reflex arc question

[yahoogoogle]

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In a reflex arc

A the sensory neuron synapses directly with the motor neuron.
B a minimum of three neurons must participate

the asnwer is B. I checked campbell and wiki and both say that reflex arc works without interneurons. does anyone have a better explanation for this problem.

 

[denticus]

I think I remember this being a DAT destroyer question and I think they're wrong. Interneurons will inhibit motor neurons which you don't want in a reflex response. Does that make sense? I think that's right, I remember looking that up too.

 

[jdoc04]

In a reflex arc you have a minimum of 3 neurons.
1. Sensory 2. Interneuron 3. Motor neuron

The classic example is the Knee jerk.....
1. The doc hits your patella tendon with the hammer--> Sensory input travels into the dorsal root
2. Sensory neuron synapses with the interneuron within the spinal cord.
3. Interneuron synapses with the motor neuron in the ventral horn
4. Motor neuron carries AP to the muscle inducing the jerking of your leg

Wiki describes this under the MONO VS POLY section

The point of a reflex arc is to by pass afferent to the brain and then the efferent from the brain. In some cases interneurons are inhibitory but not in the case of the reflex arc.

I hope this helps.

 

[Hysteria24]

It does make sense to have a motor neuron inhibited.

Lets say the reflex is to kick out your leg. You want the extensor muscle to contract, while the flexor relaxed.

In order to do this you would need (1) an afferent nerve bringing in the sensory info. (2) An efferent nerve stimulating the extensor. (3) An interneuron synapsing on the (4) efferent flexor to inhibit.

Kaplan has it show many different ways. I’m still confused

 

[Hysteria24]

whoops.. posted right after

 

[jdoc04]

http://s187.photobucket.com/albums/x237/jdoc04/?action=view&current=refarc.gif

 

[jdoc04]

http://dictionary.reference.com/browse/interneuron

 

[jdoc04]

what you are describing is more than just the reflex arc itself. the arc is just the incoming sensory -- interneuron -- motor to the muscle that will contract. the flexor-extensor is another dynamic in itself. Also the reflex is not a continuous firing of the motor neuron its a once and done situation. no need to be shut off.