spinal cord - massachusetts institute of technology€¦ · a sketch of the central nervous system...

Two Terms:

• "notochord" vs. "notocord" – (either one is accepted by me, but 1st is proper)

• "spinal chord" vs. "spinal cord" – (only the latter is proper)

A sketch of the central nervous system and its origins

G. E. Schneider 2009Part 3: Specializations in the evolving CNS;

introduction to connection patterns

MIT 9.14 Class 5The ancestor of mammals: Sketch of brain

with some basic pathways

Introduction:

Does "ontogeny recapitulate phylogeny"? (Can development tell us something about evolution?)

• Early in development, the human embryo looks nearly the sameas a pig embryo as well as the same as a monkey or ape embryo. At later stages, the divergence from pig is more evident, while the similarity to monkeys and (finally) apes remains. – It appears that the earlier the stage of embryonic development, the

greater the resemblance of different species, excluding the very early (gastrula) stages.

– Ernst Haeckel (in 1876) presented an oversimplfied drawing of this,which became very well known.

• The next slides illustrate this. The first shows the similarities first noticed by Karl Ernst von Baer (1828). The second is basedon Haeckel’s drawing.

“Ontogeny recapitulates phylogeny” from Romanes, 1901

First noticed by von Baer (1828)

Figure by MIT OpenCourseWare.

“The Developmental Hourglass” (Ernst Haeckel)

From Striedter (2005), fig. 3.11, p. 78

Ernst Haeckel

1834-1919

Early Gastrula Stages

"Phylotypic" Stage

Adults


Development does not strictly recapitulate evolutionary history, but the pictures nevertheless lead us to expect some similarities in the CNS of all vertebrates; these have indeed been found. An outline of CNS organization in animals resembling the mammal-like reptiles that were the ancestors of mammals prepares the student for understanding mammals as well.

NEXT: • Schematic outline of connections in pre-

mammalian brains • Interlude: Examples of evolution of non-olfactory

sensory specializations in various vertebrates

Schematic diagrams of the CNS • Our sketch of a brain that represents animals ancestral

to mammals is similar to one used in lectures by W.J.H. Nauta at MIT (mid 1960s through 1970s).

• Later we will illustrate the mammalian additions to the ancestral brain.

• (These schematic diagrams have been called the “Shmoo” brains. This name comes from a light-hearted student of Nauta’s who noticed the resemblance of his diagrams to a comic strip creature invented by the cartoonist Al Capp.)

Keep in mind the logic of evolution that brought about the changes in bodies and brains that led to mammals, including humans. This evolution occurred through a long series of adaptations focused on the basics of behavior enabling survival (as discussed in the previous class).

The schematic brain diagrams: two perspectives

• “Shmoo 1” could represent a generalized amphibian brain,based on the work of C.J. Herrick and more recent neuroanatomists.

• From the perspective of evolution, it could represent aCynodont of the Jurassic period, ancestral to the mammals(see Allman ch 5).

• Relevance to mammals, including human: This is the “old chassis” upon which new parts were built in evolution. Little is really discarded. (You still have that ancestral brain in the coreof your human brain!) – Note also that “old” is a relative term. The amphibian brain, for example, is

very advanced in comparison to brains of more primitive chordates.

• Later we will discuss in more detail some of the evolutionary transitions that gave rise to major changes in the CNS.

Cynodonts:Such animals existed from the late Permian through the

Triassic, the Jurassic, and into the mid Cretaceous period

Figures removed due to copyright restrictions.

Cynognathus

Cynodontia: Increasingly small-sized proto-mammals with higher metabolism. Range: from the Late Permian. It is currently unknown whether mammal-like reptiles possessed mammalian characteristics like body hair and mammary glands, as the only real evidence is provided by fossils that to date only suggest differences in skeletal structure. From web sources, 2007-8: www.palaeos.com; en.wikipedia.org/wiki/Mammal-like_reptiles See Allman on cynodonts in his ch.5.


220 One sub-group of therapsids, the cynodonts have evolved more mammal-MYA like characteristics.

The jaws of cynodonts resemble modern mammal jaws more closely and their teeth are multi-cusped and differentiated down the jaw. Cynodonts are the direct ancestors of all modern mammals.

Repenomamus

From eucynodonts (cynodonts) came the first mammals.Most early mammals were small and shrew-like animals that fed oninsects. Constant body temperature. All mammals have milk glands for their young.Neocortex has evolved in mammals. This brain region is unique tomammals.The earliest mammals include:•Eozostrodon: Triassic and Jurassic •Deltatheridium: Cretaceous •Jeholodens: mid-Cretaceous •Megazostrodon: late Triassic and early Jurassic •Triconodont: Triassic to Cretaceous •Zalambdalestes: late Cretaceous

Schematic outline of connections in such vertebrate brains

topics

• Basic subdivisions & neuron types – Introduced with schematic brain diagrams – We will identify brain regions and review various

definitions.

• Sensory channels of conduction • Overview of forebrain structures • Neocortex and its elaboration in mammals

To be identified:

• Subdivisions of the CNS • Primary sensory neurons • Secondary sensory neurons • Interneurons of the great intermediate net• Motor neurons:

– Somatic nervous system – Autonomic nervous system

• Cranial nerves: 1 and 8

subdivisions. Learn which is which!

Schematic of pre-mammalian brain

(Shmoo #1)

Study the names of these

Dorsal cortex; limbic cortex

The vertebrate brain archetype (Striedter, 2005, p. 66)

(Striedter includes some additional details)

Dorsal view

Side view Pallium = cloak (L.)

Tectum = roof Tegmentum = covering

Medulla = inner core


a. Spinal cord

b. Hindbrain (rhombencephalon)

c. Midbrain(mesencephalon)

d. ‘Tweenbrain (diencephalon)

e. Endbrain(telencephalon)

a. Spinal cord

b. Hindbrain (rhombencephalon

c. Midbrain (mesencephalon)


e. Endbrain (telencephalon)

a. Spinal cord





A MORE REALISTIC VIEW:

The thickening embryonic neural tube in proto-mammals (or in a mammalian embryo)

To be identified:




REVIEW Primary somatosensory neurons in an animal series

(from Ramon y Cajal)

1) Find one like this in the Shmoo brain (and mammals).

2) Fithe Smam

Sensory cell of the earthworm

Sensory cell of a mollusc

Sensory cell of a lower fish

Sensory cell of amphibian, reptile, bird, or mammal

nd one like this in hmoo brain (and mals)

3) Find one like this in the Shmoo brain (and mammals)


Sketch of a pre-mammalian brain

Note: Many axon were much more w branching than shown in this schema

To be identified:




Sketch of a pre-mammalian brain

Sensory channels of conduction:

• Local reflex • Lemniscal 1 • Lemniscal 2 (cerebellar)

• Terms: segmental, intersegmental, suprasegmental, dermatome, myotome, reticular formation, thalamus, decussation, cranial nerves.

Locate a local reflex channel. What function might such a pathway serve?

How do we define “local” in “local reflex”?

• In the periphery: the dermatome • In the CNS: the segment

REVIEW:

The gross anatomy: A young human

from N. GluhbegovicandT.H. Williams,1980(Harper & Row)

Note: dura mater; Spinal nerves vs CNS

Positions ofvertebrane


Dermatome map

C6

C3

C3

C5T4

T4

T5

T5

T6

T6

T7

T7

T8

T8

T9

T9

T10

T10

L1

L1

L2

L2

L3

L3

L4

L5

T1

T2

T2

C8C7

T11

T11

T12

T12

C2

C2

C4

C4

S3

S4

L5

T2

T3

T3

C4

T2

T1C6

C7

C8S3

S5

S4

S2

L3

L4L5

S1


Dermatome map, quadruped position, & spinal segments


How were the dermatomes mapped?

• Hypersensitive regions, resulting from irritation of single spinal roots, e.g., from a herniated intervertebral disk.

• Remaining sensibility, after severance of adjacent spinal nerves or dorsal roots.

• Cf. “myotome”

Sensory channels of conduction:• Local reflex • Lemniscal 1

– 1a (spinoreticular) – 1b (spinothalamic)

• Lemniscal 2 (spinocerebellar)


The oldest lemniscus: Spinoreticular

• A rostral extension of “propriospinal” fibers • Bilateral projections (largely ipsilateral) to the core

of the brainstem, the “reticular formation” • Functions (most likely):

– Sensory modulation of autonomic activity (e.g., heart rate, blood pressure, breathing rate and volume)

– Autonomic and defensive behavioral responses to pain inputs

– Temperature regulation – Inputs for control of sexual behavior patterns

• This is the least studied of the somatosensory pathways

Color coded pathways:

On right side: Spino-reticular fibers

On left side: Pontine reticulo-spinal fibers. Medullary reticulo-spinal fibers.

From M. B. Carpenter textbook

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


Some data on the most primitive living vertebrates

Comparison of a cephalochordate (top: Amphioxus) and two jawless vertebrates: (middle) Hagfish (bottom) Lamprey

o = olfactory bulb t = telencephalon d = diencephalon m = mesencephalon

Amphioxus (Branchiostoma)

1 cm

5 cm

Hagfish (Myxine)

Lamprey (Petromyzon)

5 cm

o

o

t

t

d

d

m

m


Cladogram of vertebrates

From Striedter (2005), p. 54,based on multiple sources

Vertebrates

Jawed Vertebrates

Cartilaginous FishesBony Vertebrates

Lobe-Finned VertebratesRay-Finned Fishes

Amph

ioxu

sHa

gfish

esLa

mpr

eys

Shar

ks, R

ays

Ratfi

shes

Amni

otes

Salam

ande

rsFr

ogs,T

oads

Coela

cant

hLu

ngfis

hes

Poly

pter

usSt

urgeo

nsGa

rsTe

leosts

Amia

Plac

ental

sM

arsu

pials

Mon

otre

mes

Liza

rds,

Snak

esTu

rtles

Bird

s

Croc

odile

s

Osteo

glos

som

orph

sEe

ls

Herri

ngs

Pike

sOs

tario

phys

ans

Neot

eleos

tsSa

lmon

SauropsidsMammals

Amniotes Teleosts

CLADOGRAM OF VERTEBRATES

Tetrapods


Notes on Amphioxus: brain and somatosensory inputs

• Gene studies indicate that this little creature is not brainless as once believed.

• Recent studies have indicated inputs from the body surface that enter the CNS and contact cells that distribute axons to both sides (like the spinoreticular pathways of vertebrates).

• (Earlier, we have noted evidence for visual inputs to a forebrain.)

Somatosensory pathways in hagfish and sea lampreys:

• Some secondary sensory neurons in the spinal cord have axons that extend rostrally, mostly on the ipsilateral side, reaching the brainstem.

• These axons are like the spinoreticular axons of mammals

• There is no apparent pathway ascending on the contralateral side to the brain.

Early evolution of a crossed pathway• We call a crossed lemniscal pathway from the

spinal cord the “spinothalamic tract”, although most of its axons never reach the diencephalon, as they terminate in the hindbrain and midbrain.

• It is sometimes called the “paleolemniscus” as it appears to have evolved prior to the mammals. However, the spinoreticular axons appear to be of more ancient origin.

• Why do the axons decussate? – The evolutionary origins of crossed representations

will be considered when we study the hindbrain and the visual system.

– My suggestion/hypothesis concerning this topic was introduced in the previous class. [See class 4, slide 20]

The “paleolemniscus” or “spinothalamic” tract

(Shmoo 1, side view)

Where is the spinothalamic tract in a top view?

The thickening embryonic neural tube in proto-mammals (or in a mammalian embryo)

a. Spinal cord

b. Hindbrain (rhombencephalon)




a. Spinal cord





a. Spinal cord





“Spinothalamic tract”

Note terms: decussation, reticular formation, thalamus (in ‘tweenbrain), endbrain.

Remember:

• We are not representing the earliest chordates here, but rather the mammalian brain.

• There was a long period of evolution between those early chordates and the ancestors of mammals.

Suggestion: • The spinothalamic tract probably evolved as a

specialization of a portion of the widely branching spinoreticular axons.

Questions for research on primitive somatosensory functions

• What can vertebrate animals do in the absence of somatosensory pathways to the forebrain?

• …or in the absence of all crossed ascending pathways (from the cord)?

Sensory channels of conduction:• Local reflex • Lemniscal 1

– 1a (spinoreticular) – 1b (spinothalamic)

• Lemniscal 2 (spinocerebellar)


Intro to the “cerebellar channel”, another kind of lemniscus

• A problem of multiple sensory pathways for control of the same outputs: Timing

• This problem increased with increasing body size, and increased role of head receptors in control of movements.

• It increased further with need for precise coordination of larger numbers of muscles, especially the distalmuscles of the limbs.

• Cerebellum appears to have evolved to deal with this problem: adjustments of relative timing which could be varied according to feedback.

• See the more specific but related suggestion by Allman(2000), p. 77-78: stabilizing the retinal image by comparing eye velocity with head velocity.

Cerebellum

Notes on evolution of cerebellum• The basic cell types and circuitry are highly

conserved in evolution – Layering becomes more regular – New inhibitory cell types in tetrapods – Deep nuclei as the output structures appear in tetrapods

• The size and form vary considerably – Huge expansion in the weakly electric fishes, especially in

the hyperfolded cerebellum of mormyrids – Dramatic lateral expansion and foliation as locomotor and

manipulatory abilities increased in land animals: the growth of the cerebellar hemispheres and the lateral deep nuclei

• Functions – Sensory: in animals with lateral line receptors and electroreceptors – Motor: the coordinated timing of outputs; changes with experience (learning) – Cognitive: It may function in the coordinated timing of representations in the internal

model of the external world.

Notes on evolution of cerebellum• The basic cell types and circuitry are highly conserved in evolution

– Layering becomes more regular – New inhibitory cell types in tetrapods – Deep nuclei as the output structures appear in tetrapods

• The size and form vary considerably – Huge expansion in the weakly electric fishes, especially in the hyperfolded cerebellum of mormyrids – Dramatic lateral expansion and foliation as locomotor and manipulatory abilities increased in land

animals: the growth of the cerebellar hemispheres and the lateral deep nuclei

• Functions – Sensory: in animals with lateral line receptors and

electroreceptors

– Motor: the coordinated timing of outputs; changes with experience (learning)

– Cognitive: It may function in the coordinated timing of representations in the internal model of the external world.

MIT OpenCourseWare http://ocw.mit.edu

9.14 Brain Structure and Its Origins Spring 2009

For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.

http://ocw.mit.edu/terms

http://ocw.mit.edu

spinal cord - massachusetts institute of technology€¦ · a sketch of the central nervous system...

Documents