Neuron | Research & Encyclopedia Articles

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.

Structure and function

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.

Structural classification

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.

Nerve cell body

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.

Axons

An axon is a single smooth projection arising out of the nerve cell body at a raised area called the axon hillock. Axons conduct nerve impulses away from the nerve cell body. Axons vary in length and diameter; some (such as those in the central nervous system) are very short, as small as 0.01 in (0.25 mm), while others (such as those in the peripheral nervous system) conduct impulses over long distances in the body, and can be 3 ft (1 m) long. The speed at which an impulse travels depends upon the diameter of the axon, with axons with large diameters (0.001 in/0.025 mm) conducting impulses more rapidly. Axons may have branches called axon collaterals. The main axon and its collaterals can split into smaller branches ending in small filaments called axon terminals. 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.

Dendrites

Dendrites are so named because they resemble tiny trees (the Greek word for tree is dendron). The main function of a dendrite is to receive and integrate signals from neighboring neurons and conduct these signals to the nerve cell body. Dendrites are branched extensions of the cell's cytoplasm and contain all the normal cytoplasmic structures. A nerve cell's dendrites can branch out quite extensively, thus increasing the total surface area of the neuron and making more room to receive incoming signals from other nerves. A single neuron can receive signals from hundreds of other nerves. The dendrites in sensory neurons have specialized cells called receptors, which convert stimuli into electrical signals. Under the microscope, dendrites appear hairy; the little hairs or projections are called spines, and each spine is the site of a synapse, which is the point of communication between neurons.

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.