Neurons: Structure, Function, and Types

Cell Types & Functions

• Neurons and glial cells are types of cells to know.
• Neural communication includes electrical and chemical types.
• Intracellular structures are another thing to be known.

Neuron Structure and Function

• Neurons send information to other cells.
• Dendrites receive signals.
• The soma is the cell body.
• The nucleus is located in the soma.
• The axon transmits signals, covered by the myelin sheath in segments.
• The nodes of Ranvier are gaps in the myelin sheath where action potentials occur.
• Synapses are structures that permit a neuron to pass an electrical or chemical signal to another cell.

Types of Neurons

• Sensory neurons bring information to the central nervous system.
• Interneurons associate sensory and motor activity in the central nervous system.
• Motor neurons send signals from the brain and spinal cord to muscles.
• Unipolar neurons have a single projection from the cell body.
• Bipolar neurons have two projections from the cell body.
• Pseudounipolar neurons have a single projection that splits into two branches.
• Multipolar neurons have multiple projections from the cell body.

Tracts vs. Nerves

• Tracts are bundles of axons within the CNS (Central Nervous System).
• Nerves are bundles of axons within the PNS (Peripheral Nervous System).

Glial Cells

• Glial cells support neurons and are very important to neural functions.

Types of Neuroglia

• Schwann cells are in the peripheral nervous system.
• Satellite glial cells surround neuron cell bodies in the PNS.
• Oligodendrocytes are in the central nervous system.
• Astrocytes are also located in the central nervous system.
• Microglia are located in the central nervous system as well.
• Ependymal cells line ventricles in the CNS.

Central Nervous System Glial Cells

• Ependymal cells line ventricles and assist in producing, circulating, and monitoring cerebrospinal fluid (CSF).
• Oligodendrocytes myelinate CNS axons and provide structural framework.
• Astrocytes maintain the chemical environment, provide structural support, nourish neurons, absorb and recycle neurotransmitters, form scar tissue, and establish the blood-brain barrier.
• Microglia develop from the immune system to identify and attack invaders, acting as scavengers through phagocytosis.

Blood Brain Barrier

• The blood-brain barrier is selectively permeable.

Glial Cells in the PNS

• Schwann cells myelinate axons, assist regrowth, and are involved in phagocytosis.
• Satellite cells provide structural support like astrocytes in the CNS, and assist in skeletal muscle adaptations.

Oligodendrocytes vs. Schwann Cells

• Oligodendrocytes are in the central nervous system(CNS), and Schwann cells are in the peripheral nervous system (PNS).

Intracellular Structures

• Intracellular structures are located inside of the cell.

Cell Membrane

• The cell membrane is a barrier and gatekeeper.
• It is a phospholipid bilayer that separates extracellular fluid, outside the cell, from intracellular fluid, inside the cell.
• The cell membrane has hydrophilic heads with polar regions, and hydrophobic tails which have no polar regions.
• Phosphate groups will bind to water, while fatty acid tails have no binding sites for water.

Crossing the Cell Membrane

• Ions can cross the cell membrane through appropriately shaped channels.
• Gated channels change shape to allow the passage of substances when gates are open.
• Gates prevent passage when one or both gates are closed.
• A pump transporter changes shape to carry substances across a cell membrane, requiring ATP.
• Channels are passive.
• Pumps are active.

Intracellular Organelles and Structures

• Mitochondria provides the cell with energy.
• The cell membrane separates intra- from extracellular space via a double layer of lipid molecules.
• Lysosomes use enzymes to break down waste.
• The nucleus houses chromosomes.
• Golgi bodies are responsible for protein packaging and trafficking.
• Microfilaments provide structure to the cell, helping to give it its shape.
• The nuclear membrane surrounds the nucleus.
• The endoplasmic reticulum are folded layers of membrane where proteins are assembled.
• Intracellular fluid is the fluid in which the cell's internal structures are suspended.
• Tubules are tiny tubes that transport molecules and help give the cell its shape.
• Microfilaments are threadlike fibers making up much of the cell's "skeleton".

Protein Synthesis

• During transcription, a sequence of bases is copied into mRNA.
• During translation, the mRNA is used by ribosomes to assemble a specific amino acid chain, forming a protein.
• Codons are sequences of three bases on mRNA coding for a particular amino acid.
• Ribosomes are protein structures that act as catalysts for protein synthesis.
• DNA uncoils to expose a gene, a sequence of nucleotide bases that encodes a protein.
• One strand of the gene serves as a template for transcribing a molecule of mRNA.
• The mRNA leaves the nucleus and comes in contact with ribosomes in the endoplasmic reticulum.
• A ribosome moves along the mRNA translating the bases into a specific amino acid chain to form the protein.
• Proteins that are formed in the Endoplasmic Reticulum enter the Golgi bodies, are wrapped in a membrane, and given a shipping address
• Afterwards each protein package is attached to a motor molecule and moves along a microtubule to its destination.

Protein Function

• Proteins allow communication among brain cells (receptors).
• Proteins carry oxygen in blood, with hemoglobin as an example.
• Proteins digest food, with enzymes like amylase, pepsin, and lactase as examples.
• Proteins defend the body from invading microorganisms (antibodies).
• Proteins speed up chemical reactions inside the body, with enzymes.
• Proteins provide elasticity to skin (elastin) and strength to hair/nails (keratin).
• mRNA translation, resilience, and PTSD are areas of study.
• ACE2 protein expression is increased in Alzheimer's Disease.
• COVID takes advantage of translation & replication.

Electrical Communication: Within Neurons

• Dendrites collect electrical signals.
• The cell body integrates incoming signals and generates outgoing signals to the axon.
• The axon passes electrical signals to dendrites of another cell or to an effector cell.

Membrane Potential

• "Resting" membrane potential is approximately -70 mV.
• Ions involved are Sodium (Na+), Chloride (Cl-), Potassium (K+), and Large proteins (A-).

Forces at Play

• Concentration Gradient: Molecules move from areas of high concentration to areas of low concentration.
• Electrical Gradient: Opposite charges attract; like charges repel.
• At resting potential, Na+ wants to move inside the cell due to diffusion and electrostatic pressure.
• A- cannot leave the cell, K+ experiences force of diffusion outside of cell, Cl is trying to leave cell.
• The sodium-potassium pump is more permeable to K+; requiring ATP (40% of cells resources.

Action Potential Characteristics

• An action potential can be initiated, if there is a change in internal environment from “Resting Membrane Potential” to “Threshold of Excitation” which is a ~+20mV change.
• Characteristics include "All or None" behavior and adherence to the "Rate law".
• Hyperpolarization is due to an efflux of K+, making extracellular side of membrane more positive.(more Negative)
• Depolarization is due to an influx of Na+ through Na+ channels (more Positive)
• Action potentials are generated when summed EPSPs and IPSPs cause depolarization of the membrane at the axon hillock to the threshold level.

Rate Law

• The rate law indicates the strength of a stimulus is represented by the rate of firing of an axon and the size of each action potential is always constant.

Action Potentials in Myelinated Axons

• In saltatory conduction, action potentials jump between the nodes of Ranvier.

Chemical Communication Between Neurons

• Chemical communication involves neurotransmitters communicating from one cell to another.

Two Classes of Receptors

• Ionotropic receptors are directly gated ion channels. -Transmitter binds to the binding site and then the pore opens, allowing the influx or efflux of ions.
• Metabotropic receptors use second messengers to indirectly modulate ion channels.
• Post-synaptic membrane NT effects include
• Influx of Na+ causes depolarization (EPSP)
• Efflux of K+ causes hyperpolarization (IPSP) Influx of Cl- causes hyperpolarization (IPSP)
• Influx of Ca2+ activates enzyme

Termination of Neurotransmitter Action

• Neurotransmitter action is terminated by reuptake or enzymatic deactivation.

Other factors in typical communication

• It is Neuromodulators (CNS, Typical peptides)
• Hormones act as Glands directed toward Target cells via the circulatory system
• Both of these exhibit Diffuse effects,are Typically metabotropic, slower but longer-lasting