Glial Cells: Types and Functions

• Glial cells support and connect neurons in both the peripheral nervous system (PNS) and central nervous system (CNS).

Types of Glial Cells

• Astrocytes (macroglia) support neurons, anchor them to supply lines, help determine capillary permeability (blood-brain barrier), and mop up extracellular potassium and neurotransmitters.
• Protoplasmic astrocytes have thick, branched processes that attach to neurons/vessels.
• Fibrous astrocytes have long, thin, unbranched processes in white matter.
• Microglial cells monitor neuronal health, morphing into macrophages to phagocytose pathogens or dead neurons.
• Ependymal cells form a somewhat permeable barrier between cerebrospinal fluid and brain/spinal cord tissue, with cilia to circulate cerebrospinal fluid.
• Oligodendrocytes (macroglia) create the myelin sheath within the CNS.
• Satellite cells are small cells surrounding neuronal cell bodies in the PNS, similar to astrocytes.
• Schwann cells create the myelin sheath in the PNS, forming only a single myelin segment.

Nervous System Directionality

• CNS to PNS signals are efferent.
• PNS to CNS signals are afferent.
• "SAME" acronym: Sensory (afferent), Motor (efferent).

Organization of the Nervous System

• The autonomic nervous system includes the parasympathetic, sympathetic, and enteric nervous systems.
• The somatic nervous system includes sensory (afferent) and motor (efferent) components.
• In the CNS, cell bodies are located in either nuclei or fields.
• In the PNS, cell bodies are located in ganglia.

Interneurons

• Local interneurons (CNS) transmit information over short distances.
• Projection interneurons (CNS) transmit information over long distances.
• Neuroendocrine cells release hormones into the blood supply to influence distant organs.

General Neuronal Structure

• Dendrites receive input at specialized regions (synapses).
• The cell body is responsible for the synthesis and processing of proteins and lipids.
• The axon transmits information and may be myelinated.
• Micro/neurofilaments provide rigidity.
• Microtubules facilitate transport.
• Axon terminals are located at specialized regions (synapses).

Microtubule Transport

• Retrograde transport heads towards the negative end (dynein with dynactin attached) back to the cell body.
• Anterograde transport heads towards the positive end (kinesin) to the synapse.

Myelin Sheaths

• Myelin sheaths increase action potential conduction speed.
• They are approximately 0.5-1mm in length.
• Gaps in the myelin sheath are called nodes of Ranvier.
• The space between the pre-synaptic and post-synaptic neurons is called the synaptic cleft.
• The post-synapse has post-synaptic density to receive neurotransmitters.

Transmembrane Ion Channels

• All channels have a funnel-shaped entrance region, a water-filled central pore for ion movement, and an exit region.
• Channels exhibit varying degrees of selectivity for ions based on protein structures.
• Ion movement through a channel is passive, requiring no energy expenditure.
• Movement is determined by channel openness, the ion's concentration gradient, and the electrical gradient.

Types of Ion Channels

• Non-gated channels are always open.
• Chemically-gated channels are controlled by a chemical, altering protein shape to open or close the channel.
• Voltage-gated channels are controlled by electrical charge.
• Mechanically-gated channels are controlled by stress or pressure, found in specialized sensory receptor cells.
• Refractoriness: Voltage-gated cells can become inactive, while chemically and mechanically-gated cells can be desensitized, rendering them closed and incapable of opening.

Ion Channels Details

• Ion channels are transmembrane proteins.
• Each channel includes a funnel-shaped opening, a water-filled pore, an ion-selective part of the pore, and an exit region.
• Selectivity is based on the size of the ion, considering attracted water molecules, and the charge of the ion (e.g., Na+ channels don't allow negatively-charged ions).

Resting Membrane Potential (RMP)

• Neurons have an RMP of -70mV, indicating the inside is more negative than the outside.
• The neuron's membrane is polarized.
• Depolarization occurs when the RMP becomes less negative.
• Hyperpolarization occurs when the RMP becomes more negative.

Equilibrium Potential

• Equilibrium is achieved when the rate of positive ion movement due to the concentration gradient matches the rate due to the electrical gradient.
• Nernst equation calculates the potential difference needed to balance the concentration gradient for an ion.
• A large concentration gradient requires a large equilibrium potential.
• Nerve cell membranes selectively allow ions through to maintain homeostasis.

Nernst Equation Imperfections

• Membranes can be permeable to multiple ions.
• Different ions have varying permeabilities.
• Ion permeability can change.

Goldman Equation

• The Goldman equation accounts for multiple ions and their permeabilities.
• If the permeability of K+ is highest, the equation simplifies to a Nernst equation for K+.

Active Na+/K+ Pump

• Maintains concentration gradients for Na+ and K+ using ATP to expel Na+ and bring K+ back into the cell.
• Contributes to the resting RMP of -70mV.
• Sodium has a high extracellular concentration and low intracellular concentration, causing it to enter the cell.
• Potassium has the opposite concentrations, causing it to exit the cell.
• The Sodium-Potassium ATPase pump resets ion concentrations by expelling 3 sodium ions for every 2 potassium ions pulled in.
• Chloride has a low driving force because its equilibrium potential is close to the neuronal membrane potential (-80mV).

Action Potential

• Depolarization: Membrane potential decreases relative to the resting potential, making the membrane more positive.
• Hyperpolarization: Membrane potential increases relative to the resting potential, making the membrane more negative.
• Graded potentials are depolarization signals operating over short distances.
• Action potentials are depolarization signals operating over long distances.
• Hyperpolarization (very negative membrane) occurs.
• The Na+/K+ pump restores the membrane back to the resting state of -70mV.