The cell that is the basic unit of function of the nervous system. The nervous system contains billions of neurons. There are many kinds of neurons including sensory neurons, interneu-rons, and motor neurons. Interneurons are the most common. Some neurons within these basic types have specialized structures and functions. In general, neurons share the following common features:
• Cell body, which contains the nucleus bearing the cell’s genetic instructions (deoxyribonucleic acid [DNA]), cytoplasm, neurofibrils, and mitochondria
• Dendrites, branchlike projections that receive incoming electrical signals
• Axon, a taillike structure that sends outgoing electrical signals; most axons are covered with a protective myelin sheath that insulates them
Neurons conduct electrical impulses that travel at speeds equivalent to several hundred miles an hour. Separating neurons from one another are corridors called synapses; neurotransmITters carry signals across the synapses from one neuron to another in an intricately timed sequence of events. A change in the neuron’s electrical properties, called an action potential, heralds that start of an electrical transmission and causes the sending neuron, called the presynaptic neuron, to release its neurotransmitter. The neurotransmitters cross the synaptic gap and binds with receptors in the receiving, or postsynaptic, neuron’s dendrites. This action alters the postsynaptic neuron’s electrical properties, causing a signal to which the postsynaptic neuron responds. As soon as the transmission is complete the axon and other cells, including the non-neural glia, resorb the remaining neurotransmitters and their chemical remnants, and the neuron is ready to repeat the process. As complex as the sequence is, it takes place in less than a millisecond. A single neuron can “fire” many times a second and convey messages to multiple other neurons.
The sequence requires that the correct amount of neurotransmitter be present at the correct time and for the correct duration. In Parkinson’s disease, there is a decreased number of dopaminergic neurons. This decrease causes dopamine, the neurotransmitter essential for neuron communication in parts of the brain that control movement, to be in short supply. As a result, neurons cannot properly transmit signals, and neuron communication becomes disrupted and incomplete. Messages directing movement leave the brain jumbled and confused, resulting in dysfunctional muscle response in the body such as tremors, bradykine-sia, and other dyskinesias that characterize Parkinson’s disease.
A cluster or bundle of like neurons (neurons with similar structure and function) and their supporting structures is called a ganglion (plural, ganglia) if located outside the cortex where most neurons in the brain are found. Collections of neuron bodies form gray matter, and collections of axons (fibers) form white matter. The body does not generate new neurons or replace damaged or dead neurons. The neurons present at birth are the ones that remain throughout life; neurons gradually die as a result of damage or apoptosis.
Sensory neurons are afferent (from the Latin meaning “carry toward”); they respond to external sensory stimuli such as touch and carry signals to the brain. A sensory neuron typically has a tightly focused, somewhat bulbous dendrite structure and a long (up to several feet), myelinated axon with multiple axon terminals (branches). Sensory neurons are concentrated in structures that receive sensory messages, such as the dorsal root ganglion, which conveys signals from touch receptor organs in the skin to the spinal cord and then to thalamic neurons, which complete the transmission of the sensory signals to the sensory cortex, and the retina, which conveys visual signals to neurons in the lateral geniculate nucleus of the thalamus, which in turn sends axons to the primary visual cortex.
Motor neurons are efferent (from the Latin meaning “carry from”); they convey signals from the
brain to the muscles. A motor neuron typically has a widely branched dendrite structure and a long (up to several feet), myelinated axon ending in a small cluster of axon terminals.
• Upper motor neurons originate in the brain; their axons extend from the brain and into the spinal cord. They send signals to lower motor neurons.
• Lower motor neurons originate in the anterior horn of the spinal cord; their axons extend from the spinal cord to the skeletal (striated) muscles. They send signals to muscle cells.
Parkinson’s disease disrupts upper motor neuron function. Lower motor neurons function normally but convey jumbled messages because the signals sent from the brain via the upper motor neurons are confused and incomplete.
Interneurons are the most abundant neurons in the nervous system. They are associative: They convey messages among neurons, rather than from neurons to the cells of body structures, to expedite the communication process. An interneuron typically has a short axon (no longer than a few centimeters), often unmyelinated, and sparsely placed dendrites. Interneurons are found throughout the nervous system.
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