Chapter 1

Question 1

Biological psychologists try to explain behavior in terms of

Answer 1

its physiology, development, evolution, and function.

Question 2

called the mind-brain problem or the mind-body problem

Answer 2

the question of how mind relates to brain activity; why are certain types of brain activity conscious?

Question 3

Chalmers and Rensch proposed

Answer 3

that we regard consciousness as a fundamental property of matter. A fundamental property is one that cannot be reduced to something else. For example, mass and electrical charge are fundamental properties. But consciousness isn’t like other fundamental properties. Matter has mass all the time, and protons and electrons have charge all the time. So far as we can tell, consciousness occurs only in certain parts of a nervous system, just some of the time- not when you are in a dreamless sleep, and not when you are in a coma.

Question 4

Biological psychology is

Answer 4

behavioral neuroscience; the study of the psychological evolutionary, and developmental mechanisms of behavior and experiences; Biological psychology is not only a field of study, but also a point of view. It holds that we think and act as we do because of brain mechanisms, and that we evolve those brain mechanisms because ancient animals built this way survives and reproduced. deals mostly with brain activity.

Question 5

two kinds of cells in the brain

Answer 5

the neurons and the glia. The activities of neurons and glia somehow produce an enormous wealth of behavior and experience.

Question 6

Neurons

Answer 6

• receive information and transmit it to other cells. they convey messages to one another and to muscles and glands, vary enormously in size, shape, and functions.
• The adult human brain contains approximately 86 billion neurons, the exact number varies from person to person.

Question 7

The glia

Answer 7

cells that enhance and modify the activity of neurons in many ways. generally smaller than neurons, have many functions but do not convey information over great distances.

Question 8

where does perception occur? give 2 examples

Answer 8

Perception occurs in your brain. When something contacts your hand, the hand sends a message to your brain. You feel it in your brain, not your hand. (Electrical stimulation of your brain could produce a hand experience even if you had no hand. A hand disconnected from your brain has no experience.) Similarly, you see when light comes into your eyes. The experience is in your head, not “out there.” You do NOT send “sight rays” out of your eyes, and even if you did, they wouldn’t do you any good.

Question 9

what is the relationship between mental activity and certain types of brain activity?

Answer 9

Mental activity and certain types of brain activity are, so far as we can tell, inseparable. This position is known as monism, the idea that the universe consists of only one type of being. (The opposite is dualism, the idea that minds are one type of substance and matter is another.)

Question 10

monomism

Answer 10

Mental activity and certain types of brain activity are, so far as we can tell, inseparable. Nearly all neuroscientists and philosophers support the position of monism. According to monism, your thoughts or experiences are the same thing as your brain activity. People sometimes ask whether brain activity causes thoughts, or whether thoughts direct the brain activity. According to monism, that question is like asking whether temperature causes the movement of molecules, or whether the movement of molecules causes temperature. Neither causes the other; they are just different ways of describing the same thing.

Question 11

Biological psychologists address four types of questions about any behavior.

Answer 11

• Physiological: How does the behavior relate to the physiology of the brain and other organs?
• Ontogenetic: How does it develop within the individual? - Evolutionary: How did the capacity for the behavior evolve?
• Functional: Why did the capacity for this behavior evolve? That is, what function does it serve, or did it serve

Question 12

main founders of neuroscience

Answer 12

Charles Sherrington, and the Spanish investigator Santiago Ramon y Cajal

Question 13

Camillo Golgi

Answer 13

found a way to stain nerve cells with silver salts. This method, which completely stains some cells without affecting others at all, enabled researchers to examine the structure of a single cell.

Question 14

Cajal’s research demonstrated

Answer 14

that nerve cells remain separate instead of merging into one another

Question 15

membrane (or plasma membrane)

Answer 15

The surface of a cell; a structure that separates the inside of the cell from the outside environment. Most chemicals cannot cross the membrane, but protein channels in the membrane permit a controlled flow of water, oxygen, sodium, potassium, calcium, chloride, and other important chemicals.

Question 16

nucleus

Answer 16

All animal cells have a nucleus, the structure that contains the chromosomes.

Question 17

A mitochondrion (plural: mitochondria)

Answer 17

the structure that performs metabolic activities, providing the energy that the cell uses for all activities. Mitochondria have genes separate from those in the nucleus of a cell, and mitochondria differ from one another genetically. People with overactive mitochondria tend to burn their fuel rapidly and overheat, even in a cool environment. People whose mitochondria are less active than normal are predisposed to depression and pains. Mutated mitochondrial genes are a possible cause of autism)

Question 18

Ribosomes

Answer 18

the site within a cell that synthesizes new protein molecules. Proteins provide building materials for the cell and facilitate chemical reactions. Some ribosomes float freely within the cell, but others are attached to the endoplasmic reticulum, a network of thin tubes that transport newly synthesized proteins to other locations.

Question 19

the most distinctive feature of neurons is

Answer 19

their shape- varies enormously from one neuron to another. Unlike most other body cells, neurons have long branching extensions. All neurons include a soma (cell body), and most also have dendrites, an axon, and presynaptic terminals. The tiniest neuron lacks axons, and some lack well-defined dendrites.

Question 20

soma

Answer 20

cell body; contains the nucleus, ribosomes, and mitochondria. Most of a neuron’s metabolic work occurs here. Cell bodies of neurons range in diameter from 0.005 millimeter (mm) to 0.1 mm in mammals and up to a millimeter in certain invertebrates. In many neurons, the cell body is like the dendrites- covered with synapses on its surface.

Question 21

motor neuron

Answer 21

soma in the spinal cord, receives excitation through its dendrites and conducts impulses along its axon to a muscle

Question 22

A sensory neuron

Answer 22

Specialized at one end to be highly sensitive to a particular type of stimulation, such as light, sound, or touch. Conducts touch information from the skin to the spinal cord. Tiny branches lead directly from the receptors into the axon, and the cell’s soma is located on a little stalk off the main trunk.

Question 23

Dendrites

Answer 23

branching fibers that get narrower near their ends. (The term dendrite comes from a Greek root meaning “tree.” A dendrite branches out like a tree.) The dendrite’s surface is lined with specialized synaptic receptors, at which the dendrite receives information from other neurons. The greater the surface area of a dendrite, the more information it can receive. Many dendrites contain dendritic spines, short outgrowths that increase the surface area available for synapses.

Question 24

axon

Answer 24

a thin fiber of constant diameter. (The term axon comes from a Greek word meaning “axis.”) The axon conveys an impulse toward other neurons, an organ, or a muscle. Axons can be more than a meter in length, as in the case of axons from your spinal cord to your feet. The length of an axon is enormous in comparison to its width, and in comparison, to the length of dendrites.

Question 25

myelin sheath

Answer 25

an insulating material with interruptions known as nodes of Ranvier. Invertebrate axons do not have myelin sheaths. Although a neuron can have many dendrites, it can have only one axon, but the axon may have branches. The end of each branch has a swelling, called a presynaptic terminal, also known as an end bulb (French for “button”). At that point the axon releases chemicals that cross through the junction between that neuron and another cell.

Question 26

presynaptic terminal

Answer 26

also known as an end bulb (French for “button”). At that point the axon releases chemicals that cross through the junction between that neuron and another cell.

Question 27

An afferent axon

Answer 27

brings information into a structure; . Every sensory neuron is an afferent to the rest of the nervous system; afferent starts with a as in admit

Question 28

efferent axon

Answer 28

carries information away from the structure; every motor neuron is efferent from the nervous system. efferent starts with an e as in exit;

Question 29

an interneuron

Answer 29

If a cell’s dendrites and axon are entirely contained within a single structure, the cell is an interneuron or intrinsic neuron of that structure. For example, an intrinsic neuron of the thalamus has its axon and all its dendrites within the thalamus

Question 30

variations amond neurons

Answer 30

Neurons vary enormously in size, shape, and function. The shape of a neuron determines its connections with other cells and thereby determines its function. For example, the widely branching dendrites of the Purkinje cell in the cerebellum enable it to receive input from up to 200,000 other neurons. By contrast, bipolar neurons in the retina have only short branches, and Some receive input from as few as two other cells.

Question 31

Glia

Answer 31

Glia (or neuroglia), the other components of the nervous system, preform many functions. The term glia, derived from a Greek word meaning “glue,” reflects early investigators’ idea that glia were like glue that held the neurons together. Although that concept is obsolete, the term remains. Glia outnumbered neurons in the cerebral cortex, but neurons outnumber glia in several other brain areas, especially the cerebellum. Overall, the number are almost equal.

Question 32

Oligodendrocytes

Answer 32

in the brain and spinal cord ; produce myelin sheaths that insulate certain vertebrate axons in the central nervous system.

Question 33

Schwann cells

Answer 33

in the periphery of the body build the myelin sheath that surround and insulate certain vertebrate axons. They also supply an axon with nutrients necessary for proper functioning; have a similar function as oligodendrocytes in the periphery.

Question 34

Astrocytes

Answer 34

pass chemicals back and forth between neurons and blood among neighboring neurons. They are also star shaped; wrap around the synapses of functionally related axons. By surrounding a connection between neurons, an astrocyte shields it from chemicals circulating in the surround. Also, by taking up the ions and transmitters released by axons and then releasing them back, an astrocyte helps synchronize closely related neurons, enabling their axons to send messages in waves. Astrocytes are therefore important for generating rhythms, such as your rhythm of breathing.

Question 35

Microglia

Answer 35

proliferate in areas of brain damage and remove toxic materials; act as part of the immune system, removing viruses and fungi from the brain. They proliferate after brain damage, removing dead or damaged neurons. They also contribute to learning by removing the weakest synapses.

Question 36

Radial glia

Answer 36

guides the migration of neurons and their axons and dendrites during embryonic development. When embryological development finishes, most radial glia differentiate into neurons, and a smaller number differentiate into astrocytes and oligodendrocytes.; guides the migration of neuron’s during embryological development. Glia’s have other functions as well.

Question 37

How an Astrocyte synchronizes associated axons?

Answer 37

Branches of the astrocyte surround the presynaptic terminals of related axons. If a few of them are active at once, the astrocyte absorbs some of the chemicals they release. It then temporarily inhibits all the axons to which it is connected. When the inhibition ceases, all of the axons are primed to respond again in synchrony.

Question 38

What are the four major structures that compose a neuron?

Answer 38

Dendrites, soma (cell body), axon, and presynaptic terminal.

Question 39

Which kind of glia cell wraps around the synaptic terminals of axons?

Answer 39

Astrocytes

Question 40

The Blood-Brain Barrier

Answer 40

Although the brain, like any other organ, needs to receive nutrients from the blood, many chemicals cannot cross from the blood to the brain. The mechanism that excludes most chemicals from the vertebrate brain is known as the blood-brain barrier

Question 41

Why do we need a Blood-Brain Barrier?

Answer 41

* When a virus invades a cell, mechanisms within the cell extrude virus particles through the membrane so that the immune system can find them. When the immune system cells discover a virus, they kill it and the cell that contains it. In effect, a cell exposing a virus through its membrane says, “Look, immune system, I’m infected with this virus. Kill me and save the others.” * However, certain viruses do cross the blood-brain barrier. When the rabies virus evades the blood-brain barrier, it infects the brain and leads to death. The spirochete responsible for syphilis also penetrates the blood-brain barrier, producing long-lasting and potentially fatal consequences. The microglia are more effective against several other viruses that enter the brain, mounting an inflammatory response that fights the virus without killing neuron.

Question 42

how the blood brain barrier works

Answer 42

* The blood-brain barrier depends on the endothelial cells that form the walls of the capillaries. Outside the brain, such cells are separated by small gaps but in the brain, they are joined so tightly they block viruses, bacteria, and other harmful chemicals from passage. * Most large molecules and electrically charged molecules cannot cross from the blood to the brain. A few small, uncharged molecules such as O2 and CO2 cross easily, as can certain fat-soluble molecules. Active transport systems pump glucose and amino acids across the membrane.

Question 43

“If the blood-brain barrier is such a good defense,” you might ask, “why don’t we have similar walls around all our other organs?”

Answer 43

The answer is that the barrier keeps out useful chemicals as well as harmful ones. Those useful chemicals include all fuels and amino acids, the building blocks for proteins. For these chemicals to cross the blood-brain barrier, the brain needs special mechanisms not found in the rest of the body. - Water crosses through special protein channels in the wall of the endothelial cells. For certain other chemicals, the brain uses active transport, a protein-mediated process that expends energy to pump chemical from the blood into the brain. Chemicals that are actively transported into the brain include glucose (the brain’s main fuel), amino acids (the building blocks of proteins), purines, choline, a few vitamins, and iron. Insulin and probably certain other hormones also cross the blood-brain barrier, at least in small amounts, although the mechanism is not yet known.

Question 44

Which chemicals cross the blood-brain barrier?

Answer 44

Small, uncharged molecules such as oxygen, carbon dioxide, and water cross the blood-brain barrier passively. So do chemicals that dissolve in the fats of the membrane

Question 45

Which chemicals cross the blood-brain barrier by active transport?

Answer 45

Glucose, amino acids, purines, choline, certain vitamins, and iron

Question 46

Identify one major advantage and one disadvantage of having a blood-brain barrier.

Answer 46

The blood-brain barrier keeps out viruses (an advantage) and also keeps out most nutrients (a disadvantage)

Question 47

Nourishment of Vertebrate Neurons

Answer 47

vertebrate neurons depend almost entirely on glucose, a sugar. (Cancer cells and the testis cells that make sperm also rely overwhelmingly on glucose.) Because metabolizing glucose requires oxygen neurons need a steady supply of oxygen.

Question 48

Why do neurons depend so heavily on glucose?

Answer 48

They can and sometimes do use ketones (a kind of fat) and lactate for fuel. However, glucose is the only nutrient that crosses the blood-brain barrier in large quantities. They need thiamine (vitamin B1) to use glucose.

Question 49

Our behavior emerges from

Answer 49

the communication among neurons.

Question 50

every neuron is covered by a

Answer 50

membrane which is composed of two layers (free to float relative to each other) of phospholipid molecules (containing chains of fatty acids and a phosphate group). Embedded among the phospholipids are cylindrical protein molecules through which certain chemicals can pass.

Question 51

When at rest, the membrane maintains an

Answer 51

electric gradient, also known as polarization- a difference in electrical charge between the inside and outside of the cell.

Question 52

The electrical potential inside the membrane is slightly _____ with respect to the outside, mainly because of _____ charged proteins inside the cell. This difference in voltage is called the ____________

Answer 52

negative, negatively, resting potential.

Question 53

what is the selective permeability of neurons?

Answer 53

some chemicals pass through the phospholipid bilayer more freely than others do. Oxygen, carbon dioxide, urea, and water cross freely through channels that are always open. Several biologically important ions, including sodium, potassium, calcium, and chloride, cross through membrane channels (or gates) that are sometimes open and sometimes closed. When the membrane is at rest, the sodium and potassium channels are closed, permitting almost no flow of sodium and only a small flow of potassium. Certain types of stimulation can open these channels, permitting a freer flow of either or both ions.

Question 54

The sodium-potassium pump

Answer 54

a protein complex that repeatedly transports three sodium ions out of the cell while drawing two potassium ions into it. The sodium-potassium pump is an active transport that requires energy. As a result of the sodium-potassium pump, sodium ions are more than 10 times more concentrated outside the membrane than inside, and potassium ions are more concentrated inside than outside.

Question 55

why is the sodium-potassium pump so effective?

Answer 55

only because of the selective permeability of the membrane, which prevents the sodium ions that were pumped out of the neuron from leaking right back in again. When sodium ions are pumped out, they stay out. However, some of the potassium ions in the neuron slowly leak out, carrying a positive charge with them. That leakage increases the electrical gradient across the membrane.

Question 56

The concentration gradient of a neuron

Answer 56

the difference in the distribution of ions across the membrane. Sodium is more concentrated outside than inside, so just by the laws of probability, sodium is more likely to enter the cell than leave it.

Question 57

Given that both the electrical gradient and the concentration gradient tend to move ____ ions into the cell, _____ would enter rapidly if it could. However, because the _____ channels are closed when the membrane is at rest, almost no _____ flows except for what the sodium-potassium pump forces out of the cell.

Answer 57

sodium

Question 58

potassium

Answer 58

Potassium is subject to competing forces. Potassium is positively charged, and the inside of the cell is negatively charged, so the electrical gradient tends to pull potassium in. however, potassium is more concentrated inside the cell than outside, so the concentration gradient tends to drive it out. If the potassium channels were wide open, potassium would have a small net flow out of the cell. That is, the electrical gradient and concentration gradient for potassium are almost in balance, but not quite. The sodium-potassium pump continues pulling potassium into the cell, counteracting the ions that leak out.

Question 59

_____ charged proteins inside the cell sustain the membrane’s ______.

Answer 59

negatively, polarization

Question 60

chloride

Answer 60

Chloride ions, being negatively charged, are mainly outside the cell. When the membrane is at rest, the concentration gradient and electrical gradient balance, so opening the chloride channels would produce little effect. However, chloride does have a new flow when the membrane’s polarization changes.

Question 61

When the membrane is at rest, are the sodium ions more concentrated inside the cell or outside? Where are the potassium ions more concentrated?

Answer 61

Sodium ions are more concentrated outside the cell, and potassium is more concentrated inside.

Question 62

When the membrane is at rest, what tends to drive the potassium ions out of the cell? What tends to draw them into the cell?

Answer 62

When the membrane is at rest, the concentration gradient tends to drive potassium ions out of the cell. The sodium-potassium pump also draws them into the cell.

Question 63

action potentials

Answer 63

messages sent by axons, The action potential transmits information without loss of intensity over distance. The cost is a delay between the stimulus and its arrival in the brain.

Question 64

hyperpolarization

Answer 64

When an axon’s membrane is at rest, the recordings show a negative potential inside the axon. If we now use a different electrode to apply a negative charge, we can further increase the negative charge inside the neuron. The change is called hyperpolarization, which means increased polarization.

Question 65

depolarization

Answer 65

reduce polarization towards zero. With a slightly stronger depolarizing current, the potential rises slightly higher but again returns to the resting level. Threshold of excitation produces a massive depolarization of the membrane. When the potential reaches the threshold, the membrane opens its sodium channels and lets sodium ions flow into the cell.

Question 66

what is the difference between a hyperpolarization and a depolarization?

Answer 66

a hyperpolarization is an exaggerated of the usual negative charge within a cell, to a more negative level than usual. A depolarization is a decrease in the amount of negative charge within the cell.

Question 67

What happens if the depolarization does or does not reach the threshold?

Answer 67

If the depolarization reaches or passes the threshold, the cell produces an action potential. If it is less than threshold, no action potential arises.

Question 68

All or-None Law

Answer 68

For any stimulus greater than the threshold, the amplitude and velocity of the action potential are independent of the size of the stimulus that initiated it. the amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it, provided that the stimulus reaches the threshold; puts constraint on how an axon can send a message. To signal the difference between a weak stimulus and a strong stimulus, the axon cannot send bigger or faster action potentials. All it can change is the timing. By analogy, you might send signals to someone by flashing the lights in your room on and off, varying the speed or rhythm of flashing.

Question 69

The Molecular Basis of the Action Potential

Answer 69

1. At the start, sodium ions are mostly outside the neuron, and potassium ions are mostly inside. 2. When the membrane is depolarized, sodium and potassium channels in the membrane open. 3. At the peak of the action potential, the sodium channels close.

Question 70

voltage gated channels and their role in action potential

Answer 70

The axon channels regulating sodium and potassium are voltage-gated channels. That is, their permeability depends on the voltage differences across the membrane. At the resting potential, the sodium channels are fully closed, and the potassium channels are almost closed, allowing only a little flow of potassium. As the membrane becomes depolarized, both the sodium and the potassium channels begin to open, allowing freer flow. Opening up the sodium channels makes a big difference, because both the electrical gradient and the concentration gradient tend to drive sodium ions into the neuron. When the depolarization reaches the threshold of the membrane, the sodium channels open wide enough for sodium to flow freely. Of the total number of sodium ions near the axon, less than 1 percent cross the membrane during an action potential.

Question 71

Action potential requires

Answer 71

the flow of sodium and potassium. Local anesthetic drugs, such as Novocain and Xylocaine, attach to the sodium channels of the membrane, preventing sodium ions from entering. - When a dentist administers Novocain before drilling into one of your teeth, your receptors are screaming, “pain, pain, pain!” but the axons cannot transmit the message to your brain, and so you don’t feel it.

Question 72

Propagation of the Action Potential

Answer 72

The term propagation of the action potential describes the transmission of an action potential down an axon. The propagation of animal species is the production of offspring. In a sense, the action potential gives birth to a new action potential at each point along the axon.

Question 73

how does action potential propogate down an axon?

Answer 73

- During action potential, sodium ions enter a point on the axon. Temporarily, that spot is positively charged in comparison with neighboring areas along the axon. The positive ions flow within the axon to neighboring regions. The positive charges slightly depolarize the next area of the membrane, causing it to reach its threshold and open its voltage-gate sodium channels. Then the membrane regenerates the action potential at that point. - As an action potential occurs at one point on the axon, enough sodium enters to depolarize the next point to its threshold, producing an action potential at that point. In this manner the action potential flows along the axon, remaining at equal strength throughout. Behind each area of sodium entry, potassium ions exit, restoring the resting potential.

Question 74

action potential

Answer 74

- When an area of the axon membrane reaches its threshold of excitation, sodium channels and potassium channels open. - At first, the opening of potassium channels produce little effect. - Opening sodium channels lets sodium ions rush into the axon. - Positive charge flows down the axon and opens voltage-gated sodium channels at the next point. - At the peak of the action potential, the sodium gates snap shut. They remain closed for the next millisecond or so, despite the depolarization of the membrane. - Because voltage-gated potassium channels remain open, potassium ions flow out of the axon, returning the membrane towards its original depolarization. - Everything flows logically from the facts that voltage-gated sodium and potassium channels open when the membrane is depolarized and that sodium channels snap shut at the peak of the action potential.

Question 75

myelin

Answer 75

insulating material composed of fats and proteins.

Question 76

myelinated axons

Answer 76

axons covered with a myelin sheath. In axons that are covered with myelin, action potentials form only in the nodes that separate myelinated segments. Transmission in myelinated axons is faster than in unmyelinated axons. Myelinated axons, found only in vertebrates, are covered with layers of fats and proteins. The myelin sheath is interrupted periodically by short section of axon called node of Ranvier, each one about 1 micrometer wide.

Question 77

saltatory conduction

Answer 77

 The jumping of action potentials from node to node is referred to as saltatory conduction, from the Latin word saltare, meaning “to jump.” (The same root shows up in the word somersault.) In addition to providing rapid conduction of impulses, saltatory conduction conserves energy: Instead of admitting sodium ions at every point along the axons and then having to pump them out via the sodium-potassium pump, a myelinated axons admits sodium only at its nodes.

Question 78

saltatory conduction in a myelinated axon

Answer 78

An action potential at the node triggers flow of correct to the next node, where the membrane regenerates the action potential. In reality, a myelin sheath is much longer, relative to the size of the nodes of Ranvier and to the diameter of the axon.

Question 79

the refractory period

Answer 79

at the peak of the action potential, the sodium gates snap shut. As a result, the cell is in a refractory period during which it resists the production of further action potentials. In the first part of the period, the absolute refractory period, the membrane cannot produce another action potential, regardless of the stimulation. During the second part, the relative refractory period, a stronger-than-usual stimulus is necessary to initiate an action potential. The refractory period depends on two facts: The sodium channels are closed, and potassium is flowing out of the cell at a faster-than-usual rate.

Question 80

local neurons

Answer 80

many small neurons have no axon. Neurons without an axon exchange information with only their closest neighbors. We therefore call them local neurons. Because they do not have an axon, they do not follow the all-or-none law. When a local neuron receives information from other neurons, it has a graded potential, a membrane potential that varies in magnitude in proportion to the intensity of the stimulus. - Local neurons are difficult to study because it is almost impossible to insert an electrode into a tiny cell without damaging it.

Question 81

synapses

Answer 81

Synapses are the decision-makers of your brain, but the input into these decisions makers is the on/off messages transmitting down axons.

Question 82

The sodium-potassium pump moves ___ ions out of the neuron, and ____ ions in.

Answer 82

sodium, potassium

Question 83

what happens when the membrane depolarizes to reach the cell's threshold

Answer 83

When the membrane is sufficiently depolarized to reach the cell’s threshold, sodium, and potassium channels open. Sodium ions enter rapidly, reducing and reversing the charge across the membrane. This event is known as the action potential.

Question 84

After the peak of the action potential, why does the membrane returns toward its original level of polarization?

Answer 84

After the peak of the action potential, the membrane returns toward its original level of polarization because of the outflow of potassium ions.

Question 85

what happens to the membrane immediately after an action potential?

Answer 85

the membrane enters a refractory period during which it is resistant to starting another action potential.