Neuron

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Neuron

Neurons are nerve cells (neurocytes), which, together with neuroglial cells, comprise the nervous tissue making up the nervous system . The neuron is the integral element of our five senses and of countless other physical, regulatory, and mental faculties, including memory and consciousness. A neuron consists of a nerve cell body (or soma), an elongated projection (axon), and short branching fibers (called dendrites). Neurons receive nerve signals (action potentials), integrate action potentials, and transmit these signals to other neurons or effector organs, such as muscles and glands . The structure and function of neurons is essentially the same in all animals, although the human nervous system is much more specialized and complicated than that of lower animals. Humans are born with a large, but finite, supply of neurons and those cells that are lost through aging, injury, or disease cannot be replaced.

The unique morphological and intercellular structure of the neuron is dedicated to the efficient and rapid transmission of neural signals. Within the neuron, the neural signal travels electrically. At the synapse , the gap between neurons, neural signals are conveyed chemically by a limited number of chemicals termed neurotransmitters. Specialized parts of the neuron facilitate the production, release, binding, and uptake of these neurotransmitters.

Structure and function

Although there are variations related to function, a typical neuron consists of dendrites (also termed dendritic processes), a cell body, an axon, and an axon terminus.

Dendrites are the (filamentous) terminal portions of neuron that bind neurotransmitter chemicals migrating across the synaptic gaps separating neurons. Depending on the type and function of a particular neuron, neurotransmitters may cause or inhibit the transmission of neural impulses. The cell body contains the cell nucleus and a concentration of cellular organelles. The cell body is the site of the normal metabolic reactions that allow the cell to remain viable. Neurotransmitters synthesized within the cell body are transported to the axon terminus by microfilaments and microtubules.

The nerve cell body contains a nucleus, a nucleolus, and cytoplasm containing the cell (such as mitochondria, endoplasmic reticulum, and so on). Unique to the nerve cell body are Nissl bodies, which are rough surfaced vesicles in the endoplasmic reticulum (cytoplasm located near the nucleus), and are involved with protein synthesis. Another characteristic structure of nerve cells are the neurofibrils, which are delicate threadlike structures that help to maintain the shape of the cell, and which transport substances between the cell body and the axon terminals. The plasma membrane around the cell separates the cytoplasm on the inside of the cell from the extracellular
fluid on the outside. Cell membranes of neurons contain electrically gated channels, which when properly stimulated allow electrically charged particles (such as sodium and potassium ions) to pass across the barrier. This ionic exchange is the basis for the flow, or action potential , of the nerve impulse.

The axon is a cytoplasmic continuation of the cell body specialized for the electrical conduction of neural signals. The axon may be long—up to a yard in length in humans—or short, depending upon the neuron's position and function. The cell membranes of the neural axon transmit neural signals via changes in action potentials that sweep down the membrane.

At the junction of the cell body and axon is a region termed the axon hillock. At the axon hillock, chemical signals received by the dendrites may reach a threshold level to cause a wave of electrical depolarization and hyperpolarization of the axon cell membrane. The net movements of ions across the cell membrane are responsible for these changes that move down the axon to the axon terminus as an action potential.

At the axon terminus, neurotransmitters are released into the synaptic gap. Through synaptic gaps, a typical neuron may interconnect with thousands and tens of thousands of other neurons. Axon terminals have knob-like swellings at the very end called synaptic knobs or end buttons. Each synaptic knob communicates with a dendrite or cell body of another neuron, the point of contact being a synapse. Under very high magnification, a very tiny space, the synaptic cleft or gap (about one millionth of an inch, or mm), can be detected between the synaptic knob and dendrite or cell body. Synaptic knobs contain hundreds of neurovesicles that contain a transmitter substance (or neurotransmitter). When a nerve impulse reaches the synaptic knob the neurotransmitter is ejected into the synaptic cleft and serves as a stimulus to the next adjacent neuron. The vast majority of all impulses transmitted occur at the synaptic gaps, although recent research indicates that chemical transmission can occur at other points along the axon. Many neurological diseases and psychiatric disorders result from a disturbance or alteration of synaptic activity. Drugs such as tranquilizers , anesthetics, nicotine , and caffeine target the synapse and can cause an alteration of impulse transmission.

Structural classification

Neurons exist in many shapes and sizes. Their structure, like that of other cells in the body or in nature, illustrates that structure often determines function. There are three basic structural and functional classifications of neurons.

The structural classification of a neurons depends upon the number of dendrites extending from the cell body. Multipolar neurons have several dendrites; the majority of neurons in the spinal chord and brain are multipolar. Bipolar neurons have only two processes: a single dendrite and an axon. Bipolar neurons are found in the sense organs-and in the retina of the eye and in olfactory cells. Unipolar neurons lack dendrites and have a single axon, and are also sensory neurons.

Multipolar neurons have many processes and serve principally as motor neurons. Motor neurons, efferent because they conduct impulses away from the central nervous system (the brain and spinal cord) regulate the function of muscles and glands. Afferent neural pathways that send signals to the (CNS) are generally composed of unipolar neurons. Unipolar neurons also serve as sensory neurons—their filamentous dendritic processes exposed and elaborated into or connected to sensory receptor cells. Interneurons are neurons that connect neurons along a neural pathway.

Glial cells

One cannot discuss the neuron without mentioning glial cells or neuroglia. It was once thought that these cells simply held everything together (gloios means glue, in Greek), but we now know that neuroglia are highly specialized cells. For example, neuroglia are responsible for physical support, protection against infection (through phagocytosis), and the connection of nerve cells to blood vessels. The Schwann cell (or neurolemmocyte) is a common type of glial cell found in peripheral nerve axons. Schwann cells wrap "jelly roll style" around the axon, forming a whitish phospholipid (fatty) protective and insulating cover known as the myelin sheath. The myelin sheath of the axon of peripheral nerves has interruptions or exposed gaps known as the Nodes of Ranvier. Schwann cells also make up the neurolemma, a continuous sheath that covers both the myelin sheath and the axon at the Nodes of Ranvier. Action potentials traveling down the axon occur only at the Nodes of Ranvier, jumping rapidly from gap to gap (saltatory conduction), which conducts impulses significantly faster than in nonmyelinated nerves. The neurolemma is found only in peripheral nerve fibers and plays a crucial part in nerve fiber regeneration. Damaged axons will regenerate; damaged cell bodies will not. The myelinated sheaths of the axons of neuron in the brain and spinal cord (the central nervous system) are made from different glial cells (oligodendrocytes), which lack a neurolemma, so making the regeneration of their axons impossible. Multiple sclerosis is a serious demyelinating disease of the central nervous system. Not all axons are myelinated; the presence of myelin is one difference between white matter (which has myelinated axons) and gray matter (which does not).

Functional classification

Sensory neurons transduce physical stimuli, such as smell , light , or sound, into action potentials, which are then transmitted to the spinal cord or brain. Sensory neurons, which bring information into the central nervous system, are also referred to as afferent neurons. Motor neurons transmit nerve impulses away from the brain and spinal cord to muscles or glands and are also called efferent neurons. Interneurons transmit nerve impulses between sensory neurons and the motor neurons. Interneurons are responsible for receiving, relaying, integrating, and sending nerve impulses. Interneurons are found exclusively in the central nervous system and account for almost 99% of all the nerve cells in the body.

Resources

books

Cooper, Geoffrey M. The Cell—A Molecular Approach. 2nd ed. Sunderland, MA: Sinauer Associates, Inc., 2000.

Guyton, Arthur C., and John E. Hall. Textbook of Medical Physiology. 10th ed. Philadelphia: W.B. Saunders Co., 2000.

Kandel, E.R., J.H. Schwartz, and T.M. Jessell, ed. Principles ofNeural Science. 4th ed. Boston: Elsevier, 2000.

Thibodeau, Gary A., and Kevin T. Patton. Anatomy & Physiology. 5th ed. St. Louis: Mosby, 2002.

K. Lee Lerner
Christine Miner Minderovic

KEY TERMS

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Action potential: —A transient change in the electrical potential across a membrane which results in the generation of a nerve impulse.
Afferent neuron: —A sensory neuron that carries an impulse toward the central nervous system.
Axon: —The threadlike projection of a neuron that carries an impulse away from the cell body of the neuron.
Consciousness: —A mental state involving awareness of the self and the environment.
Dendrites: —Branched structures of nerve cell bodies which receive impulses from axons and carry them to the nerve cell body.
Efferent neuron: —A motor neuron that carries an impulse from the central nervous system to muscles or glands.
Memory: —The ability to recall thoughts or events.
Myelin sheath: —A white phospholipid (fat) covering of peripheral nerve axons.
Neuroglia: —Specialized nerve cells also called glial cells. The Schwann cell is a glial cell.
Neurolemma: —Protective nerve covering made by Schwann cells.