Muscle Classification and Organization

Muscle Actions and Shapes

Muscles are classified based on their actions and shapes, which helps in identifying their function and anatomical location.

• Actions:

  • Flexor: Decreases the angle at a joint (e.g., biceps brachii).

  • Extensor: Increases the angle at a joint (e.g., triceps brachii).

  • Abductor: Moves a limb away from the midline (e.g., deltoid).

  • Adductor: Moves a limb toward the midline (e.g., adductor longus).

  • Rotator: Produces rotation (e.g., rotator cuff muscles).

  • Sphincter: Closes a passage (e.g., orbicularis oculi).

• Shapes:

  • Deltoid: Triangular shape.

  • Trapezius: Trapezoid shape.

  • Rhomboid: Rhombus shape.

  • Orbicularis: Circular (e.g., orbicularis oris).

  • Rectus: Straight fibers (e.g., rectus abdominis).

Muscle Organization (Gross to Microscopic)

Muscle tissue is organized in hierarchical layers, each with a specific connective tissue covering.

• Epimysium: Surrounds the entire muscle.

• Perimysium: Surrounds bundles of muscle fibers called fascicles.

• Fascicle: Bundle of muscle fibers.

• Endomysium: Surrounds individual muscle fibers.

• Muscle Fiber: Single muscle cell.

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

• Sarcomere: Functional contractile unit of muscle

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Major Muscles and Their Functions

Key Muscles and Actions

Understanding the major skeletal muscles and their primary actions is essential for anatomy and physiology.

• Deltoid: Abducts the arm.

• Biceps brachii: Flexes the elbow and supinates the forearm.

• Triceps brachii: Extends the elbow.

• Pectoralis major: Flexes and adducts the arm.

• Serratus anterior: Protracts the scapula.

• Sternocleidomastoid (SCM): Flexes and rotates the neck.

• Rotator cuff (SITS): Supraspinatus, Infraspinatus, Teres Minor, Subscapularis – stabilize and move the shoulder.

• Rectus abdominis: Flexes the vertebral column.

• Obliques: Rotate the trunk.

• Transverse abdominis: Compresses abdominal contents.

• Quadriceps: Extend the knee.

• Hamstrings: Flex the knee and extend the hip.

• Gastrocnemius: Plantarflexes the foot and flexes the knee.

• Soleus: Plantarflexes the foot.

• Diaphragm: Primary muscle of inspiration.

Levers in the Musculoskeletal System

Types of Levers

Muscles and bones interact as levers to produce movement. There are three classes of levers in the body:

• First-class lever: Fulcrum between effort and load (e.g., nodding the head).

• Second-class lever: Load between fulcrum and effort (e.g., standing on tiptoes).

• Third-class lever: Effort between fulcrum and load (e.g., biceps curl; most common in the body).

Key Terms:

• Fulcrum: Pivot point (joint).

• Effort: Force applied by muscle.

• Load: Resistance to movement.

Sarcomere Structure and Muscle Contraction

Sarcomere Anatomy

The sarcomere is the basic contractile unit of muscle, composed of overlapping thick and thin filaments.

• Z line: Defines the boundary of each sarcomere.

• Thin filaments: Composed of actin.

• Thick filaments: Composed of myosin.

• A band: Length of thick filaments; remains constant during contraction.

• I band: Region with only thin filaments; shortens during contraction.

• H zone: Region with only thick filaments; shortens during contraction.

Sequence of Muscle Contraction

Muscle contraction is initiated by a series of electrical and chemical events:

1. Action potential (AP) arrives at the neuromuscular junction.

2. Acetylcholine (ACh) is released and binds to receptors on the muscle fiber.

3. Muscle action potential is generated and travels along the sarcolemma and T tubules.

4. Calcium ions (Ca2+) are released from the sarcoplasmic reticulum.

5. Ca2+ binds to troponin, causing tropomyosin to move and expose binding sites on actin.

6. Myosin heads form cross-bridges with actin and perform a power stroke.

7. ATP binds to myosin, causing detachment from actin and muscle relaxation.

Types of Muscle Contractions

Isometric and Isotonic Contractions

Muscle contractions can be classified based on whether movement occurs:

• Isometric: Tension is generated without movement (muscle length does not change).

• Isotonic: Muscle changes length to produce movement.

  • Concentric: Muscle shortens while generating force.

  • Eccentric: Muscle lengthens while maintaining tension.

Nervous System Basics

Neuron Structure and Function

Neurons are the functional units of the nervous system, specialized for communication.

• Dendrites: Receive signals from other neurons.

• Soma (cell body): Contains the nucleus and organelles.

• Axon hillock: Initiates action potentials.

• Axon: Conducts action potentials away from the soma.

• Axon terminals: Release neurotransmitters to communicate with other cells.

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

• Depolarization: Na+ enters the cell, making the inside more positive.

• Repolarization: K+ exits the cell, restoring negativity.

• Hyperpolarization: Excess K+ leaves, making the cell more negative than resting.

Synapse: The junction where neurotransmitters cross the synaptic cleft to transmit signals.

Neuroglia and Myelination

Types of Neuroglia

Neuroglia are supporting cells in the nervous system, each with specialized functions.

• Astrocytes: Support neurons and form the blood-brain barrier (BBB).

• Oligodendrocytes: Produce myelin in the central nervous system (CNS).

• Schwann cells: Produce myelin in the peripheral nervous system (PNS).

• Microglia: Act as phagocytes, removing debris.

• Ependymal cells: Produce and circulate cerebrospinal fluid (CSF).

Saltatory conduction: Action potentials "jump" from node to node along myelinated axons, increasing speed.

Neurotransmitters

Major Neurotransmitters and Functions

• Acetylcholine (ACh): Stimulates muscle contraction.

• Dopamine: Involved in movement and reward pathways.

• Serotonin: Regulates mood.

• Norepinephrine: Increases alertness and arousal.

• Glutamate: Main excitatory neurotransmitter in the CNS.

• GABA (gamma-aminobutyric acid): Main inhibitory neurotransmitter in the CNS.

Brain Structure and Function

Major Brain Regions

• Frontal lobe: Controls voluntary motor functions.

• Parietal lobe: Processes somatosensory information.

• Temporal lobe: Responsible for hearing and memory.

• Occipital lobe: Processes visual information.

• Hypothalamus: Regulates homeostasis.

• Thalamus: Relay station for sensory information.

• Limbic system: Involved in emotion.

• Basal nuclei: Initiate and terminate movements.

• Cerebellum: Coordinates movement and balance.

• Arbor vitae: White matter of the cerebellum.

• Brainstem: Includes midbrain, pons, and medulla oblongata; controls vital functions.

Meninges and Cerebrospinal Fluid (CSF)

Protective Coverings and Fluid Circulation

• Meninges: Three layers covering the brain and spinal cord:

  • Dura mater: Outermost, tough layer.

  • Arachnoid mater: Middle, web-like layer.

  • Pia mater: Innermost, delicate layer.

• CSF: Found in ventricles and subarachnoid space; cushions and nourishes the CNS.

• Gray matter: Contains neuron cell bodies.

• White matter: Contains myelinated axons.

• Corpus callosum: Connects the two cerebral hemispheres.

• Spinocerebellar tracts: Carry unconscious proprioceptive information to the cerebellum.

Summary Table: Muscle and Nervous System Structures

Quick Diagrams (Text Representation)

• Muscle: Tendon == [Muscle]

• Sarcomere: Z | actin === myosin === actin | Z

• Neuron: dendrites => O ==== axon ==== >

• Brain: Frontal | Parietal | Occipital (Temporal below)

Additional info: Expanded explanations and definitions were added for clarity and completeness, as the original notes were highly condensed.