The Nervous System Flashcards
1
Q
What 2 types of cells are in the nervous system?
A
Neurons and supporting cells
2
Q
What is the nervous system divided into?
A
- Central nervous system (CNS) = brain and spinal cord
- Peripheral nervous system (PNS) = cranial nerves and spinal nerves
3
Q
What is a neuron?
A
Basic structural and functional units of the nervous system (cannot divide by mitosis)
- respond to physical and chemical stimuli
- produce and conduct electrochemical impulses
- release chemical regulators
4
Q
What is a nerve?
A
Bundle of axons located outside the CNS ; most composed of both motor and sensory fibers
5
Q
What is the cell body (perikaryon)?
A
“Nutrition center”
- Cell bodies within CNS clustered into nuclei
- Cell bodies within PNS clustered into ganglia
6
Q
What do the dendrites do?
A
Provide receptive area and transmit electrical impulses to cell body
7
Q
What does the axon do?
A
Conducts impulses away from cell body
8
Q
What is axoplasmic flow or axonal transport?
A
Proteins and other molecules are transported by rhythmic contracts to nerve endings
-Employs microtubules for transport and may occur in orthograde or retrograde direction
9
Q
What is the functional classification of neurons based upon?
A
Direction impulses conducted
10
Q
What is a sensory or afferent neuron?
A
Conduct impulses from sensory receptors into CNS
11
Q
What is a motor or efferent neuron?
A
Conduct impulses out of CNS to effector organs
12
Q
What are association or interneurons?
A
Located entirely within the CNS and serve as integrative function
13
Q
What is the structural classification of neurons based upon?
A
Based on the number of processes that extend from cell body
14
Q
What are bipolar neurons?
A
Have 2 processes
-ex: retina of the eye
15
Q
What are multi-polar neurons?
A
Have several dendrites and 1 axon
-ex: motor neuron
16
Q
What is a pseudounipolar neuron?
A
A short single process that branches like a T
-ex: sensory neurons
17
Q
What are the PNS supporting cells?
A
- Schwann cells: provide insulation; successive wrapping of the cell membrane; outer surface encased in glycoprotein basement membrane
- Nodes of Ranvier: unmyelinated areas between adjacent Schwann cells that produce nerve impulses
- Satellite cells: support neuron cell bodies within ganglia
18
Q
What are the CNS supporting cells?
A
-Oligodendrocytes: process occurs mostly post-natally
(each has extensions that form myelin sheaths around several axons, provide insulation)
-Astrocytes
19
Q
How do schwann cells function in nerve regeneration?
A
Act as phagocytes, as the distal neuronal portion degenerates. They’re surrounded by basement membrane, form regeneration tube:
- Serve as guide for axon
- Send out chemicals that attract the growing axon
- Axon tip connected to cell body begins to grow towards destination
20
Q
T or F: CNS has limited ability to regenerate.
A
True
- absence of continuous basement membrane
- oligodendrocytes molecules inhibit neuronal growth
21
Q
T or F: Proximal portion of injured nerve fiber degenerates and is phagocytosed ; Distal end of nerve fiber regenerates into tube of Schwann cells
A
False
- distal end degenerated
- proximal end regenerates
22
Q
What are neurotrophins?
A
Promote neuron growth ; factors include:
- Nerve growth factor (NGF)
- Brain-derived neurotrophic factor (BDNF)
- Glial-derived neurotrophic factor (GDNF)
- Neurotrophin-3 and neurotrophin-4/5
- *both temporal and spatial organization of NGFs determine if regeneration is possible
23
Q
What is the role of neurotrophins in the fetus?
A
Embryonic development of sensory neurons and sympathetic ganglia (NGF and neurotrophin-3)
24
Q
What is the role of neurotrophins in adults?
A
- Maintenance of sympathetic ganglia (NGF)
- Mature sensory neurons need for regeneration
- Required to maintain spinal neurons (GDNF)
- Sustain neurons that use dopamine (GDNF)
25
What do myelin-associated inhibitory proteins do?
Inhibit axon regeneration
26
What are astrocytes and what are some of their functions?
Most abundant glial cell
- Vascular processes terminate in end-feet that surround the capillaries
- Stimulate tight junctions, contributing to blood-brain barrier
- Regulate external environment of K+ and pH
- Take up K+ from ECF, NTs released from axons, and lactic acid
27
What do astrocytes play an important role in?
Regulating and monitoring glutamate, glutamine and ammonia levels
28
What is microglia?
- CNS supporting cell
| - phagocytes, migratory
29
What are ependymal cells?
- CNS supporting cell
- secrete CSF
- line ventricles
- function as neural stem cells
- can divide and progeny differentiate ("plasticity")
30
What is the blood-brain barrier?
Capillaries in brain do not have pores between adjacent endothelial cells, they are joined by tight junctions
31
Molecules within brain capillaries moved selectively through endothelial cells by:
Diffusion
Active transport
Endocytosis
Exocytosis
32
What is a resting membrane potential (RMP)?
Potential voltage difference across membrane that all cells maintain
- largely the result of negatively charged organic molecules within the cell
- limited diffusion of positively charged inorganic ions
33
What is the permeability of cell membrane due to?
- Electrochemical gradients of Na+ and K+
| - Na+/K+ ATPase pump
34
What is excitability/irritability?
Ability to produce and conduct electrical impulses
35
What is depolarization?
Potential difference reduced (become more positive)
36
What is repolarization?
Return to resting membrane potential (become more negative)
37
What is hyperpolarization?
More negative than RMP
38
What are voltage gated channels?
Part of proteins that comprise the channel; open in response to change in membrane potential
39
How many types of VG channels are there for K+?
2; 1 always open, 1 closed in resting cell
40
Howw many type of VG channels are there for Na+?
1; always closed in resting cells (some Na+ does leak into the cells)
41
What is an action potential?
Stimulus causes depolarization to threshold
42
What happens when VG Na+ channels open?
- Electrochemical gradient inward -> + feedback loop
- Rapid reversal in membrane potential from -70 to + 30 mV (DEPOLARIZATION)
- VG Na+ channels become inactivated
43
What happens when VG K+ channels open?
- Electrochemical gradient outward -> - feedback loop
| - Restore original RMP from +30 to -70 mV (REPOLARIZATION)
44
AP is produced by an increase in ___ permeability. After short delay, increase in __ permeability.
Na+ ; K+
45
Do APs require active transport?
No; depolarization and repolarization occur via diffusion
| -once AP completed, Na+/K+ ATPase pump extrudes Na+, and recovers K+
46
If the cell membrane only gets to half its threshold will an AP still occur?
No; APs are "all-or-none"
- when threshold reached, maximum potential change occurs
- amplitude does not normally become more positive than +30 mV because VG Na+ channels close quickly and VG K+ channels open
- duration is the same, only open for a fixed period of time
47
What is recruitment?
Stronger stimuli can activate more axons with a higher threshold
48
What is the absolute refractory period?
Axon membrane is incapable of producing another AP during this time
49
What is the relative refractory period?
Axon membrane can produce another AP, but requires a stronger stimulus
- VG ion channel shape alters at the molecular level
- VG K+ channels are open
50
Describe the cable properties of neurons.
Ability of neuron to transmit charge through cytoplasm
| -Axon cable properties are POOR (high internal resistance, many charges leak out of the axon through membrane)
51
Does an AP travel down the entire axon?
No; each AP is a stimulus to produce another AP in the next region of membrane with VG channels
52
How is conduction rate in an umyelinated axon?
Slow due to a ton of internal resistance -> slow for AP to propagate
- propagation occurs in 1 direction; previous region is in its refractory period
* *Multiple Sclerosis is due to demyelination -> charges move extremely slow
53
How is conduction rate in a myelinated axon?
Very fast rate of conduction due to saltatory conduction
- myelin prevents movement of Na+ and K+ through the membrane
- interruptions in myelin (Nodes of Ranvier) contain VG Na+ and K+ channels
- APs occur only at the nodes -> AP at 1 node depolarizes membrane to reach threshold at next node
54
What is a synapse?
Functional connections between a neuron and another neuron or effector cell
55
What is the direction of synaptic transmission?
One direction only: axon of first (presynaptic) -> second (postsynaptic) neuron
-Presynaptic terminal (bouton) releases a NT
56
What is an electrical synapse?
Adjacent cells electrically coupled through a channel (gap junction)
- each gap junctin is composed of 12 connexin proteins
- impulses can be regenerated without interruption in adjacent cells (fast and efficient)
- exs: smooth and cardiac muscle, brain and glial cells
57
What is a chemical synapse?
Terminal bouton is separated from postsynaptic cell by synaptic cleft -> NTs are released from synaptic vesicles -> vesicles fuse with axon membrane and NT released by exocytosis
- amount of NTs released depends upon frequency of AP
- more dominant form of transmitting signal
58
What are the most common NTs involved in chemical synapses?
Acetyl CoA and glutamate
59
What occurs in synaptic transmission?
NT release is rapid because many vesicles from fusion-complexes at "docking site"
1) AP travels down axon to bouton
2) VG Ca2+ channels open -> Ca2+ enters bouton down concentration gradient -> inward diffusion triggers rapid fusion of synaptic vesicles and release of NTs
3) Ca2+ activates calmodulin -> activates protein kinase
4) Protein kinase phosphorylates synapsins
5) NTs are released and diffuse across synaptic cleft
6) NT (ligand) binds to specific receptor proteins in postsynaptic cell membrane
7) Chemically-regulated gated ion channels open -> either EPSP (depolarization) or IPSP (hyperpolarization)
8) NT inactivated to end transmission
60
What are synapsins?
Aid in the fusion of synaptic vesicles
61
What is an EPSP?
Excitatory postsynaptic potential = depolarization
| -occurs if Na+ channels open
62
What is an IPSP?
Inhibitory postsynaptic potential = hyperpolarization
| -occurs if Cl- or GABA channels open
63
Is Acetylcholine (ACh) excitatory or inhibitory NT?
Both; depending on organ involved
64
Where are nicotinic ACh receptors found?
In autonomic ganglia and skeletal muscle fibers
-excitatory role
(clue to remember: "smokers AUTOmatically feel the nicotine in their BONES and it gets them excited")
65
Where are muscarinic ACh receptors found?
In the plasma membrane of smooth and cardiac muscle cells, and in cells of particular glands
-inhibitory role
66
What is a ligand-operated ACh channel?
Most direct mechanism; ion channel runs through receptor
| -Receptor has 5 polypeptide subunits that enclose the channel
67
How many subunits does a ligand-operated ACh channel have?
- 2 subunits contain ACh binding sites (channel opens when both sites bind to ACh -> permits diffusion of Na+ in and K+ out of postsynaptic cell)
- Inward flow of Na+ dominated -> EPSP
68
What is a G-protein operated ACh channel?
Slower mechanism; ion channels are separate proteins located away from the receptors
69
How many subunits does a G-protein operated ACh channel have?
1; binding of ACh activates alpha G-protein subunit -> Alpha subunit dissociates and diffuses through membrane until it binds to ion channel, opening it
-sometimes beta-gamma complex dissociates but typically it is the alpha subunit
70
What is acetylcholinesterase (AChe)?
Enzyme that inactivates ACh
- present on postsynaptic membrane or immediately outside the membrane
- prevents continued stimulation
- regulates contraction/relaxation
71
What is the treatment for Myasthenia Gravis?
Not enough ACh is produced -> can't produce a strong signal -> weakness and rapid fatigue of muscles
-tx: binds AChE and inhibits it so that the few ACh that are present can stick around longer
72
What poisons target AChe?
Sarin and VX gas
| -diaphragm would stop because no relaxation, everything stays in contraction
73
How does ACh communicate in the CNS?
One neuron communicates with another neuron
- Cholinergic neurons use ACh as NT
- First VG channels are located at axon hillock -> EPSPs spread by cable properties to initial segment of axon
- Gradations in strength of EPSPs above threshold determine frequency of APs produced at axon hillock
74
How does ACh communicate in the PNS?
One neuron communicates with a muscle sarcomere
- Somatic motor neurons synapse with skeletal muscle fibers
- Release ACh from boutons -> produce end-plate potential (EPSPs)
- Depolarization opens VG channels adjacent to end plate
75
What are monoamine NTs?
Epinephrine, norepinephrine, serotonin, dopamine
- Released by exocytosis from presynaptic vessels
- Diffuse across synaptic cleft
- Interact w/ specific receptors in postsynaptic membrane
76
How are monoamines inhibited?
Enzyme degradation by MAO (in the presynaptic membrane) and COMT (in the postsynaptic membrane)
-antidepressants are MAO inhibitors (serotonin will not be reuptaken -> last longer)
77
Do monoamines directly open ion channels?
No, acts through second messenger (such as cAMP)
1) Binding of norepinephrine stimulates dissociation of G-protein alpa subunit
2) Alpha subunit binds to adenylate cyclase -> ATP -> cAMP
3) cAMP activates protein kinase, phosphorylating other proteins
4) Open ion channels
78
What is serotonin derived from and what neurons does it affect?
From L-tryptophan; neurons with cell bodies in raphe nuclei
79
What does serotonin regulate?
Mood, behavior, appetite, and cerebral circulation
80
What are SSRIs?
Serotonin-specific reuptake inhibitors
- Inhibit reuptake and destruction of serotonin -> prolonging action of NT
- antidepressants
81
What neurons do dopamine affect? Where do the axons project into?
Neurons with cell bodies in midbrain; nigrostriatal dopamine system and mesolimbic dopamine system
82
What is the nigrostriatal dopamine system? Problems here are found in which disease?
Neurons in substantia nigra send fibers to corpus striatum
- initiation of skeletal muscle movement
- Parkinson's disease: degeneration of neurons in substantia nigra
83
What is the mesolimbic dopamine system?
Neurons originate in midbrain, send axons to limbic system
- Involved in behavior and reward
- Addictive drugs promote activity in nucleus accumbens
84
Where does norepinephrine function as a NT?
Both PNS and CNS
- PNS: smooth muscles, cardiac muscle and glands (increase in BP, constriction of arteries)
- CNS: general behavior
85
What amino acids are major excitatory NTs in the CNS?
Glutamic acid and aspartic acid
86
What is glutamic acid involved in?
NMDA receptor involved in memory storage
| -Excitatory
87
What is glycine involved in?
Helps control skeletal movements
- Inhibitory, produces IPSPs
- Opening of Cl- channels
88
What is the most prevalent NT in the brain?
GABA (gamma-aminobutyric acid)
- Inhibitory, produces IPSPs
- Hyperpolarizes postsynaptic membrane
- Motor function in cerebellum
89
What 5 polypeptides act as NT?
1) CCK: promote satiety following meals
2) Substance P: major NT in sensations of pain
3) Endogenous opiods
4) Neuropeptide Y
5) NO
90
What are endogenous opiods?
Brain produces its own analgesic endogenous morphine-like compounds, blocking the release of substance P
- Beta-endorphin (runner's high)
- enkephalins
- dynorphin (euphoria; coaine and heroine have similar effects)
91
What is the most abundant neuropeptide in the brain?
Neuropeptide Y
- Inhibits glutamate in hippocampus
- powerful stimulator of appetite (antagonistic to CCK)
92
How does NO exert its effects?
By stimulation of cGMP
- Macrophages release NO to help kill bacteria
- Involved in memory and learning
- Smooth muscle relaxation (i.e. Viagra)
93
What are endocannabinoids?
Act as analgesics; bind to the same receptor as THC
| -function as retrograde NT
94
What is carbon monoxide involved in?
Stimulate production of cGMP within neurons (CGMP acts as 2nd messenger to activate protein kinase)
- Promotes odor adaptation in olfactory neurons
- May be involved in neuroendocrine regulation in hypothalamus
95
Do EPSPs have a threshold?
No threshold, decreases resting membrane potential (release a small amount of NTs and membrane potential gets closer to threshold)
- Graded in magnitude
- Have no refractory period
96
Can EPSPs summate?
Yes, if sum of EPSPs = threshold -> AP
- Spatial summation: numerous boutons converge on a single postsynaptic neuron (distance; many neurons are firing)
- Temporal summation: successive waves of NT release (time; one neuron firing a lot over time)
97
What is long-term potentiation?
May favor transmission along frequently used neural pathways ("use it or lose it")
-Neuron is stimulated at high frequency, enhancing excitability of synapse -> improves efficacy of synaptic transmission
98
Neural pathways in hippocampus use ____, which activates ____ receptors.
glutamate; NMDA
| -involved in memory and learning
99
What is presynaptic inhibition?
Amount of excitatory NT released is decreased by effects of second neuron, whose axon makes synapses with first neuron's axon
100
Do IPSPs have a threshold?
No threshold; increases membrane potential (hyperpolarization)
- Can summate to reach threshold
- No refractory period
- Inhibition can be presynaptic or postsynaptic
