Muscle Histology

Smooth Muscle

Smooth muscle is found in the walls of hollow organs such as the small intestine. It is responsible for involuntary movements such as peristalsis.

• Circular Layer: The inner layer of smooth muscle fibers arranged in a circular pattern around the organ. Contraction narrows the lumen.

• Longitudinal Layer: The outer layer with fibers running lengthwise. Contraction shortens the organ.

• Example: Coordinated contraction of both layers propels food through the digestive tract.

Cardiac Muscle

Cardiac muscle is found only in the heart. It is striated and under involuntary control.

• Striations: Alternating light and dark bands due to the arrangement of actin and myosin filaments.

• Intercalated Discs: Specialized junctions connecting cardiac muscle cells, allowing rapid transmission of electrical impulses.

• Example: Intercalated discs enable the heart to contract as a coordinated unit.

Skeletal Muscle (Longitudinal Section)

Skeletal muscle is responsible for voluntary movements and is attached to bones. It appears striated under the microscope.

• Endomysium: Thin connective tissue surrounding each muscle fiber.

• Striations: Visible bands due to the organized arrangement of myofibrils.

• Sarcomere: The functional unit of muscle contraction, defined as the segment between two Z-discs.

• A-band: Dark region containing thick (myosin) filaments.

• I-band: Light region containing thin (actin) filaments only.

• Z-disc: Boundary of each sarcomere, anchoring actin filaments.

• Example: The sliding filament model explains muscle contraction as actin and myosin filaments slide past each other within the sarcomere.

Skeletal Muscle (Cross Section)

Cross sections reveal the organization of skeletal muscle into bundles and connective tissue layers.

• Epimysium: Dense connective tissue surrounding the entire muscle.

• Perimysium: Connective tissue surrounding bundles of muscle fibers called fascicles.

• Fascicles: Bundles of muscle fibers within the muscle.

• Endomysium: Connective tissue surrounding individual muscle fibers.

• Example: This organization allows for efficient force transmission and muscle contraction.

Neuromuscular Junction

The neuromuscular junction is the synapse between a motor neuron and a skeletal muscle fiber, essential for voluntary muscle contraction.

• Skeletal Muscle Fiber (Cell): The target cell that contracts in response to stimulation.

• Axon Terminal: The end of the motor neuron that releases neurotransmitters.

• Motor End Plate (Synaptic Knob): Specialized region of the muscle fiber membrane that receives the signal.

• Example: Acetylcholine is released from the axon terminal, triggering muscle contraction.

Muscles of the Head, Neck, and Trunk

Muscles of the Head

These muscles control facial expressions and mastication (chewing).

• Frontalis: Raises eyebrows and wrinkles forehead.

• Masseter: Elevates the mandible for chewing.

• Temporalis: Assists in closing the jaw.

• Orbicularis Oris: Closes and protrudes lips.

• Orbicularis Oculi: Closes eyelids.

• Zygomaticus: Raises corners of the mouth (smiling).

Muscles of the Neck

Neck muscles are involved in head movement, swallowing, and supporting the head.

• Anterior:

  • Platysma: Tenses skin of neck.

  • Sternocleidomastoid: Rotates and flexes head.

  • Sternohyoid: Depresses hyoid bone.

  • Mylohyoid: Elevates floor of mouth.

  • Digastric: Depresses mandible.

• Posterior:

  • Splenius Capitis: Extends and rotates head.

  • Semispinalis Capitis: Extends head and neck.

  • Levator Scapulae: Elevates scapula.

• Lateral:

  • Scalenes: Flex and rotate neck; elevate ribs during inspiration.

Muscles of the Chest

• Pectoralis Major: Flexes, adducts, and medially rotates the arm.

• Pectoralis Minor: Draws scapula forward and downward.

Muscles of the Thorax and Abdomen

• External Intercostals: Elevate ribs during inspiration.

• Internal Intercostals: Depress ribs during forced expiration.

• Serratus Anterior: Protracts scapula.

• Rectus Abdominis: Flexes vertebral column.

• External Obliques: Rotate and laterally flex trunk.

• Internal Obliques: Rotate and laterally flex trunk.

• Transversus Abdominis: Compresses abdominal contents.

Muscles of the Back

• Trapezius: Elevates, retracts, and rotates scapula.

• Latissimus Dorsi: Extends, adducts, and medially rotates arm.

• Rhomboideus: Retracts scapula.

• Serratus Posterior: Assists in respiration.

• Erector Spinae: Extends vertebral column.

Muscles of the Shoulder

• Deltoid: Abducts arm.

• Teres Major: Extends and medially rotates arm.

• Teres Minor: Laterally rotates arm.

• Supraspinatus: Abducts arm.

• Infraspinatus: Laterally rotates arm.

• Subscapularis: Medially rotates arm.

Muscles of the Arm, Hip, and Leg

Muscles of the Upper Arm

• Triceps Brachii: Extends forearm; has long, lateral, and medial heads.

• Biceps Brachii: Flexes elbow and supinates forearm; has long and short heads.

• Brachialis: Flexes elbow.

Muscles of the Forearm

• Anterior:

  • Pronator Teres: Pronates forearm.

  • Flexor Carpi Radialis: Flexes and abducts wrist.

  • Palmaris Longus: Flexes wrist.

  • Flexor Digitorum: Flexes fingers.

• Medial:

  • Flexor Carpi Ulnaris: Flexes and adducts wrist.

• Posterior:

  • Extensor Carpi Ulnaris: Extends and adducts wrist.

  • Extensor Digitorum: Extends fingers.

• Lateral:

  • Extensor Carpi Radialis (Longus and Brevis): Extends and abducts wrist.

  • Brachioradialis: Flexes forearm.

Muscles of the Hand

• Pollicis (Thumb):

  • Flexor pollicis brevis: Flexes thumb.

  • Abductor pollicis brevis: Abducts thumb.

  • Adductor pollicis: Adducts thumb.

• Associated with Fifth Digit:

  • Flexor digiti minimi: Flexes little finger.

  • Abductor digiti minimi: Abducts little finger.

Muscles of the Hip and Thigh

• Posterior Hip:

  • Gluteus maximus: Extends and laterally rotates thigh.

  • Gluteus medius: Abducts and medially rotates thigh.

  • Gluteus minimus: Abducts and medially rotates thigh.

• Posterior Thigh (Hamstrings):

  • Semitendinosus: Extends thigh, flexes knee.

  • Semimembranosus: Extends thigh, flexes knee.

  • Biceps Femoris: Extends thigh, flexes knee.

• Lateral:

  • Tensor Fasciae Latae: Abducts and medially rotates thigh.

  • Fascia Lata (Iliotibial Band/Tract): Stabilizes lateral knee.

• Anterior Thigh (Quadriceps):

  • Sartorius: Flexes, abducts, and laterally rotates thigh.

  • Quadriceps Femoris:

    • Rectus Femoris: Extends knee, flexes thigh.

    • Vastus Medialis: Extends knee.

    • Vastus Lateralis: Extends knee.

    • Vastus Intermedius: Extends knee.

• Medial:

  • Adductor Longus: Adducts thigh.

  • Gracilis: Adducts thigh, flexes knee.

  • Adductor Magnus: Adducts and extends thigh.

• Deep to Pelvis:

  • Psoas Major: Flexes thigh.

  • Iliacus: Flexes thigh.

Muscles of the Lower Leg and Foot

• Posterior:

  • Gastrocnemius: Plantar flexes foot, flexes knee.

  • Soleus: Plantar flexes foot.

  • Flexor Hallucis Longus: Flexes big toe.

  • Tibialis Posterior: Inverts and plantar flexes foot.

  • Flexor Digitorum Longus: Flexes toes.

• Anterior:

  • Tibialis Anterior: Dorsiflexes and inverts foot.

  • Extensor Digitorum Longus: Extends toes.

• Lateral:

  • Fibularis Longus: Everts and plantar flexes foot.

• On Foot:

  • Extensor Digitorum Brevis: Extends toes.

  • Flexor Digitorum Brevis: Flexes toes.

Additional info:

• The sliding filament theory of muscle contraction is described by the equation: where is total force, is the number of cross-bridges, and is the force per cross-bridge.

• Muscle tissue types can be compared as follows: