A sketch of the central nervous system and its originsG. Schneider 2005

Part 2: Steps to the central nervous system, from initial steps to advanced chordates

MIT 9.14 Class 3Evolution of multi cellular organisms

9.14 - Brain Structure and its OriginsSpring 2005Massachusetts Institute of TechnologyInstructor: Professor Gerald Schneider

Evolution of multi-cellular organisms: Suggestions based on phylogenetic comparisons

• TOPICS– Behaviors most fundamental for survival– Intercellular conduction in Ctenophores and

Coelenterates: suggestions about evolution of the nervous system

– A generalized conception of the CNS – The body plan of primitive chordates, as

suggested by Amphioxus (Branchiostoma)– Elaboration of the neural tube in evolution

Phylum Coelenterata: now often called Cnidaria. Animals with radial symmetry and stinging tentacles; includes free-swimming medusa forms like jellyfish, and sessile polyp forms like corals and sea anemones.

Basics of behavior enabling survival

• The most basic actions of individual organisms from amoebae to human: – Approach/avoid– Orient towards/away– Explore/forage/seek

• Each evolutionary advance had to incorporate these multipurpose actions, needed for various goal-directed activities.

• These take place on a background of maintenance activity, including respiration, temperature regulation, etc.

• In multicellular organisms, these actions require nervous system control and integration.

Early steps to a nervous system

• Sponges: responsive contractile cells without neurons, but also “myoid” and “neuroid”conduction (introduced by Nauta & Feirtag, ch. 1)

• Intercellular conduction in Ctenophores; Coelenterates: jellyfish, hydra

3 stages of nervous system evolution

Figure by MIT OCW.

Sea anemones Jellyfishes Jellyfishes & molluscs

Muscle cell Sensory neuron Motor neuron Motor neuron

Intermediate neuronSensory neuron

This leads to a generalized conception of the nervous system

(Note the influence of the S-R model.)

A note on hydra and the S-R model

• Evidence for endogenous activity: Hydra are active in a uniform, unchanging environment. (American Zoologist, 1965)– Hydra’s body and behavior: see Swanson figure, p. 17.

• This and an abundance of related evidence provides an important caveat to the S-R model.

Behavior in hydra: two patterns

FEEDING BEHAVIOR

LOCOMOTOR BEHAVIOR

Figures by MIT OCW.

Terms• Primary sensory neuron• Secondary sensory

neuron• Interneuron (or neuron

of the "great intermediate net")

• Motor neuron• Ganglia (singular:

ganglion) in PNS• Cell groups, “nuclei”, in

CNS

• Nerves in PNS• Tracts in CNS• Fasciculi (singular:

fasciculus) in CNS• Notochord• Neural tube

Amphioxus (Branchiostoma)

• Amphioxus is a tiny present-day chordate (an invertebrate Cephalochordate), but it has characteristics that suggest similarities to what the earliest chordates must have been like.

• It is sometimes called the simplest living chordate.

}1 cm

Figure by MIT OCW.

Sketch of the body plan of Amphioxus, the “simplest living chordate”:The name means “sharp at both ends”.

A dorsal nerve cord is found nervous system (CNS), in the form of a neural tube.

above the notochord: It is the central

Recent studies of Amphioxus• Studies using molecular markers of segmentation

in vertebrates:– Most of the neural tube corresponds to the brainstem

and spinal cord of vertebrates.• The three “primary brain vesicles” are present, but

there are no cerebral hemispheres.• The forebrain is mostly diencephalon with an

infundibulum and epiphysis.– Two visual inputs: pineal eye, and an unpaired light-

sensitive peripheral organ• Endbrain: There are no hemispheres; there is a

terminal or olfactory nerve.

Amphioxus frontal eye spot may correspond to the developing eye in vertebrates:

Figure by MIT OCW.

Nerves

Amphioxus Frontal Eye Spot

RetinaOptic nerve

Developing Vertebrate Eye

Pigment Cells

Pigment Epithelium

Receptor and nerve cells

Amphioxus brain, and the brain of a primitive vertebrate

Amphioxus

Frontal Eye Spot Pigment Lamellar Body: mediatesphotoperiodic behavior

Neurosecretory Cells: Control basic physiological functions,reproductionPhotoreceptors

Primitive VertebrateTelencephalon

Olfactory Bulb

RetinaHypothalamusand pituitary

Parietal Eye: mediatesphotoperiodic behavior

Optic Tectum

Figure by MIT OCW.

Based on studies by T.C. Lacelli et al. Other recent work on Amphioxus has concerned gene expression data.

Amphioxus in a transverse section

Peripheral nerves, attached to a CNS:dorsal attachments -- mostly sensory;ventral attachments -- mostly motor (the “law of roots”)

Elaboration of the neural tube in evolution

• The behavioral demands: What are the highest priorities?

• These demands resulted in progressive evolutionary changes in the neural tube: “Every brain system grows logically from the tube” (H. Chandler Elliott, 1969).

What do I mean by "evolution"?• I mean the processes of change in the way descendants

behave (and, more generally, function), and in the corresponding way their bodies and nervous systems look and function.

• The changes occur by natural selection, i.e., because certain genotypes produce more surviving offspring than others, so those genes increase in frequency and others decrease or disappear.

• The changes are genetic, and result from genetic variations. Genetic variations are enhanced by sexual selection, involving the chance re-sorting of genes and hence the expression of those genes. Evolution works firstly on the current variations in genes among various individuals of a species, and then the greater changes in evolution result from genetic variations via gene mutations.

The evolution of behavior and its underlying brain in our phylum

• Function, including behavior, is the driver of evolution.

• Functional changes result in a process of successive elaborations of the basic plan of the neural tube in its simplest form as illustrated, at least in a suggestive way, by Amphioxus.

• See the statement on the assignment page, “The origins of behavior and its brain”

Basic behavioral demands:

• Behavior for survival and reproduction • The ongoing background support• For doing these things, interfaces with

the outside world are necessary:– Sensory side– Motor side

Survival and reproduction• Approach and avoidance actions:

Motivational systems (major examples)

• Anti-predator behavior (for survival of the individual)

• Feeding and drinking (for survival of the individual)

• Reproductive behavior (for survival of the species)

• Need for interface with endocrine regulatory systems.

• Need for establishing goal hierarchies:E.g., fleeing from predator gets priority over

eating & mating.

The ongoing background support

• Stability of the internal environment• Stability in space

• These functions are supported by the "mantle of reflexes“ (a mantle we are always wearing)

Interfaces with the outside world

• Sensory detection and analysis; motor responses and their coordination

• Orienting towards or away from sources of input• Exploring, foraging, seeking• Eventually, the integrative systems of the CNS

evolved what we call cognitive abilities, for:– anticipating events (what is about to be sensed)– planning actions for achieving goals (preparing for

action).

The advantages of these functions resulted in progressive changes in the neural tube, to include:

• Sensory analyzing mechanisms• Associated motor control apparatus • “Correlation centers” – in between sensory & motor• Circuits underlying complex programs for goal-directed

activities (instinctive action patterns; learned action patterns)• Systems for modulating other brain systems, or initiating

action in them, in response to visceral & social needs (motivation systems), and other needs basic to survival

• Systems for anticipating events & planning actions (cognitive systems)

Questions to think about:

• Why did the CNS evolve the way it did?• What does it accomplish for an organism?• How is this expressed in the basic

organization of the CNS?

The following is speculation, but it is based on comparisons of a wide range of species.

Evolution of Brain 1

Increasing sophistication of sensorimotor abilities

• Sensory analyzing mechanisms– elaborated especially with the evolution of head

receptors.

• Associated motor apparatus– For directing the receptors (orienting movements)– For controlling alterations in posture and locomotion

under guidance from these receptors.

• Crucial background: maintenance of stability of the internal mileau

Structures that accomplished those functions

• Hindbrain, midbrain and forebrain mechanisms connected to head receptors are added to primitive spinal somatosensory mechanisms:

Vestibular, gustatory, olfactory, visual, vibration, electroreception and/or auditory influences added to somatosensory.

• Hindbrain & midbrain: Control of mouth, eyes, ears, head turning, added to basic spinal & hindbrain control of the body

• Forebrain: Olfactory & visual inputs; endocrine & visceral control

Evolution of Brain 2

Expansion of hindbrain: Sensory analysers:

somatosensory (face) including taste, vestibular, auditory, electroreception.

Action pattern programstriggered by specific sensory patterns

Functional adaptations cause expansions in CNS: A few illustrations from comparative anatomy (from C. L. Herrick):

• Brain of a fresh water mooneye.• Brain of a freshwater buffalo fish:

– huge "vagal lobe“ (receives input from specialized palatal organ)

• Brain of a catfish: – "facial lobe" and "vagal lobe“ (for processing taste

inputs through two different cranial nerves)

• Catfish 7th cranial nerve distribution, re: – taste senses (explains facial lobe)

Hyodontergisus

(fresh water Mooneye)

Note the size and shape ofthe hindbrain. (The hindbrainincludes the medulla oblongataand the cerebellar region.

Figure by MIT OCW.

Olfactory Bulb

Olfactory Stalk

Primitive Endbrain

Midbrain

Cerebellum

Medulla Oblongata

Carpiodestumidus

(buffalofish)

The “vagal lobe” of the hindbrain is huge. It receives and processes taste input from a specialized palatal organ.

Figure by MIT OCW.

Endbrain

Midbrain

Cerebellum

Vagal Lobe

Pilodictisolivaris(catfish)

The vagal lobe is enlarged, although less than in the buffalo fish. An enlarged “facial lobe” is also evident. It receives taste inputs from all over the body surface.

Figure by MIT OCW.

Olfactory Stalk

Primitive Endbrain

Midbrain

Cerebellum

Facial Lobe

Vagal Lobe

Amiurus melas (the small catfish):7th cranial nerve (facial nerve) innervates taste buds in skin of entire body

Based on Herrick (1903). Figure by MIT OCW.

Evolution of Brain 3 Expansion of forebrain because of adaptive value of olfactory sense for approach & avoidance functions (feeding, mating, predator avoidance, predation).……………………..Outputs: links to locomotion through the corpus stratum were most critical. These links were viathe midbrain.

(Concurrent with “Evolution 2”)

These connections were plastic:T They could be strengthened or weakened, depending on experience

Expansions, continued:Functional demands result in progressive

changes in the neural tube, to include:

• Sensory analyzing mechanisms• Corresponding motor apparatus • “Correlation centers”• Elaboration of complex programs for goal-

directed activities • Systems for modulating other brain systems in

response to visceral and social needs• Systems for anticipating events & planning actions

Selected References

Slide 5: Figure by MIT OCW. © MIT2006. Based on: Parker, George, Yale University, 1919 and from Nauta, Walle J. H., and Michael Feirtag. New York, NY: Freeman, 1986. ISBN: 0716717239. Slide 6: Drawing by Gerald Schneider. © G.E. Schneider 2006 Slide 8: Figure by MIT OCW. © MIT2006. Based on: Swanson, Larry W. Brain Architecture Understanding the Basic Plan. Oxford; New York, NY: Oxford University Press, 2003, p. 17. ISBN: 0195105052. Slide 10: Figure by MIT OCW. © MIT2006. Based on: Striedter, Georg F. Principles of Brain Evolution. Sunderland, MA: Sinauer Associates, 2005, p. 55. ISBN: 0878938206. Slide 11: Drawing by Gerald Schneider. © G.E. Schneider 2006 Slide 13: Figure by MIT OCW. © MIT2006. Based on: Allman, John Morgan. Evolving Brains. New York, NY: Scientific American Library: Distributed by W.H. Freeman and Co., 1999, p. 69. ISBN: 0716750767. Slide 14: Figure by MIT OCW. © MIT2006. Based on: Allman, John Morgan. Evolving Brains. New York, NY: Scientific American Library: Distributed by W.H. Freeman and Co., 1999, p. 70. ISBN: 0716750767 Slide 15: Drawing by Gerald Schneider. © G.E. Schneider 2006 Slide 25: Drawing by Gerald Schneider. © G.E. Schneider 2006 Slide 28: Drawing by Gerald Schneider. © G.E. Schneider 2006 Slide 30-32: Figures by MIT OCW. © MIT2006. Slide 33: Figure by MIT OCW. © MIT2006. Based on Herrick (1903) Slide 34: Drawing by Gerald Schneider. © G.E. Schneider 2006