Brain and Behavior Ch. 3

Dendrites

long, branching extensions from the cell body of a neuron. dendritic trees can take on many shapes and sizes. Dendrites are specialized for collecting information from thousands of tiny chemical signals that they receive all along their extent. dendrites collect information and pass it on to the soma.

Multipolar neurons

neurons with multiple dendrites, these are the most common class of neurons

Microglia

One of the four main types of glia. Make up 20% of the glial cell population, these small cells are the front line of immune defense in the central nervous system: they are constantly on the move, searching for any infections agents that might damage normal neural tissue. When they detect a foreign body, they consume and destroy it to prevent disease and inflammation

Three types of neurotransmitter clean-up after brief binding to receptors. neurotransmitter presence in the synaptic cleft is quickly and precisely cleaned up.

1) Degradation: the neurotransmitter molecule is broken apart by other molecules 2) Diffusion: the neurotransmitter moves out of the synapse, down its chemical concentration gradient 3) Reuptake: specialized protein transporter in the membrane will selectively pull the neurotransmitter back inside the cell, presynaptically, postsynaptically, or often, into the neighboring cells. Reuptake is by far the most common for most small neurotransmitters.

Population coding / Coalition

A new understanding emerged when researchers began to see that systems of simple units - when strung together properly - could display remarkable emergent properties. For example, the representation of an object, such as a face, can be distributed across neurons in a population, and that distributed representation is both a realistic and a flexible strategy for the nervous system. In the framework of population coding, recognition of something - say, your neighbor or college professor - is achieved by a coalition of neurons: a group of some hundreds or thousands of neurons temporarily working together as a team. A given neuron might participate in many different coalitions depending on the task and the occasion.

Metabotropic receptors

Also known as a "second-messenger-coupled" receptor. To highlight some general principles, consider one well-studied family of such receptors, the G-coupled protein receptor. G-proteins are associated with the inside face of the postsynaptic membrane, and their function is to relay information from neurotransmitter receptors to other proteins inside the cell. These in turn relay, amplify, and transform the signal. Because so many receptor types are G-protein coupled, this allows the cell to develop sophisticated signaling cascades that integrate several signals from the outside. The second messengers triggered by metabotropic receptors can serve many different functions, modulating the activity of neighboring ion channels, activating or deactivating enzymes within the cell, or changing which genes are expressed within the cell. These metabotropic receptors are a large and important family of receptors. The effects of metabotropic receptors tend to operate on a much slower time scale than ionotropic receptors.

Electrical synapses (gap junctions)

Although chemical transmission is the overwhelmingly common form of signal transmission at synapses, another mechanism also exists: electrical synapses, also known as gap junctions, allow the direct passage of an electrical signal from one cell to the next. Such connections often allow the synchronized spiking of groups of neurons. Electrical synapses are far less common, and their function is less understood.

Action potential (often called nerve impulse or spike)

An electrode placed near a neuron reveals that from time to time the voltage across the neuron's membrane suddenly reverses and then, about a millisecond later, is abruptly restored. This is known as an action potential, often called a nerve impulse or spike. Spikes are all or none, meaning that they either happen or they do not, and they are always the same size.

Inhibitory postsynaptic potential (IPSP)

If neurotransmitter binding causes the potential difference in voltage between the inside and outside of the cell to grow larger (that is, the inside to become even more negative), this change in voltage is known as an inhibitory postsynaptic potential. This can occur by allowing positively charged potassium to flow out of the cell or by allowing negatively charged chloride ions to flow into the cell, or other combinations of ions.

Afferent neuron

another term for sensory neurons, or incoming neurons that receive signals (remember a for arrive)

Depolarized / threshold / axon hillock

If the number of excitatory potentials overwhelms the number of inhibitory potentials, this can drive the voltage of cell toward more positive values, making it increasing depolarized. If the cell voltage reaches a threshold, typically about -60 mV, something special happens: an action potential is generated at the axon hillock, the part of the axon that connects to the soma. The axon hillock is the most excitable part of the neuron and therefore the location where spikes are initiated.

Something to keep in mind about neurotransmitters

It is not the neurotransmitter molecule itself that is excitatory or inhibitory, meaning that they cause positive or negative changes in the membrane voltage, but it is the action of the receptor that determines the effect.

synaptic vesicles

Just inside the membrane of a presynaptic cell, the neurotransmitter molecules are packaged inside small spherical packages called synaptic vesicles. The release of the neurotransmitter into the extracellular space occurs when the vesicle fuses with the outer membrane and the molecules spill out into the cleft.

Bipolar neurons

Neurons composed of a single dendrite on one end and a single axon on the other. these neurons are often found in sensory neurons such as the retina and inner ear.

Monopolar neurons

Neurons that have only a single extension that leaves the soma and branches in two directions. one end of a monopolar neuron receives the information and the other end serves for output. this type of neuron is typically found in sensory neurons that signal touch and pain.

Myelin sheaths

Not continuous along the length of an axon, but instead come in short segments, like a string of sausages.

Schwann cells

One of the four main types of glia. Perform a very similar function to oligodendrocytes by the process of myelination of axons, yet schwann cells are only found in the peripheral nervous system. Also, Schwann cells wrap myelin around only a single axon.

Oligodendrocytes

One of the four main types of glia. These are large cells whose main function is to wrap a layer of "insulation" around axons, the process of which is known as myelination, similar to the way that a copper wire is wrapped in rubber. Myelination speeds up the electrical signaling by neurons. A single oligodendrocyte wraps the axons of up to 50 different neurons. Oligodendrocytes are only found in the central nervous system.

Astrocyte

One of the main four types of glia. Named for its star shape. Provide structural support, but also perform critical functions in maintaining the balance of chemicals outside the neurons, the repair of injury in the central nervous system, the contribution of nutrients, the regulation of local blood flow to a region, and the release of chemical signals.

Soma

Otherwise known as cell body of the neuron. The soma contains the cell's nucleus, which is the control center of the cell that regulates cell activity, including gene expression. the soma plays a key role in integrating the signals coming in from the dendrites.

Axon

Otherwise known as the nerve fiber, this is a single, long slender process emerging from the soma. It is essentially a cable to conduct signals rapidly across long distances.

Peptide neurotransmitters

Peptides are short strings of amino acids. Neuropeptides include cholecystokinin, somatostatin, and neuropeptide Y as examples.

chemical transmission between neurons

Released chemicals are called neurotransmitters. The neurotransmitter is released by the presynaptic cell and, by diffusing from its point of release, is felt as a change of chemical concentration at the postsynaptic target.

Retrograde transmitters

Soluble gases that can carry signals in the brain. They are often produced in the dendrites of one cell and crossing the synapse backwards, to affect the axons of the presynaptic cell.

Axon terminals

Sometimes called axonal boutons or buttons, a typical axon will branch robustly at its end, typically splitting into about 10,000 of these so-called axon terminals. The terminals are identifiable as small swellings at the end tips and they contain packages of chemicals that can be released into the space between cells. The terminals are optimized for the output of signals. Axon terminals are typically found in close proximity to the dendrites and somas of other cells.

Receptors

Specialized proteins in the membrane of a cell. When the neurotransmitter molecules are released into the synaptic cleft, they exert their effects by binding to receptors. Overwhelmingly most common type of receptors are postsynaptic receptors, though they can be located on presynaptically or on neighboring cells.

membrane potential

The difference of voltage inside and outside of the cell due to the different concentrations of ions. Depending on the charge of the ions and in which direction they flow, the movement of ions across the membrane can make this potential difference smaller or larger.

Nodes of Ranvier

The gaps between myelinated segments of an axon

Local coding

The idea that all stimuli in the outside world become represented uniquely by different neurons. For example, activity in a given neuron may represent a particular geometrical shape, whereas another neuron might represent a particular animal and another still might represent a more complex stimulus, such as Ben Roethlisberger throwing a football.

Rate coding

The idea that neurons encode stimuli by the number of action potentials in a small window of time is called rate coding. In this framework, neurons are specialized to detect certain stimuli, and the detection consists of a train of spikes. Because spike trains in a neuron can be elicited as a function of the specific properties of external stimuli, encoding is based on the average rate of firing over some time interval.

Synapse

The junction (very very small) between the axon terminals of one neuron and the dendrites of another neuron (or muscle fiber, etc). The synapse links an axon to other neurons (in the central nervous system) or to a neuron, muscle, or gland (in the peripheral nervous system). Although most synapses occur at the axon terminals, they can also exist along the axon itself, and in this case they are known as en passant synapses.

synaptic cleft

The little space between the pre and postsynaptic cells. Very small, typically just 20-50 nanometers across. this small distance allows the concentration of neurotransmitters to rise and decay rapidly.

Saltatory conduction

The noncontinuous skipping of the spike (action potential) as it travels down a myelinated axon. The action potential is regenerated at each node, but not at the insulated stretches in between. The length of the myelination segments is just short enough that the depolarization at one node will be large enough to open the Na channels at the next node. This form of conduction vastly increases the speed of travel, and decreases energy expenditure.

Glial cells, or glia

The other main type of cell in the brain that is important, the first being neurons. the full functional capacity of the glial cell is still under intensive study and not fully known, though these cells are known to play several roles, providing ways to speed up the signaling from neurons, regulating the concentrations of extracellular chemicals, and determining the extent to which networks of neurons can modify their connections.

Temporal summation / Spatial summation

The small voltage changes collected in the dendrites (EPSPs and IPSPs) travel along the dendritic membrane to the cell body, where all the branches come together. Although the postsynaptic potentials are small, they can add up with one another in two ways. First, signals that arrive at the soma at the same time (or even close to the same time) will add up when they reach the soma - this is known as temporal summation. Second, signals that arrive on different branches of the dendrites will converge at the soma - this is known as spatial summation. As a result of both kinds of summation, the soma has the opportunity to integrate signals flowing into disparate parts of the dendrites. Excitatory and inhibitory postsynaptic potentials add up like a simple math problem. Two EPSPs will sum to a larger voltage change, whereas an EPSP and an IPSP arriving at the same moment will cancel each other our. Because the soma receives hundreds or thousands of such signals at any moment, the total voltage of the cell is determined not by any one incoming signal, but instead by the overall pattern of all the inputs received all overr the cell, both excitatory and inhibitory.

Ionotropic receptors

There are different concentrations of ions (charged particles) inside and outside the cells; thus, if you were to poke a hole in the membrane, ions would tend to flow in or out. An ionotropic receptor is essentially a sophisticated way of opening a temporary pore in the membrane. In its closed state, the receptor protein blocks the flow of ions; when it is opened, or gated, by the right type of neurotransmitter, the protein changes its shape and provides a pore in the membrane. Many ionotropic receptors allow only a particular type of ion to pass through; thus a receptor that binds the neurotransmitter GABA tends to selectively pass chloride ions, whereas a receptor that binds acetylcholine may selectively pass sodium ions, etc.

Differences between axons and dendrites

There is only one axon coming from a neuron, whereas there can be many dendritic extensions. Second, axons tend to remain constant in diameter all along their length, whereas dendrites are tapered. Finally, axons tend to be much longer than dendrites: dendritic trees rarely extend more than 3 mm, whereas axons carrying signals from your spinal cord to your big toe run the entire length of your leg.

Acetylcholine

This molecule serves as an excitatory neurotransmitter in the peripheral nervous system, causing muscle contractions when released at the junction between the nervous system and the muscular system.

How an action potential travels

Two ions play hey roles in making an action potential: sodium (Na+) and potassium (K+) (there are many ions and proteins involved in actuality, but the concept can be understood with these two). When a cell is at rest, there is a high concentration of Na+ on the outside of the cell and a much lower concentration on the inside; this is exactly the opposite for K+ ions. When the membrane potential rises beyond a certain threshold, it triggers the opening of voltage-gated ion channels, in this case voltage-gated Na+ channels - ion channels that selectively pass Na+ and are opened only at particular voltages across the membrane. When these channels open, Na+ ions suddenly find a way into the cell. These ions are driven in by both the concentration gradient (there are many more on the outside than on the inside) and the electrical gradient (the inside of the cell is more negatively charged than the outside of the cell, attracting the positively charged Na+ ions into the cell). Why doesn't the axon become permanently depolarized and stay there? Because voltage-gated K+ channels are not far behind in their action. The influx of Na+ depolarizes the membrane further, which triggers the opening of the K+ channels. Now, K+ ions flow down their concentration gradient (that is, there are more on the inside than on the outside, so they will tend to flow out). Because the K+ ions are positive and because they are rushing out of the cell, the inside becomes more negative - that is, is repolarizes. This return to a negative voltage shuts the voltage gated Na+ channels and ends the swing in voltage. This exchange of ions causes a voltage spike at the axon hillock, but how does an action potential travel? The answer is that the rapid voltage change gives just enough time and spreads far enough down the membrane for neighboring voltage-gated Na+ channels to open up, causing the same cycle of ion exchange to happen nearby. In this way, the cycle of depolarization and repolarization moves down the axon.

Excitatory postsynaptic potential (EPSP)

When positive ions, such as sodium, flow through a receptor into the cell (slightly reducing the difference between inside and outside, because the outside of a cell is often more positive than the inside), this is known as an EPSP.

Refractory period

Why action potentials don't travel in both directions: This is because there is a short refractory period after an action potential, during which the Na+ channels are more resistant to opening. As a result, the action potential cannot move back to a location where it has already occurred, but can only travel forward.

monoamines

a class of neurotransmitters that includes dopamine, epinephrine, norepinephrine (all known as catecholamines), serotonin, and melatonin. Dopamine, as an example, serves as the critical information-carrying molecule in the brain's reward systems and is the target of drugs of addiction such as cocaine and amphetamines. Dopamine is also the main neurotransmitter implicated in schizophrenia.

Efferent neuron

another term for motor neurons, or outgoing neurons that transmit signals (think e for exit)

Neuron

specialized cells in the brain. they have a membrane, nucleus, and specialized organelles and they produce, traffic, and secrete chemicals. because of the particular proteins on their surfaces, they can transmit electrical signals quickly over long distances. and when those electrical signals arrive at their end point(s), they trigger a specialized form of chemical signaling.

Amino acids

the building blocks of proteins, also a class of neurotransmitters. Glutamate is the most common excitatory neurotransmitter in the central nervous system. Aspartate is another excitatory amino acid neurotransmitter, whereas GABA and glycine are common inhibitory neurotransmitters.