Parts and Functional Dependencies of Neurons

1. The Cell Body

A neuron (Figs. 21E-21I) invariably has a cell body (soma) containing the cell nucleus and a more or less spherical mass of ambient cytoplasm, the perikaryon, filled with cell organelles and inclusions. During development, the soma is the first part of the neuron to arise, and throughout the life of the cell (with few exceptions, postmitotic and incapable of cell division) it is the irreplaceable trophic part that maintains the structural integrity and health of the neuron with its far-reaching processes.

2. Dendrites

Extending from the soma in most neurons are the dendrites (Figs. 21A-21B). These relatively short, initially tapering processes branch obliquely, like the limbs and branches of a tree. In varying proportions, they contain the organelles seen in the soma. In a few neurons, they arise at the end of an axon. A dorsal root ganglion cell (Fig. 15, DRG) has dendritic tufts or a specialized nerve ending (a free or an encapsulated axonal tip) in such an axon in the dermis, wall of a viscus, or other peripheral area.

Dendrites may branch elaborately (Fig. 27). They are the chief receptive processes of a neuron, even though the soma and certain proximal or distal axonal regions may also receive inputs. In their ramification, they signify the relationships of the neuron to other neurons and, hence, their integrative role. They offer points of contact (synapses) for axon terminals and for axons grazing them as they pass by (fibers of passage). They are the prime integrative components of neurons: receiving, combining, summing, or otherwise regulating their input.

In the multipolar neurons of the CNS (Fig. 15, the six dark cells), dendrites originate from the soma as one or more primary branches. These continue to ramify, becoming finer and finer, eventually forming branchlets or twigs. The surface area and volume of the dendrites may exceed those of the soma by many times. Incoming axons end on the dendritic shaft or on spines

(Fig. 16, upper and lower plates), which are minute thorns, that if present in large numbers give dendrites a studded appearance.

The role of spines is not understood, but (as once surmised) they do not serve to increase the receptive surface area; intervening shaft regions may have but few synapses. Spine synapses are the major sites of excitation; each spine usually has one input, but may have several. One function spines seem to play is isolating such afferents. But it is premature to characterize them. They have electrical properties that may be modified by slight changes in the form of the spine neck, perhaps during learning or in aging, when they show deterioration early on. They seem to be specialized ports of entry for currents into the dendritic shaft. They may be bounded by astrocytic processes (Fig. 16, lower plate). By ''spongelike'' action, astrocytes serve vital roles in neural communication: taking up excess potassium ions that might interfere with local neuronal activity, localizing glutamate release for optimal elicitation of excitation in target cells, and preventing glutamate-induced toxicity, which is potentially life-threatening to neurons.

3. The Axon and Axonal Collaterals

Somewhere about the soma of most neurons emerges a single, cablelike process called the axon or axis cylinder. It arises from a small, conical elevation (axon hillock; Fig. 21A) or from the stem of a major dendrite (Fig. 21B). Its departure may be virtually straight, presaging a clearly directed flight to a distant location (via a tract or nerve), or meandering, as if lost in the neuronal shrubbery near its origin (Fig. 22). Such direct or rambling courses provide the long-distance trunk lines and myriad local circuits over which neuronal signaling and communication take place.

The conductile axon conveys impulses to other neurons, a muscle, or a gland. In primary sensory neurons (Fig. 15, DRG) innervating the skin, impulses arise in axonal endings: naked, with minute expanded tips, or encapsulated (A), as shown. The action potentials travel to the CNS via a cranial or spinal nerve.

Compared to dendrites, the axon is relatively long (2-3 mm to 1 m), slender (0.1-20 mm), and untapering. Like dendrites, it may branch, as axon collaterals, but these typically depart at right (not acute) angles. Collaterals distribute neuronal output to several or many places, depending on circuit design and how many collaterals the axon has. Axonal collateraliza-tion is as expressive of neuronal interrelationships as

Cajal White Matter Astrocyte

Figure 21 LM potpourri: (A) Small pyramidal cell in visual cortex, axon (a), Golgi method. (B) Small neuron in dentate nucleus of cerebellum, axon (a), Golgi method. (C) Protoplasmic (velate) astrocyte in gray matter, Golgi method. (D) Oligodendrocyte in white matter, Golgi method. (E) Spinal motor neuron showing Golgi apparatus, osmium tetroxide impregnation. (F) Motor neuron in abducens nucleus showing mitochondria, Altmann-Kull method. (G) Spinal motor neuron showing neurofibrils, Cajal's silver stain. (H) Motor neuron in abducens nucleus showing Nissl bodies, thionin stain. (I) Dorsal root ganglion cell, artist's rendition. (J) Myelinated peripheral nerve fiber, showing node of Ranvier, Schmidt-Lanterman clefts, Schwann cell nucleus, and neurofibrils, artist's rendition. From The Fine Structure of the Nervous System: Neurons and Their Supporting Cells, 3rd ed., by Alan Peters, Sanford L. Palay, and Henry de F. Webster, copyright 1990 by Alan Peters. Used by permission of Oxford University Press, Inc. (illustration by Betsy A. Palay).

Figure 21 LM potpourri: (A) Small pyramidal cell in visual cortex, axon (a), Golgi method. (B) Small neuron in dentate nucleus of cerebellum, axon (a), Golgi method. (C) Protoplasmic (velate) astrocyte in gray matter, Golgi method. (D) Oligodendrocyte in white matter, Golgi method. (E) Spinal motor neuron showing Golgi apparatus, osmium tetroxide impregnation. (F) Motor neuron in abducens nucleus showing mitochondria, Altmann-Kull method. (G) Spinal motor neuron showing neurofibrils, Cajal's silver stain. (H) Motor neuron in abducens nucleus showing Nissl bodies, thionin stain. (I) Dorsal root ganglion cell, artist's rendition. (J) Myelinated peripheral nerve fiber, showing node of Ranvier, Schmidt-Lanterman clefts, Schwann cell nucleus, and neurofibrils, artist's rendition. From The Fine Structure of the Nervous System: Neurons and Their Supporting Cells, 3rd ed., by Alan Peters, Sanford L. Palay, and Henry de F. Webster, copyright 1990 by Alan Peters. Used by permission of Oxford University Press, Inc. (illustration by Betsy A. Palay).

dendritic branching. Dendrites signify the integrative power of a neuron and axons and their collaterals its distributive power, as well as the routes and addresses of impulse dissemination.

The axon is usually single, but can be double, as in bipolar neurons, or missing, as in amacrine cells (having no long processes), which intercommunicate through their dendrites. It is thinner and much longer than any dendrite, but most CNS neurons have a greater total length of dendrites than axons.

Single or double, short or long, branched or not, if an axon is present, it is the functional axis of the neuron, the hub along which nerve impulses travel. Over this main line speed the ''all-or-none'' action

Brain Cells

Figure 22 Direct and wandering axons: the pyramidal cell and stellate cell of the cerebral cortex shown here typify the two major modes of axonal departure: in the one, virtually straight, presaging flight to a distant location (e.g., the spinal cord), and in the other, meandering among the dendrites of the neuron of origin and those of others nearby. The former course characterizes the long-distance trunk lines of the CNS and the latter its myriad local circuits. Abbreviations: dendrites (d and pr.), cell body (c.b.), axons (ax.). From J. Z. Young, The Life of Mammals, 1957, copyright 1957 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (preparation by D. A. Sholl).

Pyramidal cell Stellate cell

Figure 22 Direct and wandering axons: the pyramidal cell and stellate cell of the cerebral cortex shown here typify the two major modes of axonal departure: in the one, virtually straight, presaging flight to a distant location (e.g., the spinal cord), and in the other, meandering among the dendrites of the neuron of origin and those of others nearby. The former course characterizes the long-distance trunk lines of the CNS and the latter its myriad local circuits. Abbreviations: dendrites (d and pr.), cell body (c.b.), axons (ax.). From J. Z. Young, The Life of Mammals, 1957, copyright 1957 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (preparation by D. A. Sholl).

potentials (spikes), encoding output in spike trains or bursts superimposed on a discharge frequency characteristic of the neuron under given conditions. In motor neurons and other large nerve cells, these spike potentials originate in the axon hillock and axon initial segment, the ''spike trigger zone'' (Fig. 23).

4. Axon Terminals

Specialized terminations of axons or of their preterm-inal branches, these have many shapes and names: bulbous, buttonlike, claw-shaped, and so forth. Similar beaded structures may lie along the course of an axon. All are sites where frequency-coded messages are transmitted chemically to some part of the target neuron or the muscle fiber across a synapse or neuromuscular junction and where trophic effects may pass in both directions. Axon terminals are the effector parts of neurons, whereas dendrites, the cell body, and axons are the receptive, trophic, and conductile parts, respectively.

5. Flexibility of the Parts of Neurons

The parts of neurons are adaptable. There are many exceptions to these conventional definitions, and

Structure Neuron

Figure 23 Schematic EM of neuron, its organelles, and inclusions: dendrites (Den), axon (Ax), nucleus (N), nucleolus (Nl), nucleolar satellite (NS), Nissl substance (Nis), rough-surfaced endoplasmic reticulum (not labeled), ribosomes (R), smooth-surfaced endoplasmic reticulum (SER), subsurface cisternae (SsC), Golgi apparatus (G), dense core vesicles (DV), lysosomes (Ly), lipofuscin pigment granules (LPG), microtubules (Mt), neurofilaments (Nf), dendritic thorns or spines (DT), spine apparatus (SA), synapses (Sy), synaptic vesicles (SV). From T. L. Lentz, Cell Fine Structure. An Atlas of Drawings of Whole-Cell Structure. W. B. Saunders Co., Philadelphia, 1971.

Figure 23 Schematic EM of neuron, its organelles, and inclusions: dendrites (Den), axon (Ax), nucleus (N), nucleolus (Nl), nucleolar satellite (NS), Nissl substance (Nis), rough-surfaced endoplasmic reticulum (not labeled), ribosomes (R), smooth-surfaced endoplasmic reticulum (SER), subsurface cisternae (SsC), Golgi apparatus (G), dense core vesicles (DV), lysosomes (Ly), lipofuscin pigment granules (LPG), microtubules (Mt), neurofilaments (Nf), dendritic thorns or spines (DT), spine apparatus (SA), synapses (Sy), synaptic vesicles (SV). From T. L. Lentz, Cell Fine Structure. An Atlas of Drawings of Whole-Cell Structure. W. B. Saunders Co., Philadelphia, 1971.

awareness of them enhances understanding of neural function and neurologic disease. Certain regions of the axon may serve receptive functions: with axoaxonic input at the axon hillock or on its presynaptic end bulbs. Dendrites may conduct impulses swiftly in an all-or-none manner like axons (as in the towering hippocampal pyramidal cells). They may also act as effectors (like axon terminals) and transmit activity fractionately, as in dendrodendritic synapses. Thus, the chemical membrane and subjacent organelles of a neuron constitute a functional mosaic.

It is helpful to regard neuronal design as flexible. Almost any part can perform any communication function, if circumstances or circuit design make it advantageous. The only invariant component of a neuron is the cell body. It, and only it, is the trophic part.

Flexibility is striking in relationships between neurons. Neuroscientists used to think in terms of serial progressions of neurons through axodendritic and axosomatic synapses and of similar sequences through axonal collateral channels. Now other arrangements are known (Fig. 25): parallel coupling (axoaxonic and somatosomatic synapses), reciprocal coupling (dendrodendritic synapses), parallel-serial coupling (ax-oaxodendritic synapses), and atypical coupling (somatoaxonic synapses). These more complex modes of cell-to-cell interplay, along with gap junctions and the unpredictable results of synaptic activity, illustrate the versatility of neural communication. More than

Picture Neuron

Figure 24 Axon hillock and initial segment: the axon is shown leaving the perikaryon of a pyramidal neuron at the axon hillock (AH), wherein ribosomes (r) and rough endoplasmic reticulum (ER) are seen, and descending through richly varied and epithelially organized neuropil. In the initial segment, ER is absent and ribosomes (r1) decrease in number. Mitochondria (mit) and neurofilaments enter the axon freely; microtubules (m) also enter but become bundled. Such bundling and the presence of an undercoat to the axonal plasma membrane characterize the initial segment. Three axon terminals are seen synapsing with the initial segment: two proximal (AT1) and one distal (AT2). From The Fine Structure of the Nervous System: Neurons and Their Supporting Cells, 3rded., by Alan Peters, SanfordL. Palay, and Henry deF. Webster, copyright 1990 by Alan Peters. Used by permission of Oxford University Press, Inc. (EM from collections of authors).

Figure 24 Axon hillock and initial segment: the axon is shown leaving the perikaryon of a pyramidal neuron at the axon hillock (AH), wherein ribosomes (r) and rough endoplasmic reticulum (ER) are seen, and descending through richly varied and epithelially organized neuropil. In the initial segment, ER is absent and ribosomes (r1) decrease in number. Mitochondria (mit) and neurofilaments enter the axon freely; microtubules (m) also enter but become bundled. Such bundling and the presence of an undercoat to the axonal plasma membrane characterize the initial segment. Three axon terminals are seen synapsing with the initial segment: two proximal (AT1) and one distal (AT2). From The Fine Structure of the Nervous System: Neurons and Their Supporting Cells, 3rded., by Alan Peters, SanfordL. Palay, and Henry deF. Webster, copyright 1990 by Alan Peters. Used by permission of Oxford University Press, Inc. (EM from collections of authors).

Figure 25 Neuronal coupling paradigms: for a century, neurons were considered to be coupled linearly by axodendritic and axosomatic synapses. But by 1972, as shown schematically here by David Bodian, it was clear that many paradigms of parallel coupling existed. These illustrate the adaptability of the parts of neurons, the flexibility of neuronal design, and, as Dr. Bodian observed, ''the diversity of life and its processes.'' See also text. From Neuron junctions: a revolutionary decade, D. Bodian, The Anatomical Record, Copyright © 1972. Reprinted by permission of Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc. (illustration by Elinor W. Bodian).

Figure 25 Neuronal coupling paradigms: for a century, neurons were considered to be coupled linearly by axodendritic and axosomatic synapses. But by 1972, as shown schematically here by David Bodian, it was clear that many paradigms of parallel coupling existed. These illustrate the adaptability of the parts of neurons, the flexibility of neuronal design, and, as Dr. Bodian observed, ''the diversity of life and its processes.'' See also text. From Neuron junctions: a revolutionary decade, D. Bodian, The Anatomical Record, Copyright © 1972. Reprinted by permission of Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc. (illustration by Elinor W. Bodian).

that, they show, as neuroanatomist David Bodian observed, ''the diversity of life and its processes.''

6. Polarity of Neurons

In terms of polarity, (number of cytoplasmic processes), three kinds of neurons are recognized: unipolar, bipolar, and multipolar (Fig. 26). The probable location and functional significance of each may be inferred by inspection, but an explanation is necessary.

Unipolar neurons are not found in vertebrate nervous systems, although young neurons have but one process at certain developmental stages and look like them. But in invertebrates, they represent the dominant population and, hence, the largest number of nerve cells on earth.

Bipolar neurons are simple modifications, with fusiform cell bodies, of the columnar epithelial cells from which they evolved. In humans, they form primary sensory neurons in the olfactory epithelium, retina, and vestibulocochlear ganglia. Terminal ramifications in the periphery (e.g., in the organ of Corti) respond fractionately to stimuli and may be considered distant dendrites. By contrast, the long process leading to the soma, like the other process departing, is by all criteria axonal, even having a myelin sheath for increased

Unipolar Bipolar Pseudounipolar Multipolar

Neuron Shape

Figure 26 Polarity of neurons. In number of cytoplasmic processes, three general kinds of neurons are recognized: unipolar, bipolar, and multipolar. True unipolar neurons are not found in the adult vertebrate nervous system. Bipolar neurons and a variant, pseudounipolar neurons, make up all the primary sensory neurons of the PNS. Multipolar neurons have many variably branched processes extending in many directions; as the most common type of vertebrate neuron, they are the hallmark of the human CNS. See also text. From Principles of Neuroanatomy, by Jay B. Angevine, Jr., and Carl W. Cotman, copyright 1981 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (illustration by Steven J. Harrison).

Figure 26 Polarity of neurons. In number of cytoplasmic processes, three general kinds of neurons are recognized: unipolar, bipolar, and multipolar. True unipolar neurons are not found in the adult vertebrate nervous system. Bipolar neurons and a variant, pseudounipolar neurons, make up all the primary sensory neurons of the PNS. Multipolar neurons have many variably branched processes extending in many directions; as the most common type of vertebrate neuron, they are the hallmark of the human CNS. See also text. From Principles of Neuroanatomy, by Jay B. Angevine, Jr., and Carl W. Cotman, copyright 1981 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (illustration by Steven J. Harrison).

speed of impulse conduction. By the interposition of an axonal cable between the receptive region and the cell body, spikes triggered peripherally pass swiftly to the soma, over which they flow to the other process and on to the CNS, wherein their central ramifications distribute impulses to secondary sensory neurons.

Pseudounipolar neurons are modified bipolar cells. The opposing processes shift around the soma during development and combine into one, at least proximal-ly. This process takes a short, often convoluted course and then branches like the letter T: one branch going to the periphery and the other to the CNS (Fig. 21I), as in bipolar neurons. Nerve impulses in these cells pass from one branch to the other, subsequently "backfiring" the single process and soma. Pseudounipolar and bipolar cells make up all primary sensory neurons in the PNS. Both have limited integrative domains in distant dendritic tufts and no input on their somata, which are limited to trophic and housekeeping duties.

Their central terminals, however, receive presynaptic endings, providing efferent modulation of their transmission to secondary sensory neurons. This arrangement is important in suppressing the receipt of painful stimuli. It is an enkephalinergic component of a complex brain stem analgesia system.

Multipolar neurons have many variably branched processes projecting in many directions. The most common type of vertebrate neuron, they comprise virtually all neurons of the human CNS. With their diversified input (from 104 to 3 x 105 apiece), they have tremendous integrative capacity. In the shape, size, and position of their somata, in the number, length, and branching pattern of axons and dendrites, and in neuroactive substances synthesized and released, a panoply of multipolar neurons is found in the CNS (Fig. 27). This variety is so extraordinary that neurobiologist Pasko Rakic has said, "I believe it is safe to estimate that there are more forms of cells in the

Figure 27 A panoply of neurons: a huge variety of multipolar neurons is found in the CNS. The dendrites express the integrative power of a neuron, the axon with its collaterals (or lack of same), its sphere of influence, the cell body with its minute to huge size and varied shape, its area available for axosomatic synapses, the length of its axon, and its amenability to high or low packing density. (A) Neuron of inferior olive. (B) Granule cell of cerebellar cortex. (C) Small cell of reticular formation. (D) Small gelatinosa cell of spinal trigeminal nucleus. (E) Ovoid cell of nucleus tractus solitarius. (F) Large cell of reticular formation. (G) Spindle-shaped cell, substantia gelatinosa of spinal cord. (H) Large cell of spinal trigeminal nucleus. (I) Neuron of putamen. (J) Pyramidal cell of hippocampus. (K) Cell from thalamic nucleus. (L) Cell from globus pallidus. At bottom, scaled drawings of a Purkinje cell of the cerebellar cortex and a pyramidal cell of the cerebral cortex for comparison. From Malcolm B. Carpenter and Jerome Sutin, Human Neuroanatomy, 8th ed., J. B. Lippincott, 1983 (drawings of Golgi-impregnated material by M. Stogdell for Dr. Clement Fox).

brain of a mammal than in all the rest of the entire body.'' Functionally, their vast number (100 billion, maybe 1 trillion) falls into two groups: a small number of motor neurons (about 2 million), sending axons to muscles and glands, and the remainder a host of interneurons.

As stated, the dendrites of multipolar neurons are the most intriguing variables, expressing their integra-tive power. They vary from few to profuse, from straight and smooth to curving and spiny, and from meagre shrubbery to magnificent arboreal extravaganzas. The axon comes next, being short or long, unbranched or rectilinearly, almost obsessively col-lateralized, and ruler-straight and departing the locale or sinuous and wandering through the local neuropil. Then the soma: ovoid, spherical, pyriform, fusiform, fat, skinny, big, small: miniscule (4 mm, half the size of a red blood cell) in dwarf neurons to huge (150 mm) in giants. Teased out in a tissue preparation, the Betz cells in the human cerebral cortex can be seen with the naked eye.

7. Specialization of Neurons

The variety of cell structure in the human nervous system is far more complex than in other tissues. Whereas cells elsewhere (e.g., erythrocytes, hepato-cytes) are highly redundant, neurons are highly individualized. Indispensable, decision-making command neurons are known in invertebrate nervous systems (e.g., in the ganglia of certain arthropods). But in vertebrates, a great redundancy of neurons seems the rule: to iron out individual peculiarities of cells and for safety's sake.

The human nervous system has many neuronal subpopulations. Within each, a neuron shows its individuality by form, function (speed of response, endurance, or fatigability), position in a circuit, chemical coding of impulses, and sheer size. Neuronal specialization gives our nervous system speed, flexibility, fidelity, and staggering integrative power.

8. Schemes of Classifying Neurons

To understand such complex cells as neurons, it helps to have some means of categorization. Many classifications exist, each with advantages and limitations. Neurons are grouped as to size, shape, polarity, dendritic pattern, axonal length, long-distance or local-circuit service, presence and nature of neurosecretory activity, neuroactive substances synthesized and liberated, and identifiability as a circuit component from one nervous system to another. But perhaps the most useful scheme is the position of a neuron in the fundamental circuit plan: a functional classification into sensory, motor, and intermediate neurons.

a. Sensory Neurons Primary sensory neurons are the initial neurons in sensory data processing. Derived from neural crest cells near the embryonic neural tube, their cell bodies lie outside the CNS: rod cells of the olfactory membrane, bipolar ganglion cells of the vestibulocochlear nerve (cranial nerve VIII), and pseudounipolar ganglion cells of spinal nerves and cranial nerves (CNs) V, VII, IX, and X. They are sentinals posted outside the CNS, reporting news (good and bad) from the periphery. Their number is small, perhaps 20 million.

Secondary sensory neurons are the second echelon in sensory data processing: cells in the CNS on which axons of first-order neurons terminate. Clusters of them lie in the dorsal horn of the spinal gray and brain stem nuclei (e.g., the cochlear nuclei). Whereas their afferent domain is weighted by sensory input, they are interneurons, between the first and last neurons in the plan of the nervous system. They receive input from sources beside primary sensory neurons (even from the cerebral cortex) and are multipolar, not bipolar or pseudounipolar, thereby meeting all criteria for inter-neurons.

b. Motor Neurons Multipolar in design, the cell bodies of somatic motor neurons are located entirely within the CNS. Whereas sensory neurons are the first cells in neural data processing, motor neurons are the last. They send impulses to the effector organs: muscles and glands. Like sensory neurons, their number is modest: about 2 million. Yet their role is profound. Derived from the mantle layer of the neural tube, they include the large a and small g motor neurons in the spinal cord and brainstem that innervate extrafusal and intrafusal skeletal muscle fibers: those that do work and those that report muscle stretch, respectively. They also include the visceral motor neurons of the ANS, the dual arrangement of a preganglionic motor neuron in the CNS and a postganglionic one in an autonomic ganglion.

c. Interneurons Once called internuncial or "go-between" neurons, interneurons as originally defined are neurons interposed between sensory and motor neurons. The CNS is their domain and neuroanatomy is largely the study of them and their connections. The number of interneurons is extremely large: 1011 is the figure currently cited, but because large populations of minute cerebellar and hippocampal neurons may be greatly underestimated, some see it as closer to 1012. They comprise almost all of our neurons. These countless separate, yet richly interconnected cells are more than responsive to stimuli, though they serve homeostasis with computer speed and reliability and in ways nineteenth century opponents of the neuron doctrine could hardly have comprehended. Interneur-ons are the source of endogenous neural activity, the cells that initiate new programs of behavior and, if need be, abandon old ones.

Interneuron is a useful term in invertebrate neuroscience. But it loses meaning for the staggering numbers of such cells in vertebrates, especially in the human CNS, where as noted they represent 99.9997% of the total. It is now used for neurons confined to particular regions of vertebrate nervous systems. A newer scheme of classification offers an alternative. It recognizes two major classes of neurons: projection neurons and local-circuit neurons (LCNs). Projection neurons are interneurons in the original sense, with axons running between regions of the CNS but also including sensory and motor neurons, with axons extending from the CNS to receptors and effector organs, respectively. LCNs are also interneurons, but they have axons restricted in their sphere of influence to other neurons nearby. Projection and local-circuit neurons are not classified by cell size, length of axon, or type of contact. It is only their service in long-distance or local communication that separates them.

d. Projection Neurons Most large neurons are of this class. They fit the classic mold of a neuron: multipolar, with an ample cell body, many dendrites emanating from it, and a long axon over which impulses pass to one or more distant regions, with the amino acid glutamate as the primary transmitter (usually eliciting excitation of the target cells). Pyramidal cells in the cerebral cortex and Purkinje cells in the cerebellar cortex (Fig. 27) are familiar examples. Motor neurons in the brain stem and spinal cord are others. They perform long-range signaling in the CNS (the corticospinal tract) or from CNS to muscles (the sciatic nerve). Central projection fibers (region to region), association fibers (area to area in a region), and commissural fibers (association fibers that cross the midline) arise from such cells. During neural development, projection neurons originate earlier than LCNs, as if sketching in the basic circuit plan.

e. Local-Circuit Neurons As noted, LCNs usually have a short axon or sometimes none (as in the amacrine cells of the olfactory bulb and retina). They are involved in local activity within a group of cells, not transactions between distant groups. They are far more numerous than projection neurons in the human CNS and arise later and longer in neural development. The dentate gyrus of the hippocampal formation and the cerebellar cortex have huge numbers of them. In the cerebral cortex, they are also numerous but comprise only 20-25% of the grand total; this region is noted for its long-distance, direct output (Table III). Some of these large subpopulations take a long time to produce and put in place. Indeed, millions of LCNs arise after birth and even in adulthood in some mammals, including humans.

LCNs are of great interest. With the growing appreciation that regions of the CNS are specialized for different functions, their roles in circuits peculiar to these regions are now high-priority issues. Their general structure and connections have been known for years, but until relatively recently their functions were elusive due to the difficulty of studying them. Some LCNs elicit inhibitory effects by the transmitter g-aminobutyric acid (GABA), whereas others elicit excitatory effects by glutamate. Many LCNs are recognized in the cerebral cortex, differing in location, size and branching of processes, patterns of connectivity, places of termination on target neurons, and other features, especially the presence and corelease of other transmitters. A few LCNs, however, have been understood for years: the GABAergic basket cell in the cerebellar cortex exerts surround inhibition on rows of Purkinje cells, thus providing enhanced contrast between them and the excited Purkinje cells in nearby rows.

Peripheral Neuropathy Natural Treatment Options

Peripheral Neuropathy Natural Treatment Options

This guide will help millions of people understand this condition so that they can take control of their lives and make informed decisions. The ebook covers information on a vast number of different types of neuropathy. In addition, it will be a useful resource for their families, caregivers, and health care providers.

Get My Free Ebook


Responses

  • fulgenzia
    Is nephron and neurons can be seen through naked eyes?
    1 year ago

Post a comment