Skeletal Muscles and Muscle Tissue: Study Notes (Chapters 9 & 10)

Chapter 9: Skeletal Muscles

Functions of Skeletal Muscle

Skeletal muscles are essential for various bodily functions, summarized by the acronym MPSH:

• Movement: Muscles contract to produce voluntary movements of the body.

• Posture: Continuous muscle contractions maintain body posture and position.

• Stabilize Joints: Muscles reinforce and stabilize joints during movement.

• Heat Production: Muscle activity generates heat, helping maintain body temperature.

Muscle Naming Clues

Muscle names often describe their characteristics or functions:

• Location: e.g., Frontalis (forehead)

• Shape: e.g., Deltoid (triangle-shaped)

• Size: e.g., Maximus (largest), Minimus (smallest)

• Fiber Direction: Rectus (straight), Oblique (diagonal)

• Number of Origins: Biceps (2), Triceps (3), Quadriceps (4)

• Action: Flexor (flexes), Extensor (extends), Adductor (adducts)

• Attachments: e.g., Sternocleidomastoid (attached to sternum, clavicle, and mastoid process)

Muscle Roles

• Agonist (Prime Mover): Main muscle responsible for a movement.

• Antagonist: Muscle that opposes the action of the agonist.

• Synergist: Assists the agonist in performing a movement.

• Fixator: Stabilizes the origin of the agonist for efficient movement.

Levers in the Musculoskeletal System

Muscles and bones act as levers to produce movement. Levers are classified based on the relative positions of the effort (E), fulcrum (F), and load (L):

Class	Arrangement	Example
1st Class	E-F-L (Fulcrum in middle)	Nodding head
2nd Class	F-L-E (Load in middle)	Standing on tiptoes
3rd Class	F-E-L (Effort in middle)	Biceps curl (most common)

Types of Movements

• Flexion: Decreases the angle between bones (e.g., bending elbow).

• Extension: Increases the angle between bones (e.g., straightening elbow).

• Abduction: Moves limb away from midline.

• Adduction: Moves limb toward midline.

• Supination: Palm faces up.

• Pronation: Palm faces down.

• Dorsiflexion: Toes point upward.

• Plantar Flexion: Toes point downward.

Major Muscles and Their Actions

Region	Muscle	Action
Face	Frontalis	Raises eyebrows
Face	Orbicularis oculi	Closes eyes
Face	Orbicularis oris	Closes lips
Face	Zygomaticus	Smiling
Face	Masseter	Chewing
Neck	Sternocleidomastoid	Rotates head
Upper Body	Deltoid	Abducts arm
Upper Body	Pectoralis major	Flexes/adducts arm
Upper Body	Trapezius	Raises shoulders
Upper Body	Latissimus dorsi	Extends arm
Core	Rectus abdominis	Flexes trunk
Core	External oblique	Rotates trunk
Arm	Biceps brachii	Flexes elbow
Arm	Triceps brachii	Extends elbow
Arm	Brachioradialis	Flexes forearm
Leg	Gluteus maximus	Extends thigh
Leg	Quadriceps	Extends knee
Leg	Hamstrings	Flex knee
Leg	Adductors	Adduct thigh
Leg	Tibialis anterior	Dorsiflexion
Leg	Gastrocnemius/Soleus	Plantar flexion

Chapter 10: Muscle Tissue

Functions of Muscle Tissue

Muscle tissue performs four main functions (MPSH):

• Movement

• Posture

• Stabilization

• Heat production

Types of Muscle Tissue

Type	Striations	Control	Nuclei	Location
Skeletal	Striated	Voluntary	Many	Bones
Cardiac	Striated	Involuntary	1-2	Heart
Smooth	Nonstriated	Involuntary	1	Organs

Properties of Muscle Tissue (ECCEE)

• Excitability: Ability to respond to stimuli.

• Conductivity: Ability to conduct electrical impulses.

• Contractility: Ability to shorten and generate force.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to return to original length after stretching.

Muscle Organization

Muscle tissue is organized hierarchically:

• Muscle → Fascicle → Muscle Fiber (cell) → Myofibril → Sarcomere → Filaments

Sarcomere: The functional contractile unit of muscle, composed of overlapping thick and thin filaments.

Muscle Filaments

• Thick Filament: Myosin

• Thin Filament: Actin, Troponin, Tropomyosin

• Elastic Filament: Titin

Sliding Filament Theory

• During contraction, actin filaments slide over myosin filaments.

• The sarcomere shortens, but the filaments themselves do not change length.

Resting Membrane Potential

• Inside muscle cell: High K+, negative charge

• Outside muscle cell: High Na+, positive charge

• Sodium-Potassium Pump: Maintains gradient by pumping 3 Na+ out and 2 K+ in, using ATP.

Equation:

$3\ \mathrm{Na}^+_{\text{out}} + 2\ \mathrm{K}^+_{\text{in}}$

Action Potential Sequence

1. Resting state

2. Stimulus received

3. Na+ channels open, Na+ enters (depolarization)

4. K+ channels open, K+ exits (repolarization)

5. Return to resting state

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a muscle fiber. Sequence of events:

1. Nerve impulse arrives at axon terminal

2. Acetylcholine (ACh) is released into synaptic cleft

3. ACh binds to receptors on muscle fiber

4. Na+ enters muscle cell

5. Action potential is generated

6. Muscle contraction occurs

Contraction Cycle

1. Ca2+ released from sarcoplasmic reticulum

2. Ca2+ binds to troponin

3. Tropomyosin moves, exposing binding sites on actin

4. Myosin binds to actin

5. Power stroke (myosin pulls actin)

6. ATP binds myosin, causing release from actin

7. ATP hydrolysis recocks myosin head

• Calcium exposes binding sites on actin.

• ATP is required for myosin release and resetting.

Muscle Relaxation

1. Stimulation stops

2. Ca2+ returns to sarcoplasmic reticulum

3. Tropomyosin blocks actin binding sites

4. Contraction ends

ATP Sources for Muscle Contraction

Source	Duration	Notes
ATP (stored)	Immediate	Very limited supply
Creatine phosphate	Short-term	Regenerates ATP quickly
Glycolysis	Medium-term	Anaerobic, produces lactic acid
Oxidative respiration	Long-term	Aerobic, most ATP produced

Types of Muscle Contractions

• Concentric: Muscle shortens (e.g., lifting a dumbbell)

• Eccentric: Muscle lengthens (e.g., lowering a dumbbell)

• Isometric: Muscle length does not change (e.g., holding a dumbbell still)

Key Terms to Memorize

• Agonist: Prime mover

• Antagonist: Opposes movement

• Sarcomere: Contractile unit

• Thick filament: Myosin

• Thin filament: Actin

• Calcium: Binds troponin

• ACh: Released at NMJ

• K+: High inside cell

• Na+: High outside cell

• 3 Na+ out, 2 K+ in: Sodium-potassium pump

• ATP: Releases myosin from actin

• Actin slides over myosin: Mechanism of contraction

• Concentric: Shorten

• Eccentric: Lengthen

• Isometric: No movement

Example: During a biceps curl, the biceps brachii acts as the agonist (prime mover) to flex the elbow, while the triceps brachii acts as the antagonist. The contraction is concentric as the muscle shortens to lift the weight, and eccentric as it lengthens to lower it.

Additional info: The sliding filament theory is fundamental to understanding muscle contraction. It explains how the interaction between actin and myosin filaments, regulated by calcium ions and ATP, leads to the shortening of the sarcomere and thus muscle contraction.