Anatomy and Physiology I: Skeletal, Muscular, and Nervous Systems Study Guide

Bones and Skeletal Tissues

Types and Functions of Cartilage

Cartilage is a resilient and smooth elastic tissue that covers and protects the ends of long bones at the joints and is a structural component of many body parts.

• Hyaline cartilage: The most abundant type of cartilage in the body. Found in the nose, trachea, larynx, and at the ends of long bones.

• Fibrocartilage: Has the greatest tensile strength due to its dense collagen fiber content. Found in intervertebral discs and menisci of the knee.

Functions of the Skeletal System

• Protection: Shields vital organs (e.g., skull protects the brain).

• Movement: Provides levers for muscles to act upon.

• Structural support: Maintains body shape and posture.

• Blood cell production: Occurs in red bone marrow (hematopoiesis).

• Mineral storage: Especially calcium and phosphate.

Bone Structure and Membranes

• Periosteum: Protective membrane covering the outer surface of bones.

• Sharpey's fibers: Collagen fibers that attach the periosteum to the underlying bone.

• Red bone marrow: In adults, found in flat bones (skull, sternum, ribs, pelvis) and the heads of long bones (femur, humerus). Site of blood cell formation.

Bone Cells and Matrix

• Osteocyte: Mature bone cell found in a lacuna of an osteon; maintains bone tissue.

• Osteoblast: Bone-forming cell; synthesizes osteoid (unmineralized bone matrix).

• Osteoclast: Bone-resorbing cell; releases hydrolytic enzymes to break down bone and release calcium.

• Osteoid: Unmineralized bone matrix secreted by osteoblasts.

• Hydroxyapatites: Mineral salts (mainly calcium phosphate) responsible for bone hardness.

• Alkaline phosphatase: Enzyme essential for bone mineralization and hardness.

Bone Formation and Growth

• Intramembranous ossification: Formation of flat bones from a membrane. Steps:

  1. Primary ossification center forms inside a membrane; osteoblasts secrete osteoid.

  2. Bone structure begins to form.

  3. Blood vessels invade, marrow spaces develop.

  4. Bony collar forms around the structure.

• Endochondral ossification: Formation of long bones from hyaline cartilage. Cavitation of cartilage creates the medullary cavity.

• Epiphyseal (growth) plate zones:

  1. Proliferation zone

  2. Transformation zone

  3. Ossification zone

• Hormonal regulation: Growth hormone (GH) stimulates growth during childhood; testosterone and estrogen cause growth spurts during puberty.

Bone Remodeling and Homeostasis

• Bone remodeling: Ongoing process of bone deposition and resorption on periosteal and endosteal surfaces.

• Dietary needs: Calcium is essential for bone formation.

• Osteoclasts: Secrete hydrolytic enzymes to resorb bone and release calcium.

• Calcium's role: Necessary for muscle contraction.

• Hormonal control:

  • Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts.

  • Calcitonin: Lowers blood calcium by promoting calcium deposition in bone.

Bone Healing and Disorders

• Stages of bone healing:

  1. Hematoma formation

  2. Fibrocartilaginous callus formation

  3. Bony callus formation

  4. Bone remodeling

• Osteoporosis: Condition where bone resorption outpaces bone deposition, leading to fragile bones.

Muscles and Muscle Tissue

Connective Tissue Coverings

• Epimysium: Outer connective tissue covering the entire muscle.

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

• Endomysium: Surrounds each individual muscle fiber.

Motor Units and Neuromuscular Junction

• Motor unit: A motor neuron and all the muscle fibers it innervates.

• Neuromuscular junction (NMJ): The synapse where a motor neuron communicates with a skeletal muscle fiber.

• Neurotransmitter: Acetylcholine (ACh) is released at the NMJ to stimulate muscle contraction.

Muscle Contraction Mechanism

• Requirements: Calcium and ATP are both necessary for muscle contraction.

  • Calcium binds to troponin, causing tropomyosin to shift and expose actin's binding sites for myosin.

  • ATP is required to release the actin-myosin cross-bridge.

• Action potential at axon terminal: Arrival opens calcium channels, calcium enters, and ACh is released. ACh binds to receptors on the muscle's motor end plate.

• ACh binding: Opens sodium channels, sodium influx causes depolarization and spreads the action potential along the muscle membrane.

• Filaments: Thick filaments = myosin; thin filaments = actin.

• Sarcoplasmic reticulum (SR): Stores calcium in skeletal muscle; releases calcium when stimulated by an action potential via T tubules.

• Troponin: Calcium binds to troponin to initiate contraction by moving tropomyosin.

• Acetylcholinesterase: Enzyme that breaks down acetylcholine in the synaptic cleft.

ATP Production in Muscle Cells

• Creatine phosphate (CP): Provides 1 ATP per molecule.

• Glycolysis: Produces 2 ATP per glucose; byproducts are pyruvic acid and lactic acid.

• Cellular respiration: Occurs in mitochondria, requires oxygen and pyruvic acid, yields 36-38 ATP per glucose.

Rigor Mortis

• After death, skeletal muscle cells lose oxygen and the sarcoplasmic reticulum breaks down, releasing calcium into the cytosol.

• Calcium binds to troponin, causing tropomyosin to move and expose actin binding sites.

• All actin and myosin bind, causing muscle contraction.

• Without ATP, actin and myosin cannot detach, resulting in sustained contraction (rigor mortis) until muscle proteins degrade.

Fundamentals of the Nervous System and Nervous Tissue

Neurons and Neuroglia

• Sensory (afferent) neurons: Carry information to the CNS.

• Motor (efferent) neurons: Carry information from the CNS to effectors.

• Autonomic nervous system (ANS): Involuntary; includes sympathetic (fight or flight) and parasympathetic (rest and digest) divisions.

• Synapse: Junction where two excitable cells meet (neuron-neuron or neuron-muscle).

• Neuron structure:

  • Dendrite: Receives information.

  • Cell body: Contains organelles.

  • Axon: Sends action potential to the next cell.

• Multipolar neuron: Most common neuron type in the body.

Neuroglia (Glial Cells)

• Astrocyte: Helps form the blood-brain barrier.

• Ependymal cell: Contains cilia; helps produce cerebrospinal fluid (CSF).

• Microglial cell: Acts as part of the immune system in the CNS.

• Schwann cell: Produces myelin in the peripheral nervous system (PNS).

• Oligodendrocyte: Produces myelin in the central nervous system (CNS).

White Matter vs. Gray Matter

• White matter: Composed of myelinated axons; appears white due to myelin.

• Gray matter: Contains neuron cell bodies and unmyelinated axons; appears gray.

Nuclei and Ganglia

• Nuclei: Collections of neuron cell bodies within the CNS.

• Ganglia: Collections of neuron cell bodies in the PNS.

Neuronal Membrane Potentials and Synapses

• Ligand-gated channel: Opens in response to chemical binding.

• Depolarization: Inner membrane potential becomes less negative (more positive), triggering an action potential.

• Resting membrane potential: $-70$ mV (inside relative to outside).

• Electrical synapse: Found in cardiac muscle (intercalated discs, gap junctions); allows direct ion sharing. Chemical synapse example: NMJ.

Myelination and Conduction

• Unmyelinated neurons: Conduct impulses slowly (continuous conduction).

• Myelinated neurons: Conduct impulses rapidly (saltatory conduction).

• Saltatory conduction: Action potential jumps from node to node (nodes of Ranvier) in myelinated axons; much faster than continuous conduction.

Refractory Periods

• Absolute refractory period: No new action potential can be generated, regardless of stimulus strength.

• Relative refractory period: A stronger-than-normal stimulus can initiate another action potential.

Table: Comparison of Key Neuroglia

Cell Type	Location	Main Function
Astrocyte	CNS	Blood-brain barrier formation
Ependymal cell	CNS	CSF production, cilia movement
Microglial cell	CNS	Immune defense
Schwann cell	PNS	Myelin production
Oligodendrocyte	CNS	Myelin production

Table: Types of Muscle Connective Tissue Coverings

Covering	Location	Function
Epimysium	Entire muscle	Encloses whole muscle
Perimysium	Fascicles (bundles of fibers)	Encloses fascicles
Endomysium	Individual muscle fibers	Encloses each fiber

Additional info: Some explanations and context have been expanded for clarity and completeness based on standard Anatomy & Physiology I curriculum.