Receptor
/rɪˈsɛptər/
An organ or cell able to respond to light, heat, or other external stimulus and transmit a signal to a sensory nerve.
Recruitment
/rɪˈkruːtmənt/
The process of increasing the number of active motor units to increase the force of muscle contraction.
Reflex Arc
/ˈriːflɛks ɑːrk/
The nerve pathway involved in a reflex action, including at its simplest a sensory nerve and a motor nerve with a synapse between.
Refractory Period
/rɪˈfræktəri ˈpɪəriəd/
A period immediately following stimulation during which a nerve or muscle is unresponsive to further stimulation.
Renin
/ˈriːnɪn/
An enzyme secreted by and stored in the kidneys which promotes the production of the protein angiotensin.
Repolarization
/riːˌpoʊlərəˈzeɪʃən/
The change in membrane potential that returns it to a negative value just after the depolarization phase of an action potential.
Residual Volume
/rɪˈzɪdjuəl ˈvɒljuːm/
The volume of air still remaining in the lungs after the most forcible expiration possible.
Respiration
/ˌrɛspɪˈreɪʃən/
The action of breathing; the biochemical process in cells where nutrients are converted into energy.
Reticulocyte
/rɪˈtɪkjʊloʊˌsaɪt/
An immature red blood cell without a nucleus, having a granular or reticulated appearance.
Retina
/ˈrɛtɪnə/
A layer at the back of the eyeball containing cells that are sensitive to light and that trigger nerve impulses.
Ribosome
/ˈraɪbəˌsoʊm/
A minute particle consisting of RNA and associated proteins found in large numbers in the cytoplasm of living cells; the site of protein synthesis.
Rigor Mortis
/ˌrɪɡər ˈmɔːrtɪs/
Stiffening of the joints and muscles of a body a few hours after death, lasting from one to four days.
Rod Cell
/rɒd sɛl/
A type of photoreceptor cell in the retina that can function in lower light better than cone cells.