Chapter 7: Intro to Nervous System: Neurons/Synapses

Question 1

List the divisions of the nervous system and the structures that include.

Answer 1

1. Central Nervous System (CNS)
   
   • Brain and Spinal cord
2. Peripheral Nervous System (PNS)
   
   • Cranial and Spinal nerves
   • (Also the autonomic nervous system - ANS)

Question 2

List the two types of cells in the nervous system.

Answer 2

1. Neurons
2. Supporting cells/Glial cells

(Supporting cells = PNS)
(Glial cells = CNS)

Question 3

Define a neuron.

Answer 3

Neurons are the functional units of the nervous system.

Question 4

Discuss the functional classifications of neurons. (3)

Answer 4

1. Sensory/Afferent: neurons send messages to the CNS; posterior root (incoming info)
2. Motor/Efferent: neurons carry messages out of the CNS; anterior root (info going out)
3. Asssociation/Interneurons: only in CNS and integrate functions of the nervous system

Note: Pre- and Post- synaptic neurons are different lengths depending on system

Question 5

What do neurolemmocytes (Schwann Cells) do?

Answer 5

Myelinate the axons of the neurons in the PNS.

Question 6

Name the 4 glial/supporting cells found in the CNS and their functions.

Answer 6

1. Astrocytes: Regulate the area around the neurons; Blood Brain Barrier!
2. Oligodendrocytes: Produce myelin sheath that surrounds several axons (myelin gives tissues white color = white matter)
3. Ependymal Cells: Make CSF
4. Microglia: Phagocytes; eat foreign and degenerated material

Question 7

Discuss the node of Ranvier.

Answer 7

• It’s the uninsulated gap of axon between Schwann cells

- They increase the speed of transmission

Question 8

Discuss neuron regeneration differences between CNS and PNS.

Answer 8

CNS:

• Astrocytes form glial scar physically blocking regeneration
• Oligodendrocytes produce inhibitory proteins

PNS

• Schwann cells + macrophages phagocytize cellular debris
• Schwann cells form regeneration tube and release chemicals to attract the growing tip of the axon
• Regeneration tube ensures that the axon grows back/reconnects to the correct location

Question 9

Discuss Resting Membrane Potential (RMP)

Answer 9

• At rest, cells have a negative internal charge (-70mV); considered “polarized”
• Na+/K+ Pump (3Na+ out for every 2K+ in)
• At rest Na+ conc. high outside cell and K+ conc. high inside the cell
• Cell is more permeable to K+

Question 10

Discuss the (3) changes to membrane potential.

Answer 10

1. Depolarization: pos charges flow into the cell reducing the charge difference across the membrane; (Na+ flows in)
   
   • excitatory
2. Hyperpolarization: caused by neg charges flowing into the cell or positive charges flowing out of the cell; cell becomes more neg
   
   • inhibitory
3. Repolarization: membrane potential returns to resting membrane potential (-70mV)

Question 11

Discuss an Action Potential. How do they work?

Answer 11

• it’s a wave of membrane potential change that sweeps along the axon; a nerve impulse
• occurs by rapid depolarization of membrane - Na+ influx; followed by rapid repolarization by K+ efflux
• voltage-gated channels are opened by depolarization
• Do NOT need ATP; Na+ and K+ follow their concentration gradients
• Action potentials do NOT summate; all-or-none event;
• Once they reach -55 mV the action potential goes!

Recap or charge changes:
Start: -70 mV (Resting Membrane Potential)
Increase to +30 mV (Deporlarization)
Decrease to -85 mV (Hyperpolarization)
Increase to -70 mV (Repolarization to Resting)

Question 12

What is the difference between absolute refractory period and relative refractory period?

Answer 12

Absolute Refractory Period: Membrane “absolutely” cannot produce another action potential

Relative Refractory Period: Membrane can depolarize but it requires a stronger stimulus
for it to reach threshold (because you’res starting more negative than -70… so you have to go further to get to -55)

Question 13

Discuss conduction of the UNMYELINATED axon.

Answer 13

• Axon Hillock reaches threshold and fires action potential
• Na+ influx depoarizes adjacent regions of membrane
• Generates a new action potential and the process repeats down the axon
• Conduction is SLOW

Question 14

Discuss conduction of the MYELINATED axon.

Answer 14

• Ions can’t flow across myelinated membrane because lack voltage gated channels
• Nodes of Ranvier contain many voltage gated Na+ channels
• Saltatory Conduction

Question 15

Discuss saltatory conduction.

Answer 15

• Action potential only occurs at nodes of Ranvier in myelinated axons
• Current from action potential at one node can depolarize the next node
• Fast because action potentials can skip from node to node

Question 16

Discuss speed of action potentials.

Answer 16

Speed depends on diameter of the axon and myelination

• Myelinated and large = very FAST
• Unmyelinated and small = SLOW

Question 17

Discuss a synapse.

Answer 17

• Functional connection between a neuron (presynaptic) and another cell or neuron (post synaptic)
• There are chemical and electrical synapses
• Synaptic transmission at chemical synapse occurs via neurotransmitters
• Electric synapses are RARE in the nervous system

Question 18

Discuss chemical synapses.

Answer 18

• Synaptic cleft separates terminal bouton of presynaptic from postsynaptic cel
• Neurotransmitters are in synaptic vesicles and release from bouton by exocytosis
• Amt of neurotransmitter released depends on frequency of action potentials

Question 19

Discuss Ca+ and the synaptic vesicles.

Answer 19

1. Action potential reaches axon terminals.
2. Voltage gated Ca+ channels open.
3. Ca+ binds to sensor protein in cytoplasm.
4. Ca+ protein complex stimulates fusion and exocytosis of neurotransmitters

Then: neurotransmitter (ligand) bind to receptor proteins on post-synaptic membrane which opens ligand gated ion channels (chemically regulated into channels)

Ion channels that depolarize cause EPSPs (excitatory)

Ion channels that hyper polarize cause IPSPs (inhibitory)

Question 20

Discuss graded potential.

Answer 20

• Ligand gated ion channels open and the membrane potential changes depending on which ion channel is open
• Opening Na+ or Ca2+ channels = graded depolarization = EPSP
  (More of this one? Then YES, SEND action potential)
• Opening K+ and Cl- channels = graded hyper polarization = IPSP
  (More of this one? Then NO, do NOT SEND action potential)

Question 21

Discuss EPSP and IPSPs

Answer 21

• EPSPs summate and that determines whether an action potential occurs or not
• These use neurotransmitters
• EPSP (excitatory) move membrane potential closer to threshold; may require EPSPs from several neurons to produce action potential
• Graded in magnitude
• Have no threshold
• Cause depolarization
• Capable of summation
• No refractory period
• Can be initiated by neurotransmitters
• IPSP (inhibitory) more membrane potential farther from threshold; can counter EPSPs from other neurons

Question 22

Discuss Acetylcholine (ACh).

Answer 22

• Most widely used neurotransmitter
• Excitatory or Inhibitory depending on ion channels that are open
• Has 2 receptor subtypes:
  1. Nicotinic: Always excitatory; produces EPSPs when ACh binds to it

2. Muscarinic: Excitatory or Inhibitory; uses G-proteins to open some and close other K+ channels

Question 23

What does Acetylcholinesterase (AChE) do?

Answer 23

Inactivates ACh, terminating its action

Question 24

What are “cholinergic” neurons?

Answer 24

Neurons that use ACh as a neurotransmitter

Question 25

List the 4 monoamine neurotransmitters and discuss each.

Answer 25

1. Serotonin
   - Regulation of mood, behavior, appetite and cerebral circulation
   - SSRI’s; most common type of antidepressant - slows the reuptake of serotonin
2. Epinephrine
3. Norepinephrine
   - Used in PNS and CNS
   - In PNS: sympathetic neurotransmitter - triggers fight or flight response
   - In CNS: affects general level of arousal
4. Dopamine
   - Neurons that use dopamine are highly concentrated in midbrain
   - Involved in motor control and emotional reward; control and inhibition of movements
   - Parkinson’s disease caused by degeneration of these neurons (explains by pt’s with Parkinson’s have trouble initiating movements)
   - Dopamine won’t cross blood brain barrier so treated with L-dopa which will cross and then body converts it to dopamine

Question 26

Serotonin, epinephrine, norepinephrine, and dopamine are all “catecholamines” except:

Answer 26

Serotonin

Question 27

Endorphins, enkephalins are endogenous opioid neurotransmitters which means what?

Answer 27

That they are natural pain killers.

Question 28

Discuss neural pathways and divergence vs convergence.

Answer 28

Divergence: ONE presynaptic neuron forms synapses with SEVERAL postsynaptic neurons
(Seen more in sympathetic system - fight or flight - activates wide spread area)

Convergence: SEVERAL presynaptic neurons synapse with ONE postsynaptic neuron
(Seen in parasympathetic system - can trigger one area without triggering everything)

Question 29

Summation: Know the difference between spatial summation and temporal summation.

Answer 29

Spatial Summation:

• occurs due to convergence of signals into single postsynaptic neurons
• takes place when EPSPs from diff synapses occur in postsynaptic cell at same time
• EPSPs and IPSPs add together at axon hillock

Temporal Summation:

• occurs b/c EPSPs that occur closely in time can sum before they fade
• (cell fires so many times that you get an action potential)