What is the main function of the nervous system?
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.
What are the defining features of a neuron?
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.
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?
The cell body
The cell body is the region of the neuron that holds the nucleus.
In the diagram of a human neuron shown here, the structure labeled D represents:
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.
In the diagram of a human neuron shown here, the structure labeled C represents:
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.
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.
Which part of the neuron contains the nucleus, as well as organelles like the endoplasmic reticulum and mitochondria?
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.
Which part of the neuron receives incoming signals from previous neurons in the form of neurotransmitter-containing vesicles?
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.
Which part of the neuron ends in a single terminal and can release neurotransmitters?
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.
What is the axon hillock, and what function does it serve?
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.
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.
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.
Name the two divisions of the nervous system.
The nervous system is broadly divided into the central nervous system (CNS) and the peripheral nervous system (PNS).
What two components does the central nervous system include?
The central nervous system (CNS) includes the brain and the spinal cord.
Name two main structures of the peripheral nervous system.
The peripheral nervous system (PNS) includes spinal nerves and cranial nerves.
In the PNS, neurons are organized into peripheral structures called ganglia.
Name the two divisions of the peripheral nervous system.
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.
What is the function of the somatic nervous system?
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.
What is the function of the autonomic nervous system?
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.
Name the two divisions of the autonomic nervous system.
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.”
What is the function of the sympathetic nervous system?
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.
What is the function of the parasympathetic nervous system?
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.
Name four physiological responses triggered by the sympathetic nervous system.
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
Name four physiological responses triggered by the parasympathetic nervous system.
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.
What are the main neurotransmitters involved in sympathetic activity?
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.
What is the main neurotransmitter associated with parasympathetic activity?
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.
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?
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.
When the sympathetic nervous system is activated, which hormone will be present at higher concentrations: insulin or glucagon?
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.
When the parasympathetic nervous system is activated, which hormone will be present at higher concentrations: insulin or glucagon?
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.
What function is served by an action potential?
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.
Which two gradients combine to dictate the membrane potential of a neuron?
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.
Which structures in the neuronal membrane open in response to changes in electric potential?
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.
What is the resting potential of a normal neuron, and in what units is it measured?
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.
When a neuron is at rest, where are high concentrations of sodium (Na+) located?
Outside the cell
The large amounts of Na+ outside the membrane contribute to the relative negativity of the inside of the neuron.
When a neuron is at rest, where are high concentrations of potassium (K+) located?
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.
How many ions are transferred by the Na+/K+ ATPase, and in which direction(s)?
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.
What type of transport is exemplified by the Na+/K+ ATPase?
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.
A certain cell has a resting potential of -65 mV. At what values would the potential of this cell be depolarized?
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.
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?
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.
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?
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.
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?
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.
List the steps involved in an action potential.
- The neuron begins at its resting potential with all channels closed. A stimulus, often neurotransmitter binding, triggers the opening of some Na+ channels.
- 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.
- K+ voltage-gated channels open; Na+ channels begin to close.
- The influx of K+ causes the cell to repolarize. This repolarization generally produces a hyperpolarized “undershoot” where the membrane potential dips below -70 mV.
- Leak channels and the Na+/K+ ATPase contribute to the neuron’s return to resting potential.
A certain cell has a resting potential of -65 mV. At what values would the potential of this cell be hyperpolarized?
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.
What is the main function of the cerebral cortex?
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.
What is the main function of the cerebellum?
The cerebellum controls balance, posture, and related movements.
The cerebellum evolved before the cerebral cortex, but is more recent than the brainstem.
What is the main function of the brainstem?
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.
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.
How is myelin composed, and what is its function?
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.
Oligodendrocytes produce myelin in the central nervous system. Which cells perform the same function in the peripheral nervous system?
Schwann cells make the myelin that wraps around the axons of the PNS.
What structures does the unlabeled arrow point to in the figure below?
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.
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.
Often, multiple presynaptic neurons synapse onto the same postsynaptic cell. What term describes the addition of simultaneous signals from these cells?
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.
What is the difference between an EPSP and an IPSP, and what do those abbreviations stand for?
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.
What are the differences between an electrical synapse and a chemical synapse?
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.
Which ion is required for the proper functioning of a chemical synapse?
All chemical synapses require Ca2+ for proper fusion of neurotransmitter-containing vesicles to the membrane of the axon terminal.
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.
What is a reflex arc, and what components does it include?
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.
What two main roles does the neurotransmitter acetylcholine play in the human body?
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.
What is the main excitatory neurotransmitter in the brain?
The main excitatory neurotransmitter is glutamate.
What is the main inhibitory neurotransmitter in the brain?
The main inhibitory neurotransmitter is GABA (gamma-aminobutyric acid).