Glial Cells Lecture Notes PDF

**GLIAL CELLS** **Glial cells** (*from the Greek word glia, meaning **\"glue\"**)* are non-neuronal cells in the central nervous system that provide critical support for neurons. Contrary to the belief that neurons function independently, glial cells are essential for maintaining the health and fun...

**GLIAL CELLS** **Glial cells** (*from the Greek word glia, meaning **\"glue\"**)* are non-neuronal cells in the central nervous system that provide critical support for neurons. Contrary to the belief that neurons function independently, glial cells are essential for maintaining the health and functionality of neurons. They not only surround neurons and hold them in place but also perform various other vital roles. While glial cells are primarily found in the brain and spinal cord, some types are also located in the peripheral nervous system. **Functions of Glial Cells** Glial cells perform several critical functions that are essential for the brain and nervous system: - ***Support for neurons:*** They surround and physically support neurons. - ***Manufacture nutrients:*** Glial cells produce chemicals (such as ions) that neurons need for proper functioning. - ***Absorption of toxins and waste:*** They absorb harmful toxins and waste materials that could damage or kill neurons. - ***Guiding neuron development:*** During development, glial cells guide neurons to their correct destinations in the brain. - ***Myelination:*** Certain glial cells, such as oligodendrocytes, produce the myelin sheath around axons, which insulates nerve fibers and speeds up electrical signals. - ***Outnumber neurons:*** Glial cells are more numerous than neurons, with a typical ratio of approximately 10:1. - ***Responding to nerve activity:*** Glial cells actively respond to changes in nerve activity and help regulate synaptic function. - ***Modulating communication:*** They play a role in modulating the communication between neurons, influencing learning, memory, and other brain functions. - ***Tumor formation:*** Most brain tumors arise from mutations in glial cells, particularly gliomas. **Types of Glial Cells** There are several types of glial cells, each with specialized functions: 1. **Astrocytes***:* Astrocytes are star-shaped glial cells in the central nervous system (CNS) that play a critical role in maintaining homeostasis, supporting neurons, and protecting the brain. Their wide-reaching functions contribute to the overall health and functionality of the brain. - ***Nutrient delivery:*** Astrocytes make contact with both capillaries and neurons, delivering nutrients and essential substances to neurons. They act as a \"food factory\" for neurons, helping them survive and function. - ***Ion regulation:*** They regulate the concentrations of ions (such as potassium) and chemicals in the extracellular fluid, ensuring the optimal environment for neural function. - ***Synaptic support:*** Astrocytes provide structural support for synapses and assist in maintaining the connections between neurons. - ***Blood-brain barrier formation:*** Astrocytes form a crucial part of the blood-brain barrier, which prevents harmful substances (such as toxins and pathogens) from entering the brain from the bloodstream. - ***Response to nerve activity:*** Astrocytes become active in response to changes in nerve activity, helping modulate the communication between neurons. - ***Calcium signaling:*** Astrocytes transmit calcium waves between themselves, enabling communication between different astrocytes and influencing neuronal activity. - ***Synaptic modulation:*** By regulating neurotransmitter levels and signaling, astrocytes modulate the activity of surrounding synapses, affecting learning, memory, and other brain functions. - ***Circulatory and homeostatic regulation:*** Astrocytes contribute to maintaining homeostasis, helping to regulate the balance of substances both inside (intracellular) and outside (extracellular) of cells. They play a role in ensuring equilibrium within the CNS. - ***Defense against pathogens:*** Astrocytes help defend the brain against infections such as viral and bacterial attacks, including meningitis (inflammation of the protective membranes of the brain and spinal cord). 2. **Microglia:** Microglia are the primary immune cells of the central nervous system (CNS). They play a crucial role in protecting the brain from infections and injuries. - ***Immune defense:*** Microglia act as scavengers by detecting and degrading dead or damaged cells, protecting the brain from pathogens and cellular debris. - ***Phagocytosis:*** They are involved in the process of phagocytosis, where they engulf and digest cellular debris, microorganisms, and harmful substances. - ***Neuroinflammation:*** Microglia play a role in responding to inflammation in the brain and can become activated in conditions such as neurodegenerative diseases (e.g., Alzheimer\'s or Parkinson\'s). 3. **Oligodendrocytes**: Oligodendrocytes are responsible for the formation of myelin sheaths around the axons of neurons in the CNS. Myelin sheaths are essential for insulating axons and speeding up the transmission of electrical signals. - ***Myelination in the CNS:*** Oligodendrocytes produce myelin sheaths that insulate axons, ensuring faster and more efficient electrical signal transmission. - ***Multiple connections:*** One oligodendrocyte can provide myelin to multiple axons, and one axon can be myelinated by several oligodendrocytes. 4. **Ependymal Cells**: Ependymal cells line the fluid-filled cavities (ventricles) in the brain and the central canal of the spinal cord. They play an essential role in the production and circulation of cerebrospinal fluid (CSF), which cushions the brain and spinal cord. - ***Lining of ventricles and spinal canal:*** Ependymal cells form a layer around the ventricles of the brain and the central canal of the spinal cord. - ***Cerebrospinal fluid (CSF) production:*** They are involved in producing CSF, which serves as a protective cushion for the brain and spinal cord. CSF helps protect the brain from trauma and supports its overall function. - ***CSF circulation:*** Ependymal cells have cilia (hair-like structures) that help circulate CSF between the brain and the spinal cord. - ***Component of the choroid plexus:*** Ependymal cells are a key component of the choroid plexus, the structure in the brain where CSF is produced. - - - - - - 5. **Radial Glia:** Radial glial cells are crucial during brain development as they guide the migration of new neurons to their final destinations in the brain. - ***Neuronal migration:*** Radial glia act as scaffolding, supporting developing neurons as they migrate to their target locations during embryonic brain development. - ***Progenitor cells:*** In addition to guiding neurons, radial glia can differentiate into neurons, astrocytes, and oligodendrocytes. 6. **Satellite Cells**: Satellite glial cells are found in the peripheral nervous system (PNS), where they surround and support neurons within ganglia (clusters of nerve cells). - **Nutrient supply:** They provide nutrients and metabolic support to neurons in the PNS. - **Structural support:** Satellite cells maintain the structural integrity of ganglia by surrounding neurons and regulating their chemical environment. - **Ion regulation:** They help regulate the extracellular environment, particularly by maintaining ion and neurotransmitter levels around the neurons. 7. **Schwann Cells:** Schwann cells are responsible for producing myelin in the peripheral nervous system (PNS), essential for the proper functioning of neurons. - ***Myelination in the PNS:*** Each Schwann cell myelinates a single axon by wrapping itself around the axon, forming a protective and insulating myelin sheath. This increases the speed of nerve impulse conduction. - ***Axonal repair:*** Schwann cells aid in the repair and regeneration of damaged axons in the PNS by creating pathways for axonal regrowth following injury. **Additional Notes:** - - - - **The Electrical Activity of Neurons** ***Neurons perform two essential functions:*** 1. Generate electricity that produces nerve impulses. 2. Release chemicals that enable them to communicate with other neurons, muscles, and glands. **Nerve Activation: The Three Basic Steps** 1. ***Resting potential:*** At rest, the neuron has an electrical resting potential caused by the uneven distribution of positively and negatively charged ions inside and outside the neuron. 2. ***Action potential (nerve impulse):*** When stimulated, ion channels in the neuron\'s membrane open, allowing ions to flow in and out, reversing the electrical charge and creating an action potential (nerve impulse). 3. ***Restoration of resting potential:*** After the action potential, the ionic balance is restored, and the neuron returns to its resting state, ready to fire again. **Resting State of a Neuron** ***Ion distribution:*** - Sodium (Na⁺) and potassium (K⁺) channels are closed. - The concentration of sodium ions (Na⁺) is about 10 times higher outside the neuron than inside. **When a Neuron is Stimulated** - ***Depolarization:*** Sodium channels open, allowing Na⁺ ions to rush into the cell. This causes the inside of the neuron to become positive (around +40 mV) relative to the outside, leading to an action potential or nerve impulse. - ***Action potential:*** The action potential lasts for about one millisecond (1/1,000 of a second). This brief reversal of the electrical charge triggers the nerve impulse, which travels along the axon. - ***Restoring the resting potential:*** After the action potential, sodium channels close, and potassium (K⁺) channels open, allowing K⁺ ions to exit the cell. This process restores the neuron\'s resting potential. - ***Reestablishing ion balance:*** The excess Na⁺ ions are pumped out, and K⁺ ions are brought back in, restoring the neuron\'s resting state. - ***Propagation of the action potential:*** The action potential travels down the axon to the axon terminals by opening adjacent Na⁺ channels. This flow continues along the axon, generating nerve impulses. - ***Refractory period:*** After an action potential passes, the neuron enters a brief refractory period during which the membrane is not excitable and cannot generate another impulse. **The All-or-None Principle** Action potentials follow the all-or-none principle, meaning they either occur at full intensity or not at all. - To trigger an action potential, the negative charge inside the neuron (resting potential of about -70 mV) must increase to about +50 mV (the action potential threshold) due to the influx of Na⁺. - If the change in membrane potential does not reach the +50 mV threshold, it is called a graded potential, and no action potential is triggered. **Myelin Sheath** The myelin sheath is a whitish, fatty insulation layer derived from glial cells that covers some axons. It plays a key role in speeding up the transmission of nerve impulses. - - - Damage to the myelin sheath can result in uncoordinated movements and paralysis, as seen in multiple sclerosis (MS). - In MS, the immune system attacks the myelin, disrupting the timing of nerve impulses, which affects communication between the brain and muscles.