08. Neural Tissue

A typical neuron consists of:

1. Cell Body (Soma): A large, star-shaped structure containing the nucleus.
2. Nucleus: Dark magenta in color, located at the center of the cell body.
3. Axon: A long, slender process extending from the cell body, responsible for transmitting signals.
4. Dendrites: Multiple shorter, branching processes radiating outward from the cell body, which receive signals from other neurons.
5. Glial Cells: A complex network of supporting cells surrounding the neuron, contributing to the overall structure and function of nervous tissue.

1. Cell Body (perikaryon/soma)
2. Multiple Dendrites
3. A Single Axon (fiber) with many terminals and synapses

Sensory neurons, also known as afferent neurons, carry information toward the Central Nervous System (CNS).

Interneurons serve as connectors, transmitting information between sensory and motor neurons within the Central Nervous System (CNS).

Camillo Golgi was an Italian physician and histologist known for developing the silver staining technique, which allowed for the visualization of nerve cells in detail. This method was crucial in advancing the understanding of neuronal structure.

Cresyl violet staining highlights round cells with darkly stained nuclei and lighter cytoplasm, making it useful for identifying neuronal cell bodies and assessing the morphology of brain tissue.

Franz Nissl was a German neuropathologist known for developing the Nissl staining technique, which is used to visualize neuronal cell bodies and their rough endoplasmic reticulum, aiding in the study of brain structure and pathology.

The main components of the cell body (soma) of a neuron include:

• Large, euchromatic nucleus with a prominent nucleolus
• Endoplasmic reticulum (ER):
  • Rough ER (with ribosomes) known as Nissl substance/bodies
  • Smooth ER (without ribosomes)
• Golgi apparatus
• Mitochondria
• Structural proteins: microtubules, neurofilaments, and sparse actin filaments
• Inclusions of pigmented material, such as lipofuscin, from lysosomal digestion.

Dendrites receive stimuli and conduct them towards the cell body.

Axons conduct information and impulses away from the cell body, while dendrites receive stimuli towards the cell body.

Dendrites have a rough surface with dendritic spines and usually many branches, while axons have a smooth surface and generally only one branch that extends far away from the cell body.

Myelin insulation in axons increases the speed of impulse conduction, while dendrites do not have myelin insulation.

Dendrites may have some ribosomes, whereas axons are devoid of ribosomes.

Dendritic spines are dynamic membrane protrusions that increase the surface area of dendrites. They play a crucial role in neuroplasticity, which is the ability of neural networks to reorganize and rewire, facilitating adaptation, learning, and memory.

• Anterograde transport: Moves from the perikaryon to synaptic terminals via kinesin.
• Retrograde transport: Moves from the periphery towards the soma via dynein.

Electrical synapses are very fast due to direct ion flow through gap junctions, while chemical synapses are less fast because they involve neurotransmitter release and receptor binding, which takes more time.

The three types of chemical synapses are:

1. Axodendritic - typically thousands in number, connecting axons to dendrites.
2. Axosomatic - connecting axons to the cell body.
3. Axoaxonic - less frequent, connecting axons to other axons.

Gap junctions (connexins) lead to the same or less stimulatory outcome and allow for very fast communication. In contrast, neurotransmitter signaling can result in signal amplification and has a more computational role, being either excitatory or inhibitory.

In chemical synapses, the action potential is generated in an all-or-nothing manner, meaning that once the threshold is reached, the neuron will fire completely, ensuring reliable signal transmission.

The main components of a synapse include:

1. Presynaptic Terminal: Contains synaptic vesicles filled with neurotransmitters (NTs) and mitochondria for energy.
2. Synaptic Cleft: The gap between the presynaptic and postsynaptic terminals where neurotransmitters are released.
3. Postsynaptic Density (PSD): Contains NT receptors that bind to the released neurotransmitters, facilitating signal transmission to the postsynaptic neuron.
4. Dendritic Spines: Protrusions on the postsynaptic neuron that increase the surface area for synaptic connections.

Neurotransmitters can be passed to:
i) another neuron
ii) muscle cell
iii) gland cell

Excitatory neurotransmitters, such as glutamate, promote the firing of neurons.
Modulatory neurotransmitters, like neuropeptides, can influence the strength or duration of the signal.
Inhibitory neurotransmitters, such as glycine and GABA, reduce the likelihood of neuron firing.

Excitatory synapses, such as those using glutamate, increase the membrane potential, moving it closer to the threshold of activation. In contrast, inhibitory synapses, like those using GABA, decrease the membrane potential, making it less likely for the neuron to fire an action potential. The balance between these synaptic inputs determines whether the neuron will reach the threshold and initiate an action potential.

The axon hillock is the critical region where the summation of all excitatory and inhibitory postsynaptic potentials occurs. If the summed potentials reach the threshold of activation, it triggers a rapid depolarization, leading to the generation of an action potential that travels down the axon.

The threshold of activation is the critical membrane potential that must be reached for an action potential to occur. When the excitatory postsynaptic potentials (EPSPs) surpass this threshold, it results in a rapid depolarization of the neuron, leading to the firing of an action potential.

Signal transmission in myelinated axons is approximately 125-150 m/sec, while in non-myelinated axons it is about 0.5 m/sec, making myelinated axons 300 times faster.

The Nodes of Ranvier contain voltage-gated Na+ and K+ channels, which facilitate the rapid transmission of action potentials by allowing ions to flow in and out of the axon, enabling saltatory conduction.

Louis Antoine Ranvier (1835-1922) is known for his discovery of the Nodes of Ranvier, which are crucial for the process of saltatory conduction in myelinated axons.

The ratio of glial cells to neurons in the human brain may range from approximately 1:1 to 10:1, depending on the brain region.

Glial cells provide the appropriate microenvironment for neuronal activity and support various functions such as insulation, nutrient supply, and maintenance of homeostasis.

Oligodendrocytes are responsible for the myelination of axons in the central nervous system, which enhances the speed of electrical signal transmission. They provide support and insulation to neurons by wrapping around their axons, forming a myelin sheath.

The stages of oligodendrocyte development include:

1. Oligodendrocyte Precursor Cell (OPC) - The initial stage with a blue body and short extensions.
2. Immature Oligodendrocyte (OL) - Characterized by a blue cell body with many extensions in all directions.
3. Myelinating Oligodendrocyte - The final stage where the OL wraps around the axon, forming the myelin sheath.

Oligodendrocyte development and myelination are influenced by:

• Neurotransmitters - Chemical messengers that can affect oligodendrocyte function.
• Growth factors - Proteins that promote the growth and differentiation of oligodendrocytes.
• Neurotransmitter receptors - Receptors that mediate the effects of neurotransmitters on oligodendrocytes.
• Growth factor receptors - Receptors that respond to growth factors, facilitating oligodendrocyte maturation.

Normal myelin sheaths are characterized by:

• Numerous round to oval structures with a light green interior.
• Concentric black rings that are tightly packed and evenly distributed.
• Small, dark spots scattered within the green interior.
• Closely positioned fibers separated by thin, lighter spaces.

In cases of dense degeneration, myelin sheaths exhibit:

• Disruption and unevenness in appearance.
• Varied colors including light green, dark green, blue, and yellow sections.
• Less uniform structures with swollen and misshapen fibers in reddish brown.
• Larger spaces between fibers filled with more debris, indicating degeneration.

Oligodendrocytes have a limited regenerative capacity, while Schwann cells have the ability to regenerate and support axon regrowth and remyelination.

Oligodendrocytes originate from progenitors of the neural tube, whereas Schwann cells are derived from the neural crest.

1. Myelinating Schwann Cells: These cells form the myelin sheath around a single axon, which is crucial for rapid signal transmission.

2. Non-myelinating Schwann Cells: These cells can support multiple axons without forming a myelin sheath, providing structural and metabolic support.

Fibrous Astrocytes:

• Found in white matter.
• Characterized by long, thin processes extending from a star-shaped cell body.

Protoplasmic Astrocytes:

• Found in gray matter.
• Have shorter, thicker processes that interact with axons and myelin sheaths.

Glial cells provide guidance and structural support to growing neurons, distribute nutrients and metabolites, clear waste products through the glymphatic system, and form the glia limitans, which acts as a barrier with the pia mater of the meninges.

Glial cells contribute to the blood-brain barrier (BBB) through their perivascular endfeet, which help maintain the integrity of the barrier and regulate the movement of substances between the blood and the brain.

The tripartite synapse involves the interaction between a presynaptic neuron, a postsynaptic neuron, and surrounding glial cells, which modulate synaptic activity and influence neural communication and plasticity.

Glial cells influence neural development and plasticity by participating in processes such as long-term potentiation (LTP), which is essential for learning and memory, and by providing support for the growth and differentiation of neurons.

Satellite cells form a covering layer around ganglionic neuronal bodies, providing insulation. They also assist in neurotransmitter recycling and the regulation of the microenvironment around the neurons.

Ependymal cells line the cavities of the CNS and bear cilia on their apical surface, which contribute to the movement of cerebrospinal fluid (CSF).

Ependymal cells typically lack a basal lamina, allowing them to have direct contact with the neuropil.

Microglia perform several key functions including:

1. Surveillance of the neuropil
2. Phagocytosis of debris and pathogens
3. Secretion of immunoregulatory inflammatory factors
4. Synaptic modulation and pruning of synapses

Microglia account for 4-10% of the total glial cells in the central nervous system (CNS).

Microglia originate from primitive erythro-myeloid progenitors (EMPs) in the yolk sac, which migrate to the brain parenchyma during development.

Key transcription factors involved in microglia development include RUNX1, PU.1, and IRF8.

The development of microglia involves the processes of proliferation, differentiation, and migration from the yolk sac to the brain.

Chemokines such as CXCL12, CCL2, and CCL3 are involved in the migration and differentiation of microglia during their development.

Microglia cross the blood-brain barrier as they migrate from the yolk sac to the brain parenchyma, facilitated by various signaling molecules and receptors.

The main types of cells in the nervous system include astrocytes, neurons, oligodendrocytes, ependymal cells, capillary cells, and microglial cells.