Joints and Articulations

Knee Joint

The knee joint is a complex synovial joint that allows for flexion, extension, and slight rotation of the leg. It is stabilized by various ligaments, menisci, and muscles.

• Synovial Fluid: Lubricates the joint, reducing friction and nourishing articular cartilage.

• Menisci (Lateral and Medial): Crescent-shaped fibrocartilage pads that improve joint congruency and absorb shock.

• Synovial Membrane: Lines the joint capsule and secretes synovial fluid.

• Fibrous Capsule: Encloses the joint, providing stability.

• Patella: The kneecap; protects the anterior knee and improves leverage of the quadriceps.

• Ligaments:

  • Patellar Ligament: Connects patella to tibia.

  • Collateral Ligaments: Lateral (fibular) and medial (tibial) stabilize the sides of the knee.

  • Cruciate Ligaments: Anterior (ACL) and posterior (PCL) prevent anterior/posterior displacement of the tibia.

• Bones: Femur, tibia, and patella form the joint.

• Muscles: Hamstrings (posterior thigh) and quadriceps (anterior thigh) control knee movement.

Example: The ACL is commonly injured in sports involving sudden stops and changes in direction.

Shoulder Joint

The shoulder (glenohumeral) joint is a highly mobile ball-and-socket joint, stabilized by muscles, ligaments, and the joint capsule.

• Bones: Scapula, clavicle, and humerus.

• Scapular Landmarks: Medial border, inferior/superior angles, coracoid process, spine, glenoid fossa, acromion, supraspinatus/infraspinatus/subscapular fossae.

• Clavicle: Sternal end (medial), acromial end (lateral).

• Humerus: Deltoid tuberosity, greater/lesser tubercles, intertubercular groove.

• Joints: Sternoclavicular, acromioclavicular, coracoclavicular, glenohumeral, coracoacromial.

• Ligaments: Acromioclavicular, coracoclavicular, coracohumeral, glenohumeral (superior, middle, inferior), transverse humeral, coracoacromial.

• Coracoacromial Arch: Protects the superior aspect of the humeral head.

• Muscles: Trapezius, levator scapulae, rhomboids, serratus anterior, pectoralis minor, subclavius, deltoid, coracobrachialis, rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis), teres major, latissimus dorsi, pectoralis major.

Example: The rotator cuff muscles stabilize the humeral head in the glenoid fossa during arm movements.

Hip Joint

The hip is a stable ball-and-socket synovial joint, designed for weight-bearing and movement.

• Ligaments: Iliofemoral, ischiofemoral, pubofemoral—reinforce the joint capsule.

• Labrum: Fibrocartilaginous rim that deepens the acetabulum.

• Bones: Pelvis (acetabulum) and femur.

Example: The iliofemoral ligament is one of the strongest in the body, preventing hyperextension of the hip.

Muscle Tissue and Organization

Types of Muscle Tissue

Muscle tissue is classified into three types, each with distinct structure and function.

• Skeletal Muscle: Voluntary, striated, multinucleated; attached to bones for movement.

• Cardiac Muscle: Involuntary, striated, branched, intercalated discs; found in the heart.

• Smooth Muscle: Involuntary, non-striated, spindle-shaped; found in walls of hollow organs.

Example: Smooth muscle controls peristalsis in the digestive tract.

Skeletal Muscle Organization

Skeletal muscle is organized hierarchically from the whole muscle to myofibrils.

• Whole Muscle → Fascicles → Muscle Fibers (Cells) → Myofibrils

• Connective Tissue Layers:

  • Epimysium: Surrounds the entire muscle.

  • Perimysium: Surrounds fascicles.

  • Endomysium: Surrounds individual muscle fibers.

Example: The biceps brachii is composed of multiple fascicles, each containing many muscle fibers.

Sarcomere Structure

The sarcomere is the functional unit of skeletal muscle contraction, defined by the arrangement of proteins and bands.

• Proteins: Actin (thin filament), myosin (thick filament), titin, nebulin, tropomyosin, troponin.

• Bands/Lines: A band (dark), I band (light), H zone, M line, Z line (disc).

Example: The Z lines define the boundaries of a sarcomere.

Sarcoplasmic Reticulum and T-Tubules

The sarcoplasmic reticulum (SR) stores calcium ions, while T-tubules transmit action potentials into the muscle fiber.

• Cisternae: Enlarged SR regions that store calcium.

• Triad: Structure formed by a T-tubule and two adjacent cisternae.

Sliding Filament Model

Muscle contraction occurs as thin filaments slide past thick filaments, shortening the sarcomere.

• Steps: Calcium release, troponin/tropomyosin shift, myosin-actin cross-bridge formation, power stroke, ATP binding and detachment.

Equation:

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a skeletal muscle fiber, enabling neural control of muscle contraction.

• Structure: Axon terminal, synaptic cleft, motor end plate.

• Function: Acetylcholine (ACh) release triggers muscle action potential and contraction.

Cross Bridge Cycle

The cross bridge cycle describes the interaction between actin and myosin during contraction.

• Attachment, power stroke, detachment, reactivation (ATP-dependent).

Muscle Contractions

• Isotonic: Muscle changes length (concentric = shortens, eccentric = lengthens).

• Isometric: Muscle length remains constant while tension increases.

Muscle Fiber Types

• Fast Twitch (Type II): Rapid, powerful, fatigue quickly.

• Slow Twitch (Type I): Endurance, fatigue-resistant, high myoglobin.

• Intermediate: Properties between fast and slow.

Muscle Shapes and Naming

• Shapes: Parallel, convergent, pennate, circular.

• Named by action, structure, location, or direction of fibers.

Muscle Disorders

• Muscle Sclerosis: Hardening of muscle tissue.

• Muscular Dystrophy: Genetic disorders causing muscle weakness.

• Sprains/Strains: Ligament/tendon injuries.

Other Muscle Types

• Cardiac Muscle: Involuntary, striated, intercalated discs, heart only.

• Smooth Muscle: Involuntary, non-striated, walls of hollow organs.

• Innervation: Cardiac and smooth muscle are innervated by the autonomic nervous system.

Nervous System Organization and Function

Divisions of the Nervous System

• Anatomical: Central Nervous System (CNS: brain and spinal cord), Peripheral Nervous System (PNS: nerves, ganglia).

• Functional: Afferent (sensory), efferent (motor), Somatic Nervous System (SNS), Autonomic Nervous System (ANS).

Neuronal Cell Structure

• Dendrites: Receive signals.

• Axon: Transmits impulses.

• Soma (Cell Body): Contains nucleus and organelles.

• Axon Hillock: Initiates action potentials.

• Telodendria: Terminal branches of axon.

• Perikaryon: Cytoplasm around the nucleus.

Synapse and Neurotransmitters

• Synapse: Junction between neurons; includes presynaptic terminal, synaptic cleft, postsynaptic membrane.

• Neurotransmitters: Chemical messengers (e.g., acetylcholine, norepinephrine, dopamine, serotonin, GABA).

Classification of Neurons

• Structural: Anaxonic, bipolar, unipolar, multipolar.

• Functional: Sensory, motor, interneurons; interoceptors, exteroceptors, proprioceptors.

Neuroglia (Glial Cells)

• CNS: Ependymal cells, astrocytes, oligodendrocytes, microglia.

• PNS: Satellite cells, Schwann cells.

Myelin and Ganglia

• Myelin: Insulating sheath around axons; increases conduction speed.

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

Membrane Potential and Ion Channels

• Resting Potential: Baseline charge difference across membrane (typically -70 mV).

• Depolarization: Membrane potential becomes less negative.

• Repolarization: Return to resting potential.

• Hyperpolarization: Membrane potential becomes more negative than resting.

• Na/K Pump: Maintains ion gradients by pumping 3 Na+ out and 2 K+ in per ATP.

• Types of Channels: Leak, mechanical, voltage-gated, chemically-gated, electrical.

Equation:

Events at a Cholinergic Synapse

• Action potential arrives, Ca2+ influx, ACh release, binds to receptors, postsynaptic potential generated, ACh broken down by acetylcholinesterase.

Spinal Cord and Reflexes

Spinal Cord Structure

• Regions: Cervical, thoracic, lumbar, sacral.

• Nerves: 31 pairs; dorsal (sensory) and ventral (motor) roots.

• White Matter: Myelinated axons; ascending/descending tracts.

• Grey Matter: Neuron cell bodies; dorsal, ventral, lateral horns.

Meninges

• Dura Mater: Tough outer layer.

• Arachnoid Mater: Middle, web-like layer.

• Pia Mater: Delicate inner layer.

Nerve Plexuses and Dermatomes

• Plexuses: Cervical, brachial, lumbar, sacral—networks of nerves supplying limbs.

• Dermatomes: Skin regions innervated by specific spinal nerves.

Neuronal Circuits

• Divergence, convergence, serial processing, parallel processing, reverberation.

Reflexes

• Monosynaptic: Single synapse (e.g., stretch reflex).

• Polysynaptic: Multiple synapses (e.g., withdrawal reflex).

• Examples: Babinski, tendon, withdrawal reflexes.

Brain Structure and Function

Major Brain Regions

• Cerebrum: Higher functions, sensory/motor integration.

• Diencephalon: Thalamus (relay), hypothalamus (homeostasis).

• Brainstem: Midbrain, pons, medulla oblongata (vital centers).

• Cerebellum: Coordination and balance.

• Limbic System: Emotion and memory.

Brain Waves

• Alpha: Relaxed, awake.

• Beta: Alert, active thinking.

• Theta: Light sleep, children.

• Delta: Deep sleep.

Cranial Nerves (I–XII)

• Know function, origin, and destination for each cranial nerve.

Sensory Pathways and Receptors

Adaptation

• Slow (Tonic): Receptors adapt slowly (e.g., pain).

• Fast (Phasic): Receptors adapt quickly (e.g., touch).

Types of Sensory Receptors

• Nociceptors: Pain.

• Thermoreceptors: Temperature.

• Mechanoreceptors: Physical distortion.

• Tactile, Baroreceptors, Proprioceptors, Chemoreceptors: Touch, pressure, position, chemical stimuli.

Sensory and Somatic Pathways

• Pathways for sensory input and motor output; involve relay through thalamus and cortex.

• Basal nuclei and cerebellum modulate movement.

Autonomic Nervous System (ANS) and Higher Functions

ANS Divisions

• Sympathetic: "Fight or flight"; chain ganglia, collateral ganglia, adrenal medullae; norepinephrine as main neurotransmitter.

• Parasympathetic: "Rest and digest"; craniosacral outflow; acetylcholine as main neurotransmitter.

• Dual Innervation: Most organs receive input from both divisions.

• Autonomic Tone: Baseline level of activity.

Memory and Sleep

• Short/Long-Term Memory: Memory consolidation in hippocampus.

• States of Consciousness: Deep sleep, REM sleep, wakefulness (regulated by reticular activating system).

• Aging Brain: Decreased plasticity, slower processing.

Special Senses

Olfaction and Gustation

• Olfactory Receptors: Detect odorants in nasal cavity.

• Taste Buds: Detect chemicals for taste (sweet, sour, salty, bitter, umami).

Vision

• Eye Structures: Cornea, lens, retina, optic nerve.

• Retina Organization: Rods (dim light), cones (color vision), visual pigments (rhodopsin, photopsins).

Hearing and Equilibrium

• Internal Ear Structures: Cochlea (hearing), semicircular canals (balance), perilymph (fluid).

Additional info: Some figures and tables referenced (e.g., figure 13-15, table 16-6) are not included in the source but are standard in A&P textbooks. Students should refer to their textbook for detailed diagrams.