Evolution of the human brain.
Brain architecture.
Neocortex.
Cerebrospinal fluid and blood supply.
The human brain, is like a city, with a long history.
It has its old sections which are ancient.
These ancient sections performs survival activities, required at that time.
We still retain those parts of the brain.
It has also newer sections, which developed around the older ones.
Finally, it has a modern section, which developed more recently,
and which are adapted to current functionality, of the modern human being.
It is fascinating to trace the development of the brain,
in conjunction with the evolution of human beings, from ancient forms of life.
Primitive forms of life, also had some type of primitive brains.
As more complex forms of life developed, a more complex brain also developed.
The more complex layers developed, on the foundation of the primitive layers.
Just like our DNA, carries our evolutionary history,
our brain also carries our evolutionary history.
We share parts and functionality, of our brain, with many other forms of life.
The evolution of the human brain, can be discussed broadly in these phases.
The reptilian brain.
The limbic brain.
The neocortex.
The reptilian brain first appeared in fish.
This happened nearly 500 million years ago.
The amphibians developed, a more advanced version,
of this brain, about 250 million years ago.
The limbic system first appeared, in small mammals,
about 150 million years ago.
Lastly, the neocortex began its spectacular evolution, in primates.
This happened 2 to 3 million years ago.
We homo sapiens evolved from the primates.
Our brains carry a 500 million years history of design, inside it.
It is amazing, how nature never reinvents the wheel.
For the brain, once a design was satisfactory, it carried it forward,
and built more advanced versions, over it.
The oldest part of the brain can be compared to the reptilian brain.
It controls the body’s vital functions, such as heart rate, breathing,
body temperature, etc.
Our reptilian brain, includes the main structures, found in a reptile’s brain.
They are:
The brain stem.
The cerebellum.
The limbic brain can be found in early mammals.
This part of the brain, records memories of behaviours,
that result in agreeable and disagreeable experiences.
Evolution favoured this kind of memory.
The mammals learnt to avoid disagreeable experiences,
and seek agreeable experiences.
What we call as emotions, in human beings, had its origin in this part of the brain.
The main structures of the limbic brain are :
The hippocampus.
The amygdala.
The hypothalamus.
The limbic brain is the seat of value judgements, we make.
These judgements are often unconsciously made,
and exert a strong influence on our behaviour.
Some of the primitive emotions, like fear are triggered,
in human beings, by the limbic brain.
The neocortex first assumed importance in primates.
This evolution culminated in the development of the human neocortex.
The neocortex has evolved significantly, from the first human beings,
to modern man.
It is still evolving.
The neocortex has two large cerebral hemispheres, which is responsible,
for all our higher level of thinking.
It is involved in human language, abstract thought, imagination and consciousness.
The neocortex has almost infinite learning abilities.
The neocortex has played a significant role, in the development of human culture.
The size of the neocortex has increased tremendously in primates.
The size of the neocortex has some correlation with the abilities of the brain.
Mammals which had to actively hunt other animals for food,
tend to have a more developed neocortex, compared to herbivorous animals.
This seems to give them an advantage, since it requires, a significant amount of,
cerebral activity to hunt.
The size of the brain, is not the criteria, to determine the state of evolution,
of a species of life.
Elephants and whales, have brains which are 4 to 5 times the size of a human brain.
Yet we have reason to believe that we human beings are more evolved,
than these large animals.
A larger body surface, might simply mean, that more sensory neurons are required.
The size of the brain alone, does not determine its complexity and capability.
Another physical factor that differentiates a developed brain,
is the amount of folds in the cerebral cortex.
The cerebral cortex, is folded like a frilly frock.
We wear this frilly frock, like a monkey cap in the brain.
In fact monkeys were one of the species, which developed a more advanced neocortex.
The more frilly it is, the more is the surface area of the cerebral cortex.
The more the surface area, the more is the capability for higher order thinking.
We human beings can boast of having the most frilly, cerebral cortex.
The frilly nature of the cerebral cortex, increases its surface area.
More the surface area, the more the neurons, it can accommodate.
More importantly, it can form many more connections.
When the number of connections, that the neurons can make increases,
the capacity and capability of the brain also increases.
For example, social skills like language is highly evolved in human beings.
From a evolutionary view point, a lesser brain, does not mean,
a useless form of life.
All forms of life, have their rightful place, in the web of life.
Each form of life, has a brain adapted to its needs.
The differences in the evolution, of different parts of the brain,
has a greater effect on behaviour, than just the brain size.
We can take the cerebellum as an example.
The cerebellum is part of the brain, involved in co-ordinating muscle movements.
The weight of the cerebellum, as a percentage of the brain’s total weight,
is remarkably constant in mammals, including humans.
In contrast the percentage of the neocortex varies significantly among species.
In fish and amphibian brains, there is no neocortex.
In shrews, a mole like mammal, the neocortex is about 20% of the brain’s weight.
In humans, the neocortex accounts for 80%, of the brain’s weight.
No wonder, we consider ourselves very intelligent.
It was during the transition, from primates to human beings,
that the neocortex under went the greatest development.
Within the neocortex, the prefrontal cortex, is the one part,
that expanded most in human beings.
The cortex is the thin layer of neurons,
that covers the surface of each cerebral hemisphere.
From the evolutionary perspective, the neocortex developed in three phases.
The archicortex.
The paleocortex.
The neocortex.
A large cortex contains a large number of neurons.
What increases most in mammals, is the number of connections,
between these neurons.
Neurons connect to each other with axons.
These axons are called white matter.
The amount of white matter grows steadily, from rats to human beings.
Over the course of evolution, the cerebral cortex,
has grown considerably in surface area, but very little in thickness.
The neocortex forms a significant component of the cortex.
The surface area of the human cortex, is about 1000 times,
the surface area of the cortex, of a rat.
When we trace our evolution, we are closer to monkeys.
The surface area of our cortex, is ten times that of monkeys.
In humans, the cortex is only 15% thicker than monkeys,
but the surface area is about 10 times more.
When we come to chimpanzees, however our neocortex,
is much closer to the size of chimpanzees.
Scientists believe that the superior abilities of humans,
can be attributed to specialised regions of the cortex,
and to the denser interconnections between the prefrontal cortex,
and the rest of the brain.
We seem to have built a superior corporate office, in the prefrontal cortex.
We were able to connect and put together, many things that animals could not do.
Even though our cerebellum, is comparable to mammals,
we can play a game like tennis, due to the interaction of the neocortex,
with other parts of the brain.
The main reason that the prefrontal cortex, is larger to the rest of the brain in humans,
is because we have a larger volume, of white matter,
in our prefrontal cortex.
The axons in this white matter, provide us with far greater connectivity,
between the prefrontal cortex, in the rest of the brain.
The connectivity is essential for the proper functioning of our working memory.
Working memory is involved in many cognitive abilities,
which are highly developed in humans.
When performing a task, working memory helps to,
verify the relevance of the information,
keep the objective of the task in mind, at the same time.
We can make quadrillions of connections, in our brain.
This we believe, is what makes us unique and human.
Many scientists think that the expansion of the neocortex,
might be explained by mutations in a limited number of genes,
at early stage of development.
These mutations would have resulted in the duplication of certain areas,
of the cortex.
This is analogous to a similar phenomena that is observed,
in certain genes, in the genome.
As in the case of genes, one of the possible advantage, of this duplication,
would be that particular cortical region, could evolve more rapidly,
while its duplicate continued to perform, the same basic function,
originally assigned to it.
The prefrontal cortex seems to contain, an especially large number of long axons,
which can make connections, between various regions of the cortex,
that are far from one another.
The larger the prefrontal cortex, the more longer axons it contains,
and the more likely consciousness is to emerge.
Brain imaging also shows that the prefrontal cortex, is highly active,
when tasks of memorisation, and deductive reasoning, are being performed.
Many scientists also believe that the properties, of the frontal part of the brain,
have a great deal to do with social skills.
These skills help us to develop mutual assistance, and cooperation.
It helps to strengthen group cohesion, and work towards the larger goal,
of social good.
This indeed is good news.
Human beings are the largest social group, among all forms of life.
Hopefully, this will help us evolve, into far better human beings.
The brain functions as a whole.
It is constituted of many parts.
Interestingly many parts of the brain, specialise in some specific functions.
This is analogous to a modern organisation.
Different department in the organisation, specialise in specific functions.
These departments in the brain interact with each other.
All the departments work together, to achieve the organisation’s goal.
The parts of the brain, also interact.
They communicate through neural channels, with the other parts of the brain.
This way the brain is able to function as a whole.
Some anatomically different regions of the brain, can be involved in multiple functions.
It is very useful to know, the architectural layout,
and functions of the regions, in the brain.
It will help us, try to understand the holistic function of the brain.
The brain can be viewed, as having six broad regions.
Medulla.
Pons.
Midbrain.
Cerebellum.
Diencephalon.
Cerebral hemispheres, also known as the Telencephalon.
The brain like the rest of the body, is bilateral.
It has two hemispheres.
Sometimes, it is referred to, as the left brain, and the right brain.
Each of the main regions of the brain, is present in both hemispheres.
The medulla is a direct extension of the spinal cord.
We can think of it, as the beginning of the brain region, of the central nervous system.
The medulla contains important nuclei,
involved in the control of,
breathing, blood pressure, and heart rate.
It is also involved in mediating taste, hearing, and maintenance of balance.
The pons is located, between the medulla, and the midbrain.
The cerebellum is located, behind the pons.
Nuclei in the pons, relay information, about movement and sensation,
from the cerebral cortex, to the cerebellum.
The pons also has nuclei, involved in breathing, taste, and sleep.
The midbrain is at the top of the brain stem.
Nuclei in the midbrain provide important linkages of the motor system,
particularly in the cerebellum, basal ganglia, and cerebral hemispheres.
One example, of the nuclei is the substantia nigra.
It provides important inputs, to the basal ganglia.
The basal ganglia is a collection of nuclei,
which regulates voluntary movements.
Some nuclei related to auditory, and visual systems are also located here.
Some nuclei are related to muscles, that control eye movement.
The cerebral peduncles are also located in the midbrain.
These structures are formed by massive projections, from the cerebral cortex,
to target nuclei, in the brain stem.
The medulla, the pons, and the midbrain,
are collectively referred to, as the brain stem.
Groups of related neurons, are called as nuclei.
Many basic life functions, are regulated by the brain stem.
The brain stem, is the site of origin, for most of the cranial nerves.
The spinal cord mediates sensation, and motor control, of the body trunk and limbs.
The brain stem mediates sensation, and motor control, of the head, neck, and face.
The twelve cranial nerves, are functionally analogous, to the thirty one spinal nerves.
The brain stem, is the site of entry, for information, from many specialised senses,
such as hearing, balance, and taste.
Nuclei in the brain stem mediate parasympathetic reflexes,
like cardiac functions, blood pressure, digestion, and contraction of the pupils.
The brain stem houses the motor and sensory pathways,
to and from the brain.
The reticular formation, is located in the brain stem.
The reticular formation, is a collection of nuclei, distributed in the brain stem.
The core of the brain stem, comprises of neuron cell bodies.
They are called as the reticular formation.
The reticular formation has a large number of interneurons.
They integrate information from all regions, of the central nervous system.
The reticular formation is also responsible, for the output,
of a great deal of neural information.
Most reticular formation neurons send axons for considerable distances,
up or down the brain stem, and beyond, to most regions of the brain, and spinal cord.
The reticular formation conveys information, to the upper brain.
There are two branches in the path way, from the reticular formation.
One branch goes to the thalamus.
The thalamus acts like a router in the brain.
The thalamus relays information, to different parts of the brain.
The second branch goes to the base of the fore brain.
Some of the fibres continue to the cerebral cortex.
The cerebral cortex, is the part of the brain, with the greatest thinking capability.
The reticular formation, also provides the connection,
between the spinal cord and the cerebellum.
The fibres that pass through the spinal cord,
form the reticulospinal pathways.
They influence both the afferent and efferent neurons.
Some reticular formation neurons are clustered together.
They form a nuclei, and act as integrating centres.
These centres are involved in :
Cardio vascular activity, like heart beat.
Respiratory activity, like breathing.
Some reflex activities, like swallowing, and vomiting.
The reticular formation also has nuclei, which are involved in:
Eye movement.
Reflex orientation of body, in space.
Reticular formation neurons can also be organised,
according to the neurotransmitters they use, namely,
Serotonin.
Noradrenaline.
Dopamine.
Cranial nerves are primarily nerves, which convey information,
from the head and neck areas, to and from the brain.
The nerves that comes from the eyes, ears, nose, etc, are cranial nerves.
The nerves that we use to control facial muscles, are also cranial nerves.
Most of the cranial nerves, have no need to go through the spinal cord,
instead they connect to the brain stem.
The brain stem also contains nuclei, which is involved in the processing of information,
from most of the cranial nerves.
The cerebellum is located behind the brain stem.
It plays an important role, in maintaining posture.
It is involved in coordinating head, eye and arm movements.
The cerebellum is the centre for learning motor skills.
It receives somatic sensation from the spinal cord.
It receives balance information from the vestibular organs.
It also receives, motor and sensory information, from the cerebral cortex.
It integrates all this information into intelligent coordinated muscle activity.
We can imagine that playing tennis, or classical dancing would involve,
training of the cerebellum.
Diencephalon.
The diencephalon is the collective term,
for the thalamus and the hypothalamus.
The thalamus acts like a router of sensory information.
It receives information from the sensory organs,
and relays it to sensory regions, in the neocortex.
The neocortex is the main part of the cortex.
The thalamus interconnects the cerebellum and the basal ganglia,
with regions of the cerebral cortex, concerned with movement and cognition.
The thalamus plays a major role, in the integration and processing of motor,
and sensory information, to higher brain centres.
It is likely that, along with the reticular formation,
it influences attention and consciousness.
The hypothalamus is located below the thalamus.
It is relatively quite tiny.
However, it is a crucial component for life.
It is involved in homeostatic regulation of the body.
The hypothalamus is connected to the pituitary gland.
The pituitary gland produces, and releases hormones into the blood stream.
Hormones are chemical messengers, to the other glands in the body,
which regulate their behaviour.
The hypothalamus is involved in transducing electrical signals, from the brain,
to chemical signals, in hormones.
The hypothalamus has many critical functions like:
Regulation of water balance.
Regulation of autonomic nervous system.
Regulation of eating and drinking behaviour.
Regulation of reproductive system.
Reinforcement of certain behaviours.
Regulation of circadian rhythm.
The hypothalamus is involved in somatic growth,
eating, drinking, and maternal behaviour.
The hypothalamus has extensive afferent and efferent connections,
with most regions of the central nervous system.
It is thus able to influence functioning of these regions.
The hypothalamus is central to the motivational systems of the brain.
It plays a role, in avoiding unpleasant events,
in seeking out rewarding events.
Cerebral hemispheres.
The cerebrum comprises of two cerebral hemispheres.
The cerebral hemispheres are the largest part of the human brain.
They consists of the :
cerebral cortex,
the underlying axons or white matter.
The basal ganglia.
The amygdala.
The hippocampal formation.
The cerebral hemispheres has perceptual, motor and cognitive functions.
They are involved in memory and emotion.
The corpus callosum, is a large bundle of axons,
that typically link similar regions, in the left and right side of the brain.
The corpus callosum links the left and right cerebral hemispheres.
The basal ganglia receives input, from all parts of the cerebral cortex.
The output is routed through the thalamus, to the frontal lobe.
Nuclei in the basal ganglia are involved in cognitive functions,
like learning motor skills.
The amygdala is located deep in the middle of the temporal lobe,
in front of the hippocampus.
It is involved in analysing the emotional, or motivational significance,
of sensory stimuli.
It receives input directly from the major sensory systems.
Axons from the neurons, in the amygdala, connect to:
The neocortex.
The basal ganglia.
The hippocampus.
Hypothalamus, etc.
The amygdala is involved in the expression of emotions.
When it senses danger, the amygdala automatically and unconsciously,
triggers multiple simultaneous responses.
For example, heart rate, respiration rate, and pupillary dilation.
It also results in the conscious emotional perception of fear.
This primitive evolutionary behaviour, has been carried forward,
to modern human beings.
When we react to a perceived threat, it is the amygdala,
which is responsible, for our unconscious response.
The hippocampal formation comprises of:
The hippocampus.
The dentate gyrus.
The subiculum.
Together these structures are responsible, for formation of long term memories,
about our daily experiences.
These memories are called episodic memories.
The spinal cord, brain stem, and diencephalon,
regulate many of the life sustaining functions.
The cerebral cortex, is involved in higher order thinking.
The cerebral cortex, is the outer most layer, of the cerebral hemispheres.
It is a very thin layer, about 2 to 4mm thick.
It is involved in most of the planning and execution,
of actions in everyday life.
The cerebral cortex is like a folded sheet.
It is highly folded, so as to fit in, into our skull.
The folds helps to significantly increase the surface area, of the cortex.
If the number of folds is more, more neurons can be fitted in.
More neurons in the cortex, provide for a greater capacity,
for processing information.
Highly evolved animals, including human beings, have a large number,
of folds, in the cerebral cortex.
The ridge of the fold, is called the gyrus, or gyri in plural.
The groove of the fold, is called the sulcus, or sulci in plural.
The cerebral cortex, is divided into four distinct lobes.
The frontal lobe, which is situated near our forehead.
The parietal lobe, which is situated on the top of our head.
The temporal lobe, which is situated in the sides, of our head, near our ears.
The occipital lobe, which is situated in the back of our head.
The lobes are separated by sulci or grooves.
The lateral sulcus separates the temporal lobe,
from the frontal and parietal lobes.
The central sulcus, in the middle of the head,
separates the frontal and parietal lobes.
The cerebral cortex, is divided into two cortices.
The two cortices, corresponds to the left brain, and the right brain.
The four lobes are present, in both cortices.
For example, we have a left temporal lobe, and a right temporal lobe.
The cell bodies of the neurons, are located in the cerebral cortex.
They are referred to as grey matter.
Myelinated axons emanate from these neurons, towards the inner regions.
These axons are referred to as white matter.
Each lobe has distinct functional subregions.
For example, the occipital lobe, has regions concerned with visual processing.
The cerebral cortex, is a most complex integrating area of the brain.
It is responsible for bringing together, all the afferent information,
into meaningful perceptual images.
It is also responsible for refinement of control over the motor systems,
which control the movement of the skeletal muscles.
Nerve fibres enter a particular site, in the cortex from a variety of places.
The thalamus, and the brain stem reticular formation,
is particularly very well connected to the cortex.
The thalamus and the brain stem, are important pathways to the cortex.
There are two other regions, which are considered as part of the cerebral cortex.
The cingulate cortex, surrounds the upper surface, of the corpus callosum.
The cingulate cortex, is involved in the regulation, of emotion and cognition.
The insular cortex, is buried under the frontal lobe and temporal lobe.
It is concerned with emotion.
It is also involved in the regulation of homeostasis.
Distinct functional regions, of the brain are connected, by discreet pathways.
The corpus callosum, connects the left and right regions, of the brain.
The pyramidal tracts, connect the cerebral cortex, to the spinal cord.
The other pathways, in the brain are not readily visible.
Using sophisticated neuroanatomical tracing techniques,
scientists have traced most of the major pathways.
The brain is a highly interconnected organism, of hundred billion neurons.
Tracing all these connections, is a monumental job.
Scientists are working on the connectome project,
which aims to map all the connections, in the brain.
Cranial nerves are primarily nerves, which convey information,
from the head and neck areas, to and from the brain.
The nerves that comes from the eyes, ears, nose, etc, are cranial nerves.
The nerves that we use to control facial muscles, are also cranial nerves.
Most of the cranial nerves, have no need to go through the spinal cord.
The brain stem also contains nuclei, which is involved in the processing,
of information from most of the cranial nerves.
There can be two types of pathways, in the reticular formation.
Long neural pathways.
Multi neuronal pathways.
Long neural pathways carry information over longer distances.
The pathways that carry information, between the brain and the spinal cord,
are long neuronal pathways.
The pathway between the forebrain and the brain stem,
are also long neural pathways.
Long neural pathways, consist of very few inter connected neurons.
They therefore contain very few synapses.
There is very little alterations in the information transmitted,
in long neural pathways.
Multi neuronal pathways, have many neurons in the pathway.
This results in many synaptic connections.
There are many opportunities for further neural processing,
in multi neuronal pathways.
The neocortex is the outermost layer of the cortex.
It is also the most recent,
in the evolutionary development, of the brain.
The neocortex is organised into layers and columns.
This increases the computational efficiency of the cortex.
The neocortex receives inputs from the thalamus.
It also receives information, from other cortical regions of the brain.
The neocortex in both the hemispheres, of the brain, are well connected.
The output of the neocortex, reaches out to other regions of the neocortex.
Higher level processing, which involves association and integration,
of varied information, is facilitated by these horizontal connections.
The output of the neocortex, also goes to other interior regions of the brain,
and to the spinal cord.
The interior regions include:
The basal ganglia.
The thalamus.
The pontine nuclei.
The complex input-output relationships are very well organised,
in the columns and layers of the neocortex.
Each layer contains different inputs and outputs.
Most of the neocortex is organised in 6 layers.
They are numbered as layer 1 to 6.
Layer 1 is the outer most layer, and layer 6 is the deepest layer.
Layer 1, is occupied by the dendrites, of cells located in deeper layers.
Layer 1, also has the axons, that travel throughout this layer,
and connect to other cortical regions.
The axons of the pyramidal neurons, in layer 2 and 3,
connect to other neurons, in the same cortical area, and other cortical areas.
These layers are mainly involved, in intra cortical communication.
Layer 4, is the main recipient of sensory input, from the thalamus.
It is the most prominent, in the primary sensory areas.
For example, the primary visual cortex, has a heavily neuron populated, layer 4.
Layer 5, is a major output pathway of the cortex.
It projects to other cortical areas, and subcortical structures.
Layer 6, mainly carries axons to and from the cortex.
It blends with the white matter below the cortex.
Neurons in the neocortex, are organised as columns.
Each column is a fraction of a millimetre in diameter.
Neurons within a column, act like a local processing network.
They have similar characteristics.
It is possible that these thin columns, are the fundamental computation units,
of the neocortex.
The neurons in the cortex, can be broadly classified as:
Principal or projection neurons.
Local interneurons.
Projection neurons typically have pyramid shaped neurons,
called pyramidal neurons.
These neurons use glutamate as their excitatory neurotransmitter,
to convey information, to the next synaptic relay.
Local interneurons, have axons, close to their cell bodies.
They use GABA as an inhibitory neurotransmitter.
Interneurons, are located in all layers of the neocortex.
They typically receive inputs from the same source, as the principle cells.
The neocortex has also some excitatory interneurons.
They are located mainly in layer 4.
These excitatory interneurons are the primary, recipients of sensory information,
from the thalamus.
Three major subcortical regions are located deep within the cerebral hemispheres.
They are the:
Basal ganglia.
Hippocampal formation.
Amygdala.
These three subcortical structures, coordinate to regulate cortical activity.
The subcortical regions, are organised as nuclei, or collection of neurons.
The nuclei are further organised, as a collection of sub nuclei.
Like other cells in the body, neurons in the brain,
require oxygen, and nutrition.
A significant portion of the blood circulation, goes to the brain.
The brain also consumes a large amount of energy.
The brain literally floats in a fluid called the cerebral spinal fluid.
In addition to its blood supply, the CNS is perfused with a second fluid.
This fluid is called cerebrospinal fluid.
This clear fluid fills the cerebral ventricles, and the subarachnoid space,
which surrounds the brain and spinal cord.
The CNS literally floats in a cushion of cerebrospinal fluid.
The brain and the spinal cord, are very soft, delicate tissues.
They are protected by the fluid from sudden and jarring movements.
The cerebrospinal fluid is much more than a supporting fluid.
It is very selective about the substances, that it contains.
For example, it carefully regulates the concentration of potassium,
calcium, sodium, and chloride, that it contains.
These elements play a significant role in neural signalling mechanisms.
The cerebrospinal fluid circulates through the inter-connected,
ventricular system to the brain stem.
It has small openings to the brain and spinal cord.
Through these openings it flows, into the brain.
Glucose is an essential nutrient, for the energy requirement of the brain.
The brain does not store glucose.
It requires a continuous supply of glucose.
The blood circulation to the brain, provides a continuous supply of glucose.
The blood circulation also provides, a steady stream of oxygen supply, to the brain.
The human brain is only about 2% of our body weight.
It is the size of two closed fists, kept together.
Yet, it receives about 15% of the total blood supply.
It is estimated that it uses about 25% of the energy we consume.
Substances are exchanged from the blood to the neurons, in a special way.
The kind of substances that enter the extra cellular fluid,
and the rate at which they enter it, is highly controlled.
The blood brain barrier mechanisms control this.
This barrier protects the neurons, from potentially harmful substances.
Brain nutrients such as glucose, enter the brain rapidly,
by combining with membrane carrier proteins.
Similar transport systems also move surplus substances, out of the brain.