Muscular System Lab: Structure, Function, and Major Muscles

Muscular System Lab Study Notes

Objectives of the Muscular System Lab

• Identify each of the three muscle tissue types under the microscope.

• Recognize major structures in skeletal, cardiac, and smooth muscle tissues microscopically.

• Locate and identify major structures at the neuromuscular junction and muscle fiber (cell) level.

• Identify major human muscles on models or diagrams.

Types of Muscle Tissue

Skeletal Muscle

Skeletal muscle is primarily attached to bones and is responsible for voluntary movements. It appears striated under the microscope due to the arrangement of actin and myosin protein filaments. Skeletal muscle fibers are long, cylindrical, multinucleated, and under conscious control.

• Location: Attached to bones

• Control: Voluntary

• Appearance: Striated, multinucleated

• Function: Movement of the skeleton, posture, heat production

Cardiac Muscle

Cardiac muscle is found only in the heart. It is striated like skeletal muscle but differs by being branched and having intercalated discs. Cardiac muscle is under involuntary control and is responsible for pumping blood throughout the body.

• Location: Heart wall

• Control: Involuntary

• Appearance: Striated, branched, single central nucleus, intercalated discs

• Function: Pumping blood

Smooth Muscle

Smooth muscle is found in the walls of hollow organs (e.g., intestines, blood vessels). It is not striated and is under involuntary control. Smooth muscle fibers are spindle-shaped and have a single central nucleus.

• Location: Walls of hollow organs

• Control: Involuntary

• Appearance: Non-striated, spindle-shaped, single nucleus

• Function: Movement of substances through internal organs

Microscopic Structure of Muscle Tissue

Skeletal Muscle Tissue

• Muscle fibers: Long, cylindrical, multinucleated cells

• Nuclei: Multiple, peripherally located

• Striations: Alternating light and dark bands due to myofilament arrangement

Cardiac Muscle Tissue

• Muscle fibers: Branched, usually single nucleus per cell

• Nuclei: Central

• Striations: Present

• Intercalated discs: Specialized junctions between cells for rapid electrical conduction

Smooth Muscle Tissue

• Muscle fibers: Spindle-shaped, single central nucleus

• Nuclei: Central

• Striations: Absent

Neuromuscular Junction (NMJ) Structure

The neuromuscular junction is the site where a motor neuron communicates with a skeletal muscle fiber to initiate contraction.

• Endomysium: Connective tissue surrounding each muscle fiber

• Muscle fiber: Individual muscle cell

• Myofibril: Bundles of myofilaments within muscle fibers

• Sarcomere: Functional contractile unit of muscle, defined from Z-line to Z-line

• Sarcolemma: Plasma membrane of a muscle cell

• Sarcoplasm: Cytoplasm of a muscle cell

• Motor end plate: Region of sarcolemma with receptors for neurotransmitters

• Motor neuron: Nerve cell that transmits impulses to muscle fibers

• Axon terminal: End of the motor neuron that releases neurotransmitters

• T-tubule: Invagination of sarcolemma into the cell, helps conduct impulses

• Sarcoplasmic reticulum: Stores and releases calcium ions for contraction

Neuromuscular junction (NMJ): The structural and functional connection between a motor neuron and a muscle fiber.

Muscle Naming and Classification

Muscles are named based on several characteristics:

• Action: The movement produced (e.g., flexor, extensor, adductor)

• Location: Where the muscle is found (e.g., pectoralis = chest, brachii = arm)

• Origin and Insertion: Attachment points (e.g., sternocleidomastoid: sternum, clavicle, mastoid process)

• Direction of fibers: Orientation of muscle fibers (e.g., rectus = straight, oblique = diagonal)

• Shape: (e.g., deltoid = triangle, trapezius = trapezoid)

• Size: (e.g., maximus = largest, minimus = smallest, longus = long)

• Number of origins: (e.g., biceps = two origins, triceps = three origins)

Major Human Muscles

Face and Head Muscles

• Orbicularis oculi: Closes eyelids, blinking, winking

• Orbicularis oris: Closes and protrudes lips

Neck Muscles

• Sternocleidomastoid: Flexes and rotates head

Back Muscles

• Trapezius: Elevates, retracts, and rotates scapula

• Latissimus dorsi: Extends, adducts, and medially rotates arm

Thorax Muscles

• Diaphragm: Main muscle of inspiration

• External intercostals: Elevate ribs during inspiration

• Internal intercostals: Depress ribs during forced expiration

Abdominal Muscles

• Rectus abdominis: Flexes vertebral column, compresses abdomen

• External oblique: Compresses abdomen, rotates trunk

• Transversus abdominis: Compresses abdominal contents

Shoulder Muscles

• Deltoid: Abducts arm, common site for intramuscular injections

Chest Muscles

• Pectoralis major: Flexes, adducts, and medially rotates arm

Arm and Forearm Muscles

• Biceps brachii: Flexes forearm

• Triceps brachii: Extends forearm

Buttocks Muscles

• Gluteus maximus: Extends and laterally rotates thigh

• Gluteus medius: Abducts and medially rotates thigh

Anterior Thigh Muscles

• Rectus femoris: Extends leg, flexes thigh

• Vastus lateralis: Extends leg

• Vastus medialis: Extends leg

• Vastus intermedius: Extends leg

Note: The four vastus muscles and rectus femoris make up the quadriceps femoris group.

Posterior Thigh Muscles

• Biceps femoris: Extends thigh, flexes knee

• Semitendinosus: Extends thigh, flexes knee

• Semimembranosus: Extends thigh, flexes knee

Note: These three muscles form the hamstring group.

Calf Muscle

• Gastrocnemius: Flexes knee, plantar flexes foot

Table: Comparison of Muscle Tissue Types

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Location	Attached to bones	Heart wall	Walls of hollow organs
Control	Voluntary	Involuntary	Involuntary
Striations	Present	Present	Absent
Cell Shape	Long, cylindrical	Branched	Spindle-shaped
Nuclei	Multiple, peripheral	Single, central	Single, central
Special Features	None	Intercalated discs	None

Key Terms and Definitions

• Myofibril: Bundles of myofilaments within muscle fibers responsible for contraction.

• Sarcomere: The basic contractile unit of muscle fiber, defined from one Z-line to the next.

• Sarcolemma: The plasma membrane of a muscle cell.

• Sarcoplasmic reticulum: Specialized endoplasmic reticulum that stores calcium ions.

• Motor unit: A single motor neuron and all the muscle fibers it innervates.

• Intercalated disc: Specialized junctions in cardiac muscle for rapid electrical conduction.

Example: Muscle Contraction Equation

The sliding filament theory describes muscle contraction as the sliding of actin and myosin filaments past each other, powered by ATP hydrolysis.

$\text{ATP} + \text{Myosin} \rightarrow \text{Myosin-ADP-Pi} \rightarrow \text{Power Stroke}$

Summary

• Muscle tissue is classified as skeletal, cardiac, or smooth, each with unique structure and function.

• Major muscles can be identified by location, action, and structure.

• The neuromuscular junction is essential for voluntary muscle contraction.