A sketch of the central nervous system and its origins

G. E. Schneider 2009Part 2: Steps to the central nervous system,

from initial steps to advanced chordates

MIT 9.14 Class 3

Evolution of multicellular organisms

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: critical for evolution

• The most basic actions of individual organisms from amoebae to human: – Approach/avoid – Orient towards/away – Explore/forage/seek (includes the first two, plus

instigation by a motivational state)• 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 and in Coelenterates like jellyfish and hydra: The anatomy suggests basic early stages of nervous system evolution

3 stages of nervous system evolution

Sea anemones Jellyfishes Jellyfishes & molluscs

Muscle cell Sensory neuron Motor neuron Motor neuron

Intermediate neuronSensory neuron

Figure by MIT OpenCourseWare.

Sensory neuron in epithelium

From Nauta, based onGeorge Parker, 1919(Yale Univ.)

This leads to a generalized conception of the nervous system

Primary sensory neurons

Secondary sensory neurons

Motor neurons—the final common path

interneurons CNS

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

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:

next slide

• 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

Figure by MIT OpenCourseWare.

From Swanson (2003), p. 17

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 Next slides • Neural tube

See definitions in Schneider ch 3.

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.Figure from Striedter (2005), p. 55.

}1 cm

Figure by MIT OpenCourseWare.

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

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

Amphioxus in a transverse section

Dorsal root Ventral root

Notochord

CNS (dorsal nerve cord)

Along the body axis, the peripheral nerves attach to the CNS in dorsal attachments which are mostly sensory, and ventral attachments which are mostly motor (thus following, for the most part, the “law of roots”.) At a single level one finds a dorsal root on one side and a ventral root on the other. Dorsal and ventral roots alternate sides as one moves along the body axis.

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

terminal and/or olfactory nerve.

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

Nerves

Amphioxus Frontal Eye Spot

RetinaOptic nerve

Developing Vertebrate Eye

Pigment Cells

Pigment Epithelium

Receptor and nerve cells

Figure by MIT OpenCourseWare.

From Allman (2000), p. 69

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 OpenCourseWare.

Figure from John Allman (2000), p. 70, based on studies by T.C. Lacelli et al. Other recent work on Amphioxus has concerned gene expression data.

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 a population of

animals -- in the way descendants function and behave,and in the corresponding way their bodies and nervous systems look and function.

• The changes occur by natural selection, i.e., becausecertain 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 reproduction, 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. Greater variations appear when there are genetic changes resulting from 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”

Evolutionary changes in the primitive neural tube were due to basic behavioral demands:

• Behavior critical for survival and reproduction • Support and maintenance behaviors: the ongoing

background

• For doing these things, interfaces with the outside world are necessary:– Sensory

– Motor

First, we will review briefly the behavioral demands. Then we will survey the evolutionary changes that have occurred in CNS.

-

Survival and reproduction

• Approach and avoidance actions and the underlying motivational systems (major examples)

Anti-predator behavior (for survival of the individual) - Feeding and drinking (for survival and growth of the

individual) - Reproductive behavior (for passing on the individual’s

genes)

• 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 (e.g., via regulation of blood pressure, blood oxygen and glucose)

• Stability in space (e.g., via vestibular reflexes, visuomotor reactions, proprioceptive reflexes, etc.)

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

Interfaces with the outside world

• Most basic: – Sensory detection and analysis – Orienting towards or away from sources of input;

escape and avoidance movements, approach movements

– 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 (with inputs from specialized receptors):

Sensory pathways

• Associated motor control apparatus (connected to motor neurons)

• “Correlation centers” – in between sensory & motor (e.g., midbrain tectum, cerebellum)

• Circuits for complex programs controlling goal-directed activities (instinctive action patterns; learned action patterns): Sp.cord, brainstem, endbrain

• 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: Hypothalamus & its cohorts

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

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

• Questions that led to the speculations:

– 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?

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9.14 Brain Structure and Its Origins Spring 2009

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