20 Nervous Tissue
Characteristics of Nervous Tissue
Nervous tissue is the main component of the nervous system, which includes the brain, spinal cord, and nerves.
Learning Objectives
Describe the characteristics of nervous tissue
Key Takeaways
Key Points
- Nervous tissue is one of four major classes of tissues and makes up the central nervous system and the peripheral nervous system.
- Integration and communication are the two major functions of nervous tissue.
- Nervous tissue contains two categories of cells — neurons and neuroglia.
- Neurons are highly specialized nerve cells that generate and conduct nerve impulses.
- Neuroglia are supporting cells that provide physical sport, remove debris, and provide electrical insulation.
Key Terms
- myelin: A substance produced by cells of the neuroglia that increases the speed of impulses along the axon of the neuronal fiber.
- nervous tissue: The principal constituent of the central and peripheral nervous system, comprised neurons and neuroglia cells.
- brain: The control center of the central nervous system, located in the skull.
Nervous Tissue
Nervous tissue is one of four major classes of tissues. It is specialized tissue found in the central nervous system and the peripheral nervous system. It consists of neurons and supporting cells called neuroglia.
The nervous system is responsible for the control of the body and the communication among its parts. Nervous tissue contains two categories of cells—neurons and neuroglia.
Neurons
Neurons are highly specialized nerve cells that generate and conduct nerve impulses. A typical neuron consists of dendrites, the cell body, and an axon.
Dendrites
Dendrites are responsible for responding to stimuli; they receive incoming signals towards the cell body. The axons are responsible for transmitting impulses over long distances from cell body. The cell body is like a factory for the neuron. It produces all the proteins and contains specialized organelles such as nucleus, granules and Nissl bodies.
Neuron: This image illustrates the parts of a neuron. The dendrites receive incoming signals while axons propagate signals away from the neuron cell body. The myelin sheath surrounds and insulates the axon.
Dendrite
The axon is surrounded by a whitish, fatty layer called the myelin sheath. Outside the myelin sheath there is a cellular layer called the neurilemma.
Schwann Cells
In the peripheral nervous system, Schwann cells are neuroglia cells that support neuronal function by increasing the speed of impulse propagation. The Schwann cells are underlain by the medullary sheath. The medullary sheath is interrupted at intervals by the nodes of Ranvier.
Illustration of the Schwann Cells and the Myelin Sheath: Transmission electron micrograph of a myelinated axon. The myelin layer (concentric) surrounds the axon of a neuron, showing Schwann cells.
Types of Nervous Tissue
The nervous system consists of nervous tissue, which is composed of two principal types of cells called neuron and neuroglia.
Learning Objectives
Describe the main cells that comprise nervous tissue
Key Takeaways
Key Points
- Nervous tissue is composed of neurons and supporting cells called neuroglia, or ” glial cells.”
- There are six types of neuroglia. Four are found in the central nervous system, while two are found in the peripheral nervous system.
- The four types of neuroglia found in the central nervous system are astrocytes, microglial cells, ependymal cells, and oligodendrocytes.
- The two types of neuroglia found in the peripheral nervous system are satellite cells and Schwann cells.
- Neurons are the other the other type of cell that comprise nervous tissue. Neurons have cell bodies, dendrites, and axons.
Key Terms
- neuron: The main cell type in nervous tissue.
- neuroglia: Supporting cells in nervous tissue.
Nervous tissue, one of the four main tissue types, is composed of neurons and supporting cells called neuroglia. Neuroglia are also called “glial cells.”
Neuroglia
There are six types of neuroglia—four in the central nervous system and two in the PNS. These glial cells are involved in many specialized functions apart from support of the neurons. Neuroglia in the CNS include astrocytes, microglial cells, ependymal cells and oligodendrocytes. In the PNS, satellite cells and Schwann cells are the two kinds of neuroglia.
Astrocytes
Astrocytes are shaped like a star and are the most abundant glial cell in the CNS. They have many radiating processes which help in clinging to the neurons and capillaries. They support and brace the neurons and anchor them to the nutrient supply lines. They also help in the guiding the migration of young neurons. Astrocytes control the chemical environment around the neurons.
Microglial Cells
Microglial cells are small and ovoid un shape with thorny processes. They are found in the CNS. When invading microorganism or dead neurons are present, the microglial cells can transform into a phagocytic macrophage and help in cleaning the neuronal debris.
Ependymal Cells
Ependymal cells are ciliated and line the central cavities of the brain and spinal cord where they form a fairly permeable barrier between the cerebrospinal fluid that fills these cavities and the tissue cells of the CNS.
Oligodendrocytes
Oligodendrocytes line up along the nerves and produce an insulating cover called myelin sheath. They are found in the CNS.
Satellite Cells
Satellite cells surround neuron cell bodies in the peripheral nervous system (PNS). They are analogous to the astrocytes in the CNS.
Schwann Cells
Schwann cells surround all nerve fibers in the peripheral nervous system and form myelin sheaths around the nerve fibers. They are found in the PNS. Their function is similar to oligodendrocytes.
Neurons
Neurons consist of cell body and one or more slender processes. The neuronal cell body consists of a nucleus and rough endoplasmic reticulum or Nissl Bodies. The cell body is the major biosynthetic center of a neuron and contains the usual organelles for the synthesis of proteins and other chemicals. Arm like processes extend from the cell body to all neurons.
The two types of neuron processes are called dendrites and axons. Dendrites are motor neurons that are short and have a large surface area for receiving signals from other neurons. Dendrites convey incoming messages towards the cell body and are therefore called the receptive input region.
The axon arises from the cone shaped portion of the cell body called the axon hillock. Functionally, the axon is the conducting region of the neuron and is responsible for generating and transmitting impulses typically away from the cell body. A single axon routes the nerve impulse from the cell body to another neuron or an effector organ. The axon can have many terminal branches, so each time the nerve fires, it can stimulate more than one cell.
