Dorsal Root Ganglion Function on MCAT

[justadream]

Does the DRG serve the function of the yellow line in the picture below?

Is there anything else notable about the DRG that I should know?

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[type12]

Does the DRG serve the function of the yellow line in the picture below?

Is there anything else notable about the DRG that I should know?

Whoa, where did you get that image from?! That's really off.

 

[sillyjoe]

Dorsal root ganglion are mainly the cell bodies of neurons that are carrying afferent sensory signals to the central nervous system. That's about as much as you would need to know off the top of your head for the MCAT.

 

[justadream]

@type12

lol I got it from some video I was watching about the nervous system. It didn't say that that was the DRG but that was what I had inferred. So is there a name for that yellow thing?

I know for the autonomic system, you have the pre/paraganalgionic neurons.

@sillyjoe
Does the DRG run up the spinal cord as well? The DRG counts as PNS, right?

 

[sillyjoe]

Does the DRG run up the spinal cord as well?

I believe they synapse with nerves that are in the CNS, but do not themselves go into the CNS.

The DRG counts as PNS, right?

If by PNS you mean peripheral nervous system, then yes.

If you mean parasympathetic, no.

 

[justadream]

@sillyjoe

So are DRG's involved with reflexes then?

And does the DRG carry ALL the sensory information from the body?

 

[sillyjoe]

@sillyjoe

So are DRG's involved with reflexes then?

Yes. Remember these reflex arcs don't even need to go up the spinal cord to the brain. Some happen all within the control of the spinal CNS via interneurons and efferent fibers. In many cases, the brain only serves to modulate these very basic reflexes.

And does the DRG carry ALL the sensory information from the body?

"All" is a very dangerous word to use in biology and on the MCAT. There are plenty of examples of sensory information that comes into the CNS via other mechanisms and not through the dorsal root ganglion.

 

[type12]

@type12

lol I got it from some video I was watching about the nervous system. It didn't say that that was the DRG but that was what I had inferred. So is there a name for that yellow thing?

I know for the autonomic system, you have the pre/paraganalgionic neurons.

As far as I know, it's just called a motor neuron (or somatic motor neuron), or sometimes a cholinergic neuron (to describe it's signaling).

 

[justadream]

@type12

Why doesn't the somatic part of the PNS have pre/postganglionic neurons (whereas the autonomic part does)?

@sillyjoe
"Yes. Remember these reflex arcs don't even need to go up the spinal cord to the brain."
That applies to both monosynaptic and polysynaptic reflex arcs, correct?

 

[sillyjoe]

@sillyjoe
"Yes. Remember these reflex arcs don't even need to go up the spinal cord to the brain."
That applies to both monosynaptic and polysynaptic reflex arcs, correct?

Yes.

 

[justadream]

@sillyjoe

So how does the brain finally "find out" that it has moved?

Does the something in the spinal cord (from the original stimulus) eventually reach the brain or does the information come way later (like after the muscle has moved and the muscle sends new sensory information that it has been stretched/contracted)?

 

[sillyjoe]

@sillyjoe

So how does the brain finally "find out" that it has moved?

Does the something in the spinal cord (from the original stimulus) eventually reach the brain or does the information come way later (like after the muscle has moved and the muscle sends new sensory information that it has been stretched/contracted)?

What I believe you are referring to is called proprioception, which is a whole block of the neuroscience course in medical schools. Basically, we have very specialized sensory receptors in/near our muscles that tell the CNS where exactly the muscles are in space.

If you close your eyes and move your arms you know exactly where they are in 3-dimensions. This is due to propioception.

 

[justadream]

@sillyjoe

Okay so it is proprioception that alerts our body that it has moved (not anything from the original stimulus causing the reflex).

 

[sillyjoe]

@sillyjoe

Okay so it is proprioception that alerts our body that it has moved (not anything from the original stimulus causing the reflex).

Yes. We have a separate sensory system to tell the CNS where we are in space. Just to reiterate this is WAY advanced for the MCAT. I doubt this would be touched upon even in a passage.

 

[justadream]

@sillyjoe

Somewhat unreleated but what do you think we should know about flexor/extensor muscles?

My understanding is that a stimulus causes contraction of the flexor and relaxation of the "corresponding" extensor muscle on one side of the body (e.g., the left leg that receives the stimulus) and then relaxation of the flexor and contraction of the corresponding extensor on the other side (e.g., the right leg) for balance.

 

[sillyjoe]

@sillyjoe

Somewhat unreleated but what do you think we should know about flexor/extensor muscles?

My understanding is that a stimulus causes contraction of the flexor and relaxation of the "corresponding" extensor muscle on one side of the body (e.g., the left leg that receives the stimulus) and then relaxation of the flexor and contraction of the corresponding extensor on the other side (e.g., the right leg) for balance.

You pretty much have the gist of it. This is probably more than you need to know for the MCAT. I would just review the mechanism of how it works for familiarity and not worry about memorization.

 

[type12]

@type12

Why doesn't the somatic part of the PNS have pre/postganglionic neurons (whereas the autonomic part does)?

Frankly, I don't know "why," but maybe someone else does. I haven't taken neuroscience yet!

 

[Czarcasm]

I don't know if anyone mentioned this from scanning this thread, but a ganglion is a cluster of cell bodies outside the CNS. (Inside the CNS it's called nuclei). Specifically though, the dorsal root ganglion is a cluster of cell bodies of pseudounipolar neurons. Our body receives input of some change in environment and this information is received by a specialized receptor and eventually reaches the CNS for integration:

Receptor (Reception) --> Sensory neuron (Transmission) --> Interneuron (Integration) --> Effector: Gland or Muscle (Response)

Another thing worth mentioning is that sensory neuron cell bodies are outside the CNS (hence the dorsal root ganglion), but they synapse inside the CNS on an interneuron or motor neuron (cell body or dendrite). The motor neuron's cell bodies are inside the CNS, but motor neuron axons exit the CNS into the periphery (The ventral root)

So, a common question might say: Suppose a person were to cut the ventral root of a specific nerve involved in a reflex arc for hot touch. The correct answer would be the choice which states: no response (because motor axon is cut), but the individual could still perceive the pain (because dorsal root ganglion is in tact).

 

[justadream]

@Czarcasm

Do you have a figure representing the ventral root and dorsal root relationship? I can't really seem to visualize it well.

 

[Czarcasm]

@Czarcasm

Do you have a figure representing the ventral root and dorsal root relationship? I can't really seem to visualize it well.

This is a good picture to illustrate the concept. Pay attention to where cell bodies, axons, and synapses are too.

 

[justadream]

@Czarcasm

Great diagram!

Earlier in the thread I think it was established that reflexes do not cause any stimulation to the brain directly (rather, we get indication that a muscle has been contracted/stretched after its movement).

But what "decides" whether a sensation from the receptors (traveling along the DRG) become a "reflex" or become transmitted to the brain (e.g., pain)?

EDIT: Also, for the neurons involved in sensing, if the cell bodies are in the DRG and the axons are innervating stuff in the spine, then what exactly are the sensory receptors? Are they the dendrites?

 

[Czarcasm]

@Czarcasm

Great diagram!

Earlier in the thread I think it was established that reflexes do not cause any stimulation to the brain directly (rather, we get indication that a muscle has been contracted/stretched after its movement).

But what "decides" whether a sensation from the receptors (traveling along the DRG) become a "reflex" or become transmitted to the brain (e.g., pain)?

EDIT: Also, for the neurons involved in sensing, if the cell bodies are in the DRG and the axons are innervating stuff in the spine, then what exactly are the sensory receptors? Are they the dendrites?

A series of sensory neurons synapse on interneurons - some of these synapses can be excitatory (EPSPs) or inhibitory (IPSPs). The interneuron integrates these signals and ultimately determines whether or not to fire a response either directly to a motor neuron (the reflex arc), or to additional neurons in the brain where sensory information is perceived. In fact, because of this, a lot of sensory information we receive on a daily basis is largely filtered out. I recall my teacher giving an example of walking into a home where someone is baking a cake. Initially, you smell the cake (very strong stimuli), but then other sensory neurons start firing, and eventually we adapt to the smell and barely realize it's there. But then, we can induce that stimuli if we consciously attempt to smell again. All this integration happens predominately in the CNS.

 

[justadream]

@Czarcasm

I thought some stimuli don't even involve interneurons (e.g., touching a hot plate)?

Also, for the neurons involved in sensing, if the cell bodies are in the DRG and the axons are innervating stuff in the spine, then what exactly are the sensory receptors? Are they the dendrites?

 

[Czarcasm]

@Czarcasm

I thought some stimuli don't even involve interneurons (e.g., touching a hot plate)?

Also, for the neurons involved in sensing, if the cell bodies are in the DRG and the axons are innervating stuff in the spine, then what exactly are the sensory receptors? Are they the dendrites?

Some don't but the bulk majority of reflex arc's do involve interneurons. Some sensory neuron axons synapse directly with both interneurons and motor neurons, like illustrated in this picture:

As for your second question, that really stumped me when I started studying that information for a class exam, but it terms out, sensory neurons are very odd in that they have a very long dendritic process that extends towards the cell body (long as in, it can extend from receptors in the skin (dermis) located on the surface of the body all the way to the spinal cord). It's these dendritic processes that receive and transmit the impulse to the cell body and then through the axon passing through the CNS.

 

[justadream]

@Czarcasm

So for the stimuli that don't involve interneurons, why do you feel pain from touching a hot plate? Is that felt after the reflex (from a separate indepdent stimulus - the first stimulus being the reflex, the second being from the pain receptors)?

So if the sensory receptors extend from the demis, are the sensory receptors == dendrites? Or, are the receptors something else that then connect to the dendrites?

And the dendrites would only have to extend form the skin dermis to the DRG (not to the spinal cord), correct? This is b/c the DRG is where the cell bodies of the neurons are.

 

[Czarcasm]

@Czarcasm

So for the stimuli that don't involve interneurons, why do you feel pain from touching a hot plate? Is that felt after the reflex (from a separate indepdent stimulus - the first stimulus being the reflex, the second being from the pain receptors)?

So if the sensory receptors extend from the demis, are the sensory receptors == dendrites? Or, are the receptors something else that then connect to the dendrites?

And the dendrites would only have to extend form the skin dermis to the DRG (not to the spinal cord), correct? This is b/c the DRG is where the cell bodies of the neurons are.

From experience, we know that we are able to perceive information like hot water burns, cuts, etc. and the reason for this is because all of these sensory stimuli have a feedback arc that eventually travels to the brain. I never really took advanced neuroscience classes, so I really don't know much more beyond that. But generally, from what I've seen presented, this occurs through an interneuron (which the sensory neuron synapses on); the interneuron integrates information and decides whether to respond with an action (reflex arc) and to send that information to our brain - (even though integration does occur here, it's not conscious input - the action (motor response) occurs without thought). If sufficiently stimulated and the interneuron is turned on, this will invovle an immediate (uncconscious) response via a motor neuron - meanwhile the sensory stimuli still travel up to the brain (through the brain stem, up to the sensory region of the cerebrum), from there, this information is further integrated and signaled to other areas of the brain where the pain is perceived (ie. emotional response, etc). Other reflexes, such as breathing is more direct and doesn't involve concious input. I can't really explain much more beyond that sense my knowledge is limited.