Nervous tissue is the term for groups of organized cells in the nervous system, which is the organ system that controls the body’s movements, sends and carries signals to and from the different parts of the body, and has a role in controlling bodily functions such as digestion.
Nervous tissue, also called neural tissue, is the main tissue component of the nervous system. The nervous system regulates and controls bodily functions and activity and consists of two parts: the central nervous system (CNS) comprising the brain and spinal cord, and the peripheral nervous system (PNS) comprising the branching peripheral nerves. It is composed of neurons, also known as nerve cells, which receive and transmit impulses, and neuroglia, also known as glial cells or glia, which assist the propagation of the nerve impulse as well as provide nutrients to the neurons.[rx]
Types of Nervous Tissue
Neurons
Neurons are cells that can transmit signals called nerve impulses, or action potentials. An action potential is a quick rise and fall in the electrical membrane potential of the neuron, which transmits signals from one neuron to the next. These are the different types of neurons:
- Sensory, or afferent neurons – relay information from the PNS to the CNS; different types of sensory neurons can detect temperature, pressure, and light.
- Motor, or efferent neurons – send signals from the CNS to the PNS; these signals provide information to sensory neurons to “tell” them what to do (e.g., initiate muscle movement).
- Interneurons connect sensory – and motor neurons to the brain and spinal cord; they act as connectors to form neural circuits and are involved with reflex actions and higher brain functions like decision-making.
While neurons can be specialized and look very different from one another, they each have components in common. Each neuron has a soma, or cell body, that contains the nucleus. Dendrites, finger-like projections that receive nerve impulses, branch off from the soma. The axon is a larger project that branches off from the soma. Nerve impulses travel along the axon in the form of an action potential. The axon splits into axon terminals, which branch off to other neurons. Neurotransmitters are released from the ends of the axon terminals, and these travel across the synaptic cleft to reach receptors on the dendrites of other neurons. In this way, neurons communicate with each other and can send signals that reach many other neurons.
Neuroglia
Neuroglia, or glial cells, are cells that support neurons, supply them with nutrients, and get rid of dead cells and pathogens such as bacteria. They also form insulation between neurons so that electrical signals do not get crossed, and can also aid the formation of synaptic connections between neurons. There are several types of neuroglia:
- Astroglial cells – also called astrocytes, are star-shaped cells found in the brain and spinal cord. They provide nutrients to neurons, maintain ion balance, and remove unneeded excess neurotransmitters from the synaptic cleft.
- Ependymal cells – are also found in the CNS. There are two types of ependymal cells. Non-ciliated ependymal cells form cerebrospinal fluid, while ciliated ependymal cells help the cerebrospinal fluid circulate. Cerebrospinal fluid cushions the brain and spinal cord.
- Oligodendrocytes – are found in the CNS and provide physical support to neurons. They form a myelin sheath around some neurons in the CNS. The myelin sheath is a fatty substance wrapped around the axons of some neurons; it provides electrical insulation.
- Schwann cells – also form myelin sheaths around some neurons, but they are only found in the PNS. Neurons that are myelinated can conduct electrical impulses faster than non-myelinated neurons.
- Microglial cells – or microglia, are small macrophage cells in the CNS that protect against disease by engulfing pathogens through phagocytosis (“cell eating”). They can also destroy infected neurons and promote the regrowth of neurons. All of the other types of neuroglia above are larger and collectively called macroglia.
Neurons are classified both functionally and structurally
Functional classification:[rx]
- Sensory neurons (afferent): Relay sensory information in the form of an action potential (nerve impulse) from the PNS to the CNS
- Motor neurons (efferent): Relay an action potential out of the CNS to the proper effector (muscles, glands)
- Interneurons: Cells that form connections between neurons and whose processes are limited to a single local area in the brain or spinal cord
Structural classification
- Multipolar neurons: Have 3 or more processes coming off the soma (cell body). They are the major neuron type in the CNS and include interneurons and motor neurons.
- Bipolar neurons: Sensory neurons that have two processes coming off the soma, one dendrite and one axon
- Pseudounipolar neurons: Sensory neurons that have one process that splits into two branches, forming the axon and dendrite
- Unipolar brush cells: Are excitatory glutamatergic interneurons that have a single short dendrite terminating in a brush-like tuft of centrioles. These are found in the granular layer of the cerebellum.
Neuroglia encompasses the non-neural cells in nervous tissue that provide various crucial supportive functions for neurons. They are smaller than neurons and vary in structure according to their function.[rx]
Neuroglial cells are classified as follows:[rx]
- Microglial cells: Microglia are macrophage cells that make up the primary immune system for the CNS.[7] They are the smallest neuroglial cell.
- Astrocytes: Star-shaped macroglial cells with many processes found in the CNS. They are the most abundant cell type in the brain and are intrinsic to a healthy CNS.[rx]
- Oligodendrocytes: CNS cells with very few processes. They form myelin sheaths on the axons of a neuron, which are lipid-based insulation that increases the speed at which the action potential, can travel down the axon.[rx]
- NG2 glia: CNS cells that are distinct from astrocytes, oligodendrocytes, and microglia, and serve as the developmental precursors of oligodendrocytes[rx]
- Schwann cells: The PNS equivalent of oligodendrocytes, they help maintain axons and form myelin sheaths in the PNS.[rx]
- Satellite glial cell: Line the surface of neuron cell bodies in ganglia (groups of nerve body cells bundled or connected together in the PNS)[rx]
- Enteric glia: Found in the enteric nervous system, within the gastrointestinal tract.[rx]
Classification of tissue
In the central nervous system
- Grey matter – is composed of cell bodies, dendrites, unmyelinated axons, protoplasmic astrocytes (astrocyte subtype), satellite oligodendrocytes (non-myelinating oligodendrocyte subtype), microglia, and very few myelinated axons.
- White matter – is composed of myelinated axons, fibrous astrocytes, myelinating oligodendrocytes, and microglia.
In the peripheral nervous system:[rx]
- Ganglion tissue is composed of cell bodies, dendrites, and satellite glial cells.
- Nerves are composed of myelinated and unmyelinated axons, Schwann cells surrounded by connective tissue.
The three layers of connective tissue surrounding each nerve are:[rx]
- Endoneurium – Each nerve axon or fiber is surrounded by the endoneurium, which is also called the endoneurial tube, channel, or sheath. This is a thin, delicate, protective layer of connective tissue.
- Perineurium – Each nerve fascicle containing one or more axons is enclosed by the perineurium, a connective tissue having a lamellar arrangement in seven or eight concentric layers. This plays a very important role in the protection and support of the nerve fibers and also serves to prevent the passage of large molecules from the epineurium into a fascicle.
- Epineurium – The epineurium is the outermost layer of dense connective tissue enclosing the (peripheral) nerve.
Clusters of Neuronal Cell Bodies
Clusters of cell bodies in the central nervous system are called nuclei, while the cell bodies lining the nerves in the peripheral nervous system are called ganglia.
Key Points
A cluster of neurons is called a nucleus if found in the central nervous system; it is called a ganglion if found in the peripheral nervous system (PNS).
Ganglia are the intermediate structures between the central and peripheral nervous systems.
Satellite glial cells (SGC) line the exterior surface of neurons in the PNS and surround neuron cell bodies within ganglia.
Key Terms
ganglion: A cluster of interconnecting nerve cells outside the brain.
nucleus: A cluster of neuronal bodies where synapsing occurs.
In neuroanatomy, a nucleus is a brain structure consisting of a relatively compact cluster of neurons. It is one of the two most common forms of nerve cell organization along with layered structures such as the cerebral cortex or cerebellar cortex. In anatomical sections, a nucleus shows up as a region of gray matter often bordered by white matter. The vertebrate brain contains hundreds of distinguishable nuclei varying widely in shape and size. A nucleus may itself have a complex internal structure, with multiple types of neurons arranged in clumps (subnuclei) or layers.
In addition, the term nucleus can refer to a distinct group of neurons that spread over an extended area. For example, the reticular nucleus of the thalamus is a thin layer of inhibitory neurons that surround the thalamus.
In the peripheral nervous system, a cluster of neurons is called a ganglion. One exception is the basal ganglia, located not in the periphery but rather in the forebrain. Ganglia are composed mainly of neuron cell bodies (somata) and dendritic structures. They are the intermediary connections between the peripheral and central nervous systems.
Innervation of the Autonomic Nervous System: Satellite glial cells are expressed throughout the sympathetic and parasympathetic ganglia in their respective nervous system divisions.
Satellite glial cells line the exterior surface of neurons in the PNS. Satellite glial cells (SGCs) also surround neuron cell bodies within the ganglia. They are of a similar embryological origin to Schwann cells of the PNS, as both are derived from the neural crest of the embryo during development. SGCs have a variety of roles, including control over the microenvironment of sympathetic ganglia. They are thought to have a similar role to astrocytes in the central nervous system (CNS). They supply nutrients to the surrounding neurons and also have some structural function. Satellite cells also act as protective, cushioning cells. Additionally, they express a variety of receptors that allow for a range of interactions with neuroactive chemicals.
Axon Bundles
A bundle of axons is called a nerve in the peripheral nervous system and a tract in the central nervous system.
Key Points
In the peripheral nervous system, a bundle of axons is called a nerve.
In the central nervous system, a bundle of axons is called a tract.
Each axon is surrounded by a delicate endoneurium layer.
The course connective tissue layer called perineurium binds the fibers into bundles called fascicles.
A tough fibrous sheath called epineurium encloses all the fascicles to form the nerve.
Key Terms
endoneurial fluid: A protein liquid surrounding individual nerve axons.
fascicle: A bundle of axons.
Axons
Neurons feature many long, slender projections termed axons, along which electrochemical nerve impulses are transmitted. In the central nervous system (CNS) bundles of these axons are called tracts, whereas in the peripheral nervous system (PNS) they are called nerves.
Each nerve is covered externally by a dense sheath of connective tissue, the epineurium. Underlying this layer of flat cells, the perineurium, forms a complete sleeve around a bundle of axons called fascicles. Surrounding each axon is the endoneurium. The endoneurium consists of an inner sleeve of material called the glycocalyx and an outer delicate meshwork of collagen fibers. Within the endoneurium, the individual nerve axons are surrounded by a protein liquid called endoneurial fluid. The endoneurium has properties analogous to the blood-brain barrier, in that it prevents certain molecules from crossing from the blood into the endoneurial fluid.
Axon length and diameter can vary greatly from between 1 m to 1 mm in length and 1 µm to 20 µm in diameter. The longest axons in the human body are those of the sciatic nerve, which run from the base of the spinal cord to the big toe of each foot. Axons in the central nervous system typically show complex trees with many branch points allowing for the simultaneous transmission of messages to a large number of target neurons.
Myelin
Axons are described as either un-myelinated or myelinated. Myelin is a layer of a fatty insulating substance, which is formed by two types of glial cells: Schwann cells en-sheathing peripheral neurons and oligodendrocytes insulating those of the central nervous system. Myelination enables an especially rapid mode of electrical impulse propagation called saltatory conduction. De-myelination of axons causes the multitude of neurological symptoms found in the disease multiple sclerosis.
Localization
Nerves in the PNS are typically divided into cranial and spinal nerves. There are twelve pairs of cranial nerves and thirty one pair of spinal nerves. Cranial nerves innervate parts of the head and connect directly to the brain (especially to the brainstem). They are typically assigned Roman numerals from 1 to 12, although cranial nerve zero is sometimes included. In addition, cranial nerves have descriptive names. Spinal nerves innervate much of the body, and connect through the spinal column to the spinal cord. They are given letter-number designations according to the vertebra through which they connect to the spinal column.
Gray and White Matter
The central nervous system consists of a central cavity surrounded by gray matter made of neuronal cell bodies and white matter made of myelinated axons.
LEARNING OBJECTIVES
Categorize gray versus white matter
KEY TAKEAWAYS
Key Points
- Gray matter is a major component of the central nervous system, consisting of neuronal cell bodies and other cells such as glia and dendrites.
- The white matter is composed of bundles of myelinated axons (few cell bodies) that connect various grey matter regions.
- Myelin is a thin layer, around the axons of white matter neurons and provides the white coloration.
Key Terms
- gray matter: A major component of the CNS consisting of neuronal cell bodies.
The central nervous system (CNS) is comprised of white and gray matter. In the spinal cord and cerebrum gray matter is surrounded by white matter. However, in the cerebellum and cerebral hemispheres this is reversed with the grey matter surrounding underlying white matter.
Gray Matter
Gray matter is a major component of the CNS, it refers to un-myelinated neurons and other cells of the central nervous system such as glial cells and dendrites. It is present in the brain, brainstem and cerebellum, and present throughout the spinal cord.
Gray matter contains most of the brain’s neuronal cell bodies. The grey matter includes regions of the brain involved in muscle control, and sensory perception such as seeing and hearing, memory, emotions, speech, decision making, and self-control.
Grey and White Matter: Micrograph showing grey matter, with the characteristic neuronal cell bodies (right of image – darker pink), and white matter with its characteristic fine mesh work-like appearance (left of image – lighter pink).
White Matter
A second major component of the CNS is white matter. It is composed mainly of bundles of myelinated axons, with very few neuronal bodies. White matter connects the various grey matter regions of the nervous system to each other and carries nerve impulses between neurons.
Myelin, the lipid that forms a thin layer, known as the myelin sheath, around the axons providing electrical insulation is white in color, giving rise to the name white matter.
Characteristics Of Nervous Tissue
- Nervous tissue makes up for the CNS and PNS of the nervous system
- Contains two distinct cells – neurons and glial cells
- It consists of the dendrites, cell body, axon and nerve endings.
- Neurons secrete chemical neurotransmitters which are responsible for stimulating other neurons as a result of a stimuli
- Presence of specialization at axonal terminals called synapsis
- Nerve cells live long, cannot be divided and replaced(except memory cells)
Functions
Myelinated axons conduct impulses faster than unmyelinated axons.
The function of nervous tissue is to form the communication network of the nervous system by conducting electric signals across tissue.[rx] In the CNS, grey matter, which contains the synapses, is important for information processing. White matter, containing myelinated axons, connects and facilitates nerve impulse between grey matter areas in the CNS.[rx] In the PNS, the ganglion tissue, containing the cell bodies and dendrites, contain relay points for nerve tissue impulses. The nerve tissue, containing myelinated axons bundles, carry action potential nerve impulses.[rx]
Glossary
astrocyte – glial cell type of the CNS that provides support for neurons and maintains the blood-brain barrier
axon hillock – tapering of the neuron cell body that gives rise to the axon
- axon segment – single stretch of the axon insulated by myelin and bounded by nodes of Ranvier at either end (except for the first, which is after the initial segment, and the last, which is followed by the axon terminal)
- axon terminal – end of the axon, where there are usually several branches extending toward the target cell
- axoplasm – cytoplasm of an axon, which is different in composition than the cytoplasm of the neuronal cell body
- bipolar – shape of a neuron with two processes extending from the neuron cell body—the axon and one dendrite
- blood-brain barrier (BBB) – physiological barrier between the circulatory system and the central nervous system that establishes a privileged blood supply, restricting the flow of substances into the CNS
- cerebrospinal fluid (CSF) – circulatory medium within the CNS that is produced by ependymal cells in the choroid plexus filtering the blood
- choroid plexus – specialized structure containing ependymal cells that line blood capillaries and filter blood to produce CSF in the four ventricles of the brain
- ependymal cell – glial cell type in the CNS responsible for producing cerebrospinal fluid
- initial segment – first part of the axon as it emerges from the axon hillock, where the electrical signals known as action potentials are generated
- microglia – glial cell type in the CNS that serves as the resident component of the immune system
- multipolar – shape of a neuron that has multiple processes—the axon and two or more dendrites
- myelin sheath – lipid-rich layer of insulation that surrounds an axon, formed by oligodendrocytes in the CNS and Schwann cells in the PNS; facilitates the transmission of electrical signals
- node of Ranvier – gap between two myelinated regions of an axon, allowing for strengthening of the electrical signal as it propagates down the axon
- oligodendrocyte – glial cell type in the CNS that provides the myelin insulation for axons in tracts
- satellite cell – glial cell type in the PNS that provides support for neurons in the ganglia
- Schwann cell – glial cell type in the PNS that provides the myelin insulation for axons in nerves
- synapse – narrow junction across which a chemical signal passes from neuron to the next, initiating a new electrical signal in the target cell
- synaptic end bulb – swelling at the end of an axon where neurotransmitter molecules are released onto a target cell across a synapse
- unipolar – shape of a neuron which has only one process that includes both the axon and dendrite
-
ventricle – central cavity within the brain where CSF is produced and circulates
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