Human Anatomy: Joints, Muscles, and Nerve Function

Posted on May 11, 2025 in Human Biology

Understanding Joint Structures and Types

Fibrous Joints: Immovable Connections

These are typically immovable joints where bones are united by dense fibrous connective tissue.

Sutures: Skull’s Interlocking Joints

Ridged, interlocking joints found primarily in the skull.

Syndesmoses: Ligament-Connected Bones

Bones are connected by ligaments. Fiber length varies, so movement distance also varies. For example, the connection between the fibula and tibia involves short fibers, while the connection between the radius and ulna involves longer fibers.

Gomphoses: Peg-in-Socket Tooth Joints

Peg-like sockets that hold teeth in place.

Cartilaginous Joints: Cartilage Unions

Joints where bones are united by cartilage.

Synchondroses: Hyaline Cartilage Joints

Immovable joints where bones are united by a bar or plate of hyaline cartilage (e.g., epiphyseal plates in growing bones).

Symphyses: Fibrocartilage Connections

Joints where bones are united by fibrocartilage. Hyaline cartilage is also present (e.g., intervertebral joints, pubic symphysis).

Synovial Joints: Freely Movable Articulations

Joints where articulating bones are separated by a fluid-filled joint cavity. These joints are freely movable.

Key Features of All Synovial Joints

  • Articular Cartilage: Hyaline cartilage covering the ends of bones, preventing the crushing of bone ends (e.g., in limb joints).
  • Joint Cavity: A small, fluid-filled space unique to synovial joints.
  • Articular Capsule: Composed of an external fibrous layer and an inner synovial membrane (loose connective tissue) that produces synovial fluid.
  • Synovial Fluid: Lubricates and nourishes the articular cartilage.
  • Reinforcing Ligaments: Hold the bones of the joint together while also setting limits on the joint’s range of motion.
  • Nerves and Blood Vessels: Nerves detect pain and monitor joint position and stretch. Capillary beds supply filtrate for synovial fluid.

Movements Allowed by Synovial Joints

  • Gliding: One flat bone surface glides or slips over another similar surface (e.g., wrists, ankles, vertebrae).
  • Angular Movements: Include abduction, adduction, flexion, extension, and circumduction.
  • Rotation: The turning of a bone around its own long axis, toward the midline or away from it.
  • Special Movements: Movements that do not fit neatly into other categories (e.g., supination and pronation of the radius and ulna, dorsiflexion and plantar flexion of the foot).

Common Joint Injuries: Sprains and Strains

  • Sprain: Damage to a ligament.
  • Strain: Damage to a muscle or tendon.

Anatomy of Muscles, Nerves, and Sarcomeres

Muscle Tissue Organization

  • A: Tendon
  • B: Epimysium
  • C: Perimysium
  • D: Fascicle
  • E: Muscle fiber (cell)
  • F: Sarcolemma
  • G: Myofibril
  • H: Endomysium

Sarcomere: The Contractile Unit

  • A: Thin actin filament
  • B: Z disc
  • C: H zone
  • D: Z disc
  • E: M line
  • F: I band
  • G: A band
  • H: I band
  • I: Thick myosin filament
  • J: Sarcomere

Muscle Fiber: Internal Structures

  • A: Sarcolemma
  • B: Sarcoplasmic reticulum
  • C: Terminal cisternae
  • D: T-tubule
  • E: Triad (two terminal cisternae and one T-tubule)

Neuron: Basic Structure

  • A: Dendrite
  • B: Axon
  • C: Axon Terminal
  • D: Schwann Cell (forming myelin sheath)
  • E: Node of Ranvier
  • F: Soma / Cell Body

Muscle and Nerve Physiology Fundamentals

Key Muscle Cell Components Defined

Myofibril: Contractile Rods

Rod-like contractile elements that occupy most of the muscle cell volume. They are composed of sarcomeres arranged end-to-end and appear banded.

Sarcomere: The Basic Contractile Unit

The contractile unit of a muscle fiber, composed of myofilaments made up of contractile proteins.

Myofilaments: Thick and Thin Types

Contractile myofilaments are of two types: thick and thin. Thick filaments contain bundled myosin molecules, while thin filaments contain actin molecules (primarily), along with troponin and tropomyosin.

Essential Neuromuscular Concepts (Q&A)

What is created when actin and myosin bond?

A cross bridge.

What is the resting membrane potential for a neuron?

-70 mV (millivolts).

How many ions does a sodium-potassium pump move in a single cycle?

It moves 3 sodium ions (Na⁺) out of the cell and 2 potassium ions (K⁺) into the cell.

What is the purpose of Schwann Cells (myelin sheath)?

To insulate the axon, preventing charge leakage and increasing the speed of nerve impulse transmission (saltatory conduction).

What is the connection between a nerve and a muscle called?

A neuromuscular junction (NMJ).

What types of gated channels are in a neuron?

Chemically gated (ligand-gated) channels and voltage-gated channels.

Neuromuscular Junction Events & Contraction

Six Events at the Neuromuscular Junction

  1. Action potential (AP) arrives at the axon terminal of the motor neuron.
  2. Voltage-gated calcium (Ca²⁺) channels open, and calcium enters the motor neuron’s axon terminal.
  3. Calcium entry causes the release of acetylcholine (ACh), a neurotransmitter, into the synaptic cleft via exocytosis.
  4. Acetylcholine (ACh) diffuses across the synaptic cleft.
  5. ACh binds to its receptors on the sarcolemma of the muscle fiber, opening chemically gated ion channels that allow sodium ions (Na⁺) to flow into the muscle fiber (cell), leading to depolarization and potentially an action potential in the muscle fiber.
  6. Acetylcholinesterase, an enzyme in the synaptic cleft, degrades ACh left in the synaptic cleft, terminating its effect.

Muscle Twitch Explained

A muscle twitch is a single contraction-relaxation cycle of a muscle fiber in response to a single action potential.

Action Potentials and Muscle Tension Relationship

As the frequency of action potentials increases, muscle tension also increases. This can lead to wave summation and, if the stimulation frequency is high enough to prevent muscle relaxation between individual twitches, a sustained contraction called tetanus. Essentially, more frequent nerve signals result in stronger and more sustained muscle contractions.

Types of Muscle Contractions: Isometric & Isotonic

  • Isometric Contraction: Muscle tension increases, but the muscle length does not change (e.g., trying to lift an object that is too heavy, pushing against an immovable wall).
  • Isotonic Contraction: The muscle changes length (shortens or lengthens) because muscle tension overcomes the load (e.g., picking something up – concentric; lowering something slowly – eccentric).

Six Steps in Excitation-Contraction Coupling

  1. After acetylcholine (ACh) binds to receptors on the sarcolemma, an action potential is generated and travels down the length of the sarcolemma.
  2. The action potential propagates down into the T-tubules.
  3. The action potential in the T-tubules triggers the release of calcium ions (Ca²⁺) from the terminal cisternae of the sarcoplasmic reticulum into the sarcoplasm, surrounding the myofibrils.
  4. Calcium ions (Ca²⁺) bind to troponin on the thin (actin) filaments. This causes tropomyosin to move, exposing the myosin-binding sites on actin.
  5. Myosin heads attach to the exposed binding sites on actin, forming cross-bridges, and then pivot (power stroke), pulling the actin filaments toward the center of the sarcomere.
  6. This sliding movement of actin filaments past myosin filaments results in muscle contraction (shortening of the sarcomere).