Muscles: Types, Properties, and Classifications

• The study notes cover muscles, including their types, properties, and classifications.
• The focus is on muscles of the face, thorax, abdomen, and both upper and lower limbs.

Muscle Generalities

• The human body has about 650 to 800 muscles
• Muscles are specialized contractile tissues
• Muscle tissue makes up 30–40% of body weight.
• Muscles functions to stabilize the body position, regulate organ volume, generate heat, and propel fluids/food.
• The two main types of muscle are smooth and striated.

Muscle Tissues

• The three types of muscle tissues are smooth, skeletal, and cardiac.
• The muscular system can be grouped into 3 types of muscles.
• Smooth muscle is involuntary, having continuous contractions
• Smooth muscle is influenced by the autonomic nervous system, hormones, and local metabolites.
• Smooth muscle consists of small, single-nucleus cells that are elongated, fusiform, and tapered.
• Striated skeletal muscle is voluntary and controlled by large motor nerves
• Contraction of these muscles occurs via motor units of long cells.
• Cardiac muscle has intermediate characteristics, contracting powerfully and continuously with high energy use.
• Cardiac muscle is unique to the heart and consists of short, cylindrical cells with one or two nuclei.

Muscle Properties

• Muscles have electrical excitability, responding to stimuli by producing electrical impulses.
• Muscles exhibit contractility, extensibility, and elasticity.
• Elasticity is the ability of a muscle to return to its original shape

Muscle Connective Tissues

• The epimysium is a collagenous connective tissue surrounding muscles
• The perimysium is connective tissue extending from the epimysium into the muscle, surrounding fascicles (bundles of muscle fibers)
• The endomysium is reticular connective tissue forming a layer around each muscle fiber.

Muscle Classification

• Muscles can be classified by their action: agonists, antagonists, synergists.
• Muscles can be classified by movement: flexors, extensors, adductors, abductors, rotators.
• Muscles can be classified by shape: fusiform, flat, wide, fan, circular, orbicular.
• Fusiform muscles are thick in the middle and thin at the ends.
• Orbicular muscles are similar to fusiform but have a central hole.

Tendons vs. Ligaments

• Tendons connect muscles to bones.
• Tendons transmit force generated by muscles to bones.
• Ligaments, mostly made of collagen, connect bones or organs to form articulations.
• Ligaments functions to limit excessive movement of articulations.

Facial Muscles

• Key muscles include the occipitofrontalis, procerus, depressor supercilii, orbicularis oculi, levator labii superioris, nasalis, zygomaticus major, orbicularis oris, risorius, masseter, depressor anguli oris, depressor labii inferioris, and mentalis.
• The occipitofrontalis muscle consists of the occipital belly which originates from the superior nuchal line of the occipital bone, and the frontal belly that originates from the epicranial aponeurosis.
• The occipitofrontalis muscle inserts into the epicranial aponeurosis (occipital belly) and the skin/subcutaneous tissue of the eyebrows and forehead (frontal belly).
• The occipitofrontalis raises the eyebrows/wrinkles the forehead and retracts the scalp.
• The orbicularis oculi muscle originates from the medial margin of the orbit (palpebral ligament and lacrimal bone) and inserts into the skin of the orbit and eyelids.
• The levator labii superioris alaeque nasi muscle originates from the maxillary bone and inserts into the alar cartilage of the nose and the skin of the upper lip.

Thorax Muscles

• Key muscles include the pectoralis major and minor, serratus anterior, and intercostals.
• The pectoralis major originates from the medial clavicle, the anterior sternum, the first 7 costal cartilages, and the anterior surfaces of the costal cartilages.
• The pectoralis major inserts into the proximal third of the humerus and it functions to adduct and medially rotate the arm.
• The pectoralis minor originates from the 3rd, 4th, and 5th ribs, and inserts into the coracoid process of the scapula.
• The pectoralis minor functions to move the scapula forward and downward, and to aid in inspiration.

Abdominal Muscles

• Key muscles include the rectus abdominis, external and internal obliques, transversus abdominis.
• The rectus abdominis originates from the crest of the pubis and inserts into the costal cartilages of the 7th rib as well xiphoid process.
• This muscle divides the left and right sides is divided by the linea alba (white line).
• The external oblique originates from the cara lateral of the ribs 5-12 and it inserts from the iliac crest to the rectus abdominis.
• This muscle is used for contralaterally rotating the torso.
• The internal oblique originates from the iliac crest and inserts in the linea alba.
• This muscles flexes the trunk.
• The transversus abdominis originates from the internal surface of the lower five ribs and the transverse processes of the lumbar vertebrae.
• Transversus Abdominis inserts the lineage media.
• This muscles constricts and supports the abdominal wall.

Back Muscles

• The key muscles in the back include trapezius, latissimus dorsi, rhomboids, and levator scapulae.
• The trapezius is a large muscle spanning the upper back and neck.
• It's divided into descending, transverse, and ascending portions, each with distinct origins, insertions, and functions involving scapular movement
• Origin of the descending portion includes the superior nuchal line
• Insertion of the descending portion includes the clavicle, functions to elevate the scapula

Muscles of Upper Limb

• The muscles around the shoulder include the deltoids, subscapularis, supraspinatus, infraspinatus, teres minor and major.
• The deltoid muscle oritinates the clavicle and insers to the deltoid tuberosity of humerus
• The muscles in the upper arm and elbow include biceps brachii, brachialis, triceps brachii, and anconeus.
• The hand has palmar muscles and extensor muscles.
• The thenar muscles (abductor pollicis brevis, flexor pollicis brevis, opponens pollicis) control thumb movements.
• The hypothenar muscles (abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi) control little finger movements.