#Meninges 🧠

🧠 The dura mater, tough mother, is the most outer layer of the meninges, and inside of that layer is the #aractnoid. And the arachnoid is the filmy-like substance and it covers the entire surface of the brain. It doesn't invaginate with every hill and valley of the brain.
Pia, the final layer of the meninges, is too fine to be seen.

🧠 The dura is separated from the brain by a layer of fluid, and what that does is that it circles the brain and it in-cases the brain in this bag of fluid. So that, as the brain moves around, it's cushioned -- it can't bang against the skull because it's cushioned by the fluid.

🧠 In the #cranium, the dura is right up against the skull; there's no separation. They can't be separated, except if there's a bleed. And that is a very dangerous situation, that's a medical emergency.
If there's a bleed between the skull and dura. If there's some injury and blood gets in there that's a medical emergency.

🧠 In contrast to the situation in the cranium, where the dura is right up against the bone, the dura in the spinal cord is not up against the #VertibralColumn. So there's a lot of space. It's not up against the bone. And therefore, you can imagine that increasing pressure in the cranium is a much bigger deal.

🧠 If the dura is tough, the dural folds are doubly tough, because they are actually double, double folds -- folds made of two layers of dura.

🧠 There's a fold called the Falx Cerebri, and it goes between the two parts of the brain, between the two hemispheres.
Tentorium is the dural fold that goes between the cerebrum and the cerebellum.

🧠 The most important point of these folds, or one of the benefits of these folds, is that they separate problems, pressure problems, into three compartments.

🖼️ Image source: https://in.pinterest.com/pin/191614159128229061/?lp=true

🔸 The #meninges form a very effective barrier against toxins, viruses, against all sorts of damage, so that the peripheral nervous system tends to be far more vulnerable than the central nervous system.

🔸 In addition, the two regions have very different capacities for repair.

🔸 If you cut an axon in the peripheral nervous system, it can repair itself -- It will reconnect. If there's a traumatic injury to the central nervous system, the same does not happen. So, the ability to repair is far, far greater in the periphery than in the central nervous system.

🔸 A large molecule called botulinum toxin, which comes from spoiled food, can get in and it primarily will affect the peripheral; it will only affect the peripheral neurons. It will not get past the meninges.

🔸 #PolioVirus actually gets in right at the synapse between the motor neuron and the voluntary muscle, and it goes back. It travels back and it does a clever thing -- It gets through the meninges, but it gets through the meninges by getting in through an axon of a motor neuron. And what does it do then? For thanks, it actually kills this motor neuron. So now, that motor neuron is going to die.

🔸 Let's look at another virus: herpes zoster.
Herpes zoster is a virus that produces, what's commonly called as, #shingles. And in herpes zoster, the virus gets into the dendrites of sensory neurons, and it gets transported back inside the axon in these sensory neurons, and then it goes and lives in synaptic terminals. And if all is great, it lives in there and then it never talks again.

🔸 But, under some circumstances, the virus can decide to reproduce and blossom, and it will make copies of itself so that it actually sends it back out. And what you get is a virus all throughout the sensory territory and what you get is a rash.