Directional terminology
In biology, directional terms
are given with respect to the
organism’s body axis.
These terms are important for
navigating creatures with
unusual body plans or
lifestyles.
Structure of the Nervous System
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Directional terminology
Coronal
section
Organization of the nervous system
• The nervous system is organized into
the central and peripheral divisions.
• Central Nervous System (CNS)
CNS
PNS
– Brain
– Spinal cord
Horizontal
section
• Peripheral Nervous System (PNS)
– Somatic Nervous System
– Autonomic Nervous System
Sagittal
section
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Organization of the nervous system
Organization of the PNS
• The PNS has two divisions, each of
which has its own sub-divisions:
• Somatic nervous system:
– Efferent (outgoing) nerves: Motor nerves
that connect the CNS to the skeletal
muscles.
– Afferent (incoming) nerves: Sensory
nerves that carry information from the
sense organs to the CNS.
• Autonomic nervous system: Regulates
homeostasis.
– Sympathetic nervous system (SNS):
Arousing. “Fight or flight.”
– Parasympathetic nervous system (PNS):
Calming. “Rest and digest.”
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Organization of the autonomic nervous system
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The ventricular system
The brain is cushioned and supported by a series of inter-connected
hollow spaces called “ventricles.”
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The ventricular system
The meninges & cerebrospinal fluid (CSF)
The ventricles are clearly visible in coronal and horizontal sections of
the brain.
• The brain and spinal cord are
protected by special membranes
called meninges.
Lateral ventricles
– When these membranes are infected,
meningitis results.
• Cerebrospinal fluid (CSF) fills the
ventricles and circulates around the
brain and spinal cord.
– Like plasma, CSF contains glucose,
various salts, and minerals, but unlike
plasma it contains very little protein.
– CSF possesses a similar density to the
brain. This allows the brain to float
comfortably in the skull.
IIIrd ventricle
Horizontal section
Coronal section
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The meninges & cerebrospinal fluid
A few millimeters of skin, muscle, and bone are all that separate the
brain from the dangers of the outside world.
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Blood-Brain Barrier
The tight gaps between endothelial cells (cells that line the blood
vessels) prevent large molecules from passing into the brain
One of the protective mechanisms of the nervous system; prevents
potentially lethal substances from entering the brain
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Two main types of brain cells
Brain Cells
• Neurons
– The basic information processing
units of the brain.
– Approximately 80 billion in the
human brain.
• Glial cells – “glue (G.)”
– Support and modulate neurons’
activities.
– Creates the myelin sheath.
– Approximately 100 billion in the
human brain.
Pyramidal neuron.
Santiago Ramón y Cajal – 1899
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Introducing the neuron
Neurons transmit information
• Neurons are specialized cells that are
specialized for transferring information
from one place to another.
• Neurons have specialized structures for
this purpose.
– Dendrites – “tree (G.)”
• Gather information from other neurons.
Dendrites
Cell body
Axon hillock
Axon
– Cell body
• Core region; contains the nucleus and DNA.
– Axon hillock
• Junction of the cell body and axon, where the
action potential begins.
Terminal button
– Axon
• Carries information to be passed onto other cells.
– Terminal button
• Knob at the tip of an axon that conveys
information to other neurons.
• Connects with dendrites of other neurons.
Golgi stained pyramidal neurons in the cerebral cortex.
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Neurons transmit information
Grey matter & white matter
• Axons, especially longer ones, are
usually covered with a myelin sheath.
• Grey matter
– Areas of the nervous system
composed of cell bodies and
blood vessels.
• Myelin is a fatty substance produced
by glial cells.
Grey matter
White matter
• Ex. Cerebral cortex, subcortical
nuclei, etc.,
Oligodendrocytes myelinate axons in the CNS
• Myelin insulates the axon, increasing
the speed and efficiency of electrical
signal conduction.
• White matter
– Areas of the nervous system rich
in fat-sheathed neural axons.
• Ex. Subcortical white matter, corpus
callosum, etc.,
• The fatty nature of myelin gives white
matter its color.
Coronal section
Schwann cells myelinate axons in the PNS
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Grey matter & white matter
• Grey matter
– Areas of the nervous system
composed of cell bodies and
blood vessels.
Groupings of axons: tracts & nerves
Grey matter
White matter
• Tract
– Large collections of axons
in the CNS are called
tracts.
– Tracts connect nuclei to
each other in the brain.
– White matter consists
mostly of tracts
(myelinated axons)
• Ex. Cerebral cortex, subcortical
nuclei, etc.,
• White matter
– Areas of the nervous system rich
in myelinated neural axons.
Axons in a tract
Axons in a nerve
• Nerve
• Ex. Subcortical white matter, corpus
callosum, etc.,
– A large collection of axons
forming connections in the
PNS.
Coronal section
• Examples: Phrenic nerve
(arising from C3-C5, it
controls the diaphragm).
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Groupings of cell bodies: nuclei
Groupings of cell bodies: cortex
• Nucleus (pl. nuclei)
• The brain’s outer layer of grey
matter is called the cerebral cortex
- “bark (L.)”
– A distinct cluster of neural cell bodies
(grey matter) forming a functional group.
Nuclei can be distinguished by their
structure, chemical composition, and
function.
• Examples: Ventral Tegmental Area (VTA),
Arcuate Nucleus (ARC) etc.,
– It is often just called ‘the cortex’ for
short.
Coronal section of mouse brain.
• The cerebral cortex is part of a
larger structure called the
cerebrum.
The cortex is the part we usually imagine when
picturing the brain.
Cell-dense areas are called nuclei.
• The cortex is the site of our higher
level functions, consciousness, and
many other important things.
Nissl staining
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The name `cortex’ comes from a word meaning
bark. Like bark on a tree, the cortex is the
outermost layer of the brain.
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Neurons form networks
Contrasting cortex with other brain regions
•
•
The cerebral cortex has a
characteristically wrinkled appearance.
•
This is a space-saving tactic. By
crumpling up, the cortex is able to fit more
grey matter into the same amount of
space.
Neurons in the cortex tend to
have a uniform, grid-like
organization.
– This is rather like how suburbs
are organized.
– The comparison makes sense
because both suburbs and the
cerebral cortex are the most
recently developed parts of
cities and brains respectively.
•
On the other hand, neurons in
subcortical and brainstem
nuclei tend to have a more
irregular organization.
– To continue the analogy, this is
like how the innermost parts of
old cities often have a
confusing and sporadic layout.
– The inner city, like the deepest
regions of the brain, shows
evidence of an ancient history.
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• Gyrus (pl. gyri) – “ring, circle (G.)”
– A bump or convolution between
grooves.
This suburb has an especially ‘cortical’ appearance. The
artificial peninsulas are constructed in such a way that the
maximum number of houses can have water frontage.
Gyrus
• Sulcus (pl. sulci) – “furrow, trench
(L.)”
Sulcus
– A groove between gyri.
Sulcus
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External features of the brain
The cerebral cortex
• Cerebrum - “brain (L.)”
• The cerebral cortex is divided into
two hemispheres, each of which
has four lobes.
– Major structure of the forebrain
consisting of two virtually identical
hemispheres (left & right).
• Frontal lobe
– Motor control.
– Executive functions.
• Cerebellum – “little brain (L.)”
– Involved in motor coordination, and
possibly other mental processes.
• Brainstem
– Comprises the deep structures of the
brain. Connects the brain to the
spinal cord.
– Critical for sustaining life (respiration,
blood pressure, etc.,)
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The corpus callosum
• Occipital lobe – “back of skull (L.)”
– Vision
• Parietal lobe – “wall (L.)”
– Touch sensation.
– Sense of self in space.
• Temporal lobe
– Auditory sensation.
– Language perception.
– Gustatory (taste) functions.
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The limbic system
• The limbic system – “edge/border
(L.)” – is important for emotions,
memories, and can be affected by
psychological disorders and drug
addiction. It consists of three major
parts.
• Cingulate cortex – “encircling/belt (L.)”
– Involved in emotional processing and
memory.
• Amygdala – “almond (G.)”
– Involved in fear, aggression, and
emotionally charged memories.
The corpus callosum - “firm body (L.)” - connects the two cerebral
hemispheres, allowing both sides of the brain to work together.
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• Hippocampus – “seahorse (G.)”
– Involved in the formation of long-term
memories.
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The basal ganglia
The brainstem
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•
The brainstem is divided into three basic
regions.
•
Diencephalon:
The basal ganglia are involved in
controlling movement. They are also
important in learning and memory, and in
particular learning “habits”. The basal
ganglia consist of three main parts:
•
The caudate nucleus – “tail (L.)” - and
putamen – “shell (L.)” - are together
known as the striatum – “striped (L.)”.
•
The globus pallidus – “pale globe (L.)”
•
The substantia nigra – “black substance
(L.)” - is part of the midbrain. It contains
numerous dopaminergic neurons that
project into the basal ganglia.
– Thalamus – “inner chamber (L.)”
• All sensory information (except smell) passes
through the thalamus en route to the cortex.
– Hypothalamus – “below/under (G.) thalamus”
• Controls homeostasis, regulates hormone secretion
from the pituitary gland.
•
– Contains neurons that produce dopamine that
project to various other brain regions.
•
Hindbrain:
– Pons – “bridge (L.)”
• Connects the cerebellum to the brainstem.
– Medulla – “marrow (L.)”
• Controls breathing & heart rate.
• Connects the brain to the spinal cord.
– Parkinson’s Disease involves the death of
DAergic neurons in the substantia nigra.
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Midbrain:
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A way to think about the brain
•
The brain, like everything else in the body, is a product of
evolution.
•
By its nature, evolution does not tend to remove old things
that still work.
– It may add new improvements on top of the old standbys, but
it typically leaves old things alone as long as they aren’t
causing any problems.
•
Newly evolved regions like the cortex sit atop older regions
(the limbic system), and those in turn sit atop even older
regions (the brainstem).
•
Inevitably, things do not work perfectly in this system.
– The brain was not engineered all at once – it was built up bit
by bit and the end result of this is a very fragile and rickety
machine.
•
As we will see, our brains and genes are not always wellsuited to the modern world. This is often the source of our
troubles…
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