Muscles | Zoology Optional Notes for UPSC PDF Download

Introduction

The human body comprises three distinct types of muscle tissue, each characterized by unique microscopic structures, locations in the body, and functions, which are controlled either voluntarily or involuntarily. These three types are skeletal muscles, smooth muscles, and cardiac muscles.

Skeletal Muscles

Skeletal muscles, which attach to the skeleton, are responsible for voluntary body movements. The fibers are multi-nucleated and transversely striated, organized in parallel bundles known as fasiculi. Notably, skeletal muscles are the only voluntary muscles in the body.

Smooth Muscles

Smooth muscles are primarily found in blood vessels and tubular organs of the GI tract, reproductive, urinary, and respiratory systems. These muscles feature unstriated fibers with a single nucleus and are under autonomic control, making them involuntary.

Cardiac Muscle

Cardiac muscle is exclusive to the heart wall, exhibiting auto-rhythmicity. Its contractions are involuntary with an intrinsic rhythm, requiring no external stimuli. Intercalated discs hold adjacent cells together, facilitating the transmission of contraction forces. Cardiac muscles are also striated.

Skeletal Muscle System

When discussing the muscular system, the focus is on skeletal muscles, of which there are over 600 in the body. Each skeletal muscle is considered an organ, composed of muscle tissue, nerve, and connective tissue.

Functions of the Muscular System

Movement

The primary function of skeletal muscles is to facilitate movement, even in small areas like the eyeball and ear. Skeletal muscle contraction is vital for various functions such as breathing, bodily fluid movement, and maintaining tonus, essential for blood and lymph circulation.

When muscles are cut off from nerve supply, tonus is lost, leading to flaccidity and eventual atrophy. Involuntary contraction of smooth and cardiac muscles is crucial for fluid and material movement in the body.

Body Heat Production

Muscle metabolism produces heat as a byproduct, contributing significantly to the body's ability to maintain a constant temperature. Muscles, constituting 40-45% of body weight, serve as the primary source of body heat. Increased muscle activity raises the rate of heat production, explaining the challenges faced by individuals with reduced muscle mass in staying warm.

Posture and Support Of Body

While the skeletal system provides a framework for body support, muscles perform the heavy lifting. Skeletal muscles maintain posture, stabilize joints, and support internal organs. Postural muscles, especially those in the head, neck, and trunk, work continuously, even when seemingly relaxed.

Properties of Muscle

Irritability (Electrical Excitability)

Muscles respond to electrical stimulation from nerve impulses, showcasing their irritability or electrical excitability.

Contractility

Muscles respond to stimuli by contracting lengthwise or shortening, demonstrating their contractility.

Extensibility

After relaxation, muscles can be stretched beyond their resting length by the contraction of opposing muscles. Muscle fibers can be stretched up to 30% of their resting length.

Elasticity

Muscle fibers, once stretched, exhibit a tendency to recoil to their original resting length, showcasing elasticity.

Structure of Skeletal Muscle

Muscle Attachments

Tendons

• Dense connective tissue attaching muscles to bones.
• Muscle contraction shortens, putting tension on tendons and bones.
• Muscle tension causes movement at synovial joints.

Origin

• Less moveable muscle attachment, often at girdles and appendages.
• Proximal muscle attachment is termed the origin.

Insertion

• More moveable muscle attachment at synovial joints.
• Distal attachment is called the insertion, and movement at the joint causes one bone to move more than the other.

Belly

• Fleshy, thick part of the muscle, also known as the gaster.

Aponeuroses

• Flattened sheet-like tendons.

Retinaculum

• Strong connective tissue band covering groups of tendons.
• Prevents bowing during muscle contraction.
• Found at the wrist and ankle.
  

Associated Connective Tissue

Endomysium

• Outer covering of individual muscle fibers.
• Binds fibers together and supports capillaries and nerve endings.

Perimysium

• Sheath covering binding muscle fibers into bundles called fasciculi.
• Supports nerves and blood vessels for surrounding fasciculi.

Epimysium

• Sheath covering the entire muscle, part of the tendon.

Fascia

• Fibrous connective tissue covering muscles and attaching to the skin.

a) Superficial Fascia

• Secures skin to underlying structures.
• Thickness varies, with adipose tissue in areas like the abdomen.

b) Deep Fascia

• Extension of superficial fascia to deeper surfaces.
• Lacks adipose tissue, blends with epimysium.
• Composed of dense connective tissue, compartmentalizing and binding adjacent muscles.

c) Subserous Fascia

• Extends between deep fascia and serous membranes.
• Composed of loose connective tissue.
  

Muscle Groups Based On Their Actions

Synergistic

Prime Movers

• Muscles primarily responsible for a movement.

Fixators

• Muscles aiding prime movers by stabilizing joints to prevent unwanted movements.

Examples

• Most large movements require several synergistic muscles.

Antagonistic

• Muscles with opposing actions located on opposite sides of a joint.
• Example: Biceps brachii and brachialis (flexors) opposed by triceps (extensor).
  

Muscle Types by Fiber Arrangement

Parallel (Longitudinal)

• Strap-like with long excursion and good endurance.
• Examples: Sartorius, rectus abdominus.

Convergent (Radiate)

• Fan-shaped, focuses contraction force on a single point.
• Stronger than parallel.
• Example: Pectoralis muscles.

Sphincteral (Circular)

• Fibers concentrically arranged around an orifice.
• Acts as a sphincter when contracted.
• Examples: Orbicularis oris, orbicularis oculi.

Pennate (Feather)

• Tendons run through the muscle body.
• More fibers than parallel types, capable of generating more tension.
• Examples: Unipennate (e.g., gluteus maximus), Bipennate (e.g., deltoid), Multipennate (e.g., deltoid).

Blood and Nerve Supply to the Muscle

• Muscles require extensive blood supply for metabolic activity.
• Skeletal muscles need nerve impulses for contraction.
• Each muscle fiber connected to a nerve cell.

Neural Pathways

Motor (Efferent) Neurons

• Conduct impulses to stimulate muscle fiber contraction.

Sensory (Afferent) Neurons

• Conduct impulses away from muscle to the CNS.
• Respond to muscle fiber activity.
• Muscle fibers weaken if not periodically stimulated to contract.

Structure of a Skeletal Muscle Fiber

Sarcolemma

• Cell membrane of the muscle fiber.

Sarcoplasmic Reticulum

• Network of membranous channels extending through the cytoplasm.

Sarcoplasm

• The cytoplasm of the muscle fiber.

Tubules (Transverse Tubules)

• System of tubules running perpendicular to the sarcoplasmic reticulum.

Myofibrils

• Contractile elements of skeletal muscles.
• Diameter: 1-2 μm.
• Three smaller structures within myofibrils:
  • i) Thin filament
  • ii) Thick filament
  • iii) Elastic filament
    
• Thin and thick filaments overlap during contraction and relaxation.
• Filaments do not extend the entire length but create sarcomeres, the basic functional units.
• Z-discs, narrow plate-shaped regions of dense material, separate sarcomeres.
• Within sarcomeres:
  • A-band: Dark area, thick filament overlapped by thin filaments.
  • I-band: Lighter and less dense, consisting only of thin filament.
  • H-band: Center of each A-band, consists only of thick filament.
  • M-line: Centrally divides each H-band.
• Myosin proteins (thick filament) resemble two twisted golf clubs, with a head and tail portion.
• Actin (thin filament) has a myosin binding site for attachment during contraction and relaxation.
  Structure of & myosin (thick filament) & actin (thin filament)

Physiology of Muscle Contraction And Relaxation

Contraction of Muscles

Sliding Filament Model

• Describes muscle contraction.
• Myosin head attaches to thin filaments (actin) and slides the muscle inward.
• Sarcomeres shorten as Z-discs come closer.
• Length of filaments does not change.
• Sliding filaments and sarcomere shortening cause overall muscle fiber shortening.

Role of Calcium in Contraction

• Increased Ca⁺² concentration enhances contraction.
• Decreased Ca⁺² concentration decreases sliding (contraction).

Role of ATP in Contraction

• In the relaxed state, ATP is attached to myosin head.
• Hydrolysis of ATP releases phosphate and converts to ADP, providing energy to myosin.
• Sarcoplasmic reticulum releases Ca⁺², freeing tropomyosin for myosin binding.
• Myosin head, powered by energy, binds to actin, producing a power stroke that pulls the thin filament and contracts the muscle.
• After the power stroke, ATP binds again for another attachment.
  

Relaxation of Muscles

• Ca⁺² is the main mediator for muscle contraction.
• In relaxation, Ca⁺² concentration in sarcoplasm decreases.
• Two ways of decreasing concentration:
  • a) Acetylcholine (Ach), the mediator for Ca⁺² release, is degraded by acetylcholinesterase enzyme.
  • b) Sarcoplasmic membrane's Ca⁺² pump moves outside Ca⁺² into the sarcoplasm, and calsequestrin binds to the pump, removing Ca⁺² from the sarcoplasm.

Muscles of Facial Expression

The muscles in this group are crucial for expressing a variety of emotions such as surprise, fear, and happiness. These muscles are located within the layers of superficial fascia in the face and play a significant role in moving the skin, not joints, when contracting.

a) Occipitofrontalis Muscle

• Covers parts of the skull.
• Consists of two bellies: Occipital belly (near the occipital bone) and Frontal belly (near the frontal bone).
• Connected by a flat tendon.
• Frontalis muscles raise the eyebrows and create transverse wrinkles on the forehead upon contraction.

b) Orbicularis Oris

• Situated at the corner of the mouth.
• Closes and compresses lips against teeth.
• Shapes the lips during speech.

c) Orbicularis Oculi

• Surrounds the palpebral fissure.
• Contraction closes the eyelids.
• Not attached to the bone.

d) Zygomaticus

• Inserts in the skin at the angle of the mouth and orbicularis oris.
• Pulls the corners of the mouth upward and outward, as in smiling and laughing.

e) Levator Labii Superioris

• Inserts in the skin at the angle of the mouth and orbicularis oris.
• Assists in elevating the upper lips.

f) Depressor Labii Inferioris

• Present in the skin of the lower lips.
• Depresses (lowers) the lower lips.

g) Buccinator

• Situated in the cheek.
• Contracts the cheeks.

h) Mentalis

• Present in the skin of the chin.
• Elevates and protrudes the lower lips.
• Pulls the skin of the chin up, as in pouting.

i) Platysma

• Present around the angle of the mouth and the skin of the lower face.
• Draws the outer parts of the lips inferiorly and posteriorly, as in pouting.

j) Risorius

• Present in the skin at the angle of the mouth.
• Draws the angle of the mouth laterally, as in tenseness.

k) Corrugator Supercilii

• Present in the skin of the eyebrow.
• Draws the eyebrow inferiorly, as in frowning.

l) Levator Palpebrae Superioris

• Present in the skin of the upper eyelids.
• Elevates the upper eyelids, opening the eyes.

Muscles that Move the Mandible (Lower Jaw)

Muscles involved in moving the mandible are known as muscles of mastication, contributing to biting and chewing. These muscles are attached to the skull and bone.

a) Masseter

• Situated between the process and external surface of the corner of the lower jaw.
• Provides side-to-side movements of the mandibles.

b) Temporalis

• Present in the temporal part.
• Raises (moves up) the lower jaw.

c) Medial Pterygoid

• Elevates and moves the mandible side to side.

d) Lateral Pterygoid

• Protracts (prolongs or extends) and moves the mandible side to side, opening the mouth.

Muscles that Move the Eyeballs - Extrinsic Muscles

There are two types of muscles present in the eyeballs: extrinsic muscles, originating outside the eyeballs and inserting on their outer surface; and intrinsic muscles, originating and inserting within the eyeball.

Extrinsic Muscles

Movements of the eyeballs are controlled by three pairs of extrinsic muscles.

Rectus Muscles

Two pairs for movement or rotation:

• Superior
• Inferior
• Lateral
• Medial

Oblique Muscles

One pair for movement or rotation:

• Superior
• Inferior

a) Superior Rectus

• Present in the superior and central part of the eyeball.
• Rolls the eyeball superiorly.

b) Inferior Rectus

• Present in the inferior and central part of the eyeball.
• Rolls the eyeball inferiorly.

c) Lateral Rectus

• Present in the lateral side of the eyeball.
• Rolls the eyeball laterally.

d) Medial Rectus

• Present in the medial side of the eyeball.
• Rolls the eyeball medially.

e) Superior Oblique

• Present in the eyeball between superior and lateral recti.
• Rotates the eyeball on its axis.
• Directs cornea inferiorly and laterally.

f) Inferior Oblique

• Located between inferior and lateral recti.
• Rotates the eyeball on its axis.
• Directs cornea superiorly and laterally.

Muscles that Move the Tongue - Extrinsic Muscles

The tongue is divided into lateral halves by a median fibrous septum, extending throughout its length and attached interiorly to the hyoid bone.

Extrinsic Muscles

Genioglossus

• Present on the undersurface of the tongue and hyoid bone.
• Depresses (lowers) the tongue and thrusts (pushes) it anteriorly.

Styloglossus

• Present on the side and undersurface of the tongue.
• Elevates the tongue and draws it posteriorly.

Palatoglossus

• Present on the side of the tongue.
• Elevates the tongue posteriorly and draws the soft palate inferiorly onto the tongue.

Hyoglossus

• Present on the side of the tongue.
• Depresses the tongue and draws its sides inferiorly.

Muscles of the Floor of the Oral Cavity

These muscles, known as suprahyoid muscles, lie superior to the hyoid bone.

Suprahyoid Muscles

a) Digastric

• Elevates the hyoid bone and depresses the mandible, as in opening the mouth.

b) Stylohyoid

• Elevates the hyoid bone and draws it posteriorly.

c) Mylohyoid

• Elevates the hyoid bone and floor of the mouth and depresses the mandible.

d) Geniohyoid

• Elevates the hyoid bone and draws it anteriorly and depresses the mandible.

Muscles that Act on the Abdominal Walls

Anterolateral Abdominal Wall

The anterolateral abdominal wall comprises skin, fascia, and four pairs of flat and sheet-like muscles:

• Rectus Abdominis
• External Oblique
• Internal Oblique
• Transversus Abdominis

The rectus abdominis muscles are interrupted by three transverse fiber bands called tendinous intersections. They compress the abdomen and contribute to urination, forced expiration, and childbirth.

Posterior Abdominal Wall

The posterior abdominal wall is formed by lumbar vertebrae, hip bone, and quadratus lumborum muscles. Quadratus lumborum muscles aid in forceful expiration and deep inspiration.

Muscles Used in Breathing

These muscles, crucial for respiration, are attached to the ribs. Their contraction and relaxation play a pivotal role in altering the size of the thoracic cavity during breathing. Two key groups of muscles involved are:

a) Inspiratory Muscles

• Diaphragm: Primary muscle for inspiration.
• Internal Intercostals: Assist in inspiration.

b) Expiratory Muscles

• External Intercostals: Aid in expiration.

Muscles that Move the Pectoral (Shoulder) Girdle

Muscles responsible for depressing, rotating, or elevating the clavicle or scapula are crucial for shoulder movement. They can be categorized into:

a) Anterior Groups

• Subclavius
• Pectoralis Minor
• Serratus Anterior

b) Posterior Groups

• Trapezius
• Levator Scapulae
• Rhomboid Major
• Rhomboid Minor

Muscles that Move the Humerus (Arm)

The arm's movement involves nine muscles, with two being axial and seven scapular muscles.

Axial Muscles

• Pectoralis Major
• Latissimus Dorsi

Scapular Muscles

• Deltoid
• Subscapularis
• Supraspinatus
• Infraspinatus
• Teres Major
• Coracobrachialis

Muscles that Move the Radius and Ulna (Forearm)

The forearm's movement is managed by flexor and extensor groups.

a) Flexors

• Biceps Brachii
• Brachialis
• Brachioradialis

b) Extensors

• Triceps Brachii
• Anconeus

Muscles that Move the Wrist, Hand, and Fingers

Muscles are divided into anterior and posterior compartments based on location and function.

a) Anterior Compartment Muscles (Flexors)

i) Superficial

• Flexor Carpi Radialis
• Palmaris Longus
• Flexor Carpi Ulnaris
• Flexor Digitorum Superficialis

ii) Deep

• Flexor Digitorum Profundus
• Flexor Pollicis Longus

b) Posterior Compartment Muscles (Extensors)

i) Superficial

• Extensor Carpi Radialis Longus
• Extensor Carpi Radialis Brevis
• Extensor Digitorum
• Extensor Digit Minimi
• Extensor Carpi Ulnaris

ii) Deep

• Abductor Pollicis Longus
• Extensor Pollicis Brevis
• Extensor Pollicis Longus
• Extensor Indicis

Intrinsic Muscles of the Palm

Muscles within the palm are intrinsic, aiding in digit movement and divided into three groups:

• Thenar
• Hypothenar
• Intermediate

These groups contribute to flexion, extension, abduction, adduction, and opposition.

Muscles of Thigh

Muscles of the thigh are classified into three groups based on location.

a) Anterior (Extensor) Compartment

Comprising muscles like the Quadriceps Femoris and Sartorius, it extends the leg and flexes the thigh, innervated by the femoral nerve.

b) Posterior (Flexor) Muscles

These muscles, collectively called hamstrings (Biceps Femoris, Semitendinosus, and Semimembranosus), primarily flex the leg and sometimes extend the thigh, innervated by the sciatic nerve.

c) Adductors (on the Medial Side)

These muscles, innervated by the obturator nerve, include Adductor Magnus, Adductor Longus, Adductor Brevis, and Pectineus. Gracilis, an outer muscle in this compartment, not only adducts the thigh but also flexes the leg.

Muscles of Hip

Muscles of the hip can be classified based on location, development, and function.

a) Anterior Dorsal Hip Muscles

• Psoas Major
• Psoas Minor
• Iliacus

b) Posterior Dorsal Hip Muscles

• Gluteus Maximus
• Gluteus Minimus
• Gluteus Medius

c) Ventral Hip Muscles

• Obturator Internus
• Obturator Externus
• Superior Gemellus
• Inferior Gemellus
• Quadratus Femoris

These muscles facilitate rotation, extension, abduction, and lateral rotation of the thigh.

Muscles that Move the Foot and Toes

All leg muscles attached to the foot can be classified into anterior and posterior groups, separated by the tibia, fibula, and interosseous membrane.

a) Anterior Group

i) Extensors

• Tibialis Anterior
• Extensor Digitorum Longus
• Extensor Hallucis Longus

ii) Peroneals

• Peroneus Longus
• Peroneus Brevis

b) Posterior Group

i) Superficial

• Triceps Surae (Soleus and Gastrocnemius)

ii) Deep

• Tibialis Posterior
• Flexor Hallucis Longus
• Flexor Digitorum Longus

Disorders of Muscles

Muscle-related disorders encompass various conditions affecting muscle function, often leading to discomfort and functional impairment.

Cramp

• Definition: Painful sensation due to prolonged or sudden extreme muscle contraction.
• Causes: Electrolyte imbalance, fatigue, stress, and poor posture.
• Symptoms: Associated with muscular stiffness.

Muscular Dystrophy

• Definition: Group of inherited diseases causing muscle fiber degeneration and replacement by fatty tissue.
• Consequences: Increasing weakness, loss of ability, varying severity.
• Impact: Damage to intercostal muscles or myocardium, potentially fatal.

Poliomyelitis

• Definition: Infectious viral disease affecting the central nervous system.
• Manifestations: Muscular stiffness, weakness, eventual paralysis.

Strain

• Occurrence: Excessive use or stretching of muscles.
• Damage: May tear muscle fibers or tendons, causing pain and swelling.
• Severity: In extreme cases, complete tissue tearing with hemorrhage and loss of function.

Tendinitis

• Definition: Painful inflammation of a tendon sheath due to excessive muscle use.
• Causes: Overuse, bacterial infection, rheumatic disease.

Tension Headache

• Description: Common headache resulting from muscular contraction in the neck.
• Trigger: Emotional stress or other factors causing muscular spasm.
• Complications: Muscular spasm may constrict blood vessels, worsening the condition.

Migraine

• Definition: Severe headache similar to tension headache but with more severe symptoms.
• Initiation: Often triggered by factors, occasionally preceded by an "aura."
• Physiological Impact: Blood vessel constriction followed by dilation, causing intense headache, visual disturbance, dizziness, vertigo, and vomiting.

Related Terminology

1. Atrophy:

• Definition: Wasting away of muscle due to lack of use.

2. Hypertrophy:

• Definition: Increase in the size of the muscle cell.

3. Intramuscular Injection:

• Description: Medication injected into skeletal muscle (deltoid, vastus lateralis, or gluteus medius).

4. Muscle Spasm (Cramp):

• Definition: Sustained contraction of a muscle, usually due to overuse.

5. Myalgia:

• Definition: Muscle pain.

6. Hernia:

• Description: Organs protrude through a weakened muscle. Hiatal hernia involves the stomach pushing through the diaphragm.

7. Flat Feet (Talipes):

• Definition: Weakening of leg muscles supporting the arch, leading to flattened arches. Improved by exercise, massage, and corrective shoes.

8. Tetanus (Lockjaw):

• Description: Infectious disease causing continuous spasms of voluntary muscles. Prevented by anti-toxoid vaccine.

9. Torticollis (Wry Neck):

• Causes: Inflammation of the trapezius and/or sternocleidomastoid muscle.

10.Myasthenia Gravis:

• Description: Progressive muscular weakness and paralysis, sometimes fatal.

11. Tennis Elbow:

• Description: Inflammation of the lateral epicondyle due to overuse. Treated with pain relief and ice.

12. Shin Splints:

• Description: Injury to the muscle tendon in the front of the shin, caused by jogging. Proper running shoes with arch support are recommended.

13. Rotator Cuff Disease:

• Description: Inflammation of tendons around the shoulder joint due to repetitive overhead swinging. Treated with rest and physical therapy.

14. Cumulative Trauma Disorders (Repetitive Motion Disorders):

• Causes: Repeated muscle use, e.g., keyboard use.

15. Ergonomics:

• Study of the application of biology and engineering to the relationship between workers and their environment.

16. Tendonitis:

• Inflammation of a tendon.

17. Bursitis:

• Inflammation of a bursa between bony prominence and muscles or tendons, common in the shoulder or knee.

18. Contracture:

• Tightening or shortening of a muscle.