The Muscular System and Joints

• The muscular system enables movement through the contraction of skeletal muscles.

Joints

• Joints, or articulations, are where bones meet.
• Some joints provide firm connections, while others allow for free movement.
• Three major categories of joints exist: fibrous, cartilaginous, and synovial.

Fibrous Joints

• Bones are united by dense connective tissue.
• These joints are relatively immovable or rigid.
• Most fibrous joints occur in the skull and are sutures.

Cartilaginous Joints

• Found between epiphyses and diaphysis of juvenile long bones.
• Most are temporary and disappear after growth ceases as cartilage is replaced by bone.
• Examples include intervertebral disks and the pubic symphysis.
• Permit more movement than fibrous joints but less than synovial joints.

Synovial Joints

• Bones are separated by a fluid-filled space called the joint cavity.
• The cavity is lined by a synovial membrane of delicate connective tissue.
• Articular surfaces are covered with thin layers of cartilage.
• Synovial membranes are strengthened by a fibrous capsule and ligaments.
• Ligaments restrict movements to required directions and extents.
• Synovia, a viscous fluid within the cavity, provides lubrication and nutrition.
• It is produced by the cellular component of the synovial membrane.

Bursa

• A small fluid-filled sac lined by synovial membrane with inner capillary layer of viscous synovial fluid.
• Reduces friction between tissues.
• Located between skin and bones, tendons and bones, muscles and bones, and ligaments and bones.

Intervertebral Disk

• Provides a little chance to move.
• Not very frequent and found only in growing (juvenile) bones.
• Cells produce fluid.

Long Bones

• Not in direct contact and enclosed by a region--a capsule filled with synovial fluid.
• The surface has a level of cartilage.

Dense Regular Connective Tissue

• Connects one bone to another
• Allows for precise movement (not random)
• This tissue is typical of large mammals.

Nuchal Ligament

• Runs from the head to the thoracic vertebrae.
• Supports the heavy head.
• Acts as a facilitator of movement.
• Is a paired structure with one on the left and one on the right.

Movements

• Flexion decreases joint angles
• Extension increases joint angles (+180°)
• Overextension is a flexed position to the other limit
• Abduction moves extremities away from the median plane
• Adduction moves extremities towards the median plane.
• Rotation spins a part on its own axis.

Skeletal Muscles

• Cause movement through contraction.
• The musculoskeletal system operates as a system of levers with joints acting as fulcra

Muscle Terminology

• Origin: the proximal or more central attachment.
• Insertion: the more distal or peripheral attachment.
• Agonist (prime mover): muscle that moves bones
• Antagonist: muscle that opposes the movement of the agonist.
• Deep muscles close to joints are postural and stabilize joints.
• Superficial muscles create power and large movement.
• Movement comes from muscle groups working together.

Muscle Types

• Polygastric muscles have several bellies separated by short tendons.
• Certain muscles of the body wall form thin flat layers with broad tendon sheets (aponeuroses).
• Adapted to support abdominal organs.
• Some muscles arise with two, three, or four separate heads.
• These are named biceps, triceps, and quadriceps.

Muscle Fiber Architecture

• The way fascicles attach to tendons creates different skeletal muscle sizes and shapes
• Parallel fascicles lie parallel to one another
• Fusiform muscles narrow at each end
• Convergent patterns are found with triangular-shaped muscles
• Pennate patterns are attached like plumes of a feather
• Circular patterns are characteristic of sphincter muscles

Thin Muscular Layers

• Cutaneous muscles are thin layers intimately connected to fasciae.
• Forms a contractile sheath covering most of the body.
• Contained within the superficial fascia, it tenses and twitches the skin
• Allows movement of the skin.

Abdominal Wall

• Consist of thin abdominal muscles, not involved in movement, but support the abdominal wall.

Connective Tissue

• Superficial layer of connective tissue covers and supports external surfaces.
• Not directly connected to bones.
• They are but connected to the connective tissue.

Dense Fasciae

• Extensive sheets of connective tissue between skin and deeper structures.
• They covers, passes between, and separates muscles.
• Attaches, stabilizes, and imparts strength
• It also protects a group of muscles.

Ventrolateral Abdominal Wall Muscles

• Comprises of three broad fleshy sheets each with differing fiber orientations.
• Continued ventrally by an aponeurotic tendon that leads to a fibrous cord called the linea alba.
• A fourth muscle forms a broad band close the the linea alba.

Muscles Involved in Forward Movement

• Not on do they work in pairs and groups, they work in chains
• The extensor (dorsal) chain influences an extension
• The flexor (ventral) chain induces a flexion

Extensor Chain of Muscles

• The muscles above the spine and behind the hip are involved in all forward movement.

Muscles of the Forelimb

• Girdle muscles join the forelimb to the trunk. Stabilizing the scapula, during progression they control the swing of
• Intrinsic muscles acting on the shoulder: the lateral group comprises the supraspinatus and the infraspinatus
• As well as supporting the abdominal viscera, aiding breathing and defecation these muscles also work together to help to create flexion (lift) and latero-flexion of the thoraco-lumbar region of the spine.
• This muscle group is one of the main contributors to hip flexion and movement of the pelvis through flexion of the lumbo-sacral junction.
• Along with the rectus femoris (part of the quadriceps muscle group) this muscle is the main hip flexor, really recruited when advancing the limb forwards.