Muscular System

Gross Anatomy of Muscle

The gross anatomy of a muscle refers to its overall structure as seen with the naked eye, including the arrangement of muscle fibers, connective tissue layers, blood vessels, and nerves.

• Muscle cross-section: Reveals bundles of muscle fibers (fascicles) surrounded by connective tissue.

• Connective tissue layers: Epimysium (surrounds entire muscle), perimysium (surrounds fascicles), and endomysium (surrounds individual muscle fibers).

• Blood supply and innervation: Muscles are richly supplied with blood vessels and nerves for nutrient delivery and control.

• Example: In a cross-section of the biceps brachii, you can identify fascicles, connective tissue, and the central location of blood vessels.

Sarcomere Structure and Function

The sarcomere is the fundamental contractile unit of striated muscle, defined by the area between two Z-lines.

• Components: Thick filaments (myosin), thin filaments (actin), Z-lines, M-line, A-band, I-band, H-zone.

• Function: Sarcomeres shorten during muscle contraction, producing force.

• Representation: The arrangement of actin and myosin filaments gives skeletal muscle its striated appearance.

• Example: The sliding of actin over myosin shortens the sarcomere, leading to muscle contraction.

Muscle Contraction: Sliding Filament Theory

The Sliding Filament Theory explains how muscles contract at the molecular level.

• Mechanism: Myosin heads bind to actin, forming cross-bridges, and pull the actin filaments toward the center of the sarcomere.

• ATP Role: ATP is required for myosin head detachment and re-cocking.

• Calcium Role: Calcium ions bind to troponin, exposing binding sites on actin.

• Equation:

• Example: During a biceps curl, the sliding of filaments shortens the muscle, lifting the weight.

Muscle Types and Units

Muscles are classified by structure and function. A motor unit consists of a motor neuron and all the muscle fibers it innervates.

• Muscle types: Skeletal, cardiac, and smooth muscle.

• Motor unit: The basic functional unit of muscle contraction; varies in size depending on the muscle's function.

• Example: Eye muscles have small motor units for fine control; leg muscles have large motor units for power.

Muscle Fiber Types

Muscle fibers can be classified based on contraction speed and metabolic properties.

• Type I (slow-twitch): Fatigue-resistant, high endurance, rich in mitochondria.

• Type IIa (fast-twitch oxidative): Intermediate properties.

• Type IIb/x (fast-twitch glycolytic): Fatigue quickly, generate high force.

• Example: Marathon runners have more Type I fibers; sprinters have more Type II fibers.

Muscle Hypertrophy and Atrophy

Hypertrophy is the increase in muscle size due to increased fiber size; atrophy is the decrease in muscle size due to disuse or disease.

• Hypertrophy: Caused by resistance training, increased protein synthesis.

• Atrophy: Caused by immobilization, aging, or disease.

• Example: Immobilizing a limb in a cast leads to muscle atrophy.

Muscle Attachments and Actions

Muscles attach to bones via tendons and produce movement by contracting and pulling on bones.

• Origin: The fixed attachment point.

• Insertion: The movable attachment point.

• Action: The movement produced (e.g., flexion, extension).

• Example: The biceps brachii originates on the scapula and inserts on the radius, flexing the elbow.

Muscle Nomenclature

Muscles are named based on location, shape, size, direction of fibers, number of origins, and action.

• Examples: Rectus abdominis (straight muscle of the abdomen), deltoid (triangular shape).

Nervous System

Overview and Divisions

The nervous system is responsible for coordinating body activities and consists of two main divisions.

• Central Nervous System (CNS): Brain and spinal cord.

• Peripheral Nervous System (PNS): All neural tissue outside the CNS, including cranial and spinal nerves.

• Functional divisions: Sensory (afferent) and motor (efferent) pathways.

• Example: Touching a hot object activates sensory neurons (PNS), which relay information to the CNS for processing.

Neurons and Neuroglia

Neurons are the functional cells of the nervous system, while neuroglia provide support and protection.

• Neuron structure: Cell body, dendrites, axon.

• Types of neurons: Sensory, motor, interneurons.

• Neuroglia: Astrocytes, oligodendrocytes, Schwann cells, microglia, etc.

• Example: Schwann cells form myelin sheaths in the PNS.

Classification of Nervous System

The nervous system can be classified by structure and function.

• Structural: CNS vs. PNS.

• Functional: Somatic (voluntary control of skeletal muscles) vs. autonomic (involuntary control of smooth/cardiac muscle and glands).

• Autonomic subdivisions: Sympathetic (fight or flight) and parasympathetic (rest and digest).

Electrical Signaling in Neurons

Neurons communicate via electrical impulses called action potentials.

• Resting membrane potential: The difference in charge across the membrane at rest, typically -70 mV.

• Action potential: A rapid change in membrane potential that travels along the axon.

• Propagation: The action potential moves from the axon hillock to the axon terminals.

• Equation:

• Example: The nerve impulse that causes muscle contraction is an action potential traveling down a motor neuron.

Comparison of CNS and PNS

The CNS and PNS differ in structure and function.

Types of Neurons

Neurons are classified by structure and function.

Additional info:

• Some details, such as specific muscle names and origins/insertions, were not provided and should be supplemented with textbook or lecture material.

• For muscle fiber types, the classification into Type IIa and IIb/x is based on current physiological understanding.