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.

A neuron is broadly composed of:

• Dendrites, or branched structures that receive incoming signals
• A cell body, which contains the nucleus
• An axon, or projection along which the signal propagates away from the cell body

Question 3

Name the two divisions of the nervous system.

Answer 3

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

Question 4

What two components does the central nervous system include?

Answer 4

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

Question 5

Name two main structures of the peripheral nervous system.

Answer 5

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

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

Question 6

Name the two divisions of the peripheral nervous system.

Answer 6

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 7

What is the function of the somatic nervous system?

Answer 7

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 8

Define:

sensory neuron

Answer 8

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 9

Define:

motor neuron

Answer 9

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 10

Define:

reflex

Answer 10

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 MCAT, remember that reflexes are unconscious and do not involve the brain.

Question 11

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

Answer 11

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 12

A student in your study group mentions that reflexes cannot occur if signal transmission to and from the brain is blocked. What, if anything, is wrong with this statement?

Answer 12

Reflexes do not require any input from the brain. They are fully unconscious and involve only a sensory neuron, a motor neuron, and one or more synapses in the spinal cord.

Avoiding the brain allows the reflex to take place rapidly, which is especially important in dangerous situations.

Question 13

What is the function of the autonomic nervous system?

Answer 13

The autonomic nervous system is involved in the involuntary control of muscles and glands. It can promote a variety of functions including peristalsis, changes in heart rate, vasodilation and vasoconstriction, and more.

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

Question 14

Of the following structures, which does not fall mainly under autonomic control?

• cardiac muscle in the heart
• smooth muscle in blood vessel walls
• skeletal muscle in the quadriceps
• smooth muscle in the small intestine

Answer 14

Skeletal muscle, whether in the quadriceps or elsewhere, is generally controlled by the somatic nervous system.

The other choices (smooth muscle and cardiac muscle) are involuntary and would fall under autonomic control.

Question 15

Name the two divisions of the autonomic nervous system.

Answer 15

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 16

What is the function of the sympathetic nervous system?

Answer 16

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 17

Name four physiological responses triggered by the sympathetic nervous system.

Answer 17

The sympathetic system:

• increases heart rate
• increases blood flow to skeletal muscle
• decreases blood flow to the digestive system; inhibits peristalsis
• dilates pupils

Question 18

What is the function of the parasympathetic nervous system?

Answer 18

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 19

Name four physiological responses triggered by the parasympathetic nervous system.

Answer 19

The parasympathetic system:

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

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

Question 20

What is the main neurotransmitter associated with parasympathetic activity?

Answer 20

Acetylcholine is the parasympathetic neurotransmitter at both the preganglionic and postganglionic synapses.

Note that the parasympathetic function of acetylcholine is distinct from its role in the promotion of muscle contraction at the neuromuscular junction.

Question 21

What are the main neurotransmitters involved in sympathetic activity?

Answer 21

Epinephrine (commonly called adrenaline) and norephinephrine (commonly called noradrenaline) are the sympathetic neurotransmitters at the postganglionic synapse.

While acetylcholine is released at the preganglionic synapse, it is not commonly tested in relation to the sympathetic system.

Question 22

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

Answer 22

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 23

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

Answer 23

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 24

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 24

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 25

Name the four lobes of the cerebral cortex.

Answer 25

The four lobes of the brain are the frontal, parietal, temporal, and occipital lobes.

Question 26

What is the main function of the cerebral cortex?

Answer 26

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 27

What is the main function of the cerebellum?

Answer 27

The cerebellum controls balance, posture, and related movements.

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

Question 28

What is the main function of the brainstem?

Answer 28

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 29

Name the three major parts of a neuron.

Answer 29

A neuron is broadly composed of dendrites, a cell body, and an axon.

• Dendrites are branched structures that receive incoming signals.
• The cell body contains the nucleus and organelles.
• The axon is a projection along which the signal propagates away from the cell body.

Question 30

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

Answer 30

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 31

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

Answer 31

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 and create an action potential if they surpass a certain threshold.

Question 32

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

Answer 32

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 33

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

Answer 33

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 34

How is myelin composed, and what is its function?

Answer 34

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 35

In the brain, what is the difference between white matter and gray matter?

Answer 35

White matter is mainly composed of myelinated axons, which give it its color. Regions associated with communication, like the thalamus, are made of white matter.

Gray matter is mainly composed of the cell bodies of neurons and does not contain myelinated projections. Regions made of gray matter generally function in cognitive processes like language.

About 60% of the brain’s volume is white matter; the rest is gray matter.

Question 36

Define:

glial cells

Answer 36

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 37

Name three types of glial cells that are present in the central nervous system.

Answer 37

• Astrocytes have a variety of roles, including detoxification, provision of nutrients, and recovery from injury.
• Microglia perform an immune function and act similar to macrophages.
• Oligodendrocytes produce myelin around the axons in the white matter.

Question 38

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

Answer 38

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

Question 39

Which debilitating disease is the result of the deterioration of the myelin surrounding nerve cells in the central nervous system?

Answer 39

Multiple sclerosis (MS)

Question 40

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

Answer 40

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 41

Define:

synapse

Answer 41

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 MCAT, synapses tested are generally chemical.

Question 42

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

Answer 42

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 MCAT, it is more important to be familiar with chemical synapses.

Question 43

A certain synapse is determined to be cholinergic. Which ion is required for its proper functioning?

Answer 43

Ca²⁺ is required at this synapse.

Since this synapse is described as cholinergic, it must be a chemical synapse; more specifically, it must use acetylcholine as its neurotransmitter. All chemical synapses require Ca²⁺ for proper fusion of neurotransmitter-containing vesicles to the membrane of the axon terminal.

Question 44

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

Answer 44

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 45

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

Answer 45

-70 mV

The resting potential is negative because more cations are present outside the cell than inside. Note that one mV, or millivolt, is one thousandth of a volt.

Question 46

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

Answer 46

Outside the cell

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

Question 47

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

Answer 47

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 48

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

Answer 48

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 49

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

Answer 49

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 50

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

Answer 50

The cell would be depolarized, or “less polar,” at values greater than (or in other words, 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 51

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 51

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 52

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 52

This phase is called repolarization.

Like its name implies, repolarization references the act of becoming “more polar.” Specifically, after depolarization, the cell must repolarize to become negative again.

Question 53

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

Answer 53

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 54

What function is served by an action potential?

Answer 54

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 55

List the steps involved in an action potential.

Answer 55

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 efflux (outflow) 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 56

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

Answer 56

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 57

What are the two types of signal summation, and how do they differ?

Answer 57

The two types of summation are spatial summation and temporal summation.

Spatial summation occurs when multiple presynaptic cells synapse onto the same postsynaptic neuron. When these distinct cells fire in close succession, their signals can sum to produce an action potential.

Temporal summation occurs when a single presynaptic cell synapses onto a postsynaptic neuron. When this cell fires multiple times in a row, its signals can sum to produce an action potential.

Question 58

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

Answer 58

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 59

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 59

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 60

What is the main excitatory neurotransmitter in the brain?

Answer 60

The main excitatory neurotransmitter is glutamate.

Question 61

What is the main inhibitory neurotransmitter in the brain?

Answer 61

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