Neuron Types and Neuroglia

Structural Classification of Neurons

• Neurons are classified based on the number of processes extending from their cell body.
• Multipolar neurons typically have multiple dendrites and one axon, found predominately in the brain and spinal cord, including motor neurons.
• Bipolar neurons possess one main dendrite and one axon, located in the retina of the eye, inner ear, and olfactory area.
• Unipolar neurons have fused dendrites and an axon forming a continuous process from the cell body and also referred to as pseudounipolar neurons, originating as bipolar neurons in the embryo.

Neuroglia

• Neuroglia, or glia, constitute about half the CNS volume.
• Neuroglia actively participate in nervous tissue activities.
• Neuroglia are more numerous than neurons, glias do not generate or propagate action potentials, and can multiply and divide in mature nervous systems.
• Six types of neuroglia reside in the CNS: astrocytes, oligodendrocytes, microglia, and ependymal cells.
• Schwann cells and satellite cells are present in the PNS.

Astrocytes

• Astrocytes are star-shaped, numerous, and the largest of the neuroglia.
• Protoplasmic astrocytes have short, branching processes and are found in gray matter.
• Fibrous astrocytes have long, unbranched processes and are located in white matter.
• Astrocyte processes interact with blood capillaries, neurons, and the pia mater.

Astrocyte Functions

• Astrocytes support neurons with their microfilaments.
• These isolate CNS neurons from harmful blood substances by secreting chemicals that maintain capillary endothelial cell permeability.
• Astrocytes secrete chemicals that regulate growth, migration, and interconnection of the brain's neurons in the embryo.

Oligodendrocytes

• Oligodendrocytes resemble astrocytes but are smaller with fewer processes.
• Oligodendrocyte processes form and maintain the myelin sheath around CNS axons.
• The myelin sheath is a lipid and protein layer that insulates some axons and boosts nerve impulse speed; axons covered in myelin are called myelinated.

Microglia

• Microglial cells are small with slender processes and spine-like projections.
• Microglia act as phagocytes, removing cellular debris and phagocytizing microbes and damaged tissue like tissue macrophages.

Ependymal Cells

• Ependymal cells are cuboidal to columnar cells arranged in a single layer, possessing microvilli and cilia.
• These line the ventricles of the brain and spinal cord's central canal, filled with cerebrospinal fluid for protection and nourishment.
• Ependymal cells help in the circulation of cerebrospinal fluid and also help to form the blood-cerebrospinal fluid barrier.

Neuroglia of the PNS

• Neuroglia in the PNS completely surround axons and cell bodies.
• Schwann cells and satellite cells are the two glial cell types in the PNS.
• Schwann cells surround PNS axons which forms the myelin sheath around axons like oligodendrocytes.
• A single oligodendrocyte myelinates several axons.
• One Schwann cell myelinates a single axon, and can enclose up to 20 unmyelinated axons.
• Schwann cells assist in axon regeneration, being more effective in the PNS than in the CNS.

Satellite Cells

• Satellite cells are flat cells that surround PNS ganglia neuron cell bodies.
• They provide structural support, satellite cells regulate material exchange between neuronal cell bodies and interstitial fluid.

Action Potentials

• An action potential is a sequence of events that rapidly decreases and reverses the membrane potential and restores it to the resting state.
• Action potentials involve a depolarizing and repolarizing phase.

Action Potential Detailed Phases

• During depolarization, the negative membrane potential becomes less negative, reaches zero, and then becomes positive.
• During repolarization, the membrane potential returns to the resting state of −70 mV, and after-hyperpolarization might occur, where the membrane temporarily becomes more negative.
• Voltage-gated Na+ channels open first, allowing Na+ to enter that causes the membrane to depolarize.
• Voltage-gated K+ channels open, allowing K+ to exit that causes repolarization.
• The after-hyperpolarizing phase occurs when the voltage-gated K+ channels remain open after repolarization.
• For action potential generation, depolarization must reach a threshold of around −55 mV.
• An action potential is not generated in response to subthreshold stimuli.
• Action potentials happen in response to a threshold stimulus, and multiple action potentials form following suprathreshold stimuli.
• Each action potential triggered by a suprathreshold stimulus has the same amplitude as one caused by a threshold stimulus.
• Once generated, the action potential amplitude is constant; however, the stimulus strength above threshold affects action potential frequency, up to a maximum limit determined by the absolute refractory period.

Depolarizing Phase Breakdown

• When the axon membrane is depolarized to threshold by a graded potential or another stimulus, voltage-gated Na+ channels open quickly.
• The subsequent Na+ inrush causes depolarization.
• Na+ inflow shifts the membrane potential from −55 mV to +30 mV.
• The inside of the membrane at the action potential's peak is 30 mV more positive than the outside.
• Voltage-gated Na+ channels have an activation and inactivation gate.
• In the resting state, the inactivation gate is open, but the activation gate is closed, preventing Na+ from going through.
• At threshold, voltage-gated Na+ channels activate, opening both gates and beginning Na+ inflow.
• More channels then open, which further depolarizes the membrane, causing more Na+ channels to open.
• The flow of 20,000 Na+ ions during the few milliseconds of voltage-gated Na+ channel being open considerably changes the membrane potential.
• Extracellular fluid has millions of Na+ which keeps the change in Na+ concentration negligible.
• Sodium-potassium pumps counter single action potentials by removing 20,000 Na+ ions to maintain the low concentration of Na+ inside the cell.

Repolarizing Phase Breakdown

• Inactivation gates close and voltage-gated Na+ channels inactivation happens shortly after the activation gates open.
• A threshold-level depolarization opens voltage-gated K+ channels.
• The voltage-gated K+ channels open slower, their opening happens around the same time that the voltage-gated Na+ channels are closing.
• Slow K+ channel opening and pre-existing Na+ channel closing causes the repolarizing phase of the action potential.
• Na+ inflow reduces as Na+ channels become inactivated.
• K+ channels opening accelerates K+ outflow, and the membrane potential changes from +30 mV to -70 mV.
• Inactivation of Na+ channels also allows them to enter the resting state which causes repolarization.

After-Hyperpolarizing Phase Detailed

• K+ outflow through voltage-gated K+ channels can induce an after-hyperpolarizing stage of the action potential.
• The voltage-gated K+ channels remain open, and the membrane potential becomes more negative (−90 mV).
• The membrane then returns to -70 mV as the voltage-gated K+ channels close.
• Voltage-gated Na+ channels exhibit inactivated states, most voltage-gated K+ channels switch between resting and activated states.

Refractory Period Details

• The refractory period is the time during which an excitable cell cannot produce another action potential, following an action potential in response to a normal threshold stimulus.
• A very huge stimulus cannot cause a second action potential at this time, during the absolute refractory period.
• The period of Na+ channel activation and inactivation coincides with this period.
• Inactivated Na+ channels must go back to a resting state before reopening.
• Graded potentials have no such refractory period.

Role of Ion Channel Gates

• Open ion channels enable particular ions to move down electrochemical gradients.
• Ions move from a high to low concentration.
• Cations move toward areas of negativity, and anions move toward areas of positivity.
• The movement of ions produces electric currents that alter membrane potential.
• Gates control when ions channels open and close.
• Neurons and muscle fibers utilize four ion channel types to generate electrical signals: leak, ligand-gated, mechanically gated, and voltage-gated channels.

Ligand-Gated Channels

• Ligand-gated channels open and close in response to a chemical ligand to bind.
• Neurotransmitters, hormones, and particular ions are examples of such ligands.
• The neurotransmitter acetylcholine, for instance, activates cation channels, allowing Na+ and Ca2+ to enter and K+ to exit.
• Pain receptors, interneurons, as well as motor neuron dendrites and cell bodies all have ligand-gated channels.

Voltage-Gated Channels

• A voltage-gated channel is opened by a change in the membrane potential (voltage).
• Throughout all neuron types, voltage-gated channels aid with action potential development and conduction.