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Reflex arc

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	Reflex arc
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	Topics Reflex arc Topic Home A reflex arc is the neural pathwayNeural pathway Overview A neural pathway is a neural tract connecting one part of the nervous system with another, usually consisting of bundles of ... that mediates a reflex actionReflex action Summary A reflex action is an automatic neuromuscular action elicited by a defined stimulus.... . In higher animals, most sensory neurons do not pass directly into the brainBrain Summary In animals, the brain, or encephalon , is the control center of the central nervous system.... , but synapse in the spinal cordSpinal cord In vertebrates, the spinal cord is the part of the central nervous system that is enclosed in and protected by the vertebral... . This characteristic allows reflex actions to occur relatively quickly by activating spinal motor neurons without the delay of routing signals through the brain, although the brain will receive sensory input while the reflex action occurs. Monosynaptic vs. polysynaptic When a reflex arc consists of only two neurons in an animal (one sensory neuronSensory neuron Sensory neurons are nerve cells within the nervous system responsible for converting external stimuli from the organism's en... and one motor neuronMotor neuron In vertebrates, motor neurons are efferent neurons that originate in the spinal cord or brain stem and synapse with muscle f... ), it is defined as monosynaptic. Monosynaptic refers to the presence of a single chemical synapse. In the case of peripheral muscle reflexes, brief stimulation to the muscle spindleMuscle spindle A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons.... results in contraction of the agonist or effector muscle. By contrast, in polysynaptic reflex pathways, one or more interneurons connect afferentAfferent nerve In the nervous system, afferent neurons--otherwise known as sensory or receptor neurons--carry nerve impulses from receptors... and efferentEfferent nerve In the nervous system, efferent nerves--otherwise known as motor or effector neuron--carry nerve impulses away from the ... signals. All but the most simple reflexes are polysynaptic, allowing processing or inhibition of polysynaptic reflexes within the spinal cord. The Patellar Reflex Patellar reflexPatellar reflex The patellar reflex or knee jerk is a reflex employing only three neurons.... : when the patellar tendon is tapped just below the knee, the patellar reflexPatellar reflex The patellar reflex or knee jerk is a reflex employing only three neurons.... is initiated and the lower leg kicks forward (via contraction of the quadriceps). The tap initiates an action potentialAction potential Overview An action potential is a wave of electrical discharge that travels along the membrane of a cell.... in a specialised structure known as a muscle spindleFacts About Muscle spindle A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons.... located within the quadriceps. This action potential travels to the spinal cord, via a sensory axon which chemically communicates by releasing glutamate (see synapse) onto a motor nerveMotor nerve Motor nerves enable the brain to stimulate muscle contraction.... . The result of this motor nerve activity is contraction of the quadriceps muscleQuadriceps muscle Summary The quadriceps femoris includes the four remaining muscles on the front of the thigh.... , leading to extension of the lower leg at the knee. The sensory input from the quadriceps also activates local interneurons that release the inhibitory neurotransmitter glycine onto motor neurons, blocking the innervation of the antagonistic (hamstring) muscle. The relaxation of the opposing muscle facilitates extension of the lower leg. External links - "Physical Examination"