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Information about Reflex Arc

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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.

Monosynaptic vs. polysynaptic

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.

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.

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, blocking the innervation of the antagonistic (hamstring) muscle. The relaxation of the opposing muscle facilitates extension of the lower leg.

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.

In lower animals reflex interneurons do not necessarily reside in the spinal cord, for example as in the lateral giant neuron of crayfish.

External links

Reference

  • Ganong, W.F. 2001. Review of Medical Physiology. McGraw-Hill Publishing, New York, p. 123.
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.
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ReFLEX is a wireless protocol developed by Motorola which is used for two-way paging. It is based on the one-way FLEX protocol and comes in two variants, ReFLEX25 and ReFLEX50. Later version 2.7 of the ReFLEX protocol was released. Devices compliant with ReFLEX 2.7.
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In animals, the brain or encephalon (Greek for "in the skull"), is the control center of the central nervous system, responsible for behavior. The brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing,
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spinal chord is a thin, tubular bundle of nerves that is an extension of the central nervous system from the brain and is enclosed in and protected by the bony vertebral column.
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Sensory neurons (neurones) are nerve cells within the nervous system responsible for converting external stimuli from the organism's environment into internal electrical motor reflex loops and several forms of involuntary behavior, including pain avoidance.
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Location Ventral horn of the spinal cord
Function Excitatory projection (to NMJ)
Neurotransmitter ACh
Morphology Projection neuron
Presynaptic connections M1 via the Corticospinal tract
Postsynaptic connections
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patellar reflex or knee-jerk is a stretch reflex.

Mechanism

Striking the patellar tendon with a tendon hammer just below the patella stretches the quadriceps tendon.
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The ankle jerk reflex, also known as the Achilles reflex, occurs when the Achilles tendon is tapped while the foot is dorsi-flexed. A positive result would be the jerking of the foot towards its plantar surface.
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Muscle structure is innervated by both sensory and motor neuron axons. The Muscle Spindle's functions are to send proprioceptive information about the muscle to the central nervous system, and to respond to muscle stretching.
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afferent neurons--otherwise known as sensory or receptor neurons--carry nerve impulses from receptors or sense organs toward the central nervous system. This is the case vice versa as well. This term can also be used to describe relative connections between structures.
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sensory system: in this case, vision, for the visual system. ]]

A sensory system is a part of the nervous system responsible for processing sensory information.
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efferent nerves – otherwise known as motor or effector neurons – carry nerve impulses away from the central nervous system to effectors such as muscles or glands (and also the ciliated cells of the inner ear).
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MUSCLE (multiple sequence comparison by log-expectation) is public domain, multiple sequence alignment software for protein and nucleotide sequences.
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patellar reflex or knee-jerk is a stretch reflex.

Mechanism

Striking the patellar tendon with a tendon hammer just below the patella stretches the quadriceps tendon.
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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.

It is a strong, flat, ligamentous band, about 8 cm.
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patellar reflex or knee-jerk is a stretch reflex.

Mechanism

Striking the patellar tendon with a tendon hammer just below the patella stretches the quadriceps tendon.
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An action potential is a "spike" of electrical discharge that travels along the membrane of a cell. Action potentials are an essential feature of animal life, rapidly carrying information within and between tissues. They also occur in some plants.
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Muscle structure is innervated by both sensory and motor neuron axons. The Muscle Spindle's functions are to send proprioceptive information about the muscle to the central nervous system, and to respond to muscle stretching.
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synapse. Synapses allow nerve cells to communicate with one another through axons and dendrites, converting electrical impulses into chemical signals.]]

Chemical synapses
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Motor nerves allow the brain to stimulate muscle contraction. A motor nerve is an efferent nerve that exclusively contains the axons of somatic and branchial motoneurons, which innervate skeletal muscles (that ensure locomotion) and branchial muscles (that motorize the face and
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"Quads" redirects here. For other uses see Quad
The quadriceps femoris (quadriceps, quadriceps extensor, guads or quads) includes the four prevailing muscles on the front of the thigh.
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For the plant, see .


Glycine (abbreviated as Gly or G)[1] is the organic compound with the formula HO2CCH2NH2.
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The lateral giant neuron (LG) is an interneuron in the abdominal nerve cord of crayfish. It is part of the system that controls a special kind of escape reflex of crayfish.
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Astacoidea
Latreille, 1802
Parastacoidea
Huxley, 1879

Families
Astacoidea
  Astacidae
  Cambaridae
Parastacoidea
  Parastacidae

Crayfish, often referred to as crawfish or crawdad
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eMedicine is an online clinical medical knowledge base that was founded in 1996 by Scott Plantz and Richard Lavely, two medical doctors. It was sold to WebMD in January 2006.
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Dorland's is the brand name of several different medical dictionaries and ancillary products, chiefly Dorland's Illustrated Medical Dictionary (currently in its 31st edition) and Dorland's Pocket Medical Dictionary (currently in its 27th edition).
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'The Merck Manual of Diagnosis and Therapy' (The Merck Manual or Merck's) is the world's best-selling medical textbook, now in its 18th edition.

It is recommended by librarians in standard references such as the "Brandon/Hill selected list of books and
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