Nervous System

Question 1

What is the main function of the nervous system?

Answer 1

The nervous system controls the body through electrical and chemical signals. Its roles include both sensory and motor functions, as well as the control of basic life processes.

The nervous system involves both voluntary and involuntary control.

Question 2

What are the defining features of a neuron?

Answer 2

Neurons are the functional units of the nervous system. They synapse with other neurons and transmit electrical and chemical signals.

The main components of a typical neuron are dendrites, a cell body, and an axon.

Question 3

In the diagram of a human neuron shown here, the structure labeled A represents the nucleus of the cell. What part of the neuron does the label B represent?

Answer 3

The cell body

The cell body is the region of the neuron that holds the nucleus.

Question 4

In the diagram of a human neuron shown here, the structure labeled D represents:

Answer 4

The dendrites

Dendrites are projections that extend from the cell body of a neuron. These structures receive information from adjacent neurons and carry it toward the cell body.

Question 5

In the diagram of a human neuron shown here, the structure labeled C represents:

Answer 5

The axon

The axon is a relatively long projection along which signals can propagate away from the cell body. The end of the axon, or axon terminal, can send chemical signals to an adjacent neuron through a gap known as a synapse.

Question 6

Define:

synapse

Answer 6

A synapse is a region where one nerve cell adjoins another. Here, signals can pass between the two, generally from the axon of the presynaptic cell to the dendrite of the postsynaptic one.

Synapses can be electrical or chemical. On the AP Biology exam, synapses tested are generally chemical.

Question 7

Which part of the neuron contains the nucleus, as well as organelles like the endoplasmic reticulum and mitochondria?

Answer 7

The cell body or soma serves as the location for the nucleus and organelles.

The region of the cell body immediately next to the axon is called the axon hillock. Here, incoming signals are summed (added together) and create an action potential if they surpass a certain threshold.

Question 8

Which part of the neuron receives incoming signals from previous neurons in the form of neurotransmitter-containing vesicles?

Answer 8

Dendrites receive signals from adjacent neurons.

A neuron releases neurotransmitters, held inside membrane-bound vesicles, into the synapse in a calcium-dependent process. These vesicles reach the dendrites of another cell and activate its receptors.

Question 9

Which part of the neuron ends in a single terminal and can release neurotransmitters?

Answer 9

The axon releases neurotransmitters at its terminal.

These NTs can range from glutamate to acetylcholine, but all must be held in membrane-bound vesicles. Calcium influx promotes the fusion of these vesicles to the cell membrane, allowing their contents to be sent into the synapse.

Question 10

What is the axon hillock, and what function does it serve?

Answer 10

The axon hillock is the region of the cell body situated immediately before the axon.

Incoming signals from the dendrites meet here, are summed, and create an action potential if they are greater in magnitude than a certain threshold.

Question 11

Define:

sensory neuron

Answer 11

A sensory neuron receives information from a peripheral receptor, then transmits it as an electrical signal to the spinal cord. This information could relate to the external environment or the body’s interior.

Sensory neurons are also called afferent neurons. Afferent structures always travel toward the organ or system in question; here, afferent neurons travel toward the spinal cord.

Question 12

Define:

motor neuron

Answer 12

A motor neuron transmits an electrical signal from the spinal cord to an effector muscle to initiate movement.

Motor neurons are also called efferent neurons. Efferent structures always travel away from the organ or system in question; here, efferent neurons travel away from the spinal cord.

Question 13

Name the two divisions of the nervous system.

Answer 13

The nervous system is broadly divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

Question 14

What two components does the central nervous system include?

Answer 14

The central nervous system (CNS) includes the brain and the spinal cord.

Question 15

Name two main structures of the peripheral nervous system.

Answer 15

The peripheral nervous system (PNS) includes spinal nerves and cranial nerves.

In the PNS, neurons are organized into peripheral structures called ganglia.

Question 16

Name the two divisions of the peripheral nervous system.

Answer 16

The PNS is divided into the somatic and the autonomic nervous systems.

The somatic system involves the conscious movement of skeletal muscles, while the autonomic system deals with the involuntary functioning of internal organs.

Question 17

What is the function of the somatic nervous system?

Answer 17

The somatic nervous system is generally associated with voluntary skeletal muscle movement. However, it also includes sensory neurons that transmit signals to the central nervous system.

Sensory neurons are afferent, meaning that they carry signals toward the spinal cord. Motor neurons are efferent and travel from the spinal cord to effector muscles.

Question 18

What is the function of the autonomic nervous system?

Answer 18

The autonomic nervous system is involved in the involuntary control of muscles and glands. It can promote a variety of functions including peristalsis (muscle movement in the intestines), changes in heart rate, dilation and constriction of blood vessels, and more.

The ANS includes two main divisions: the sympathetic nervous system and the parasympathetic nervous system.

Question 19

Name the two divisions of the autonomic nervous system.

Answer 19

The autonomic nervous system is divided into the sympathetic and parasympathetic systems.

The sympathetic system is commonly known to activate “fight-or-flight” responses, while the parasympathetic system relates to “resting and digesting.”

Question 20

What is the function of the sympathetic nervous system?

Answer 20

The sympathetic nervous system serves to activate certain bodily processes when in cases of stress or danger. It is commonly known as the “fight-or-flight” system.

Broadly, the sympathetic system speeds up heart rate, increases blood flow to muscles, and inhibits digestion.

Question 21

What is the function of the parasympathetic nervous system?

Answer 21

The parasympathetic nervous system serves to activate certain bodily processes when at rest and in safe conditions. It is especially active after eating and is commonly known as the “rest-and-digest” system.

Broadly, the parasympathetic system slows down heart rate and increases blood flow to the digestive system.

Question 22

Name four physiological responses triggered by the sympathetic nervous system.

Answer 22

The sympathetic system:

• increases heart rate
• increases blood flow to skeletal muscle via vasodilation
• decreases blood flow to the digestive system via vasoconstriction
• dilates pupils

Question 23

Name four physiological responses triggered by the parasympathetic nervous system.

Answer 23

The parasympathetic system:

• decreases heart rate
• decreases blood flow to skeletal muscle via vasoconstriction
• increases blood flow to the digestive system via vasodilation
• constricts pupils

Other effects include the stimulation of sexual arousal, salivation, and urination.

Question 24

What are the main neurotransmitters involved in sympathetic activity?

Answer 24

Epinephrine (commonly called adrenaline) and norephinephrine (commonly called noradrenaline) are the main sympathetic neurotransmitters.

Technically, these hormones are only released by the postganglionic synapse, which runs from the ganglion (nerve center) to the target organ. Acetylcholine is released at the preganglionic synapse, which travels from the spinal cord to the ganglion. However, the AP Biology exam typically does not test this distinction.

Question 25

What is the main neurotransmitter associated with parasympathetic activity?

Answer 25

Acetylcholine is the main parasympathetic neurotransmitter.

In this capacity, acetylcholine promotes “rest and digest” functions. However, note that acetylcholine also has an entirely distinct role in the muscular system, where it promotes skeletal muscle contraction.

Question 26

During the examination of a subject, it was found that the smooth muscle of her artery walls was temporarily dilated. Is this effect associated with sympathetic or parasympathetic activation?

Answer 26

Both.

To discern which division of the autonomic nervous system was active in this patient, we would need to know which artery walls are being described. Vasodilation of the arteries leading to the digestive system would imply parasympathetic activity, while dilation of the arteries that supply skeletal muscle is an effect of the sympathetic system.

Question 27

When the sympathetic nervous system is activated, which hormone will be present at higher concentrations: insulin or glucagon?

Answer 27

Glucagon will be more highly present.

The release of both glucagon and cortisol is correlated with sympathetic activity. Both hormones act to increase blood glucose levels, making energy more available for fight-or-flight activity.

Question 28

When the parasympathetic nervous system is activated, which hormone will be present at higher concentrations: insulin or glucagon?

Answer 28

Insulin will be more highly present.

Insulin release from the beta islet cells of the pancreas can be directly promoted by parasympathetic stimulation. More importantly, simply remember that insulin is released after meals to promote the storage of glucose, while the parasympathetic system is also activated after eating to promote digestion.

Question 29

What function is served by an action potential?

Answer 29

An action potential is an electrical event that allows a signal to propagate down the axon of a neuron.

Action potentials involve a brief variation in membrane potential from its resting value of -70 mV. This process is facilitated by voltage-gated sodium and potassium channels.

Question 30

Which two gradients combine to dictate the membrane potential of a neuron?

Answer 30

Electrical and chemical gradients combine to form the electrochemical gradient.

The electrical potential is produced by the difference in charge across the membrane, while the chemical gradients are formed by differences in ion concentration between the two sides. Like any gradients, both of these are under pressure to dissipate.

Question 31

Which structures in the neuronal membrane open in response to changes in electric potential?

Answer 31

Voltage-gated ion channels, specifically sodium (Na⁺) and potassium (K⁺) channels

For the sake of the AP Biology exam, just think of electric potential as relating to the difference in electrical charge inside and outside the axon. When this difference in charge changes, voltage-gated channels open or close.

Question 32

What is the resting potential of a normal neuron, and in what units is it measured?

Answer 32

-70 mV

The resting potential is negative because more cations (positive ions) are present outside the cell than inside. This makes the inside of the neuron relatively negative. Note that one mV, or millivolt, is one thousandth of a volt.

Question 33

When a neuron is at rest, where are high concentrations of sodium (Na⁺) located?

Answer 33

Outside the cell

The large amounts of Na⁺ outside the membrane contribute to the relative negativity of the inside of the neuron.

Question 34

When a neuron is at rest, where are high concentrations of potassium (K⁺) located?

Answer 34

Inside the cell

Though K⁺ concentrations are much greater inside the neuron than outside, the distribution of other ions, especially Na⁺, causes the interior of the cell to be relatively negative.

Question 35

How many ions are transferred by the Na⁺/K⁺ ATPase, and in which direction(s)?

Answer 35

The Na⁺/K⁺ ATPase, sometimes called the sodium-potassium pump, transfers three sodium ions out of the cell and two potassium ions in.

Though the pump is present in other cell types, it is especially relevant in neurons, where it contributes to the high Na⁺ concentration outside the cell and the high K⁺ concentration inside.

Question 36

What type of transport is exemplified by the Na⁺/K⁺ ATPase?

Answer 36

The Na⁺/K⁺ ATPase is a classic example of active transport.

Active transport requires energy, generally in the form of ATP, and transports molecules or ions against their concentration gradients.

Question 37

A certain cell has a resting potential of -65 mV. At what values would the potential of this cell be depolarized?

Answer 37

The cell would be depolarized, or “less polar,” at values less negative than -65 mV.

In the nervous system, depolarization is one step of an action potential. During this step, the membrane potential moves from -70 mV to values as high as +40 mV.

Question 38

During one step of an action potential, the membrane potential drops from +40 mV to a value near its resting potential of -70 mV. What name is given to this phase?

Answer 38

This phase is called repolarization.

Like its name implies, repolarization references the act of becoming “more polar.” In neurons, we can think of repolarization as simply becoming more negative. Specifically, after depolarization, the cell must repolarize to return to its negative resting potential.

Question 39

Under certain conditions, even a strong excitatory stimulus will not trigger an action potential. In this case, the neuron is said to be in what stage?

Answer 39

The neuron is experiencing a refractory period. This happens during and immediately after an action potential, when another action potential is difficult or impossible to produce.

Refractory periods can be either absolute, in which sodium channels are inactivated and no stimulus can trigger an action potential, or relative, in which a stronger stimulus than usual is necessary.

Question 40

Why is an action potential triggered when a cell is depolarized from -70 mV to -50 mV, but not when it moves from -70 mV to -60 mV?

Answer 40

A neuron must reach a certain depolarization threshold to trigger an action potential. This threshold is around -55 mV in a normal cell.

Neurons are commonly referred to as “all-or-none.” In other words, a full action potential will occur when threshold is surpassed, but nothing will happen if that value is not reached.

Question 41

List the steps involved in an action potential.

Answer 41

1. The neuron begins at its resting potential with all channels closed. A stimulus, often neurotransmitter binding, triggers the opening of some Na⁺ channels.
2. If this Na⁺ influx depolarizes the cell past its threshold, Na⁺ channels continue to open, further depolarizing the cell to a value of around +40 mV.
3. K⁺ voltage-gated channels open; Na⁺ channels begin to close.
4. The influx of K⁺ causes the cell to repolarize. This repolarization generally produces a hyperpolarized “undershoot” where the membrane potential dips below -70 mV.
5. Leak channels and the Na⁺/K⁺ ATPase contribute to the neuron’s return to resting potential.

Question 42

A certain cell has a resting potential of -65 mV. At what values would the potential of this cell be hyperpolarized?

Answer 42

The cell would be hyperpolarized, or “more polar,” at values more negative than -65 mV.

In the nervous system, hyperpolarization occurs as an “undershoot” after the repolarization phase of an action potential. During this step, the membrane potential briefly moves below its resting value of -70 mV.

Question 43

What is the main function of the cerebral cortex?

Answer 43

The cerebral cortex controls higher thought processes including language, memory, and consciousness.

In terms of evolutionary time, the cerebral cortex arose more recently than the cerebellum or brainstem.

Question 44

What is the main function of the cerebellum?

Answer 44

The cerebellum controls balance, posture, and related movements.

The cerebellum evolved before the cerebral cortex, but is more recent than the brainstem.

Question 45

What is the main function of the brainstem?

Answer 45

The brainstem maintains essential life processes such as breathing and heart rate. It also plays a homeostatic role through its promotion of thirst and hunger and maintenance of body temperature.

The brainstem is simpler than, and evolved before, the cerebellum and the cerebral cortex.

Question 46

Define:

glial cells

Answer 46

Glial cells are structural or support cells abundant in both the CNS and PNS. Unlike neurons, they do not transmit impulses, but perform a variety of other functions.

Different types of glia can produce myelin, provide nutrients to other cells, and facilitate the maintenance of the blood-brain barrier. More glial cells are present in the brain than actual neurons.

Question 47

How is myelin composed, and what is its function?

Answer 47

Myelin is a white, lipid-based material that contains cholesterol and various proteins. It insulates the axon, allowing the action potential to propagate faster.

Myelin is produced by glial cells in both the CNS and PNS.

Question 48

Oligodendrocytes produce myelin in the central nervous system. Which cells perform the same function in the peripheral nervous system?

Answer 48

Schwann cells make the myelin that wraps around the axons of the PNS.

Question 49

What structures does the unlabeled arrow point to in the figure below?

Answer 49

These structures are two of the nodes of Ranvier.

The myelin sheath insulates the axon, allowing nerve impulses to travel quickly. However, this insulation prevents Na⁺ and K⁺ ions from entering and exiting the cell. Nodes of Ranvier are gaps in the myelin where this ion exchange is free to occur.

Question 50

Define:

saltatory conduction

Answer 50

Saltatory conduction is a method of action potential propagation in which the potential “jumps” down the axon. This is observed in myelinated axons.

The potential “jumps” because ion exchange can only occur at the nodes of Ranvier, or gaps in the myelin coating the axon. For this reason, it appears to “jump” from each node to the next.

Question 51

Often, multiple presynaptic neurons synapse onto the same postsynaptic cell. What term describes the addition of simultaneous signals from these cells?

Answer 51

Summation involves the addition of incoming signals. If the summed value is sufficient to overcome the postsynaptic cell’s threshold, an action potential will be produced.

Note that inputs can be either excitatory or inhibitory. Often, both types of signals are present at the same time.

Question 52

What is the difference between an EPSP and an IPSP, and what do those abbreviations stand for?

Answer 52

An EPSP, or excitatory postsynaptic potential, occurs when inputs to a neuron stimulate it to depolarize. An EPSP increases the likelihood of an action potential occurring.

An IPSP, or inhibitory postsynaptic potential, occurs when inputs to a neuron cause it to hyperpolarize. An IPSP decreases the likelihood of an action potential occurring.

Question 53

What are the differences between an electrical synapse and a chemical synapse?

Answer 53

Electrical synapses involve the direct transmission of a signal between cells connected by gap junctions. This type of synapse is prevalent in the heart.

Chemical synapses are small gaps between cells. The presynaptic cell releases neurotransmitters in membrane-bound vesicles; these molecules cross the gap and bind to receptors on the postsynaptic cell.

For the AP Biology exam, it is more important to be familiar with chemical synapses.

Question 54

Which ion is required for the proper functioning of a chemical synapse?

Answer 54

Calcium (Ca²⁺)

All chemical synapses require Ca²⁺ for proper fusion of neurotransmitter-containing vesicles to the membrane of the axon terminal.

Question 55

Define:

reflex

Answer 55

A reflex is a quick, involuntary action that is provoked by a specific stimulus. Examples include the knee-jerk reflex and blinking when startled.

For the AP Biology exam, remember that reflexes are unconscious and do not involve the brain.

Question 56

What is a reflex arc, and what components does it include?

Answer 56

A reflex arc is the set of neurons that promotes a specific reflex. The neurons involved always synapse in the spinal cord.

Some reflex arcs are monosynaptic, which include only a sensory and a motor neuron. More complicated reflexes often require polysynaptic arcs, which contain at least one interneuron as well.

Question 57

What two main roles does the neurotransmitter acetylcholine play in the human body?

Answer 57

Acetylcholine stimulates parasympathetic functions and promotes skeletal muscle contraction.

Acetylcholine is the most important neurotransmitter to remember for the AP Biology exam. You should also know the name of the enzyme that breaks down extra acetylcholine in the synapse; this enzyme is acetylcholinesterase.

Question 58

What is the main excitatory neurotransmitter in the brain?

Answer 58

The main excitatory neurotransmitter is glutamate.

Question 59

What is the main inhibitory neurotransmitter in the brain?

Answer 59

The main inhibitory neurotransmitter is GABA (gamma-aminobutyric acid).