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The structure marked X is the pubic symphysis and the type of cartilage seen is fibrocartilage.

Fibrocartilage contains a mixture of white fibrous tissue and cartilaginous tissue. This mix of fibrous and cartilaginous components make this tissue extremely tough and

resilient. Fibrocartilage is present in the following sites:

• Pubic symphysis
• Manubriosternal joint
• Articular discs of temporomandibular and sternoclavicular joints
• Anulus fibrosus of intervertebral discs
• Menisci of the knee
• Acetabular labrum
• Glenoid labrum
• The lining of bony grooves for tendons

Fibrocartilage predominantly contains type I collagen. Hyaline and elastic cartilages mainly contain type II collagen.

Collag en

Type I

Locations

Bone, Fibrocartilage, Fascia, Tendon, Ligaments

Mnemonic B-one

Collag en

Type II

Type III

Type IV

Locations Mnemonic

Hyaline and Elastic cartilage Car-two-lage Reticular tissue Re-three-cul

ar

Basement membrane Floor-four

The symphysis is not a fibrous joint, but a secondary cartilaginous joint. Structural classification of joints is as follows:

Type

Fibrous joints

Subtypes

Cranial suturesSyndesmosisG omphosis

Type Subtypes

Cartilaginous joints Primary cartilaginous joints (

or synchondrosis)Secondary cartilaginous joints (or symp hysis)

Synovial joints Ball-and-socket or spheroidal

jointsSellar or saddle jointsC ondylar or bicondylar jointsE llipsoid jointsHinge jointsPiv ot or trochoid jointsPlane joi nts

Synovial joints are freely movable joints and therefore functionally classified as diarthrosis. Functional classification of joints:

• Synarthroses - These are immovable. e.g., fibrous joints
• Amphiarthroses - They are slightly mobile. e.g., cartilaginous joints
• Diarthroses - These are freely mobile. e.g., synovial joints.

A joint with three or more articular surfaces is classified as a compound joint. Classification of joints based on the number of articulating surfaces:

• Simple joints have 2 articulation surfaces. e.g., shoulder joint, hip joint.
• Compound joints have 3 or more articulation surfaces. e.g., elbow joint, wrist joint.
• Complex joints have an intra-articular disc between the articular surfaces which divides the joint space. e.g., temporomandibular joint, sternoclavicular joint.

The incudostapedial joint is a ball and socket type of joint.

In ball and socket type of joints, the articular surfaces include a globular head (male surface) fitting into a cup-shaped socket (female surface). These joints are multi-axial and movements occur around a number of axes. Flexion, extension, abduction, adduction, medial rotation, lateral rotation, and circumduction, all occur quite freely. E.g., shoulder joint, hip joint,

talocalcaneonavicular joint, and incudostapedial joint.

Note: The other joint between the incus and malleus (incudomalleolar joint) is a saddle joint.

The deformity shown is hallux valgus. This is caused due to an excessive lateral deviation at the first metatarsophalangeal joint. It is an example of an ellipsoid joint.

Ellipsoid joints have an oval, convex (male) surface fitting into an elliptical concave (female) surface. They show biaxial movements such as flexion-extension, and abduction–adduction. A combination of these movements results in circumduction. Movement on the third axis is prevented due to the general articular shape.

E.g., Wrist (radiocarpal) joint, atlanto-occipital joints, metacarpophalangeal joints, and metatarsophalangeal joints.

Forward and backward movement of the mandible that occurs at the temporomandibular joints are called as protraction and retraction, respectively. Protraction occurs when the lower jaw is pushed forward, while retraction pulls the lower jaw backward.

Protraction and retraction also occur in the scapula. During protraction, the scapula is moved laterally and anteriorly along the chest wall, such as during rowing movements. Retraction is bringing scapula back to the original position after protraction.

Option A: Opposition is the thumb movement that brings the tip of the thumb in contact with the tip of a finger. Returning the thumb to its anatomical position next to the index finger is called reposition.

Option B: Excursion is the side to side movement of the mandible.

Option D: Depression and elevation are downward and upward movements of the mandible (or scapula), respectively.

The styloid process is a sharp elevation of bone. Bony elevations can be sharp or rounded:

• Rounded or irregular elevations include a tubercle, tuberosity, epicondyle, malleolus, and trochanter.
• Sharp elevations of bones include styloid process, spine, cornu (horn), and hamulus (hook-like).

The joint marked in the above image is the costotransverse joint, which is a synovial type of joint. The various types of joints in the thoracic wall include:

Sternocostal joints: These are the joints between the costal cartilages of the first seven ribs and the sternum.

• The joint between the 1st costal cartilage and manubrium is a fibrocartilaginous joint

(synarthrosis).

• The joints between the 2nd to 7th costal cartilages and the sternum are synovial joints.

The interchondral joint between the costal cartilages of the 9th and 10th ribs is never synovial and is usually absent.

Type of Joint Synovial joint

Primary cartilaginous joint/S ynchondrosis

Secondary cartilaginous/Sym physes joint

Examples

Costovertebral jointCostotran verse jointInterchondral joint s

Costochondral joint

Manubriosternal and xiphiste rnal joints

The origin of the muscle is fixed, while the insertion moves during contraction.

Skeletal muscles exert their force on the bones through their tendon attachments and the force generated helps in generating movements or in maintaining posture.

Skeletal muscles are multinucleated, while smooth muscles are uninucleated. There are three major muscle types in the human body:

• Skeletal muscle
• Smooth muscle
• Cardiac muscle

The muscle fibres are surrounded by a delicate network of connective tissue called the endomysium.

Muscles are supported by connective tissue in various layers namely:

• Endomysium - surrounds individual muscle fibers.
• Perimysium - surrounds a bundle of muscle fibers (fasciculi).
• Epimysium - surrounds whole muscle or muscle groups.

The endomysial, perimysial and epimysial layers fuse at locations where the muscles connect to adjacent structures such as tendons, aponeuroses, and fasciae.

The rectus abdominis muscle contains parallel fasciculi. The other muscles listed have oblique fasciculi.

Muscles can be classified based on the arrangement of muscle fibers relative to the direction of pull as follows:

• Parallel - muscle fasciculi are parallel to the line of pull.
• Oblique - muscle fasciculi are oblique to the line of pull.
• Spiral - muscle fasciculi have a spiral or twisted arrangement.

Parallel muscles can be further grouped based on their shape as follows:

• Quadrilateral - parallel fibers run for a short distance.
• Strap-like - parallel fibers run for a long distance.
• Strap-like with tendinous intersections - parallel fibers are interrupted by tendinous
• insertions.
• Fusiform - parallel fibers in the muscle belly that converge at the tendon region.

The trapezius muscle individually is a flat, triangular muscle that extends over the back of the neck and upper thorax. When paired together, they resemble a trapezium and hence the name.

Muscles with oblique fibers are further subclassified as follows:

• Triangular - They are triangular in shape.
• Pennate - They are feather-shaped. Based on their complexity, they can have the following subtypes:
• Unipennate - fascicles on one side of the tendon.
• Bipennate - fascicles on two sides of the tendon.
• Multipennate - central tendon branches into two or more branches within the muscle.
• Circumpennate (radial) - muscle fibers converge to a central tendon in a radial fashion.

The muscle marked X in the cadaveric image is the pectoralis major, which is a spiral muscle. Spiral muscles have fiber architecture as follows:

• The muscle fibers have a spiral or twisted arrangement. e.g., pectoralis major (sternocostal fibers) and latissimus dorsi that undergo a 180° twist between their medial and lateral attachments.
• The entire muscle takes a spiral course around a bone. e.g., supinator winds obliquely around the proximal radial shaft.

Cruciate is a type of spiral muscle where two or more planes of muscle fibers are arranged in differing directions. e.g., sternocleidomastoid, masseter and adductor magnus are all partially spiral and cruciate.

The tibialis anterior muscle has oblique muscle fibers arranged in a circumpennate (radial) fashion and not parallel fibers. The muscle fibers start from the walls of osteofascial compartment in the leg and converge obliquely in a radial fashion on a central tendon.

The other muscles listed (sartorius, rectus abdominis, and sternohyoid) have parallel fibers.

A muscle with two bellies with an intervening tendon is called a digastric muscle. Examples include the digastric muscle (digastricus) that has anterior and posterior belly with an intermediate tendon and omohyoid that has superior and inferior belly with an intermediate tendon.

Diarthric muscles are those which extend over two joints. Biaxial is a term used for joints that have movements in 2 axes.

The following image shows an illustration of the anterior and posterior belly of the digastric muscle with an intervening tendon:

The condition shown above is Dupuytren's contracture. It is the contracture of the flat or broad sheet-like tendon known as the aponeurosis, which leads to flexion deformities of the fingers, especially the ring and little fingers.

The aponeurosis is a pearly-white sheet of fibrous tissue that replaces tendons in sheet-like muscles having wide areas of attachment. They are whitish-silvery in colour and shiny. It is similar to tendons in histology and are sparingly supplied with blood vessels and nerves.

Examples of aponeuroses include:

• Palmar aponeuroses on the palms of the hands
• Plantar aponeuroses on the plantar aspect of the foot
• Anterior abdominal aponeuroses superficial to the rectus abdominis muscles
• Anterior and posterior intercostal membranes between the ribs
• Scalp aponeuroses or galea aponeurotica in the scalp

Option A: The retinaculum is a strong, short, and broad fibrous band around the tendons which holds them in place.

Option B: The linear fibrous band formed by interdigitation of the tendinous or aponeurotic ends of the muscles is known as raphe.

Option D: Fascia is a generic term applied to sheaths, sheets or other dissectible masses of connective tissue that are large enough to be visible to the unaided eye.

Z disc or Krause's membrane is present in the middle of the I band of the myofibril.

Each myofibril has alternating dark (Anisotropic or A-band) and light (Isotropic or I-band) regions.

• A band
• The A-band contains both thick and thin filaments
• In the middle of the A-band, there is an H-band with only thick filaments
• In between the H-band, there is the dark M line
• I band
• The I-band contains only thin filaments
• In the middle of the I-band, there is a dark Z-disc or Krause's membrane
• The segment of the myofibril between two Z-discs is known as the sarcomere

The following photomicrograph of the skeletal muscle shows alternating dark and light regions with the Z-disc:

Autonomic efferents supply the smooth muscle fibres of the blood vessels.

Every skeletal muscle is supplied by a motor nerve, which actually is a mixed nerve that contains motor and sensory fibres.

The motor supply of muscle contains the following efferents:

• Extrafusal muscle fibers are supplied by α-efferents that are large and myelinated.
• Intrafusal muscle fibers of muscle spindle are supplied by γ-efferents that are small and myelinated.
• Vascular smooth muscle fibers get autonomic efferents that are fine and unmyelinated.

Flower spray endings present in the muscle spindles are the secondary sensory nerve endings. The sensory nerve supply of muscle is as follows:

• Muscle spindles are supplied by thickly myelinated (IA) and thinly myelinated (II) afferents. The sensory nerve endings in the muscle spindle are of the following types:
• Primary sensory endings (type IA - Annulospiral endings) present in the central nuclear region of the intrafusal fibres.
• Secondary sensory endings (type II- Flower spray endings) present on either side of the nucleus of the intrafusal fibres.
• Golgi tendon organ is supplied by large, myelinated (IB) afferents.
• Connective tissue sheaths are supplied by fine, myelinated and unmyelinated axons that carry pain and other sensations.

A small motor unit has 5-10 muscle fibers.

A motor unit (myone) is a single α motor neuron together with the muscle fibers supplied by it. The size of motor unit depends upon the precision of muscle control. Muscles that require fine control have fewer muscle fibers innervated by each neuron, muscles that participate in less controlled movements may have many fibers innervated by one neuron.

Motor units can be classified as follows:

• Small motor units (5–10 muscle fibers) are found in muscles of fine movements (such as extraocular muscles).
• Large motor units (100–2000 muscle fibers) are found in muscles of gross movements (such as proximal limb muscles).

Flexor carpi ulnaris is not a hybrid muscle as it is supplied by a single nerve (ulnar nerve).

A muscle supplied by two different nerves with different root values is called a composite or hybrid muscle. These muscles usually have more than one set of fibers and get their supply from different nerves for the different sets of fibers.

Examples of composite or hybrid muscles include:

• Brachialis
• Pectoralis major
• Iliopsoas
• Subscapularis
• Biceps femoris
• Adductor magnus
• Digastric
• Flexor pollicis brevis
• Flexor digitorum profundus
• Pectineus
• Opponens pollicis
• Trapezius

Mnemonic: BP IS BAD FOR FU™ POT

Postural muscles predominantly contain Type I (slow-twitch) fibers that demonstrate slow, tonic contraction.

Muscle fibers belong to two main categories:

• Type I fibers (slow contracting or slow-twitch)
• Type II fibers (fast contracting or fast-twitch) - further subdivided to 2 types:
• Type IIA fibers
• Type IIX or IIB fibers

The following light micrograph shows fast-twitch fibers and myoglobin rich slow-twitch fibers: