The Muscular System
The Muscular System Overview
The muscular system plays a crucial role in various aspects of bodily functioning.
Functions include:
Movement of the body
Maintaining posture and body position
Communicating through facial expressions
Executing a variety of involuntary functions
Basics of Muscle Structure
Tendons:
Attach muscles to bones.
Muscles contract or shorten, which pulls on the tendons, thereby moving the bones at joints.
Number of Skeletal Muscles
Humans possess between 650 and 700 skeletal muscles.
The exact count varies depending on the method of counting.
Types of Muscle Tissue
There are three main types of muscle tissue:
Cardiac muscle
Skeletal muscle
Smooth muscle
Anatomy of Muscle Tissues
Smooth Muscle
Characteristics:
Found mostly in the walls of hollow, visceral organs
Involuntary and lacks striations
Composed of spindle-shaped cells in sheets or layers
Characterized by very slow, sustained contractions that move substances through an organ or along a tract.
Arrangements of Smooth Muscle Layers:
Typically consist of two layers:
Circular layer
Longitudinal layer
Annotated Diagram of Smooth Muscle
Key Structures:
Vein
Submucosal plexus (Meissner's plexus)
Glands in submucosa
Lymphatic tissue
Mucosa and muscularis layers
Cardiac Muscle
Characteristics:
Located only in the heart
Involuntary with striations
Formed with branching cells
Contains intercalated discs
Exhibits slow, steady, rhythmic contractions.
Heart Function:
The heart is a muscular organ responsible for pumping blood throughout the body.
Heart muscle tissue specifically referred to as myocardium.
Skeletal Muscle
Characteristics:
Attaches to bones
Voluntary and striated appearance
Fibers are single, long, cylindrical multinucleate cells
Capable of strong contractions
Contraction can be either slow or fast but is not rhythmic.
Muscle Fiber Organization
Skeletal muscle fibers are organized into bundles called fascicles.
Fascicles form the organizational units of skeletal muscles.
Connective Tissue Layers:
Each muscle fiber is surrounded by endomysium.
Each fascicle is surrounded by perimysium.
Each muscle is encapsulated by epimysium.
Epimysia blend into strong, cord-like tendons that attach muscles to bones.
Tendons are made of fibrous connective tissue.
Fascicle Arrangements
Different arrangements of fascicles influence the muscle's strength and range of motion. The arrangements include:
Parallel
Fusiform
Multipennate
Bipennate
Unipennate
Convergent
Circular
Muscle Terminology
Origin and Insertion of Muscles:
The origin of a muscle is its attachment to the immovable (or less movable) bone.
The insertion is where the muscle attaches to the movable bone.
During contraction, the insertion moves towards the origin.
Naming of Skeletal Muscles:
Based on multiple factors, such as:
Direction of the muscle fibers
Relative size of the muscle
Location or origin & insertion of the muscle
Number of origins
Shape
Function or action performed by the muscle
Examples of Muscle Naming
Gluteus maximus: Largest in its group.
Biceps brachii: Named for having two origins and being situated in the brachial area.
Skeletal Muscle Grouping
Major Skeletal Muscles (anterior):
Sternocleidomastoid, Pectoralis Major, Gluteus Medius, Rectus Femoris, Tibialis Anterior, etc.
Major Skeletal Muscles (posterior):
Trapezius, Latissimus Dorsi, Biceps Femoris, Gastrocnemius, etc.
Muscle Interaction During Movement
Most body movements result from multiple muscles acting together or opposing each other.
Prime mover (agonist): The muscle producing a specific movement.
Antagonist: Produces the opposite effect on the same bones.
Synergists: Aid in stabilizing a movement, either by enhancing the same movement or reducing undesirable movements.
Fixators: Stabilize the origin of a prime mover.
Muscle Fiber Structure
Each muscle fiber functions as a cell.
Sarcolemma: Membrane surrounding muscle cells.
Muscle fibers contain myofibrils composed of two types of myofilaments:
Actin: Thin filaments
Myosin: Thick filaments
Myofibrils are organized into functional units called sarcomeres.
Sliding Filament Theory
The sliding filament theory explains muscle contraction:
When muscle fibers contract, thin (actin) and thick (myosin) filaments slide past each other, resulting in the sarcomere shortening.
Myosin molecules have club-shaped heads that attach to actin binding sites facilitated by calcium ions and regulatory proteins.
Muscle Physiology
Muscle tissues exhibit special properties essential for function:
Irritability: Capability to receive and respond to stimuli.
Contractility: Ability to shorten when stimulated.
Functions of Muscles
Producing Movement:
Primary function of skeletal muscles in locomotion and manipulation.
Facial muscles allow for emotional expression.
Maintaining Posture:
Support to hold body upright against gravity and stabilize joints.
Essential for balance.
Generating Heat:
Heat is a by-product of muscle activity, helping maintain body temperature.
Transporting Substances:
Cardiac and smooth muscles assist in moving materials like blood or food within the body.
Muscle Mechanics
Muscles usually cross at least one joint (with a few exceptions).
The muscle’s bulk is generally proximal to the joint crossed.
Muscles always pull; they do not push; during contraction, the insertion moves toward the origin.
Neural Control of Muscles
Motor Units: Consist of one motor neuron and all the skeletal muscle cells it stimulates.
At the neuromuscular junction, where the neuron communicates with a muscle cell, acetylcholine (ACh) serves as the neurotransmitter triggering muscle contraction.
Action potentials generated due to ionic changes initiate contraction of the muscle fiber.
Contraction Mechanism
A nerve impulse triggers the release of ACh into the synapse.
ACh binds to receptors on the sarcolemma, changing membrane permeability.
Calcium ions are released from the sarcoplasmic reticulum, initiating contraction.
Myosin heads form crossbridges with actin, leading to muscle shortening as the sarcomere contracts.
Muscle Contraction Dynamics
Upon receiving stimulation, muscle fibers contract completely (the all-or-none law).
Graded Responses: Varying degrees of muscle contraction can occur based on changing stimulation frequency or the number of activated muscle cells.
Muscle Energy Sources
Energies for contraction are produced through:
Breakdown of creatine phosphate
Aerobic cellular respiration
Anaerobic glycolysis
Muscle Fatigue
Resulting from oxygen debt during prolonged activity.
Muscles fail to contract despite stimulation; endurance relies heavily on blood supply.
Types of Muscle Contractions
Isotonic contractions: The muscle shortens, causing movement.
Isometric contractions: The muscle remains unchanged in length; no movement occurs.
Tone: Continuous partial contractions mark well-conditioned muscles. Regular exercise increases muscle size, strength, and endurance.
Muscle Atrophy
Muscles that are not engaged regularly become less toned, leading to atrophy or wasting away.
Exercise Effects
Aerobic Exercise: Increases strength and flexibility but has minimal impact on size.
Resistance Training: Enlarges individual muscle cells by promoting new contractile filaments.
Developmental Aspects of the Muscular System
Infants initially exhibit gross reflex movements; as the nervous system matures, they gain better control over fine muscle movements.
In adolescents, skeletal muscle control matures throughout childhood and peaks by mid-adolescence.
Aging causes muscles to become stringy due to increased connective tissue and decreased muscle tissue, leading to reduced strength and mass.
Diseases of the Muscular System
Muscle spasms: Sudden involuntary contractions may result from muscle overuse, dehydration, strain, or prolonged positions.
Muscle strains: Damage due to tearing of fibers in a muscle or its tendons.
Muscle paralysis: Results when nerve supply to a muscle is destroyed, leading to flaccidity and eventual atrophy.
Muscular dystrophy (MD): A group of genetic disorders causing progressive muscle weakness, with Duchenne MD being the most common and serious type.