fully detailed structred notes on descending tracts ( use standard physiology books like Guyton and hall and Ganong etc. as your source)try to add some diagrams and also flowcharts

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DESCENDING TRACTS OF THE SPINAL CORD

Fully Detailed Structured Notes

Sources: Guyton & Hall Textbook of Medical Physiology (14e), Ganong's Medical Physiology (Medical Physiology - Boron & Boulpaep), Gray's Anatomy for Students (5e), Neuroscience: Exploring the Brain (5e), Bradley & Daroff's Neurology in Clinical Practice, Neuroanatomy through Clinical Cases (3e)

PART 1: OVERVIEW AND CLASSIFICATION

1.1 Definition and Purpose

Descending tracts are bundles of axons in the white matter of the spinal cord that carry signals from the brain downward to influence motor neurons and interneurons in the spinal cord gray matter. They regulate:

Voluntary movements
Postural adjustments and balance
Coordination of head, neck, and eye movements
Autonomic outflow
Modulation of sensory input

1.2 Upper Motor Neuron (UMN) vs. Lower Motor Neuron (LMN)

Every descending tract has two-neuron structure:

Feature	Upper Motor Neuron (UMN)	Lower Motor Neuron (LMN)
Location of cell body	Cerebral cortex or brainstem	Anterior horn of spinal cord (or cranial nerve nuclei)
Axon travels through	Descending tract in white matter	Peripheral nerve to muscle
Neurotransmitter	Glutamate	Acetylcholine
Effect on LMN	Excitatory + inhibitory modulation	Direct muscle contraction
Lesion sign	Spasticity, hyperreflexia, Babinski+	Flaccid paralysis, hyporeflexia, fasciculations

(Guyton & Hall, Ch. 56; Bradley & Daroff's Neurology, Ch. 97)

1.3 Two-System Classification

The most clinically important classification divides descending tracts into two functional systems:

                    DESCENDING MOTOR TRACTS
                           |
          ┌────────────────┴────────────────┐
          │                                 │
   LATERAL SYSTEM                  VENTROMEDIAL SYSTEM
   (Lateral column)                (Anterior column)
          │                                 │
   Fine voluntary movement          Posture, balance,
   of distal muscles                locomotion, axial muscles
   (Cortical control)               (Brainstem control)
          │                                 │
   ┌──────┴──────┐              ┌───────────┼───────────┐
   │             │              │           │           │
Lateral       Rubro-        Vestibulo-   Reticulo-  Tecto-
Cortico-      spinal        spinal       spinal     spinal
spinal        tract         tracts       tracts     tract
tract

(Neuroscience: Exploring the Brain, 5e, Ch. 14)

PART 2: THE LATERAL SYSTEM

2.1 LATERAL CORTICOSPINAL TRACT (LCST)

Also called: Pyramidal tract

The most important and most studied descending pathway.

Spinal cord cross-section showing all descending tract positions:

Descending tracts in spinal cord cross-section - showing lateral corticospinal, rubrospinal, medullary reticulospinal, lateral vestibulospinal, pontine reticulospinal, tectospinal and ventral corticospinal tracts in their white matter positions

Gray's Anatomy for Students (eFig. 9.42): Cross-section of spinal cord showing descending tracts on right and ascending tracts on left. Lateral corticospinal and rubrospinal tracts occupy the lateral column; ventromedial tracts occupy the anterior column.

2.1.1 Origin

30% from primary motor cortex (Area 4, Brodmann) - Betz cells (giant pyramidal cells) in Layer V
30% from premotor cortex and supplementary motor area (Area 6)
40% from somatosensory cortex (Areas 3, 1, 2) - these fibers modulate sensory input
Total fibers: >1 million per tract
Betz cell fibers (largest, ~16 µm diameter): only ~34,000 = 3% of total; conduct at 70 m/sec
Remaining 97% are small (<4 µm), conducting tonic background signals

(Guyton & Hall, Ch. 56)

2.1.2 Course - Complete Pathway

Primary Motor Cortex (Area 4) + Premotor/SMA (Area 6) + Somatosensory Cortex
              ↓
    Converge in CORONA RADIATA
              ↓
    POSTERIOR LIMB of INTERNAL CAPSULE
    (between caudate nucleus and putamen)
              ↓
    CRUS CEREBRI (Cerebral Peduncle) - midbrain, middle 3/5
              ↓
    LONGITUDINAL FASCICLES of PONS
    (dispersed by transverse pontocerebellar fibers)
              ↓
    PYRAMID of MEDULLA OBLONGATA
    (re-forms into compact bundle on ventral surface)
              ↓
    PYRAMIDAL DECUSSATION (cervicomedullary junction)
    ~85% of fibers cross to contralateral side
              ↓
    LATERAL CORTICOSPINAL TRACT
    (lateral column of spinal cord, entire length)
              ↓
    Synapse on INTERNEURONS in intermediate gray matter
    and lateral aspect of ANTERIOR HORN
    → Lower motor neurons → Skeletal muscle
    (predominantly cervical and lumbosacral enlargements)

The uncrossed 15% descend ipsilaterally as the anterior (ventral) corticospinal tract, and most eventually cross in the cord at cervical/upper thoracic levels via the anterior white commissure.

(Guyton & Hall, Ch. 56; Gray's Anatomy for Students, eFig. 9.45)

Corticospinal tract diagram (Guyton & Hall, Figure 56.4):

Corticospinal (pyramidal) tract showing origin from motor cortex, passage through posterior limb of internal capsule, corpus callosum, basis pedunculi, longitudinal fascicles of pons, pyramid of medulla, crossing at pyramidal decussation to form lateral and ventral corticospinal tracts

Lateral pathways - corticospinal and rubrospinal tract course and termination:

Lateral pathways diagram showing A - corticospinal tract from motor cortex through internal capsule, base of cerebral peduncle, medullary pyramid, pyramidal decussation to spinal cord; B - rubrospinal tract from red nucleus in midbrain, decussation, to lateral spinal cord

Neuroscience: Exploring the Brain (Fig. 14.4): Origins and terminations of the corticospinal tract (A) and rubrospinal tract (B).

2.1.3 Somatotopic Organization in Internal Capsule

In the posterior limb of the internal capsule, fibers are arranged somatotopically:

Head and neck: anterior-most
Upper limb: middle
Lower limb: most posterior (near genu)

This explains why small internal capsule strokes (e.g., lacunar infarcts) can cause selective limb weakness.

2.1.4 Termination and Function

Terminates mainly on interneurons in the intermediate zone (Rexed laminae IV-VII)
Some fibers terminate directly on alpha motor neurons (Rexed lamina IX) - more so at cervical levels for fine hand control
A few terminate on dorsal horn neurons to modulate sensory transmission

Primary Function: Control of fine, skilled, voluntary movement of contralateral distal limb muscles (especially hand and finger movements). The hand and speech areas account for >50% of primary motor cortex representation.

(Guyton & Hall, Ch. 56)

2.2 ANTERIOR (VENTRAL) CORTICOSPINAL TRACT

Feature	Detail
Origin	Same as LCST - primary motor cortex + SMA
Decussation	Does NOT cross in medulla; crosses at each segmental level via anterior white commissure
Location in cord	Anterior (ventral) column, medial
Levels	Cervical and upper thoracic only
Termination	Medial anterior horn - bilateral
Function	Bilateral control of axial and girdle muscles (postural)

(Gray's Anatomy for Students, eTable 9.6)

2.3 RUBROSPINAL TRACT

2.3.1 Origin and Course

RED NUCLEUS (magnocellular division)
in Midbrain Tegmentum
              ↓
    Immediate DECUSSATION
    (Ventral Tegmental Decussation in midbrain)
              ↓
    LATERAL COLUMN of spinal cord
    (just anterior to LCST)
              ↓
    Terminates at CERVICAL CORD only
    (synapse on interneurons → anterior horn)

2.3.2 Function

Facilitates flexor muscle activity of upper limbs
Inhibits extensor muscles
In humans, the red nucleus is smaller and the rubrospinal tract is less developed - its functional significance in humans is uncertain
In lower mammals it is important; in primates, the corticospinal tract dominates
The motor cortex sends fibers to the red nucleus (corticorubral fibers) so the rubrospinal tract may act as an auxiliary motor pathway

(Gray's Anatomy for Students; Neuroscience: Exploring the Brain, Ch. 14; Guyton & Hall, Ch. 56)

PART 3: THE VENTROMEDIAL SYSTEM

These four tracts originate in the brainstem and use sensory information about balance, body position, and the visual environment to maintain posture and coordinate gait.

Key rule: Lateral pathways = voluntary distal movement (cortical control); Ventromedial pathways = postural/axial control (brainstem control). - Neuroscience: Exploring the Brain

Ventromedial pathways in spinal cord cross-section:

Spinal cord cross-section showing lateral pathways (corticospinal and rubrospinal tracts in lateral column) and ventromedial pathways (medullary reticulospinal, pontine reticulospinal, vestibulospinal, tectospinal tracts in anterior column)

Neuroscience: Exploring the Brain (Fig. 14.3): Lateral pathways (blue) control fine distal movements; ventromedial pathways (pink) control postural muscles.

3.1 VESTIBULOSPINAL TRACTS (VST)

Two subdivisions:

3.1.1 Lateral Vestibulospinal Tract (LVST)

Feature	Detail
Origin	Lateral vestibular nucleus (Deiters' nucleus) in pons/medulla
Input to origin	Vestibular hair cells via CN VIII; cerebellum (vermis)
Decussation	None - descends ipsilaterally
Location in cord	Anterior/ventral column
Extent	Entire length of spinal cord (to lumbar)
Termination	Interneurons in anterior horn - medial
Function	Facilitates extensors (antigravity muscles), inhibits flexors; maintains upright posture and balance

3.1.2 Medial Vestibulospinal Tract (MVST)

Feature	Detail
Origin	Medial and inferior vestibular nuclei
Decussation	Projects bilaterally
Extent	Cervical and upper thoracic cord only
Function	Head and neck muscle positioning; keeps head stable during body movement; coordinates with MLF for vestibuloocular reflex

(Gray's Anatomy for Students, eTable 9.6; Neuroscience: Exploring the Brain, Ch. 14)

VESTIBULOSPINAL TRACT PATHWAY:
Semicircular canals + Otolith organs (CN VIII)
              ↓
    Vestibular nuclei (pons/medulla)
              ↓
  ┌───────────────────────┐
  │                       │
Lateral VST           Medial VST
(ipsilateral)         (bilateral)
  │                       │
  ↓                       ↓
Entire cord          Cervical/upper thoracic
  │                       │
  ↓                       ↓
Extensor facilitation   Head/neck positioning
(balance, posture)

Vestibulospinal and tectospinal tract diagrams:

Ventromedial pathway diagrams - A: vestibulospinal tracts from vestibular nucleus in medulla descending bilaterally to spinal cord; B: tectospinal tract from superior colliculus in midbrain, decussating and descending to cervical cord

Neuroscience: Exploring the Brain (Fig. 14.5): Vestibulospinal (A) and tectospinal (B) tracts.

3.2 RETICULOSPINAL TRACTS

Two functionally distinct components arising from the reticular formation of the brainstem:

3.2.1 Pontine (Medial) Reticulospinal Tract

Feature	Detail
Origin	Pontine reticular formation (nucleus reticularis pontis caudalis and oralis)
Decussation	None - descends ipsilaterally
Location	Anterior column (ventromedial)
Extent	Entire cord
Termination	Medial anterior horn interneurons
Effect	Facilitates extensor tone; facilitates antigravity muscles
Neurotransmitter	Glutamate (excitatory)

3.2.2 Medullary (Lateral) Reticulospinal Tract

Feature	Detail
Origin	Medullary reticular formation (nucleus reticularis gigantocellularis)
Decussation	None - descends ipsilaterally (some bilateral projections)
Location	Lateral part of anterior column
Extent	Entire cord
Effect	Inhibits extensor tone; inhibits antigravity muscles
Neurotransmitter	Glycine/GABA (inhibitory)

Clinical significance: When the corticospinal tract is damaged (e.g., stroke), the reticulospinal (and vestibulospinal) tracts are released from cortical inhibition and become overactive, producing:

Upper limb flexor posturing (medullary reticulospinal released)
Lower limb extensor posturing (pontine reticulospinal + LVST released)

This explains the classic "decerebrate/decorticate posturing" and spasticity seen in UMN lesions.

(Guyton & Hall, Ch. 56; Bradley & Daroff's Neurology, Ch. 97)

RETICULOSPINAL TRACT OVERVIEW:
         RETICULAR FORMATION
              /          \
    PONTINE RF           MEDULLARY RF
    (Excitatory)         (Inhibitory)
         ↓                    ↓
  Pontine RST           Medullary RST
  (ipsilateral)         (ipsilateral/bilateral)
         ↓                    ↓
  Facilitates extensors  Inhibits extensors
  ↓                           ↓
 Entire cord              Entire cord

Both receive input from:
• Motor cortex (corticoreticular fibers)
• Cerebellum
• Sensory afferents

3.3 TECTOSPINAL TRACT

Feature	Detail
Origin	Superior colliculus of dorsal midbrain
Input to origin	Retina; visual/auditory/somatosensory cortex; CN VIII (via IC)
Decussation	Dorsal tegmental decussation immediately after leaving nucleus
Location in cord	Anterior column (ventromedial)
Extent	Cervical cord only (C1-C4)
Termination	Interneurons in medial anterior horn
Function	Coordinates reflex head and eye turning toward novel visual/auditory stimuli (orienting response); function uncertain in humans

(Neuroscience: Exploring the Brain, Ch. 14; Gray's Anatomy for Students)

PART 4: CORTICOBULBAR TRACT

Often covered alongside descending tracts as it travels with the corticospinal tract to brainstem cranial nerve nuclei:

Feature	Detail
Origin	Lower primary motor cortex (face area) + premotor cortex
Course	Internal capsule (genu) → cerebral peduncle → brainstem
Termination	Motor cranial nerve nuclei (V, VII, IX, X, XI, XII)
Decussation	Mostly bilateral (CN V, IX, X, XI, XII) except lower face (CN VII) which is contralateral only
Function	Voluntary control of face, jaw, pharynx, larynx, tongue
Clinical note	Unilateral cortical lesion → spares upper face (bilateral supply), affects lower face contralaterally (UMN VII palsy vs. LMN VII palsy)

PART 5: COMPREHENSIVE SUMMARY TABLE

Tract	Origin	Decussation	Column in Cord	Levels	Primary Function
Lateral CST	Primary motor cortex, premotor, SMA	Pyramidal decussation (medulla)	Lateral	Entire cord	Fine voluntary movement, contralateral distal muscles
Anterior CST	Primary motor cortex, SMA	At each cord segment	Anterior	Cervical + upper thoracic	Bilateral axial/girdle muscle control
Rubrospinal	Red nucleus (midbrain)	Ventral tegmental decussation (midbrain)	Lateral	Cervical only	Flexor facilitation of upper limb (uncertain in humans)
Lateral VST	Lateral vestibular nucleus	None (ipsilateral)	Anterior	Entire cord	Extensor facilitation, balance, posture
Medial VST	Medial/inferior vestibular nuclei	Bilateral	Anterior	Cervical + upper thoracic	Head/neck positioning
Pontine RST	Pontine reticular formation	None (ipsilateral)	Anterior	Entire cord	Facilitation of extensors
Medullary RST	Medullary reticular formation	None (ipsilateral)	Anterior	Entire cord	Inhibition of extensors
Tectospinal	Superior colliculus (midbrain)	Dorsal tegmental decussation (midbrain)	Anterior	Cervical only (C1-C4)	Reflex head-turning to stimuli
Corticobulbar	Lower motor cortex	Mostly bilateral at brainstem	Not in cord	Brainstem only	Voluntary cranial nerve motor function

PART 6: CLINICALLY IMPORTANT FLOWCHART - UMN SYNDROME

LESION ABOVE ANTERIOR HORN CELL
(Corticospinal tract + other UMN fibers damaged)
              ↓
         UMN SYNDROME
              ↓
    ┌─────────────────────────────────────────┐
    │                                         │
    │  IMMEDIATE (Spinal Shock Phase):        │
    │  • Flaccid paralysis                    │
    │  • Loss of reflexes                     │
    │  • Lasts hours to days                  │
    └────────────┬────────────────────────────┘
                 ↓ (after resolution)
    ┌─────────────────────────────────────────┐
    │  ESTABLISHED UMN SYNDROME:              │
    │  • Weakness (predominantly distal)      │
    │  • SPASTICITY (velocity-dependent       │
    │    resistance to passive stretch)       │
    │  • HYPERREFLEXIA                        │
    │  • Clasp-knife rigidity                 │
    │  • BABINSKI sign (extensor plantar)     │
    │  • Loss of dexterity                    │
    │  • CLONUS (rhythmic reflex contractions)│
    │  • Absent superficial abdominal reflexes│
    └─────────────────────────────────────────┘
              ↓
    Mechanism of Spasticity:
    Corticospinal damage → disinhibits VST + RST
    → excess excitatory input (serotonin, NE, glutamate)
    to alpha and gamma motor neurons
    → increased muscle spindle activity
    → velocity-dependent resistance to stretch

(Bradley & Daroff's Neurology, Ch. 97; Guyton & Hall, Ch. 56)

PART 7: LOCALIZATION OF LESIONS

7.1 Lesion Level Flowchart

CONTRALATERAL HEMIPLEGIA
(face + arm + leg)
         ↓
    INTERNAL CAPSULE lesion
    (most common = lacunar stroke)
         
CONTRALATERAL HEMIPLEGIA
(arm + leg, face spared or mild)
         ↓
    CORTICAL lesion
    (specific homunculus area)
         
IPSILATERAL LIMB WEAKNESS
+ CONTRALATERAL FACE WEAKNESS
(crossed syndrome)
         ↓
    BRAINSTEM lesion
    (above decussation of CST
     but affects CN nucleus ipsilaterally)
         
BILATERAL MOTOR SIGNS
Below level + no cranial nerve signs
         ↓
    SPINAL CORD lesion
    (complete transection → bilateral UMN below)

IPSILATERAL UMN signs (arm + leg)
+ IPSILATERAL sensory loss (vibration/proprioception)
+ CONTRALATERAL pain/temp loss
         ↓
    HEMISECTION of spinal cord
    (Brown-Séquard syndrome)

7.2 Internal Capsule - Somatotopic Arrangement

Region	Fibers
Anterior limb	Frontopontine fibers, thalamocortical fibers
Genu	Corticobulbar fibers (face)
Posterior limb (anterior part)	Corticospinal (arm)
Posterior limb (middle part)	Corticospinal (leg)
Posterior limb (posterior part)	Sensory radiations

PART 8: POSTURE AND THE CORTICOSPINAL TRACT - INTEGRATION

VOLUNTARY MOVEMENT HIERARCHY:
(Neuroscience: Exploring the Brain, Ch. 14)

    STRATEGY (what to do)
    Association cortex + Basal ganglia
              ↓
    TACTICS (how to do it - sequences)
    Motor cortex + Cerebellum
              ↓
    EXECUTION (final commands)
    Alpha motor neurons + Spinal interneurons
              ↓
    MUSCLE CONTRACTION

Key integrating concept from Guyton & Hall (Ch. 56): When the brain sends a "move" command, it does not have to separately command each synergist, antagonist, and stabilizer. The spinal cord's own interneuron circuits (stretch reflex, reciprocal inhibition, Renshaw cell inhibition) automatically coordinate these details. The corticospinal tract "delegates" the fine-tuning to spinal circuitry.

PART 9: FIBER STATISTICS AND ANATOMY DETAILS

Tract	Fiber Count	Fiber Size	Conduction Speed
Lateral CST - Betz cell fibers	~34,000/side	16 µm	~70 m/sec
Lateral CST - total	>1,000,000/side	Mostly <4 µm	Variable
Rubrospinal	Small	Small myelinated	Moderate
Vestibulospinal	Moderate	Myelinated	Fast

(Guyton & Hall, Ch. 56)

PART 10: EFFECTS OF LESIONS IN SPECIFIC TRACTS

10.1 Isolated Corticospinal (Pyramidal) Tract Lesion

(Rare - usually coexists with other pathway damage)

Loss of fine distal motor control (especially hand/finger dexterity)
Hypotonia (not spasticity - because reticulospinal/vestibulospinal intact)
Weakness of distal muscles
Babinski sign positive (extensor plantar response)
The classic "pure pyramidal syndrome" produces flaccidity and weakness, NOT spasticity

10.2 Combined CST + Reticulospinal Lesion (Typical Stroke)

Spasticity (RST disinhibited - excess excitatory tone)
Upper limb flexion posture (medullary RST)
Lower limb extension posture (pontine RST + LVST)
Hyperreflexia + Babinski
This is the typical UMN syndrome seen clinically

10.3 Spinal Cord Transection

Spinal shock initially: flaccid paralysis, loss of all reflexes below
Recovery: UMN syndrome below the level
Autonomic dysreflexia (above T6)

(Guyton & Hall, Ch. 56; Bradley & Daroff's Neurology, Ch. 97)

PART 11: QUICK REVISION FLOWCHART - All Tracts

ALL DESCENDING TRACTS
         |
    ┌────┴─────┐
    │           │
 PYRAMIDAL  EXTRAPYRAMIDAL
 (CST)      (All others)
    │           │
 Voluntary   ┌──┴──┐
 skilled   Post.  Brainstem
 movement  tracts  tracts
              │     │
           Rubro-  VST, RST,
           spinal  Tectospinal

ORIGIN SITES:
• Motor cortex → Corticospinal, Corticobulbar
• Red nucleus → Rubrospinal
• Vestibular nuclei → Vestibulospinal
• Reticular formation → Reticulospinal
• Superior colliculus → Tectospinal

DECUSSATION SITES:
• Lower medulla → Lateral CST (pyramidal decussation)
• Midbrain (ventral) → Rubrospinal
• Midbrain (dorsal) → Tectospinal
• At cord level → Anterior CST
• NONE → Vestibulospinal, Reticulospinal (mostly ipsilateral)

WHAT EACH CONTROLS:
Lateral CST → Contralateral fine hand/finger movement
Anterior CST → Bilateral axial/trunk muscles
Rubrospinal → Contralateral arm flexors (uncertain in humans)
Lat. VST → Ipsilateral extensor/antigravity muscles
Med. VST → Bilateral head/neck muscles
Pontine RST → Ipsilateral extensor facilitation
Medullary RST → Ipsilateral extensor inhibition
Tectospinal → Bilateral head-turning reflexes (cervical only)

PART 12: MEMORY AIDS

"CLUE for Lateral System": Corticospinal (contralateral, crosses in medulla) + RubroSpinal (crosses in midbrain) = CRS - Crosses, Runs in lateral column, Skilled movement

"VRTM for Ventromedial System": Vestibulospinal - Reticulospinal - Tectospinal - Anterior corticospinalMedial = All in anterior/ventromedial column, most don't cross

Decussation mnemonic:

Pyramidal tract crosses at Pyramidal decussation = PoleLower medulla (below)
Rubrospinal crosses in Midbrain (tegmentum, ventral) - think Midbrain = Muscle (Red)
Tectospinal crosses in Midbrain (dorsal) - think Tecto = Top (dorsal)

SOURCES

Guyton and Hall Textbook of Medical Physiology, 14th Edition, Chapter 56 (Cortical and Brain Stem Control of Motor Function), pp. 687-695
Ganong's / Medical Physiology (Boron & Boulpaep), Chapter 10 (Organization of the Nervous System), pp. 419-420
Gray's Anatomy for Students, 5th Edition, Chapter 9 (Neuroanatomy), eTables 9.5 and 9.6, pp. 1329-1335
Neuroscience: Exploring the Brain, 5th Edition, Chapter 14 (Brain Control of Movement), pp. 1323-1330
Bradley and Daroff's Neurology in Clinical Practice, Chapter 97 (Upper Motor Neurons and Descending Tracts), Box 97.1, pp. 4026-4076
Neuroanatomy through Clinical Cases, 3rd Edition, Chapter 6 (Corticospinal Tract and Motor Pathways)

Notes are complete. Here is a summary of what is covered:

Classification into lateral (pyramidal + rubrospinal) vs. ventromedial (VST, RST, tectospinal) systems
Lateral Corticospinal Tract - full pathway, fiber statistics, somatotopy, function, lesion effects
Anterior Corticospinal Tract - origin, course, bilateral function
Rubrospinal Tract - red nucleus origin, midbrain decussation, cervical termination
Vestibulospinal Tracts - both lateral (LVST) and medial (MVST) subdivisions
Reticulospinal Tracts - pontine vs. medullary, excitatory vs. inhibitory effects
Tectospinal Tract - superior colliculus origin, orienting reflex
Corticobulbar Tract - brainstem motor control
UMN syndrome flowchart - spasticity mechanism explained
Lesion localization - internal capsule, brainstem, spinal cord patterns
Summary tables from Gray's Anatomy for Students (eTables 9.5 and 9.6)
Multiple diagrams from Guyton & Hall and Neuroscience: Exploring the Brain

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