Divisions and general organisation of the brain and spinal cord

Question 1

What are the general divisions of the nervous system?

Answer 1

CNS

• brain
• spinal cord

PNS

• somatic (sensory and motor)
• autonomic (parasympathetic and sympathetic)

Question 2

How do we classify if something is in the PNS or the CNS?

Answer 2

CNS neurone is it is wholly contained within brain or spinal cord. This can include neurons that travel between the brain and spinal cord or those connecting different regions of brain or spinal cord.
If part of the neuron lies outside the brain or spinal cord, this is the peripheral nervous system neuron. Doesn’t have to be the whole neuron, just a part of the neuron outside to be a PNS neuron.

Question 3

What is the difference between grey and white matter?

Answer 3

Grey matter - cell bodies
White matter - axon tracts

White matter is this name as the myelin in the axon tracts gives the axon tracts a milky white appearance.
Cell bodies have little myelin so appear grey.

Brain:
Much of the grey matter is arranged on the outside (cortex of brain). Axon tracts come from the centre.

Spinal cord:
White matter on outside and grey matter on inside.

Question 4

Where does the sympathetic and parasympathetic part of the body come from in the spinal cord?

Answer 4

Sympathetic system - largely arrises from the thoracic cord (T1 to L2) continuous column

Parasympathetic system - from brain stem and sacral spinal cord

Question 5

What 3 parts is the brain divided into?

Answer 5

(bottom upwards)
Hindbrain
Midbrain
Forebrain

Inferior part of CNS - spinal cord extending from inferior point of spinal cord up to the frenum mangum where the spinal cord ends. This then transitions into a part of the brain called the hindbrain.
Hindbrain - 3 main components: medulla oblongata, cerebellum, pons (can be see due to the bulge).

The cerebellum sits behind (in the brain). It is small but has as many neurons as the rest of the brain put together.

Midbrain - small area characterised by two bumps on dorsal surface and a channel running in front of the bumps.
Forebrain - everything else in the brain

Question 6

What are the main planes we use in the body?

Answer 6

Anterior is towards the front and posterior towards the back. (Towards head and legs). Can also be called rostral and caudal.

Ventral is towards the belly and dorsal is towards the back. Can also call this anterior and posterior.

Also have superior and inferior suggesting the top and bottom.

Coronal (like a crown). Parallel to front of face.
Sagittal. Runs through midline
Horizontal. Cuts into top and bottom.

Question 7

In terms of ventral and dorsal in the brain, which was round is it and why?

Answer 7

Underside of brain is its ventral and upper side is the dorsal surface.

Why?
During development the brain and spinal cord are a tube with a ventral and Dorsal surface. This remains same for spinal cord but changes for brain as the brain goes through a 90* angle change so now they have switched.

Question 8

How many spinal nerves are there and which part of the spinal cord do they come from?

Answer 8

31 pairs of spinal nerves
Each are associated to a vertebrae and exit through the corresponding vertebral foramen.

8 cervical
12 thoracic 
5 lumbar 
5 sacral
1 coccygeal

Question 9

When does the spinal cord end and what does this mean?

Answer 9

Spinal cord ends at disc between L1 and L2. 31 Spinal nerves therefore extend further than the 31 segments of the spinal cord.

End of cord - ends at level of disc. Spinal cord segments giving rise to nerves below L1-L2 have to travel down some distance below level of cord before they can escape to corresponding vertebral foramen.

Question 10

What is the cauda equina?

Answer 10

We have a region full of spinal nerves and cerebral spinal fluid called the caudal equina. This means that fluid can be accessed by a syringe going in to access it for conditions such as meningitis. The needle can be inserted without fear of damaging the cord itself.

Question 11

Explain how the nerves in the individual vertebrae work?

Answer 11

They are mixed spinal nerves.
Sensory information in through dorsal routes.
Motor information out through ventral routes.

Motor = cell bodies of motor neurons are in the ventral horns of the spinal cord and they project their axons out through ventral routes that join with dorsal routes to form the mixed spinal nerve.
These motor axons go on and terminate in the neuro muscular junction on skeletal muscle.

Sensory = comes in from a sensory specific receptor (e.g. touch, vibration, pain, temperature). The axons come in through the dorsal routes into the dorsal horns.
The cell bodies of sensory neuron is not in the dorsal horns of the spinal cord but in the dorsal root ganglia. These are groups of cells on each side of the spinal cord which are associated with each segment. They can see seen as bugles as you run down the cord.
Once the sensory information comes into the dorsal horns, what happens next depends on the type of sensation. Some cases it will synapse and cross the midline and then ascend to give the brain the sensory information. In other cases it won’t synapse.

Question 12

Explain what is meant by the nerves entering and leaving the spinal cord called rootlets

Answer 12

The motor nerves don’t leave the cord as a single nerve and sensory nerves don’t enter as a single nerve.

Instead, they enter and exit as a series of rootlets.

Can also see that as we go down the cord, the rootlets leave at an increasing oblique angle. At the top of the cord they leave almost horizontally. This is because of the difference between the length of the cord and the length of the vertebral column.
The cord is shorter than the vertebral column. Each spinal cord segment is associated with a particular vertebrae has to leave through the corresponding vertebral foramen so the nerves leave progressively obliquely until you reach the end of the cord where the spinal nerves have to travel downwards before they find a corresponding vertebral foramen to escape.

Question 13

What are the different lobes of the cerebrum called?

Answer 13

Frontal
Motor
Sensory
Parietal 
Temporal 
Occipital

Movement is largely associated with frontal lobe and vision with the occipital lobes.
There are other processes associated with the frontal lobe such as mood.
The temporal lobe has important roles in language and memory.
Most functions are divided up into different lobes. e.g. movement with frontal and parietal lobe.

No function is located just to one lobe and no one lobe is just one function!

Question 14

What is the structure of the brain?

Answer 14

Large number of folds. This increases SA to provide more material being able to fit into cranial vault.

These folds are forms by ridges called gyri and the grooves between them are known as sulci.

There are deeper divisions in a few places in the brain called fissures. Can see the lateral fissure clearly separating the temporal lobe below from the frontal and parietal lobes above.

Outer part of cerebrum = cerebral cortex.

Question 15

What is the ventricular system?

Answer 15

interconnected chambers within the brain which have a role in production and movement of cerebral spinal fluid.

Since the brain is composed of chambers called ventricles it means that the brain is hollow. The chambers are continuous with the central canal of the spinal cord.

The central canal is closed caudally. Rostrally it is continuous with the ventricular system of the brain.

Question 16

What are the component parts of the ventricular system?

Answer 16

Lateral ventricle
Cerebral aqueduct
3rd ventricle 
4th ventricle
Central canal

In each hemisphere we have a large lateral ventricle.
You can’t normally see it as it is covered by a septum. It extends back and drops down into the temporal lobe.
This lateral ventricle is continuous with the third ventricle.

The third ventricle has the dienecephalon formed on the thalamus above and hypothamulus below. There is a dinecepohalon on each side which forms the walls of the third ventricle. This is continuous with the lateral ventricles on both sides through a little hole between the ventricles called the interventricular foramen.

Fourth ventricle - sits in the midline and is much bigger in depth than third ventricle. It sits between the pons ventrally and the cerebellum dorsally and it connects to the third ventricle via a thin canal called the cerebral aqueduct.

n the fourth ventricle, the cerebral spinal fluid which is present within the ventricles escapes through a number of routes (through central canal of medulla which is continuous with the central canal of the spinal cord or it can escape through 3 different holes medially and laterally one on each side).
At the level of the fourth ventricle, we have connections outside of the brain or staying within the brain stem within the central canal of the medulla.

Question 17

What is the organisation and shape of the ventricular system?

Answer 17

The lateral ventricle extends as a C shape back towards to occipital lobe and down into the temporal lobe. It does not reach the occipital lobe. It also has a little tail called posterior horns.

There is one in the left and one in the right hemisphere. They join together via the intervertebral foramen with the slit in the midline of the third ventricle that then connects though the cerebral aqueduct to the fourth ventricle.

There is a lateral foramen on each side of lushska and the medial foramen of Magendie between the two lateral foramen.

Next slide:
There are two lateral ventricles joined via the intervertebral foramen with the hole shown. The thin slit connects them to the third ventricle in the midline, down through the cerebral aqueduct into the fourth ventricle.

Two lateral ventricles arranged obliquely so the front of the paternal ventricle (anterior horns) are closer together than the temporal horns at back. They meet in the midline with the third ventricle. You can see the continuation through the cerebral aqueduct into the fourth ventricle. There are three foramen by which the CSF (cerebral spinal fluid) escapes. It is also continuous with the central canal of the medulla and then the spinal cord (this is how it gets to the spinal cord).

Question 18

What is inside of the venticles?

Answer 18

Clusters of tissue within it called the choroid plexus. This is specialised ependymal cells that make cerebral spinal fluid.

Once it as been made, CSF can extend throughout the lateral ventricles and third ventricle and fourth ventricle and the spinal canal as all of these are continuous.

At level of fourth ventricle, CSF can escape from brain meaning that CSF is within the brain and immediately surrounding.

Question 19

What are the 3 main functions of the CSF?

Answer 19

1. Buoyancy as brain is surrounded by and contains CSF. Important as the average weight of a human brain is 1.3kg and this is too heavy and would squash neurons on the ventral surface so the brain is therefore floating in CSF giving brain a weight of 30 grams (protects cells on ventral surface)
2. Protection (brain is floating which creates a hydrostatic buffer to stop the brain crashing against the side of the cranium
3. Removal of waste products (made by neurones and glia to enter CSF and eventually return to the venous blood stream)

Question 20

How much CSF do we make each day and what happens if we do not reabsorb this?

Answer 20

550ml

CSF is constantly being made and absorbed.
If the CSF isn’t reabsorbed correctly, it can result in a condition such as hydrocephalus.

The skull is so large as the CSF builds up, usually within ventricles out the brain, it pushes the cortex outwards. The cortex then pushes against the skull plates and becomes in neonate (newborn child), the skull plates aren’t fused, they move apart and the head and grow to massive sizes.
In many cases it is treatable but putting a stent in to remove the excess CSF or treat the cause of the problem surgically.

Question 21

What are the 3 meningeal layers of the CNS?

Where does the CSF leave the ventricles of the brain?

Answer 21

1. Pia mater (innermost layer)
2. Arachnoid mater (middle layer)
3. Dura mater (outer layer)

The CSF leaves the ventricles of the brain and exits into the space between layer 1 and 2 called the subarachnoid space.

Closest to cortex = the Pia Mater. This is a thin delicate layer. It can’t be seen via the naked eye. It is only 2 cells thick.

There is then a substantial gap between the Pia mater and the arachnoid mater. This is called the subarachnoid space. This is important as this is the space that CSF is found once it has left the fourth ventricle and exited into the brain. There are 3 foramen in the Pia mater in the fourth ventricle where it enters the subarachnoid space.

Arachnoid Mater - takes its name from its spider web appearance

Dura mater - a tough layer. it is closely associated with the arachnoid layer and has a big role in protecting the brain

Question 22

Give details on the Pia mater

Answer 22

The innermost meningeal layer.

Protects the CNS (together with the other meninges)

A fine, vascular membrane that also allows entry of blood vessels into the CNS.

Provides an impermeable layer for containing CSF.

The Pia matar cells sit on top of the outer layer of the brain.
The outer layer of the brain cortex is made out of non-neuronal cells (glial cells e.g. astrocytes). Thin layer of astrocytes on surface of the cortex is called the glia limitans.

Question 23

Give details on the arachnoid matar

Answer 23

The middle meningeal layer.
Protects the CNS.
A spiders-web like membrane that forms the upper limits of the sub-arachnoid space.
Is closely associated with the dura mater.

Question 24

Give details on the dura mater

Answer 24

The outer meningeal layer.
Protects the CNS.
In the skull, composed of two layers which separate in places to form sinuses carrying venous blood.

Surrounding the brain, the dura mater is closely attached to the periosteum of the cranium: surrounding the spinal cord =, it hangs loosely, only attached at the foramen magnum.

In the cranium, the dura matar is attached to the lining of the cranium itself.
The spinal cord is attached to the foramen magnum and hangs down loosely inside the vertebral column with the spinal cord inside it.
The dura matar is actually two layers.

Question 25

How does CSF drain out the brain?

Answer 25

At the top of the head, the dura has split into two at the midline. It has formed a cavity full of venous blood. This returns deoxygenated blood via the jugular vein back to the heart. This is called the superior sagittal sinus.

Below the dura we have the arachnoid and the Pia matar. CSF is in the subarachnoid space.

At the position of the superior sagittal sinus, the arachnoid layer has specialisations that poke into the superior sagittal sinus itself and these are known as arachnoid granulations (or arachnoid villi). The CSF within the subarachnoid space (the space all the way around the brain and spinal cord) will get pumped out from the arachnoid villi into the superior sagittal sinus.

CSF is continually being made and continually being reabsorbed into the venous blood stream. This was the body recovers important components of the CSF such as electrolytes.

Question 26

Where is CSF found?

Answer 26

CSF is found in the ventricles and the subarachnoid space.