Download Organization of Nervous System

Organization of Nervous System
Topics in this Section
1. How the brain is organized
2. How sensory information gets into the
nervous system.
Basic Unit of Nervous System
The neuron is the basic unit of the nervous
system.
Neurons are usually not actually connected to
one another. They are separated by a small
gap. This gap is called a synapse.
Four Main Parts
There are four main parts to the neuron:
1.
2.
3.
4.
Cell body (soma)
Dendrite
Axon
Axon terminal (bouton)
As I said, neurons are not usually connected
to one another. Therefore, they need some
way to communicate. The bouton contains
the chemicals that allow neurons to
communicate with each other.
Bouton
Neurotransmitters
The chemicals that allow one neuron to
transmit information to another neuron are
called neurotransmitters.
There are several kinds of neurotransmitters,
which we will discuss in later lectures.
Pre- and Post- Synaptic
In a given case, the neuron that is sending the
signal is called the presynaptic neuron. It is
the neuron before the synapse.
The neuron receiving the message is called
the postsynaptic neuron. It comes after the
synapse.
One final thing to notice, for the
neurotransmitter to have an effect, the
postsynaptic neuron must be receptive to
the neurotransmitter.
Receptors allow the postsynaptic neuron to be
receptive to the neurotransmitter.
Bouton
Basic Structure of Nervous
System
One can divide up the nervous system into
two main divisions:
Central Nervous System
Peripheral Nervous System
Central Nervous System
The two main components are the brain and
the spinal cord.
Cortex
Let us start with an overview of the outside of
the brain.
The outside of the brain is called the cortex (if
it helps, this comes from the Latin for “bark
of tree”)
Neocortex: the convoluted covering or the
cerebrum. The cortex is responsible for the
higher mental activities, complex problem
solving, etc.
One of the first things to notice is that the
cortex is not smooth. It is full of
convolutions.
Gyri and Sulci
The sulci are the grooves in the brain. The
gyri are the are the raised portions between
the grooves.
Researchers have used sulci and gyri, and
other features, as landmarks to divide the
cortex into different areas.
The largest of the areas are called lobes or
_____ cortex. The blank usually indicates
location or function.
Main Sulci (for now)
Central Sulcus
Lateral Sulcus
Main Lobes (Cortices)
Frontal
Motor
Somatosensory
Temporal (Auditory)
Parietal
Occipital (Visual)
Cerebellar (for the cerebellum)
Central Sulcus
The central sulcus is one important landmark.
It divides the motor cortex from the
somatosensory cortex.
Lateral Sulcus
The lateral sulcus separates the temporal lobe
from the rest of the cortex.
Importance of Division into
Different Cortices
The different cortices tend to perform
different functions.
This means that there is some localization of
function in the brain.
It will prove useful to notice that this
localization of function is not confined to
one, or even a small portion of the brain.
Examples of Localization
I want to discuss an example of localization to
illustrate some key points of how the brain
works:
Regular organization
“Cortical amplification”
Sensory and Motor Cortex
The motor cortex control the movement of the
limbs on the opposite side of the body.
The somatosensory cortex receives the
sensory information from the opposite side
of the body.
Homunculus
One important thing to note is that the
somatosensory and motor cortex are laid out
in a regular fashion.
The area of these cortices controlling the body
are in (roughly) the order they are in the
body.
Cortical Amplification
The other thing to notice is that areas are not
represented on the cortex by their actual
proportion of the body. The hands, for
example, are much smaller than the back,
but take up more space on the cortex.
Explanation
Cortical amplification relates to how many
neurons are necessary to control a given are
or how sensitive we are to sensory
stimulation in that area (which relates to the
number of neurons in that area of the body)
Recap
1. Main component of nervous system is the
neuron.
2. The neuron has four basic components: dendrite,
soma, axon, and bouton.
3. Neurons communicate with each other with
neurotransmitters.
4. The brain is divided up into functional areas
(e.g., somatosensory and motor cortex).
5. Cortical areas governed by regular organization
and cortical amplification.
Since we will cover them in later chapters, I
will only refer to the main subcortical
features.
The corpus callosum is one of the structures
that connects the two hemisphere of the
brain.
This means that they two halves share
information.
Limbic and other Structures
There are several important subcortical
structures that we should get a feel for,
because they will show up again.
Location of these Structures
There are several main brain
areas
Thalamus: this is a sensory relay station in
the brain.
Hypothalamus: Small structure located
under the thalamus which controls basic
survival needs (e.g., fighting, eating,
emotions).
Basal Ganglia: composed of many areas;
involved in movement.
Limbic System: involved in emotion.
Hippocampus: mostly associated with
memory.
Reticular Formation: arousal
Cerebellum: fine motor control
Spinal Cord and Brain Stem
There are several important structures in the
spinal cord and brain stem. We will discuss
them when we get to the relevant chapters.
Peripheral Nervous System
This is divided into autonomic and somatic.
We will focus, right now, on the autonomic.
Autonomic Nervous System
Involved in many aspects of arousal and
consciousness.
Can be simply understood by its role in
preparing the body to either fight or flee
from a scene (fight or flight).
Two Major Components of
Autonomic
Sympathetic: Prepares the body for action.
Parasympathetic: prepares the body to
conserve energy.
The peripheral nervous system influences
many organ systems.
Simplifying This
In order to simplify the effects of the
sympathetic and parasympathetic system on
a given organ, ask yourself this:
In order to prepare for a fight, what would the
sympathetic nervous system need to do?
Quite often, the parasympathetic does the
opposite.
How Neurons Work
Before we discuss how we can image the
brain, we need to discuss how neurons
work.
Action Potential
The most important concept is the action
potential.
The action potential is the signal from the
axon to the bouton that causes
neurotransmitters to be released.
For the moment, there are three ideas about
the action potential that are most important
and will crop up again.
1. The action potential is all-or-none
(digital).
2. The action potential causes electrical
discharges that can be detected on the
brain’s surface (by EEG).
3. Neurons cannot store much energy. So.
when they “fire” (have an action potential)
they require a sudden burst of energy.
Brain Activity Detection Tools
There are multiple way to discover what is
happening in a living brain.
Electroencephalograph (EEG)
Remember, the neuron gives off weak
electrical signals. These are informative
for two reasons:
1. They indicate states of consciousness.
2. They indicate what brain areas are active.
CT Scan
Measures density of tissue by X-ray
absorption.
This allows one to get a rough idea of what
the brain looks like.
Cerebral Blood Flow
As I indicated, neurons cannot store energy,
so they require a constant flow of blood. We
can use this to see what areas are active.
This involves introducing a tracer, such as a
radioisotope, and looking for absorption.
PET
Inject a radioactive substance that releases
positrons. These collide with electrons to
release gamma rays.
These are useful for comparing active blood
flow to blood flow at a resting state, since
one can measure the rate at which the
subject uses oxygen.
MRI
Measures the responses of magnetized protons
to pulses of radio waves.
This allows one to get good structural and
functional data.
Note how in the previous slide and in the one
on PET, specific areas responded to
particular stimuli.
This is another piece of support for
localization of function.
Two Issues You Can Answer
1. Do we use 10% of our brains?
2. Why doesn’t caffeine sober someone up?
Do we use 10% of our Brains?
The idea that we use 10% of our brains has
been used to claim that psychic powers are
possible to those who learn to use more. It
is somewhat a nice thought to think that
there is so much untapped brain potential.
Reasons Why Incorrect
1. Consider strokes and other brain damage.
Nearly wherever these occur they generally
cause damage proportionate to the size of
the area damaged.
2. A brain 10 times too big would have been
unlike to have evolved.
3. Looking at brain images, many areas of
brain lit up in normal tasks and nearly all of
it from one task or another.
In Our Left or Right Minds?
For free, you can also see the problem with
describing someone as “right brained” or
trying to selectively train one hemisphere
over the other.
The corpus callosum connects both
hemispheres and transfers information.
Caffeine and Alcohol
Caffeine is a stimulant, and alcohol is a
depressant. Therefore, many people think
that if you want to sober someone up you
can give them caffeine.
This does not work.
Why doesn’t caffeine sober
someone up?
The short answer is that caffeine and alcohol
affect neurons in different ways.
Remember I told you that neurons release
neurotransmitters that affect postsynaptic
cells?
One of these neurotransmitters is called
glutamate. It tends to increase the activity of
postsynaptic cells.
Presynaptic Receptors
As it turns out, there are also receptors on the
bouton itself. These receptors modulate the
release of neurotransmitters.
Adenosine is a neurotransmitter that acts on
the presynaptic receptor. It inhibits the
release of glutamate.
How Caffeine Works
Caffeine has its effect by blocking the
receptors for the neurotransmitter adenosine.
Thus, by inhibiting the inhibitor, the
original action can be completed—
Lots of glutamate gets released to excite the
postsynaptic neuron. This is what causes the
increased arousal.
How Alcohol Works
Alcohol affects many systems throughout the
brain. It interferes with the ability of
neurons to function. It also alters a type of
inhibitory receptor GABA to make it more
responsive. Since it is more responsive, it
causes the post-synaptic neuron to become
more inhibited. This is what causes the
nervous system depression.
So, as you can see, the two affect different
systems and different groups of neurons.
This is why giving caffeine to a drunk only
gives you an awake drunk.