Cogs107A - Week 2 - Lectures 3 &4 (polished version)

What are glial cells?

Glial cells are non-neuronal cells that support, protect, and regulate neurons. They are essential for normal brain function. Glia outnumber neurons in many brain regions. They do more than just provide support. Simple explanation: Glia are helper cells of the nervous system.

What are the main types of glial cells in the CNS?

The main CNS glial cells are ependymal cells, astrocytes, microglia, and oligodendrocytes. Each type has specialized functions. Together they support neuron health and communication. Simple explanation: Different glia have different jobs.

What are ependymal cells?

Ependymal cells line the ventricles of the brain and the central canal of the spinal cord. They are involved in producing and circulating cerebrospinal fluid. Some are modified to form the choroid plexus. Simple explanation: They line fluid-filled spaces in the brain.

What is the choroid plexus?

The choroid plexus is an epithelium composed of modified ependymal cells. It lines the ventricles. It produces cerebrospinal fluid (CSF). Its tight junctions form the blood-CSF barrier. Simple explanation: It makes and protects brain fluid.

What is cerebrospinal fluid (CSF)?

CSF circulates nutrients such as glucose and ions. It removes waste products from the brain. It cushions the brain within the skull. It helps maintain a stable environment. Simple explanation: CSF is the brain's protective fluid.

What is the blood-CSF barrier?

The blood-CSF barrier is formed by tight junctions in the choroid plexus. It controls which substances pass from blood into CSF. It protects the brain environment. Simple explanation: A filter between blood and brain fluid.

What are microglia?

Microglia are the immune cells of the CNS. They defend the brain and sculpt neural circuits. They respond rapidly to injury. They remove damaged cells and synapses. Simple explanation: Brain immune defenders.

What is phagocytosis?

Phagocytosis is the process by which a cell engulfs other cells or particles. Microglia use phagocytosis to remove debris. Astrocytes can also participate. Simple explanation: Cellular cleanup.

What happens when microglia become activated?

Activated microglia change shape and migrate toward injury sites. They release immune molecules. They engulf damaged tissue. They respond to ATP released by injured neurons. Simple explanation: They switch into emergency mode.

What role does ATP play in microglial activation?

Damaged neurons release high levels of ATP. ATP is sufficient to trigger microglial responses. Microglia detect ATP and move toward the source. Simple explanation: ATP acts as a distress signal.

What do microglia remove during development and plasticity?

Microglia remove synapses and neurites. They can also remove entire stressed or damaged neurons. This helps neural circuits adapt. Simple explanation: Microglia prune connections.

What are oligodendrocytes?

Oligodendrocytes are glial cells that form myelin in the CNS. They wrap axons with myelin sheaths. This increases signal speed and provides support. Simple explanation: CNS insulation cells.

What is the myelin sheath?

The myelin sheath is a spiral wrapping of glial membrane around an axon. It electrically insulates axons. It greatly increases conduction speed. Simple explanation: Insulation around wires.

What is the difference between oligodendrocytes and Schwann cells?

Oligodendrocytes myelinate axons in the CNS. Schwann cells myelinate axons in the PNS. One oligodendrocyte can wrap multiple axons, while one Schwann cell wraps one axon. Simple explanation: Same job, different locations.

What are astrocytes?

Astrocytes are star-shaped glial cells. They interact with neurons and blood vessels. They regulate synapses and the brain environment. They are highly abundant. Simple explanation: Multi-purpose support cells.

What is the blood-brain barrier (BBB)?

The BBB is formed by endothelial cells, pericytes, and astrocytes. It restricts passage of substances from blood to brain. Astrocyte end-feet help maintain it. Simple explanation: A protective wall around the brain.

What substances can cross the blood-brain barrier?

Fat-soluble substances like alcohol, nicotine, and caffeine cross by passive diffusion. Glucose crosses via active transport. Many drugs cannot cross easily. Simple explanation: Only select substances get through.

What are advantages of the blood-brain barrier?

It protects the brain from toxins. It keeps CNS and PNS neurotransmitters separate. It tightly regulates ion composition. Simple explanation: Strong protection for neurons.

What are disadvantages of the blood-brain barrier?

Glucose delivery is more energetically costly. Drug delivery to the brain is difficult. Treatment of brain disorders is challenging. Simple explanation: Protection comes at a cost.

What is the tripartite synapse?

The tripartite synapse includes a presynaptic neuron, postsynaptic neuron, and surrounding astrocytes. Astrocytes influence synaptic transmission. They shape the chemical environment. Simple explanation: Synapses involve three partners.

How do astrocytes influence synaptic transmission structurally?

Astrocytes segregate neighboring synapses. They isolate chemical release. They alter the geometry between cells. Simple explanation: They organize synapse spacing.

How many synapses can one astrocyte contact?

One astrocyte can contact approximately 100,000 synapses. This allows broad regulation of neural activity. Simple explanation: One astrocyte manages many connections.

How do astrocytes use calcium signaling?

Astrocytes show Ca2+ increases in response to neurotransmitters. These occur in microdomains. Calcium waves can propagate across astrocytes. Simple explanation: Calcium is their communication signal.

What are gliotransmitters?

Gliotransmitters are substances released by astrocytes. Examples include glutamate, GABA, and ATP. They influence synaptic transmission. Simple explanation: Chemical messages from glia.

How do astrocytes influence synapse development?

Astrocytes release factors like thrombospondins and cholesterol. These promote new synapse formation. They can modify synapse strength. Simple explanation: Astrocytes help build synapses.

How do astrocytes participate in phagocytosis?

Astrocytes signal microglia to remove synapses indirectly. They can also directly recognize and engulf silent synapses. Simple explanation: Astrocytes help with cleanup.

How do glial cells communicate with each other?

Astrocytes and oligodendrocytes communicate via gap junctions. Calcium waves can propagate between astrocytes. This allows network-level signaling. Simple explanation: Glia talk to each other.

What is secondary injury in traumatic brain injury?

Glia can contribute to secondary injury after trauma. They release pro-inflammatory cytokines. They release glutamate, causing excitotoxicity. Simple explanation: Damage spreads after initial injury.

How are microglia involved in Alzheimer's disease?

Microglia are activated by amyloid-beta (Aβ). This activation can contribute to neuron loss. Chronic inflammation worsens disease progression. Simple explanation: Immune response can become harmful.

What role do glia play in multiple sclerosis (MS)?

MS involves inflammation, demyelination, and microglial activation. Oligodendrocytes are damaged. Loss of myelin slows or stops signals. Simple explanation: Myelin loss disrupts communication.

Why does demyelination impair neural signaling?

Myelin increases conduction speed. Loss of myelin slows or blocks action potentials. Signal transmission becomes unreliable. Simple explanation: Signals leak without insulation.

What are the key functions of glia overall?

Glia form barriers like the BBB and blood-CSF barrier. They regulate synapses and neural communication. They can also signal and respond to injury. Simple explanation: Glia support, protect, and regulate neurons.

What does the word "glia" mean historically?

Glia comes from the Greek word for "glue." Early neuroscientists believed glial cells merely held neurons together and played no active role. Simple explanation: They were thought to be brain filler.

How has the modern view of glia changed?

Glia are now recognized as active participants in brain function. They influence development, signaling, metabolism, plasticity, and disease. Simple explanation: Glia do real brain work.

What are the core functions of glial cells?

Glia guide brain development, modulate synaptic signaling, provide metabolic support, shape plasticity, and contribute to disease processes. Simple explanation: They support and regulate neurons.

Do glial cells outnumber neurons 10 to 1?

No. The human brain contains roughly equal numbers of neurons and glia (about 86 billion neurons to 85 billion glia). Simple explanation: The 10:1 ratio is a myth.

How does the glia-to-neuron ratio vary across brain regions?

The cerebral cortex is glia-heavy (~4:1), the cerebellum is neuron-heavy, and the brainstem and ganglia are glia-heavy. Simple explanation: Different regions need different support.

What are ependymal cells?

Ependymal cells line the ventricles of the brain and the central canal of the spinal cord. They play a key role in producing and regulating cerebrospinal fluid. Simple explanation: They line fluid spaces.

What is the choroid plexus?

The choroid plexus is a structure made of modified ependymal cells that produces cerebrospinal fluid (CSF). Simple explanation: It makes brain fluid.

What is the blood-CSF barrier?

The blood-CSF barrier is formed by tight junctions between ependymal cells. It prevents direct leakage from blood into CSF, forcing regulated transport through cells. Simple explanation: A controlled filter.

What role do cilia play on ependymal cells?

Cilia beat to circulate cerebrospinal fluid through the ventricular system. This helps distribute nutrients and remove waste. Simple explanation: They stir the fluid.

What are the main functions of cerebrospinal fluid (CSF)?

CSF circulates nutrients and ions, removes waste, and cushions the brain so it floats and avoids compression. Simple explanation: Feed, clean, protect.

What are microglia?

Microglia are the immune cells of the central nervous system. They monitor, defend, and sculpt neural tissue. Simple explanation: Brain immune guards.

What do microglia look like in their resting state?

Resting microglia have thin, branching processes that constantly survey their environment. They are active even when "resting." Simple explanation: Always watching.

What happens when microglia become activated?

Activated microglia become bloated and migrate to sites of injury or infection. They change shape and function. Simple explanation: They rush to damage.

What is phagocytosis in microglia?

Phagocytosis is the process by which microglia engulf and digest debris, dead cells, or unwanted synapses. Simple explanation: Cellular cleanup.

What signal can trigger microglial activation?

Damaged neurons release ATP, which alone is sufficient to attract and activate microglia. Simple explanation: ATP is a distress signal.

What is synaptic pruning?

Synaptic pruning is the removal of unnecessary synapses by microglia during development and plasticity. Simple explanation: Circuit refinement.

What is the function of oligodendrocytes?

Oligodendrocytes provide myelin insulation for axons in the central nervous system. This increases signal speed and efficiency. Simple explanation: They insulate wires.

What is myelin?

Myelin is a fatty sheath that wraps around axons, preventing ion leakage and speeding electrical signaling. Simple explanation: Electrical insulation.

What are nodes of Ranvier?

Nodes of Ranvier are gaps between myelin segments where voltage-gated ion channels cluster. Action potentials jump between nodes. Simple explanation: Signal relay points.

How do oligodendrocytes differ from Schwann cells?

Oligodendrocytes myelinate axons in the CNS and can myelinate multiple axons. Schwann cells myelinate the PNS and wrap only one axon. Simple explanation: One-to-many vs one-to-one.

Why does myelin increase conduction speed?

Myelin restricts ion exchange to nodes of Ranvier, allowing saltatory conduction where signals jump node to node. Simple explanation: Faster jumps.

What are astrocytes?

Astrocytes are highly branched glial cells that regulate synapses, blood flow, and brain chemistry. Simple explanation: Brain regulators.

What structures form the blood-brain barrier (BBB)?

The BBB is formed by endothelial cells, pericytes, and astrocyte end feet. Together they protect the CNS. Simple explanation: A protective wall.

What is the function of the BBB?

The BBB restricts harmful substances while allowing essential nutrients into the brain. Simple explanation: Selective protection.

What substances cross the BBB by passive diffusion?

Small, fat-soluble substances like alcohol, nicotine, and caffeine cross freely. Simple explanation: Fat gets through.

How do nutrients cross the BBB?

Essential nutrients like glucose cross via active transport mechanisms. Simple explanation: Special carriers.

How does the BBB differ from the blood-CSF barrier?

The BBB is regulated by astrocyte-endothelial interactions, while the blood-CSF barrier relies on tight junctions between ependymal cells. Simple explanation: Different control systems.

What is the tripartite synapse?

It includes the presynaptic neuron, postsynaptic neuron, and surrounding astrocyte. All three contribute to signaling. Simple explanation: Three-part communication.

How do astrocytes regulate neurotransmitter spread?

Astrocytes physically encapsulate synapses, preventing neurotransmitter spillover. Simple explanation: Chemical containment.

How do astrocytes respond to synaptic activity?

They have neurotransmitter receptors that trigger localized calcium increases within astrocytes. Simple explanation: They listen in.

What are gliotransmitters?

Gliotransmitters are chemicals released by astrocytes that modulate synaptic signaling. Simple explanation: Glial feedback signals.

How do astrocytes modify synapses?

They regulate receptor number, vesicle release probability, and help create or eliminate silent synapses. Simple explanation: Fine-tuning circuits.

Can astrocytes remove synapses?

Yes. Astrocytes can directly phagocytose synapses or recruit microglia to remove them. Simple explanation: Controlled pruning.

How do glial cells communicate with each other?

Astrocytes and some oligodendrocytes connect via gap junctions that share intracellular space. Simple explanation: Direct cell linking.

What are calcium waves?

Calcium waves occur when calcium increases spread across astrocytes through gap junctions. This coordinates glial networks. Simple explanation: Shared signaling waves.

What is secondary injury in traumatic brain injury (TBI)?

Secondary injury is delayed damage caused by overactive glia releasing cytokines and excess glutamate. Simple explanation: Damage after damage.

What is excitotoxicity?

Excitotoxicity is neuron death caused by excessive glutamate overstimulation. Simple explanation: Overfiring kills cells.

What is the vicious cycle hypothesis in Alzheimer's disease?

Microglia activate in response to Aβ plaques, release toxins, damage neurons, and trigger further microglial activation. Simple explanation: Self-feeding degeneration.

What happens in multiple sclerosis (MS)?

MS involves immune-mediated destruction of oligodendrocyte myelin in the CNS. Simple explanation: Loss of insulation.

Why does demyelination cause neurological symptoms?

Without myelin, action potentials slow or fail, disrupting communication. Symptoms depend on lesion location. Simple explanation: Signals break down.

Who was Luigi Galvani and what did he discover?

Luigi Galvani showed that frog muscles twitch when nerves are connected with different metals, even after death. He concluded that living tissue contains its own electricity, rejecting the idea of "animal spirits." Simple explanation: He discovered biological electricity.

What experiment did Galvani perform with frog legs?

Galvani completed a circuit between a frog nerve and muscle using two metals, causing muscle contraction. He also used atmospheric electricity (lightning) to trigger contractions. Simple explanation: Circuits made muscles move.

What is electrophysiology?

Electrophysiology is the study of electrical properties of biological tissues. Galvani's work established this field. Simple explanation: Studying electricity in living systems.

How did Alessandro Volta challenge Galvani's conclusion?

Volta argued the electricity came from the metals, not the tissue. He showed that two different metals create a voltage difference. Simple explanation: Metals generate electricity.

What was Volta's lasting contribution?

Volta's work led to the invention of the first battery. Although he was right about metals, Galvani was correct that neurons use electrical signals. Simple explanation: He built the battery.

How does the nervous system transmit information over long distances?

The nervous system uses electrical signals called action potentials that travel along axons. These signals rely on membrane properties. Simple explanation: Electricity moves messages.

What is a reflex arc?

A reflex arc is a fast, automatic response pathway involving sensory input, spinal processing, and motor output. Simple explanation: A quick response loop.

What happens when you step on a tack?

Sensory neurons send signals to the spinal cord. One pathway goes to the brain for pain perception, while another activates motor neurons to withdraw the foot. Simple explanation: Feel pain and pull away.

Why are reflexes faster than conscious actions?

Reflexes are processed in the spinal cord without waiting for brain involvement. Simple explanation: Shorter pathway.

What generates electrical signals in neurons?

Electrical signals arise from ions dissolved in water that are separated by a lipid membrane. Simple explanation: Charged particles + barrier.

What is a polar molecule?

A polar molecule has uneven electron distribution, creating positive and negative ends. Water is a classic example. Simple explanation: Unequal charge.

What is a nonpolar molecule?

A nonpolar molecule has even electron distribution and no charge separation. Carbon dioxide is an example. Simple explanation: No charged ends.

What are hydration shells?

When salts dissolve, water molecules surround ions to keep them dissolved. This organized shell stabilizes ions in solution. Simple explanation: Water shields ions.

What does "like dissolves like" mean?

Polar molecules dissolve in water, while nonpolar molecules cluster together. This explains why oils avoid water. Simple explanation: Similar types mix.

What is the structure of a phospholipid?

A phospholipid has a hydrophilic (polar) head and hydrophobic (nonpolar) tails. Simple explanation: Water-loving head, water-fearing tails.

Why do phospholipids form a bilayer?

Heads face water inside and outside the cell, while tails pack inward away from water. This creates a stable barrier. Simple explanation: Self-assembling wall.

Why does the membrane block ions?

The membrane's hydrophobic core repels charged ions and water. Ions require channels to cross. Simple explanation: Charges can't pass through oil.

What is diffusion?

Diffusion is the movement of molecules from high to low concentration until equilibrium is reached. Simple explanation: Spreading out.

What is a concentration gradient?

A concentration gradient is a difference in ion concentration across the membrane. It drives diffusion. Simple explanation: Unequal distribution.

Why are ion channels necessary?

The lipid bilayer blocks ions, so channels provide selective pathways for diffusion. Simple explanation: Doors through the wall.

What is electrical potential difference (voltage)?

Voltage is stored energy caused by separation of charge. It represents the potential to do work. Simple explanation: Electrical pressure.

How does the water reservoir analogy explain voltage?

A pump lifts water to create stored energy. When released, water flows and does work—just like ions flowing down voltage. Simple explanation: Height equals energy.

What is electrical current (I)?

Current is the movement of electrical charge (ions). It is measured in amperes. Simple explanation: Flow of charge.

What is electrical conductance (g)?

Conductance is how easily charge moves across the membrane. More open channels increase conductance. Simple explanation: Openness.

What is electrical resistance (R)?

Resistance is the opposition to charge movement. It equals 1/g. Simple explanation: Blockage.

How do neurons increase current flow?

They open more ion channels, increasing conductance and lowering resistance. Simple explanation: Open more doors.

How is membrane potential measured?

A voltmeter uses one electrode inside the neuron and a reference electrode outside. Simple explanation: Compare inside to outside.

What is the resting membrane potential?

The inside of the neuron is negative relative to the outside at rest. Simple explanation: Inside is more negative.