Chapter 7 Flashcards
What is the nervous system divided into?
- central nervous system (brain and spinal cord)
- peripheral nervous system (cranial and spinal nerves)
What are the 2 cells of the nervous system?
neurons and supporting cells (glial cells)
What are neurons?
functional units of the nervous system
What do glial cells do?
maintains homeostasis
- 5x more common than neurons
How do neurons gather and transmit information?
- responds to stimuli (sensory)
- produces and sends electrochemical impulses (electrical conduction)
- releases chemical messages (neurotransmitters)
What are neurons made of? Where’s the nucleus?
1) cell body (with nucleus)
2) dendrites
3) axon
What is the cell body and it’s function?
nutritional center
- makes macromolecules
What are groups of cell bodies called in the CNS? PNS?
CNS: nuclei
PNS: ganglia
What do dendrites do? How?
receive information and convey it to cell body through ligand gated channels
What do axons do? How?
conduct impulses away from cell body through voltage gated channels
What are the 2 special transports provided by long axon length?
1) axoplasmic flow: moves soluble compounds toward nerve endings via rhythmic contractions of axon
- protein transportation
2) axonal transport: transport of vesicles, mitochondria, ion channels
What does axonal transport move? Which way?
large and insoluble compounds
- bidirectionally along microtubules
- FAST!
What does anterograde transport move? How?
moves materials away from cell body
- uses molecular motor kinesin
What does retrograde transport move? How?
moves materials toward cell body
- uses molecular motor dynein
How do viruses and toxins enter the CNS?
through retrograde transport
What are afferent neurons? Where do impulses go?
sensory neurons
- impulses INTO CNS
What are efferent neurons? Where do impulses go?
motor neurons
- impulses OUT of CNS
What are interneurons? Where are they located?
- association
- integrate nervous system activity
- inside CNS only
What is a pseudounipolar neuron? Example?
cell body sits along side of single process
- sensory neurons
What is a bipolar neuron? Example?
dendrite and axon arise from opposite ends of cell body
- retinal neurons
What are multipolar neurons? Example?
have many dendrites and one axon
- motor neurons
What is the PNS equivalent of the supporting/glial cells?
Schwann cells: myelination of PNS axons
Satellite cells
How big are Schwann cells? What do they do?
1mm of 1 axon and wraps around
- electrically insulates axon
What are the supporting cells of the CNS?
oligodendrocytes, microglia, astrocytes, ependymal cells
What do oligodendrocyte do?
myelinates several CNS axons
What do ependymal cells do?
neural stem cells
What cells are involved in nervous system mantenance?
glial cells
What is the node of Ranvier?
unmyelinated gap between Schwann cells
Which nervous system does axon regeneration occur? Why?
PNS easier than CNS
- oligodendrocytes produce proteins that block regrowth
What happens when an axon in PNS is cut?
- distal part degenerates
- Schwann cells survive and form regeneration tube
- tube releases chemical to attract growing axon
- tube guides regrowing axon to synaptic site
What do neurotrophins do?
- promote fetal nerve growth
- required for survival of many adult neurons
- important for regeneration
What is the most common glial cell?
astrocytes
What do astrocytes do?
- buffering K+ levels
- recycling neurotransmitters
- regulating adult neurogenesis
- releasing transmitters that regulate neuronal activity
What is the blood-brain barrier?
allows only specific compounds to enter brain
What makes up the blood-brain barrier?
capillary specializations in brain
- astrocyte induction
- gaps between adjacent cells are closed by tight junctions
What is resting membrane potential?
all cells have a negative internal charge and unequal distribution of ions
What causes the resting membrane potential?
- large anions trapped inside cell
- Na+/K+ pump keeps Na+ high outside the cell
- K+ is very permeable and high inside cell (attracted by negative charges inside)
What affects resting membrane potential?
excitable cells
- rapid changes in permeability to ions
- neurons and muscles activate to generate and conduct electrical impulses
How is membrane potential measured?
1 electrode in the cell and 1 electrode outside
What is depolarization?
membrane potential is more positive
What is hyperpolarization?
membrane is more negative than resting potential
What is repolarization?
membrane returns to resting potential
Which leak channels are always open?
potassium (K+)
How are voltage-gated channels regulated?
opened by depolarization
Are K+ and Na+ channels open in resting cells?
No they’re closed
- both voltage-gated
How does the action potential graph look?
wave formed by rapid depolarization by Na+ influx and repolarization by K+ efflux
Which way does action potential travel in an axon
axon hillock to synapse
What happens during depolarization?
- voltage-gating Na+ channels open
- Na+ driven inward by electrochemical gradient -> depolarization = more channels open
- positive feedback loop
- causes rapid change from -70 to 30mV
What happens during repolarization?
- voltage-gated Na+ channels close and K+ opens
- electrochemical gradient drives K+ outward
- repolarizes axon back to RMP
How does depolarization and repolarization occur?
diffusion
- after action potential, sodium-potassium pump extrudes Na+ and recovers K+
What does it mean for an action potential to be all-or-none?
Once membrane potential reaches threshold, action potential is irreversibly fired
What is true about action potentials?
- propagate without decrement
- produced by voltage gated channels
- cannot summate
- constant amplitude
Where are voltage-gated channels found?
plasma membrane of axons
What does increased stimulus intensity cause?
more action potentials are fired
- size of APs remain constant
What is the absolute refractory period?
membrane cannot produce another action potential because Na+ channels are inactivated
What is the relative refractory period?
voltage-gated K+ channels are open
- harder to depolarize to threshold
How does a new action potential get generated?
process repeats all along axon
- amplitude is the same throughout
How fast is conduction of an unmyelinated axon?
2 mph
How fast is conduction in myelinated membranes?
0! Ions can’t flow through myelination
Where do action potentials occur in myelinated axons?
in Nodes of Ranvier
- voltage-gated Na+ channels are present in the nodes
How does action potential work in myelinated axons?
current from action potential at one node can depolarize next node to threshold
- saltatory conduction
What is synapse?
connection between neuron (presynaptic) and another cell (postsynaptic)
What is the common synapse in the nervous system?
chemical synapses via neurotransmitters
- electrical synapses are rare
Where does depolarization flow in an electrical synapse?
presynaptic into postsynaptic cell through gap junctions
What are gap junctions formed by? Where are they found?
connexin proteins
- found in smooth and cardiac muscles, brain, and glial cells
What does the synaptic cleft separate?
terminal bouton of presynaptic from postsynaptic cell
What is found in synaptic vesicles?
neurotransmitters
How do neurotransmitters get released?
vesicles fuse with bouton membrane
- exocytosis
What affects how much neurotransmitter is released?
directly related by frequency of action potentials
How does synaptic transmission work?
- action potential travels down axon to depolarize bouton
- opens voltage-gated calcium channels in bouton (driven inward by gradient)
- triggers exocytosis of vesicles and releases neurotransmitters
How does calcium trigger exocytosis of vesicles?
activates calmodulin then protein kinase
- kinase phosphorylates regulatory proteins that promote fusion and exocytosis of vesicles
What happens when a neurotransmitter (ligand) diffuses across the synaptic cleft?
- binds to receptor proteins on postsynaptic membrane (opens ligand gated channels)
What causes excitatory postsynaptic potentials (EPSPs)?
depolarization
What causes inhibitory postsynaptic potentials (IPSPs)?
hyperpolarization
Why are EPSPs and IPSPs graded potentials?
- ligand gated channels
- dendrite and neuron cell body only
- variable in amplitude
- propagate with decrement
- summation
What is the most widely used neurotransmitter?
acetylcholine
- used in brain and autonomic nervous system and all neuromuscular junctions
What are the acetylcholine receptor subtypes?
nicotinic and muscarinic
What do ligand-gated channels have?
a neurotransmitter receptor site and an ion channel
How do ligand-gated channels open?
when a ligand/neurotransmitter binds
What is a nicotinic acetylcholine channel formed by?
5 polypeptide subunits
How does a nicotinic acetylcholine channel work?
opens when 2 acetylcholines bind (only 2 subunits contain binding sites)
- permits diffusion of sodium in and potassium out of postsynaptic cell
- inward flow of sodium dominates
What signal do nicotinic channels produce?
EPSPs
What are g protein-coupled channels?
1 subunit membrane polypeptide
- activates ion channel indirectly through g-proteins
How do muscarinic acetylcholine channels work?
- binding of 1 ACh activates g-protein cascade and affects gated potassium channels
- open = hyperpolarization
- close = depolarization
What do acetylcholinesterase do?
inactivates acetylcholine
- stops action
- located in cleft
What are the large synapses on skeletal muscles called?
end plates or neuromuscular junctions
What do end plates produce?
large EPSPs (end-plate potentials)
- opens voltage gated channels beneath end plate
- cause muscle contraction
What blocks ACh action at neuromuscular junctions?
curare
What are examples of monoamine neurotransmitters?
serotonin
norepinephrine
dopamine
What is serotonin derived from?
tryptophan
What can tyrosine make?
norepinephrine and dopamine
- catecholamines
What are monoamine neurotransmitters inactivated by?
presynaptic reuptake
- breakdown by monoamine oxidase (MAO)
What are MAO inhibitors?
antidepressants
What is serotonin involved in?
regulation of mood, behavior, appetite, and cerebral circulation
What are SSRIs?
serotonin-specific reuptake inhibitors (antidepressants)
- block reuptake of serotonin, prolonging its action
What are the 2 major dopamine systems in brain?
1) nigrostriatal dopamine system
2) mesolimbic dopamine system
Where does nigrostriatal dopamine system originate from? What is it involved in? Degeneration causes what?
- substrantia nigra
- motor control
- Parkinson’s disease
What is the mesolimbic dopamine system involved in? Overactivity leads to what?
- behavior and emotional reward
- schizophrenia (treated by anti-dopamine drugs)
What is norepinephrine used for in PNS? CNS?
PNS: sympathetic neurotransmitter
CNS: affects general level of arousal
- amphetamines stimulate pathways
What are the main CNS excitatory neurotransmitters?
glutamate and aspartate (acidic)
What is an example of an inhibitory neurotransmitter?
glycine
How does glycine work as inhibitor? What inhibits glycine?
- opens Cl- channels to hyperpolarize
- Strychnine blocks glycine receptors
What happens when strychnine works?
causes spastic paralysis
What is GABA and its function? Degeneration can cause what?
gamma-aminobutyric acid (most common in brain)
- inhibitory
- opens Cl- channels
- Huntington’s disease
What is true about EPSPSs?
- graded in magnitude
- no threshold
- depolarization
- summate
- no refractory period
What is spatial summation?
EPSPs from different synapses occur in postsynaptic cell at the same time
What is temporal summation?
EPSPs that occur closely in time can summate before they fade
What is postsynaptic inhibition?
GABA and Glycine produce IPSPs
- IPSPs dampen EPSPs
- hard to reach threshold
When does presynaptic inhibition occur?
1 neuron synapses onto axon or bouton of another neuron
- inhibits release of its neurotransmitter
