BIO134 - Neurobiology Midterm

What are gap junctions called, and what do they do?

Connexons - they allow current to flow directly from one cell to another through a continuous pore formed by hemichannels from each cell. Present in electrical synapses.

How is a connexon structured?

A hexamer of connexin proteins forming a pore. Two hemichannels dock to form a complete channel spanning a 8nm wide gap.

What is a rectifying electrical synapse?

One in which two hemichannels are different proteins; only one has a voltage sensitive gate. This allows current to flow preferentially in one direction only (unidirectional as opposed to bidirectional; demonstrated in giant crayfish synapse).

Why does the presynaptic cell need to be larger than the postsynaptic cell at electrical synapses?

High membrane resistance causes current loss. A larger presynaptic cell generates enough current to overcome resistance and drive the postsynaptic cell.

What is a key trade-off of electrical synapses vs. chemical synapses?

Electrical synapses are more reliable (signals passed faithfully) but less modifiable. Chemical synapses are slower but highly modulatable.

What was Otto Loewi’s experiment, and what did it prove?

He stimulated the vagus nerve of a perfused heart, then applied that perfusate to the second heart, which also slowed. This proved the vagus nerve releases a chemical (ACh) to slow the heart.

What did curare reveal at the neuromuscular junction?

By blocking nAChRs, curare revealed the subthreshold depolarization (EPSP) that is normally obscured by the large muscle action potential.

What are the key steps in chemical synaptic transmission?

1. AP invades terminal.

2. Depolarization opens voltage-gated calcium channels, causing calcium influx.

3. Vesicles fuse (exocytosis).

4. Transmitters diffuse across synaptic cleft.

5. Transmitter bind postsynaptic receptors.

6. Receptor binds channel or activates G-protein.

What is an EPSP and how does it differ from an action potential?

An excitatory postsynaptic potential (EPSP) is a graded, decremental depolarization caused by transmitter binding. Unlike APs, EPSPs decay with distance. At the NMJ (neuromuscular junction), it is called an end plate potential (EPP) or excitatory junction potential (EJP).

What are MEPPs and what makes them ‘quantal’?

MEPPs (miniature end-plate potentials) are spontaneous, small, fixed-amplitude voltage changes caused by the release if single vesicles (quanta) of ACh.

How did researchers calibrate the number of ACh molecules in a MEPP?

They used a micropipette to inject known quantities of ACh directly at the junction, matching voltage deflection of one MEPP to calculate ~100 ACh molecules per quantum.

Why couldn’t researchers match MEPP size by dropping dilute ACh droplets onto the junction?

Acetylcholinerase in the cleft rapidly degrades ACh that diffuses from a distance; only molecules released directly at the junction reach receptors effectively.

What triggers vesicle fusion at the presynaptic terminal?

Depolarization opens voltage-gated calcium channels; calcium enters the terminal and triggers a cascade of protein interactions leading to vesicle fusion.

Why does stepping voltage UP decrease calcium entry even when channels are open?

A more depolarized voltage reduces the electrochemical driving force for calcium (smaller gradient). Stepping back down opens channels with a larger calcium gradient, increasing entry.

Why must calcium enter the presynaptic terminal for vesicle fusion to occur?

Postsynaptic response increases with depolarization but is abolished at ECa (when there is no net calcium flow); calcium must be present before, not after, the pulse. The ‘off’ response is not delayed whereas the ‘on’ response is (Katz & Miledi).

What is the SNARE complex and how does it work?

SNARE proteins mediate vesicle fusion; synaptobrevin (v-SNARE on vesicle) zippers together with syntaxin and SNAP-25 (t-SNARES on plasma membrane), pulling two lipid bilayers together. Calcium binding to synaptotagmin triggers final fusion.

How are synaptic vesicles recycled after exocytosis?

Membrane is retrieved by clathrin-mediated endocytosis (10-20 sec), forming endosomes. Vesicles bud from endosomes (~1 min), are refilled with transmitter via transporters, and dock/prime at the active zone for rerelease.

How do neuropeptides differ from small-molecule transmitters?

Peptides are synthesized in the soma (ribosomes), packaged in large, dense-core vesicles, and transported to terminals; they work for slow, modulatory, long-term signaling. Small molecules are synthesized locally at terminals; they work for fast, rapid, short-term signaling. Peptides are cleaved from larger precursor proteins and degraded by extracellular peptidases. Small molecules are recycled via reuptake.

How is anandamide released, and what receptor does it target?

Anadamide is not released by vesicles; it’s produced on-demand from membrane fatty acids enzymatically. It’s the endogenous ligand for cannabinoid receptors (pre- and postsynaptically).

What is nitric oxide and why is it called a ‘volume transmitter’?

Nitric oxide is a gas synthesized on demand from arginine by nitric oxide synthase (requires calcium/calmodulin). It freely diffuses across membranes, activating guanylyl cyclase in neighboring cells; not confined to a synaptic cleft, hence ‘volume transmitter’.

What are the major categories of transmitter?

1. Biogenic amines - 5HT, dopamine, NE, epinephrine, histamine, octapamine

2. Amino acids - glutamate, GABA, glycine

3. Cholines - just ACh

4. Purines - ATP, adenosine

5. Neuropeptides - neuropeptide Y, opioids, substance P

6. Lipid transmitters - anandamide

7. Gases - NO

How is serotonin (5HT) synthesized?

Tryptophan (amino acid) → tryptophan hydroxylase → 5-hydroxytryptophan → (amino acid decarboxylase) → 5-hydroxytryptamine (serotonin/5HT)

How are catecholamines synthesized?

Tyrosine → tyrosine hydroxylase → L-DOPA (aromatic L-amino acid decarboxylase) → Dopamine → dopamine β-hydroxylase → Norepinephrine → phenylethanolamine N-methyltransferase (PNMT) → epinephrine

What is synaptic plasticity?

A change in synaptic efficacy over time that reflects the history of presynaptic activity. It can be intrinsic (activity-based) or extrinsic (modulated by other hormones/neurons).

What causes synaptic facilitation?

Residual calcium left in the presynaptic terminal after an AP adds to calcium from the next AP, increasing likelihood of vesicle release. Prominent at high-frequency stimulation and short-term inter-stimulus intervals.

What causes synaptic depression?

Depletion of the readily releasable pool of vesicles during high-frequency stimulation. Recovery requires vesicle re-docking and priming.

What is post-tetanic potentiation (PTP)?

A long-lived (minutes) but transient increase in synaptic efficacy following a high-frequency tetanic stimulus train. Caused by an accumulation of calcium in the presynaptic terminal during the tetanus.

What is the difference between presynaptic inhibition and facilitation (extrinisic)?

Presynaptic inhibition: a neuron increases gK, shortening AP and reducing calcium influx → less transmitter released.
Presynaptic facilitation: decreases gK, broadening AP and increasing calcium influx → more release

What is paired-pulse facilitation and why is it homosynaptic?

Two action potentials at the same terminal in quick succession produce a larger, second EPSP due to residual calcium. It’s homosynaptic because both pulses must arrive at the SAME terminal.

How does a synaptic potential travel along a postsynaptic cell?

Passively and decrementally - the same cable properties that govern dendrites apply. Amplitude decays with distance from the synapse due to current leakage through the membrane. Unlike APs, synaptic potentials are not regenerated.

What did Takeuchi & Takeuchi (1967) demonstrate about nAChR synaptic current?

Using a voltage clamp at the NMJ, they showed synaptic current is inward at negative voltages and reverses to outward near 0mV, proving it is driven by two ions (Na+ in, K+ out) with an intermediate Erev rather than a single ion.

What does it mean that ACh-gated channels open in an all-or-none fashion?

Individual nAChR channels open to a fixed conductance step - they are never half-open. Macroscopic current reflects number of open channels, not graded single-channel conductance. Demonstrated by patch clamp.

Why is the nAChR pore much less ion-selective than voltage-gated channels?

The nAChR pore us much larger than the K+ channel, and allows both K+ and Na+ to pass. Its selectivity is determined by charge, not size.

What is shunting inhibition, and why does it matter even when an IPSP is ‘invisible’?

Opening Cl- channels clamps Vm near ECl and dramatically reduces membrane resistance. Even if ECl equals Vrest (no voltage change), the increased gCl shunts any concurrent EPSPs — current injected by excitatory channels leaks out through open Cl- channels instead of depolarizing the cell.

Under what conditions can GABA be excitatory?

If extracellular Cl- is unusually high, making ECl positive to resting potential, opening GABAA channels drives Cl- out, depolarizing the cell. This occurs in immature neurons and in some adult neurons where the KCC2 transporter is absent or downregulated.

What is ectopic neurotransmission?

Transmitter release or receptor activation occurring outside the formal synaptic contact zone. Transmitters can diffuse from release sites and act on receptors not directly apposed to the terminal (volume transmission). (Demonstrated computationally by Coggan et. al. 2005, at nicotinic synapses).

What is α-bungarotoxin and how was it used to study nAChRs?

A toxin that binds irreversibly to α subunits of nicotinic AChRs. Radiolabeled toxin + gel separation allowed isolation and quantification of the receptor.

What are ionotropic vs. metabotropic receptors?

Ionotropic: ligand-gated channels; fast, direct.

Metabotropic (GPCRs): 7 transmembrane domains; signal via G-proteins and second messengers; slower, but more diverse effects.

How do neuronal nAChRs differ from muscle nAChRs?

Neuronal nAChRs only have α and β (non α) subunits (10 α types, 4 β subtypes), vs the muscle receptor’s α, β, γ, δpentamer. Neuronal subtypes can: 1) be highly calcium-permeable, 2) desensitize rapidly, and 3) have single-channel conductances ranging wide from 5-50pS. Muscle nAChRs often act presynaptically to modulate transmitter release.

What is the evolutionary relationship between ionotropic receptor families?

All ligand-gated ion channels (nAChR, GABAA, glycine, 5-HT3, glutamate, purinergic P2X) are evolutionary relatives - they share the same basic pentameric or trimeric structure and a common ancestor. This is visible in phylogenetic trees that cluster them across species from nematodes to vertebrates.

What are the four main Gα subunit classes and what does each target?

αs: stimulates adenylyl cyclase (+AC).

αi/o: inhibits AC, opens Kir channels, closes CaV1/2 channels.

αq/11: activates PLCβ(+), opens KV7/KCNQ channels.
α12/13: activates small Rho-family GTP-ases.

How does the βγ dimer of a G protein contribute to signaling?

The free βγ dimer (released when the α unit dissociates) is itself an active signaling molecule. It can directly gate ion channels (ex. GIRK/Kir channels), inhibit certain AC isoforms, activate PLC β1-3, and modulate calcium channels, all independent of the α subunit pathway.

When does catecholamine (ex. norepinephrine) bind on a GPCR like the β-adrenergic receptor?

Within the bundle of transmembrane helices, in a binding pocket formed by TM helices 3, 5, 6, and 7 on the extracellular face. This is distinct from ionotropic receptors where the transmitter binds at the extracellular N-terminal domain.

Outline the cAMP signaling cascade.

Transmitter → receptor → G-protein (GDP → GTP) → α subunit activates adenylyl cyclase → ATP → cAMP → PKA activation (regulatory subunits release catalytic subunits) → phosphorylation. Terminated by phosphodiesterase (cAMP → AMP).

How does cAMP activate PKA, and what are its downstream effects beyond ion channels?

cAMP binds the regulatory subunits of PKA, releasing the active catalytic subunits. Catalytic subunits can translocate to the nucleus and and phosphorylate transcription factors (ex, CREB), altering gene expression and giving second messenger pathways long-term effects beyond acute channel modulation.

How does NO activate its intracellular targets, and what does that produce?

No diffuses through membranes and binds the hime group of soluble guanylyl cyclase (SGC), activating it. Activating sGC converts GTP → cGMP, which then activates cGMP-dependent protein kinase (PKG) and can open cyclic nucleotide-gated ion channels.

What are the two forms of guanylyl cyclase and what activates each?

Membrane-associated guanylyl cyclase: activated by the extracellular peptide ligands binding the receptor domain.

Soluble (cytoplasmic) guanylyl cyclase: activated by NO binding to its heme group.

Both convert GMP → cGMP.

What are the two branches of the PIP2/PLC pathway?

Pathway 1 (α subunit): PLC cleaves PIP2 → IP3 (releases ER calcium) + DAG (activates PKC with calcium).

Pathway 2 (βγ subunit): PLA2 → arachidonic acid → activates lipoxygenases and COX (inflammation/signaling).

What does PKC (protein kinase C) require for activation, and what does it do?

PKC requires BOTH DAG (anchors PKC to the membrane) AND elevated calcium for full activation. Once active, it phosphorylates serine/threonine residues on target proteins, modulating channel activity, receptor sensitivity, and many other cellular processes.

What is calmodulin and how does it transduce calcium signals?

Calmodulin is a ubiquitous calcium-binding protein. When intracellular calcium rises, calcium binds calmodulin, causing a conformational change that allows it to bind and activate calcium/calmodulin-dependent protein kinase (CaMK). CaMK then phosphorylates many downstream substances.

What does the arachidonic acid branch of the PIP2 pathway produce?

PLA2 (activated by βγ or αi/o) cleaves DAG to release arachidonic acid. AA is then metabolized by: 12-lipoxygenase → 12-HPETE and active metabolites; 5 lipoxygenase → 5-PETE → leukotrienes; cyclooxygenase (COX) → prostaglandins and thromboxanes. NSAIDs (ex. ibuprofen, carprofen) block COX.

What are receptor tyrosine kinases (RTKs) and how do they differ from GPCRs?

RTKs (ex. Trk neurotrophin receptors) are single-pass transmembrane proteins with an extracellular ligand-binding domain and an intracellular kinase domain. Ligand binding causes receptor dimerization and autophosphorylation on tyrosine residues, then recruits adaptor proteins - no G-protein intermediary.

What are the Trk receptors and which neurotrophins activate them?

TrkA binds NGF (β-NGF).

TrkB binds BDNF and NT-4/5.

TrkC binds NT-3 (also NT-4, NT-7).

Once activated, Trks signal through three main pathways: PI3K → AKT (survival/proliferation), MAPK → ERK (gene expression/growth), and PLCγ(calcium signaling).

What is the C-start behavior in fish?

A rapid escape response where the fish bends its body into a C-shape (taking ~25ms) before propelling away from a threat. It is mediated primarily by the Mauthner cell.

Where are Mauthner cells located and how many are there?

There is one pair of Mauthner cells (M-cells), one on each side, located in the hindbrain/brainstem of the fish. They are among the largest neurons in vertebrates.

What sensory inputs activate the Mauthner cell?

Auditory and vestibular via the eighth (VIII) nerve from the sacculus hair cells.The lateral dendrite receives electrical excitation via club endings (gap junctions) and chemical excitations.

What is the axon cap and its function?

A specialized region around the Mauthner cell axon hillock. Collateral interneurons synapse here to produce electrical (field effect) inhibition — current flow in the cap hyperpolarizes the axon initial segment, preventing refiring.

What are the two inhibitory mechanisms in the Mauthner circuit?

1. Self-inhibition (recurrent collaterals → interneurons → axon cap) prevents the M-cell from firing repeatedly.

2. Contralateral inhibition ensures only one M-cell fires, producing a unidirectional turn.

How does the tentacled snake exploit the Mauthner circuit?

The snake twitches its J-shaped body, triggering the fish’s Mauthner-mediated C-start toward the snake’s head. The snake’s strike (25ms) is timed to intercept the fleeing fish.

When stimulating the eighth nerve, what components appear in the Mauthner cell response?

A fast, short latency EPSP (gap junctions, no delay), followed by a slower chemical EPSP (synaptic delay). Together these can summate to trigger an action potential.

How is chemical inhibition demonstrated in the Mauthner cell?

IPSPs reverse polarity at the equilibrium potential for Cl-/K+. When membrane potential is more negative than the reversal potential (~-76mV), the IPSP depolarizes; more positive → hyperpolarizes.

What is a ‘reduced preparation’ and why is it used to study leech behavior?

A reduced preparation isolates part of the nervous system (ex. a semi-intact or dissected preparation) while preserving the circuit driving a behavior. It allows direct electrophysiology on identified neurons while the behavior (ex. swimming, local bending) is still expressed.

What are the three classes of leech mechanosensory neurons and what do they detect?

T (touch) cells respond to gentle skin touch; adapts rapidly (phasic).
P (pressure) cells respond to stronger pressure; adapts slowly (tonic).

N (nociceptive) cells respond to noxious/harmful stimuli.

What is a receptive field in the context of leech sensory neurons?

The region of skin from which a sensory neuron can be activated.

What is the difference between tonic and phasic sensory response?

Tonic neurons fire persistently throughout a sustained stimulus, encoding stimulus magnitude and duration. Phasic neurons fire a brief burst at stimulus onset (and/or offset) and then adapt - encoding changes in stimulus intensity rather than steady state. Leech T cells show phasic adaptation to sustained touch.

What is annulus erection in the leech and what circuit coordinates it?

Annulus erection is a local reflex where the small annular muscles contract around the site of a touch, stiffening the skin segment. It is driven by mechanosensory input (T/P cells) acting through motor neurons within the segmental ganglion - a simple behavior coordinated by synapse-specific properties within a single ganglion.

How does the leech swim, and what central pattern generator (CPG) drives it?

The leech swims by propagating a dorsoventral bending wave from head to tail. A CPG in the segmental ganglia, involving oscillator interneurons, produces rhythmic alternating dorsal/ventral motor neuron bursts. The swim-initiating neuron (cell 204) can trigger swimming with a single spike.

How do synapse- and neuron-specific properties coordinate leech local bending?

Local bending requires some muscles to contract while others relax in a spatially appropriate pattern. This is achieved not by a higher command center but by the specific synaptic weights between P cells and motor neurons - P cells on one side excite ipsilateral longitudinal motor neurons and inhibit contralateral ones, producing a bend toward the stimulus through hardwired connectivity.

What is the stomatogastric ganglion (STG)?

A small ganglion in crustaceans containing ~30 neurons that controls the rhythmic movements of the stomach (gastric mill and pylorus). It is a key model for central pattern generator research.

What are the two motor rhythms generated by the STG?

1. Gastric mill rhythm (slow, ~1min) - controls teeth-like structures that grind food.

2. Pyloric rhythm (fast, ~1-2Hz) - controls pyloric valve that filters food into the gut.

What are the key pyloric neurons and their roles?

AB - anterior burster; pacemaker, electrically coupled to PD
PD - pyloric dilator
VD - ventricular dilator
LP - lateral pyloric; constrictor
PY - pyloric; constrictor

AB/PD fire first; LP and PY fire in sequence after inhibition is released.

What types of synaptic connections dominate the pyloric circuit?

Almost all chemical connections in the pyloric circuit are inhibitory. AB and PD are connected via electrical synapses. The pattern arises from inhibitory interactions and post-inhibitory rebound.

What happens when the STG is isolated from the commissural gangliam(CoGs)?

The pyloric rhythm weakens significantly or stops. Reconnection to the CoGs (which contains ~400 modulatory neurons) restores robust rhythmic activity, showing that neuromodulatory input is required for full circuit function.

How can neuromodulators change the pyloric rhythm?

By changing synaptic strength, intrinsic membrane properties, or phase relationships (ex. dopamine hyperpolarizes PD, advancing PY phase; IVN stimulation delays PY phase). Modulators include 5-HT, DA, octapamine, GABA, and many peptides.

What is special about neurons in the STG (except AB)?

Most STG neurons are both motor neurons AND interneurons. Action potentials are mainly used to communicate with muscles, not between neurons.

What is the AB neuron and why is it important?

The AB (anterior burster) cell is an intrinsic oscillator (calcium-dependent) that drives rhythmic activity in the pyloric circuit.

Define cycle, phase, and period in the context of the pyloric rhythm.

Cycle - duration/period (one full repetition).

Period - time for one full cycle,

Phase - delay/period ( a point’s position within the cycle, expressed as a fraction).

What is the gastric mill and what movements does it produce?

The gastric mill is a set of calcified teeth inside the stomach. Two patterns:

1. Squeeze - lateral teeth close medially.

2. Cut and grind - lateral teeth move medially while the medial tooth moves anteriorly, shearing food.

Why is the connectome alone insufficient to predict circuit function?

The same wiring can produce completely different behaviors depending on which neuromodulators are present, the history of activity, and intrinsic membrane properties. Connections must be ‘functionally defined’ in the context of ongoing modulatory state.

What is network degeneracy in neural circuits?

Multiple distinct parameter combinations (different ion channel densities, synaptic strengths) can produce the same functional output. This means the nervous system is not uniquely ‘tuned’ - many solutions exist for the same behavior.

What is a central pattern generator (CPG)?

A neural circuit capable of generating rhythmic, patterned motor output without sensory feedback or rhythmic input. Examples: STG pyloric/gastric circuits, locomotor CPGs in spinal cord.

How can the same neuron participate in two different CPG networks?

Some neurons (ex. IC, VD in the STG) are components of both the pyloric and gastric circuits simultaneously. Different neuromodulators can shift the relative weighting of a neuron’s participation between networks.

What is ‘graded’ vs. ‘spike-dependent’ transmitter release?

Spike-dependent release occurs in all-or-none bursts triggered by action potentials. Graded release occurs proportionally to the amplitude of subthreshold membrane potential changes - important in neurons like STG cells that communicate via slow oscillations.