A sketch of the central nervous system and its origins

G. E. Schneider 2009Part 4: Development and differentiation, spinal level

MIT 9.14 Class 8

CNS structure at the spinal level; autonomic and enteric nervous systems

Survey of adult human spinal cord

• Different levels, illustrated • The sensory channels (reflex, spinocerebellar

and spinothalamic tracts, origin of dorsalcolumn axons)

• Major descending pathways (cortico-, rubro-, reticulo-, and vestibulo-spinal)

• “Propriospinal” fibers.

Left: Internal

structure ofspinal cord

Right:

Levels, rostral to caudal

INTERNAL STRUCTURE OF THE SPINAL CORD

Dorsal RootFibers

Fasc.Gracilis

Nuc cornucommissuralis PosteriorSubstantia Gelatinosa

Nuc. Posteromarginalis

Zone of LissauerLat. Corticospinal Tr.

Nuc. Proprius Cornu DorsalisNuc. ReticularisNuc. Dorsalis (of Clarke)

Ant. Spinocerebellar Tr.Lat. Spinothalamic Tr.

Nuc. Motorii LateralisFasc. Proprius

Ant. Spinothalamic Tr.Vestibulospinal Tr.

Nuc. Motorii MedialisVentral Root Fibers

Internal Structure

Spinal Cord Segment C1

Segment C5

Segment C8

Segment T2

Segment T10

Segment L1

Segment L4

Segment S4

III

IIIIV

V

VI

VII

VIIIIX

IXIX

IX

X M

Figure by MIT OpenCourseWare.

Different levels, illustrated: Note the following things

• Gray vs. white matter. Gray matter: dorsal and ventral horns

• Changes in amount of white matter, rostral to caudal– More and more descending axons leave the white matter – More and more ascending axons join the white matter

• Cervical and lumbar enlargements – See Striedter, p. 222, for Brontosaurus cord

• Presence of “lateral horn” in thoracic and upper lumbar cord

Comparisons, speculations

• Myelin – “A crucial vertebrate innovation” (Allman p. 78):

Why? – Not found in any invertebrate or in the jawless

vertebrates (hagfish, lampreys) • How does spinal cord of humans differ from

spinal cords of other mammals?

Spinal cord cross sectionLumbar level

Note the laminae of Rexed. See Per Brodal, p. 80f.

Figure by MIT OpenCourseWare.

Zona Terminalis

Substantia Gelatinosa

IIIIII

IV

V

VI

VII

VIIIIX

Drawing based on cell - body stain

Note the laminae ofRexed.

Survey of adult human spinal cord

• Different levels, illustrated • The sensory channels (reflex, spinoreticular &

spinothalamic, and spinocerebellar tracts; origin of dorsal column axons)

• Major descending pathways (reticulo-, vestibulo-, rubro-, and corticospinal)

• “Propriospinal” fibers

Termination of Dorsal Root Fibers

Again, note the laminae of Rexed.

Adult spinal cord, schematic frontal section:reflex and lemniscal channels

Spinothalamic tract

Dorsal columns

Dorsal root fibers of various sizes

Adult spinal cord, schematic frontal section:reflex and lemniscal channels

Dorsal root fibers of various sizes

Spinothalamic tract

Dorsal columns

Spinoreticular axons

Remaining from very early chordates: Spinoreticular fibers (on right in blue)

Ascending reticular projections tointralaminar thalamic nuclei andhypothalamus

Superior colliculus

Red nucleus

Midbrain

interior colliculus

Medial lemniscus Tegmentum of midbrain

Collateral endings inreticular formation

Ascending collateral fibers

Collateral endings inreticular formation

Facial nerve

Abducens nerve

Ascending collateral fibers

Pontine reticular formation(nucleus reticularis pontiscaudalis)

Medullory reticular formation(nucleus reticularisgigantocellularis)

Medulla

Pontine reticulospinal fibers

Medullary reticulospinal fibers Ascending spinoreticular fibers

Hypoglossal nerve

Lateral reticular nucleus

Descending nucleus and tractof trigeminal nerve

Pontine reticular formation(nucleus reticuloris pontis oralis)

Trigeminal nerve

Pons

Ascending reticular fiber system

Figure by MIT OpenCourseWare.

Spinoreticular axons include a few “spinothalamic” axons

Ascending reticular projections tointralaminar thalamic nuclei andhypothalamus

Superior colliculus

Red nucleus

Midbrain

interior colliculus

Medial lemniscus Tegmentum of midbrain

Collateral endings inreticular formation

Ascending collateral fibers

Pontine reticular formation(nucleus reticuloris pontis oralis)

Trigeminal nerve

Pons

Ascending reticular fiber system

Figure by MIT OpenCourseWare.

At the rostral end of the spinal cord:

The dorsal column nuclei, namely, the gracile and cuneate nuclei (nuc. gracilis and nuc. cuneatus)

What sensory cell group is found just lateral to these nuclei in the caudal hindbrain and upper cervical spinal cord? With these cells, there is a representation of the entire body surface at this level around the junction of spinal cord and hindbrain.

The body surface represented at the hindbrain-cord boundary

Cerebellar channel: Clarke’s Column, illustrated at 5th thoracic segment (Nauta & Feirtag, Fig. 64)

t ino jlexmpo cn ofoInr ewnts from lovememo

unk rlimbs & t

Figure by MIT OpenCourseWare.

Survey of adult human spinal cord

• Different levels, illustrated • The sensory channels (reflex, spinocerebellar and

spinothalamic tracts, origin of dorsal column axons) • Major descending pathways (cortico-, rubro-,

reticulo-, and vestibulospinal) • “Propriospinal” fibers.

Adult spinal cord:some descending and intrinsic axons

Proprio-spinal Corticospinal axons axons

(after their decussation)

Rubrospinal axons

Reticulospinal, Vestibulospinal, Fastigiospinal (from Cb), Tectospinal

Corticospinal axons, uncrossed (variable in quantity)

Ascending and descending long axons of primate spinal cord

Proprio- Ascending spinal axons axons

Ascending and descending axons

Corticospinal axons (after their decussation)

Rubrospinal axons

Reticulospinal, Vestibulospinal, Fastigiospinal (from Cb), Tectospinal

Corticospinal axons, uncrossed (variable in quantity)

Intermission: The ventricular system; the meninges and glia

• Remember: the origins of the ventricle in the formation of the neural tube

• The importance of the cerebrospinal fluid in the mature CNS:– Nutrients

– Fluid balance regulation via specific cell regions

– Also a communication medium (because of chemical secretions into it and diffusion from it)

• Where the fluid is made and how it flows:

Ventricular system: The foramena of Luschka (lateral apertures), and the foramen of Magendie (median aperture)

Choroid plexus: specialized ependymalcells which make cerebrospinal fluid

Interventricular Foramen

Choroid Plexus of Lateral Ventricle

Choroid Plexus ofFourth Ventricle

Central Canal

Median Aperture

Lateral Aperture

Aq

Choroid Plexus of Third Ventricle

Figure by MIT OpenCourseWare.

The Meninges

1. Define "dura mater" and "pia mater": meaning of the Latin terms, and basic anatomy.

2. Define "arachnoid membrane" and "subarachnoid space".

See Nauta & Feirtag, ch. 10; also P. Brodal, ch. 1, and other texts

Meninges & Glia (P. Brodal, ’98, p. 19)

Figure by MIT OpenCourseWare.

Picture taken with transmission electron microscope (EM): Astroctyes, pial cells, subarachnoid space(Peters, Palay & Webster, 1976)

SS = subarachnoid space PM = pial membrane Col = collagen fibers SM = smooth muscle GL = glia limitans (astrocyte processes) B = basal lamina As = astrocyte arrows, lower fig: attachment points

Figures removed due to copyright restrictions.

Autonomic nervous system (ANS)

• Overview of functions • Schematic overview of structure • Formation of sympathetic ganglia from the

neural crest • Sympathetic innervation pattern (thoracico-

lumbar system) • Parasympathetic innervation (cranio-sacral

system); dual innervation of smooth muscles and glands.

• Chemical mediation at synapses (first discovered by Otto Loewi in 1921).

Important functions of some autonomic pathways

PARASYMP. FUNCTIONS SYMPATHETIC FUNCTIONS GLAND, TISSUE

Constricts pupil (miosis) Dilates pupil (mydriasis) Iris

Stimulates secretion Little effect on secretion Lacrimal gland

Secretion reduced; less watery Secretion increased; watery Salivary glands

Little effectStimulates secretion (ACh) Sweat glands Constricts Dilates the lumen Lungs, bronchi

Slows heart rate Speeds heart rate; increased Heart ventricular contraction

Stimulates motility, secretions Inhibits motility & secretions Stomach, intestines Relaxes Constricts except with very Anal sphincters

intense activation Vasodilation, engorgement of Orgastic contraction of ductus Sex organs erectile tissue deferens, seminal vesicle, prostatic or

uterine muscles; vasoconstriction

Contracts bladder, relaxes Relaxes wall of bladder; Urinary bladder sphincter, promotes emptying constricts internal sphincter,

inhibits emptying

Important functions of some autonomic pathways

GLAND, TISSUE SYMPATHETIC FUNCTIONS PARASYMP. FUNCTIONS

Adrenal medulla Stimulates secretion Little or no effect Blood vessels, trunk & extremities

Constricts ---

Autonomic pathways: schematic of structural arrangements

(From Truex & Carpenter textbook, p.

s

237)

Note the CNS locationof the preganglionic motor neurons of the two divisions of the ANS.

Figure by MIT OpenCourseWare.

Another schematic view of ANS (Nolte, ’88 textbook, p. 253)

Figure by MIT OpenCourseWare.

Autonomic pathways:

a simpler schematic view (partial)

Step back a moment: Arrangement of motor neurons in the three major divisions of the motor system (Swanson, 2003, p. 99)

Somatic: Synaptic Autonomic: Paracrine Neuroendocrine: Endocrine

S

E

S P

Figure by MIT OpenCourseWare.

Arrangements within the ANS: Autonomic (paracrine) innervation(P. Brodal & others)

GANGLION

GANGLION

PREGANGLIONIC POSTGANGLIONIC

CNS

EFFECTOR

SYMPATHETIC

PARASYMPATHETIC

Figure by MIT OpenCourseWare.

Autonomic innervation of the intestine in several vertebrate classes.

From Bullock (1977), based on Burnstock (1969)

Fish

Fish

Reptiles

Amphibians Mammals

CholinergicAdrenergic

8th

Figure by MIT OpenCourseWare.

More on the ANS next time

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9.14 Brain Structure and Its Origins Spring 2009

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