Donette Steele, M.A. / Clinical Psychology

Communication within the Nervous System

CHAPTER 2

COMMUNICATION WITHIN THE NERVOUS SYSTEM

 

The Cells That Make Us Who We Are

How Neurons Communicate with Each Other

 

THE CELLS THAT MAKE US WHO WE ARE

Neurons are specialized cells that convey sensory information into the brain, carry out the operations involved in thought and feeling and action, and transmit commands out into the body to control muscles and organs. 

 

The most prominent part of the neuron is the cell body or soma.

The nucleus is the largest organelle inside the cell body and contains the cell’s chromosomes.

Dendrites are extensions that branch out from the cell body to receive information from other neurons. 

The axon extends like a tail from the cell body and carries information to other locations. 

 

Branches at the end of the axon culminate in swellings called end bulbs or terminals.

The terminals contain chemical neurotransmitters, which the neuron releases to communicate with a muscle or an organ or the next neuron in a chain.

 

There are three major kinds of neurons and variations within those types. 

A motor neuron carries commands to the muscles and organs.

A motor neuron’s axon and dendrites extend from the cell body, which is why it is called a multipolar neuron.

 

Sensory neurons carry information from the body and from the outside world into the brain and spinal cord.

A sensory neuron can be unipolar with a single short stalk from the cell body that divides into two branches, or bipolar with an axon one side of the cell body and a dendritic process on the other.

Interneurons connect one neuron to another in the same part of the brain or spinal cord.

 

The most critical factor in the neuron’s ability to communicate is the membrane that encloses the cell.

The membrane holds the cell together, and controls the environment within and around the cell.

Some molecules, like water, oxygen, and carbon dioxide, can pass through the membrane freely. 

Many other substances are barred from entry.

Still others are allowed limited passage, through protein channels that open and close under different circumstances. 

 

This selective permeability contributes to the most fundamental characteristic of neurons, polarization, which means that there is a difference in electrical charge between the inside and outside of the cell. 

A difference in electrical charge between two points is also called a voltage.

 

The difference in charge between the inside and outside of the membrane of a neuron at rest is called the resting potential. 

This is typically around -70 mV.

The resting potential is due to the unequal distribution of electrical charges on the two sides of the membrane. 

The charges come from ions, atoms that are charged because they have lost or gained one or more electrons. 

 

Sodium ions (Na+) and potassium ions (K+) are positively charged.

Chloride ion (Cl-) are negative, and so are certain proteins and amino acids that make up the organic anions (A-).

The fluid outside the neuron is high in Sodium (Na+) and Cloride (Cl-) ions, while there is more Potassium (K+)  and Organic Anions (A-) on the inside. There are two forces that work to balance the location of the ions. Because of the concentration gradient, ions move through the membrane to the side where they are less concentrated.  As a result of the electrical gradient ions are attracted to the side that is oppositely charged.

 

In the neuron’s resting state, both the sodium and potassium channels are closed, but a few ions still trickle through.

These ions that make it through are returned by the sodium-potassium pump, which consists of large protein molecules that move sodium ions through the cell membrane to the outside and potassium ions back inside. Its exchange rate of three sodium ions for every two potassium ions helps keep the inside of the membrane more negative than the outside.

 

An excitatory signal causes a partial depolarization, which means that the polarity in a small area of the membrane is shifted toward zero.  This partial depolarization disturbs the ion balance in the adjacent membrane, so the disturbance flows down the dendrites and across the cell membrane. 

A partial depolarization is decremental – it dies out over distance – for this reason it is often called the local potential. 

 

If the local potential exceeds the threshold for activation of that neuron it will cause the normally closed sodium channels in that area to open, which triggers an action potential.

The action potential is an abrupt depolarization of the membrane that allows the neuron to communicate over long distances.

 

Opening the sodium channels allows the sodium ions in that area to rush into the axon at a rate 500 times greater than normal.

A small area inside the membrane becomes fully depolarized to zero; the potential overshoots to around +30 or +40 mV, making the interior at that location temporarily positive.

At the peak of the action potential the sodium channels close, so there is no further depolarization, and about the same time, the potassium channels begin to open.

The positive charge inside the membrane and the force of the concentration gradient combine to move potassium ions out; this outward flow of potassium ions returns the axon to its resting potential. 

 

The action potential causes nearby sodium channels to open as well, triggering a new action potential right next to the first one.

That action potential triggers another farther along, creating a chain reaction of action potentials that move through the axon.

Thus, a signal flows from one end of the neuron to the other.

 

The action potential differs in two important ways from the local potential that initiates it.

First, the local potential is a graded potential, which means that it varies in magnitude with the strength of the stimulus that produced it. 

The action potential, on the other hand, is ungraded; it operates according to the all-or-none law, which means that it occurs at full strength or it does not occur at all. 

A second difference is that the action potential is non-decremental; it travels down the axon without any decrease in size, propagated anew and at full strength at each successive point along the way.

 

Right after the action potential occurs, the neuron goes through the absolute refractory period, a brief time during which it cannot fire again.

This occurs because the sodium channels cannot reopen.

During the relative refractory period the neuron can be fired again, but only by a stronger than threshold stimulus.

This is due to the potassium channels remaining open for a few milliseconds following the absolute refractory period, and the continued exit of potassium making the inside of the neuron slightly more negative than usual.

The axon encodes stimulus intensity not in the size of its action potential but in its firing rate, an effect called the rate law.

 

Glial cells are non-neural cells that provide a number of supporting functions to neurons.

Glial cells produce myelin, a fatty tissue that wraps around the axon to insulate it from the surrounding fluid and from other neurons.

Myelin is produced in the brain and spinal cord by a type of glial cell called oligodendrocytes, and in the rest of the nervous system by Schwann cells. 

 

The gaps in the myelin sheath are called nodes of Ranvier.

Action potentials jump from node to node in a form of transmission called saltatory conduction.

Myelination and the resulting saltatory conduction increase conduction speed.

 

During fetal development one kind of glial cell forms a scaffold that guides new neurons to their destination. 

Later on, glial cells provide energy to neurons and respond to injury and disease by removing cellular debris.

Others contribute to the development and maintenance of connections between neurons.

 

HOW NEURONS COMMUNICATE WITH EACH OTHER

In the 1800s, Camillo Golgi developed a new tissue-staining method that helped anatomists see individual neurons by randomly staining some entire cells without staining others.

With this technique the Spanish anatomist Ramón y Cajal (1937) was able to see that each neuron is a separate cell.

HOW NEURONS COMMUNICATE WITH EACH OTHER

The connection between two neurons is called a synapse.

The neurons are not in direct physical contact at the synapse but are separated by a small gap called the synaptic cleft.

The neuron transmitting to another is called the presynaptic neuron.

The receiving neuron is the postsynaptic neuron.

 

HOW NEURONS COMMUNICATE WITH EACH OTHER

The German physiologist Otto Loewi isolated the hearts of two frogs and applied electrical stimulation to the vagus nerve attached to one of the hearts, which made the heart beat slower.

He then extracted salt solution from that heart and placed it in the second heart.

If the neurons used a chemical messenger, the chemical might have leaked into the salt solution.

The second heart slowed too, just as Loewi expected.

 

Then he stimulated the accelerator nerve of the first heart, which caused the heart to beat faster.

When he transferred salt solution from the first heart to the second heart, this time it speeded up.

So, Loewi had demonstrated that transmission at the synapse is chemical, and that there are at least two different chemicals that carry out different functions.

 

At chemical synapses, the neurotransmitter is stored in the terminals in membrane-enclosed containers called vesicles.

When the action potential arrives at the terminal it opens channels that allow calcium ions to enter the terminals from the extracellular fluid.

The calcium ions cause the vesicles clustered nearest the membrane to fuse with it; the membrane opens there and the transmitter spills out and diffuses across the cleft.

 

On the postsynaptic neuron the molecules of neurotransmitter dock with specialized chemical receptors that match the molecular shape of the transmitter molecule and cause channels in the membrane to open.

Ionotropic receptors open the channels directly to produce immediate reactions required for muscle activity and sensory processing.

Metabotropic receptors open channels indirectly and slowly to produce longer-lasting effects.

 

 

Hypopolarization is excitatory and facilitates the occurrence of an action potential.

Hyperpolarization is inhibitory and makes an action potential less likely to occur.

If the receptors open sodium channels, this produces hypopolarization of the dendrites and cell body, which is an excitatory postsynaptic potential (EPSP).

Other receptors open potassium channels, chloride channels, or both; as potassium moves out of the cell or chloride moves in, it produces a hyperpolarization of the dendrites and cell body, or an inhibitory postsynaptic potential (IPSP).

 

The output of a single neuron is not enough by itself to cause a postsynaptic neuron to fire; the postsynaptic neuron must combine potentials from many neurons in order to fire.

These potentials are combined at the axon hillock in two ways:

Spatial summation combines potentials occurring simultaneously at different locations on the dendrites and cell body.

Temporal summation combines potentials arriving a short time apart.

 

The neurotransmitter’s story does not end when it has activated the receptors.

Typically the transmitter is taken back into the terminals by a process called reuptake; it is repackaged in vesicles and used again.

At some synapses, transmitter in the cleft is reabsorbed by glial cells.

Transmitters can also be broken down by an enzyme that splits the molecule into its components.

 

A nervous system that controls complex behavior must have several ways for regulating itself.

One of the ways is axoaxonic synapses, which  result in presynaptic excitation or presynaptic inhibition, which increases or decreases, respectively, the presynaptic neuron’s release of neurotransmitter onto the postsynaptic neuron.

 

Autoreceptors on the presynaptic terminals sense the amount of transmitter in the cleft; if the amount is excessive, the presynaptic neuron reduces its output.

Glial cells also contribute to the regulation of synaptic activity by preventing neurotransmitter from spreading to other synapses and sometimes absorbing the neurotransmitter in the synaptic cleft and recycling it for the neuron’s use.

 

Having a variety of neurotransmitters multiplies the effects that can be produced at the synapses.

The fact that there are different subtypes of receptors adds even more.

 

For decades neurophysiologists labored under the erroneous belief, known as Dale’s principle, that a neuron was capable of releasing only one neurotransmitter.

We learned only fairly recently that many neurons ply their postsynaptic partners with two to four and perhaps even more neurotransmitters.

 

Artificial neural networks, which consist of simulated neurons that carry out cognitive-like functions, learn how to perform the task like we do, by trial and error.

These networks mimic human and animal behavior surprisingly well.

The real networks are very complex and inaccessible, but the artificial networks allow us to test hypotheses experimentally.

 

Communication within the Nervous System
Chapter 2

 

 

How neurons communicate with one another

The Cells That Make Us Who We Are

  How many are there?

  Neurons: 100 billion

÷ Make up 10% of brain volume

  Glial cells: Many more!

÷ Make up 90% of brain volume

  Neurons:

  convey sensory information to the brain;

  carry out the operations involved in thought and feeling;

  transmit commands out to the body to control muscles and organs.

  Cell body or soma

  Contains the nucleus

÷ The nucleus contains the genetic material (DNA).

  Contains organelles in cytoplasm

÷ These convert nutrients to fuel, construct proteins, and remove waste.

  Dendrites

  Extensions that branch out from the cell body

  Receive information from other neurons and sensory cells

  Axon

  Extends some distance from the cell body

  Carries information to other neurons and to organs

 

 

The Cells That Make Us Who We Are

  Terminals or end bulbs are swellings found at the end of the axon.

  Neurotransmitters are chemical substances found inside the terminals.

  The neurotransmitters communicate with:

  other neurons;

  muscles;

  organs.

 

 

 

  Three major types of neurons that vary by shape:

  Multipolar neuron

  A multipolar neuron (or multipolar neurone) is a type of neuron that possesses a single (usually long) axon and many dendrites, allowing for the integration of a great deal of information from other neurons. These dendritic branches can also emerge from the nerve cell body. Multipolar neurons constitute the majority of neurons in the brain and include motor neurons and interneurons

 

  Unipolar neurons have but one process from the cell body. However, that single, very short, process splits into longer processes (a dendrite plus an axon). Unipolar neurons are sensory neurons - conducting impulses into the central nervous system.

  Bipolar neurons have two processes - one axon & one dendrite. These neurons are also sensory. For example, biopolar neurons can be found in the retina of the eye.

 

  Unipolar neurons have but one process from the cell body. However, that single, very short, process splits into longer processes (a dendrite plus an axon). Unipolar neurons are sensory neurons - conducting impulses into the central nervous system.

 

  Bipolar neurons have two processes - one axon & one dendrite. These neurons are also sensory. For example, biopolar neurons can be found in the retina of the eye.

 

  Three major types that vary by function

  Motor neuron

÷ Motor neurons are multipolar.

  Sensory neuron

÷ Either bipolar or unipolar.

  Interneuron

÷ Multipolar

 

The Cells That Make Us Who We Are

  The most critical factor in the neuron’s ability to communicate is the cell membrane.

  Composition of the cell membrane

  Lipid molecules and proteins make up the cell membrane.

  Lipid molecule heads are attracted to the fluid in and outside the cell; the tails are repelled by liquid.

  Some of the lipids orient their heads toward the extracellular fluid and some orient toward the intracellular fluid.

  This creates a double-layer membrane.

  The cell membrane holds the cell together and controls the environment in and around the cell.

Cell Membrane of a Neuron


The neuron’s membrane varies in permeability.

  Water, oxygen, and carbon dioxide pass freely

  Many other substances are barred from entry.

  Others can pass through protein channels in the membrane under certain circumstances.

  Polarization results from this selective permeability of the membrane.

  Polarization is the difference in electrical charge between the inside and outside of the cell.

  This difference in electrical charge is referred to as a voltage.

 

The Cells That Make Us Who We Are

  The resting potential is the difference in charge between the inside and outside of the membrane when the neuron is at rest.

 

  Excitatory signals received by the neuron cause a partial depolarization, or hypopolarization, in a small area of the membrane.

  The depolarization is caused by a change in ion balance, which also affects the adjacent membrane.

  This spreading depolarization diminishes over distance, so it is often referred to as a local potential.

  At the axon, depolarization that reaches threshold (around -60 mV) will cause sodium channels to open, triggering an action potential.

 

  The action potential is an abrupt change in membrane potential that allows the neuron to communicate over larger distances.

  Sodium ions enter the open sodium channels due to force of diffusion and electrostatic pressure.

  The rate of entry of sodium is 500 times greater than normal.

  The result is complete depolarization.

  Polarity “overshoots” to around +30 to +40 mV, making the interior of the cell temporarily positive.

  Sodium channels close at the peak of the action potential and no further depolarization is possible.

 

  Near the peak of the action potential, potassium channels open.

  Potassium ions move out due to diffusion.

  Potassium also moves out due to electrostatic pressure because the inside of the cell is temporarily positive.

  The membrane returns to near resting potential.

  This entire event takes approximately 1 millisecond.

  Because nearby sodium channels open, a new action potential is triggered at the adjacent patch of membrane.

  When the action potential reaches the terminals it passes the message on to the next cell “in line”.

  The local potential is a graded potential, but the action potential follows the all-or-none law.

  All-or-none means that the action potential always occurs at full strength and does not vary with stimulus intensity.

  The action potential also is nondecremental; it does not decrease over distance.

  This makes it possible for the message to travel over long distances.

  Absolute Refractory Period

  When the sodium channels close during the action potential, that part of the axon cannot fire again.

  This limits how frequently the neuron can fire.

  This also prevents backward spread of depolarization, so action potentials move only toward the terminals.

  Relative Refractory Period

  Potassium channels remain open; continued K+ outflow polarizes the membrane beyond the resting potential.

  A stronger stimulus is required to trigger an action potential.

  Rate law: Stronger stimuli trigger new action potentials earlier in recovery, so the axon encodes intensity as rate of firing.

  Neurotoxins affect ion channels involved in the action potential.

  The  (fugu)  fish produces tetrodotoxin, which blocks sodium channels.

  Scorpion venom keeps sodium channels open, prolonging the action potential.

  Glial cells produce myelin, a fatty tissue that surrounds axons, providing insulation and support.

  CNS: oligodendrocytes make the myelin.

  PNS: Schwann cells make the myelin.

  Myelin increases the conduction speed in axons.

  Myelinated axons have gaps called nodes of Ranvier; action potentials occur only at the nodes.

  Transmission between nodes (under the myelin) is by local potential, which moves faster than action potentials and uses less energy.

  Because the action potential “jumps” from node to node this is called saltatory conduction.

  Multiple sclerosis is a disease in which myelin is destroyed, resulting in reduction of conduction speed.

 

Myelin from Glial Cells
Other Glial Functions

  During development, glial cells provide scaffolds for neurons to migrate to their final destinations.

  Glial cells also respond to injury and disease by removing cellular debris.

  Glial cells provide energy to neurons.

  When glial cells are present, neurons make seven times as many connections with other cells

  As behavioral complexity increases, the ratio of astrocytes (a type of glial cell) to neurons also increases.

 

  A synapse is the connection between the presynaptic neuron and postsynaptic neuron.

  The neurons are separated by a space or cleft.

  The earlier view was that the nervous system was a continuous web.

  Using Golgi staining, which stains only a portion of neurons, Cajal was to see that each neuron is an individual cell.

  At that time, physiologists also believed neurons used electric current to communicate across the synaptic cleft.

 

How Neurons Communicate With One Another

  Loewi’s Discovery of Chemical Transmission

  Loewi isolated two frog hearts (heart A and heart B).

  Stimulating the vagus nerve of  heart A slowed the heart.

  Salt solution collected from heart A caused heart B to slow.

  Stimulating the accelerator nerve caused heart A to beat faster.

  Transferring this salt solution caused heart B to beat faster.

  Loewi concluded that each nerve released a different chemical into the fluid—one excited the heart and one inhibited it.

  This experiment demonstrated synaptic transmission by chemical neurotransmitters (though electrical transmission also occurs).

  Transmitters are stored in vesicles in the axon terminals.

  When the action potential arrives at the terminals, calcium channels open and calcium enters the cell.

  This causes vesicles to fuse with the membrane, which results in release of transmitter into the cleft.

  Transmitters cross the cleft and influence receptors on the postsynaptic neuron.

 

 

How Neurons Communicate With One Another

  Activation of receptors on the postsynaptic cell has two possible effects on the membrane potential.

  Hypopolarization creates an excitatory postsynaptic potential (EPSP).

÷ This makes the postsynaptic neuron more likely to fire.

  Hyperpolarization creates an inhibitory postsynaptic potential (IPSP).

÷ This makes it less likely an action potential will occur.

 

How Neurons Communicate With One Another

  Their effects can accumulate over a short time, an event referred to as temporal summation.

  Spatial summation combines inputs arriving at different locations on the dendrites and cell body.

  The neuron acts as

  an information integrator, by the processes of summation, and

  a decision maker, by combining excitatory and inhibitory inputs algebraically, determining whether to fire.

 

 

  Usually, the transmitter’s effects must be terminated to allow frequent responding and to prevent it from affecting nearby synapses.

  Reuptake takes the transmitter back into the terminals to be used again.

  Glial cells can reabsorb transmitters at some synapses.

  Acetylcholine is broken down by acetylcholinesterase.

  Some drugs work by enhancing or reducing transmitter effects. For example:

÷ some antidepressants block reuptake of transmitters;

÷ others prevent their degradation.

 

 

How Neurons Communicate With One Another

  Synaptic activity is regulated in several ways.

  One process occurs via axoaxonic synapses.

÷ Presynaptic inhibition decreases the release of transmitter.

÷ Presynaptic excitation increases the release of transmitter.

÷ This regulation occurs by affecting calcium entry into the terminal.

  Autoreceptors sense the amount of transmitter in the cleft and cause the presynaptic neuron to reduce excessive output.

  Glial cells

÷ prevent transmitter from spreading to other synapses;

÷ absorb and recycle transmitter for the neuron’s reuse;

÷ release glutamate to regulate presynaptic transmitter release.

 

 

  Having a variety of transmitters multiplies the effects that can be produced at the synapse.

  Different subtypes of receptors add even more complexity.

  For example, nicotine acts at nicotinic and muscarinic receptors for acetylcholine.

  Some neurons release more than one transmitter:

  a fast-acting transmitter and a slower-acting neuropeptide;

  or two fast-acting transmitters;

  or an excitatory transmitter and an inhibitory transmitter.

 

 

  Some representative neurotransmitters:

  Acetylcholine

  Monoamine transmitters

÷ Examples: dopamine, norepinephrine, and serotonin

  Amino acid transmitters

÷ Examples: glutamate, GABA, and glycine

  Peptide transmitters

÷ Examples: endorphins, substance P, neuropeptide Y

  Gaseous transmitters

÷ Examples: nitric oxide, carbon monoxide