PSYC216L2: Cells of the Nervous System

Approximately how many neurons are in the human brain?

About 86 billion.

What is the overall neuron-to-glia ratio in the human brain?

Roughly 1:1, though it varies by region.

In the cerebral cortex, neurons vs glia ratio?

Glia outnumber neurons 4:1

In the cerebellum, neurons vs glia ratio?

Neurons outnumber glia 4:1 (opposite of cortex).

Beyond brain size, list four neuronal factors associated with higher intelligence.

1. Number of neurons in the cerebral cortex (more is better)

2. Thickness of the cerebral cortex (thicker is better)

3. Whether those neurons are densely packed or loosely packed (density is better)

4. Axonal conduction velocity (faster is better)

Where are Von Economo neurons predominantly located?

Anterior cingulate gyrus and insula

Key features of Von Economo neurons?

·       Specialized neuron found in humans and great apes + elephants and cetaceans

·       Long, slender neurons with elongated cell bodies

·       0.5 million in human brain

·       Conduct nerve impulses rapidly

·       May be involved in social information processing

Primary function of neurons?

Communication/Information-Processing

Name three general roles glial cells play.

Support & insulation, myelination, metabolic/nutrient support (plus many others).

Which glial cell type buffers extracellular K⁺ and helps form the blood‑brain barrier?

Astrocytes.

Which CNS glial cell forms myelin?

Oligodendrocytes.

Which PNS glial cell forms myelin and guides axon regrowth?

Schwann cells.

Which glial‑like cells act as the brain’s immune scavengers?

Microglia.

Give two ways astrocytes modulate synapses.

Store/release neurotransmitters & promote synaptogenesis.

What is the main input region of a neuron?

Dendrites (often with dendritic spines).

What cellular process occurs in dendritic spines?

Synaptic reception & local integration of signals.

Which neuron part integrates signals and contains the nucleus?

Cell body (soma).

What is an axon collateral?

A branch of the main axon allowing communication with multiple targets.

What proteins power fast axonal transport anterograde & retrograde?

Kinesin (anterograde) and dynein (retrograde) moving along microtubules.

Terminal boutons contain high densities of what organelle for energy?

Mitochondria.

Which enzyme inside terminals degrades monoamine neurotransmitters?

Monoamine oxidase (MAO) in mitochondria or vesicles.

What is the role of transporter proteins in presynaptic terminals?

Load neurotransmitters into vesicles & reclaim them from the cleft.

Define autoreceptor.

Presynaptic receptor that monitors & regulates its own neurotransmitter release.

What does the nucleolus produce?

Ribosomal RNA → ribosome assembly.

Function of rough endoplasmic reticulum in neurons?

Protein synthesis due to attached ribosomes.

Two key roles of smooth ER in neurons?

Lipid synthesis & glucose metabolism/calcium storage.

What does the Golgi apparatus do for neurotransmitters?

Packages proteins/lipids into vesicles for transport or secretion.

What is the main energy currency produced by mitochondria?

ATP via oxidative phosphorylation.

Describe a unipolar neuron.

Single process that splits into central & peripheral branches—common in somatosensory ganglia.

Primary function of bipolar neurons?

Sensory—e.g., retina & olfactory epithelium.

What type of neuron carries information to muscles?

Motor (efferent) neurons, typically multipolar & myelinated.

What is an interneuron?

Neuron whose processes remain within a single brain region; integrates local circuits.

Why are myelinated axons faster?

Myelin increases membrane resistance & decreases capacitance → saltatory conduction at nodes.

What is a nucleus (in neuroanatomy)?

A cluster of neuronal cell bodies within the CNS.

What are projection fibers?

Bundles of long axons carrying information between brain regions.

Typical resting membrane potential value?

≈ –70 mV (inside negative).

Which ions are more concentrated outside the neuron at rest?

Na⁺ and Cl⁻.

Which ions dominate inside the neuron at rest?

K⁺ and large anions (A⁻, proteins).

What transport protein maintains ionic gradients at rest?

Na⁺/K⁺‑ATPase (pumps 3 Na⁺ out / 2 K⁺ in).

Approximately what percentage of brain energy powers Na⁺/K⁺ pumps?

≈ 40 %.

Name the two passive forces on ions across membranes.

Diffusion (concentration gradient) & electrostatic pressure (charge).

Why is the membrane called selectively permeable?

Different ion channels confer varying permeabilities to specific ions.

Where is the action potential initiated?

Axon hillock/initial segment where Na⁺ channel density is highest.

Define threshold in neuronal firing.

Membrane potential at which inward Na⁺ current exceeds outward leaks, triggering an AP (≈ –55 mV).

Explain spatial vs temporal summation.

Spatial: inputs from multiple synapses combine; Temporal: repeated inputs from one synapse add over time.

During AP rising phase, which ion rushes in?

Na⁺ via voltage‑gated Na⁺ channels.

Which ion contributes to repolarization?

K⁺ leaving through voltage‑gated K⁺ channels.

Why is there an after‑hyperpolarization?

K⁺ channels stay open briefly after repolarization, driving Vm more negative than rest.

Define absolute refractory period.

Time during which a second AP cannot be generated because Na⁺ channels are inactivated.

Define relative refractory period.

Period when a stronger‑than‑normal stimulus can evoke an AP as membrane recovers.

Does AP amplitude change along the axon?

No—APs regenerate fully, obeying the all‑or‑none law.

What is cable theory in neurons?

Describes passive spread of voltage along dendrites/axons with resistance and capacitance constraints.

Define saltatory conduction.

AP jumps node‑to‑node along myelinated axons, boosting speed & saving energy.

Two advantages of saltatory conduction?

"1. Faster conduction 

2. Lower ATP cost (fewer Na⁺/K⁺ pumps needed)."

How does increasing axon diameter speed conduction?

Reduces internal (axial) resistance, allowing current to spread faster.

Capacitance relates to what property of a membrane?

Ability to store electric charge (lower capacitance → faster voltage change).

Where are Na⁺ channels most densely clustered in myelinated axons?

Nodes of Ranvier (~10× ordinary axonal density).

Relative abundance of Na⁺ channels in the membrane overall?

Fewer than 1 in 1 million membrane molecules are Na⁺ channels.

Which pufferfish toxin blocks voltage‑gated Na⁺ channels?

Tetrodotoxin (TTX).

Name a marine dinoflagellate toxin with similar Na⁺ channel block.

Saxitoxin.

Local anesthetics like novocaine act by blocking which channels?

Voltage‑gated Na⁺ channels in peripheral nerves.

Which compound selectively blocks voltage‑gated K⁺ channels and is sometimes used experimentally to broaden APs?

Tetraethylammonium (TEA).

What spider toxin blocks K⁺ channel pores?

Tarantula toxin.

Scorpion venom affects K⁺ channels how?

Inhibits the voltage sensor, preventing opening.

Which autoimmune disease destroys CNS myelin?

Multiple sclerosis (MS).

Core deficit in amyotrophic lateral sclerosis (ALS)?

Degeneration of motor neurons leading to muscle weakness & paralysis.

Possible link between ALS and trauma?

Head/axonal trauma may trigger programmed cell death in vulnerable motor neurons.

What is a 'maintained discharge'?

Baseline spontaneous firing rate of neurons even without apparent stimuli.

According to the rate law, how is stimulus intensity encoded?

By frequency of action potentials, not amplitude.

Contrast leaky (passive) vs voltage‑gated channels.

Leaky channels are always open, setting resting conductances; voltage‑gated open/close with membrane potential changes to generate APs.

Where can you watch animations of resting & action potentials for review?

Sumanas interactive animations (links in lecture outline).