Nervous System Terms

Schwann Cells

Schwann Cells are found only in the peripheral nervous system. They support nerve fibers and form myelin sheaths. The Schwann cell's nucleus and cytoplasm are squeezed to the perimeter to form the neurilemma. Neurilemma is essential to the regeneration of injured nerve fibers.

Neurons are also classified according to the direction in which they transmit impulses

Sensory neurons transmit impulses to the spinal cord and brain and are also known as afferent neurons. Motor neurons transmit impulses away from the brain and spinal cord and are also known as efferent neurons. Interneurons are connecting neurons and conduct impulses from sensory neurons to motor neurons. They are located only in the central nervous system.

Ependymal Cells

are ciliated cells that form a sheet that usually lines fluid cavities in the brain.

Astrocytes

hold neurons and small blood vessels close to each other. They are star shaped, with many branches, which accounts for their name astrocyte.

Oligodendrocytes

produce the myelin sheath for nerve fibers in the central nervous system.

Microglia

provide phagocytosis for the system.

Nerve

A nerve is a group of peripheral nerve fibers (axons) bundled together like the strands of a cable and held together by several layers of connective tissue.

Saltatory conduction

Rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelin-sheathed regions of membrane.

PNS nerves have myelin sheaths

The nerves of the peripheral nervous system (PNS) usually have myelin sheath and are therefore predominantly white matter.

Neurons are classified according to the number of their extensions from the cell body.

Multipolar neurons have only one axon but several dendrites. Bipolar neurons have only one axon and also only one highly branched dendrite, and unipolar neurons have a single process extending from the cell body.

Neurons within the CNS have axons

Neurons within the CNS have axons that are missing the outer cell membrane of a Schwann cell—the neurilemma—which is necessary for the regeneration of nerve cells. This is a primary reason for the lack of regeneration of nerves when there is damage or trauma to the CNS.

Axon outside of CNS

On many neurons outside the CNS, the axon is surrounded by a white, fatty substance known as myelin that is formed by Schwann cells. Nodes of Ranvier are indentations between adjacent Schwann cells. The outer cell membrane of a Schwann cell (the nuclei and cytoplasm) is called the neurilemma and is necessary for the regeneration of nerve cells.

several layers of connective tissue surround each nerve fiber.

Surrounding each nerve fiber is a delicate layer of fibrous connective tissue called endoneurium. Bundles of fibers, called fascicles, are held together by a connective tissue layer called the perineurium. Numerous fascicles are then held together to form a complete nerve by a fibrous layer called the epineurium.e

autonomic nervous system or ANS.

The involuntary division of PNS is composed of the autonomic neurons and is known as the autonomic nervous system or ANS.

Glia cells

The other cell type found in the nervous system is the glia. Glia do not transmit impulses but, instead, serve as supporting cells to hold the functioning neurons of the CNS and PNS together and protect them. Five main types of glia serve special protective purposes for the nervous system. The first four glial types in this list are located in the CNS. Only the Schwann cells are located in the PNS.

neurilemma

The outer cell membrane of a Schwann cell (the nuclei and cytoplasm) is called the neurilemma and is necessary for the regeneration of nerve cells.

Membrane Potentials

The outside is slightly positive and the inside is slightly negative. This difference in electrical charge is called "potential" because it is a type of stored energy. A membrane that exhibits a membrane potential is said to be polarized. When a neuron is not conducting electrical signals, it is said to be "resting." The membrane potential maintained by a nonconducting neuron's plasma membrane is called the resting membrane potential (RMP).

somatic nervous system.

The voluntary division of PNS is composed of somatic or voluntary motor neurons and is sometimes referred to as the somatic nervous system.

The nerve fibers of the CNS are white and gray:

The white matter appears white due to the fatty substance myelin, which forms the myelin sheath. The gray matter is primarily cell bodies, interneurons, and unmyelinated fibers, which have a gray appearance. In the CNS, small distinct regions of gray matter are referred to as nuclei. Similar regions in the PNS are referred to as ganglia.