Patellar reflex. Note that this image includes an interneuron in the pathway of the patellar reflex for purposes of illustration. The inhibitory component of the reflex involving the hamstring muscle is not shown.

A reflex arc is the neural pathway that mediates a reflex action. In higher animals, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. 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. Image File history File links ReflexArc1. ... Image File history File links ReflexArc1. ... A neural pathway is a neural tract connecting one part of the nervous system with another, usually consisting of bundles of elongated, myelin insultated neurons, known collectively as white matter. ... The mechanism of the reflex arc. ... In animals the brain, or encephalon (Greek for in the head), is the control center of the central nervous system, responsible for thought. ... The Spinal cord nested in the vertebral column. ...

When a reflex arc consists of only two neurons in an animal(one sensory neuron and one motor neuron), it is defined as monosynaptic. Monosynaptic refers to the presence of a single chemical synapse. In the case of peripheral muscle reflexes (patellar reflex, achilles reflex), brief stimulation to the muscle spindle results in contraction of the agonist or effector muscle. The mechanism of the reflex arc Sensory neurons (or neurones) are nerve cells within the nervous system responsible for converting external stimuli from the organisms environment into internal [[electrical poop is responsible for it aright underlie motor reflex loops and several forms of involuntary behavior, including pain avoidance. ... ⅔In vertebrates, the term motor neuron (or “motoneuron”) classically applies to neurons located in the central nervous system (CNS) which project their axons outside the CNS and directly or indirectly control muscles. ... The patellar reflex or knee jerk is a monosynaptic reflex. ... The ankle jerk reflex, also known as the Achilles reflex, occurs when the Achilles tendon is tapped while the foot is dorsi-flexed. ... A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons. ...

By contrast, in polysynaptic reflex pathways, one or more interneurons connect afferent (sensory) and efferent (motor) signals. All but the most simple reflexes are polysynaptic, allowing processing or inhibition of polysynaptic reflexes within the spinal cord. The mechanism of the reflex arc In the nervous system, afferent neurons--otherwise known as sensory or receptor neurons--carry nerve impulses from receptors or sense organs toward the central nervous system. ... This article or section may be confusing for some readers, and should be edited to be clearer or more simplified. ... In the nervous system, efferent nerves otherwise known as motor or effector neuron carry nerve impulses away from the central nervous systemto effectors - either muscles or glands. ... Structure of a skeletal muscle Muscle is one of the four tissue types. ...

Classic Example: The Patellar Reflex

Patellar reflex: when the patellar tendon is tapped just below the knee, the patellar reflex is initiated and the lower leg kicks forward (via contraction of the quadriceps). The tap initiates an action potential in a specialised structure known as a muscle spindle 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 nerve. The result of this motor nerve activity is contraction of the quadriceps muscle, 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 that innervate the antagonistic (hamstring) muscle. The relaxation of the opposing muscle facilitates extension of the lower leg. The patellar reflex or knee jerk is a monosynaptic reflex. ... The Patellar ligament (anterior ligament) is the central portion of the common tendon of the Quadriceps femoris, which is continued from the patella to the tuberosity of the tibia. ... The patellar reflex or knee jerk is a monosynaptic reflex. ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons. ... Illustration of the major elements in a prototypical synapse. ... Motor nerves enable the brain to stimulate muscle contraction. ... Quads redirects here. ...

Notes

It should be noted that 'simple' monosynaptic reflexes do also have additional components. As the sensory axon enters into the spinal cord, it sends out a collateral axon which synapses onto an inhibitory interneuron. When activated, this IA inhibitory interneuron releases glycine which inhibits the motor activity of the antagonist muscle. The result is reinforced activity of the agonist muscle by removing tonic activity. For the plant, see Glycine (plant). ...

In lower animals reflex interneurons do not necessarily reside in the spinal cord, for example as in the lateral giant neuron of crayfish. The lateral giant neuron (LG) is an interneuron in the abdominal nerve cord of crayfish. ... Families Astacoidea   Astacidae   Cambaridae Parastacoidea   Parastacidae Crayfish, often referred to as crawfish or crawdad, are freshwater crustaceans resembling small lobsters, to which they are closely related. ...

External links

• Ole K. Andersen, SMI, Aalborg University - Physiological and Pharmacological modulation of the human nociceptive withdrawal reflex at smi.auc.dk
• Somatic Reflex Arc at vetmed.vt.edu
• Dictionary at eMedicine Reflex+arc
• Dorlands/Elsevier a_56/12149511
• Overview at sfsu.edu
• Overview at rutgers.edu (with animation)
• Merck Home 06-077c - "Physical Examination"
• Tutorial at wisc-online.com

eMedicine is an online clinical medical knowledge base that was founded in 1996. ... Elseviers logo. ... The Merck Manual of Diagnosis and Therapy (often referred to simply as The Merck Manual) is one of the worlds most widely used medical textbooks. ...

Reference

• Ganong, W.F. 2001. Review of Medical Physiology. McGraw-Hill Publishing, New York, p. 123.