ABSTRACT
A. Neuron Structure 1. Neurons have same components as other cells: endo-
plasmic reticulum, ribosomes, lysosomes, peroxisomes, mitochondria, nucleus, cell membrane
2. Characteristic features: axon and dendrites 3. Nucleus: large, spherical with a nucleolus 4. Cell membrane: a phospholipid bilayer
a. Membrane proteins have hydrophobic regions usually made of alpha helices
b. Spectrin-ankyrin network provides link between cytoskeleton and cell membrane
5. Integral proteins: span cell membrane (Fig. 2-1) a. Single transmembrane proteins
1) Receptor tyrosine kinases a) Important in signal transduction b) Many are associated with receptors
2) Cell adhesion molecules (CAMs) a) Important for direct interaction between cell
membrane and intracellular and extracellular elements
b) Critical for cell function, development, plasticity, myelination, response to injury
c) Immunoglobulin (Ig) family: have extracellular Ig-like domains that may bind membranes to
surfaces or initiate signal transduction pathway i) May have one Ig-like domain, e.g., myelin pro-
tein PO ii) Some have two Ig-like domains, e.g., Trk recep-
tors that bind neurotrophic factors iii) This includes sialic-acid binding proteins such
as myelin-associated glycoprotein (MAG) and Schwann cell myelin protein (SMP)
iv) Responsible for calcium-independent cell adhesions: L1 and neural cell adhesion molecule (NCAM)
d) Cadherins i) Transmembrane glycoproteins ii) Responsible for calcium-dependent adhesions
(adherens junctions) of adjacent cells via homophilic interactions (dimerization) among cadherins of same kind
iii) Important in cell-cell interactions and axonal growth
e) Integrins: adhesions to extracellular matrix proteins (laminin, fibronectin, proteins of Ig family)
f) Neuroligins i) Interact with β-neurexins of neuronal presynap-
tic membranes to form intercellular (interneuronal) junctions
ii) Important for linking presynaptic and postsynaptic cells
b. Multitransmembrane domain proteins: ion channels, pumps, transporters, connexins 1) Ion channels: cysteine residue family, glutamate
ionotropic family, G protein-coupled receptor family 6. Cytoskeletal proteins (Fig. 2-1)
a. Microfilaments 1) Polymers of actin: form double-stranded helix 2) Found throughout cytoplasm: aggregate under cell
membrane 3) Found in growth cones, dendritic spines, axons:
important in plasticity
Cell membrane: rich in phospholipids and gangliosides
Integral membrane proteins and connexins are involved in ion homeostasis, electrical activity, and signaling
Adhesion molecules are responsible for interactions of neurons with glial cells and extracellular matrix
4) Actin-binding proteins determine structural arrangement of actin microfilaments and cross-linking and interactions with cell membrane and extracellular matrix a) Spectrin: acts to cross-link actin oligomers and
forms complex with ankyrin, actin, and other cytoskeletal proteins, forming stable submembranous cytoskeletal complex, stabilizing cell membranes
b) Ankyrin c) Fibrin: may be important in formation of filo-
podia in growth cones d) Profilin: promotes actin polymerization e) Gelsolin: cytoplasmic, calcium-regulated, actin-
modulating protein that binds to and severs existing actin filaments and can cap F-actin; mutations of gelsolin lead to familial amyloidosis type IV, Finnish type
b. Microtubules 1) Polymers of α-and β-tubulin 2) Action of guanosine triphosphatase (GTPase) is
important for assembly and disassembly 3) Critical role in intracellular transport: provide infra-
structure for bidirectional axonal transport 4) Microtubule-associated proteins
a) Bind to microtubule polymers or monomers and
facilitate polymerization and assembly b) Function: regulated by state of phosphorylation c) Kinesin and dynein: microtubule-associated pro-
teins important for axonal transport (see below) d) Tau protein
i) Six isoforms produced by alternative splicing of tau gene on chromosome 17
ii) Three isoforms contain three microtubule-binding repeats: 3R (3-repeat) forms
iii) Other three isoforms contain four microtubulebinding repeats: 4R (4-repeat) forms
iv) Normal ratio of 3 tau:4 tau is 1:1 v) Accumulation of a particular hyperphosphory-
lated tau isoform distorts the ratio and reduces microtubule binding, producing characteristic tau filamentous inclusions
vi) Accumulation of hyperphosphorylated 4R tau yields paired helical filaments that form neurofibrillary tangles of Alzheimer’s disease
vii) Example of 3R tauopathy: Pick’s disease viii) Examples of 4R tauopathies: corticobasal degen-
eration, progressive supranuclear palsy, argyrophilic grain disease
c. Neurofilament proteins 1) Polymers of neurofilamin 2) Neuron-specific intermediate filaments in axons
3) Determine axon caliber 4) Function depends on phosphorylation: undergo
ongoing, extensive phosphorylation that slows their transport and obligates them to become an integral part of axonal cytoskeleton (important determinant of axonal caliber)
5) Important role in axonal growth (growth cones and collateral axonal sprouting) and axonal transport
6) Defects/mutations in neurofilament (NEFL) gene cause neuropathy (i.e., Charcot-Marie-Tooth [CMT] type 2e)
7. Perikaryon (soma, cell body): polymorphic; may be stellate, pyramidal, or globular
8. Dendrites: afferent component of neuron a. Dendritic spines: critical sites of plasticity and depend
on clustering of excitatory and inhibitory receptors (see below)
b. Growth cone dynamics depend on 1) Synaptic activity 2) Growth factors
a) Interact through receptor tyrosine kinase B (Trk B) b) Include nerve growth factor (NGF), brain-derived
neurotrophic factor (BDNF), neurotrophin 3 (NT3) and NT4/5
Microfilaments Subunit = actin Location = growth cone, dendritic spine, axons Associated proteins = spectrins, gelsolin, profilin, others Main function = interaction with membrane proteins Dynamic turnover
Microtubules Subunit = tubulin Location = soma, dendrites, axons Associated proteins = microtubule-associated proteins, tau Main function = axonal transport Dynamic turnover
Neurofilaments Subunit = Neurofilament proteins (NF)-L, NFM, NF-H Location = axons Main function = maintenance of axon caliber No dynamic turnover
3) Ligands for Notch (cell membrane protein that promotes dendritic branching, but inhibits dendritic growth)
4) Rho GTPases: regulate actin cytoskeleton; control extension of growth cone (Rho GTPase-dependent transduction)
9. Axon a. Functions
1) Conduction of action potentials 2) Synaptic transmission 3) Fast axonal transport: 200 to 400 mm/day
a) Anterograde: from cell body to axonal terminal i) Transport of vesicles, organelles, receptors, and
other proteins ii) Involves motor protein kinesin
b) Retrograde: from terminal to soma i) Important for membrane recycling and retro-
grade signaling ii) Involves motor protein dynein
4) Slow axonal transport a) Slow component A: 0.2 to 1 mm/day; transports
microtubules and microfilaments b) Slow component B: 2 to 8 mm/day; transports
actin microfilaments and metabolic enzymes 5) Intermediate axonal transport: 50 to 100 mm/day
a) Bidirectional b) Transports mitochondria
10. Myelin a. Compacted myelin
1) Minor dense line or intraperiod line: formed by juxtaposed outer leaflets
2) Major dense line: composed of juxtaposed inner leaflets
3) Proteolipid protein (PLP) and myelin basic protein (MBP): myelin compaction in central nervous system (CNS)
4) Myelin protein zero (MPZ) and peripheral myelin protein (PMP)-22: myelin compaction in peripheral nervous system (PNS)
5) Defects or mutations in compact myelin proteins
Dendritic spines are dynamic structures that have a critical role in synaptic plasticity
Dendritic growth during development depends on synaptic activity, growth factors, ligands for Notch, and cytoskeletal changes triggered by Rho GTPase
lead to both CNS and PNS diseases a) PNS diseases: CMT1b (MPZ), CMT1a (PMP-
22 duplication), hereditary neuropathy with liability to pressure palsies (PMP-22 deletion)
b) CNS disease: Pelizaeus-Merzbacher disease (PLP, linked to chromosome Xq22)
b. Noncompacted myelin 1) Lacks fusion of intracellular loops of myelin 2) Present at paranodes and juxtaparanodes 3) MAG mediates axon-glia adhesions important for
myelination by Schwann cells (PNS) and oligodendrocytes (CNS); monoclonal anti-MAG antibodies occurs in neuropathies associated with paraproteinemias
4) Connexin-32 a) Gap junction protein at paranodal region (gene on
chromosome X) that allows communication within periaxonal space and across myelin sheath
b) Mutation of gene encoding connexin-32 protein is responsible for X-linked form of CMT (CMT-X)
c. Autoimmunity to myelin proteins and gangliosides cause PNS disease 1) Demyelinating neuropathy: monoclonal anti-MAG
antibodies 2) Multifocal motor neuropathy: monoclonal anti-
GM1 antibodies 3) Acute motor axonal neuropathy: polyclonal anti-
GM1 antibodies 4) Miller Fisher variant of acute inflammatory demyeli-
nating polyradiculopathy: polyclonal anti-GQ1B antibodies
11. Nodes of Ranvier
a. High concentration of voltage-gated sodium channels is important for saltatory conduction
b. Ankyrin G: linker protein that clusters voltage-gated sodium channels at these points
c. Also contain CAMs, Na+/K+ adenosine triphosphatases (ATPases), and GM1 gangliosides
12. Paranodal and juxtaparanodal region a. Myelin loops are anchored to axon by contactin and
contactin-associated protein (Caspr)/paranodin) b. Express voltage-gated and inward rectifier potassium
channels that have role in regulating axonal excitability 13. Internode
a. Length of internode varies proportionally with axon diameter
b. Covered by compacted myelin 14. Axonal growth or sprouting
a. Like dendritic growth cones, axons rely on synaptic activity, growth hormones, and Rho GTPases (Rho GTPase-dependent transduction)
b. Growth cone: finger-like extensions called filopodia, with lamellipodia in between these extensions
c. Growth cone guided by gradient of attracting agents (e.g., netrin, laminin), repelling agents (e.g., ephrin, semaphorin), or directing agents (e.g., Robo-Slit inhibits crossing midline)
d. MAG and proteins such as Nogo expressed by fully differentiated oligodendrocytes may inhibit regeneration of CNS axons
B. Glial Cells 1. Astrocytes: large glial cells
Axonal transport Fast anterograde: transort of vesicular proteins (ion channels, neuropeptides) at 200-400 mm/day using kinesin
Fast retrograde: transport of endocytic vesicles (nerve growth factor, toxin, virus) at 200-300 mm/day using dynein
Slow: transport of microtubules, microtubuleassociated proteins, neurofilaments, actin at 0.28 mm/day
Intermediate: bidirectional transport of mitochondria at 50-100 mm/day
Myelin proteins Proteolipid protein (PLP) is involved in central myelin compaction; defect in this protein can lead to Pelizaeus-Merzbacher disease
Defects of myelin protein zero (MPZ) and peripheral myelin protein (PMP)-22 in compacted peripheral myelin cause inherited neuropathies
Noncompacted myelin proteins may be associated with inherited neuropathies (connexin-32 defect) or acquired neuropathies (anti-MAGassociated neuropathy)
MAG, myelin-associated glycoprotein
a. Role in neuronal function 1) Increase glycolysis and production of lactate (a fuel for
neurons) 2) Nitric oxide (NO)-mediated increased blood flow
preferentially to active neuronal microenvironment a) NO: synthesized in neurons, glial cells, endothelial
cells b) NO: rapidly diffusible and important in vasodila-
tation (e.g., penile erection), oxidative injury (acts as free radical), and neuronal plasticity
c) NO: produced by NO synthetase (NOS) d) Arginine is substrate for NOS e) Astrocytes express arginine transporter and provide
bulk of arginine supplies to neurons f) Influx of calcium into neuron through N-methyl-
D-aspartate (NMDA) receptors (activated by glutamate) stimulates neuronal NOS to produce NO
g) NO activates cytoplasmic guanylate cyclase, producing cyclic guanosine monophosphate (GMP)
b. Prevention of excitotoxicity (see below) 1) Buffering extracellular potassium 2) Uptake of excess glutamate and conversion to gluta-
mine-neurons convert it back to glutamate 3) Prevention of free radical-induced damage
a) Glutathione source for mitochondrial antioxidation of H2O2
c. Role in neuronal networking 1) Form neuronal migration paths away from ventricular
layer during development (radial glial cells, Bergmann glia in cerebellum)
2) Calcium wave propagation through interastrocytic gap junctions help networking between neurons supported by astrocytes
d. Formation of blood-brain barrier e. Glial response to injury (gliosis)
2. Oligodendrocytes: myelin-forming cells in CNS a. Vimentin: marker of early oligodendrocyte development
(oligodendrocyte progenitors and pre-oligodendrocytes) b. PLP and MBP: expressed only in mature
oligodendrocytes c. May myelinate a segment of several axons d. Perineuronal oligodendrocytes are also in gray matter
(function unclear) 3. Schwann cells: myelin-forming cells in PNS
a. Nerve growth factor receptor (NGFRt): expressed in Schwann-cell precursors
b. Myelinating Schwann cells express MBP, MPZ, PMP-22 c. Unmyelinating Schwann cells express glial fibrillary
acidic protein (GFAP) and NCAM d. One Schwann cell myelinates a segment of only one axon
e. Many small unmyelinated fibers may be invested by one Schwann cell
f. Peripheral nerve fibers have continuous basement membrane over Schwann cells
4. Ependymal cells a. Single layer of ciliated neuroepithelial cells lining the
ventricles b. Connected by tight junctions to form selective barrier
5. Microglia a. Monocytic phagocytic cells b. Derived from mesoderm, not neuroepithelium c. Few in number, but proliferate rapidly after insult to
nervous system (e.g., ischemia) and become macrophages, acting as scavenger cells to remove damaged tissue and debris
d. No role in maintaining blood-brain barrier
C. Neuronal Microenvironment 1. Cerebral blood flow (CBF)
a. About 750 mL/min b. Increased metabolic demand: proportionate increase in
CBF of cortical gray matter c. Extrinsic regulators of CBF
1) Dependent on cardiac output, systemic blood pressure, blood viscosity, and stimulation of baroreceptors in carotid sinus and aortic arch
2) Sympathetic input from superior cervical ganglion causes vasoconstriction via norepinephrine, neuropeptide Y, and adenosine triphosphate (ATP)
3) Parasympathetic input from sphenopalatine ganglion causes vasodilation via NO, acetylcholine (ACh), and vasoactive intestinal polypeptide (VIP)
4) Sensory innervation arises from trigeminal ganglion
Glial cells Astrocytes are important for potassium buffering, uptake of glutamate, and response to injury in central nervous system
Oligodendrocytes are myelin-forming cells in central nervous system
Schwann cells are myelin-forming cells in peripheral nervous system
Ependymal cells line the ventricles
Microglia are monocytic phagocytic cells
and releases substance P and calcitonin gene-related peptide (CGRP) to produce vasodilation
d. Intrinsic regulators of CBF 1) Regional (local) metabolic regulation with vasodilator
metabolites: cause vasodilatation and increased CBF (e.g., NO, amino acid metabolites, adenosine, potassium, oxygen free radicals)
2) Regional (local) chemical regulation a) Cerebral blood vessels are very sensitive to any
change in the local carbon dioxide tension (PaCO2): increased PaCO2 causes vasodilation, reduced PaCO2 vasoconstriction (mediated primarily by change in extracellular pH)
b) Change in local oxygen tension (PaO2) causes opposite effect: increased PaO2 produces vasoconstriction, reduced PaO2 vasodilation (much less pronounced than PaCO2 effect)
c) Increased regional lactic acid levels in region of ischemia (low regional blood pH) induce regional vasodilation
d) Hyperventilation may be used as temporary measure to reduce intracranial pressure by inducing a reduction in the PaCO2 and causing vasoconstriction (this effect is short-lived and CBF usually returns to baseline and may overshoot)
3) Autoregulation a) Ability to maintain stable CBF despite fluctuations
in mean arterial blood pressure b) Occurs with fluctuations of systemic mean arterial
blood pressures between 60 and 160 mm Hg, acting to preserve constant perfusion pressures to brain
c) In patients with long-standing hypertension, both upper and lower limits of mean arterial pressure in which autoregulation occurs are raised
d) Intraluminal pressure-responsive myogenic reflexive action of vascular wall: inducing vasoconstriction with increased intraluminal pressures and vasodilation with reduced intraluminal pressures
2. Blood-brain barrier a. Anatomy
1) Endothelial cells: primary site of the barrier are relatively deficient in mechanisms for vesicular transport
2) Tight junctions between endothelial cells (composed of occludin, claudins, cingulins, and zonula occludens proteins) make it necessary for substances to pass through the lipid bilayer membrane of endothelial cells (rather than in between endothelial cells)
3) Layer of pericytes surrounds endothelial cells and is delineated by glial foot processes
b. Blood-brain barrier absent in circumventricular organs (subfornicial organ, median eminence, neurohypophysis, vascular organ of lamina terminalis, pineal body, subfornical organ, area postrema)
c. Passage through blood-brain barrier 1) Highly lipid-soluble molecules (carbon dioxide,
volatile anesthetics, circulating steroids, phenytoin, ethanol, nicotine) use simple diffusion through endothelial cell membranes
2) Water-soluble compounds (glucose and amino acids) pass by carrier-mediated facilitated transport
3) Glucose transporter isotype 1 (GLUT1) a) Responsible for transporting glucose down con-
centration gradient b) Driven by relatively higher plasma concentration
of glucose c) Not energy-dependent d) GLUT1 deficiency: patients often normal at birth
(may have microcephaly) and develop intractable seizures, developmental delay; treated with antiepileptic agents and ketogenic diet
4) Large neutral amino acids use a sodium-independent, energy-independent transporter (transported down concentration gradient)
5) Small nonessential amino acids use energy-dependent, sodium-dependent active transport
6) Na+/K+ ATPase: energy-dependent ion channel located at endothelial cell membrane, responsible for energy-dependent exchange of extracellular potassium with intracellular sodium
7) Proteins (insulin, transferrin, vasopressin) by transcytosis through saturable systems
8) Vasogenic edema a) Accumulation of interstitial fluid that occurs with
abnormal, leaky blood-brain barrier b) Malignant brain neoplasms: abnormal permeabil-
ity of tumor endothelial cells
Neuronal Microenvironment Depends On Astrocytic network
Buffers potassium Produces lactate and glutamine
Cerebral blood flow Regulated by NO, amino acid metabolites, adenosine, and autonomic innervation
Blood-brain barrier Formed by tight junctions
c) Meningitis: breakdown of blood-brain barrier from inflammatory response
d) Associated with early vacuolization and swelling of myelin sheaths
e) Often responsive to treatment with corticosteroids f) In contrast, cytotoxic edema refers to primary neu-
ral or glial intracellular swelling, often not responsive to corticosteroids: movement of water molecules in extracellular space is limited and their diffusion is restricted, appearing as high signal in diffusion-weighted imaging
3. Cerebrospinal fluid (CSF) a. Formed primarily by choroid plexus in lateral ventricles b. Rate of production: 0.35 mL/min, about 500 mL/day c. Estimated total CSF volume: 140 mL d. Normal CSF pressure: between 80 and 180 mm H2O e. Course of flow: lateral ventricles through foramina of
Monro into third ventricle and through cerebral aqueduct into fourth ventricle; exits fourth ventricle through single foramen of Magendie in midline and paired foramina of Luschka in lateral walls of fourth ventricle to enter subarachnoid space
f. CSF flows in thecal sac and around brain convexities and tracks along blood vessels into Virchow-Robin spaces
g. CSF is absorbed into superior sagittal sinus through arachnoid granulations (pacchionian granulations) 1) Clusters of villi are essentially protrusions of arach-
noid membrane through dura mater into superior sagittal sinus
2) Functional one-way pressure valves: blood-brain interface with one-way flow of CSF and its contents into venous system, when CSF pressure is maintained above a certain threshold
h. Blood-CSF interface formed by cuboidal or columnar epithelial cells surrounding fenestrated capillaries
i. Apical surface of choroidal cells contains ATPase which acts to secrete sodium ions into ventricular CSF at the apical membrane in exchange for potassium
j. Carbonic anhydrase produces bicarbonate (from water and carbon dioxide), which is transported across apical membrane via bicarbonate-chloride exchange and acts to neutralize positive sodium charge
k. CSF production depends on active ion transport with secretion of sodium by Na+/K+ ATPase and chloride by the Cl−/HCO3− exchanger
l. Carbonic anhydrase inhibitors such as acetazolamide act to reduce CSF production
D. Trophic Factors and Neuronal Plasticity 1. Coincidence detection
a. Temporal coincidence (co-occurrence) of events strengthens synaptic connection: “neurons that fire together wire together,” strengthening functionally relevant connections and eliminating exuberant connections
b. Sensory input can be critical during brief critical period 1) Harlow: demonstrated that isolated young monkeys
could not form normal social interactions 2) Hubel and Wiesel: showed that monocular visual
deprivation during a specific postnatal period decreased formation of axonal connections for deprived eye and prevented formation of normal eye dominance columns
c. Hebbian process 1) Repeated stimulation of specific receptors leads slowly
to formation of “cell-assemblies” that can act as closed system after stimulation ends
2) High-frequency correlated activity produces longterm potentiation (LTP) (discussed below)
3) Low-frequency uncorrelated activity produces longterm depression (LTD) (discussed below)
4) Lack of long-term stimulation or use of network leads to loss of function and cell death: “use it or loose it”
2. Neurotrophism (Fig. 2-2) a. Innervating neurons compete for limited supply of tar-
get-derived neurotrophic factors (e.g., NGF, BDNF, NT-3, cytokines)
b. Growth factor is released by the target and binds to receptors that transduce a signal, which is transmitted retrogradely to the cell body and is important for neuron’s survival
c. Neurotrophic receptors 1) Receptor tyrosine kinases (Trk A, Trk B, Trk C) are
Occipital horn normally has a high degree of variability and asymmetry: most often rudimentary and not clinically significant
Innervating neurons compete for a restricted quantity of target-derived trophic factor
Neurotrophins control many critical decisions in development and death of neurons
Loss of neurotrophic support from a target of neural innervation may be responsible for many neurodegenerative diseases
low-affinity receptors when activated alone and relatively specific to individual growth factors
2) Common neurotrophin receptor (p75NTR) increases affinity of tyrosine kinases when activated together but is a nonspecific receptor
3) Specific cytokine receptors coupled with a receptor kinase
d. Neurotrophic pathways 1) Mitogen-activated protein kinase (MAPK)
a) Extracellular signal-regulated kinase (ERK 1/2): activated by growth factors
b) High osmolality glycerol-induced kinase (p38/HOG): activated by stress
c) C-jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK): activated by stress
2) Phosphoinositide-3 kinase (PI3K), phospholipase C (PLCγ), protein kinase C (PKC)
3) Janus kinase/signal transducer and activator of transcription (JAK/STAT)
3. Cell death (Fig. 2-2) a. Necrosis
1) Often precipitated by energy failure 2) Cell swelling (cytotoxic edema) and membrane
rupture 3) Likely to produce inflammatory response
b. Apoptosis 1) Normal developmental process of programmed cell death 2) Cellular and nuclear shrinkage (DNA fragmentation)
but no membrane rupture 3) Does not produce inflammation 4) Involves activation of apoptotic receptors, activation
of caspases, alteration of interaction and balance between proapoptotic and antiapoptotic factors, mitochondrial failure, and oxidative stress
5) Mitochondrial failure: increased intramitochondrial calcium, release of cytochrome c and other apoptotic factors into the cytosol
6) Cytochrome c activates Procaspase 9 via interaction with apoptosis activating factor-1 (Apaf-1)
7) Activation of caspase 9 is inhibited by inhibitor of apoptosis proteins (IAPs), which are in turn inhibited by mitochondrial-derived Smac (second mitochondrial activator of caspases)
8) Caspases eventually act to mediate DNA fragmentation and lipid peroxidation
c. Apoptotic receptors 1) Glutamatergic calcium channels: excitotoxicity 2) Death receptors (e.g., Fas, tumor necrosis factor recep-
tor [TNFR], low-affinity nerve growth factor receptor [p75], TRAIL)
d. Apoptotic pathways 1) Calcium-induced excitotoxicity
a) DNA degradation via endonucleases b) Phospholipase activation c) Protein degradation via calpain d) Lipid peroxidation via nitration (role of
calcium/calmodulin and NOS) (see above) i) NOS: activated by increased intracellular calci-
um (and calcium bound to calmodulin); calcium influx mediated by NMDA receptors
ii) NO reacts with superoxide (O2−), producing peroxynitrite (ONOO−)
iii) Peroxynitrite acts as highly reactive radical e) Mitochondrial energy failure via inhibition of
Krebs cycle 2) Cellular effectors of apoptosis (caspase cascade)
a) Caspases (cysteine proteases) i) Direct effector pathway for death receptors ii) Caspases activate other caspases and cleave pro-
teins leading to DNA destruction and lipid peroxidation
b) Bcl-2 family members i) Proapoptotic members (Bad, Bax, Bid): form
pores (MPT) in mitochondrial membrane; cytochrome c leaking out of mitochondria stimulates caspase cascade via Apaf 1
ii) Antiapoptotic members (Bcl-2, Bcl-XL): inhibit cytochrome c leakage
3) Availability of glutathione via astrocytes stimulates conversion of H2O2 to H2O and prevent respiratory chain failure-associated lipid peroxidation a) Stimulation of poly (ADP-ribose) polymerase
(PARP) induces NADP depletion and failure of conversion of H2O2 to H2O, leading to lipid peroxidation
b) PARP also activates proapoptotic nuclear factorkappa B and p53
4. Protein denaturation and folding (Fig. 2-3) a. Heat shock proteins (HSPs)
1) Molecular chaperones responsible for preventing premature folding and destruction of native proteins
2) Usually expressed continually, but expression is induced at times of cellular stress (e.g., ischemia)
3) Transcription of HSP gene is mediated by binding of the heat shock factor to heat shock element (DNA sequence)
4) HSP-70 binds to misfolded or denatured proteins to prevent folding and destruction and induces ATPdependent refolding of denatured proteins
b. Ubiquitin-proteasome system
1) Catalyzes degradation of damaged or misfolded proteins
2) Ubiquitination of proteins and ubiquitin-dependent degradation of cell proteins important for maintenance of cell cycle and apoptosis (ATP-dependent process)
3) Proteins are flagged for degradation by attachment of polyubiquinated chain
4) Three-enzyme system: protein undergoes activation (enzyme E1), conjugation (enzyme E2), ligation (enzyme E3); mutation of the PARK2 gene encoding parkin protein (an E3 ubiquitin ligase) is responsible for an inherited parkinsonism with autosomal recessive inheritance
5) End result: polyubiquinated protein destined for 26 S proteasome, which in turn cleaves protein complex to ubiquitin and protein fragments
6) Protects against accumulation of denatured proteins and apoptosis
c. Abnormal inclusions in neurodegenerative disorders (Table 2-1) 1) Aggregation of nascent proteins that have lost normal
function and are resistant to degradation 2) Abnormal ubiquitination and impaired ubiquitin-
dependent degradation can cause aggregation of ubiquitin cellular inclusions characteristic of many neurodegenerative conditions (Alzheimer’s disease and Parkinson’s disease)
E. Synapses (Fig. 2-4) 1. Presynaptic component
a. Synaptic vesicles 1) Small clear vesicles: γ-aminobutyric acid (GABA),
glycine, glutamate, or ACh 2) Intermediate-size vesicles: monoamines 3) Large dense-core vesicles: neuropeptides 4) Uptake of neurotransmitters in the nerve terminal is
through vesicular protein ATPases specific to the neurotransmitter
b. Vesicle mobilization 1) Synapsin binds vesicles to cytoskeleton at active zone 2) Phosphorylation of synapsin allows mobilization of
vesicles 3) Rab 3 proteins are monomeric GTP-binding proteins
implicated in control of regulated exocytosis c. Docking, priming, fusion
1) Depends on interaction between SNARE complex proteins a) Vesicular (V)-SNARE is synaptobrevin b) Transmembrane (T)-SNARE is syntaxin and
SNAP-25 2) Clostridial toxins hydrolyze SNARE proteins, pre-
venting neurotransmitter release (Fig. 2-4): botulinum toxins (BTX) A and E cleave SNAP25; BTX C acts on syntaxin; BTX B and F cleave synaptobrevin
d. Exocytosis 1) Depends on calcium influx through N and P/Q type
calcium channels 2) Synaptotagmin acts as calcium sensor for vesicle
e. Endocytosis and recycling 1) Clathrin coats site of release, forming a pit 2) Dynamin causes fission from cell membrane
a) This is regulated by endophilin, amphiphysin, and adaptins (clathrin adaptor proteins, which promote assembly of clathrin network)
b) Synaptojanin and HSC-70 are involved in uncoating of vesicle to make it ready for neurotransmitter uptake
2. Postsynaptic component a. Anchoring of presynaptic and postsynaptic active zones:
adhesion molecules anchor active zones together in junctional folds
b. Nicotinic ACh receptors (nAChRs) in neuromuscular junction: clustered at postjunctional folds from multiple regulatory proteins 1) Expression of nAChR: activated by neuroligins 2) Clustering is triggered by agrin 3) Laminin, MuSK (sarcoglycan in T-tubes), and rapsyn
Synaptic vesicles Small clear vesicles store and release γ-aminobutyric acid, glutamate, glycine, and acetylcholine
Intermediate dense-core vesicles store and release monoamines
Large dense-core vesicles store and release neuropeptides
Vesicle mobilization Storage of neurotransmitters depends on transporter-selective vesicular protein ATPases
Synapsin phosphorylation facilitates vesicle mobilization
Membrane docking, priming, and fusion depends on SNARE protein complex, which includes synaptobrevin, syntaxin, and SNAP-25
Influx of calcium in depolarized axon terminal triggers exocytosis of vesicle through interaction with synaptotagmin
Table 2-1. Inclusion Bodies in Central Nervous System Neurodegenerative Disorders
Cellular Implicated Inclusion (protein) localization disorder
(ankyrin in T-tube): also critical proteins for nAChR clustering
4) Anti-MuSK antibodies have been associated with myasthenia gravis
5) Different phenotypes of myopathies arise depending on type of regulatory protein deficiency or malfunction (Fig. 2-4)
F. Ion Channels (Fig. 2-5 A) 1. Basic electrophysiology
a. The membrane potential (Vm) 1) Vm: determined by the difference between intracel-
lular and extracellular potentials
Vesicle endocytosis and recycling depends on Vesicle coating by clathrin
Fission by dynamin
Regulation by endophilin, amphiphysin, and synaptojanin
2) Equilibrium potential of ion (Eion): determined by ion’s concentration gradient a) Sodium, calcium, and chloride: higher extracellu-
lar concentrations b) Potassium and other anions: higher intracellular
concentrations c) Ionic gradient depends on ATP-driven ion pumps
and regulation of neuronal microenvironment glial cells
d) Nernst equation can be used to calculate Eion:
Eion = RT/zF ln [ion]o/[ion]i = 58 log [ion]o/[ion]i
e) Vm can be determined by the Goldman equation:
RT ln PK[K+]o + PNa[Na+]o + PCl[Cl−]iVm = _______________________________ F PK[K+]i + PNa[Na+]i + PCl[Cl−]o
3) Resting membrane potential (RMP): Vm at steady state (no net flow of ions across the membrane) a) For neurons: between –60 and –80 mV b) For skeletal muscle: –85 to –95 mV
4) Opening an ion channel brings Vm closer to Eion of that ion a) Opening a sodium channel with ENa of +70 mV
or a calcium channel with an ECa of +150 mV: depolarizes cell
b) Opening a potassium channel with EK of –100 mV: hyperpolarizes cell
c) Opening a chloride channel with ECl of –75 mV: may hyperpolarize or depolarize cell
b. Action potential 1) Once depolarization reaches the threshold for voltage-
gated sodium channels, there is rapid depolarization (fast sodium spike)
2) Any stimulus greater than threshold: produces action potential independent of its intensity; an all-or-none event
3) Voltage-gated sodium channels rapidly inactivate 4) Slow activation of potassium channels repolarizes
membrane and produces transient hyperpolarization 5) This transient hyperpolarization produces refractory
period, preventing retrograde propagation of action potentials
6) Propagation of the action potential depends on the resistance of the axoplasm and the outward leakage of current a) Length constant (λ): distance along dendrite
where ΔVm has decayed to 37% of its value at the site of stimulation; usually between 0.1 and 1.0 mm
λ = rm/ra b) Time constant (τ): amount of time for Vm to
move 63% of way to its final value c) Conduction velocity: higher in large diameter
fibers because transverse membrane resistance is higher while longitudinal axioplasm resistance is lower
2. Voltage-gated channels (Fig. 2-5 A) a. Depolarization opens channel (sliding helix model)
1) Four subunits: α subunit forms pore and determines ion selectivity and voltage sensitivity
2) α unit of the voltage-gated sodium and voltage-gated calcium channels are made of four domains in tandem (TMI-IV), each composed of six transmembrane regions
Ion channels are multimetric transmembrane proteins that form hydrophobic pores through which ions can passively flow
Activation of ligand-gated ion channels produces excitation or inhibition
Sodium channels Fast inactivating (tetrodotoxin sensitive) are responsible for spike of action potential
Slow inactivating (tetrodotoxin resistant) are responsible for plateau or pacemaker potentials
Calcium channels L-type: slow depolarization and excitationcontraction coupling in skeletal muscle
P/Q and N types: cause neurotransmitter release
T type: produce low-threshold spike (oscillatory burst firing)
Potassium channels Delayed rectifying repolarization after action potential
Type A (fast)
2-Pore domain
Slow calcium activated: sets excitability in response to neuron’s energy state
Inward rectifier: responsible for resting membrane potential
Channelopathies
Channel Disorders
Disorders associated with ligand-gated channels
Channel Disorders
b. Potassium channels 1) Responsible for hyperpolarization and reduction of
neuronal excitability 2) Categorized on basis of number of transmembrane
domains 3) Inward rectifier potassium channels (KIR)
a) Two transmembrane domains b) Responsible for maintaining resting membrane
potential: when hyperpolarization of membrane potential “overshoots” below resting membrane,
Kir is activated and conducts K current into cell, bringing Vm toward EK
c) May be ATP-gated or G protein-coupled 4) Two-pore (2P) domain potassium channels
a) Four transmembrane domains b) Help to determine resting membrane, influences
action potential duration, and important in modulation of neuronal excitability
c) TASK channel currents: very sensitive to changes in extracellular pH
Vm is the elecrical potential inside the cell relative to the outside
Equilibrium potential is the voltage difference that offsets the tendency of an ion to move down its concentration gradient
Opening an ion channel brings the membrane potential toward the equilibrium potential of that ion (opening sodium and calcium channels causes depolarization; opening potassium channels causes hyperpolarization)
G protein-coupled receptors have neuromodulatory effects, which are determined by G protein molecular switch
G protein Action Receptors Gαs cAMP β-adrenergic, D1,
production 5-HT4-7, H2, VIP, Activation of CGRP PKA
Gαi/o Opens potas-GABAB, mGluR2-4, sium channels α2-adrenergic, D2,
Closes calcium 5-HT1, opioid, channels somatostatin; Y2,
endocannabinoids Gq/11 Activates PKC mGluR1,5; M1;
Releases cal-α1-adrenergic; cium from 5-HT2; H1; subintracellular stance P; angiostores tensin II; bradykinin
CGRP, calcitonin gene-related protein; GABA, γ-aminobutyric acid; 5-HT, hydroxytryptamine; PKA, protein kinase A; PKC, protein kinase C; VIP, vasoactive intestinal polypeptide.
