during expiration.

Diaphragm

Major movement of inspiration; flattens abdominal cavities.

Injury

Muscle stretching.

Thoracic Cavity

Increases the volume during contractions.

Origin or Head

Muscle end attached to more stationary of two bones, usually more proximal.

Insertion

Muscle end attached to bone with greatest movement, usually more distal.

Belly

Largest portion of the muscle between origins and insertion; some have multiple bellies.

Tendons

Attach muscles to bone; CT that extends past the level of the muscle fibers and then attaches into the bone.

Aponeurosis

A broad tendon that is chord-like or sheet-like.

Agonist

Muscle that, when it contracts, causes an action.

Antagonist

A muscle working in opposition to the agonist.

Synergists

Muscles that work together to cause a movement.

Prime Mover

Muscle that plays a major role in accomplishing movement.

Fixators

Muscles that stabilize joint/s crossed by the prime mover; prevent movement of the origin of the prime mover.

Muscle bellies

Muscles with two or three origins.

Nomenclature

Muscles are named according to location, size, shape, action/function, orientation/direction of fibers, origins/insertion, and number of heads.

Location

Muscle naming based on terms like pectoralis, gluteus, brachial, inferior, superior, etc.

Size

Muscle naming using terms like maximus, minimus, longus, brevis (shortest), vastus (huge).

Shape

Muscle naming based on terms like deltoid, quadratus, teres, trapezius, etc.

Action/Function

Muscle naming based on terms like abductor, adductor, masseter.

Orientation or Direction of Fibers

Muscle naming using terms like rectus (parallel to midline).

Number of Heads or Muscles

Muscle naming based on terms like biceps, triceps, quadriceps.

What is the primary function of the abdominal wall muscles?

Compression of the thoracic and abdominal cavities, aiding in forced expiration, vomiting, defecation, urination, and childbirth.

Name a muscle that flexes and rotates the vertebral column.

Rectus abdominis

What is the role of the external abdominal oblique?

Flexion and rotation of the vertebral column, as well as compression of the abdominal cavity.

Which muscles are involved in scapular movements?

Trapezius, levator scapulae, rhomboideus, serratus anterior, and pectoralis minor.

What is the function of the trapezius muscle?

Elevates, depresses, and rotates the scapula.

What does the serratus anterior muscle do?

Holds the scapula in place and assists in scapular protraction.

How do the internal abdominal obliques contribute to abdominal function?

They assist in flexing and rotating the vertebral column and compressing the abdominal cavity.

What is the significance of the crossing pattern of abdominal muscles?

It adds strength to the abdominal wall to support organs and aid in movements.

What is the role of the levator scapulae muscle?

Elevates the scapula and helps stabilize it.

Which ribs do the rhomboideus muscles originate from?

The 3rd to 5th ribs.

What is the function of the pectoralis minor muscle?

Stabilizes the scapula and assists in scapular movements.

Arm Movements

Muscles that attach arm to thorax

Abduction

Movement away from the midline of the body

Medial Rotation

Rotation towards the midline of the body

Lateral Rotation

Rotation away from the midline of the body

Pectoralis Major

Muscle that acts as a flexor of the shoulder

Deltoid

Muscle that abducts and rotates the arm

Rotator Cuff

Group of muscles holding humerus in the glenoid cavity

SITS Muscles

Subscapularis, Infraspinatus, Teres Minor, Supraspinatus

Forearm Movement

Movements at the elbow

Extension

Movement that increases the angle between body parts

Flexion

Movement that decreases the angle between body parts

Supination

Rotation of the forearm so the palm faces up

Pronation

Rotation of the forearm so the palm faces down

What movements are facilitated by the muscles on the anterior surface of the forearm?

Flexion of the wrist, hand, and fingers, as well as abduction and adduction of the wrist.

What is the role of the muscles on the posterior surface of the forearm?

They facilitate extension of the wrist, hand, and fingers, and abduction of the wrist.

What are extrinsic muscles of the hand?

Muscles that move the fingers and wrist but originate outside of the hand.

What are intrinsic muscles of the hand?

Muscles that both originate and insert in the hand, responsible for movements like abduction, adduction, and opposition of the fingers.

What movements are associated with thigh flexion?

Flexion, abduction, and stabilization of the femur when standing.

Which muscles are primarily responsible for flexing the hip?

Iliacus and Psoas Major, often referred to as iliopsoas.

What is the function of gluteal muscles?

They are involved in the extension and abduction of the thigh, as well as stabilizing the femur.

What is the significance of the iliopsoas muscle?

It flexes the hip and allows for lateral rotation of the thigh.

What are deep thigh rotators?

Muscles that assist in the rotation of the thigh.

What is the role of the tensor fasciae latae?

It assists in thigh abduction and stabilization.

What does 'opposition' refer to in hand movements?

The ability to touch the thumb to the tips of the other fingers.