Reflex Arcs

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•  Autonomic and involuntary responses are referred to as reflexes

•  Reflex arcs comprise the neurons that mediate reflexes

The basic pathway for a nerve impulse is described by the stimulus response model

  • stimulus is a change in the environment (either external or internal) that is detected by a receptor
  • Receptors transform the stimuli into nerve impulses that are transmitted to the brain where decision-making occurs
  • When a response is selected, the signal is transmitted via neurons to effectors, promoting a change in the organism

Some responses may be involuntary and occur without conscious thought – these actions are called reflexes

  • Reflex actions do not involve the brain – instead sensory information is directly relayed to motor neurons within the spine 
  • This results in a faster response, but one that does not involve conscious thought or deliberation

Stimulus–Response Pathway

reflex pathway

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•  Withdrawal reflex of the hand from a painful stimulus

Reflex actions are particularly beneficial in survival situations, when quick reactions are necessary to avoid permanent damage

  • Because reflex arcs don’t involve the brain (only the spine and possibly brainstem), reflex actions are more rapid
  • Reflex responses also include autonomic actions such as modifications to heart rate, breathing and pupil accommodation

A common example of a reflex action is the patellar reflex (‘knee jerk’ response) that occurs when the patellar tendon is tapped

  • The patellar reflex is a common test employed by doctors to determine the presence of spinal lesions

Example of a Reflex Arc

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•  Drawing and labelling a diagram of a reflex arc for a pain withdrawal reflex

In a pain withdrawal reflex arc:

  • A pain stimulus is detected by a receptor (nocireceptor) and a nerve impulse is initiated in a sensory neutron
  • The sensory neuron enters the spinal cord via the dorsal root and synapses with a relay neuron in the grey matter
  • The relay neuron synapses with a motor neuron, which leaves the spinal cord via the ventral root
  • The motor neuron synapses with a muscle (effector), causing it to contract and remove the limb from the pain stimulus

Pain Withdrawal Reflex

pain withdrawal arc