The Reflex Arc

How a Stimulus Elicits a Response

A Knee-Jerk Response• What happened?

• Why?

• When the hammer hit the kneethe foot jerked up.

Reacting to Changes• You need to keep the conditions inside your body constant.

Doing this is called homeostasis. Small changes insideyour body can cause its cells to be damaged or destroyed.Yet, there are big changes going on outside your body.

• You need to detect a change in the environment (astimulus) and react to the change (a response) in a waythat maintains homeostasis. When you do this withoutthinking, it is called a reflex.

Reacting to Changes

• It can get very hot or very cold outside, but the temperatureinside your body stays the same. How?

• When it gets cold outside (stimulus) you shiver (response)and keep the temperature inside your body from dropping.

• When it gets hot outside (stimulus) you perspire (response)and keep the temperature inside your body from rising.

Posture

• In order to maintain your posture (even bad posture - stopslouching) your muscles are constantly monitoring theirshape. A change in shape of a muscle (the stimulus) causesthe muscle to readjust its shape (the response) andmaintain your posture.

• The knee-jerk reflex is base on the hammer changing theshape of a muscle.

Revisiting the Knee-Jerk Response

• What is the stimulus?The hammer hits the tendon.

The muscle contracts, causingthe foot to jerk upward.

• What is the response?

Other Reflexes

BlinkingAn insect flying towards youreye

Pupils get smallerA bright light shining in youreye

NauseaA nasty odor

SalivationThe aroma of your favoritefood

ResponseStimulus

How is a Stimulus Detected?• Some cells are specialized to react to a specific stimulus.

These are called receptors (they receive a stimulus). Thereceptor cells of your eyes are stimulated by light.

The Response

• When the receptor is stimulated, it sends a message to apart of your body that effects the correct response. This iscalled the effector.

How is the Hammer Tap Detected?

• The muscles in your leghave stretch receptors.They react to a change inlength of the muscle.When the hammer hits thetendon at the knee, itmakes a muscle in thefront of your thigh longer(stretches it). Thatstimulates the stretchreceptors in that muscle.

The Knee-Jerk Response• When the stretch receptors

are stimulated, they send amessage to the muscles ofyour thigh.

• The muscles in the front ofyour thigh contract.

• The muscles in theback of your thigh relax.

• Your foot jerks.

Change in Muscle Length

• Here is a similar reflex inthe arm, showing musclelength.

• The weight dropping intothe hand is the stimulus. Likethe hammer tapping the knee,it stretches a muscle.

• The response is the musclecontracting, jerking the arm up.

How the Message Travels From theReceptor to the Effector.

• Nerve cells (neurons)carry the message fromthe stimulated receptorsto the correct effectors.

• A sensory neuron carriesthe message from the receptorto the central nervous system(the spinal cord and brain).

• A motor neuron carries themessage from the centralnervous system to the effector.• This is a reflex arc.

Reflex Arcs• In a knee-jerk reflex arc the

sensory neuron directlyconnects to the motor neuronin the spinal cord. This iscalled a simple reflex arc.

• Follow the sensory neuronfrom the spindle (receptor) towhere it connects with the motorneuron in the spinal cord.

• Follow the motor neuron tothe muscle (effector).

Reflex Arcs• In most reflex arcs

the sensory neuronconnects to motorneurons throughassociation neurons(interneurons) inthe central nervoussystem.

• Note the interneuronin the spinal cord.

The Correct Pathway.• If you put your finger on a

hot stove, what is thestimulus?

• What is the correctresponse?

• Would it help your fingerif the response was your footmoving?

The Correct Pathway.

• The correct connection betweenthe sensory neuron carrying themessage from the receptor andthe motor neuron carrying themessage to the effector is thework of the interneurons of thecentral nervous system. Makingthe right connections is calledintegration.

A Conscious Stimulus-Response

• We react to all stimuliin basically the sameway as a reflex. Theintegration just getsmore complex.

• Complex behaviorinvolves complexintegration in thebrain.

Making the Right Connection

• Integration in the centralnervous system works likethe central switchingoffice (CSO) of atelephone system

• When you phone a friend,the call is not directlycarried by a wire goingfrom your phone to yourfriend’s.

Making the Right Connection

• The wire from your phonegoes to the CSO.

• The CSO connects yourwire to the wire goingbetween the CSO andyour friend’s phone(integration).

• Hello.

Review• When the receptor detects

the stimulus, it excites asensory neuron.

• The message travelsthrough the sensoryneuron to an interneuronin the central nervoussystem (labeled controlcenter).

Review

• The message travelsthrough the interneuron toa motor neuron.

• The message travelsthrough the motor neuronto the effector.

• The effector is stimulatedand its reaction is theresponse.

Name the Neurons

• Neuron 2

Sensory Neuron

Name the Neurons

• Neuron 3

Interneuron

Name the Neurons

• Neuron 4

Motor Neuron

What is “the message” we have beentalking about?

• Tune in next lesson tofind out.