BIO404 Revised TT1

what is Vertebrate brains?

Extremely complex — contain billions of neurons and trillions to quadrillions of synapses; genetic redundancy makes it hard to link single genes to phenotypes.

what is genetic redundancy?

The presence of genes with overlapping functions so that removing one gene may not change phenotype because others compensate.

what is the challenge of linking decision making to biology?

Decision making is complex and cannot easily be attributed to a single gene or single neuron.

what is necessary to understand the nervous system?

Mapping neural connections and understanding how neural circuits give rise to observable behaviors.

what is invertebrate brain complexity?

Simpler than vertebrates — typically thousands of neurons and thousands to hundreds of thousands of synapses yet can show complex behaviors.

what is Metazoa?

All animals except unicellular organisms.

what are the three axes used to classify metazoa?

Body symmetry, developmental patterns, and genetic distance.

what are major types of body symmetry?

Bilateria (bilateral), radiata (radial), biradial (ctenophora), and no symmetry (placozoa, porifera).

what are the developmental pattern groups?

Deuterostomes, Protostomes, and early-diverging animals.

what are protostome subdivisions?

Lophotrochozoa and Ecdysozoa.

what is Lophotrochozoa?

A protostome group that includes lophophorates (organisms with lophophores — feeding structures of tentacles with motile cilia) and trochophorates (organisms with trochophore larval stage).

what is Trochophorata?

A developmental stage with free-swimming larvae having bands of motile cilia.

what is Ecdysozoa?

Protostome group that undergoes ecdysis (molting) of cuticle/exoskeleton and generally lack motile cilia.

what are deuterostomes?

A major animal group that includes chordates (vertebrates) and non-chordate deuterostomes such as echinoderms and hemichordates.

what is Echinodermata?

Sea urchins and starfish; pentaradial (five-sided) symmetry and a water vascular system used for locomotion.

what is the nervous system of cnidarians and ctenophores?

Comprised of diffuse nerve nets and generally lacking highly centralized neural structures.

what is neural condensation in Bilateria?

The formation of ganglia (clusters of neurons) connected by nerves; likely the brain arose by fusion of anterior segmental ganglia.

what is Choanoflagellates?

The closest unicellular relatives of animals; have actin collars surrounding a motile cilium used for feeding.

what is the significance of choanoflagellates for multicellularity?

Some form colonies (rosettes), giving clues to the transition from unicellular to multicellular life and early cellular differentiation (proliferative vs motile cells).

what is the microtubule organizing center role in choanoflagellates?

It can act as a basal body for flagellar synthesis or as a mitotic spindle for chromosome segregation.

what major neural-related genes existed before animals?

Genes for cell adhesion, signaling, and synaptic proteins were present in choanoflagellates prior to metazoans.

what does gene repurposing mean in this context?

Proteins that had ancestral (non-neural) functions in unicellular ancestors were repurposed to neural roles in animals.

what is the choanoflagellate cell adhesion toolkit?

A core set of cell adhesion and extracellular matrix genes shared with metazoans.

what is the choanoflagellate synaptic toolkit?

Presence of many genes for cell–cell signalling, including receptor tyrosine kinases and G-protein coupled receptors, and genes associated with synapse formation/function.

what is Homer protein in choanoflagellates?

A post-synaptic scaffolding protein that forms stable tetramers and contains amino acids required for interactions with ion channels, receptors, cytoskeleton and other scaffolds.

what unusual localization of Homer was found in Salpingoeca (a choanoflagellate)?

Homer localized to the cell nucleus rather than the membrane and interacted with flotillin 1 and 2 (membrane-raft associated proteins).

what implication did Homer–flotillin nuclear localization have?

Comparative studies showed homer and flotillin could co-localize to nuclei of rat hippocampal astrocytes in vitro, revealing potential new neural functions.

what are Poriferans?

Early-diverging animals (sponges) that lack neurons but have choanocytes (collar cells) similar to choanoflagellates and generate feeding currents.

what is notable about poriferan synaptic genes?

Sponges are enriched for synaptic genes (especially presynaptic active zone genes) compared to choanoflagellates despite lacking neurons.

can poriferans generate action potentials?

Yes — they can generate action potentials, but they lack classic NaV (voltage-gated sodium) and KV (voltage-gated potassium) channels.

how do poriferans generate APs without NaV/KV channels?

They have CaV (voltage-gated calcium) channels enabling calcium-dependent action potentials; the repolarization mechanism is not well understood.

what is the role of Ca2+ action potentials in hard (glass) sponges?

Voltage-gated Ca2+ channels generate spikes that arrest ciliary beating (a "sneeze" reflex) to protect from particulates; spikes propagate via a syncytium.

what is a syncytium in this context?

A network of fused cells allowing electrical or chemical signals to propagate across cells without traditional synaptic boundaries.

what causes sponge sneezing/deflation behavior?

Relaxation of actomyosin stress fibers and involvement of nitric oxide signalling, acting as a protective contraction.

what neurotransmitters/modulators affect sponge behavior?

GABA, glutamate, acetylcholinesterase (AChE) modulators, and serotonin can modulate sponge contractions; no peptidergic signalling (neuropeptides) had been discovered in notes.

what are sensory cilia in the osculum of sponges?

Primary cilia at the osculum serve as sensory structures controlling contractions (removing osculum halts contractions until it regenerates).

what ions are required for sponge contractions?

Sodium (Na) and calcium (Ca) are required; replacing Na with K does not stop deflation, suggesting APs may not directly initiate contraction.

what is phototaxis in sponges?

Sponges can show movement toward/away from light without classical photoreceptors, indicating early sensory coordination (unclear direction in notes).

what is a typical poriferan life cycle?

Adults release sperm that fertilize eggs in other sponges; larvae develop, disperse via water currents, settle and metamorphose into adults.

what are Placozoans?

Very simple animals (e.g., Trichoplax) that show motile behaviors without neurons, reproduce by binary fission, and lack synapses yet show coordinated behaviors.

what is notable about Trichoplax synaptic genes?

Trichoplax has more synaptic genes than choanoflagellates and homologues with required domain structures for synapse function.

what ion channels does Trichoplax express?

A rich repertoire, expressing most ion channels needed for fast electrical signalling.

how many GPCRs does Trichoplax express?

Over 665 GPCRs for slow cellular signalling.

what is neuropeptide biosynthesis?

In nervous systems, neuropeptides are produced by cleaving precursor pro-proteins using prohormone convertases and post-translational modifications (e.g., N-terminal pyroglutamination and C-terminal amidation by PAM).

what are prohormone convertases?

Enzymes that cleave precursor proteins into multiple mature neuropeptides.

what is PAM?

Peptidylglycine alpha-amidating monooxygenase — the enzyme that catalyzes C-terminal amidation of peptides.

what are protoneurons in placozoans?

Neuropeptidergic cells identified by expression of neuropeptide precursors and their predicted GPCRs (examples include FFNP & SIFGa).

what is Endomorphin-like peptide (ELP) in Trichoplax?

A pre-pro peptide discovered in transcriptomes (named TadELP) with QDYPFFGX repeats similar to vertebrate YPFFG endomorphins.

what does TadELP do?

ELP arrests ciliary beating (pauses locomotion) and induces contagious pausing in adjacent animals, demonstrating neuropeptidergic signalling without neurons.

what evidence suggests ELP is secreted?

The pausing is contagious (adjacent animals pause without contact), indicating a diffusible secreted factor.

what other behaviors do secreted peptides elicit in Trichoplax?

Application of other secreted peptides leads to contracting and rotating behaviors.

what amino acids control feeding in Trichoplax?

Glutamate stimulates feeding whereas glycine and GABA suppress feeding via respective receptors.

what are Ctenophores?

Comb jellies — early-diverging animals that possess synapses and nervous systems and unique cell types like collocytes.

what are collocytes?

Ctenophore-specific cells that secrete sticky substances to capture prey.

how do ctenophores swim?

By beating cilia on comb plates arranged in 8 comb rows (4 pairs), each comb row composed of hundreds of cilia.

what is the aboral organ in ctenophores?

A sensory structure at the aboral pole that functions as a gravity sensor and coordinates ciliary beating.

what typical neurotransmitters are ctenophores reported to lack?

Classical transmitters and/or their receptors such as acetylcholine, GABA, glycine, serotonin, and dopamine.

what neural components are expanded in ctenophores?

Neuropeptides, peptide-gated ion channels, glutamate receptors, and gap junction proteins (innexins).

what is unusual about ctenophore presynaptic vesicles?

Presynaptic vesicles appear to bud off the smooth endoplasmic reticulum (ER), suggesting an ER origin rather than Golgi-derived vesicles.

what is a syncytial nerve net observed in some ctenophore larvae?

A subepithelial nervous system in which neurons are fused into a meshwork (no clear borders) with mesogleal neurons deeper in body.

what are ctenophore sensory cell distributions?

Sensory cells are ciliated and randomly distributed across the animal’s surface; comb cells and tentacles are innervated by different sensory cells.

what is the structure and function of the aboral organ statolith system?

Statoliths (stones) rest on balancer structures (groups of ciliated cells) covered by dome cilia; balanced beating keeps upright; tilt presses stone onto one balancer which increases beating on that side to restore orientation.

how does the aboral organ detect tilt?

It senses gravity perpendicular to the body midline, detecting left-right tilt and adjusting ciliary beating to correct posture.

what is the ion dependence of balancer activity in ctenophores?

High K+ seawater depolarizes balancers causing faster ciliary beating; cobalt (a Ca2+ channel blocker) prevents excitation, showing calcium channels at cilia bases are critical.

what did single-cell transcriptomics show in ctenophores?

Single-cell RNA sequencing failed to cluster neurons distinctly, contradicting morphology and suggesting high sequence variability prevents homology-based clustering.

what role did machine learning play in ctenophore neuropeptide discovery?

ML algorithms identified neuropeptide precursors by features (signal peptide; small mature sequences separated by cleavage sites) where homology searches had failed.

what are features of ML-identified neuropeptide precursors?

Presence of signal peptide and small mature peptides separated by cleavage sites; sequences were very different from known neuropeptides.

where are many ctenophore neuropeptides localized?

Probes showed many neuropeptides localized to the aboral organ.

what are Cnidarians?

Animals including jellyfish, corals and sea anemones that can have multiple life stages with different nervous system organizations and specialized cnidocytes for prey capture.

what is a cnidocyte?

A phylum-specific stinging cell used by cnidarians for prey capture and defense.

what are the two main cnidarian groups discussed?

Medusozoa (jellyfish + hydrozoans) and Anthozoa (corals + sea anemones).

what characterizes the Medusozoan life cycle?

A medusa stage (free-swimming jellyfish that reproduce sexually) and a polyp stage (sessile, asexual budding creates medusae).

what characterizes Anthozoa life cycles?

No medusa stage — the life cycle ends at the polyp stage, and polyps produce gametes directly.

what is a planula larva?

A ciliated free-swimming larva whose job is to find a settlement site and develop into a polyp; the aboral pole often concentrates ciliary and sensory cells.

what is neuroactive signaling significance at the planula aboral pole?

The aboral pole contains more GPCRs and transcripts; aboral halves can complete metamorphosis whereas oral halves cannot; GLWamide application stimulates metamorphosis.

what is GLWamide?

A neuropeptide that can stimulate metamorphosis in cnidarian larvae (planula).

what effect does retinoic acid have on GLWamide cells and larvae?

Retinoic acid reduces GLWa-expressing cells, causing larvae to fail to orient to light and preventing polyp formation.

what is the nervous system of a polyp like?

A simple meshwork with some centralization near the mouth and tentacles; epithelial contractile cells drive body contractions.

what evidence is there for learning in polyps?

Classical conditioning experiments (light paired with electric shock) produced stronger responses to light in paired groups, indicating associative learning.

why are small jellyfish (Scyphozoa) used as NS models?

They have prominent nervous systems with giant axons and are easy for electrophysiology (electrodes can be inserted easily).

what drives jellyfish locomotion?

A nerve ring (annulus) with giant axons stimulates bell contraction, which pushes water for propulsion.

what is pacemaker activity in jellyfish?

Regular action potential firing in pacemaker neurons stimulates swimming by causing EPSPs in motor giant neurons that produce Ca++ spikes.

how is swimming inhibited in jellyfish?

Touch inputs from feeding or obstacles inhibit the pacemaker (so the animal stops moving to feed or avoid collisions).

what are jellyfish escape responses?

Stronger and faster bell contractions triggered during predator attack; tentacle APs electrically couple to the nerve ring for rapid responses.

what is the conduction challenge in large jellyfish bells?

Signals traveling tens of centimeters cause asynchronous contractions unless the system compensates (e.g., reducing synaptic delays with distance).

what is a typical electrophysiology setup for jellyfish studies?

Six electrodes — three in motor neurons and three in postsynaptic epithelial cells.

what happens to stimuli strength with distance in jellyfish tissue?

Stimuli diminish with distance due to electrical leakage.

what is the relationship between AP strength and contraction in jellyfish?

Strong APs produce small contractile responses with large delay; weak APs produce larger contraction with smaller delay — suggesting adjustments (progressive reduction of synaptic delay) to coordinate contractions across the bell.

what are box jellyfish eyes like?

Box jellyfish (e.g., Tripedalia cystophora) have complex eyes — 4 rhopalia each containing 6 eyes including upper and lower lens eyes with retina, lens, cornea capable of image formation.

what are ULE and LLE in box jellyfish?

Upper lens eye (ULE) and lower lens eye (LLE) — lens eyes capable of image formation; LLE involved in obstacle avoidance.

what are slit eyes in box jellyfish?

Light-sensitive slit eyes present in rhopalia that detect light intensity/patterns.

what behavioral adaptation do box jellyfish eyes support?

Obstacle avoidance (e.g., avoiding mangrove roots); experiments show they avoid black bars/edges and tend to stay centered.

what is the overall summary of nervous system evolution in non-bilaterians?

There are competing hypotheses about how and when nervous systems evolved, especially regarding ctenophores and poriferans, and several evolutionary scenarios are considered (homology, convergence, parallelism).

what is the porifera-first phylogeny hypothesis?

Porifera diverged first and the nervous system evolved once later, with nervous systems lost in some lineages such as Placozoa.

what is the ctenophore-first phylogeny idea?

Ctenophores might branch off early — interpretations include nervous system evolving once with loss in Placozoa/Porifera or nervous systems evolving independently in Ctenophora and Cnidaria/Bilateria.

what does the homology scenario for NS evolution propose?

Neurons were present in the last common ancestor (LCAA) and later diverged significantly in descendant lineages (ctenophore vs cnidaria/bilateria).

what does the convergence scenario for NS evolution propose?

The LCAA lacked neurons and neurons originated separately in ctenophore and cnidaria/bilateria lineages from different ancestral cell types (independent origins).

what does the parallelism scenario for NS evolution propose?

The LCAA lacked neurons and neurons originated separately in ctenophore and cnidaria/bilateria lineages from the same ancestral cell type but via parallel evolution.

what is LCAA?

Last common ancestor of animals (often shorthand for the animal ancestor being discussed).

what is innexin?

A protein forming gap junction channels in non-vertebrates (noted in ctenophores as present).