How Muscles Are Named

• Muscles are named based on characteristics like direction, size, shape, action, number of origins, location, and origin/insertion.
• Direction: Rectus muscles run parallel to the midline, such as the Rectus Abdominus.
• Size: Maximus muscles are the largest, like the Gluteus maximus.
• Shape: Deltoid muscles are triangular.
• Action: Flexor muscles decrease joint angle, such as the Flexor carpi radialis.

• of Origins: Biceps muscles have two origins, like the Biceps brachii.

• Location: Temporalis muscles are near the temporal bone.
• Origin & Insertion: Sternocleidomastoid originates from the sternum & clavicle and inserts on the mastoid process of the temporal bone.

Muscle Contraction Types

• Isometric contraction involves no change in muscle length, but there is load.
• Isotonic contraction maintains constant tension while the muscle changes length.
• Eccentric contraction involves the muscle lengthening under tension.
• Concentric contraction involves the muscle shortening under tension.

Sequence of Action Potentials in Skeletal Muscle Contraction

• Action potentials in skeletal muscle contraction travel through the following sequence: Motor neuron → Neuromuscular junction → Muscle fiber.
• The process begins with an action potential generated by the somatic motor division of the nervous system.
• The action potential travels down the motor neuron to the sarcolemma of the muscle fiber, continuing as a wave of excitation.
• It then enters the t-tubules, prompting the sarcoplasmic reticulum (SR) to release calcium ions (Ca++).
• The surge of Ca++ leads to binding with the troponin-tropomyosin complex on actin filaments, exposing myosin binding sites.
• Myosin heads attach to these sites, pulling the actin filaments and powering the muscle contraction through the sliding filament theory.

Calcium's Role in Muscle Contraction

• Calcium must be released to start muscle contraction.

Motor Unit

• Consists of a somatic motor neuron and all the skeletal muscle fibers it stimulates.
• The motor unit represents the basic functional unit of muscles.
• Groups of motor units work together to coordinate muscle contraction.
• Motor units are dispersed throughout the muscle and not clustered together.
• The total strength of a contraction depends on the size and the number of activated motor units.

Autorhythmic Fibers

• Cardiac muscle is autorhythmic because it has pacemaker cells that generate spontaneous action potentials.
• Autorhythmic cardiac fibers are self-excitable and initiate their own electrical impulses without needing an external stimulus.

Hyperplasia

• Hyperplasia refers to an increase in the number of fibers in a muscle.

Rigor Mortis

• Rigor mortis is the post-mortem stiffening of muscles caused by the depletion of adenosine triphosphate (ATP).
• Calcium ions leak out of the SR, preventing myosin from unbinding from actin.
• It begins 3-4 hours after death and lasts about 24 hours, during which cross-bridges cannot detach from actin.

Lever Systems and Leverage

• A lever is a rigid structure that can move around a fixed point (fulcrum).
• The fulcrum (F) is the fixed point in a lever system.
• Effort (E) causes the movement.
• Load (L) is the resistance that opposes the movement.
• First-class levers have the fulcrum positioned between the effort and the load (scissors, seesaws).
• Second-class levers have the load positioned between the fulcrum and the effort.

Muscle Coordinators

• Prime Mover/Agonist: contracts to cause an action.
• Antagonist: stretches/relaxes and yields to the effects of the prime mover.
• Synergist: contracts and stabilizes intermediate joints to aid the movement of a prime mover.

ROM (Range of Motion) of Ankle

• Dorsiflexion and Plantar Flexion (inversion and eversion done at the intertarsal level)

Degree of ROM for Joint Types

• Hinge joints allow flexion and extension in the sagittal plane (uniaxial), like the elbow, ankle, and interphalangeal joints.
• Plane joints allow biaxial movement side to side and back and forth, with triaxial joints adding rotation (intercarpal/tarsal, sternocostal, vertebrocostal).
• Saddle joints allow biaxial flexion/extension and abduction/adduction (radiocarpal/metacarpophalangeal). Ball and socket joints allow triaxial flexion/extension, abduction/adduction, and rotation (shoulder/hip).

Tendons, Bursae, and Menisci

• Tendon Sheath: is a tubelike bursae that wraps around tendons to reduce friction between tendons and bones.
• Bursae: are saclike structures that reduce friction in joints (like the shoulder and knee) between moving parts.
• Meniscus: is a layer of cartilage in the knee joint that stabilizes the joint and prevents bony degeneration.

Suture Joints

• Suture joints feature a thin layer of dense irregular connective tissue, found in skull bones, and are immovable/slightly movable.

Syndesmosis Joints

• Syndesmosis joints are characterized by a varying amount of dense irregular connective tissue, usually a ligament or membrane, and allow slight movement. Examples include the distal tibiofibular joint and interosseous membrane between the tibia and fibula.

Cartilaginous Joint Classification

• Synchondrosis joints consist of hyaline cartilage and are slightly movable/immovable (1st rib & manubrium)
• Symphysis joints feature a broad, flat disc of FIBROUS cartilage and are slightly movable/immovable (pubic symphysis & intervertebral joints)

Aging & Joints

• Aging leads to decreased production of synovial fluid.
• Articular cartilage becomes thinner with age.
• Ligaments shorten & lose flexibility.
• Osteoarthritis is a degenerative disease where joint cartilage is gradually lost.
• Stretching and aerobic exercises aimed at achieving full ROM can minimize the effects of aging on joints.