Neurons are the cells that make up the nervous system. Neurons are large, permanent cells that do not divide during adulthood and spend most of their lives in the G0 phase of the cell cycle. If part of a neuron is damaged, it undergoes Wallerian degeneration, meaning that the neuron degenerates distal to the injury, and does not undergo reactive gliosis in response to injury. Astrocytes, a type of glial cell, do this.

A peripheral nerve is composed of three layers: the endoneurium (inner layer around a single nerve fiber), perineurium (middle layer that surrounds a fascicle of nerve fibers), and the epineurium (dense outer layer that surrounds an entire nerve).

Monosynaptic reflexes means that the afferent neuron directly stimulates an efferent neuron, which directly stimulates a muscle to contract. This means that the information never interfaces with the brain to process. The knee-jerk reflex is known as a monosynaptic reflex because only one neural synapse is involved in the reaction.

Bipolar neurons have 1 dendrite and 1 axon. Unipolar neurons have 1 extension, which splits into 1 dendrite and 1 axon. Multipolar neurons have many dendrites and 1 axon. Sensory neurons are neurons within the neural system that specifically work within the sensory system.

Microglial cells are a macrophage of the brain and spinal cord. They function in phagocytosis, making them an immune defense of the central nervous system (CNS). Micorglia make up approximately 10-15% of all cells found in the brain.

Astrocytes, named for their star shape, provide protection and support. They provide support to the endothelial cells of the blood brain barrier, help maintain ion balances in the CNS, and aid in repair and healing of the CNS after injuries (specifically making scar tissue).

Ependymal cells line the cerebrospinal fluid filled ventricles of the brain and the central canal of the spinal cord.

Lastly myelin is produced by oligodendrocytes and Schwann cells.

Astrocytes, named for their star shape, provide protection and support. They provide support to endothelial cells of the blood brain barrier, help maintain ion balances in the CNS, and aid in repair and healing of the CNS after injuries (specifically in forming scar tissue).

Microglial cells are a macrophage of the brain and spinal cord. Their function is phagocytosis, making them an immune defense of the central nervous system (CNS). Micorglia make up approximately 10-15% of all cells found in the brain.

Ependymal cells line the cerebrospinal fluid filled ventricles of the brain and the central canal of the spinal cord. Lastly, myelin is produced by oligodendrocytes and Schwann cells.

The dorsal root transmits sensory information only and is thus responsible for the afferent sensory root of a spinal nerve. The dorsal root of spinal nerves emerge from the posterior side of the spinal cord and joins with the ventral root to form a mixed spinal nerve. The ventral root comes from the anterior side of the spinal nerve and is the efferent motor root of a spinal nerve. 

There are two types of support cells that myelinate axons in the nervous system: oligodendrocytes and Schwann cells. The difference between these two cell types is their location in the nervous system. Oligodendrocytes myelinate axons in the central nervous system, and Schwann cells myelinate axons in the peripheral nervous system.

Ependymal cells secrete cerebrospinal fluid and astrocytes help form and regulate the blood-brain barrier.

Myelin insulates axons and functions to increase the speed of a nerve impulse as it travels down an axon. Central nervous system axons are myelinated by oligodendrocytes, whereas peripheral nervous system axons are myelinated by Schwann cells. When an action potential interfaces with a myelinated axon, sodium influxes at the regions between myelin sheathing. These regions without myelin are called nodes of Ranvier. The depolarization at a node can quickly be transmitted to the next node, rather than traveling fluidly down the whole axon. This process of jumping between nodes is known as saltatory conduction, and serves to greatly increase the transmission of action potentials. Loss of myelin can lead to numerous neurodegenerative disorders.