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The Peripheral Nervous System (PNS) > Reflexes
Reflexes
• Components of a Reflex Arc
• Spinal Reflexes
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The Peripheral Nervous System (PNS) > Reflexes
Components of a Reflex Arc
• Reflexes, or reflex actions, are involuntary, almost instantaneous movements in
response to a specific stimulus.
• Reflex arcs which contain only two neurons, a sensory and a motor neuron, are
considered monosynaptic. Examples of monosynaptic reflex arcs in humans
include the patellar reflex and the Achilles reflex.
• Most reflex arcs are polysynaptic, meaning multiple interneurons (also called relay
neurons) interface between the sensory and motor neuron in the reflex pathway.
Reflex arc
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The Peripheral Nervous System (PNS) > Reflexes
Spinal Reflexes
• The stretch reflex is a monosynaptic reflex that regulates muscle length through
neuronal stimulation at the muscle spindle; alpha motor neurons cause
contraction to resist stretching, and gamma motor neurons control the sensitivity
of the reflex.
• The stretch and Golgi tendon reflexes work in tandem to control muscle length
and tension. Both are examples of ipsilateral reflexes, meaning the reflex occurs
on the same side of the body as the stimulus.
• The crossed extensor reflex is a contralateral reflex which allows the body to
compensate on one side for a stimulus on the other. For example, when one foot
steps on a nail, the crossed extensor reflex shifts the body's weight onto the other
Jendrassik maneuver
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foot, protecting and withdrawing the foot on the nail.
• The withdrawal reflex and the more specific pain withdrawal reflex involve
withdrawal in response to a stimulus (or pain). When pain receptors, called
nociceptors, are stimulated, reciprocal innervations stimulate the flexors to
withdraw and inhibit the extensors to ensure they are unable to prevent flexion
and withdrawal.
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Appendix
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The Peripheral Nervous System (PNS)
Key terms
• alpha motor neuron Alpha motor neurons (α-MNs) are large lower motor neurons of the brainstem and spinal cord. They
innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor
neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.
• golgi tendon reflex The Golgi tendon reflex is a normal component of the reflex arc of the peripheral nervous system. In a Golgi
tendon reflex, skeletal muscle contraction causes the agonist muscle to simultaneously lengthen and relax. This reflex is also
called the inverse myotatic reflex because it is the inverse of the stretch reflex. Alhough muscle tension is increasing during the
contraction, alpha motor neurons in the spinal cord supplying the muscle are inhibited. However, antagonistic muscles are
activated.
• motor neuron A neuron located in the central nervous system that projects its axon outside the CNS and directly or indirectly
control muscles.
• reflex arc A reflex arc is a neural pathway that controls an action reflex. 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. There are two types of reflex arcs: autonomic reflex arc (affecting inner organs) and
somatic reflex arc (affecting muscles)
• sensory neuron Sensory neurons are typically classified as the neurons responsible for converting various external stimuli that
comes from the environment into corresponding internal stimuli.
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The Peripheral Nervous System (PNS)
Reflex arc
The path taken by the nerve impulses in a reflex is called a reflex arc, shown here in response to a pin in the paw of an animal, but equally adaptable to
any situation and animal (including humans).
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Wikimedia. "Anatomy and physiology of animals A reflex arc." CC BY 3.0
http://commons.wikimedia.org/wiki/File:Anatomy_and_physiology_of_animals_A_reflex_arc.jpg View on Boundless.com
The Peripheral Nervous System (PNS)
Jendrassik maneuver
The Jendrassik maneuver is a medical maneuver wherein the patient flexes both sets of fingers into a hook-like form and interlocks those sets of fingers
together (note the hands of the patient in the chair). This maneuver is used often when testing the patellar reflex, as it forces the patient to concentrate
on the interlocking of the fingers and prevents conscious inhibition or influence of the reflex.
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Wikipedia. "Gehuchten Jendrassik maneuver." Public domain http://en.wikipedia.org/wiki/File:Gehuchten_Jendrassik_maneuver.JPG View on Boundless.com
The Peripheral Nervous System (PNS)
Golgi tendon organ
The Golgi tendon organ, responsible for the Golgi tendon reflex, is diagrammed with its typical position in a muscle (left), neuronal connections in spinal
cord (middle), and expanded schematic (right). The tendon organ is a stretch receptor that signals the amount of force on the muscle and protects the
muscle from excessively heavy loads by causing the muscle to relax and drop the load.
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Wikipedia. "Tendon organ model." Public domain http://en.wikipedia.org/wiki/File:Tendon_organ_model.jpg View on Boundless.com
The Peripheral Nervous System (PNS)
Which of the following statements about reflex arcs is true?
A) Most reflex arcs involve more than 5 neurons
B) Reflex arcs are usually slow and inefficient
C) The brain never receives reflex sensory input
D) Reflex arcs do not require the brain to function
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The Peripheral Nervous System (PNS)
Which of the following statements about reflex arcs is true?
A) Most reflex arcs involve more than 5 neurons
B) Reflex arcs are usually slow and inefficient
C) The brain never receives reflex sensory input
D) Reflex arcs do not require the brain to function
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The Peripheral Nervous System (PNS)
Which of the following spinal reflexes functions to maintain a
muscle at a constant length?
A) Golgi tendon reflex
B) Stretch reflex
C) Crossed extensor reflex
D) Withdrawal reflex
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The Peripheral Nervous System (PNS)
Which of the following spinal reflexes functions to maintain a
muscle at a constant length?
A) Golgi tendon reflex
B) Stretch reflex
C) Crossed extensor reflex
D) Withdrawal reflex
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The Peripheral Nervous System (PNS)
Attribution
• Wikipedia. "Reflex arc." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Reflex_arc
• Wikipedia. "Reflex." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Reflex
• Wikibooks. "Anatomy and Physiology of Animals/Nervous System." CC BY-SA 3.0
http://en.wikibooks.org/wiki/Anatomy_and_Physiology_of_Animals/Nervous_System#Reflexes
• Wiktionary. "motor neuron." CC BY-SA 3.0 http://en.wiktionary.org/wiki/motor+neuron
• Wikipedia. "reflex arc." CC BY-SA 3.0 http://en.wikipedia.org/wiki/reflex%20arc
• Wikipedia. "sensory neuron." CC BY-SA 3.0 http://en.wikipedia.org/wiki/sensory%20neuron
• Wikipedia. "Stretch reflex." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Stretch_reflex
• Wikipedia. "Pain withdrawal reflex." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Pain_withdrawal_reflex
• Wikipedia. "Withdrawal reflex." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Withdrawal_reflex
• Wikipedia. "Golgi tendon reflex." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Golgi_tendon_reflex
• Wikipedia. "Crossed extensor reflex." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Crossed_extensor_reflex
• Wikipedia. "alpha motor neuron." CC BY-SA 3.0 http://en.wikipedia.org/wiki/alpha%20motor%20neuron
• Wikipedia. "golgi tendon reflex." CC BY-SA 3.0 http://en.wikipedia.org/wiki/golgi%20tendon%20reflex
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