The Big Picture
The musculoskeletal system gives the body its framework, shape, and ability to move. It also helps protect the body.
This system includes bones and muscles, along with ligaments, tendons, and cartilage. Each part has a different job, but the parts work together to support movement.
Start Here If You’re Overwhelmed
Focus on these core ideas:
- The human body has 206 bones, grouped into five main types.
- Joints are places where two or more bones meet, and different joints allow different amounts of movement.
- Cartilage cushions joints so bone ends do not rub together.
- Freely movable joints include six types, such as ball-and-socket joints, hinge joints, and pivot joints.
- The body has more than 600 muscles, grouped as skeletal, smooth, or cardiac muscle.
- Muscles contract and relax to move body parts, maintain posture, and produce body heat.
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Course Notes: Key Ideas You Need to Know
What the Musculoskeletal System Does
The musculoskeletal system forms the body’s framework and gives the body its shape. It also protects the body and allows movement.
The system includes:
- bones
- muscles
- ligaments
- tendons
- cartilage
Bones Are Living Structures
Bones are hard and rigid, but they are living tissue. Living bone cells are surrounded by minerals deposited by those cells.
A thin, strong membrane called the periosteum covers the entire surface of each bone. The periosteum joins with the ligaments and tendons attached to the bone.
Bones need oxygen and nutrients because they are living tissue. Blood vessels inside the bones supply them.
Compact Bone, Spongy Bone, and Bone Marrow
Compact bone is the hard, dense layer that forms the outer surface of every bone.
Inside compact bone is spongy bone, also called trabecular bone. It is softer and less dense, with many small hollow spaces.
These spaces contain bone marrow. Blood cells are made in bone marrow.
The Five Types of Bones
The human body has 206 bones. These bones fall into five main groups.
Long Bones
Long bones are longer than they are wide. They are hard and dense and provide strength, structure, and mobility.
Examples include:
- humerus, radius, and ulna in the arms
- femur, tibia, and fibula in the legs
- metacarpals in the palms
- metatarsals in the feet
- phalanges in the fingers and toes
Short Bones
Short bones are cube-shaped. They allow skill and ease in movement.
The bones of the wrists and ankles are short bones.
Flat Bones
Flat bones are thin, flat, and curved. They protect organs.
Examples include:
- ribs and sternum, or breastbone
- skull
- pelvic bones
- scapulae, or shoulder blades
Irregular Bones
Irregular bones have uneven shapes with different bony projections and features that interact with other bones.
They provide places for muscles to attach. They also allow different amounts of movement and flexibility and provide some support.
Examples include:
- hip bones
- vertebrae in the spinal column
- some bones of the skull
Sesamoid Bones
Sesamoid bones are usually short or irregular bones located inside a tendon. They protect the tendon.
The patella, or kneecap, is a sesamoid bone connected to the quadriceps tendon.
What Is a Joint?
A joint is the place where two or more bones meet. Joints allow movement.
Several structures help a joint work smoothly.
Cartilage is connective tissue that cushions the joint. It keeps the ends of bones from rubbing together.
The synovial membrane lines the joint and releases synovial fluid. This fluid works as a lubricant and helps the joint move smoothly.
Strong bands of connective tissue called ligaments hold bones together at a joint.
Three Groups of Joints
Joints are grouped according to how much movement they allow.
Fibrous Joints
Fibrous joints are immovable.
The sutures of the skull are an example. Fibrous connective tissue binds these joints.
Cartilaginous Joints
Cartilaginous joints are slightly movable.
Movement between the cartilaginous intervertebral discs of the spinal column is an example.
Synovial Joints
Synovial joints are freely movable. They allow more flexibility and movement than fibrous or cartilaginous joints.
Structures in a Typical Synovial Joint
A typical synovial joint contains several structures.
- Articular cartilage: Covers the surfaces where the two bones meet and creates a smooth joint surface.
- Joint capsule: A strong, sleeve-like covering made from fibrous connective tissue that encloses the joint.
- Synovial membrane: Lines the joint capsule and releases synovial fluid into the joint cavity.
- Synovial fluid: Lubricates the bones and reduces friction inside the joint.
- Bursae: Small sacs filled with synovial fluid that provide cushioning and a gliding surface between the joint and tendons crossing over it.
The singular form of bursae is bursa. Excessive joint use may cause painful inflammation of a bursa. This inflammation is called bursitis.
Six Types of Freely Movable Synovial Joints
The body has six types of movable synovial joints. They are classified according to the movement each joint allows.
Ball-and-Socket Joints
Ball-and-socket joints allow movement in all directions.
The rounded end of one bone fits into a hollow area in another bone. The hip and shoulder joints are examples.
These joints allow a wide range of movement, but they are also more easily displaced. For example, a fall can cause the shoulder to dislocate.
Hinge Joints
Hinge joints allow movement in one direction around a single axis.
Their movement is similar to a door hinge.
Examples include:
- elbows
- knees
- fingers
Pivot Joints
Pivot joints allow turning from side to side.
A rounded projection on one bone fits into a depression on another bone. The joint connecting the skull to the spine is a pivot joint.
Saddle Joint
A saddle joint allows a wide range of motion between two saddle-shaped bones that meet.
The base of the thumb contains a saddle joint called the carpometacarpal joint. This joint gives the thumb a wide range of motion, including opposable thumb movement, and helps the hand grasp objects firmly.
Gliding Joint
A gliding joint forms between two matching flat bones held together by ligaments.
The bones glide past each other along the joint surface. Movement may occur:
- up and down
- left and right
- diagonally
This produces limited but complex movement.
Gliding joints are found in:
- the wrist, at the intercarpal joints
- the ankles, at the intertarsal joints
- the vertebral column
Condyloid Joint
A condyloid joint forms when the oval joint surface of one bone fits into an oval depression in another bone.
Examples include:
- the wrist, or radiocarpal joint
- the knuckles at the base of the fingers, or metacarpophalangeal joints
The Three Types of Muscle Tissue
The human body has more than 600 muscles.
Muscle tissue falls into three groups:
| Muscle Type | Main Features |
|---|---|
| Skeletal muscle | Attached to bones, striated, voluntary |
| Smooth muscle | Smooth appearance, involuntary |
| Cardiac muscle | Found only in the heart, striated, involuntary |
Skeletal Muscle
Skeletal muscles attach to bones.
They are striated, meaning they have a striped or streaked appearance. They are also voluntary, which means you consciously control their movement.
Arm and leg muscles are examples.
Smooth Muscle
Smooth muscles do not have the striped appearance of skeletal muscles.
They are involuntary, meaning they work automatically and are not consciously controlled.
Smooth muscle is found in areas such as:
- the stomach
- the intestines
- blood vessels
- other body organs
Cardiac Muscle
Cardiac muscle is found only in the heart.
It looks striated like skeletal muscle, but it is involuntary.
What Muscles Do
Muscles do only two things:
- Contract
- Relax
Most muscles contract and relax quickly.
Muscles have three important body functions:
- Movement of body parts
- Maintenance of posture
- Production of body heat
Tendons Connect Muscles to Bones
Tendons are strong, tough connective tissues that connect muscles to bones.
When a muscle contracts and becomes shorter, the tendons at each end of the muscle cause the bone to move.
The body contains many tendons. The Achilles tendon is one example.
Muscles Help Maintain Posture
Some muscles stay contracted to help maintain the body’s posture.
This ongoing muscle activity helps the body hold its position.
Muscles Produce Body Heat
Muscles need energy when they contract. They metabolize nutrients to produce this energy, and this process produces heat.
More muscular activity produces more body heat. This is why the body becomes warmer during exercise.
Shivering also produces heat. When the body is exposed to cold, rapid muscle contractions create the sensation of shivering.
Fast Review
Use these points for a quick review before a quiz or exam:
- The musculoskeletal system gives the body structure, shape, protection, and movement.
- The body has 206 bones, divided into long, short, flat, irregular, and sesamoid bones.
- Compact bone forms the hard outer layer, while spongy bone contains spaces holding bone marrow.
- Blood cells are made in bone marrow.
- A joint is the place where two or more bones meet.
- Fibrous joints are immovable, cartilaginous joints are slightly movable, and synovial joints are freely movable.
- Cartilage cushions joints, ligaments hold bones together, and synovial fluid reduces friction.
- The six movable synovial joint types are ball-and-socket, hinge, pivot, saddle, gliding, and condyloid.
- The body has more than 600 muscles.
- Skeletal muscle is striated and voluntary. Smooth muscle is involuntary. Cardiac muscle is striated, involuntary, and found only in the heart.
- Muscles contract and relax to move body parts, maintain posture, and produce body heat.
- Tendons connect muscles to bones.
Final Pep Talk
There are many bone and joint names in this topic, so focus first on the big groups and what makes each one different. Once those patterns feel familiar, the smaller details have a clearer place to fit.