3.1 Cells of Nervous System and Their Functions

The two types of cells found in the nervous system are called neurons or nerve cells and neuroglia, which are specialized connective tissue cells. Neurons conduct impulses, whereas neuroglia supports neurons.

3.1 Neuron

Neurons, also called nerve cells, are highly specialized to transmit messages (nerve impulses) from one part of the body to another.

• Structural units of the nervous system
• Composed of a body, axon, and dendrites
• Long-lived, amitotic, and have a high metabolic rate

1. 3.1.1 Parts of a Neuron
2. Cell Body or Soma
   • The cell body is the metabolic center of the neuron.
   • Contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components.
   • Contains the nucleus and a nucleolus
   • Major biosynthetic center
   • Focal point for the outgrowth of neuronal processes
   • There are no centrioles (hence its amitotic nature)
   • Well developed Nissl bodies (rough ER)
   • Axon hillock – cone-shaped area from which axons arise
   • Cell bodies form the grey matter of the nervous system and are found at the periphery of the brain and in the centre of the spinal cord.
   • Groups of cell bodies are called nuclei in the central nervous system and ganglia in the peripheral nervous system.
   • The soma is the primary site of integration within the neuron.
3. Axon
   • Each nerve cell has only one axon.
   • Armlike extensions from the soma.
   • Carrying nerve impulses away from the cell body.
   • They are usually longer than the dendrites, sometimes as long as 100 cm.

Structure of an axon

• • • The membrane of the axon is called axolemma.
    • Large axons and those of peripheral nerves are surrounded by a myelin sheath.
    • This consists of a series of Schwann cells arranged along the length of the axon.
    • Between the layers of plasma membrane there is a small amount of fatty substance called myelin.
    • The outermost layer of Schwann cell plasma membrane is sometimes called neurilemma.
    • There are tiny areas of exposed axolemma between adjacent Schwann cells, called nodes of Ranvier, which assist the rapid transmission of nerve impulses.
    • Generate and transmit action potentials.
    • Secrete neurotransmitters from the axonal terminals.

1. Dendrites
   • Armlike extensions from the soma
   • Short, tapering, and diffusely branched processes.
   • They are the receptive, or input, regions of the neuron.
   • Electrical signals are conveyed as graded potentials (not action potentials).
2. Nissl bodies
   • The rough Endoplasmic Reticulum present in soma.
3. Neurofibrils
   • Intermediate filaments that are important in maintaining cell shape.
4. Axon hillock
   • Conelike region of the cell body from where axon arises.
5. Axon terminals
   • Branched terminal ends of the axon.
6. Neurotransmitters
   • Axon terminals contain hundreds of tiny vesicles, or membranous sacs, that contain chemicals called neurotransmitters.( As we said, axons transmit nerve impulses away from the cell body. When these impulses reach the axon terminals, they stimulate the release of neurotransmitters into the extracellular space.)
7. Synaptic cleft
   • Each axon terminal is separated from the next neuron by a tiny gap called the Synaptic cleft.
8. Synapse
   • Functional junction of neurons.
9. Myelin Sheaths
   • Most long nerve fibers are covered with a whitish, fatty material called myelin, which has a waxy appearance.
   • Myelin protects and insulates the fibers and increases the transmission rate of nerve impulses.
   • Axons outside the CNS are myelinated by Schwann cells.
10. Neurilemma
    • Neurilemma (also known as neurolemma, sheath of Schwann, or Schwann’s sheath) is the outermost nucleated cytoplasmic layer of Schwann cells (also called neurilemmocytes) that surrounds the axon of the neuron.
    • It forms the outermost layer of the nerve fiber in the peripheral nervous system.
11. Nodes of Ranvier
    • The myelin sheath is formed by many individual Schwann cells, it has gaps, or indentations, called Nodes of Ranvier, at regular intervals.

Figure 1- Neuron Figure 2- Relationship of Schwann cells to axons in the peripheral nervous system

• The cell body carries out most of the metabolic functions of a neuron, so if it is damaged, the cell dies and is not replaced.
• Small collections of cell bodies called ganglia are found in a few sites outside the CNS (i.e. in the PNS).
• Bundles of nerve fibers (neuron processes) running through the CNS are called tracts.
• Bundles of nerve fibers (neuron processes) running through the PNS are called nerves.
• The white matter consists of dense collections of myelinated fibers (tracts).
• Gray matter contains mostly unmyelinated fibers and cell bodies.