Skeletal, Muscular, and Nervous Systems: Structure, Function, and Integration

Chapter 5: The Skeletal System

Bone Shapes and Examples

The human skeleton is composed of bones with distinct shapes, each adapted for specific functions.

• Long Bones: Longer than they are wide; primarily function as levers. Examples: Femur, humerus.

• Short Bones: Approximately equal in length and width; provide stability and support. Examples: Carpals (wrist bones), tarsals (ankle bones).

• Flat Bones: Thin, flattened, and often curved; protect internal organs and provide surfaces for muscle attachment. Examples: Sternum, ribs, scapulae.

• Irregular Bones: Complex shapes that do not fit other categories. Examples: Vertebrae, some facial bones.

Structure of a Long Bone

Long bones have specialized regions and internal structures:

• Epiphysis: The expanded ends of a long bone, containing spongy bone and red marrow.

• Diaphysis: The shaft or central part of a long bone, composed mainly of compact bone.

• Spongy Bone: Porous bone found mainly in the epiphyses; contains red marrow for blood cell production.

• Compact Bone: Dense bone forming the outer layer; provides strength and support.

• Endosteum: Thin membrane lining the medullary cavity inside the bone.

• Periosteum: Tough, fibrous membrane covering the outer surface of bone; contains nerves and blood vessels.

• Osteon (Haversian System): Structural unit of compact bone, consisting of concentric lamellae around a central canal.

• Lamellae: Rings of bone matrix within an osteon.

• Lacunae: Small spaces between lamellae housing osteocytes.

• Central Canal: Channel in the center of each osteon containing blood vessels and nerves.

Bone Cells and Their Functions

• Osteoblasts: Bone-forming cells; synthesize and secrete bone matrix.

• Osteoclasts: Bone-resorbing cells; break down bone tissue to release minerals.

• Osteocytes: Mature bone cells; maintain bone tissue and communicate with other bone cells.

Bone Formation: Endochondral Ossification

Most bones form through endochondral ossification, where cartilage is replaced by bone.

• Epiphyseal Plate: Also known as the growth plate, it is a layer of hyaline cartilage where bone growth occurs in children and adolescents. It allows for longitudinal growth until it ossifies and becomes the epiphyseal line in adults.

Hormonal Regulation of Calcium

• Parathyroid Hormone (PTH): Secreted by the parathyroid glands; increases blood calcium levels by stimulating osteoclast activity, enhancing calcium reabsorption in kidneys, and activating vitamin D.

Axial vs. Appendicular Skeleton

• Axial Skeleton: Consists of the skull, vertebral column, and thoracic cage; supports and protects the central axis of the body.

• Appendicular Skeleton: Includes the limbs and girdles (pectoral and pelvic) that attach them to the axial skeleton.

Chapter 5 Part II: Joints

Definition and Classification of Joints

A joint (articulation) is a site where two or more bones meet. Not all joints allow movement.

• Ligament: Connects bone to bone; stabilizes joints.

• Tendon: Connects muscle to bone; transmits force for movement.

Joint Movement Vocabulary

• Synarthrosis: Immovable joint (e.g., skull sutures).

• Amphiarthrosis: Slightly movable joint (e.g., intervertebral discs).

• Diarthrosis: Freely movable joint (e.g., shoulder, knee).

Structural Classification of Joints

Type	What Connects Bones	Functional Class	Example
Fibrous	Dense connective tissue	Synarthrosis or Amphiarthrosis	Sutures (skull), syndesmosis (distal tibiofibular joint), gomphoses (teeth in sockets)
Cartilaginous	Cartilage	Synarthrosis or Amphiarthrosis	Synchondrosis (epiphyseal plate), symphysis (pubic symphysis)
Synovial	Joint capsule and synovial fluid	Diarthrosis	Shoulder, knee, hip

Types of Synovial Joints

• Plane: Gliding movements (e.g., intercarpal joints).

• Hinge: Flexion and extension (e.g., elbow).

• Pivot: Rotation (e.g., proximal radioulnar joint).

• Condyloid: Movement in two planes (e.g., wrist joint).

• Saddle: Similar to condyloid but with greater movement (e.g., thumb joint).

• Ball and Socket: Multiaxial movement (e.g., shoulder, hip).

Types of Joint Movements

• Gliding: Sliding movements between flat surfaces.

• Flexion: Decreases the angle between bones (e.g., bending the elbow).

• Extension: Increases the angle between bones (e.g., straightening the knee).

• Abduction: Movement away from the midline (e.g., raising the arm sideways).

• Adduction: Movement toward the midline.

• Circumduction: Circular movement combining flexion, extension, abduction, and adduction.

Chapter 6: The Muscular System

Types of Muscle Tissue

Type	Location	Control	Features
Skeletal	Attached to bones	Voluntary	Striated, multinucleated
Cardiac	Heart	Involuntary	Striated, intercalated discs
Smooth	Walls of hollow organs	Involuntary	Non-striated, spindle-shaped

Anatomy of a Skeletal Muscle

• Epimysium: Surrounds the entire muscle.

• Perimysium: Surrounds bundles of muscle fibers (fascicles).

• Endomysium: Surrounds individual muscle fibers.

• Fascicle: Bundle of muscle fibers.

• Muscle Fiber: Single muscle cell.

• Myofibril: Rod-like unit within muscle fiber, composed of sarcomeres.

• Myofilament: Protein filaments (actin and myosin) responsible for contraction.

Muscle Contraction and Relaxation

Muscle contraction is initiated by a nerve impulse and involves several steps:

1. Action Potential: Electrical signal travels down the motor neuron.

2. Neurotransmitter (Acetylcholine): Released into the synapse at the neuromuscular junction.

3. Receptor Activation: Acetylcholine binds to receptors on the muscle fiber membrane, generating an action potential in the muscle.

4. Calcium Release: Action potential triggers the sarcoplasmic reticulum to release calcium ions.

5. Cross-Bridge Formation: Calcium binds to troponin, exposing binding sites on actin; myosin heads attach to actin.

6. Power Stroke: Myosin heads pivot, pulling actin filaments inward (contraction); ATP is required for myosin head detachment and re-cocking.

7. Relaxation: Calcium is pumped back into the sarcoplasmic reticulum, and the muscle fiber returns to its resting state.

Muscle Movement Terminology

• Origin: The fixed attachment point of a muscle.

• Insertion: The movable attachment point where the muscle exerts force.

Chapter 7: The Nervous System

Organization and Function

The nervous system is divided into central and peripheral components, each with specific functions.

• Central Nervous System (CNS): Brain and spinal cord; processes information and coordinates responses.

• Peripheral Nervous System (PNS): All neural tissue outside the CNS; transmits signals to and from the CNS.

• Sensory (Afferent) Division: Carries sensory information to the CNS.

• Motor (Efferent) Division: Carries commands from the CNS to effectors (muscles and glands).

• Somatic Nervous System: Controls voluntary movements (skeletal muscles).

• Autonomic Nervous System: Controls involuntary functions (smooth and cardiac muscle, glands).

• Sympathetic Division: Prepares the body for "fight or flight" responses.

• Parasympathetic Division: Promotes "rest and digest" activities.

Neuron Classification

• Sensory (Afferent) Neurons: Transmit impulses from sensory receptors to the CNS.

• Motor (Efferent) Neurons: Transmit impulses from the CNS to effectors.

Action Potential Transmission

An action potential is an electrical impulse that travels along the axon of a neuron:

• Depolarization occurs as sodium ions enter the neuron.

• The impulse propagates down the axon to the synaptic terminal.

• Neurotransmitters are released to communicate with the next cell.

Spinal Cord Anatomy

• Consists of gray matter (cell bodies) and white matter (myelinated axons).

• Central canal contains cerebrospinal fluid (CSF).

Brain Coverings and CSF

• Dura Mater: Tough, outermost meningeal layer.

• Arachnoid Mater: Middle, web-like layer.

• Pia Mater: Thin, innermost layer adhering to the brain surface.

• Cerebrospinal Fluid (CSF): Cushions and nourishes the brain and spinal cord; found in ventricles and subarachnoid space.

Major Brain Structures and Functions

Structure	Function
Medulla Oblongata	Controls vital autonomic functions (heart rate, breathing)
Pons	Relays signals between cerebrum and cerebellum; regulates breathing
Midbrain	Processes visual and auditory information
Thalamus	Relay station for sensory information
Hypothalamus	Regulates homeostasis, endocrine activity, and autonomic functions
Cerebellum	Coordinates movement and balance
Frontal Lobe	Voluntary movement, planning, reasoning
Primary Motor Cortex	Controls voluntary muscle movements
Prefrontal Cortex	Complex cognitive behavior, decision making
Broca’s Area	Speech production
Parietal Lobe	Sensory perception and integration
Primary Somatosensory Cortex	Processes tactile information
Temporal Lobe	Auditory processing, memory
Primary Auditory Area	Receives sound information
Auditory Association Area	Interprets sounds
Olfactory Area	Processes smell
Occipital Lobe	Visual processing
Primary Visual Area	Receives visual information
Visual Association Area	Interprets visual stimuli
Pineal Gland	Secretes melatonin, regulates circadian rhythms
Pituitary Gland	Master endocrine gland; regulates other glands

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