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.

The Cells That Make Us Who We Are

Neurons are specialized cells that convey sensory information into the brain and spinal card and produce all the things we do

Motor Neuron

 1) Basic Structure:  Motor Neuron carries commands to the muscles and organs

 (a)The soma, or cell body, contains the nucleus and carries out basic cell activities

 (b) Dendrites receive information from ether neurons

 (c) Axons carry information to other neurons (or muscles or organs)

 (d) A sheath of myelin surrounds each axon

 (e) Branches at the end of the axon terminate in swellings called axon terminals

  (f) The connection between two neurons is called a synapse

Other Types of Neurons:

(a) Sensory neurons bring information into the central nervous system from the outside world

(b) Interneurons connect neurons over short distances

(c)  Projection neurons are similar to interneurons, but here longer axons and cover greater distances

The Neural Membrane

A cell's membrane is made of fat and protein

Holds a cell together

Controls the environment within and around the cell

The Resting potential refers to the difference in electrical charge between the inside and outside of an inactive neuron

The Action Potential is a brief depolarization of the neural membrane that enables the neuron to communicate

 Occurs in the axon

 Sodium channels open, sodium ions flow into the cell,  making the electrical charge of the cell more  positive

 Once the cell has depolarized the sodium channels close,  and potassium channels open. The  positive charge of the cell moves the potassium out. As potassium leaves, the  cell recovers its  resting potential

An Action Potential operates according to the All or None law meaning they occur  at full strength or not at all

Action potentials are Non‑decremental, meaning they do not lose intensity or strength as they progress down the axon

Refractory Periods

 Refractory Periods are periods of reduced or zero responsiveness

  Absolute Refractory Period  - the neuron cannot fire aain because the sodium channels  cannot reopen

Relative  Refractory Period occurs after the absolute refractory period. The neuron can fire but only with a stronger than threshold stimulus

Rate Law: Due to the relatively refractory period, the neuron's firing rate is  proportional to stimulus intensity 

 Myelination and Conduction Speed

  Gaps between pieces of myelin are called nodes of Ranvier

 Action potentials jump from one Node of Ranvier to another in a very rapid form of transmission called Salutatory Conduction 

 Other Glial Functions

(a) Guide new neurons to target cells during fetal development

(b) Later provide physical support for neurons

(c) Probably help regulate activity in neurons

(d) Contribute to development and maintenance of connections between neurons at synapses

How Neurons Communicate with Each Other

Chemical Transmission at the Synapse: When the potential arrives at the  presynaptic terminal calcium ions enter, causing the release of  neurotransmitter from vesicles across the synaptic cleft onto the  postsynaptic membrane 

Postsynaptic Integration

 A neuron receives input from about a thousand neurons and the input of many      neurons is required to initiate an action potential, so the postsynaptic neuron must combine these inputs

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

 Temporal summation combines potentials  arriving a short time apart

Terminating Synaptic Activity

Most neurotransmitters are taken back into the terminals in a process called reuptake

Enzyme   Acetylcholine is broken down by acetylcholinesterase, an enzyme.  The acetate and choline are then taken back into the terminals

 Excess transmitter is also absorbed by glial cells

     Many drugs exert their effect by controlling the amount of  neurotransmitter  in the synapse

Synaptic Modulation

     At axoaxonic synapses a. third neuron r

releases transmitter onto the terminals of the presynaptic neuron, producing presynaptic excitation  or presynaptic inhibtion (often by regulating the amount of Ca2 entering the terminal

    Autoreceptors on the terminals sense the amount of neurotransmitter in the  synaptic cleft and the terminal adjusts its output of transmitter

   Postsynaptic receptors increase in number or sensitivity to compensate for increases  or decreases in neurotransmitter

Neurotransmitters

1) Having a variety increases the effects that can be produced

2) Some neurons can release at least 4 neurotransmitters, including both excitatory and inhibitory transmitters

Computer Models of Neural Processing

1) Computers are good serial processors, humans are efficient parallel processors

2) The human brain is considered to be substantially more powerful than the world's most powerful computer, but computers are gaining ground rapidly

(a) An artificial neural network is a group of simulated neurons that carry out cognitive‑like functions

(b) The researcher trains the network by presenting it with a series of inputs and giving it the correct output to compare with its own output

(c) Such networks work like the brain in several ways and may help us understand the brain