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A motor neuron (or motoneuron) is a nerve cell (neuron) whose cell body is located in the spinal cord and whose fiber (axon) projects outside the spinal cord to directly or indirectly control effector organs, mainly muscles and glands.〔(How Stuff Works )〕 Motor neurons' axons are efferent nerve fibers that carry signals from the spinal cord to the effectors to produce effects.〔Schacter D.L., Gilbert D.T., and Wegner D.M. (2011) Psychology second edition. New York, NY: Worth〕 Types of motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons. There are upper motor neurons and lower motor neurons, with the cell type described above being a lower motor neuron. Upper motor neurons are cortico-spinal interneurons that arise from the motor cortex and descend to the spinal cord where they activate the lower motor neurons through synapses. The term 'motor neuron' is usually restricted to the efferent neurons that actually innervate muscles (the lower motor neurons). A single motor neuron may innervate many muscle fibres and a muscle fibre can undergo many action potentials in the time taken for a single muscle twitch. As a result, if an action potential arrives before a twitch has completed, the twitches can superimpose on one another, either through summation or a tetanic contraction. In summation, the muscle is stimulated repetitively such that additional action potentials coming from the somatic nervous system arrive before the end of the twitch. The twitches thus superimpose on one another, leading to a force greater than that of a single twitch. A tetanic contraction is caused by constant, very high frequency stimulation - the action potentials come at such a rapid rate that individual twitches are indistinguishable, and tension rises smoothly eventually reaching a plateau. == Anatomy and physiology == According to their targets, motor neurons are classified into three broad categories: Somatic motor neurons, which originate in the central nervous system, project their axons to skeletal muscles (such as the muscles of the limbs, abdominal, and intercostal muscles), which are involved in locomotion . Special visceral motor neurons, also called ''branchial motor neurons'', which directly innervate branchial muscles (that motorize the gills in fish and the face and neck in land vertebrates). General visceral motor neurons (''visceral motor neurons'' for short) which indirectly innervate cardiac muscle and smooth muscles of the viscera ( the muscles of the arteries): they synapse onto neurons located in ganglia of the autonomic nervous system (sympathetic and parasympathetic), located in the peripheral nervous system (PNS), which themselves directly innervate visceral muscles (and also some gland cells). In consequence: * the motor command of skeletal and branchial muscles is ''monosynaptic'' (involving only one motor neuron, respectively, ''somatic'' and ''branchial'', which synapses onto the muscle). * the command of visceral muscles is ''disynaptic'' (involving two neurons: the ''general visceral motor neuron'' located in the CNS, which synapses onto a ganglionic neuron, located in the PNS, which synapses onto the muscle). It could be argued that, in the command of visceral muscles, the ganglionic neuron, parasympathetic or sympathetic, is the real ''motor neuron'', being the one that directly innervates the muscle (whereas the ''general visceral motor neuron'' is, strictly speaking, a ''preganglionic'' neuron). But, for historical reasons, the term motor neuron is reserved for the CNS neuron. All vertebrate motor neurons are cholinergic, that is, they release the neurotransmitter acetylcholine. Parasympathetic ganglionic neurons are also cholinergic, whereas most sympathetic ganglionic neurons are noradrenergic, that is, they release the neurotransmitter noradrenaline. (see Table) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「motor neuron」の詳細全文を読む スポンサード リンク
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