Brent Cornell

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

• A 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 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

     
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