Reflexes are automatic responses to stimuli that involve a reflex arc consisting of sensory receptors, sensory neurons, processing in the central nervous system, motor neurons, and effectors. Reflexes maintain homeostasis, carry out automatic actions like swallowing and sneezing, and maintain balance and posture through spinal reflexes controlling trunk and limb muscles. There are different types of reflexes including flexor reflexes which cause limb flexion in response to stimuli, and crossed extensor reflexes which cause the opposite limb to extend about 0.2 to 0.5 seconds after a flexor reflex is elicited in one limb. Reciprocal inhibition occurs when a stretch reflex excites one muscle while simultaneously inhibiting the antagonist muscle through reciprocal innervation.

1. Reflex action and reflexes

2. • Reflexes are rapid automatic responses to stimuli
• Neural reflex involves sensory fibers to CNS and
motor fibers to effectors
An introduction to reflexes

3. Reflex arc
• Wiring of a neural reflex
• Five steps
– Arrival of stimulus and activation of receptor
– Activation of sensory neuron
– Information processing
– Activation of motor neuron
– Response by effector

4. Components of a Reflex Arc

5. Properties of reflexes
• Reflexes are automatic, unconscious to changes,
either inside or outside the body.
a. Reflexes maintain homeostasis (autonomic
reflexes) – heart rate, breathing rate, BP,
digestion.
b. Reflexes also carry out the automatic actions of
swallowing, sneezing, coughing, vomiting.
c. Reflexes maintain balance and posture; e.g.,
spinal reflexes control trunk and limb muscles.
d. Brain reflexes involve reflex center in brainstem;
e.g., reflexes for eye movement.

6. Methods of Classifying Reflexes

7. Flexor reflex and withdrawal
• Any type of cutaneous sensory stimulus from
a limb is likely to cause the flexor muscles of
the limb to contract
• Withdrawing the limb from the stimulating
object called flexor reflex
• Also called nociceptive or pain reflex

8. Neuronal mechanism of flexor reflex

9. • The pathway for eliciting the flexor reflex do
not pass directly to the anterior motor
neurons
• 1st interneuron pool of neurons
• Then to the motor neurons
Basic steps in the circuits:
a. diverging circuit to spread the reflex to the
necessary muscles for withdrawal
b. inhibit antagonist muscles
c. cause afterdischarge
 Pattern of withdrawal

12. Crossed extensor reflex
• About 0.2 to 0.5 sec after a stimulus elicits a
flexor reflex in one limb, the opposite limb
begins to extend
• Push the entire body away

14. The Flexor and Crossed Extensor
Reflexes

15. Reciprocal inhibition and innervations
• When a stretch reflex excites one muscle, it
often simultaneously inhibits the antagonist
muscles called reciprocal inhibition
• The neuronal circuit that causes this relation is
called reciprocal innervation