Chapter 3 - Signaling in the Nervous System

Question 1

Types of electrical impulses

Answer 1

• local (restricted to the area that received the stimulus like post-synaptic membrane)
• propagated (may travel through the neuron and axon aka action potentials)

Question 2

Action potentials

Answer 2

• propagated electrical impulses that travel down the axon

- result of coordinated opening and closing of Na+ and K+ ion channels

Question 3

Membrane potential at rest

Answer 3

• intracellular environment is rich in K+
• extracellular environment is rich in Na+
  aka salty banana

Question 4

Cell charge at rest

Answer 4

• difference in ion concentrations produces electrical potential across membrane leading the inside of the neuron to have a charge of -70mV (so the inside of the neuron is negative)

Question 5

Two passive forces maintaining equilibrium of Na+ and K+ across the membrane

Answer 5

• chemical force moves Na+ and K+ from the compartment containing high concentration to the compartment containing low concentration (diffusion force (entropy) moves Na+ inward and K+ outward)
• electrical force counters the diffusion of Na+ and K+ as the result of changing charge inside vs outside the neuron (electrochemical force (enthalpy) halts inward Na+ and outward K+ diffusion)
• when the chemical and electrical forces are equally strong => equilibrium potential

Question 6

Na+/K+ pump

Answer 6

• maintains difference in ion concentrations inside and outside of the cell
• imbedded in cell membrane
• moves 3 Na+ out of cell and 2 K+ into cell (leads to net negative charge inside neuron)
• this is an energy-consuming process (most predominant energy consuming process in the brain (uses a lot of ATP))

Question 7

Three stages of action potentials

Answer 7

• rapid depolarization (charge inside axon increases past zero, approaches +50mV –> overshoot) due to Na+ rushing into the cell
• rapid hyperpolarization (charge inside axon decreases past zero, around -80mV –> undershoot) due to K+ rushing out of the cell
• gradual return to the resting potential (-70mV) with the Na+/K+ pump

Question 8

Rapid depolarization

Answer 8

• predictable all or nothing electrical event in the axon only
• rapid increase in positive electrical charge in a segment of the axon

Question 9

Na+ and K+ ion channels

Answer 9

• voltage/charge sensitive (open in response to charge change inside the axon)

Question 10

Na+/K+ ion currents drive the action potential

Answer 10

• Na+ begins to enter the axon (inward positive current; peaks fast)
• K+ begins to exit the axon (lengthy outward positive current)
• Na+ channels close
• K+ leaves axon
• K+ channels close

Question 11

Myelin

Answer 11

• insulates the conductance of the action potential down the axon
• interrupted by Nodes of Ranvier where axon is exposed

Question 12

Na+ channel distribution down axon

Answer 12

clustered in Nodes of Ranvier (gNa occurs in exposed areas of axon)

Question 13

K+ channel distribution down axon

Answer 13

clustered under the myelin (gK occurs under myelin sheath)

Question 14

Saltatory conduction

Answer 14

• positive charge jumping from one node to the next (due to Na+ channels at nodes)
• speeds up electrical signal conductance/movement down the axon

Question 15

Action potentials in demyelinated axons

Answer 15

there is a loss of gNa through the damaged myelin, leading to conduction velocity being reduced and the action potential fails to propagate

Question 16

Pre-synaptic Ca++ channels

Answer 16

• action potential becomes longer in shape when it enters the terminal
• Na+ rushes into the pre-synaptic terminal and that positive charge causes the Ca++ to enter into the terminal
• the Ca++ enters the terminal slower and for longer than the Na+
• Ca++ facilitates linkage between the neurotransmitter vesicle and the pre-synaptic membrane (with twisting), leading the neurotransmitters to be diffused into the synaptic cleft to receptors on the post-synaptic cell
• Ca++ activates the protein that unbinds the bound vesicles from the cytoskeleton, turning them into the free pool of neurotransmitter containing synaptic vesicles, restarting the process

Question 17

Bound reserve pool of neurotransmitter containing synaptic vesicles

Answer 17

• bound to cytoskeleton

- Ca++ frees the vesicles by activating a protein that unbinds them

Question 18

Free pool of neurotransmitter containing synaptic vesicles

Answer 18

• Ca++ facilitates linkage between these vesicles and the membrane through twisting
• once emptied, these vesicles are returned to the bound reserve pool

Question 19

Neurotransmitters

Answer 19

• acetylcholine
• glutamate
• GABA
• Biogenic amines
• – catecholamines
• —– norepinephrine
• —– dopamine
• – indolamines
• —– serotonin
• —– melatonin
• neuropeptide transmitters
• – endorphins
• – enkephalins

Question 20

Acetylcholine

Answer 20

• N receptor; opens Na+ and other small ion channels (excitatory)
• M receptor; cAMP or IP3, DAG second messengers; opens or closes Ca++ channels (excitatory or inhibitory)

Question 21

Glutamate

Answer 21

• NMDA receptor; open channels which permit Ca++ influx if membrane is depolarized (senses simultaneous activity of 2 synaptic inputs; may trigger molecular changes that strength synapse (LTP))
• Kainate receptor; opens Na+ channels (excitatory)
• AMPA receptor; opens Na+ channels (excitatory)
• Metabotropic receptor; IP3, DAG second messenger (excitatory raises intracellular Ca++)

Question 22

Dopamine

Answer 22

• D1 receptor; cAMP second messenger; closes K+ channels (stays inside); (excitatory)
• D2 receptor; cAMP second messenger; opens K+ channels (goes out); (inhibitory)

Question 23

GABA

Answer 23

• GABAA receptor; opens Cl- channels (inhibitory - postsynaptic)
• GABAB receptor; IP3, DAG second messenger; closes Ca++ channels, opens K+ channels (inhibitory - presynaptic)

Question 24

Glycine

Answer 24

opens Cl- channels (inhibitory)