longest cell in human body

[sshiflet]

What is the longest cell in the human body?

[DrDNA]

Hints.

Carry impulses.

Part of a book.

[Fred56]

Is it a neuron, by chance?

[DrDNA]

Is it a neuron, by chance?

 

Bingo!

Neurons in the spine can be quite long.

[Fred56]

I wonder how long they were in like, an apatosaur.

[DrDNA]

I wonder how long they were in like, an apatosaur.

 

I dunno. How big were they?

 

It inspired me to look up how long a whale neuron might be...

 

"A cell in the lumbar posterior root ganglion has a running length greater than the height of the individual. The ratio of the total length to the diameter of the cell body is in terms of at least a million. Needless to say such a cell in a whale would be 100 to 150 feet long. "

 

http://cyberlectures.indmedica.com/show/111/1/Neuron

 

That's amazing, don't you think?

[Fred56]

Some dinosaurs were pretty big. What about the mosasaur or the spinosaur. Didn't a really big fossil get discovered recently? I wonder how many know about these long spindly things that go all the way to, er, the extremities?

[Glider]

Bingo!

Neurons in the spine can be quite long.

Peripheral neurons are longer. The spinal cord is only about 45cm, but the snsory nerve that runs from your toe (for example) to the dorsal horn in the spine is longer (can be >1 metre).

[Fred56]

Glider:

How are such efferent (is that the correct term?) neurons connected at the "dorsal horn"?

There must be a path all the way to the brain, but does it consist of several axons, or what (or don't we know enough yet)?

[Glider]

Hey Fred56:

The nerves entering at the dorsal horn are Afferent (sensory). Efferent (motor) nerves exit via the ventral horn.

 

As you say, there must be a path all the way to the brain, and most sensory pathways are three neuron pathways.

 

The axon branches of the first neuron is tipped with sensory 'organs' (pacinian corpuscles, ruffini endings, Meissner's corpuscles, free nerve ending or whatever). The axons of these unipolar afferents go from these endings, all the way to the dorsal root at the spine, where the cell body is conected to the axon by a single projection coming sideways out of the axon.

 

These afferent cell bodies collectively form the the dorsal root ganglion (the swelling in the dorsal root). The axons, once past the ganglion are technically dendrites, but really, they're just the part of the single fibre that exists after the somatic projection.

 

These continue on into dorsal horn of the spinal cord, where they synapse with large spinothalamic projection neurons. It's a bit more complex in that there are interneurons and things involved, but primarily, these first afferents connect the periphery to the large spinal neurons in the CNS.

 

Spinal projection neurons (mainly spinothalamic, although there are other large tracts in the spinal cord) project from the dorsal horn of the spinal cord, to the thalamus (lateral and medial thalamic nuclei) travelling up the dorsal white matter of the spinal cord (cuneate and gracile fascicles). All sensory information (except smell) passes through the thalamus.

 

From the thalamus, shorter projection neurons carry the information to appropriate parts of the brain; primary somatosensory cortex and so-on.

 

So, in most cases, sensory pathways consist of only three neurons: From the periphery to the spine, from the spine to the thalamus and from the thalamus to higher CNS areas.

 

I've attached a slide showing a transverse section of the spinal cord to give a better idea. You can see the afferent enter the dorsal root, how the cell bodies of these unipolar neurons form the dorsal root ganglion and where these neurons synapse in the darker area of the spinal cord at the centre (darker (grey matter) because it contains large numbers of spinal projection neuron cell bodies) and the lighter (white matter) tracts that form the cortex; dorsal ascending pathways and ventral descending pathways. The ventral roots (not shown) contain the efferent fibres that project out to their respective effectors.

[DrDNA]

Glider,

Isn't the dorsal root gang the one that comes out of the L4-L5 space (or near that area)?

Also refered to the sciatic nerve?

The one which runs all the way down to the foot.....

Compression of the sciatic nerve causes sciatica (which is pain in the leg, behind the knee cap and even the big toe).

If the sciatic nerve is compressed due to degenerative disc disease, for example, it hurts like hell. Believe me, I know.....

[Glider]

The large sciatic nerve is comprised of a collection of a number spinal nerves that project from the L3 to S1 roots (compression of each gives different symptoms, but all are classed as sciatica), but there are 31 pairs of spinal nerves (i.e. 31 on each side of the spine).

 

Each spinal nerve contains both afferent and efferent nerves, which enter and exit the vertibrae via the dorsal and ventral roots (respectively). So, there are 31 pairs of dorsal and ventral roots from projecting from C1 to CO.

 

The dorsal roots have swellings (ganglia) because they contain the cell bodies of the unipolar afferent neurons. The ventral roots don't have swellings because the cell bodies of the multipolar efferent neurons are in the ventral horn of the spinal cord.

 

So, the dorsal root ganglia are simply the swellings (collections of cell bodies) that can be found in any of the 31 dorsal roots.

[Revenged]

Glider,

Isn't the dorsal root gang the one that comes out of the L4-L5 space (or near that area)?

Also refered to the sciatic nerve?

 

The sciatic nerve doesn't come out of the L4-L5 space... It is made up from nerves from all the spaces between L4 and S3 ('sacral plexus')...

 

http://anatomy.med.umich.edu/images/sacral_plexus.gif

 

The sciatic nerve then splits into the tibial nerve and common peroneal nerves...

 

The tibial supplies the muslces within the posterior compartment of the lower leg... The common peronal supplies muslces within the lateral and anterior compartments of the lower leg...