Sunday Studies - Neuroanatomy

I'm starting to get a little better appreciation for the basics of neuroantomy. It's very name suggest we are learning the anatomy of neurons and a neuron is just a specialized type of cell commonly referred to as a nerve cell. Now, let's start thinking in terms of plugs and extension cords to help us understand the anatomy of nerve cells and the whole concept of neuroanatomy. Readers may wish to refer to the picture in this blog labeled "Structure of a Typical Neuron". There are only 4 or 5 parts we need to concern ourselves with at this time.
OK, imagine you're holding a plug in your hand. The prongs that get stuck into an outlet are analogous to the dentrites of a cell. The little plug part you have your fingers on when you stick the prongs into an outlet are like the cell body. Then we have a cord. This cord is like the axon which sticks out of the plug and ends up terminating or ending somewhere. Let's say, we have a lamp and decide to plug the lamp into the wall. The prongs we push into the wall are the dendrites, the part of the plug we grasp to push in the prongs into the outlet is the cell body, the cord leading to the lamp is the axon and the end of the lamp where we screw in the light bulb is the axon terminal.

That's about it. Well, that's four things. You've probably noticed insulation around wires, the insulation around the wires (axons) in nerve cells is called myelin.

With our lamp plugged in, the electricity flows from the plugs in the outlet to the light bulb receptacle in the lamp the same way electricity or an action potential travels from the dendrites of a nerve cell to the axon terminals.

What if our cord isn't long enough? Then we may use an extension cord. At one end of the cord, we have prongs (dendrites) to stick into an outlet and at the other end we have slots where another set of prongs can be plugged in (the axon terminal end). And, as many of us know around Christmas time when we are stringing up lights we may have to use several extension cords to get everything the way we want it. This is much like the neuroanatomy in the human body. Hearing, for example has four sets of extension cords running from the ears to the part of the brain which perceives sound.

With our lamp, we put a light bult at the axon terminal end then, when the power is turned on, we get light.

Our nerve cells don't go to light bulbs but may may go to things like sweat glands, eye balls or even our heart.

Now, the fun thing about neuroanatomy is that all those plugs or cell bodies are given different names.

Hearing has four sets of extension cords therefore it has four plugs or cell bodies.

Plug 1 = spiral ganglion

Plug 2 = cochlear nerve

Plug 3 = inferior colliculus

Plug 4 = medial geniculate nerve

But, I need to focus on other sets of extension cords and wiring. In the last two days of class we covered to full chapters - chapter 11 the somatosensory system which pertains to touch, proprioception, pain and temperature both in the spinal cord and via cranial nerves (since each has a different pathway) and Chapter 15, The Gustatory and Olfactory Systems (taste & smell).

If nothing else, I'd like to get the pathways for all those things listed here - it will make for a good review.

Something else I forgot to mention - the actual path or way an axon or cord travels is usually given a name as well. If we run our cord to the lamp along the wall, we might call that the "wall tract" if we run it across the middle of the floor we might call it the Uncle Harry Tract because uncle harry is the crazy person who decided the cord should run across the floor.

OK - we've got 31 sets of spinal nerves which go out of and into our spinal cord. The nerves which go out are motor (efferent) nerves (they help move things and make things happen). The nerves which go into our spinal cord are sensory (or afferent) nerves which help us perceive the outside world and let us know what the heck is going on.

We also have 12 cranial nerves which may also be motor, sensory or a bit of both.

If we perceive something from our arms, legs or torso then that information is going to be transmitted to our spinal cord.

Let's start with pain & temperature nerves that get transmitted through our spinal cord. Now, instead of saying Plug 1, Plug 2 ...we will refer to these neurons in the order they appear by calling them 1st order neurons, 2nd order neurons, etc. We may also call them primary, secondary, tertiary and so forth.

You step on a nail! OUCH! A-beta fibers in effect - The action potential elicited by the pain shoots up the nerve from our foot to an area just outside our spinal cord known as the spinal ganglion which is where our first order neuron lives. From there, the cell body of our next cord lives just inside the spinal cord and it's called the Tract of Lissauer (2nd order nucleus).

Now, if you stepped on a nail with your right foot then the spinal gangion and Tract of Lissauer are both on the right side of your body but, after the nerve impulse leaves the Tract of Lissauer, the impulse will cross over to the left side of the spinal cord via an exit ramp known as the "Ventral White Commissure" then the axon continues up the spinal cord and here the axon road is called the Spinothalamic Tract. Let's look at that word, spinothalamic, and take it apart - we basically have "spine" and "thalamic" which indicates that this road travels from the spine to the thalamus. Lots of words are spliced together like that by simply adding the letter "o" and squeezing the two other words together.

For instance, that Ventral White Commissure we mentioned, it travels "ventromedially" to the central canal of the spinal cord. Take that word apart (ventromedially) and we can see that it's simply the words "ventral" and "medial" smashed together with the letter "o" between them. Ventral means front or anterior and medial means towards the middle so ventromedially means the ventral white commissure crosses the middle of the spinal cord, in front of the central canal.

Incidentally, the central canal is literally, a canal or space that runs down the middle of our spinal cord and is where cerebrospinal fluid runs ...hey - there's another one of those words with the letter "o" in the middle ...cerebrospinal = cerebrum + o + spinal. Generally speaking when a word is put together like that it can tell you where something is coming from and where it's going to.

Let's get back on track with the spinothalamic tract which runs from the spine to the thalamus.

The thalamus contains the third order neurons - these neurons can be medial (towards the middle) or lateral (towards the sides or away from the middle)

Spinal pathways meaning impulses that would come in from our arms, legs or throrax for example go to the ventral posterolateral nucleus in the thalamus which is then the 3rd order neuron.

OK - I'm having a little bit of an Ahah moment. One thing we've learned in school is that proprioception inhibits nociception which doesn't mean a darn unless you know what proprioception and nocicption are - but proprioception and touch go together and nociception refers to pain and what I've noticed is that pain/temperature nerves and touch/proprioception nerves all go into the ventral posterolateral nucleus of the thalamus. This bit of neuroanatomy starts to lend credence to the notion that touch inhibits pain or (proprioception inhibits nociception, same thing)

To help remember this concept, I've noticed some students write down "Mommy kisses boo boo" Mom's kiss is the touch or proprioception and the boo boo is pain or nociception.

Keep in mind, as mentioned in a previous blog - the nerves used for touch perception are larger and have better insulation than nerves used for pain which means nerve impulses travel faster for touch than for pain. Think of the thickness of nerve fibers like highways as compared to two lane dirt roads - the bigger and better the road, the less resistance there is to traveling fast.

Anyway - from the ventral posteriolateral nucleus of the thalamus we have a tract of cords called the posterior limb internal capsule which travels to the SI area of the cerebral cortex. The SI area is also known as the primary somatosensory area of the brain (another word with "o" in the middle!) Somato means body so primary somatosensory area refers to the part of the brain that perceives senses of the body.

I've included a brain picture to show exactly where these somatosensory areas are located in the human brain. It's an area known as the postcentral gyrus. In the brain, we have gyrus and sulcus. The gyrus are the hills and the sulcus are the little ridges or valleys between the bumps/gyrus. Looking at the side of a human brain you would be able to notice a long sulcus or groove running down the side of the brain which is called the central sulcus - the gyrus on either side of the sulcus are named according to their relative position to the central sulcus. So, we have the precentral gyrus and the postcentral gyrus - our final destination for our touch/proprioception & pain/temperature senses are in the postcentral gyrus.

btw - here is an interesting video on senses and brain development http://videos.scienceofecd.com/unisa/bd3_mustard1.htm

Let's get things summarized here -

For spinal pain/temperature, here is the path ...

1. spinal ganglion - 1st order neuron
2. Tract of Lissauer - 2nd order neuron
3. ventral white commissure - where the nerve axons dessucate or cross over to the other side of the body.
4. spinothalamic tract - name the axon is given as it ascends up the spinal cord into the thalamus
5. ventral posterolateral nucleus - 3rd order neuron
6. posterior limb of internal capsule
7. Primary somatosensory area of the brain (post central gyrus)

For spinal touch/proprioception, here is the path

1. spinal ganglion - 1st order neuron
2. gracile/cuneate tract
3. dorsal column nucleus - 2nd order neuron
4. internal arcuate fibers - where the fibers decussate or cross over to the other side (this happens in the medulla which is wehre the 2nd order neurons are located.
5. medial lemniscus - name given to axon tract that ascends to the thalamus
6. ventral posteriolateral nucleus - 3rd order neuron
7. posterior limb of internal capsule
8. primary somatosensory area of the brain (post central gyrus)

Well, those are the paths if you sense something with an arm or leg since those nerves go to the spinal cord but, I'm sure we've all felt the stinging pain of a sunburn on our face or the tickle of a fly that's landed on our nose. Those are different neuron pathways and are referred to as Cranial pathways - so ....

For Cranial touch/proprioception we have

1. trigeminal ganglion - 1st order neuron (in the pons)
2. principal trigeminal nucleus - 2nd order neuron (still in the pons)
3. decussation occurs in the pons
4. ventral postereomedial nucleus of the thalamus - 3rd order neuron
5. posterior limb of internal capsule
6. primary somatosensory area of the brain (post central gyrus)

For Cranial pain/temperature we have

1. trigeminal ganglion
2. a desending tract called the spinal trigeminal tract
3. caudal spinal trigeminal nucleus - 2nd order neuron
4. decussation in the internal acrcuate fibers
5. ascending tract called the trigeminal thalamic tract
6. ventral postereomedial nucleus of the thalamus - 3rd order neuron
7. posterior limb of internal capsule
8. primary somatosensory area of the brain (post central gyrus)

Cool - now we have touch/proprioceptoin and pain/temperature pathways summarized ...next would be taste and smell ...

Taste is a bit interesting in that it has three primary or 1st order neurons. The anterior 2/3 of the tongue is innervated by the Facial nerve, cranial nerve (CN) number 7 or VII ...cranial nerves are most properly denoted by Roman numerals. The posterior 1/3 of the tongue is innervated by the glossopharyngeal nerve (CN IX) and the eppiglotal region is innervated by the Vagus nerve (CN X)

so ....let's try the pathway ....

1. geniculate ganglion of the facial nerve - 1st order neuron
2. inferior or petrosal ganglion of the glossopharyngeal nerve - 1st order neuron
3. inferior or nodose ganglion of the bagus nerve - 1st order neuron
4. those three 1st order neurons then meet in the solitary or gustatory nucleus - 2nd order neuron
5. the axon then ascends up to the ventral posteromedial nucleus - 3rd order neuron
6. then head out via the gustatory radiation to ...
7. the primary gustatory area in the parietal operculum (another part of the "outer" brain, just below the postcentral gyrus)

Smell pathway ....I'm not entirely certain but will do my best ...

1. mitral cells in the olfactory bulb - probably 1st order neurons
2. olfactory tract
3. olfactory trigone
4. lateral olfactory striae
5. primary olfactory cortex
6. ends in the entorhinal area of the brain which is so deep in the brain that it's right next to the brain stem

the olfactory association area is more towards the front of the brian - I'm just not clear on this neurological pathway.

The preceeding pathways represent the two chapters we went over last week in class. I think I already mentioned the auditory pathway (chapter 12) at the beginning of this blog. Chapter 14 was the visual pathway - I think I have that one memorized - let's see ...

Ugh ...I'm already thinking I could write a better neuro book - certainly at least help clarify the darned subject matter - Things such as 1st, 2nd, 3rd ...order neurons should really be listed or shown with a picture. It's difficult when things that should be grouped together aren't.

OK - the first two order neurons are located in the retina - bipolar cells are primary and ganglion cells are secondary. Honestly, I might consider the rods and cones as two different primary nucleus but, I didn't discover this stuff so ....

the lateral geniculate nucleus is the third order neuron - recalling the medial geniculate nucleus was for hearing (4th order for hearing)

I think that's it for the visual pathway because after the lateral geniculate nucleus we have optic radiation paths which go to the visual cortex. OK - here it is -

Visual pathway -

1. bipolar cells - 1st order neurons
2. ganglion cells - 2nd order neurons
3. lateral geniculate nucleus - 3rd order neurons

I better get to bed -

The one picture I haven't talked about yet shows the density of neuron connections at birth, at 7 and age 15. We hit our peak of neuronal connections in the brain about age 10 then the brain starts to destroy the weakest synapses and preserves only those that have been "magically" transformed by experience - once again ...use it or lose it. From the pictures it's clear that there is a greater density of neurons at age 7 than at age 15.

That's also true and applicable to things like cochlear implants. When a child receives a cochlear implant it works much better than say, for an adult, who has never been able to hear. The portion of our brain used for hearing - if not ever used is taken over by other areas of the brain so, when a person loses one sense and then we notice other senses becoming more acute and sharp there is a physiological reason since the areas that are sharper may now make use of the area of the brian which is no longer used for the lost sense. As a child, not as much time has elapsed for other portions of the brain to take over the lost senses area of the brain.

Here is a link for the site where I got a couple of the pictures http://www.unisanet.unisa.edu.au/Resources/101766/Online%20Brain%20Development%20course/Topic%201/Lecture.htm

this link below shows how neurons can be destroyed by mercury

Neuron Degeneration via Mercury

One final word - I'd like to thank my parents for all the help and support they give me throughout the week. When I come home from school and I see the work they've done to help me out the first words in my head - every time - are "thank you"

It really does help a lot. :)