November 2015
 Objectives:
 To describe the pathway nervous impulses travel
through a reflex
 To observe reflexes and reaction times
 Journal: List the layers of protection around the central
nervous system from the vertebrae to the spinal cord.
Reflexes
• Involuntary reactions that protect you
• Response is proportionate to the
stimulus
• Only the spinal cord is necessary
Reflex Arc
 Nerve impulses, or action potential, travel routes made up of
neurons
 Paths are called neuron pathways
 Reflex arc is specialized type of neuron pathway
 Allow impulse conduction in only one direction
 Receptors - beginning of dendrites of sensory neurons, where
impulse conduction starts
 Located far from spinal cord (tendons, skin, etc)
 Causes muscle contraction or gland secretion
Two-Neuron Arc
 Simplest type of reflex arc – only involves sensory and
motor neuron with one synapse
 Most common is the “knee jerk”
 Sensory neuron sends message (chemicals) across
synapse to motor neuron
 Motor neuron forms synapse with an Effector – organ that
puts nerve signals “into effect”


Usually muscles or glands
Response is called a Reflex
Three-Neuron Arc
 Involves a sensory, motor, AND interneuron
 More complex response
 Two synapses – between sensory and interneuron
and between interneuron and motor
 All interneurons are in the brain or spinal cord
 Example: withdrawal reflex
 Since in contact with CNS, may feel pain