Muscle sys Notes (from gwen)

Skeletal Muscle ; like puppet strings you control

• Location: attached and ON BONES

• Striated (STRIPED) , voluntary (YOU CONTROL IT)

• Long, cylindrical fibers

• Multiple nuclei per cell

• Function: body movement, helps posture, makes heat

Cardiac Muscle ; like a clock (always ticking to a rhythm unless something is wrong)

• Location: heart walls

• Striated, involuntary (AUTOMATIC)

• Branched fibers

• Single central nucleus

• Intercalated discs connect cells for electrical signals

• Function: pumps blood through the heart

Smooth Muscle ; like an automatic conveyer belt

• Location: walls of hollow organs (stomach, intestines, blood vessels)

• Non-striated, involuntary

• Spindle-shaped cells

• Single nucleus

• Function: moves things through organs, like food moving through intestines, squeezing of the bladder.

Skeletal Muscle Tissue Hierarchy & Connective Tissue Coverings

• Muscle → surrounded by epimysium (dense irregular connective tissue)

• Fascicle → group of muscle fibers, surrounded by perimysium

• Muscle fiber (cell) → surrounded by endomysium

• Myofibril → bundles of myofilaments inside fibers

• Myofilaments → actin and myosin filaments that make contraction happen

Sliding Filament Theory

• Myosin heads attach to actin, forming cross-bridges

• Myosin pulls actin toward the center of the sarcomere, shortening the muscle

• ATP (from mitochondria) is needed to detach and reset myosin heads

• Result: muscle shortens and produces force

Three Types of Skeletal Muscle Fibers

Tortoise 🐢 Slow Oxidative (Type I):

• Contract slowly

• Fatigue-resistant - can keep going all day

• Red color from high myoglobin

• Used for endurance activities like posture and marathons

Horse 🐴 Fast Oxidative-Glycolytic (Type IIa):

• Contract quickly

• Moderately fatigue-resistant, can go longer than a cheetah

• Pinkish color

• Used for moderate-intensity activities, galloping or jumping

Cheetah 🐆 Fast Glycolytic (Type IIb or IIx):

• Contract very quickly, built for explosive sprints

• Fatigue rapidly

• White color (less myoglobin) focused on speed, not endurance

• Used for short, powerful bursts like sprinting or lifting weights

Muscle Naming Criteria

• Size (e.g. gluteus maximus)

• Maximus, medius, major, minor, longus, breves

• Location (e.g. intercostals between ribs)

• Intercostals (between ribs, costa=ribs) Temporalis (near the portal bone of skull) Tibialis anterior (along the front of tibia) Brachialis (in the arm, brachium=arm) Femoris (in the thigh, femur region) Abdominis (in the abdomin)

• Action (e.g. flexor carpi radialis)

• Flexor carpi radialis (flexes the wrist toward radius bone) Extensor digitorum (extends the fingers, digits) Abductor pollicis longus (abducts the thumb) Levator scapulae (elevates the scapula) Depressor anguli oris (pulls the mouth corners downward (frowning)

• Shape (e.g. deltoid, triangular)

• Deltoid (triangular) Trapezius (trapezoid shape) Rhomboid (diamond) Orbicularis oculi (circular) Serratus anterior (serrated knife) Platysma (flat, sheet like) Teres (round-like)

• Number of origins (e.g. biceps has two heads)

• Triceps, Quadriceps, Digastric (two bellies)

• Direction of fibers (e.g. rectus means straight)

• Transverse, Oblique (internal/external), Oris

• Attachment sites (e.g. sternocleidomastoid)

• Coracobrachialis → attaches to coracoid process of scapula and humerus (brachium)

• Iliocostalis → attaches to ilium (hip) and ribs (costal)

• Stylohyoid → attaches from styloid process of temporal bone to hyoid bone

Tendon of Origin vs. Insertion

• Origin: fixed attachment, less movement. Can be more than one (bi, tri,quad)

• Insertion: movable attachment, moves when the muscle contracts

Muscle action

Neuromuscular Junction = Like a Doorbell at a Party

• Imagine your muscle fiber is a room full of dancers waiting for music.

• The motor neuron is a guest ringing the doorbell.

• The doorbell sound is like acetylcholine.

• When acetylcholine rings the “doorbell,” it sends an electrical signal into the room.

• The music starts, and the dancers (muscle fibers) begin dancing (contracting).

Motor Unit = Light Switch and Light Bulbs 💡

• Sequence of recruiting motor units is a learned behavior

• A motor neuron is like a light switch.

• The muscle fibers it controls are light bulbs.

Small motor unit = a tiny light switch turning on a few bulbs. ex. moving eyes 👀

• Controls only a few muscle fibers

• Like turning on a nightlight → precise, gentle light (e.g. moving your eyes, playing a guitar, facial expressions).

Large motor unit = big switch turning on lots of bulbs. ex. kicking legs 🦵

• Controls many muscle fibers

• Like flipping a switch for stadium lights → big, powerful illumination (e.g. kicking with your legs).

Specific Muscle Groups

Rotator Cuff Muscles:

• Supraspinatus

• Infraspinatus

• Teres minor

• Subscapularis

Quadriceps Muscles:

• Rectus femoris

• Vastus lateralis

• Vastus medialis

• Vastus intermedius

Hamstrings Muscles:

• Biceps femoris

• Semitendinosus

• Semimembranosus

Agonist, Antagonist, Synergist, Fixators

• Hammer 🔨 ; Agonist (Prime mover): muscle responsible for the main action (e.g. biceps brachii flexing the elbow)

• Pry bar ; Opposite direction Antagonist: muscle that opposes the agonist (relaxes) (e.g. triceps brachii extends the elbow, opposing the biceps)

• Clamp ; Steady and secure. Synergist: assists the agonist and helps stabilize movements (e.g. brachialis helps the biceps during elbow flexion)

• Fixators ; Fixators are muscles that hold a body part steady so other muscles can work smoothly.

  When you write with a pencil: Your shoulder muscles act as fixators to hold your shoulder steady. So your hand and fingers can move smoothly to write.

Relationship Between Muscles, Bones, CT, Nerves, and Joints

• Muscles pull on bones to create movement at joints

• Bones are levers, anchoring points, are what gets moved

• Connective tissue layers hold everything together, includes tendons, ligaments, joint capsule, periosteum, epimysium.

• Joints allow movement between two bones, the fulcrum.

• Nerves give signal to contract via neuromuscular junction

• Muscles coordinate with bones, joints, and connective tissues for smooth body movement

Fascicle Arrangement and Function

• Parallel: fibers run parallel to the long axis → more movement range (e.g. sartorius)

• Pennate: fibers attach at angles → greater strength but less range (e.g. rectus femoris)

• Convergent: Fan shape, fibers converge toward a single tendon → versatile direction of pull (e.g. pectoralis major)

• Circular: fibers arranged in rings → close openings (e.g. orbicularis oris)

Mobility vs. Strength:

• Parallel fibers favor mobility and range of motion (Thigh/Satorius, Arm/Biceps brachii, Abs/Rectus abdominis) Like crisscrossing your legs, bending and rotates forearm, bending your torso.

• Pennate fibers favor strength and force production (Thigh/Rectus femoris, Deltoid/Shoulder, Flexor pollicis longus/Thumb muscle) Like straightening your knee powerfully, lifting your arm strongly in many directions, strong thumb flexion.