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. Cell body (soma)

2. Dendrite

3. Axon

4. 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)

Recap1. 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 ThisIn 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.