Peripheral Neuroglia
Schwann cells are the type of neuroglia most commonly identified as being found outside the brain. Here's the thing — they are located in the peripheral nervous system (PNS), which consists of the cranial nerves, spinal nerves, peripheral sensory receptors, and ganglia outside the brain and spinal cord. Which means more precisely, the PNS also contains satellite glial cells and enteric glial cells. Understanding the distinction between peripheral and central neuroglia explains how nerves outside the brain are supported, protected, and able to communicate with the rest of the nervous system That's the part that actually makes a difference..
Neuroglia and the Two Divisions of the Nervous System
Neuroglia, also called glial cells, are supporting cells that surround neurons. Although neurons perform most of the information-processing work of the nervous system, neuroglia provide the structural, metabolic, and immune conditions that allow neurons to function Easy to understand, harder to ignore..
The nervous system is divided into two major regions:
- The central nervous system (CNS) includes the brain and spinal cord.
- The peripheral nervous system (PNS) includes nerves and ganglia outside the brain and spinal cord.
Each region contains different types of neuroglia. The major neuroglia of the CNS are astrocytes, oligodendrocytes, microglia, and ependymal cells. The principal neuroglia of the PNS are Schwann cells and satellite cells.
This distinction is important because the CNS and PNS have different structural and physiological needs. To give you an idea, oligodendrocytes and Schwann cells both produce myelin, but they do so in different ways and under different conditions.
Schwann Cells: The Main Neuroglia Outside the Brain
Schwann cells are the main answer when asked which type of neuroglia is found outside the brain. Named after the German physiologist Theodor Schwann, these cells wrap around axons in peripheral nerves Worth keeping that in mind..
Some Schwann cells form a multilayered covering called myelin. In real terms, this fatty insulating layer helps electrical impulses travel rapidly along the axon. Myelin also reduces the amount of energy required to transmit a signal And that's really what it comes down to..
A single myelinating Schwann cell wraps around one segment of one axon, and successive Schwann cells cover the axon along its length. Still, small gaps between these cells are called nodes of Ranvier. These gaps allow the action potential to appear to “jump” from one node to another, a process known as saltatory conduction Worth keeping that in mind..
This arrangement differs from that of oligodendrocytes in the CNS. Also, one oligodendrocyte can extend several processes and myelinate portions of multiple axons. By contrast, each myelinated peripheral axon requires a series of individual Schwann cells.
Schwann cells also have important roles beyond myelination. They:
- Provide metabolic support to peripheral axons.
- Guide the growth of developing peripheral nerves.
- Remove damaged or cellular debris after injury.
- Help repair peripheral nerves.
- Regulate the environment surrounding axons.
- Form part of the blood–nerve barrier.
Nonmyelinating Schwann Cells
Not every Schwann cell produces myelin. Nonmyelinating Schwann cells instead surround groups of small, unmyelinated axons. They are especially common in autonomic nerves and in certain sensory pathways.
Rather than wrapping each axon in thick layers of myelin, these cells form a protective covering around several axons at once. Although they do not create the same strong electrical insulation as myelinating Schwann cells, they still contribute to nerve maintenance, organization, and repair That alone is useful..
The distinction between myelinating and nonmyelinating Schwann cells demonstrates that a glial cell can provide physical support and regulate the local environment without producing myelin Small thing, real impact..
Satellite Glial Cells in Peripheral Ganglia
Although Schwann cells are the usual textbook answer, they are not the only neuroglia outside the brain. Satellite glial cells are found in peripheral ganglia, including the dorsal root ganglia and sympathetic ganglia.
Ganglia are collections of neuron cell bodies located outside the CNS. That said, satellite glial cells closely surround these cell bodies, much as astrocytes surround neurons in the CNS. They act as a protective and regulatory layer around each neuronal soma That's the part that actually makes a difference..
Their functions include:
- Maintaining the chemical environment around neuronal cell bodies.
- Controlling the movement of ions and neurotransmitters.
- Providing structural support.
- Responding to injury, inflammation, and changes in neural activity.
- Regulating synaptic transmission within ganglia.
Satellite glial cells are especially abundant in sensory ganglia, where they help maintain a stable environment for sensory neurons that transmit information such as touch, temperature, and pain It's one of those things that adds up..
Enteric Glial Cells in the Digestive System
Another peripheral type is the enteric glial cell. These cells are located in the enteric nervous system, a large network of nerves embedded in the walls of the digestive tract That's the part that actually makes a difference. Still holds up..
The enteric nervous system is sometimes called the “second brain” because it can control many digestive functions independently
