Understand 1st year medicine

Made by a (former) student for students!

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CNS (Central nervous system) and PNS (Peripheral nervous system)

Tyrosine + ENZYME Tyrosine hydroxylase  →  L-DOPA + ENZYME dopa decarboxylase →  Dopamine + ENZYME Dopamine hydroxylase →  NE (norepinephrine ) → EP (epinephrine)

NE: CNS neurotransmitter, acts predominantly on alpha receptors, more positive inotropy than EP and is used in septic shock

EP: Sympathetic effect, acts on alpha and beta receptors, positive inotropy but rarely used in septic shock because apparently its alpha-1 vasoconstrictive action (vasopressor) reduces organ perfusion causing lactic acidosis

Summary of GPCRs G protein coupled receptors (QISS QUQly til your SIQ of SQS - sick of sex) e.g. Gq or Gs or Gi
alpha1=Q (eyes, blood vessel)
beta1=S (heart) ... 1 heart 2 lungs
beta2=S (lungs)
M1=Q (stomach)
m4 - eye
H2=S (stomach)

Somatic (skeletal muscles) - only have one synapse (ACh x nicotinic receptor on muscle)


Autonomic - sympathetic and parasympathetic


1) Sympathetic


Preganglionic: Release ACh (acetyl-choline) which interacts with nicotinic receptor on postganglion
Adrenal medulla directly releases EP (epinephrine) to circulation without synapsing in the coeliac ganglion


  • Heart muscle: Release NE x B1 receptor on target organ
  • Sweat gland: Release ACh x muscarinic receptor on target tissue

Effects of EP

  • Increased plasma glucose (Inhibit insulin secretion, stimulate glycogenolysis in liver and muscle, stimulate glycolysis)
  • Increased HR (heart rate), SV (stroke volume) thus CO (cardiac output) since [CO = HR x SV]
  • Used in cardiac arrest: Increase TPR total peripheral resistance via alpha1 vasoconstriction and increase CO via B1
  • Treat anaphylaxis due to vasoconstrictive effects (vasopressor!). Bronchodilator for asthma
  • In local anesthetic e.g. lidocaine to vasoconstrict to retard the tissue distribution

2) Parasympathetic

Preganglionic: ACh x nicotinic receptor
Postganglionic: ACh x muscarinic receptor


Spinal Cord


Ganglion: Cell bodies outside CNS
  • Inflow - Dorsal root ganglion: Afferent cell bodies
  • Outflow
  • 1) Sympathetic: Prevertebral or paravertebral cell bodies of postganglionic sympathetic neurons
  • 2) Parasympathetic: Cell bodies of postganglionic parasympathetic neurons

Nucleus: Cell bodies in CNS

Dorsal Root: Afferent to spinal cord (i.e. sensory)

Ventral Root: Efferent to spinal cord (i.e. motor)




Splanchnic nerves - innervate viscera; carrying fibers of motor (autonomic) and sensory. All splanchnic fibers are sympathetic except for pelvic splanchnic (parasympathetic)

Diaphragm - has medial and lateral arcuate ligaments



The sympathetic trunks pass under the medial arcuate ligaments of the diaphragm, on top of the psoas major muscles, to enter the abdomen.

Sympathetic from (greater, lesser and least) thoracic splanchnic and upper lumbar splanchnic nerve pass through sympathetic chain without synapsing and synapses at ganglia (cell body outside spinal cord); all these ganglia lie prevertebral (preaortic) except for the paired ganglia (e.g. aorticorenal)

  • Greater thoracic splanchnic: T5-9 pass to
    • Celiac ganglia
    • synapse directly to suprarenal medulla (preganglion!)
      • ACh released from (preganglionic) splanchnic nerve to N1 (nicotinic) receptors on medulla →  Release of E (and a bit of NE) from medulla to capillaries
  • Lesser thoracic splanchnic: T10-11 pass to
    • Superior mesenteric ganglia
    • Aorticorenal ganglia located at renal artery
      • Post-synaptic fibers either pass laterally to form renal plexus along renal vessels or inferiorly to form intermesenteric plexus lying on the aorta between superior and inferior mesenteric artery
  • Least thoracic splanchnic: T12 pass to
    • Inferior mesenteric ganglia
  • Lumber splanchnic: L1-L4
    • Upper (L1-2): to inferior mesenteric ganglia
    • Lower (L3-4): to Superior hypogastric plexus → pelvis


The thoracic and lumbar splanchnic nerves pass into prevertebral (preaortic) ganglia (the ganglia all are SYMPATHETIC as PS ganglia are found intramural; paired celiac, the superior mesenteric, paired aorticorenal, and the inferior mesenteric) as preganglion. The postsynaptic fibers pass into perivascular plexuses (sympathetic nerves that run along vessels to supply target organ or the vessel itself).



Sensory and Motor of Sympathetic Ganglia (presynaptic)


(paired) Celiac - from greater thoracic splanchnic

S: Pain from stomach, liver, biliary system, spleen, upper half of duodenum, pancreas (basically foregut!)
M: Vasculature of celiac trunk



Autonomic plexus: Collection of parasympathetic and sympathetic fibers that coalesce to innervate viscera


  • Converge to form plexus
    • Cervical plexus: C1-5
    • Brachial plexus: C5-T1
      • marmu
      • musculocutaneous, axillary, radial, median, ulnar
    • Lumbar plexus: L1-L4
      • iiglof (interested in getting laid on friday?)
      • iliohypogastric, ilioinguinal, genitofemoral, lateral cutaneous, obturator, femoral
    • Sacral plexus: L5-S1



Ganglia (presynaptic sympathetic) → Plexus (postsynaptic sympathetic)



1) Superior mesenteric plexus - from lesser thoracic splanchnic

S: Pain of the below
M: Vasculature of midgut (second part of duodenum to ⅔ transverse colon)

2) (paired) Aorticorenal plexus - from  lesser thoracic splanchnic

S: Pain from kidney, suprarenal, upper ureter
M: vasculature of renal arteries

3) Inferior mesenteric - from least thoracic splanchnic

S: Pain from hindgut (descending colon, sigmoid colon and rectum)
M: Sympathetic for hindgut
  • Sourced from
    • Intermesenteric plexus (sympathetic)
  • Consists of network of post-synaptic sympathetic



PS from vagus and pelvic splanchnic

The branches also pass through the preaortic ganglia but do not synapse there as the ganglia are located within walls of viscera

Anterior vagal trunk branches

Posterior vagal trunk branches

Foregut (mouth to second part of duodenum) rotation

  • right vagus nerve of esophageal plexus through esophageal hiatus (T8) of diaphragm to form posterior vagal trunk to supply posterior surface of stomach
  • left vagus nerve forms anterior vagal trunk to supply anterior surface of stomach

Foregut + Midgut (up to splenic flexure) by vagus

Hindgut (distal third of colon to anal canal) by pelvic splanchnic nerves (PS fibers from S2-4)
  • from lateral horn of the spinal cord at the S2, S3, & S4 levels and exit at the ventral primary rami of S2, S3, and S4 to form the inferior hypogastric plexus, located laterally of rectum. These presynaptic fibers synapse in the colon wall on postsynaptic neurons that innervate the gut smooth muscle to cause peristalsis in the hindgut.

Superior hypogastric plexus lies on abdominal aorta below the inferior mesenteric artery, located at the aorta bifurcation

S: Pain from pelvic viscera
M: Sympathetic for pelvic viscera
  • Source
    • Intermesenteric plexus
    • Lower lumbar splanchnic (L3-L4 - postganglion sympathetic)
  • Consists of network of
    • post-synaptic sympathetic
  • Gives rise to r. and l. hypogastric nerve which form the inferior hypogastric plexus
    • Hypogastric nerve
      • Interconnect the superior and inferior hypogastric plexuses and contain no ganglia
      • Conveys majority of sympathetic contribution to inferior hypogastric plexus (mainly superior hypogastric plexus, minor is sacral splanchnic)

Pelvis autonomic nerve

Inferior hypogastric plexus - plexus for pelvis
Located on either side of rectum lying medial to internal iliac vessels
  • Sympathetic
  • from superior hypogastric plexus (no PS [parasympathetic], just S [sympathetic] - from lower splanchnic and intermesenteric plexus)
  • sacral splanchnic
  • post-ganglionic
    • The further a fiber travels away from thoracolumbar region, the more likely it is a postganglionic.
      • Thoracic splanchnic: Primarily preganglionic that synapses in prevertebral ganglion
      • Lower level splanchnics: Postganglionic that have already synapsed
  • Parasympathetic
    • from pelvic splanchnic nerves that leave the lateral horn of S2-4 and leave the ventral horn → spinal nerve → ventral ramus

Rectal plexus: An extension of the posterior portion inferior hypogastric plexus
  • Autonomic for internal anal sphincter surrounding upper ⅔ of anal canal
    • Most of the time IAS (internal anal sphincter) Is contracted
  • Somatic for external anal sphincter for distal ⅓ of anal canal
    • Pudendal nerve (exits greater sciatic foramen): Voluntary EAS (external anal sphincter) is contracted
  • Visceral afferent
  • Hypogastric, Pudendal nerve and pelvic splanchnic

Vesical Plexus- Bladder
  • Anterior portion of the inferior hypogastric plexus
  • Loops around ureter and bladder wall

Visceral afferent stimulates stretch receptors

Parasympathetic fibers (pelvic splanchnics) travel through inferior hypogastric plexus to
  • Contract (stimulatory) Detrusor muscle: M3 (ACh), ATP
  • Relax urethra SM: NO
  • Relax (inhibitory) internal urethral sphincter

Sympathetic travel through inferior hypogastric plexus to
  • Relax detrusor muscle: B3 (NA)
  • Contract internal urethral sphincter: alpha1 (NA)

Somatic in pudendal nerve: ACh
  • Contract external sphincter (skeletal muscle)

Vesical plexus also branches to seminal vesicle, ducturs deferens and ejaculatory duct in male.

Continence: When bladder is filling and sphincters are contracted

Uterovaginal plexus: An extension of the intermediate portion inferior hypogastric plexus

Visceral afferent fibers
  • fundus and body of uterus: sympathetics (hypogastric n. and superior hypogastric plexus) to reach lower thoracic segment of spinal cord
  • upper vagina and cervix: pelvic splanchnics (S2-S4) and pudendal nerve

Greater sciatic foramen gives exit to
  • pirformis
  • 7 nerves (sciatic, posterior femoral cutaneous, superior gluteal, inferior gluteal, pudendal, nerve to the obturator internus, and nerve to the quadratus femoris), and to three groups of vessels (internal pudendal, superior gluteal, and inferior gluteal).

Pudendal nerve (S2-4) block: Anesthetic to the nerve
  • Blocks dermatomes of S2-4 (includes perineum, lower one-fourth of the vagina.)
    • Does NOT block pain felt from body or fundus of uterus as those afferents travel along sympathetic to thoracic region, thus, a mother can still feel contractions and play an active role in labor.
  • Caudal epidural block: Catheter to sacral canal to bathe the sacral nerve roots - pudendal nerve

Prostate plexus: anterior portion of the inferior hypogastric plexus

Innervate prostate gland, the prostatic urethra and the ejaculatory duct, cavernous nerves of the penis (mainly PS - ParaSympathetic)

PS to penis: relaxation of smooth muscle allowing blood to flow into cavernous spaces in the corpora of the penis resulting in erection

S to penis: Ejaculation and vasoconstriction resulting in remission of an erection.

Parasympathetics puts it up and Sympathetics squirts it


Sensory and Movement


Ascending Tracts

all to VPL (Ventral posterolateral) nucleus of thalamus and project to postcentral gyrus of cerebral cortex

Pathway: Dorsal root ganglion and ascend in white matter (anterolateral portion) →  VPL of thalamus →  Post-central gyrus (primary sensory area)

Cross over at spinal lemniscus

Anterior: Ascending collateral pathway ascend white matter
  • CRUDE touch and pressure

Lateral sPinoThalamic: Immediate reflex to motor neuron (PAIN!!!, TEMPERATURE)
  • Pain and temperature
    • A-delta and C fibers
    • C-fibers for itch too!

Transection above decussation causes contralateral loss of pain and temperature below lesion

A fibers (BAD): In decreasing order of diameter and myelination (thus decreasing speed)
- alpha: aa............ proprioception
- beta: b touched!!!
- delta: pain and temp.

Note: Spinothalamic tract is close to sympathetic pathway (ipsilateral!). Therefore a lesion can cause Horner’s syndrome (HORNy PAM) - ptosis, anhidrosis (lack of sweating), miosis

Posterior column
  • Fasciulus gracilis (gracilis/grass is a muscle in lower limb): Sensory for lower trunk!
  • Fasciuculus cuneatus (like cervical!): Sensory for upper trunk
  • From nucleus gracilis/cuneatus to medial lemniscus via internal arcuate fiber
  • Conscious (goes to cerebral cortex) proprioception (unconscious is spinocerebellar), discriminative touch (crude touch = anterior spinothalamic tract!)
    • Pacini
    • Meissner
    • Joint receptor
    • Muscle spindle
    • Golgi tendon organ
  • Transection above decussation: Contralateral loss
  • Trasection below decussation: Ipsilateral loss
Spinocerebellar tract
- Unconscious proprioception (does not go to cerebral cortex!)
- Posterior (shortcut): Ipsilateral
- Anterior: Contralateral
- Second order (and final!) neurons = Mossy fiber

Descending Tracts

Anterolateral corticospinal tract
- Mainly lateral tract - contralateral
- Minor anterior tract - ipsilateral
- NOT fully myelinated until 2nd year (Babinsky’s sign in babies is normal!)


Pyramidal FIBERS (i.e. those from pyramid - still ipsilateral) leave the pyramids and decussate in the anterior median fissure of medulla (pyramidal/motor decussation) which is CAUDAL (below) the pyramids (contralateral)

- Transection above motor decussation i.e. anterior median fissure of medulla: Contralateral
  • e.g. internal capsule lesion in left side -> Right hemiplegia)
- Transection below motor decussation: Ipsilateral

Causes hemiplegia (TOTAL paralysis in one side of body), hemiparesis (weakness only in one side)

Stroke in internal capsule affects corticospinal neuron (i.e. UMN - see below): Sequences as described in spinal shock
PhaseTimePhysical exam findingUnderlying physiological event
10-1dAreflexia/HyporeflexiaLoss of descending facilitation
21-3dInitial polysynaptic -> monosynaptic reflex returnDenervation supersensitivity
  • more receptors for neurotransmitters expressed -> easier to stimulate
31-4wHyperreflexia (initial)Axon-supported synapse growth
41-12mHyperreflexia, SpasticitySoma-supported synapse growth

Upper motor neuron (e.g. corticospinal neuron): From cerebral cortex to spinal cord level
  • Do NOT directly innervate muscle
  • Lesion: NO muscle atrophy

Lower motor neuron (e.g. ventral horn): Spinal cord level to muscle
  • Innverates muscle
  • Lesion: Muscle ATROPHY

Blood supply of spinal cord
  • Spinal artery (from vertebral artery)
    • 1 anterior spinal artery: Anterior ⅔ of cord
      • Occlusion is DANGEROUS → Ischemia causes corticospinal (paraplegia), spinothalamaic (loss of pain and temperature), autonomic
    • 2 posterior spinal artery
      • The posterior spine has extensive anastomoses so occlusion wil only have very little effect
    • Radicular arteries from thoracic and abdominal aorta
      • Great radicular artery of Adamkiewicz: Major vessel to lower ⅔ of spinal cord
        • Occlusion causes paraplegia (paralyzed lower half of body)

J - aorta
I - posterior intercostal artery
A - segmental spinal artery
B - anterior radicular artery
C - posterior radicular artery
D - posterior spinal artery
E - posterior branch of right posterior intercostal artery
F - segmental medullary artery
G - posterior branch of left posterior intercostal artery
H - anterior spinal artery

Does not directly innervate muscle
Synapses to LMN which innervates muscle

Innervated not just by UMN but also by other synapses

Thus UMN damage: does NOT cause muscle atrophy as there are still other connections to LMN

5 senses (眼耳口鼻+senses...)

- Visual
- Auditory, vestibular
- Gustatory
- Olfactory
- Somatosensory - touch, pain, proprioception

Transduction: Conversion from one form  Second messenger (e.g. cAMP) Visual system

Receptive field: Area which (X to chemical) stimuli causes a change in firing of that neuron
Pressure receptors on back: Large (hard to discriminate)

Properties of stimuli
- Type: determines pattern of firing
- Duration
- Strength: The stronger, the higher the frequency of action potential (not strength!)
  • Maximum AP reached and then recruit more nerve fibers (principle of motor unit recruitment...)

- Location: Topographic

Special Receptors

Golgi tendon organ
Golgi tendon organ (Simple Ib and Gr... Inhibitory)
  • proprioceptive sensory receptor organ located in the tendons of skeletal muscle.
  • consists of collagen fibers intertwined with Ib afferent neurons
    • When muscle contracts, collagen fibers stretch, thus the stretch-sensitive cation channels in Ib afferent neurons open, causing the axon to be depolarized and to fire action potential.
  • Disynaptic
    • Autogenic inhibition: Synapses with inhibitory interneuron x alpha motor neurone of that skeletal muscle (grrr..... angry dog.. inhibitory and drop!)
    • Reciprical stimulation: Synapse with excitatory interneuron x alpha motor neuron of ANTAGONISTIC skeletal muscle

Muscle spindle: Involved in stretch reflex (muscle contraction in response to muscle stretch). It is a monosynaptic (imagine tendon tap.. no need to think about it!) and disynaptic reflex
  • Extrafusal fiber: Entire portion is contractile, surrounds intrafusal
  • Intrafusal fiber
    • Contractile portions only at ends
    • Central non-contractile: Surrounded by sensory afferents
      • 1a: Primary (Centrally Wraps around)
      • 2: Secondary (On both ends of primary)

  • Monosynaptic: DIRECTLY synapse to alpha-MN for agonist (extrafusal) muscle (e.g. tendon tap -> femoral nerve -> quadricep femoris)
  • Disynaptic: Synapse with Inhibitory Interneuron -> alpha-MN to antagonist muscle (e.g. tendon tap -> interneuron -> sciatic nerve for hamstring) RECIPRICOL INHIBITION

- Alpha-motor neurone (extrafusal), makes up 70% of ventral root
  • Active muscle contraction (but if only alpha-MN is activated → only extrafusal muscle contracts, the muscle spindle located in intrafusal muscle does not get stretched - in fact it gets slack so both afferent neurons stop firing to continuously stimulate the alpha-MN (Motor Neurone)

- Gamma-motor neurone (extrafusal): Involved in alpha-gamma coactivation, makes up 30% of ventral root
  • Gamma-MN innervates the contractile portions of muscle spindle (intrafusal) only (i.e. not center)
  • Activated at the same time as alpha-MN is
  • Thus when Gamma-MN is stimulated, the ends contract, causing the central portion of the spindle to be stretched
    • Stretch-sensitive ions channel -> Sensory afferents continue to fire!!!

Passive muscle stretch: Tendon tap
Active muscle stretch:

Iso means EQUAL (e.g. isotope)

Types of active contractions:
  • Isotonic: Tension during contraction remains the same but muscle length changes
    • concentric: muscle tension exceed resistance so muscle shortens
    • Eccentric: Muscle elongates while under tension as the opposing force is greater than force generated by muscle (act to decelerate the joint)
  • Isometric: Tension during contraction increases but length of muscle remains the same
    • keep body upright!

Autonomic System

  • Cell body of preganglionic neuron in lateral grey horn from T1-L2 -> Preganglionic sympathetic fibers leave the spinal cord via the anterior horn of the spinal cord and leaves via preganglionic white ramus  (unmyelinated) and either:
  • synapse in the sympathetic chain at the same level. by going out using grey ramus (unmyelinated)
  • ascend or descend in the sympathetic chain prior to synapsing at another level.
  • pass through the sympathetic chain without synapsing (greater, lesser, least splanchnic nerve) and travel to prevertebral ganglia (eg. celiac, superior mesenteric, inferior mesenteric, aorticorenal)

Cell bodies of the postganglionic sympathetic fibers:
  • Paravertebral ganglia (sympathetic chain/trunk)
  • Prevertebral ganglia (if no synapse at paravertebral → celiac, superior mesenteric, inferior mesenteric ganglia, aorticorenal)

Sympathetic pathway - IPSILATERAL!

- Anterior: Parasympathetic
- Posterior: Sympathetic

Hypothalamospinal tract: Hypothalamus to spinal cord
  • From hypothalamus down to T1 -> Paravertebral ganglion - superior cervical ganglion -> Carotid plexus to join V1 of Trigerminal nerve -> from nasociliary nerve to long ciliary nerve to innervate ciliary body, dilator pupillae and superior tarsal muscle

Horner’s syndrome (Horny PAM)
  • No sympathetic (superior cervical ganglion -> Long ciliary nerve -> Dilator pupillae and ciliary body) - cause: Damaged spinothalamic tract - lesion close to sympathetic
  • Uncontrolled PS!
    • Miosis (Pupil dilation: Dilator pupillae - sympathetic)
    • Anhydrosis (Lack of sweating: M3 - sympathetic)
    • Ptosis (Droopy eyelid: Superior tarsal muscle/Muller’s muscle - SM - sympathetic)
    • Vasodilation (Alpha-adrenoceptor causes vasoconstriction- sympathetic) - hot face

Terms to Know
Ptosis: Droopy eyelid
Proptosis: Stuck out eye
Anhydrosis: Dry sweat glands
Miosis: Pupil constriction
Mydriasis: Pupil dilation
Cell bodies of preganglionic parasympathetic fibers:
  • Nuclei associated with CN (cranial nuclei) 3, 7, 9, 10 in midbrain
  • S2 3 4 → Pelvic splanchnic nerve to ganglia (contain preganglionic parasympathetic)

Inferior hypogastric plexus: Provides autonomic control within pelvis, found on either side of rectum lying medial to internal iliac vessel. Sympathetic contribution stems from superior hypogastric plexus (PURE sympathetic - from lumbar from intermesenteric plexus [nerve plexus on the abdominal aorta, between the exits of the superior and inferior mesenteric artery] and from lumbar splanchnic nerve.

intermesenteric plexus lies along the abdominal aorta between the superior and inferior mesenteric arteries. It is primarily formed by the upper lumbar splanchnic nerves (L1 and L2), but also receives fibers from the aorticorenal and superior mesenteric ganglia. However, it is important to note that it does not contain any vagal parasympathetic fibers - vagal fibers do not descend, on the surface of the aorta, below the superior mesenteric and renal plexuses. That is why the hindgut, supplied with blood by the inferior mesenteric artery, is also supplied with parasympathetic nerves via the pelvic splanchnic nerves.
The superior hypogastric plexus is bifurcation of the aorta, descends into the pelvis and bifurcates as the right and left hypogastric nerves and connect to the inferior hypogastric plexuses. The hypogastric nerves convey the majority of the sympathetic contribution from the superior hypogastric plexus into the inferior hypogastric plexuses.
The superior hypogastric pleux has contributions to the ureter and epididymis.

Pelvic splanchnic nerve: from S2-4 ventral rami that emerge from ventral sacral foramina, giving preganglionic parasympathetic fibers.

The pelvic splanchnic (PS) joins the lumbar splanchnic nerves in superior hypogastric plexus to form inferior hypogastric plexus (i.e. contain both PS and S)

Pelvic splanchnic provides PS to SM and glands in pelvis and perineum
- Empty bladder (detrusor), peristalsis and glandular secretion in hindgut, sense feces and facilitate its emptying by relaxing internal anal sphincter and contraction of ampulla in rectum, erection (dilation of helicine arteries), secretory of prostate, seminal vesicle
whose cell bodies are located in a small lateral horn of gray matter in spinal cord levels S2, 3, and 4.

Defecation reflex
Peristalsis via myenteric (Auerbach) plexus [Meissner is submucosal]
  • Mass movement stimulated by food in stomach and chyme in duodenum
  • Local reflex stimulated by feces in rectum
Above pectinate line
superior rectal artery
superior rectal vein
internal hemorrhoids (not painful)
inferior hypogastric plexus
Below pectinate line
Stratified squamous
middle and inferior rectal arteries
middle and inferior rectal vein
external hemorrhoids (painful)
inferior rectal nerve

Visceral afferent fiber: Distended rectum and anal canal above pectinate line to sacral spinal cord via pelvic splanchnic
Pelvic splanchnic: Peristalsis - stimulate contraction of smooth muscle (SM) in ampulla of rectum
Internal and external anal sphincters inhibited to allow emptying
  • Internal sphincter: Circular involuntary smooth muscle
  • External sphincter: Striated voluntary muscle

Cell body of postganglionic parasympathetic fibers:
  • Ganglia close to target organ to contribute inferior hypogastric plexus

Plexus: Large collection of sympathetic and parasympathetic
  • thorax: cardiac, pulmonary, esophageal
  • abdomen: aorta and its branches (superior mesenteric, inferior mesenteric)
  • Pelvis: Superior and inferior hypogastric plexus
    • superior: plexus of nerves below the bifurcation of the abdominal aorta in front of the last lumbar vertebra and the promontory of the sacrum
    • inferior: autonomic control within the pelvis located on either side of rectum

Ventral root -> white (myelinated) ramus communicans in only T1-L2 (provides connection between pre and post-ganglionic) -> grey ramus communicantes (unmyelinated)  at all spinal nerve levels

Intermediate Grey Matter: Nucleus dorsalis, T1-L2 preganglionic sympathetic (Intermediolateral column - the pointy lateral horn -> axons leave through the ventral roots), S2 to S4 (sacral PS but insufficient to form distinct lateral horn -> leaves via ventral root)
Marginal zoneAllISome spinothalamic tract cells
Substantia gelatinosaAllIIModulate pain and temperature
Body of posterior hornAllIII-VISensory processing
Nucleus dorsalisT1-L2/L3VIIPosterior spinocerebellar tract cells
Intermediolateral columnT1-L2/L3VIIPreganglionic sympathetics
Sacral parasympathetic nucleusS2-S4VIIPreganglionic parasympathetics --> pelvic viscera
Accessory nucleusMedulla-C5IXMotor neurons --> trapezius and sternocleidomastoid
Phrenic nucleusC3-C5IXMotor neurons --> diaphragm

Enteric Nervous System
  1. Smaller is the submucosal plexus (of Meissner) - stimulate secretion; absent in esophagus and stomach
  2. The myenteric plexus (of Auerbach) is large, between inner circular and outer longitudinal and muscularis externa; found throughout the entire gastrointestinal tract - motility (persistalisis)

ENS neurones come from the neural crest.


Detrusor muscle arranged in spiral, longitudinal & circular bundles
  • smooth lining of bladder at trigone
  • rugae

Ureters run obliquely through detrusor muscle

internal sphincter – formed from detrusor muscle at bladder neck
  • smooth muscle

external sphincter
  • skeletal muscle: voluntary control

Storage stage: Sympathetics postganglionic neurons release NE
  1. Stretch receptors in pelvic splanchnic, pudendal and hypogastric nerve → activate bladder storage center in CNS
  2. Sympathetic from T11-L2 from lateral grey horn via hypogastric plexus
    1. Alpha-receptors1 in bladder neck (internal sphincter): contracting the internal sphincter
    2. Beta-receptors3 in bladder wall: relax detrusor
    3. Compresses oblique entry point  of ureter to prevent urine reflux

Threshold (200mL) reached: Perception of bladder fullness and pain fibers to thoracic and lower lumbar splanhnic - referred pain to T11-L2 dermatome

Emptying stage: PS dominated and Somatic

PS: Postganglion release ACh
  • CNS miturition center activated: Pelvic splanchnic from S2,3,4 to ganglia → postganglion facilitate detrusor contraction M3

Somatic nicotinic ACh
  • Pudendal nucleus of S2-4 to external urethral sphincter (somatic control)
  • Abdominal muscles contract to increase intrapelvic pressure of bladder
  • Infants not fully developed corticospinal connections


Glutamate - principle excitatory neurotransmitter in brain (over half of all brain synapse releases this)
  • doesn't cross BBB (blood brain barrier) thus neurons must synthesize it (from glutamine -(glutaminase) -> Glutamate and NH3)
  • glutamine is released from glial cells and enter presynaptic terminal where it is converted to glutamate and packaged into vesicle via VGLUT.

Upon depolarization of the presynaptic terminal, glutamate is released. It is recycled via:
  1. glutamate taken up by glial cell
  2. converted to glutamine by glutamine synthase into glutamine
  3. glutamine again back into presynaptic terminal

Glutamate receptors are Ionotropic, all of which are nonselective cation channels:
  • AMPA - Mostly Na+, works at low receptor potentials
  • NMDA - Mg2+ block but when postsynaptically sufficiently depolarized, it is rejected allowing huge calcium influx (plasticity - long term potentiation)
    • Both glutamate AND glycine must be bound to the receptor

Glutamate excitotoxicity
Ischemic cascade →  ATPase fails → Cell depolarized →  Voltage-gated calcium channels open → Calcium influx →  Glutamate release to synaptic cleft → Glutamate binds to post-synaptic neuron →  Calcium influx →  Free radical, ROS (reactive oxygen species), endonuclease, ATPase, phospholipase [mitrochondrial breakdown release apoptotic factors (caspase dependent)] → allow even more calcium influx!

ModalityTypeFiber type
TouchRapidly adapting cutaneous mechanoreceptors (Meissner corpuscle end-organs
Pacinian corpuscle end-organs
hair follicle receptors
some free nerve endings)
Aβ fibers
Touch & pressureSlowly adapting cutaneous mechanoreceptors (Merkel and Ruffini corpuscle end-organs
some free nerve endings)
Aβ fibers (Merkel and Ruffini's), Aδ fibers (free nerve endings)
VibrationMeissners and Pacinian corpuscle end-organsAβ fibers
TemperatureThermoreceptorsAδ fibers (cold receptors)
C fibers (warmth receptors)
PainFree nerve ending nociceptorsAδ fibers (Nociceptors of neospinothalamic tract)
C fibers (Nociceptors of paleospinothalamic tract)

Mnemonic for basis of difference: STORM Baby

Also remember: In a Lower motor neuron lesion everything lowers

Basis of Difference (STORM Baby)UMNLLMNL
S = StrengthLowersLowers
T = ToneIncreases (spastic)Decreases (flaccid)
O = OthersSuperficial reflexes absent
Reaction of degeneration
R = Reflexes = DTR or Deep tendon reflexesIncreasedDecreased
M = Muscle MassSlight loss onlyDecreases / Atrophy
Baby = Babinski SignPositive (toe up)Negative (toe down)