Chapter 48: Neurons

Question 1

Neuron structures

Answer 1

Most of a neuron’s organelles are in the cell body or soma

Dendrites are highly branched extensions that receive signals from other neurons

The axon is typically a much longer extension that transmits signals to other cells at synapses

The cone-shaped base of an axon is called the axon hillock

Most neurons are nourished or insulated by cells called glia or glial cells

Question 2

Information processing

Answer 2

Sensory input

Sensory cells detect external stimuli and internal conditions and transmit information along sensory neurons

Integration

Sensory information is sent to the brain or ganglia where interneurons integrate the information

Motor output

Motor output leaves the brain or ganglia via motor neurons which trigger muscle or gland activity

Processing of information takes place in simple clusters of neurons called ganglia or a more complex organization of neurons called a brain

Question 3

Membrane potential

Answer 3

A voltage across the plasma membrane

The resting potential is the membrane potential of a neuron not sending signals

• Typically -60 to -80mV

Question 4

Formation of the resting potential

Answer 4

A mammalian neuron at resting potential has a higher K⁺ concentration inside the cell and a higher Na⁺ concentration is highest outside the cell

Sodium-potassium pumps use ATP to maintain these K⁺ and Na⁺ gradients across the plasma membrane

• Pump 3 Na⁺ out for 2 K⁺ pumped in

A neuron at resting potential contains
many open K⁺ channels and fewer open Na⁺ channels allowing K⁺ diffuse out of the cell

Question 5

Hyperpolarization

Answer 5

An increase in magnitude of the membrane potential

Gated K⁺ channels open, allowing K⁺ to diffuse out and making the inside of the cell more negative

Question 6

Graded potentials

Answer 6

Changes in polarization where the magnitude of the change varies with the strength of the stimulus

If a depolarization shifts the membrane potential sufficiently it results in a massive change in membrane voltage called an action potential

• Action potentials have a constant magnitude, are all-or-none and transmit signals over long distances

Question 7

Action potential generation

Answer 7

1. Resting potential- most voltage-gated Na⁺ and K⁺ channels are closed; some K⁺ channels are always open
2. Depolarization- stimulus-gated Na⁺ channels open first and Na⁺ flows into the cell
3. Rising phase- the threshold is crossed and the membrane potential increases opening more voltage-gated Na⁺ channels
4. Falling phase- Na⁺ channels become inactivated by the inactivation loop and voltage-gated K⁺ channels open; K⁺ flows out of the cell
5. Undershoot- membrane permeability to K⁺ is at first higher than at rest, then voltage-gated K⁺ channels close and resting potential is restored

Question 8

Refractory period

Answer 8

The period after an action potential when a second action potential cannot be initiated

The refractory period is a result of a temporary inactivation of the Na⁺ channels

Question 9

Conduction of action potentials

Answer 9

At the axon hillock site where the action potential is generated an electrical current depolarizes the neighboring region of the axon membrane

Action potentials travel in only one direction toward the synaptic terminals

Inactivated Na⁺ channels behind the zone of depolarization prevent the action potential from traveling backwards

Differences in signal frequency are the only variable that allows information to be encoded

Question 10

Axon structure

Answer 10

Axons are insulated by a myelin sheath which causes an action potential’s speed to increase

Myelin sheaths are made by glial cells

• Oligodendrocytes in the CNS
• Schwann cells in the PNS

Question 11

Action potential conductance

Answer 11

Action potentials are formed at the nodes of Ranvier, gaps where voltage-gated Na⁺ channels are located

Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction

Question 12

Saltatory conduction

Answer 12

Action potentials in myelinated axons jump between the nodes of Ranvier

Action potentials are formed at the nodes of Ranvier, gaps where voltage-gated Na⁺ channels are located

Question 13

Presynaptic neuron

Answer 13

Synthesizes and packages neurotransmitters in synaptic vesicles located in the synaptic terminal

The action potential causes the release of the neurotransmitter by opening voltage gated Ca²⁺ channels that allow Ca²⁺ to diffuse in

The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell

Question 14

Generation of postsynaptic potentials

Answer 14

Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell

Neurotransmitter binding causes ion channels to open generating two kinds of postsynaptic potential

1. Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward threshold; both K⁺ and Na⁺ permeable
2. Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold; either K⁺ or Cl⁻ permeable

Question 15

Summation of postsynaptic potentials

Answer 15

Most neurons have many synapses on their dendrites and cell body

A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron

Postsynaptic potentials combine to produce a larger postsynaptic potential

• Temporal summation- two EPSPs are produced in rapid succession
• Spatial summation- EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together

The axon hillock is the integrating center that represents the membrane potential at any instant in relation to EPSPs and IPSPs

• When the potential at the axon hillock reaches threshold an action potential is generate

Question 16

Metabotropic receptors

Answer 16

Neurotransmitter does not bind directly to an ion-gated channel

Neurotransmitter activates a signal transduction pathway in the postsynaptic cell whish phosphorylates specific ion channel proteins

Effects of second-messenger systems have a slower onset but last longer

Question 17

Neurotransmission termination

Answer 17

Receptor activation and postsynaptic response cease when neurotransmitters are cleared from the synaptic cleft

Neurotransmitters are removed by:

• Simple diffusion
• Inactivation by enzymes
• Recapture into the presynaptic neuron

Question 18

Acetylcholine

Answer 18

Involved in muscle stimulation, memory formation, learning

Vertebrates have two major classes of acetylcholine receptor

• Ligand gated- neuromuscular; degraded by acetylcholinesterase
• Metabotropic- CNS and heart; has an inhibitory rather than excitatory effect

Question 19

Amino acid neurotransmitters

Answer 19

Active in the CNS and PNS

CNS excitatory neurotransmitter

• Glutamate

CNS inhibitory neurotransmitters

• Gamma-aminobutyric acid (GABA)- increases membrane permeability to Cl⁻ in the brain
• Glycine- parts of the CNS outside of the brain

Question 20

Biogenic amines

Answer 20

Synthesized from amino acids

Epinephrine- hormone that acts outside of the nervous system

Norepinephrine- Excitatory neurotransmitter in the autonomic nervous system

Dopamine & Serotonin-released in the brain; affect sleep, mood, attention, and learning

Question 21

Neuropeptides

Answer 21

Relatively short chains of amino acids that function as neurotransmitters

Operate via G protein-coupled receptors

Substance P & endorphins- affect our perception of pain

• Opiates bind to the same receptors as endorphins

Question 22

Gaseous neurotransmitters

Answer 22

Local regulators in the PNS

Nitric oxide- causes smooth muscle relaxation and vasodilation

• Is not stored in cytoplasmic vesicles but is synthesized on demand
• Broken down within a few seconds of production

Carbon monoxide- regulates release of hypothalamic hormones