Myelin Sheath

[Guest jasmine]

Hi!

 

I have several questions that I have been wondering about... I'll start with the first one:

Why is there only myelin sheath on the white matter (the outer part of the brain) and not on the grey matter (the inner part of the brain), why does not the grey matter need the insulation, what is its function?

 

Thanks....

[neo007]

Hi,

myelin sheath acts as an insulator to axons, composed of neurones, and speeds up the speed of impulses.

As far as i know, white matter is involved in the transmission of impulses. This would mean that grey matter is not involved in impulse transmission as much as white matter.

Grey matter is a diffuse network of brain regions thought to be involved in complex information processing.

Hope that answered your questions

 

This link might be of some use to you

http://www.merck.com/mmhe/search.html?qt=white+grey+matter+brain&qp=%2Bsite%3Awww.merck.com+%2Burl%3A%2Fmmhe+-url%3Aprint%2F+-url%3Aindex%2F+-url%3Aresources%2Fpronunciations+-url%3Amultimedia%2F&charset=utf8&la=en&start=0

[Glider]

Hi!

 

I have several questions that I have been wondering about... I'll start with the first one:

Why is there only myelin sheath on the white matter (the outer part of the brain) and not on the grey matter (the inner part of the brain)' date=' why does not the grey matter need the insulation, what is its function?

 

Thanks....[/quote']

White matter (the inner part of the brain and the outer region of the spinal cord) is white due to high concentrations of myelin (which is largely fat).

 

White matter is composed mainly of tracts; bundles of nerve fibres (axons) carrying information from one place to another. These fibres need myelination partly to insulate each fibre from every other fibre, and partly to increase conduction rates. The conduction rate of a large motor nerve can be 300 metres per second. The conduction rate of small, non-myelinated fibres is around 0.5 metres per second. Whilst diameter has an effect on conduction velocity, myelination has the greater effect, given the relative differences in axon diameter we are talking about. You can see the importance of myelination in, for example, multiple sclerosis which results in the progressive demyelinates of nerves.

 

Grey matter (the outer part of the brain and the central 'core' of the spinal cord) is grey because it contains a large number of neuronal cell bodies.

 

The cerebral cortex (the outer part of the brain) is largely grey matter. It is composed of up to 6 layers of cells (laminae). The darker layers (grey matter) contains the cell bodies of large and small pyramidal cells and granular cells. The lighter layers contain more myelin which insulates the projections of granular cells across the cortex.

 

Below the grey matter of the cortex is the white matter of the brain which is due to the myelination of the axons of cortical projection neurons as they project to deeper, subcortical regions.

 

The grey matter of the central spinal cord is grey because it contains many cell bodies. These are the cell bodies of descending motor neurons (ventral horns) and ascending spinal projection neurons (dorsal horns). This is a bit oversimplified, but it'll do.

[Nevermore]

Go Glider! Go Glider! Heheheh

[Glider]

And all before breakfast too

[Guest jasmine]

Hi!

 

I have another question; Why does the nervous system consume a large amount of energy? Is it because the nervous system is used for so many activities in the body?

 

another question i have been thinking about is; why does the synapse exist, why couldn't there just have been one long neuron instead?

 

Thank You for your replies!

[rakuenso]

There are millions of synapses at working every second, even as I'm typing these. And Neurons generally transmit information through electric signals, so I suppose that might be the reason as to why it requires large amounts of energy.

[Glider]

Hi!

 

I have another question; Why does the nervous system consume a large amount of energy? Is it because the nervous system is used for so many activities in the body?

The brain uses so much energy because it is constantly active. Plus' date=' there are around 146,000 neurons per square mm in the cortical surface which has a total surface area of ~2,200 cm^2. All these cells require energy.

 

another question i have been thinking about is; why does the synapse exist, why couldn't there just have been one long neuron instead?

 

Thank You for your replies!

Because synapses allow information processing to take place.

 

One long neuron is capable of transmitting information from one point to another, but that's it. Most (though not all) sensory pathways consist of three neurons; one from the receptor to the dorsal horn of the spinal cord (this neuron can have an axon over a meter in length), from the dorsal horn to the thalamus and from the thalamus to the appropriate area of the cerebral cortex. But the function of this pathway is simply to get information from the receptor to the brain. Once in the brain, the information needs to be processed. This requires synapses.

 

The action at synapses can be varied. Synaptic transmission can be excitatory, resulting in an excitatory post synaptic potential (EPSP) or inhibitory, resulting in an inhibitory post synaptic potential (IPSP). Each cortical cell has many synapses. On a single cortical pyramidal cell there can be between 20,000 and 1,000,000 synapses (there are around 6x10^8 synapses per square mm in the cortex).

 

Each cell requires input from many synapses in order reach its firing threshold. This input can take the form of many incoming signals from a few cells in a short time (temporal summation), or incoming signals from many diverse cells to dendrites all over the cell body (spatial summation).

 

Given that the incoming signals can also be either excitatory or inhibitory, you can begin to see the potential for processing at this level. When you add to all that the different actions of different transmitter substances (which are only released at synapses), you can begin to see the potential for truly huge processing power, and why synapses are necessary.

[Guest jasmine]

Thankyou for you thorough answers!

I have one last question to ask

 

How does the sympathetic and parasympatethic systems maintain homeostasis, Could you give examples of how the autonomic nervoussystem effect specific endocrine glands? Has norepinepherine something to do with this?

 

Thanks

[Glider]

Thankyou for you thorough answers!

I have one last question to ask

 

How does the sympathetic and parasympatethic systems maintain homeostasis' date=' Could you give examples of how the autonomic nervoussystem effect specific endocrine glands? Has norepinepherine something to do with this?

 

Thanks[/quote']

I thought you said one last question? These (three) questions are starting to read like test questions to me. The answers you seek can be found in most introductory neurosciences book. Try Biological Psychology by James Kalat. That's a good one.

[Auburngirl05]

I have always wondered about how many calories are burned just "thinking". Would you burn significantly more sitting and working, say, difficult math problems than you would if you were just sitting, "vegging out" for the same period of time? Obviously you're always thinking of something, but does more strenuous brainwork expend significantly more calories?