Week 1 Topic 2 - Neuroanatomy, neural systems and brain function

Question 1

How many neurons do we have?

Answer 1

To start with, let’s think about what the starting material is. We have about 100 billion neurons in
the adult human brain.

Question 2

How many synapses do we have?

Answer 2

Now, this equates to up to 100,000 trillion synapses. That’s a vast amount of
potential given that each of these is effectively a binary unit that contributes to the computing power
of the brain.
But that is only a part of the story. The greatest complexity comes, not so much from these big
numbers, but from some of their properties that we’ll look at now.

Question 3

What is the property of convergence?

Answer 3

The first is convergence. This is the ability of many different cells to send their inputs to one target
cell. That is, the single cell receiving the inputs is receiving it from multiple sources. In fact, the
average neuron in the human brain receives 10,000 different inputs– not necessarily from 10,000
different cells, but there will be 10,000 different synapses on it.

Question 4

What is the property of divergence?

Answer 4

The other side of the coin from convergence is divergence, and here, this is the ability of a single cell
to project to multiple cells. Here again, we’re looking at large numbers, maybe up to 1,000 different
axon terminals from one single neuron. So we have the ability of the cell to receive multiple signals as
well as to send out multiple signals. This creates what we know as neural networks.

Question 5

What does the peripheral nervous system relate to?

Answer 5

The peripheral nervous system includes the

1. autonomic,
   which is the component that we are not aware of– maintaining our heart rate, maintaining our
   breathing rate, maintaining gut peristalsis for example– and the
2. somatic nervous system, things
   we are consciously aware of– things like movement, things like feeling temperature or fine touch or
   vibration.

Question 6

Autonomic Nervous System and why it is not idea to describe the parasympathetic system as rest and digest (because it also controls the ocular control - constriction of the pupil in bright light)

The sympathetic system dilates the pupil in dim light.

Answer 6

autonomic nervous system. Now, you might be familiar with this
as often described as being divided into the

1. parasympathetic component, and its complement, the
2. sympathetic component.

These are often referred to as ‘rest and digest’ or ‘fight and flight’ divisions.

I would encourage you not to think along those lines as there are too many exceptions to these rules.

For example, the parasympathetic nervous system via the action of one of the cranial nerves– in
fact, the third cranial nerve, the ocular motor nerve– is responsible for constriction of the pupil in
bright light.

The complement via the sympathetic nervous system is to dilate the pupil in dim light.

It’s
not really related to ‘rest and digest’ or ‘fight and flight.’

And there are a large number of exceptions
to these.

But generally, they do work in opposite ways. They work against each other. Dilation or constriction is
a common feature of the two nervous systems.

But the important point here is that although they send peripheral branches, they’re not restricted
to the periphery alone.

So in the brain, there will be pathways that are wholly concerned with either
parasympathetic or sympathetic function.

Therefore, the earlier description of the parasympathetic
and sympathetic nervous system not being related to the brain is probably not ideal.

Question 7

What does it mean for a cell to be part of the CNS?

Answer 7

I would think of the nervous system as being divided up into central nervous system where we have
this definition whereby a neuron is a member of the central nervous system if it’s wholly contained
within the brain or spinal cord. So this means it might be within the brain, within the spinal cord, or
travel between the brain and spinal cord.
But no part of it– not the cell body, not the dendrites, not the axons– project outside of those two
structures. If any part of it does, then it’s a peripheral nervous system neuron. And that’s the way I
would suggest you think about these terms.

Question 8

What do ventral, anterior, dorsal, and posterior mean?

Answer 8

‘ventral’ and ‘anterior’ on the one hand, and ‘dorsal’ and ‘posterior’ on the other, become synonymous.

Ventral/anterior: underside of an animal or plant; abdominal, belly side

Dorsal/posterior: of, on, or relating to the upper side or back of an animal, plant, or organ

Question 9

What is cranial flexion?

Answer 9

What this means is the long axis of the spinal cord is approximately at right angles to the long axis of the brain.

And this occurs during embryological development.

So what this means is, if we are describing the front of the spinal cord, we can refer to it, for example, as the ventral surface.

This would be this surface of the spinal cord.

But because of cranial flexion, the ventral surface of the brain now becomes the underside of the brain.

Similarly, the dorsal surface of the spinal cord is continuous with the dorsal surface, which now is the top, the superior surface of the brain.

So pay attention to these terms, because it might not be immediately obvious.

If people are talking about the dorsal aspects of the brain, they’re really talking about the top, and if they’re talking about the ventral surface of the brain, they’re really talking about the inferior, lower surface because of this
cervical flexion, or cranial flexion, that occurs during development.

Question 10

What are the main three planes that are used to describe sections through the
brain?

Answer 10

1. horizontal, which is effectively transverse sections;
2. sagittal, which runs through the midline,
   from Sagittarius the archer, the stance an archer would take; and
3. coronal, or a frontal, section,
   parallel with the plane of the face. Corona is Latin for a crown. So it’s imagining putting a crown on
   your head.

Question 11

What are the anatomical four parts of the brain?

Answer 11

1. the spinal cord, which ends approximately at the level of the foramen magnum, the large hole in the base of the skull;
   which is then continuous with the
2. hindbrain, which is made up of these three parts we’ll meet in a little
   bit more detail later,
3. midbrain the medulla, the cerebellum, and the pons; and above that, this region called the midbrain; and above this, the much bigger area, which represents the
4. forebrain - cerebral hemispheres of the forebrain.

Question 12

What are two things about the spinal cord?

Answer 12

1. The first is the white matter, composed of axons, is largely on the outside of the spinal cord. This
   is the opposite arrangement to the brain, where on the outside, it’s mainly the grey matter, which
   is the cell bodies. The grey matter in the spinal cord sits in the middle. The exact shape will change
   according to where you are in the spinal cord. But it’s roughly the shape of the letter ‘H’ no matter
   where you are.
2. The other thing to note is that the nerves associated with the spinal cord are mixed. So this nerve
   that’s going out to the periphery will have both sensory neurons within it, they’re bringing information
   in with cell bodies in the dorsal root ganglia, projecting their information into the cord, as well as
   axons projecting their information out to skeletal muscle.

So each of the spinal nerves is mixed - has both sensory and motor information. They enter and leave
the cord as a series of rootlets, multiple entry and exit points for each spinal nerve.

Question 13

How many spinal nerves do we have?

Answer 13

We have 31 spinal nerves, most individuals do. A few exceptions, where some have one or more, one
fewer or one more. But the vast majority of people have 31, of which there are 8 cervical, 12 thoracic,
5 lumbar, 5 sacral. And if there’s any variation, it comes in the coccygeal nerves, but most of us only
have one.

Question 14

What’s the foramina?

Answer 14

Where the spinal nerves exit. They exit through holes - foramina - between the vertebra, the intervertebral foramina.

Question 15

How far down the vertebral column does the spinal cord go down?

Answer 15

Now the spinal cord isn’t as big as the vertebral column. In fact, it’s only about 2/3 of the length of the
vertebral column.

It stops growing at the same rate by about the end of the first trimester.

After that,
the vertebral column is growing faster.

Now that has important consequences, some of which are exploited clinically.

What this means is
that the spinal cord is ending at about the level of the intervertebral disc between L1 and L2.

But
because each spinal nerve exits through a corresponding intervertebral foramen, it means that any
nerve below L1 and L2 has to travel down the spinal cord until it finds its corresponding intervertebral
foramen and exits.

Question 16

What is the cauda equina?

Answer 16

The horse’s tail where the group of nerves that are below the S1 and S2 have to exit… Since the spinal cord is longer than the vertebral spine

S1, for example, segment S1 of the spinal cord is here, but the first sacral vertebra is here. So this
nerve has to project down through a space below the level of the cord before it exits. This means
that you have a group of nerves at the base of the spinal cord where there’s no neural tissue, no cell
bodies involved. This gives the name to this structure, the cauda equina, the horse’s tail, because
that’s exactly how it appears in a fresh specimen.
And clinically this is important, because it means a needle can be put into the spinal cord below the
level of L1, L2 - for example, to drain cerebrospinal fluid diagnostically, or if build up of pressure -
knowing that you’re going below the level of the spinal cord itself and therefore won’t damage it. And
this, of course, is done in lumbar punctures.

Question 17

What is the medulla?

Answer 17

The medulla is the oldest part of the hindbrain in evolutionary terms, and in fact, it’s the oldest part of
the brain in all. It contains life-supporting centres - this is the part of the brain that keeps you alive on
a minute-to-minute basis.

Let’s move up a little higher. The spinal cord, as I mentioned, ends at the intervertebral foramen, and
above that, we come to the region of the hindbrain, the medulla, the cerebellum, and the pons.

Question 18

What does the midbrain do?

Answer 18

Above the hindbrain is a relatively smaller area, the midbrain, separated by a small channel. Small,
but an important area of the brain. Amongst other things, it’s a very important relay between activity
in the forebrain above and the hindbrain below.

Question 19

What does the thalamus do?

Answer 19

Virtually all sensory
information, whether it’s special sensory, such as taste or vision or hearing, go through the thalamus.

Or somatosensation, fine touch, coarse touch, vibration, pain, temperature, go through the thalamus.

Sitting on top of the midbrain is a structure called the thalamus, and we’ll meet this again later.

This is a part of the brain called the diencephalon,

And it’s a very important relay.

And it’s also, as we’ll see, an important relay for descending information, motor information as well.

You will find that there are some texts that include the thalamus in the brain stem, and there are
evolutionary reasons why you might do so.

There’s functional reasons why you might do so.

But in general, in the UK, we tend to regard the thalamus as a separate structure to the rest of the brain stem.

In American texts, the tendency is to include it with the brainstem.

Question 20

What is the brainstem?

Answer 20

Let’s think about the brainstem. That area shown in red on the rotating skull here is the most
important part of the brain.

It’s the reason why brainstem activity is used as a clinical descriptor of
life.

I’m sure you’ve heard of the term ‘brainstem death’, which is used to clinically define whether
somebody is capable of independent life or not.

And it’s complicated because there’s a lot going on.

There are ascending, somatosensory, and
descending motor pathways going through it.

Also, there are lateral connections between the stem of
the brainstem and the cerebellum dorsal to it.

A lot of the cranial nerve nuclei– remember you have
12 pairs of cranial nerve nuclei– most of those are contained within it. it’s an important centre for
chemo reception, as well as a number of cranial reflexes, such as salivation, mastication, swallowing.

We could also include the gag reflex and suckling in infants amongst these.

It’s important for a number of vital life-supporting roles, which we’ve already mentioned the
cardiovascular and respiratory ones sitting in the medulla.

But there are others as well, not least
those concerned in arousal - arousal as wakefulness, keeping you out of a comatose state.

All of
these are important activities of the reticular formation, which runs throughout the brainstem.

Question 21

What are three important nuclear groups in the brainstem?

Answer 21

Finally, there are three important nuclear groups:

1. the raphe,
2. the locus coeruleus, and the
3. substantia
   nigra, which also reside in the brainstem.

These are the sites of very important monoaminergic
pathways within the brain.

Question 22

What does the raphe do?

Answer 22

The raphe, for example, is the centre for all the serotonergic pathways within the brain. If you use
serotonin as your neurotransmitter, they originate from a site within the brainstem.

Question 23

What is the locus coeruleus?

Answer 23

The locus coeruleus is the site of all the adrenergic pathways - pathways that use adrenaline,
noradrenaline, as the neurotransmitter, originates from the locus coeruleus.

Question 24

What is the substantia nigra?

Answer 24

And the substantia nigra is the important centre for dopaminergic neurons within the brain, very
important in movement control.

Parkinson’s disease is the loss of the dopaminergic neurons from
one part of the substantia nigra.

So the brainstem is responsible for a very large range of important
functions to keep us alive.

Question 25

What are the folds in the forebrain?

Answer 25

If we move up to the forebrain, the first thing you notice is that it’s large and thrown into a large
number of folds.

The folds increase the surface area of the brain, so we can fit more neuronal tissue
into the cranium, and the ridges are known as a gyrus.

1. This, for example, is the middle temporal
   gyrus, plural gyri.
2. And the grooves, here we have the central sulcus, plural sulci.
3. There are bigger
   divisions too. Here, for example, is the lateral fissure.
4. Separating the cerebellum from the cerebrum,
   we have a transverse, or horizontal, fissure.

Question 26

What is a “gyrus”?

Answer 26

ridges in the brain are known as a gyrus

Question 27

What are the four lobes?

Answer 27

Now we can divide the brain up into lobes, as I’m sure you know, and the lobes are named after the
associated bone of the cranium.

So normally, the brain is described as having four lobes,

1. an occipital lobe,
2. parietal lobe,
3. frontal lobe,
   and
4. temporal lobe.
5. You may also see some texts that include a limbic lobe, which is a part of the
   brain concerned with determining your emotional state, and that’s buried deep within the brain. But
   that’s less common. It’s much more common to refer to these four.

Question 28

Do lobes act independently?

Answer 28

What’s important to note here is that no single function of the body is concerned with any one lobe.

So if we look at the frontal lobe here, for example, this has many functions.

But one of the main ones
is movement, since much of the motor cortex resides within the frontal lobe.

But for effective movement control, you also need the activity of other lobes.

For example, the
parietal lobe is very important in receiving feedback from the rest of the body, or the skeletal
musculature of the body, to perform effective motor functions.

So motor functions can be regarded
as spread over both of these lobes.

Similarly, the parietal lobe has important roles in determining your interpretation of what you see.

So that indicates that the parietal lobe has multiple functions.

The occipital lobe also is important in
vision, houses the primary visual cortex.

So the take-home message here is that you cannot allocate
a single function to any one lobe, and no one lobe has just one function.

Question 29

Cortex
Cerebral hemispheres
Nuclei
Basal Ganglia
Hippocampus
Diencephalon
Thalamus
Hypothalamus

Answer 29

When we look at the outside of the brain, we’re looking at the cortex, the outer layer.

But there’s
more to the cerebral hemispheres than the cortex.

Buried deep within it are other structures, and
we refer to groups of cell bodies within the nervous system as nuclei.

Now this has nothing to do with the term used to house your genetic material within a cell.

In
neuroanatomy, a nucleus is just a group of cell bodies of similar function, and buried deep within the
cerebral cortex, we have a number of nuclei.

Some here, for example, are nuclei of the basal ganglia, a very important group of cells involved in
movement control.

There are also various small nuclei dotted around that aren’t shown on this figure,
but also associated with the basal ganglia and other parts of the brain.

In particular, the one that’s not shown here is the hippocampus.

That would be sitting deep in the
temporal lobe.

We can’t really see that.

That’s an important nucleus, concerned, amongst other
things, with memory.

In addition to these nuclei, at the top of the brain stem is the diencephalon, a group of nuclei of
great range of functions.

This one you’ve met already.

This is the thalamus.

I mentioned that that was
important for motor and sensory relay.

Below it is the hypothalamus, under the thalamus.

The hypothalamus, an important structure,
amongst other things, concerned with autonomic control such as reproductive behaviour, thirst,
measuring glucose levels, a number of other important homeostatic functions.

Question 30

What are we talking about when we say nuclei?

Answer 30

Groups of cell bodies within the nervous system - e.g. the hippocampus is a nuclei of the brain.

Question 31

What is the Diencephalon?

Answer 31

division of the forebrain (embryonic prosencephalon), and is situated between the telencephalon and the midbrain (embryonic mesencephalon). It consists of structures that are on either side of the third ventricle, including the thalamus, the hypothalamus, the epithalamus and the subthalamus.

Question 32

What are the ascending connections to the cortex?

Answer 32

Now the connections to the cortex are many.

Firstly, we have ascending connections.

Sensory
connections.

These are coming up, largely, from the thalamus.

Remember its important role as a
relay.

So, sensory information from the body enters the spinal cord, and is then transferred via the
thalamus to parts of the cortex, in particular, the parts of the cortex responsible for processing
sensory information, the somatosensory cortex.

In addition, the special senses I mentioned also go through various components of the thalamus - all
of them, in fact, except smell.

Smell - olfaction - is the most primitive of all our senses, and this goes
directly into the olfactory cortex, and it’s the only sense which doesn’t go through the thalamus.

There is a component that goes through the thalamus, but that’s just to tell us whether we like or
don’t like the particular odour.

It doesn’t tell us what that odour is.

This route directly into the olfactory cortex with very little processing reflects the very primitive
nature of olfactory senses.

Question 33

What are the descending connections from the cortex?

Answer 33

Now
these are mainly motor, to the spinal cord, largely through the corticospinal tract, and to nuclei within
the brain stem.

If they the terminate on the nuclei of cranial nerves, motorcranial nerves, to innervate
facial muscles - muscles of the head and neck - they will be going via the corticobulbar tract.

There
are also pathways concerned with higher levels of motor control, and go to the basal ganglia and the
cerebellum.

.

Question 34

What are the cortical projections to the limbic system?

Answer 34

And finally, there’s projections to the limbic system.

This is the part of the brain concerned with
determining our emotional states, and there are large areas of the cortex that impact on this.

What
you can see will have an impact on emotional state, so there’s projections from the visual cortex
to the limbic system.

What you’re physically doing can have an impact on your emotional state, so
you’ll have projections from the motor cortex to the limbic system.

In fact, there are multiple cortical
projections to the limbic system in determining our emotional state and emotional behaviour.

Question 35

What are the connections within the cerebral cortex?

And what are “association files”?

Answer 35

Now importantly, there are also connections within the cerebral cortex.

If they occur on the same side, the same cerebral hemisphere, we refer to them as association files.

So for example, this can link parts of the auditory cortex with the visual cortex to help us determine what we’re seeing and how we recognise what it is.

We might be getting visual as well as auditory clues, for example.

Or,
connecting taste and smell senses in appreciating exactly the nature of
something that we’re eating.

Question 36

What senses go through the thalamus and which doesn’t but which part of it does?

Answer 36

In addition, the special senses I mentioned also go through various components of the thalamus - all
of them, in fact, except smell.

Smell - olfaction - is the most primitive of all our senses, and this goes
directly into the olfactory cortex, and it’s the only sense which doesn’t go through the thalamus.

There is a component that goes through the thalamus, but that’s just to tell us whether we like or
don’t like the particular odour.

It doesn’t tell us what that odour is.

Question 37

What are the connections between the hemispheres of the cortex?

What is a commissure? What is the largest commissure?

Answer 37

We also have connections between the hemispheres.

So between, for example, the somatosensory
cortex on one side of the brain with the somatosensory cortex on the other.

This is via tracts called
commissures– commissures are just the names for pathways that connect one side of the brain with
the other, and the most prominent of these that you may have come across is the corpus callosum,
the large band of white matter that you can see in a hemisection of the brain.

Question 38

What’s an association file?

Answer 38

So let’s just have a little look at these, and they are shown here. The association fibres are shown in
blue, and you can see them linking different parts of the brain on one side.

Question 39

What is a commissure?

Answer 39

The commissures cross the midline. Here, for example, is the big commissure of the corpus
callosum. Although it’s only showing a little bit of it here, it will be radiating out, such that equivalent
areas of the cortex on one side are connected to equivalent areas on the other, literally letting
one side of the brain know what the other side is thinking or is doing. Inferiorly, we have another
commissure, the anterior commissure, which does very similar things, but for the more ventral parts
of the brain. Here they’re shown connecting the two temporal lobes.

Question 40

What are projection neurons?

Answer 40

In addition to the association and commissural neurons, we also have projection neurons, and these
are neurons that extend, typically, long distances, and frequently connect structures from the brain
to the spinal cord, or to the spinal cord from the brain.

These are shown in red. Here, they’re running
through a structure called the internal capsule, which is the main pathway that some major ascending
and descending axons make.

Question 41

What is the internal capsule?

Answer 41

the main pathway that some major ascending

and descending axons make.

Question 42

What are the three of the major classes of neurons

that we will find connecting cortical structures to other parts of the brain?

Answer 42

So these projection fibres will include motor fibres that are descending from the motor cortex down
to the spinal cord, and will also include ascending somatosensory fibres that are bringing information
in from the spinal cord and taking it to the cortex. So these are three of the major classes of neurons
that we will find connecting cortical structures to other parts of the brain: association, commissural,
and projection neurons.

Question 43

Tractography

Diffusion tensor tractography

Answer 43

Now a great deal of advances have been made recently in neuroimaging, and one of the major
advances is in the field of tractography.

This has really helped us understand much more about
the ways in which parts of the brain are connected to another, and in particular, a technique called
diffusion tensor tractography– it’s a type of magnetic resonance imaging, a type of MRI– that relies
on looking at the ways in which water diffuses through structures.

It’s particularly useful in enabling us to image myelin.

So we can plot individual pathways.

Here, the so-called seed point– part has been labelled– in the right frontal lobe– and we can follow the axons that
are present here as they enter the corpus callosum.

Remember, this is a commissural neuron that
crosses the midline– and here are the extensions of these axons from the right frontal lobe across to
the left frontal cortex.

And you can see them ramifying out, very beautifully.

Multiple pathways can be
labelled in this way, and then imaged, using computer graphics to build up a pattern of tracts within
the brain.

This is an example of computer-enhanced diffusion tensor imaging, where a large number of
pathways have been labelled.

You can see some which are crossing from one side to the other.

Commissural neurons, commissural axons and others, which are coming down here, entering
the pons - and in fact, if we follow these purple ones down, we’d see them going below.

Examples
of projection neurons here.

This approach has been used both in surgical planning, and in
understanding, increasingly, how different parts of the brain are speaking to each other.

They’re
helping us greatly understand how the brain is connected, how the parts of the brain are connected,
and therefore giving us bigger clues into how it’s functioning.

Question 44

Summary of Week 1 Topic 2

Answer 44

So let’s summarise. What have we seen?

1. We’ve seen that we can divide the nervous system by
   functional and anatomical criteria.
2. We have learned that the CNS and PNS can be divided up, and
   that a good definition of the CNS is an axon or a neuron which is wholly contained within it, and that’s
   perhaps a better one than simply saying a brain or spinal cord neuron.
3. We’ve seen that the CNS can be divided up into the hindbrain, midbrain, and forebrain, with the
   spinal cord below also being a part of the peripheral nervous system.
4. We’ve seen that the hindbrain
   consists of the pons, medulla, and cerebellum.
5. We’ve seen that we can derive a large amount of the very complex circuitry in the brain by means
   of neural networks, and this is a property of both the very large numbers of neurons we are dealing
   with, together with the properties of convergence, whereby a neuron can receive multiple signals,
   as well as divergence, whereby a single neuron can innovate and form synapse with multiple other
   neurons.
6. And finally, we’ve seen that we’re starting to learn more about how these structures are put
   together with some recent advances in neuroimaging. Thank you