27. Cell Signaling in the Nervous System

Question 1

Explain what neurons are in relation to the nervous system.

Answer 1

Neurons (nerve cells) are the structural and functional units of the nervous system.

Question 2

Describe dendrites (shape and function).

Answer 2

Shape: Branches of the cell body (give tree-like shape)
Function: Receive signals from other neurons through axonal contacts (synapses) formed on their surface.

Question 3

Describe axons (shape and function).

Answer 3

Shape: very thin nerve fibers
* varying in length

Function: carry nerve impusles away from a neuron to another neuron
* every neuron has one axon that connects it with other neurons/muscles/gland cells.

Question 4

Describe myelin sheaths (shape and function).

Answer 4

Shape: fatty protective substance that covers axon.

Function: acts as insulation for axons –> helps send signals over long distances
* less common in neurons whose axons remain in the local region.

Question 5

Explain how action potentials progagate along axons.

Answer 5

1. Na+ rushes into the cell through the voltage-gated Na+ channels –> region becomes positively charged.
2. K+ leaves the cell through the voltage-gated K+ channels –> the cell becomes negatively charged.
3. Repeat.
4. Action potential propagation is mediated by progressive opening and closing of ion channels.

Question 6

Describe the differences between dendrites and axons.

Answer 6

Question 7

What are the two types of neuron fibers?

Answer 7

Sensory fibers and motor fibers.

Question 8

What are the characteristics of sensory fibers?

Answer 8

Function: pass impulses or messages from sensors to the brain and toward the CNS.
* responsible for sensations like interpreting touch, pressure, temp., and pain
* contain only afferent fibers, long dendrites of sensory neurons.

Question 9

What are the characteristics of motor neurons?

Answer 9

Function: send messages to muscles and glands in response to stimuli, including damage or physical traumas (accidents)
* reason that you tense your shoulders in response to a potential threat
* contain only efferent fibers, long axons of motor neurons

Question 10

Describe where synapses are located.

Answer 10

AKA neuronal junctions
* located at the end of an axon

Includes:
* presynaptic fiber
* synaptic cleft (0.02um)
* postsynaptic fiber

Question 11

Describe cell signaling at the synapse.

Answer 11

• Arrival of a nerve impulse at the presynaptic terminal causes release of neurotransmitters.
• Neurotransmitters diffuse across the synaptic cleft and bind to receptor molecules on the postsynaptic membrane (nerve impulse is transmitted).
• Chemical binding action alters the shape of receptors –> initate series of reactions that open channel-shaped protein molecules.
• Ions flow into or out the neuron
• Sudden shift of electrical charge across the postsynaptic membrane changes electric polarization of the membrane –> produces postsynaptic potential.
• Large net flow of positive ions = excitatory postsynaptic potential –> generation of a new nerve impulse (aka action potential)

Question 12

In the human brain, how many synapses, neurons, and neurotransmitters are there?

Answer 12

• synapses - trillion
• neurons - million
• neurotransmitters - hundred

Question 13

Describe what a neurotransmitter is.

Answer 13

• Chemical substance that is released at the end of a nerve fiber because of a nerve impulse.
• Diffuses across synapse or junction –> causes transfer of impulse to another nerve fiber, muscle fiber, or other structure

Question 14

Describe an excitatory neurotransmitter.

Answer 14

Activation of an excitatory neurotransmitter in the presynaptic neuron increases the probability of an action potential occuring in the postsynaptic neuron.

Question 15

Describe an inhibitory neurotransmitter.

Answer 15

Activation of a neurotransmitter in the presynaptic neuron less likely increases the probability of an action potential occuring in a postsynaptic neuron.

Question 16

Identify the different small molecule neurotransmitters.

Answer 16

• cholinergic: acetylcholine
• biogenic amines: dopamine, norepinephrine, epinephrine, serotonin
• amino acids: glutamate, GABA

Question 17

Identify large molecule neurotransmitters.

Answer 17

neuropeptides: endorphin

Question 18

Explain the 5 different possible drug actions on neurotransmitters.

Answer 18

• alter biosysthesis of a neurotransmitter in the presynaptic neuron.
• alter release of a neurotransmitter from a presynaptic neuron to the synaptic cleft (efflux)
• alter reuptake of a neurotransmitter back to the presynaptic neuron (reuptake)
• alter uptake of neurotransmitter by the postsynaptic neuron
• alter degradation of a neurotransmitter in a presynaptic neuron and synaptic cleft

Question 19

Describe the characteristics of a neurotransmitter receptor.

Answer 19

Membrane receptor protein at the postsynaptic membrane of dendrite, cell body, and axon activated by a neurotransmitter.
* after activation, the receptor can trigger other events to occur in the postsynaptic neuron.

Question 20

What are the two types of neurotransmitter receptors?

Answer 20

Ionotropic and metabotropic

Question 21

Explain the characteristics of ionotropic receptors.

Answer 21

AKA ligand-gated ion channels - changes shape when it is bound to a ligand.
* change in shape –> creates a channel that allows ions to flow through
* no second messenger is involved
* response is rapid

Question 22

Explain the characteristics of metabotropic receptors.

Answer 22

AKA GPCR - activates g-protein –> activates second messenger
* activation of metabotropic receptor may or may not result in opening of ion channels (depends)
* response is slow

Question 23

What are the three possible drug actions on neurotransmitter receptors?

Answer 23

• agonist
• antagonist
• modulator

Question 24

Explain how an agonist drug would work on a neurotransmitter receptor.

Answer 24

Drug action as an agonist: occupy neurotransmitter receptors and activate them.
* no need to wait for endogenous ligand to bind to receptor.

Question 25

Explain how an antagonist drug would work on a neurotransmitter receptor.

Answer 25

Drug action as an antagonist: occupy neurotransmitter receptor but do not activate them
* competitively bind to endogenous ligand site

Question 26

Explain how a modulator drug would work on a neurotransmitter receptor.

Answer 26

Drug action as a modulator: occupy neurotransmitter receptors at different binding sites, then occupy ligand binding sites –> alter physiological functions of receptors.

Question 27

What percentage of prescription drugs act on neurotransmitters and receptors?

Answer 27

60% of prescription drugs

Question 28

In the human brain, how many synapses, neurons, and neurotransmitters are there?

Answer 28

• synapses - trillion
• neurons - million
• neurotransmitters - hundred