Nervous System - Gray's Anatomy for Students

Overview and Structural Divisions

• Central Nervous System (CNS) - brain and spinal cord (both develop from the neural tube)
• Peripheral Nervous System (PNS) - all nervous structures outside the CNS; connects CNS to the body; develops from neural crest cells and outgrowths of the CNS. Includes spinal nerves, cranial nerves, visceral nerves and plexuses, and the enteric system.

• Somatic part - innervates skin and skeletal muscle (structures from somites); concerned with the external environment
• Visceral part - innervates organ systems, smooth muscle, and glands; concerned with the internal environment

Fig. 1.34 - The CNS (brain + spinal cord, shown in green) and PNS (cranial and spinal nerves, shown in pink) together form the complete nervous system.

Central Nervous System

Brain

• Cerebral hemispheres - outer gray matter (cell bodies), inner white matter (axonal tracts/pathways), and ventricles (CSF-filled spaces)
• Cerebellum - two lateral lobes plus a midline portion
• Brainstem - classically includes the diencephalon, midbrain, pons, and medulla; in common usage "brainstem" usually refers to midbrain, pons, and medulla only

Spinal Cord

Meninges

Functional Subdivisions of the CNS

Somatic Part of the Nervous System

• Anteriorly - form muscles of limbs and trunk (hypaxial muscles) and associated dermis
• Posteriorly - form intrinsic back muscles (epaxial muscles) and associated dermis

• Medial processes into the posterior spinal cord
• Lateral processes into the differentiating dermatomyotome

Somatic Sensory Fibers (Somatic Afferents / GSAs)

Somatic Motor Fibers (Somatic Efferents / GSEs)

Dermatomes

Fig. 1.38 - Dermatomes, showing C6 as an example. The skin on the lateral forearm and thumb is supplied by C6. The autonomous zone of C6 (where overlap is least likely) is the pad of the thumb.

Myotomes

Visceral Part of the Nervous System

• Sensory - monitors changes in viscera
• Motor - regulates activities of the viscera (smooth muscle, cardiac muscle, glandular epithelium)

Fig. 1.43 - The CNS regions associated with visceral motor output. Sympathetic outflow is from T1-L2; parasympathetic outflow is from brainstem (CN III, VII, IX, X) and S2-S4.

Sympathetic System

1. Leave the spinal cord in anterior roots from T1-L2
2. Enter spinal nerves, pass through anterior rami
3. Enter the sympathetic trunk via white rami communicantes (myelinated, appear white)

1. Synapse in the paravertebral ganglion at the same level - postganglionic fibers return to the anterior ramus via gray rami communicantes (unmyelinated), distributed to periphery in spinal nerves (to sweat glands, arrector pili muscles, blood vessel smooth muscle)
2. Ascend in the trunk to synapse at a higher ganglion (serving cervical and upper thoracic regions)
3. Descend in the trunk to synapse at a lower ganglion (serving lumbar and sacral regions)
4. Pass through the trunk without synapsing - form splanchnic nerves that travel to prevertebral ganglia (celiac, superior mesenteric, etc.) to synapse there, with postganglionic fibers then distributed to abdominal and pelvic viscera

Fig. 1.44 - The sympathetic nervous system. The paravertebral chain runs the length of the spinal cord. Postganglionic fibers go to peripheral structures (glands, smooth muscle) and to viscera (heart, abdominal organs, pelvic viscera) via plexuses.

Parasympathetic System

• CN III, VII, IX - preganglionic fibers synapse in one of four ganglia near branches of CN V; postganglionic fibers travel with CN V branches to: salivary glands, mucous glands, lacrimal gland, pupillary constrictor, ciliary muscle
• Vagus nerve [X] - most extensive parasympathetic nerve; provides visceral branches to plexuses of thoracic and abdominal viscera (heart, lungs, GI tract to the mid-colon)

• Preganglionic fibers form pelvic splanchnic nerves, entering pelvic extensions of the prevertebral plexus
• Distributed to pelvic and abdominal viscera mainly along blood vessels
• Postganglionic neurons sit in the walls of the target organs
• Targets include inferior abdominal viscera, pelvic viscera, and erectile tissue arteries

Visceral Sensory Innervation (Visceral Afferents)

• Enter the spinal cord at similar levels to the motor output, though may enter at higher or lower levels (e.g., cardiac pain fibers can enter above T1)
• Primarily detect pain

• CN IX - chemoreceptors and baroreceptors in walls of major neck arteries; receptors in pharynx
• CN X - cervical viscera, thoracic vessels and viscera, abdominal viscera
• S2-S4 - pelvic viscera and distal colon
• These primarily relay information about normal physiological status and reflex activities

Referred Pain

• Inflamed appendix stimulates visceral sensory fibers that enter at T10
• Pain is referred to the T10 dermatome (umbilical region)
• Pain is diffuse and colicky (each peristaltic wave triggers it)
• Later, when the appendix contacts the parietal peritoneum (innervated by somatic nerves), pain localizes to the right iliac fossa and becomes constant - this somatic pain overrides the referred visceral pain

Fig. 1.52 - The mechanism for referred pain from an inflamed appendix to the T10 dermatome. Visceral sensory nerves from the appendix converge on the same spinal cord level as somatic sensory nerves from the umbilical skin, causing the brain to interpret the pain as originating in that skin region.

The Enteric System

• Collections of neurons in the walls of the GI tract
• Intrinsic circuits capable of regulating gut function independently of CNS input
• Connections with both sympathetic and parasympathetic fibers

Protection of the Nervous System

1. Bony encasement - skull (brain) and vertebral column (spinal cord)
2. Meninges - three connective tissue layers (dura, arachnoid, pia)
3. CSF - fills the subarachnoid space and central canal; acts as a hydraulic shock absorber
4. Blood-brain barrier - tight junctions between CNS capillary endothelial cells restrict passage of many substances

Summary Table: Somatic vs. Visceral Nervous System

Circulatory System

THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System

Overview: The Cardiovascular System as an Organ System

• Hemolymphatic system
• Endocrine system
• Respiratory system
• Digestive system
• Urinary system
• Reproductive system

Section 1.6 - Early Embryonic Circulation and the Development of Major Blood Vessels

A. Circulatory System of a 3- to 4-Week-Old Human Embryo

1. Intraembryonic systemic circulation - ventral and dorsal aorta, branchial arch and aortic arches, anterior and posterior cardinal veins
2. Extraembryonic vitelline circulation - omphalomesenteric arteries and veins (supplying the yolk sac)
3. Placental circulation - umbilical arteries and veins (connecting to the placenta)

Fig. 1.6A - Lateral view of the 3- to 4-week embryo showing all three circulatory systems. Red = arteries; Blue = veins. Note the heart, dorsal aorta, aortic arches (in the pharyngeal pouches), anterior and posterior cardinal veins, vitelline vessels draining the yolk sac, and umbilical vessels connecting to the chorionic plate of the placenta.

B. Development of the Arteries Derived from the Aortic Arch

• Typical branching pattern (77%): brachiocephalic trunk + left common carotid + left subclavian arise separately from the arch
• Second most common (13%): left common carotid also arises from the brachiocephalic trunk
• Right-sided aortic arch: ~0.1%
• Duplicated aortic arch: ~0.1%

Fig. 1.6B(c) - The adult aortic arch and its branches. The ligamentum arteriosum is the fibrous remnant of the ductus arteriosus (derived from the 6th aortic arch). It connects the pulmonary artery to the aortic arch.

C. Development of the Cardinal Venous System (Weeks 5-7 to Birth)

• Replace the posterior cardinal veins
• Receive blood from the intercostal veins
• Form the future azygos system (azygos and hemiazygos veins)

• Develop to drain the kidneys
• Right subcardinal vein → middle part of the inferior vena cava
• Transverse anastomosis → left renal vein
• Distal left subcardinal vein persists as a gonadal vein (testicular or left ovarian vein)

• Develop during formation of the lower limbs
• Transverse anastomosis → left common iliac vein

• Between anterior cardinal veins → future left brachiocephalic vein
• Right anterior + common cardinal veins → superior vena cava
• Left common cardinal vein → coronary sinus (venous drainage of the heart)

D. Development of the Vitelline and Umbilical Veins

Fig. 1.6D - Four stages (a = 4th week; b = 5th week; c = 2nd month; d = 3rd month). The vitelline (omphalomesenteric) veins form a plexus around the duodenum, perfuse the liver anlage, and create the first hepatic sinusoids. The right umbilical vein regresses during the 2nd month; the left umbilical vein takes over all placental return. Blood flows through the ductus venosus - a shunt bypassing the liver - into the inferior vena cava. The distal right vitelline vein becomes the future portal vein, carrying blood from the unpaired abdominal organs (via superior/inferior mesenteric veins and splenic vein) to the liver.

Section 2.2 - Structural Organization: Body Cavities and the Heart

• Visceral layer (epicardium) - directly invests the heart
• Parietal layer - lines the outer wall of the pericardial sac

Section 4.1 - The Arteries of the Lower Limb

Overview of Arterial Segments

Fig. 4.1A - The arterial supply to the lower limb, color-coded by segment. (a) Right leg, anterior view; (b) Right leg, posterior view. From proximal to distal: abdominal aorta → common iliac → external iliac → femoral → popliteal → anterior tibial / posterior tibial → dorsal pedal / plantar arteries.

Arterial Segments in Detail

Branch Summary Tables

• Inferior epigastric artery
• Cremasteric artery
• Artery of the round ligament of the uterus
• Pubic branch
• Deep circumflex iliac artery

• Superficial epigastric artery
• Superficial circumflex iliac artery
• Superficial and deep external pudendal arteries
• Deep artery of the thigh (profunda femoris): medial/lateral circumflex femoral arteries + perforating branches
• Descending genicular artery

• Posterior tibial recurrent artery
• Medial and lateral superior genicular arteries (form the arterial anastomotic network of the knee)
• Sural arteries
• Middle genicular artery
• Medial and lateral inferior genicular arteries

• Anterior tibial recurrent artery
• Anterior lateral and medial malleolar arteries
• Dorsal pedal artery (with lateral tarsal, arcuate, dorsal metatarsal, and dorsal digital branches)

• Fibular artery (with perforating branch and calcaneal branches)
• Medial plantar artery
• Lateral plantar artery (forms the plantar arch with the deep plantar artery)

Section 4.2 - The Veins of the Lower Limb

Overview: Three Venous Systems

Deep Veins of the Lower Limb

Fig. 4.2A - (a) Anterior view of thigh, leg, and dorsum; (b) Posterior view of leg; (c) Plantar view of sole. The deep venous system (running alongside the arteries) handles ~85% of venous return. Key deep veins include the femoral, popliteal, anterior/posterior tibial, and fibular veins.

• Femoral vein
• Deep vein of thigh
• Medial and lateral circumflex femoral veins
• Popliteal vein
• Sural veins and genicular veins
• Anterior and posterior tibial veins
• Fibular veins
• Dorsal and plantar metatarsal veins
• Plantar digital veins

Superficial Veins of the Lower Limb

Fig. 4.2C - Superficial veins of the right lower limb. (a) Anterior view - showing the great saphenous vein ascending from the dorsal venous network of the foot along the medial aspect of the limb to empty through the saphenous opening into the femoral vein. (b) Posterior view - showing the small saphenous vein and the femoropopliteal vein.

• Great saphenous vein - longest vein in the body; ascends along the medial thigh and leg; empties into the femoral vein through the saphenous opening
• Accessory saphenous vein
• Posterior arch vein
• Small saphenous vein - ascends along the posterior leg; drains into the popliteal vein
• Femoropopliteal vein
• Dorsal venous network and arch (foot)
• Plantar venous network and arch
• Superficial epigastric, circumflex iliac, and external pudendal veins (at the thigh root)

Perforating (Communicating) Veins

Clinical Box: Varicose Disease of the Superficial Leg Veins

• Spider veins - tiny intradermal varices
• Reticular varices - web-like dilations of small subcutaneous veins
• Great saphenous varicosity - true varicose veins along the great saphenous territory
• Small saphenous varicosity - varicosities along the small saphenous territory

• Primary (idiopathic) varices (75%): Degeneration of the vein wall → incompetence of venous valves → pooling
• Secondary (symptomatic) varices: Chronic occlusion of the deep venous system → incompetence of perforator veins → reversal of flow direction

• Thrombophlebitis - acute superficial venous system inflammation
• Deep venous thrombosis (DVT) - acute disease of the deep venous system

Section 4.3 - The Lymphatic Vessels and Lymph Nodes

• Collectors follow the course of superficial veins (great and small saphenous)
• Two bundles:
  • Anteromedial bundle (runs along great saphenous vein → superficial inguinal lymph nodes) - drains skin and subcutaneous tissue of most of the lower limb
  • Posterolateral bundle (runs along small saphenous vein → superficial popliteal lymph nodes) - drains lateral foot and a narrow strip of the calf

• Follows the deep veins (popliteal and femoral)
• Drains muscles, joints, and nerves
• Deep popliteal lymph nodes → deep inguinal lymph nodes

• Superficial inguinal nodes: arranged horizontally (along the inguinal ligament) and vertically (along the great saphenous vein)
• Deep inguinal nodes: medial to the femoral vein, partly behind the inguinal ligament

Section: Vascular Supply to the Skin (B)

Summary: Key Principles of the Circulatory System