Muscle Anatomy and Mechanics: Study Notes for Anatomy & Physiology (EXS 111)

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Muscle Tissue: Overview

Definition and Types

Muscle tissue is a primary tissue in the human body, responsible for movement and force generation. It is composed of specialized cells that contract in response to stimulation. There are three main types of muscle tissue:

Skeletal Muscle Tissue: Attached to bones; responsible for voluntary movements.
Cardiac Muscle Tissue: Found only in the heart; responsible for pumping blood.
Smooth Muscle Tissue: Located in walls of hollow organs (e.g., intestines, blood vessels); controls involuntary movements.

Additional info: Muscle tissue is primarily composed of actin and myosin proteins, which interact to produce contraction.

Functions of Muscle Tissue

Major Roles

Movement: Skeletal muscles move bones; smooth muscles move substances through hollow organs.
Stabilization: Muscles stabilize joints and maintain posture.
Heat Production: Muscle contractions generate heat, helping maintain body temperature.
Valve Function: Muscles act as valves to regulate passage of substances through body passageways.

Example: Skeletal muscle contraction allows walking, while smooth muscle contraction moves food through the digestive tract.

Muscle Tissue: Specific Terminology

Key Terms

Sarcolemma: The plasma membrane of muscle cells.
Sarcoplasm: The cytoplasm of muscle cells.
Sarcoplasmic Reticulum: Specialized endoplasmic reticulum in muscle cells, stores calcium ions necessary for contraction.
Mitochondria: Organelles that produce ATP, the energy source for muscle contraction.

Example: The sarcolemma surrounds each muscle fiber, while the sarcoplasmic reticulum releases calcium ions to initiate contraction.

Organization of Skeletal Muscle

Structural Hierarchy

Skeletal muscle is organized into several levels, each surrounded by connective tissue:

Muscle: Entire organ, surrounded by epimysium.
Fascicle: Bundle of muscle fibers, surrounded by perimysium.
Muscle Fiber (Myofiber): Individual muscle cell, surrounded by endomysium.
Myofibril: Contractile element within muscle fiber, composed of repeating units called sarcomeres.
Sarcomere: Functional unit of contraction, defined by Z discs and containing actin (thin) and myosin (thick) filaments.

Example: The biceps brachii muscle contains many fascicles, each made up of numerous muscle fibers.

Microscopic Anatomy of Skeletal Muscle

Muscle Fiber Structure

Multinucleated: Skeletal muscle fibers have multiple nuclei located peripherally.
Striations: Alternating light and dark bands due to arrangement of actin and myosin.
Sarcomeres: Repeating segments within myofibrils; boundaries marked by Z discs.

Example: Striations are visible under a microscope and are characteristic of skeletal and cardiac muscle.

Connective Tissue Sheaths

Types and Functions

Epimysium: Surrounds entire muscle.
Perimysium: Surrounds fascicles.
Endomysium: Surrounds individual muscle fibers.

These sheaths provide support, elasticity, and transmit force generated by muscle contraction to tendons and bones.

Muscle Attachments

Origins and Insertions

Origin: Less movable attachment point.
Insertion: More movable attachment point.
Direct Attachment: Muscle attaches directly to bone via connective tissue.
Indirect Attachment: Muscle attaches via tendons or aponeuroses.

Example: The brachialis muscle originates on the humerus and inserts on the ulna.

Microscopic Structure: Sarcomeres

Components and Bands

Z Disc: Boundary of each sarcomere.
Thin Filaments (Actin): Extend from Z disc toward center.
Thick Filaments (Myosin): Located in center of sarcomere.
A Band: Dark band; length of thick filament.
I Band: Light band; region with only thin filaments.
H Zone: Center of A band; only thick filaments.
M Line: Center of H zone; holds thick filaments together.

Example: Sarcomeres are responsible for the striated appearance and contractile function of skeletal muscle.

Sarcoplasmic Reticulum and T-Tubules

Role in Contraction

Sarcoplasmic Reticulum (SR): Stores and releases calcium ions () necessary for muscle contraction.
T-Tubules: Invaginations of the sarcolemma that transmit electrical impulses deep into the muscle fiber.
Triad: Structure formed by a T-tubule flanked by two terminal cisterns of the SR.

Example: When stimulated, the SR releases , triggering the sliding filament mechanism of contraction.

Mechanisms of Muscle Contraction

Types of Contraction

Concentric Contraction: Muscle shortens to produce movement.
Eccentric Contraction: Muscle lengthens while generating force; acts as a brake to resist gravity.

Example: Lowering the body during a pushup involves eccentric contraction of the chest muscles.

Muscle Fiber Types

Classification by Energy Production and Contraction Speed

Slow Oxidative Fibers (Type I): High myoglobin, many mitochondria, fatigue-resistant, aerobic metabolism.
Fast Oxidative Fibers (Type IIa): Intermediate properties; aerobic and anaerobic metabolism.
Fast Glycolytic Fibers (Type IIx): Low myoglobin, few mitochondria, powerful, fatigue quickly, anaerobic metabolism.

Fiber Type	Myoglobin	Mitochondria	Fatigue Resistance	Metabolism
Type I (Slow Oxidative)	High	Many	High	Aerobic
Type IIa (Fast Oxidative)	Intermediate	Many	Moderate	Aerobic & Anaerobic
Type IIx (Fast Glycolytic)	Low	Few	Low	Anaerobic

Example: The soleus muscle is rich in slow oxidative fibers, while the rectus femoris contains more fast glycolytic fibers.

Muscle Naming and Classification

Criteria for Naming Muscles

Location: e.g., brachialis (arm).
Shape: e.g., deltoid (triangular).
Size: e.g., maximus (large), minimus (small), longus (long).
Direction of Fibers: e.g., rectus abdominis (straight), transversus abdominis (transverse).
Number of Origins: e.g., biceps (two), triceps (three), quadriceps (four).
Location of Attachments: e.g., sternocleidomastoid (sternum, clavicle, mastoid process).
Action: e.g., flexor, extensor, adductor, abductor.

Example: The biceps brachii is named for its two origins and location in the arm.

Muscle Mechanics: Levers and Attachments

Lever Systems in the Body

Lever: Rigid bar (bone) that moves on a fixed point (fulcrum).
Fulcrum: Joint around which movement occurs.
Effort: Force applied by muscle contraction.
Load: Resistance moved by the lever.

Lever Class	Arrangement	Example	Advantage
First-Class	Fulcrum between effort and load	Neck extension	Mechanical advantage/disadvantage
Second-Class	Load between fulcrum and effort	Standing on tiptoe	Mechanical advantage
Third-Class	Effort between fulcrum and load	Biceps curl	Mechanical disadvantage

Example: The biceps brachii acts as a third-class lever during elbow flexion.

Muscle Actions and Interactions

Roles in Movement

Agonist (Prime Mover): Muscle responsible for a specific movement.
Antagonist: Muscle that opposes the action of the agonist.
Synergist: Muscle that assists the prime mover.
Fixator: Synergist that stabilizes a bone or joint.

Example: During elbow flexion, the biceps brachii is the agonist, the triceps brachii is the antagonist, and the brachialis acts as a synergist.

Muscle Position and Joint Movement

General Rules

Muscle crossing the anterior side of a joint produces flexion.
Muscle crossing the posterior side produces extension.
Muscle crossing the lateral side produces abduction.
Muscle crossing the medial side produces adduction.

Example: The pectoralis major flexes the shoulder, while the latissimus dorsi extends it.

Compartments of the Limbs

Organization and Function

Connective tissue divides limb muscles into compartments.
Muscles in the same compartment act as synergists.
Muscles in opposing compartments act as antagonists.
Each compartment is innervated by a single nerve.

Example: The anterior compartment of the thigh contains quadriceps muscles (extensors), while the posterior compartment contains hamstrings (flexors).

Muscle Development and Aging

Changes Over the Lifespan

Muscle fibers develop from myoblasts.
Muscle mass differs between males and females due to hormonal influences.
With aging, connective tissue increases and muscle fibers decrease, leading to reduced muscle mass and strength.
Sarcopenia: Age-related muscle wasting.

Example: By age 80, muscular strength may decrease by up to 50% due to sarcopenia.

Summary Table: Muscle Tissue Types

Type	Location	Control	Striations	Function
Skeletal	Bones	Voluntary	Yes	Movement, posture
Cardiac	Heart	Involuntary	Yes	Pumping blood
Smooth	Hollow organs	Involuntary	No	Move substances

Additional info: These notes provide a comprehensive overview of muscle anatomy and mechanics, suitable for college-level Anatomy & Physiology students.