Compare the central (CNS) and peripheral nervous system (PNS) and include the names given to a collection of nerve cell bodies; groups of nerve fibers; types of supporting cells present in each region.
Early in embryonic development, neuroectoderm and neural crest cells give rise to neuroepithelial stem cells of the CNS and PNS, respectively. The neuroepithelial stem cells differentiate into neuroblasts and glioblasts, which eventually give rise to two principal cell types: nerve cells (neurons) and neuroglial (glial or glia) cells ().
Neurons serve as the structural and functional link between the nervous system and body. Neurons rapidly transmit information as electrical impulses and convey that information to other neurons and effector cells at specific points of contact called synapses.
Neurons exhibit considerable variation in shape and size; however, each neuron consists of a cell body and a variable number of neurites or cell processes that extend from the cell body. The cell processes are designated as dendrites and axons.
The structural components of neurons are comparable to other cells in the body; however, neurons exhibit a regional distribution of cytoskeletal proteins and organelles between the neuron cell body, the dendrites, and the axon ( and ) (, , ).
Fig. 3.2 Schematic showing the structural organelles of a neuron by electron microscopy. The nucleus, nucleolus, golgi apparatus, rough endoplasmic reticulum, mitochondria, along with microtubules and microfilaments, which are also known as neurofibrils, are depicted. The normal histological features of a neuron and the distribution of the cellular structural components may change with age and disease. (Modified with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
Fig. 3.3 Light microscopy of a multipolar (motor) neuron (40 × magnification; hematoxylin and eosin [H/E] stain). Note the centrally placed nucleus, prominent nucleolus, and abundant rough endoplasmic reticulum (Nissl substance). These structural features are characteristic features associated with cells undergoing active protein synthesis. Neurons exhibit polarity based on the flow of electrical signals along their membrane. Signals are received at the dendrites, passed along the cell body, and propagated along the axon toward the target.
Fig. 3.4 Schematic of a neuron cell body and nerve cell processes (neurites). Neurons may contain one or more unmyelinated dendrites that function to receive, integrate, and transmit an electrical signal toward the neuron cell body. Each neuron contains only one axon which may be myelinated or unmyelinated and functions to generate and transmit action potentials away from the neuron cell body. The axon hillock represents the site of origin of the axon from the cell body. The unmyelinated initial segment of the axon lies just distal to the hillock and represents an area of high Na+ channel density and the point of initiation for an action potential. The distal part of the axon exhibits branching into several collateral axons that end as specialized presynaptic terminals known as terminal boutons. (Modified with permission from Silbernagl S, Despopoulos A. Color Atlas of Physiology. 6th edition. © Thieme 2009.)
Myelin increases the propagation speed of the electrical impulse. The myelin sheath surrounding an axon is discontinuous, containing small gaps along the length of the axon which are devoid of myelin. These regions, called nodes of Ranvier, contain Na + ion channels and serve as sites of axonal membrane depolarization. Nodes of Ranvier permit the rapid spread of a nerve impulse to move from one node to the next through the process of saltatory conduction.
In unmyelinated axons of peripheral nerves, the plasma membrane of the neuroglial cell invests the axon as a single layer, and the nerve impulse propagates at a slower rate through continuous conduction.
Fig. 3.5 (a) Schematic of cells responsible for axon myelination in the central nervous system (CNS) and peripheral nervous system (PNS). Neuroglial cells deposit a lipid sheath, known as myelin around axons. In the CNS the oligodendrocytes, a type of neuroglial cell, myelinates multiple sections known as an internodal segment on multiple axons. The ability of a single cell to myelinate more than one axon helps to conserve space within the CNS. In the PNS, the neuroglial Schwann cell myelinates one internodal segment of a single axon. (b) Schematic image and (c) light microscopy demonstrating a node of Ranvier in a myelinated axon (40 × magnification; Osmium stain). Small gaps in the myelin sheath, known as nodes of Ranvier, contain exposed Na+ ion channels and serve as sites for membrane depolarization. Myelin insulates the axon, increasing the electrical resistance of the membrane, and prevents decay of the action potential. Nodes of Ranvier increase the conduction speed by permitting the action potential to “jump” rapidly from one node to the next through the process of saltatory conduction. (a: Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker. b: Reproduced with permission from Kahle W, Frotscher M. Color Atlas of Human Anatomy, Vol 3. 6th edition. © Thieme 2011.)
The speed at which a nerve impulse may be transmitted is dependent on the myelin sheath surrounding axons in the CNS and PNS. Oligodendrocytes and Schwann cells are responsible for myelination of axons in the CNS and PNS, respectively. Demyelination of axons occurs in Multiple Sclerosis and Guillain-Barre syndrome, leading to somatic and autonomic deficits. Symptoms may manifest as weakness, numbness, and tingling, GI disturbances, cardiac arrhythmia, or ventilation problems.
Multiple sclerosis is an inflammatory condition restricted to the CNS and is characterized by the production of autoantibodies directed against proteins in the myelin sheath. Destruction of the myelin sheath leads to scar formation in the areas of myelin damage and progressive axonal degeneration.
In comparison, Guillain–Barre syndrome is associated with acute inflammatory processes in the PNS, leading to the demyelination of peripheral nerve fibers and root, with little damage to the underlying axon.
Axonal transport is the process through which metabolic byproducts and organelles of the neuron may pass between the soma and the terminal end of the axon. The mechanism of axonal transport is essential for neuronal survival, synaptic transmission, regeneration, and neurite outgrowth ().
Fast anterograde axonal transport typically involves the movement of endocytic vesicles containing substances that have a functional role at the axon terminus. The enzymes, proteins, and macromolecules necessary for neurotransmitter release from the synaptic terminal utilize this transport mechanism.
Slow anterograde axonal transport involves the movement of soluble cytoplasmic substances, motor proteins, and cytoskeletal proteins which are necessary for proper synaptic transmission. Slow axonal transport of cytoskeletal proteins is essential for axonal outgrowth during development and axonal regeneration following injury.
Fast retrograde axonal transport may involve the passage of exogenous soluble (trophic) growth factors from the axon terminal, or the transport of worn-out synaptic membrane vesicles to be carried toward the cell body for degradation or recycling. Viruses and neurotoxins may infect axon terminals of peripheral nerves and pass to cell bodies by retrograde transport.
Neurons are excitable, polarized cells that receive and transmit electrochemical signals in one direction. The signal received by the dendrite passes to the cell body, and then to the terminal part to the axon where transmission of the impulse to another cell occurs.
In electrical synapses, two neurons are physically and electrically coupled by small intercellular channels, called gap junctions. Gap junctions provide low electrical resistance and allow for the rapid propagation of an action potential to pass directly from the presynaptic neuron to a postsynaptic neuron.
In chemical neurotransmission, a small intercellular space, or synaptic cleft separates the presynaptic neuron and postsynaptic target. Chemical synapses involve the release of a chemical mediator, called a neurotransmitter from the presynaptic terminal of the neuron. The neurotransmitter diffuses across the synaptic cleft and then binds to specific receptors located on the postsynaptic membrane.
A detailed description of neurotransmission is covered in Chapter 4.
Fig. 3.7 (a,b) Schematic illustration of electrical (a) and chemical (b) synapses. Synapses are sites of functional contact between two neurons, a neuron and an effector cell, or a neuron and a sensory receptor. Synapses may be excitatory or inhibitory and consist of a presynaptic membrane, synaptic cleft, and a postsynaptic membrane. Electrical synapses (a) consist of a physical and electrical coupling of two neurons together by gap junctions. The gap junction allows ions to flow bidirectionally between cells. Chemical synapses (b) are unidirectional and use neurotransmitters released from the presynaptic terminal to bind to the receptors on the postsynaptic membrane and creates either an excitatory membrane depolarization (EPSP) or inhibitory (IPSP) hyperpolarization of the postsynaptic membrane. EPSP increases the potential of generating an action potential by the postsynaptic neuron, whereas an IPSP decreases the chance of the postsynaptic neuron firing.
Fig. 3.8 Schematic showing points of synaptic contact. Synapses may be classified by the site of synaptic contact. Synaptic contact occurs most often between a presynaptic axon and a postsynaptic dendrite. Other points of synaptic contact may occur between two axons (axon-axonal) or between the axon and neuron cell body (axosomatic). (Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker.)
Alzheimer disease is the most common form of senile dementia and is characterized by progressive memory loss that results from synaptic damage, degeneration of nerve processes, plaque formation within the cerebral cortex, and the accumulation of neurofibrillary tangles associated with neuronal cell death.
Plaques represent abnormal build-up of clusters of β-amyloid proteins between synaptic terminals which may disrupt synaptic signaling and lead to degeneration of nerve processes. Plaques form as nerve processes degenerate at presynaptic terminals and become wrapped with aggregations of β-amyloid proteins. Neurofibrillary tangles develop from the improper folding of the microtubule associated protein, Tau. Defects in Tau protein synthesis lead to disruption in axonal transport, neuronal cell death, and the extracellular accumulation of Tau as neurofibrillary tangles.
Fig. 3.9 (a–d) Location of neuron cell bodies and nerve fibers in the central nervous system (CNS) and peripheral nervous system (PNS). The regions containing neuron cell bodies (pink) and axons (green) in the CNS are analogous to structures in the PNS. In the CNS, groups of functionally related neuron cell bodies form nuclei and reside in the gray matter (pink) of the brain and spinal cord (a). Clusters of myelinated axons form tracts (green) and reside within the white matter of the CNS (b). In the PNS, groups of neuron cell bodies form ganglia (pink) (c), whereas groups of axons are referred to as peripheral nerves. (Modified with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
In the CNS, the cell bodies of neurons and dendrites reside the gray matter and may be organized in laminae (layers/sheets) or aggregate to form functionally associated groups of cell bodies known as nuclei (a).
The axons extending from the cell bodies may be myelinated or unmyelinated and form functionally related bundles of fibers called tracts or fasciculi which pass through the white matter of CNS. Tracts are usually named and connect nuclei between different regions of the brain and spinal cord (b).
In the PNS, aggregations of functionally related neuronal cell bodies lie outside the CNS and form encapsulated structures known as ganglia (c). Ganglia are associated with the autonomic nervous system, the dorsal roots of the spinal nerves, and some of the sensory cranial nerves (CN V, VII, VIII, IX, and X) (, ).
All spinal nerves as they emerge from the vertebral column are classified as mixed nerves and carry both motor (efferent) and sensory (afferent) fibers. In comparison, some cranial nerves may be mixed nerves, while others may carry only afferent (sensory) fibers or only efferent (motor) fibers.
Afferent fibers of spinal or cranial nerves convey sensory information from peripheral receptors toward the CNS. The neuronal cell bodies for these afferent (sensory) fibers are in sensory ganglia located in the PNS.
Efferent fibers transmit motor impulses away from the CNS to peripheral effectors such as muscle or glands. The cell bodies of efferent (motor) fibers reside in either the spinal cord or as cranial motor nuclei within the brainstem of the CNS. Additionally, some efferent (motor) cell bodies associated with the autonomic nervous system lie in autonomic ganglia.
Fig. 3.10 Light microscopy demonstrating visceral motor neuron cell bodies in an autonomic ganglion. Wall of the ileum (20 × magnification; hematoxylin and eosin [H/E] stain). Autonomic ganglia associated with the peripheral nervous system (PNS) may be found near an organ or within the wall (intramural) of visceral organs. Autonomic neuron cell bodies are part of two neuron relay chain. One cell body resides in nuclei of the central nervous system (CNS) and the second cell body is found in the PNS. The autonomic nervous system, which includes the parasympathetic and sympathetic systems, consists of neuron cell bodies and axons. Neuron cell bodies shown in this image appear as large, pink cells. The nucleus and nucleolus are visible.
Fig. 3.11 Light microscopy demonstrating sensory neuron cell bodies in the dorsal root (spinal) ganglion. (4 × magnification; silver stain of dorsal root ganglion and spinal cord). Sensory cell bodies appear as large round cells with a centrally placed nucleus. Sensory nerve fibers are shown passing through ganglion and continue as dorsal root afferent fibers. The dorsal root ganglion and sensory (afferent) nerve fibers are in the peripheral nervous system (PNS). The afferent fibers convey sensory information toward the central nervous system (CNS). Nerve fibers carrying motor (efferent) impulses from the CNS are shown passing through the ventral root toward peripheral targets.
Fig. 3.12 Structural components and connective tissue coverings of a peripheral nerve in the peripheral nervous system (PNS). (a) Schematic of peripheral nerve in cross-section. Peripheral nerves represent cranial or spinal nerves that carry sensory, motor, or autonomic input. Nerve fibers may be classified by function, fiber diameter, the extent of myelination, and conduction speed. Spinal nerves are functionally classified as mixed nerves that carry both motor (efferent) and sensory (afferent) fibers. In comparison, some cranial nerves may be classified as mixed nerves, or only as sensory, or only motor. (b) Light microscopy demonstrating the connective tissue layers surrounding a peripheral nerve (20 × magnification; hematoxylin and eosin [H/E] stain). Peripheral nerves consist of bundles of myelinated or unmyelinated axons enveloped with three layers of connective tissue (CT). The CT layers arranged from superficial to deep, include an endoneurium, perineurium, and epineurium. The individual axons in the section shown appear as small pink circles surrounded by the myelin sheath (white). The endoneurium surrounds each axon, and is important in axonal regeneration following nerve damage. The middle layer of CT, known as the perineurium, bundles groups of individual axons together to form fascicles. The outer CT layer is the epineurium which bundles nerve fascicles together. (a: Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 1. © Thieme 2014. Illustrations by Markus Voll and Karl Wesker.)
Fig. 3.13 Schematic diagram showing morphological changes to the neuron cell body and the proximal and distal regions of a myelinated axon following nerve injury. Potential outcomes depend on the extent and location of the nerve injury; however, they may include chromatolysis of the cell body, degeneration of the distal axon (Wallerian; anterograde axonal degeneration), and loss of synaptic contact with the target. This may cause a loss of sensation, muscle atrophy, or motor dysfunction. Typically, the proximal part of the axon remains attached to the cell body and continues to live, while the section distal to the injury degenerates. The proximal axon may potentially regenerate distally and reform synaptic connections with targets if the endoneurium and Schwann cells surrounding the axon remain viable. If the proximal axon is unable to penetrate the existing Schwann cell sheath, the axon exhibits disorganized growth and forms a traumatic neuroma.