Central Nervous System, Thoracic Cage, Vertebral Column, and Associated Muscles: Structured Study Notes

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Central Nervous System (CNS)

Functional Organization of the CNS

The Central Nervous System (CNS) is composed of the brain and spinal cord. It serves as the main control center for processing and relaying information throughout the body.

Sensory Input: Transmits information from sensory receptors (e.g., pain, temperature, touch, pressure, proprioception) to the brain.
Motor Output: Sends commands from the brain to skeletal muscle, smooth muscle, cardiac muscle, and glands.

Inferior End of the Spinal Cord

Conus Medullaris: The tapered, cone-shaped end of the spinal cord, typically ending at the L1–L2 vertebral level.
Cauda Equina: A bundle of spinal nerves extending below the conus medullaris, resembling a horse's tail.
Filum Terminale: A thin strand of connective tissue anchoring the spinal cord to the coccyx.

Memory aids: Conus = cone-shaped; Cauda Equina = horse's tail; Filum Terminale = final anchor.

Meninges

The spinal cord is protected by three connective tissue layers, from outermost to innermost:

Dura Mater
Arachnoid Mater
Pia Mater

Memory trick: DAP (Dura, Arachnoid, Pia)

Central Canal

Runs through the center of the spinal cord.
Contains cerebrospinal fluid (CSF) for cushioning, nutrient transport, and waste removal.

Lumbar Puncture (Spinal Tap)

Performed in the lumbar region to collect CSF.
Used to diagnose conditions such as meningitis, multiple sclerosis, and Guillain-Barré syndrome.
Can also deliver medications into the CSF.

White Matter Tracts

White matter contains myelinated axons organized into tracts:

Fasciculus Gracilis: Carries touch and pressure from the lower body.
Fasciculus Cuneatus: Carries touch and pressure from the upper body.
Spinocerebellar Tracts: Carry proprioceptive information to the cerebellum for balance and coordination.
Spinothalamic Tracts: Carry pain and temperature sensations to the thalamus.

High Yield: The spinothalamic tract carries pain and temperature.

Gray Matter

Gray matter is organized into three horns:

Horn	Function
Posterior	Sensory input (somatic and visceral)
Lateral	Visceral motor output (autonomic nervous system)
Anterior	Somatic motor output (skeletal muscle)

Spinal Nerves

Sequence: Anterior rootlets → Anterior root; Posterior rootlets → Posterior root → Spinal nerve → Anterior and Posterior rami
Dorsal Root Ganglion: Located on the posterior root; contains cell bodies of sensory neurons.
Important Rule: Posterior = Sensory; Anterior = Motor

Nerve Plexuses

Plexus: A network of interwoven nerves.
Major plexuses: Cervical, Brachial, Lumbar, Sacral
Function: Redistribute spinal nerves to form peripheral nerves.

Autonomic Nervous System (ANS)

The ANS controls involuntary functions and has two divisions:

Division	Origin	Main Effects
Sympathetic (Fight or Flight)	Thoracic & Lumbar spinal cord	Increases heart rate, dilates pupils/bronchi, decreases digestion
Parasympathetic (Rest and Digest)	Cranial nerves & Sacral spinal cord	Decreases heart rate, increases digestion/salivation, constricts pupils

Thoracic Cage

Functions and Components

The thoracic cage protects vital organs (heart, lungs, great vessels) and provides attachment for respiratory muscles.

Sternum
12 pairs of ribs
Thoracic vertebrae
Costal cartilage

Sternum

Manubrium: Superior portion; includes jugular (suprasternal) notch and clavicular notch.
Body: Largest part; attachment for ribs.
Xiphoid Process: Inferior, smallest portion; mostly cartilage in youth.
Sternal Angle: Junction of manubrium and body; important anatomical landmark.

Ribs

Type	Pairs	Attachment
True (Vertebrosternal)	1–7	Directly to sternum via costal cartilage
False (Vertebrochondral)	8–10	To costal cartilage of rib 7 (not directly to sternum)
Floating (Vertebral)	11–12	Only to thoracic vertebrae; no anterior attachment

Head: Articulates with thoracic vertebra.
Tubercle: Also articulates with thoracic vertebra.
Costal Angle: Lateral bend of rib.
Body: Longest portion; forms wall of thoracic cage.

High Yield: Both head and tubercle articulate with thoracic vertebrae.

Vertebral Column

Regions and Functions

Cervical: 7 vertebrae (C1–C7)
Thoracic: 12 vertebrae (T1–T12)
Lumbar: 5 vertebrae (L1–L5)
Sacrum: 5 fused vertebrae (counted as one bone)
Coccyx: 3–5 fused vertebrae

Functions: Supports body weight, protects spinal cord, allows movement, provides muscle attachment.

Normal Curves

Type	Curves	Development
Primary	Thoracic, Sacral	Before birth
Secondary	Cervical, Lumbar	After birth (for head/body support)

Typical Vertebra Anatomy

Body: Weight-bearing portion.
Pedicle & Lamina: Form vertebral arch.
Vertebral Foramen: Contains spinal cord.
Spinous Process: Posterior projection; palpable.
Transverse Processes: Lateral projections; muscle/rib attachment.
Superior/Inferior Articular Processes: Interlock with adjacent vertebrae.
Intervertebral Foramen: Passage for spinal nerves between vertebrae.

Vertebral Regions: Unique Features

Cervical: Transverse foramen (for vertebral artery/vein), bifid spinous process, Atlas (C1) supports skull, Axis (C2) has dens for head rotation.
Thoracic: Costal facets for rib attachment, long spinous processes.
Lumbar: Largest, thickest vertebrae; hatchet-shaped spinous process.

Spinal Disorders

Kyphosis: Excessive thoracic curvature ("hunchback"). Causes: osteoporosis, poor posture.
Lordosis: Excessive lumbar curvature ("swayback"). Causes: pregnancy, obesity.
Scoliosis: Lateral curvature; can be structural, habitual, or idiopathic (most common in females).

Spina Bifida

Failure of lamina to form properly; often linked to inadequate folic acid during development.
Types:
- Occulta: Mildest, often asymptomatic.
- Meningocele: Meninges protrude.
- Meningomyelocele: Most severe; spinal cord and meninges protrude.

Sacrum and Coccyx

Sacrum: Five fused vertebrae; features include sacral promontory, ala, sacral foramina, sacral canal, sacral hiatus, median sacral crest, and transverse lines.
Coccyx: Tailbone; usually 3–5 fused vertebrae.

Muscles of the Back, Thorax, and Abdomen

Erector Spinae Muscle Group

The erector spinae are the primary extensors of the vertebral column and neck, maintaining posture.

Iliocostalis: (cervicis, thoracis, lumborum) – extends vertebral column, assists with lateral flexion.
Longissimus: (capitis, cervicis, thoracis) – extends vertebral column, head, and neck.
Spinalis: (capitis, cervicis, thoracis) – extends vertebral column.

Memory trick: I Love Spaghetti (Iliocostalis, Longissimus, Spinalis)

Quadratus Lumborum

Located in the lower back; responsible for lateral flexion of the vertebral column (not extension).

Abdominal Muscles

External Oblique: Compresses abdomen, depresses ribs, flexes vertebral column ("hands in pockets" fiber direction).
Internal Oblique: Beneath external; same function as external oblique.
Transversus Abdominis: Deepest; compresses abdomen, stabilizes trunk.
Rectus Abdominis: "Six-pack" muscle; flexes vertebral column, compresses abdomen, depresses ribs.
Tendinous Intersections: Fibrous bands crossing rectus abdominis, forming the "six-pack" appearance.
Rectus Sheath (Aponeurosis): Connective tissue surrounding rectus abdominis; linea alba is the midline thickening.

Serratus Anterior

"Boxer's muscle"; protracts scapula, allowing forward punching motion.

Diaphragm

Primary muscle of inspiration; contracts to enlarge thoracic cavity and draw air into lungs.

Pectoralis Major

Origin: Cartilage of ribs 2–6, sternum, medial clavicle.
Insertion: Greater tubercle/intertubercular groove of humerus.
Actions: Flexes, adducts, and medially rotates humerus.
Innervation: Pectoral nerves.
Clinical: Prone to tears during heavy bench pressing.

Deltoid

Origin: Acromion and spine of scapula.
Insertion: Deltoid tuberosity.
Main action: Abduction of the shoulder.
Innervation: Axillary nerve.

Trapezius

Origin: Occipital bone, spinous processes C7–T12.
Insertion: Acromion, spine of scapula, lateral clavicle.
Actions: Elevates, retracts, depresses scapula; extends neck.
Innervation: Accessory nerve (Cranial Nerve XI).

Rhomboids (Major & Minor)

Located deep to trapezius; retract (adduct) scapula; important for posture.

Teres Major

Actions: Extends, adducts, and medially rotates humerus.
Note: Not a rotator cuff muscle (only teres minor is).

Latissimus Dorsi

Origin: Spinous processes of T8–T12 and lumbar vertebrae.
Insertion: Intertubercular groove of humerus.
Actions: Extends, adducts, medially rotates humerus.
Innervation: Thoracodorsal nerve.
Clinical: Antagonist to shoulder abductors/flexors (e.g., deltoid).

Subclavius

Small muscle beneath clavicle; depresses and protracts shoulder.

Pectoralis Minor

Deep to pectoralis major; depresses and protracts scapula.

Intercostal Muscles

External Intercostals: Elevate ribs during inspiration.
Internal Intercostals: Depress ribs during forced expiration.

Muscle Summary Table

Muscle	Main Action	Innervation (if emphasized)
Erector spinae	Extend spine/neck	—
Quadratus lumborum	Lateral flexion	—
External oblique	Compress abdomen, flex trunk	—
Internal oblique	Compress abdomen, flex trunk	—
Transversus abdominis	Compress abdomen	—
Rectus abdominis	Flex trunk, compress abdomen	—
Serratus anterior	Protract scapula	—
Diaphragm	Inspiration	—
Pectoralis major	Flex, adduct, medially rotate humerus	Pectoral nerves
Deltoid	Abduct shoulder	Axillary nerve
Trapezius	Elevate/retract/depress scapula; extend neck	Accessory nerve (CN XI)
Rhomboids	Retract scapula	—
Teres major	Extend, adduct, medially rotate humerus	—
Latissimus dorsi	Extend, adduct, medially rotate humerus	Thoracodorsal nerve
Subclavius	Depress, protract shoulder	—
Pectoralis minor	Depress, protract scapula	—
External intercostals	Elevate ribs	—
Internal intercostals	Depress ribs	—