BIO404

Vertebrate brains are very complex because

they have billions of neurons and trillions–quadrillions of synapses; genetic redundancy makes gene functions hard to pinpoint.

Why do phenotypes sometimes persist if one gene is removed

Because other genes with similar functions can compensate due to genetic redundancy.

Why is decision making hard to study in brains

It involves complex, interconnected neural networks not easily traced to single genes.

Why are invertebrates useful to study

They have fewer neurons/synapses but still show complex behaviours, making it easier to link behaviour to neural activity.

Ways to classify Metazoa:

by symmetry, developmental patterns, and genetic data.

Two main groups of Protostomes:

Lophotrochozoa (have lophophore feeding structures) and Ecdysozoa (undergo ecdysis/molting of cuticle).

What were early nervous systems like

Diffuse, lacking centralized structures; gradually condensed into brains in vertebrates.

Choanoflagellates are important to NS evolution because:

they have no NS but possess many genes used later for nervous systems in animals.

What does this suggest about NS genes in choanoflagellates

Proteins were repurposed and gained new functions in metazoans.

What does Homer protein do

It is a scaffolding protein at the post-synapse that organizes receptors, enzymes, and cytoskeleton for proper function.

Where is Homer protein found in choanoflagellates

In the nucleus instead of the membrane.

Poriferans have:

no nervous system but many synaptic genes and calcium-based action potentials.

How do poriferans sneeze

By body contractions mediated by primary cilia and neurotransmitters, triggered from the osculum.

What does cutting the osculum do

Stops contractions until it regenerates.

How are ions involved in poriferan contractions

Na⁺ and Ca²⁺ are essential; removing Na⁺ and replacing with K⁺ does not stop deflation, so APs are unlikely to initiate contractions.

How do poriferans reproduce

Females release larvae which swim to find a place to settle and form a new sponge colony.

Placozoans have:

2 epithelial layers, move by ciliary beating, and reproduce by binary fission.

Why are placozoans studied in NS evolution

They have cell types that behave like neurons and more synaptic genes than choanoflagellates/sponges.

What is peptide maturation

Cleavage + posttranslational modifications (pyroglutamination and amidation via PAM enzyme).

What does endomorphin do in placozoans

Causes them to pause ciliary movement, mimicking feeding behaviour.

How do placozoans influence each other’s behaviour

One secretes chemicals that cause nearby animals to also pause moving.

Effect of glutamate on placozoans:

induces feeding behaviour.

Effect of glycine & GABA on placozoans:

suppress feeding behaviour.

Ctenophores use which cells to catch prey

Collocytes (glue-secreting cells).

How do ctenophores swim

Using 8 comb rows of fused cilia controlled by the aboral organ.

Where is the oral pole in ctenophores

The side with the mouth.

Where is the aboral organ

On the opposite pole from the mouth.

Do ctenophores use neurotransmitters like other animals

No, they lack canonical NT systems.

Where do their synaptic vesicles come from

Believed to bud directly from the ER.

What is the subepithelial nervous system of ctenophores

A syncytium—neurons fused with no borders.

What sensory structures do ctenophores have

Ciliated sensory cells, ciliated furrows, mesogleal neurons, and tentacle sensory cells.

How does the aboral organ sense gravity

With balancer cells under dome cilia and heavy statolith stones.

How do balancers respond to tilt

Deflection excites them, causing faster beating on one side to right the animal.

What ions control balancer activity

Ca²⁺ is critical; cobalt (Ca²⁺ channel blocker) stops excitation.

What happens if sea water has more K⁺

Causes depolarization and increases ciliary beating frequency.

What happens if a ctenophore is tilted 90°

Top balancers beat faster, aboral organ reorients downward, helping the animal move down.

Single cell RNA-seq in ctenophores found:

neuron cells didn’t cluster (sequence divergence prevents homology detection).

How were neuropeptides found in ctenophores

Machine-learning algorithm found dozens of novel precursors with signal peptides and cleavage sites.

Where are many neuropeptides expressed in ctenophores

At the aboral organ.

Cnidarians have which two life stages

Polyp (sessile, asexual) and medusa (motile, sexual).

Which groups have polyps only

Anthozoans (corals, anemones).

Which groups alternate polyp and medusa

Medusozoans (hydrozoa, scyphozoa).

What are cnidarian larvae called

Planulae.

How do planulae move

Cilia on the epiderm sweep them through water.

What is the aboral pole of planulae important for

Has more neuroactive cells and GPCR transcripts, needed for metamorphosis.

What does GLWa neuropeptide do

Stimulates planula metamorphosis into polyps.

What does retinoic acid (RA) do to planulae

Reduces GLWa cells, prevents light attraction, and stops polyp formation.

What does separating planula poles show

Only aboral halves can form polyps; oral halves cannot.

How complex are cnidarian polyps’ nervous systems

Very simple meshwork of neurons around mouth/tentacles.

What cells cause polyp contractions

Mild epithelial cells.

Example of learning in cnidarians: pairing light (CS) with shock (US) leads to contraction from light alone.

Jellyfish swim using: a nerve ring with giant motor axons around the bell.

What does the nerve ring do

Sends APs to bell muscles to contract and push water out, propelling the jellyfish.

What are pacemaker neurons in jellyfish

Neurons that fire APs regularly to drive motor axons but are inhibited by touch on the mouth or umbrella.

What is the fast escape response in jellyfish

Giant axons fire strong Na⁺ APs through tentacles for fast, powerful synchronized contraction.

Why do giant axons conduct faster

Larger diameter and electrical coupling allow rapid AP spread.

What problem does slow AP spread cause

If not compensated, contraction starts only on the stimulated side first.

How do jellyfish solve this

Faster conduction in distal regions synchronizes contraction.

What are rhopalia

4 sensory structures on box jellyfish each with 4 eyes (slit, pit, upper lens, lower lens).

What do lens eyes in box jellyfish do

Form real images to avoid obstacles like mangrove roots.

Where do box jellyfish prefer to swim

In the center of mangroves to avoid bumping into roots.

Why is ctenophore placement on the tree important

They are now seen as the sister group to all other animals (not sponges as once thought).

Why is their neurotransmission puzzling

Most canonical neurotransmitters are missing or not found.