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12.10A: Components of a Reflex Arc

A reflex arc defines the pathway by which a reflex travels—from the stimulus to sensory neuron to motor neuron to reflex muscle movement.

 

LEARNING OBJECTIVES

 

Describe the components of a reflex arc

 

KEY TAKEAWAYS

Key Points

 

  • Reflexes, or reflex actions, are involuntary, almost instantaneous movements in response to a specific stimulus.
  • Reflex arcs that 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 neurons in the reflex pathway.

 

Key Terms

 

  • motor neuron: A neuron located in the central nervous system that projects its axon outside the CNS and directly or indirectly control muscles.
  • sensory neuron: These are typically classified as the neurons responsible for converting various external stimuli that come from the environment into corresponding internal stimuli.
  • reflex arc: 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).

Description

A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. When a person accidentally touches a hot object, they automatically jerk their hand away without thinking. A reflex does not require any thought input.

The path taken by the nerve impulses in a reflex is called a reflex arc. 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.

Most reflex arcs involve only three neurons. The stimulus, such as a needle stick, stimulates the pain receptors of the skin, which initiate an impulse in a sensory neuron. This travels to the spinal cord where it passes, by means of a synapse, to a connecting neuron called the relay neuron situated in the spinal cord.

The relay neuron in turn makes a synapse with one or more motor neurons that transmit the impulse to the muscles of the limb causing them to contract and pull away from the sharp object. Reflexes do not require involvement of the brain, although in some cases the brain can prevent reflex action.

This is a drawing that diagrams a reflex arc—the path taken by the nerve impulses. This picture shows a pain in the paw of an animal, but it is equally adaptable to any situation and animal (including humans). The picture shows how the nerve impulse travels from the pin prick to a sensory neuron, to a synapse, to a relay neuron, then to a motor neuron that activates a muscle movement.

 

Reflex arc: The path taken by the nerve impulses in a reflex is called a reflex arc. This is shown here in response to a pin in the paw of an animal, but it is equally adaptable to any situation and animal (including humans).

Types of Reflex Arcs

There are two types of reflex arcs:the   autonomic reflex arc, affecting inner organs, and the somatic reflex arc, affecting muscles. When a reflex arc consists of only two neurons, 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 the contraction of the agonist or effector muscle.

By contrast, in polysynaptic reflex arcs, one or more interneurons connect afferent (sensory) and efferent (motor) signals.
For example, the withdrawal reflex (nociceptive or flexor withdrawal reflex) is a spinal reflex intended to protect the body from damaging stimuli. It causes the stimulation of sensory, association, and motor neurons.