Anatomy & Physiology: Muscular System Study Guide

Muscular System Overview

Definition & Function of Excitability

Excitability is the ability of muscle tissue to receive and respond to stimuli. This property allows muscles to react to neural input and initiate contraction.

• Excitability: The capacity of muscle cells to respond to chemical signals, stretch, or other stimuli.

• Function: Enables muscles to contract in response to nervous system signals.

Features & Functions of Skeletal Muscle

Skeletal muscle is responsible for voluntary movements and is characterized by its striated appearance and multinucleated fibers.

• Source of Energy: ATP is the primary energy source for muscle contraction.

• Functions: Movement, posture maintenance, joint stabilization, and heat generation.

Structure of a Sarcomere

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

• Components: Filaments (actin and myosin), regulatory proteins (troponin, tropomyosin), A band, I band, H zone, M line, Z disc.

Equation:

$\text{Sarcomere length} = \text{distance between two Z-discs}$

Muscle Atrophy

Muscle atrophy is the decrease in muscle mass due to inactivity, disease, or aging.

• Causes: Disuse, denervation, malnutrition, aging.

• Effects: Reduced strength and endurance.

Myasthenia Gravis

Myasthenia gravis is an autoimmune disorder that impairs communication between nerves and muscles, leading to muscle weakness.

• Mechanism: Antibodies block or destroy acetylcholine receptors at the neuromuscular junction.

• Symptoms: Muscle weakness, especially in the eyes, face, and throat.

Muscle Classification & Structure

Functions of Intrinsic Muscles

Intrinsic muscles are located within the structure they move, such as the hand or foot, and are responsible for fine motor control.

• Example: Intrinsic muscles of the hand control finger movements.

Muscle System Organization

• Types of Muscles: Skeletal, cardiac, and smooth.

• Muscle Roles: Agonist (prime mover), antagonist, synergist, and fixator.

Cranial & Smooth Muscles

• Cranial Muscles: Muscles of facial expression and mastication.

• Smooth Muscles: Involuntary muscles found in walls of hollow organs.

Cascades and Muscle Involvement

Muscle cascades refer to the sequential activation of muscle groups during complex movements.

• Example: Walking involves coordinated activation of leg, hip, and trunk muscles.

Muscles of the Head

• Superficial vs. Deep Muscles: Superficial muscles are closer to the skin; deep muscles lie beneath them.

• Example: The masseter (superficial) vs. the medial pterygoid (deep).

Muscle Identification & Function

Muscles of Mastication

These muscles are responsible for chewing movements.

• Major Muscles: Masseter, temporalis, medial pterygoid, lateral pterygoid.

• Innervation: Mandibular branch of the trigeminal nerve (CN V).

Muscles of Facial Expression

• Examples: Orbicularis oculi (closes eyelids), zygomaticus major (smiling), buccinator (compresses cheek).

Muscles of Respiration

• Main Muscle: Diaphragm (primary muscle of inspiration).

• Accessory Muscles: External intercostals (inspiration), internal intercostals (expiration).

Muscles of the Abdominal Wall

• Superficial to Deep: External oblique, internal oblique, transversus abdominis.

• Tendinous Intersections: Segments of the rectus abdominis, forming the "six-pack" appearance.

Muscles of the Back

• Multifidus: Stabilizes vertebrae during local movements of the vertebral column.

• Erector Spinae: Extends and laterally flexes the spine.

Upper Limb Muscles

Muscles of the Shoulder

• Deltoid: Abducts, flexes, and extends the arm.

• Rotator Cuff: Supraspinatus, infraspinatus, teres minor, subscapularis (stabilize shoulder joint).

Muscles of the Arm & Forearm

• Biceps Brachii: Flexes elbow and supinates forearm.

• Triceps Brachii: Extends elbow.

• Pronator Teres: Pronates forearm.

• Flexors & Extensors: Flexor group (anterior forearm), extensor group (posterior forearm).

Lower Limb Muscles

Muscles of the Hip & Thigh

• Gluteus Maximus: Extends and laterally rotates the thigh.

• Hamstrings: Biceps femoris, semitendinosus, semimembranosus (flex knee, extend hip).

• Quadriceps Femoris: Rectus femoris, vastus lateralis, vastus medialis, vastus intermedius (extend knee).

• Adductors: Adductor longus, adductor brevis, adductor magnus (adduct thigh).

Muscles of the Leg

• Anterior Compartment: Tibialis anterior (dorsiflexes foot), extensor digitorum longus.

• Posterior Compartment: Gastrocnemius, soleus (plantarflex foot).

• Lateral Compartment: Fibularis longus and brevis (evert foot).

Muscle Actions & Clinical Applications

Muscle Actions

• Flexion: Decreases the angle between two body parts (e.g., bending the elbow).

• Extension: Increases the angle (e.g., straightening the knee).

• Abduction: Moves a limb away from the midline.

• Adduction: Moves a limb toward the midline.

Superficial vs. Deep Muscles

• Superficial Muscles: Located closer to the skin, often responsible for gross movements.

• Deep Muscles: Located beneath superficial muscles, often responsible for stabilization and fine control.

Clinical Terms

• RICE: Rest, Ice, Compression, Elevation—standard treatment for acute muscle injuries.

• Largest Muscle: The gluteus maximus is the largest muscle in the human body.

Muscle Tables

Comparison of Major Muscle Groups

Muscle Group	Main Function	Example Muscle	Innervation
Facial Expression	Move facial skin	Orbicularis oculi	Facial nerve (CN VII)
Mastication	Chewing	Masseter	Trigeminal nerve (CN V)
Respiration	Breathing	Diaphragm	Phrenic nerve
Upper Limb	Arm movement	Biceps brachii	Musculocutaneous nerve
Lower Limb	Leg movement	Quadriceps femoris	Femoral nerve

Superficial vs. Deep Muscles of the Forearm

Layer	Example Muscle	Action
Superficial	Flexor carpi radialis	Flexes wrist
Deep	Flexor digitorum profundus	Flexes fingers

Additional info: Some content was inferred and expanded for academic completeness based on standard Anatomy & Physiology curricula.