Muscle Structure and Function

Structural and Functional Organization of Muscles

• Muscle cells produce force on organs and tissues for movement.
• Tissue types include skeletal, cardiac, and smooth, with at least five main functions:
  • Movement
  • Stability
  • Control of openings/passages
  • Heat production (85% of body heat)
  • Glycemic control (stabilizing blood glucose levels)
• Skeletal muscles contain connective tissue, nerves, and blood vessels.
• A muscle contains four connective tissue components:
  • The endomysium surrounds each muscle fiber
  • The perimysium wraps muscle fibers into fascicles
  • The epimysium surrounds the entire muscle
  • The fascia separates muscles/groups and subcutaneous tissue.

Fascicle Orientation

• The orientation of fascicles impacts muscle strength and pull direction and are classified into five categories:
  • Fusiform muscles are thick in the middle and tapered at ends
  • Parallel muscles have a uniform width and parallel fascicles (strap-like or quadrilateral)
  • Triangular (convergent) muscles are fan-shaped (broad origin, narrow insertion)
  • Pennate muscles are feather-shaped, with oblique fascicle insertion on a tendon running the muscle's length, including:
    • Unipennate: fascicles approach tendon from one side
    • Bipennate: fascicles approach tendon from both sides
    • Multipennate: shaped like a bunch of feathers with quills converging
  • Circular muscles (sphincters) form rings around openings.
• Muscle compartments are groups of related muscles separated by connective tissue fascia
• Intermuscular septa are thick fascia separating one compartment from another.

Skeletal Muscle Attachments

• Skeletal muscles attach to bones via connective tissue extensions:
  • Indirect attachment: muscle ends short, gap bridged by a tendon
  • Direct (fleshy) attachment: muscle tissue appears to emerge directly from bone
• Most skeletal muscles attach to a different bone at each end, crossing at least one joint.
• The muscle's origin is at the stationary end, while the insertion is at the mobile end, although this terminology is imperfect.
• The belly is the thicker middle region of the muscle.
• Some muscles insert on fascia, tendon, or collagen fibers of the dermis instead of bone.

Muscle Actions

• Skeletal muscle groups coordinate joint control.
• Muscles are classified into these categories based on action:
  • Prime mover (agonist): produces most of the force
  • Synergist: aids the prime mover by steadying or modifying direction
  • Antagonist: opposes the prime mover to control speed/range, and prevent injury
  • Fixator: steadies a bone to allow another muscle to pull on something else
• Intrinsic muscles are contained within a region, while extrinsic muscles act on a region from elsewhere.
• Innervation is the identity of the nerve that stimulates a muscle.

Nerve Types

• There are two nerve types:
  • Spinal: emerge from the spinal cord and innervate below the neck, identified by letter-number corresponding to adjacent vertebrae and each branch.
  • Cranial: emerge from the brain base and innervate head and neck, identified by roman numerals, but not all innervate skeletal muscle.
• Working muscles require good blood supply for glucose, fatty acids, and oxygen.
• Muscle names use Latin to describe structure, location, or action
• Learning the muscular system requires various tactics.

Muscles of the Head and Neck

• Muscles of the head and neck can be grouped as:
  • Muscles of facial expression
  • Muscles of chewing and swallowing
  • Muscles that move the head as a whole
• Facial muscles perform facial expressions and aid oral functions.
• All but one facial muscle is innervated by the facial nerve (CN VII).
• The muscles of the scalp (frontalis and occipitalis) are connected by the galea aponeurotica.
• Other facial and nasal muscles of the orbital region include:
  • Orbicularis oculi muscle
  • Levator palpebrae superioris
  • Corrugator supercilii
  • Nasalis
• The oral region contains eight muscles, including:
  • Orbicularis oris
  • Levator labii superioris
  • Levator anguli oris
  • Zygomaticus major
  • Zygomaticus minor
  • Risorius
  • Depressor anguli oris
  • Depressor labii inferioris
• The mental (chin) and buccal (cheek) regions include:
  • Mentalis muscle
  • Buccinator muscle
• The platysma is located in the cervical and mental region.
• Muscles of chewing and swallowing aid in food manipulation and tongue movement.

Tongue Muscles

• The tongue includes intrinsic and extrinsic muscles.
• Four extrinsic muscles connect the tongue to the rest of the head.
• Three of the four extrinsic tongue muscles are innervated by the hypoglossal nerve (CN XII).
• There are four extrinsic muscles of the tongue:
  • The genioglossus
  • The hyoglossus
  • The styloglossus
  • The palatoglossus
• Four pairs of muscles produce biting and chewing movements:
  • Temporalis
  • Masseter
  • The medial pterygoid
  • The lateral pterygoid
• The suprahyoid group contains four pairs of hyoid muscles assisting in chewing and swallowing, including the:
  • Digastric
  • Geniohyoid
  • Mylohyoid
  • Stylohyoid
• The infrahyoid group contains four pairs of muscles inferior to the hyoid bone aiding in swallowing, including the:
  • Omohyoid
  • Sternohyoid
  • Thyrohyoid
  • Sternothyroid
• The muscles of the pharynx consist of three pairs that act as pharyngeal constrictors.

Muscles of the Head

• Muscles that move the head originate on the vertebral column, thoracic cage, and pectoral girdle and insert on the cranial bones.
• Different contractions of the head can cause contralateral or ipsilateral movements.
• The flexors of the neck include four muscles whose prime mover is the sternocleidomastoid, innervated by the accessory nerve and spinal nerves.
• The three Scalenes (anterior, middle, and posterior), when contracted unilaterally, tilts head toward the same shoulder or rotates face away.
• The extensors of the neck are in the nuchal region.
• The trapezius extends and laterally flexes the neck.
• The splenius capitis and splenius cervicis produce ipsilateral flexion and extend the head.
• The semispinalis capitis and semispinalis cervicis extend and contralaterally rotate the head.

Muscles of the Trunk

• Trunk muscles fall into four categories:
  • Respiration
  • Abdominal wall support
  • Vertebral column movement
  • Pelvic floor support
• Muscles of respiration include the diaphragm, external intercostal, internal intercostal, and innermost intercostal muscles.
• The diaphragm separates the thoracic and abdominal cavities.
• Eleven pairs of external and internal intercostal muscles aid with respiration and are innervated by intercostal nerves.
• Muscles of the anterior abdominal wall also include layers of broad flat muscles.
• Abdominal muscles are innervated by the anterior rami of spinal nerves.
• Tendons of oblique and transverse muscles are aponeuroses that enclose the rectus abdominis in the rectus sheath.
• Three lateral abdominal muscles assist with posture and movements of the vertebral column:
  • External abdominal oblique supports the abdominal viscera against the pull of gravity, stabilizes the vertebral column, maintains posture, and compresses abdominal organs. Unilateral contraction causes contralateral rotation of the spine.
  • Internal abdominal oblique action is the same as the external oblique except that unilateral contraction causes ipsilateral rotation of the waist.
  • Transverse abdominal compresses abdominal contents but does not contribute to movements of the vertebral column.
• Rectus abdominis muscles are divided into segments by transverse tendinous intersections.
• Muscles of the back extend, rotate, and laterally flex the vertebral column.
• The most prominent superficial back muscles, such as the latissimus dorsi and trapezius, are concerned with upper limb movements.
• Deeper back muscles include the:
  • Iliocostalis
  • Longissimus
  • Spinalis
• Major deep muscles that assist with back movement include:
  • Semispinalis thoracis
  • Quadratus lumborum
  • Erector spinae muscles
  • Multifidus

Pelvic and Shoulder Muscles

• The floor of the pelvic cavity is mainly formed by the levator ani.
• The levator ani can also be divided into three portions:
  • Ischiococcygeus
  • Iliococcygeus
  • Pubococcygeus
• Upper and lower limbs contain numerous muscles for body movement and object manipulation, organized into compartments separated by fasciae.
• The upper limb has anterior and posterior compartments, while the lower limb has anterior, posterior, medial, and lateral compartments.
• Muscles acting on the pectoral girdle originate on the axial skeleton; they are divided into anterior and posterior groups.
• Major muscles of the anterior group are responsible for the abduction of the arm.
• Major muscles making up the posterior group include the superficial trapezius and three deep muscles: the levator scapulae, rhomboid minor, and rhomboid major.

Muscles of the Arm

• Muscles acting on the arm are made up of muscles that cross the shoulder joint and insert on the humerus.
• Two of the nine muscles are considered axial muscles because they originate in the axial skeleton; the other seven comprise the scapular muscles, which originate on the scapula
• These muscles include the prime movers of the shoulder-joint and axilla.
• Two axial muscles include the pectoralis major and the latissimus dorsi.
• Of the seven scapular muscles, three flex, extend, and rotate the arm, and four make up the rotator cuff.
• Three scapular muscles include the:
  • Deltoid
  • Teres major
  • Coracobrachialis

Rotator Cuff Muscles

• The tendons of the four remaining scapular muscles form the rotator cuff
• The first three muscles lie on the posterior side of the scapula, and the fourth occupies the subscapularis fossa on the anterior surface of the scapula
  • The supraspinatus
  • The infraspinatus
  • The teres minor
  • The subscapularis

Muscles of the Forearm

• Muscles acting on the forearm are in both the arm and the forearm itself
• The elbow and forearm can perform four motions: flexion, extension, pronation, and supination.
• Muscles with bellies in the arm (brachium) include elbow flexors in the anterior compartment and the elbow extender in the posterior compartment.
• Most forearm muscles (antebrachium) act on the wrist and hand, synergists in elbow flexion/extension and function in pronation/supination.
• Extrinsic muscles are in the forearm, and intrinsic muscles are in the hand itself
• Forearm muscles are divided into anterior and posterior compartments, each further divided into superficial and deep layers.
• The superficial layer of the anterior (flexor) compartment are wrist and finger flexors and arise from a common tendon on the humerus
• Muscles in the superficial layer of the anterior (flexor) compartment include the:
  • The flexor carpi radialis
  • The flexor carpi ulnaris
  • The flexor digitorum superficialis
  • The palmaris longus
• The deep layer of the anterior (flexor) compartment includes two flexors: The flexor digitorum profundus and the flexor pollicis longus
• The superficial layer of the posterior (extensor) compartment has five muscles: The extensor carpi radialis longus, the extensor carpi radialis brevis, The extensor digitorum, The extensor digiti minimi, and the extensor carpi unlaris
• The deep layer of the posterior (extensor) compartment: the abductor pollicis longus the extensor pollicis brevis, the extensor pollicis longus, and the extensor indicis

Intrinsic Muscles

• Intrinsic muscles of the hand assist flexors and extensors in the forearm and make finger movements more precise.
• The three groups of intrinsic muscles of the hand are:
  • The thenar group
  • The hypothenar group
  • The midpalmar group
• The hypothenar group muscles form a fleshy mass and is concerned with movement of the little finger. There are also palmar interosseous muscles.
• The muscles are organized into four layers.
• Ventral 1 is the layer encountered beneath the plantar aponeurosis.
  • The flexor digitorum brevis is a stout muscle on the midline that flexes digits II-IV and supports the arches. Ventral 2 assists in the extensor hallucis longus Ventral layer 3 contains muscles that mostly serve solely to bend particular toes or to bend digits rather than the joints where the digits join the tarsus.

Muscles of the Hip and Lower Limb

• The largest muscles make actions on the femur and hip joint, the leg and knee joint, extrinsic (leg) muscles acting on the foot and ankle joint, and intrinsic (foot) muscles acting on arches and toes.
• Muscles acting on the hip and femur mostly originate in the hip bone
• Anterior muscles that operate in the hip are mainly the iliacus and psoas major.
• Lateral and posterior hip assist the gluteal muscles.
• There are a total of six lateral hip muscles.
• Medial muscles assist with the hip to aid movement.
• The muscles acting in the knee and leg create actions on the knee joint. Some cross both the hip and knee joints; the other muscles make it easier to move. There are certain muscles known as the hamstrings. The popliteus also unlocks the knees and dislocation of the tibia
• Muscle actions that operate on the foot also aid in the return of blood from the legs. Anterior also prevents the toes on scuffing the ground.
• The crural and peroneal are two leg muscles assisting the foot.
• The intrinsic of the foot act to help support arches. Ventral layer 2,3,4.