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| Sarcoma |
| | A malignant tumor arising in connective or muscle tissue. They are usually spread by the blood stream and frequently metastasize to the lung. |
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| Saxitoxin |
| | A potent neurotoxin present in some shellfish poisoning that produces its effect by blocking sodium channels. |
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| Schwann Cells |
| | A very important glial cell present in the peripheral nervous system. They wrap themselves around all axons outside the CNS and form myelin, which serves like insulation. |
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| Sensory Neurons |
| | Neurons that carry information from sensory receptors (usually processes of the neuron) to the CNS. They are also known as afferent neurons. |
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| Skeletal System |
| | The organ system that supports and moves the body, protects internal organs, provides for mineral storage, and provides for blood formation. The main organs are the bones, cartilage, ligaments, and bone marrow. |
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| Somatic Nervous System |
| | The portion of the nervous system that carries motor information to skeletal muscles. Also known as the Voluntary System. |
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| Somatic System |
| | The part of the nervous system under voluntary control. |
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| Stable Cells |
| | Body cells that have a long lifespan with normally a low rate of division but the ability to rapidly divide upon demand. Examples are liver cells, alveolar cells of the lung, and kidney tubule cells. |
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| Stimulus |
| | A change in the environment, such as an irritant, loss of blood, or presence of a foreign chemical. |
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| Strychnine |
| | An extremely poisonous natural substance that inhibits the neurotransmitter glycine at postsynaptic sites, resulting in an increased level of neuronal excitability in the CNS. |
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| Synapse |
| | The junction between the axon of one neuron and the dendrite of another neuron or an axon and a connection with a muscle cell (neuromuscular junction). |
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