Neuroscience Chapter: Glial Cells and Neurons

Neuronal Signaling

• Neuronal signaling is the process by which neurons communicate with each other and other cells.
• The nervous system is composed of the central nervous system (CNS) and the peripheral nervous system (PNS).

Cells of the Nervous System

• The functional unit of the nervous system is the neuron.
• Neurons generate electrical signals that cause the release of chemical messengers (neurotransmitters).
• Neurotransmitters allow neurons to communicate with other cells.
• Most neurons act as integrators, their output reflecting the balance of inputs they receive.
• Glial cells are non-neuronal cells that provide supportive functions, like retaining the ability to divide throughout their lifespan.

Structure of Neurons

• Cell body (soma): contains the nucleus, ribosomes, and organelles.
• Dendrites: branched outgrowths that receive incoming information. Branching increases cell surface area.
• Axon: extends from the cell body and carries outgoing signals.
• Axon hillock (initial segment): the location where electrical signals are generated.
• Collaterals: branches of the axon. Branching occurs near their endpoints.

Myelination of Axons

• Many neuron axons are myelinated.
• Myelin: 20-200 layers of plasma membrane wrapped around axons by supporting cells.
• Myelin sheaths are formed by oligodendrocytes (CNS) and Schwann cells (PNS).
• Nodes of Ranvier: spaces between myelin sections where the axon's plasma membrane is exposed to extracellular fluid.
• Myelin sheaths speed up electrical signal conduction and conserve energy.

Axonal Transport

• Axonal transport: movement of organelles and materials between the cell body and axon terminals.
• Kinesin (anterograde): transport from cell body to axon terminals, moves nutrients, enzymes, and neurotransmitter-filled vesicles.
• Dynein (retrograde): transport from axon terminals to cell body, moves recycled membrane vesicles, growth factors, and other chemical signals.

Functional Classes of Neurons

• Afferent neurons: carry information from tissues and organs to the CNS.
• Efferent neurons: carry information away from the CNS to effector cells (like muscles and glands).
• Interneurons: connect neurons within the CNS.
• For each afferent neuron entering the CNS, there are ~10 efferent neurons and ~200,000 interneurons.
• Afferent neurons have peripheral sensory receptors that respond to stimuli.
• Afferent neurons typically have a single axon with a central process that enters the CNS and a peripheral process.
• Efferent neurons have their cell bodies in the CNS.
• Groups of afferent and efferent neuron axons, connective tissue, and blood vessels form the nerves of the PNS.

Synapses

• Synapse: the junction between two neurons where one neuron alters the electrical and chemical activity of another neuron.
• Signals are transmitted via neurotransmitters, which combine with receptor proteins on the receiving neuron's membrane.
• Most synapses occur between an axon terminal of one neuron and a dendrite or cell body of another neuron.
• A postsynaptic neuron can have thousands of synaptic junctions.

Glial Cells

• Astrocytes: regulate extracellular fluid composition (remove K+ and neurotransmitters), stimulate the blood-brain barrier formation, and support neurons metabolically (provide glucose, and remove wastes)
• Microglia: specialized, macrophage-like cells that perform immune functions.
• Ependymal cells: line fluid-filled cavities and regulate cerebrospinal fluid production and flow.
• Schwann cells: glial cells in the PNS, responsible for producing myelin sheaths.

Basic Principles of Electricity

• The differences in charge across the cell membrane allow for the flow of current.
• The potential difference is the work that can be done as separated charges come together.
• Electrical potential is measured in millivolts.

Concentration Data

• Ion concentration differs significantly between intracellular and extracellular fluids of a neuron. -Na+, Cl-, and K+ most significant

The Resting Membrane Potential

• At rest, the inside of a neuron is negatively charged relative to the outside.
• The resting membrane potential (RMP) is determined by the concentration and permeability differences of different ions across the cell membrane.

Nature and Magnitude of the Vm

• The RMP of neurons ranges from -40 to -90 mV.
• The RMP exists due to the small excess of negative ions inside the cell and positive ions outside the cell.
• Excess charged particles gather in a thin layer on both sides of the cell membrane

Major Ions Across the PM

• The concentration differences for Na+ and K+ are mainly due to the Na+/K+-ATPase pump.
• The RMP primarily results from K+ diffusion down its concentration gradient.

Contribution of Ion Concentration Differences

• Difference in concentration across the membrane sets up a driving force; the membrane potential opposes the concentration gradient.
• At equilibrium, there's no net movement of ion.

Contribution of Ion Permeability to Membrane Potential

• The Goldman-Hodgkin-Katz (GHK) equation accounts for the different permeability of various ions across the membrane as well as their concentration gradients.

Action Potential in the Neuron

• Action potentials are large, rapid changes in membrane potential.
• APs are mediated by voltage-gated Na+ and K+ channels.

Development of a Resting Membrane Potential

• K+ permeability is far greater than that of Na+ or Cl¬
• K+ mostly responsible for RMP
• A small number of open Na+ channels slightly pull the RMP towards the Na+ equilibrium potential.

Graded Potentials

• Graded potentials are short-distance signals.
• Their magnitude varies with the triggering event's strength and diminishes with distance.
• Graded potentials allow for the summation of stimuli, potentially triggering further events.

Action Potentials

• Action potentials are long-distance signals.
• They have a threshold and are all-or-none.
• Initiation depends on sum total of excitatory and inhibitory graded potentials.

Action Potential Propagation

• The current generated during an AP can be enough to depolarize adjacent membranes to threshold, triggering a chain reaction.
• Membranes that have just undergone an AP are refractory and don't fire again immediately, preventing backward propagation.

Speed of AP Propagation

• Larger fiber diameter reduces resistance to local current, increasing conduction speed.
• Myelination (saltatory conduction): speeds up AP propagation by allowing the signal to jump between nodes of Ranvier

Saltatory Conduction

• This mode of propagation is faster due to signal transmission along myelinated sections of the neuron.

Generation of Action Potentials

• Stimuli initiate action potentials by bringing the membrane to threshold.
• In afferent neurons, depolarization to threshold is due to a receptor potential.

Synaptic Integration

• A single excitatory signal by itself is often not enough to cause an action potential.
• Temporal summation: multiple EPSPs (excitatory postsynaptic potentials) occurring close together in time sum to create a larger depolarization.
• Spatial summation: EPSPs from multiple synapses arriving close in space sum.
• EPSPs and IPSPs (inhibitory postsynaptic potentials) can cancel each other out.

Modification of Synaptic Transmission by Drugs

• Drugs influence synaptic mechanisms by impacting neurotransmitter release, synthesis/degradation, or receptor function.

Neurotransmitters and Neuromodulators

• Neurotransmitters are involved in rapid actions (excitation/inhibition).
• Neuromodulators modulate the effects of neurotransmitters, influencing slower, more complex events like developmental states.

Biogenic Amines

• These (e.g., dopamine, norepinephrine, and serotonin) are small, charged molecules derived from amino acids and play essential roles in various neurological functions and mental states.

Acetylcholine

• It's a major neurotransmitter in the PNS (neuromuscular junctions) and CNS (various cognitive functions)
• It plays roles in learning, attention, learning, and memory functions of the brain.

Alzheimer's Disease

• It affects a substantial percentage of aging individuals (10-15% over age 65, 50%+ over 85).
• Loss of cholinergic neurons and the associated neurotransmitter (ACh) contributes to the cognitive decline observed in this disease.

Amino Acids: GABA

• GABA is the major inhibitory neurotransmitter in the brain (involved in neural circuit activities).
• GABA increases Cl- influx, causing hyperpolarization, which inhibits further firing.

Neuropeptides

• These are chains of two or more amino acids influencing various physiological responses, frequently modulating actions of other neurochemicals.

Gases

• Gases like nitric oxide (NO) are important signaling molecules, transmitting signals in a localized fashion due to diffusion, affecting several neuronal and cellular functions.

Test your knowledge on the functions of glial cells and key aspects of neuronal structure and function. This quiz covers topics such as the role of the axon hillock, myelin sheath, and various types of glial cells. Perfect for students studying neuroscience or related fields.