Chapter 2 - Fundamentals of neurobiology Flashcards
Cell theory
- The nervous system is composed of individual cells
- Supported by Ramon y Cajal
Reticular theory
- The nervous system forms a continuous network (‘reticulum’) of fused processes
- Supported by Camillo Golgi
Camillo Golgi (1843-1926) and the Golgi silver impregnation technique (1873)
Golgi used a new staining technique that stains fewer cells instead of all of them, allowing us to see full neurons
Santiago Ramon y Cajal (1852-1934)
Used the Golgi method to decide between the cell theory and reticular theory, and found that there was strong support for the cell theory, but it could not yet be proved because at that time light microscopy was being used, so the smallest measurements that could be seen were around 1µm, so synaptic gaps could not be seen
1 nm (nanometer)
10 x -9 m (meter)
1 µm (micrometer)
10 x -6 m (meter)
Proof of cell theory
Occurred in the 1950s when electron microscopy was used to prove the existence of synaptic gaps
The major parts of the neuron are…
- Soma / cell body
- Dendrites
- Axon
Where are axon potentials initiated?
The axon hillock
Length of axons vs dendrites
Axons are much longer than dendrites in most cases
The three major features of neurons are…
- Polarization: receptive dendrites, axons with synaptic terminals; flow of impulses in one direction
- Electrical and chemical excitability
- Specialized secretory properties
Morphology of the neuron
- Soma (cell body)
- Perikaryon (cytoplasm around nucleus)
- Axon
- Axon hillock
- Collaterals (axonal branches)
- Boutons (axonal terminals)
- Dendrites
- Dendritic spines (input zones for synaptic information)
Dendritic spine
- Short dendritic extension, exhibiting different morphologies (thin, mushroom, branched, stubby)
- Specialized zone for synaptic input
- Neck of spine restricts diffusion between head of spine and rest of dendrite (‘compartmentalization’)
- Serves to increase area of neuron available for synaptic input
- As many as 40,000 spines are present on largest pyramidal neurons (meaning it receives input from 40,000 synapses) –> one spine : one pre-synaptic synapse
- Typically each spine contains one excitatory synapse
- The approximate density of excitatory input on a neuron can be inferred from an estimate of its number of spines
Myelin
- Not all neurons are myelinated
- Myelin sheaths originate from non-neuronal cells (glial cells): oligodendrocytes in the CNS and Schwann cells in the PNS
- It provides axon insulation and facilitates rapid impulse transmission
- Myelin is not continuous –> the areas in between are Nodes of Ranvier
- Nodes of Ranvier have many Na+ channels
- Internodes are the areas in between Nodes of Ranvier, and are covered in myelin
Multiple Sclerosis (MS)
- An autoimmune condition in which the immune system attacks the CNS, leading to demyelination
- Onset of diseases usually in young adults; more common in women
- Name refers to scars produced by loss of myelin in white matter (axons) of CNS
- Loss of myelin causes distorted signal conduction in axons
Projection of a neuron
Route taken by its axon from the origin site to the target site
Anterograde and retrograde tracer substances
- Reveals connections between different brain areas
- Anterograde –> cell body to axon (normal flow of information)
- Retrograde –> axon terminal to cell body
Sir Charles Sherrington (1857-1952)
Coins the term ‘synapse’ in 1897
Synapse
- Discontinuity in the circuitry of the nervous system
- Specialized contact zone in the nervous system where one neuron communicates with another
George Palade and Sanford Palay
In 1954, they applied electron microscopy to synapses and synaptic vesicles, providing final proof for synapses and the cell theory
Gap junction
- Provided some support for Golgi’s reticular theory, since the cytoplasm is essentially continuous
- Made up of 2 units, one from cell A and one from cell B, and the center has a pore which can open and close
Connexons and connexins
- Family of transmembrane proteins that assemble to form gap junctions in vertebrates
- Each gap junction is composed of 2 hemichannels (connexons), which are themselves each constructed out of 6 connexin molecules
- The connexin molecules are four-pass transmembrane proteins (M1-M4) with both C and N cytoplasmic termini, a cytoplasmic loop (CL), and 2 extracellular loops (EL-1 and EL-2)
– Typical molecular weight is 26-60 kDa
1 gap junction = 2 connexons = 2 x 6 connexins
1 connexin = 4-pass transmembrane proteins (M1-M4)
Otto Loewi (1873-1961)
His classical experiment in 1921 demonstrated transmission of acetylcholine using hearts in boxes and measuring contraction force and number of beats
Synaptic transmission
- Passive diffusion across the synapse
- The gap is small enough (20-40 nm) that passive diffusion is very fast
Binding of transmitter to postsynaptic receptors leads to…
The generation of excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs)
Fusion of a synaptic vesicle with the pre-synaptic membrane
- Synaptic vesicles move down the axon and bind to release site on the pre-synaptic membrane via vesicle-membrane proteins (v-SNAREs) and target-membrane proteins (t-SNAREs)
- SNARE = soluble NSF-attachment protein receptor
- This SNARE complex interacts with both NSF (N-ethylmaleimide sensitive fusion protein) and SNAP (soluble NSF-attachment protein) to form a fusion complex
- Action potential propagation induces calcium influx at the pre-synaptic membrane, which, in addition to ATP hydrolysis by NSF, results in disassembly of the SNARE complex and membrane fusion
- Following neurotransmitter release, synaptic vesicle membrane components are recycled via an endocytic process
Modes of intercellular signaling
- Non-specialized signaling (humoral, paracrine, autocrine)
- Ephaptic signal transmission
- Transmission via electrical synapses
- Transmission via chemical synapses
A neurotransmitter is defined more by its _________ than by its own structure.
Receptor
Direct gating is mediated by…
Ionotropic receptors
Indirect gating is mediated by…
Metabotropic receptors
The response of a neuron or a muscle cell to a neurotransmitter is largely determined by the type of ________.
Receptor
Nicotinic acetylcholine receptor (nAChR)
Plant alkaloid nicotine can bind to the ACh binding site and activate the receptor, thus, nicotine acts as an agonist of ACh
Ionotropic receptors are comprised of 2 families…
- Different types of glutamate receptors
- nAChR, γ-aminobutyric acid A receptor, glycine receptor, and a subclass of serotonin receptors
Synthesizing and breaking down ACh
- If ACh is unable to be broken down convulsions occur (used in WWII nerve gas)
- ACh is broken down by acetylcholesterase
- ACh is synthesized locally (in synaptic bouton) instead of in the cell
Jean-Pierre Changeux (1936-)
He and his coworkers isolated and purified nAChR from electric organ of electric eel by employing α-bungarotoxin (snake toxin) in the 1970s
Muscles are a rich source of ______.
ACh
Shosaku Numa (1929-1992)
- He and his coworkers molecularly identified nAChR using electric organ of Torpedo ray in the 1980s
- Cloning of the sodium channel from the electric organ of the electric eel
Ionotropic receptor
- A relatively large, multisubunit complex typically composed of five individual proteins that combine to form an ion channel through the membrane
- Opening of the channel is achieved through rapid conformational changes
- 5 subunits
- Each subunit consists of 4 transmembrane-spanning segments referred to as TM1-TM4
- The membrane-spanning segments that line the pore are the 5 TM2 regions, one contributed by each subunit
– The amino acids that compose the TM2 segment are arranged in such a way that 3 rings of negatively charged amino acids are oriented toward the central pore of the channel; these rings appear to provide a selectivity filter so that only cations (mainly Na+ and K+) can pass through the central channel
Structure of nAChR
- Molecular weight is ~290 kDa
- 5 subunits
– 2 alpha
– 1 beta
– 1 gamma
– 1 delta - The subunits assemble in the lipid bilayer of the cell membrane to form a ring enclosing a central pore
- Each receptor complex has 2 ACh binding sites that reside in the extracellular domain, formed for the most part by 6 amino acids in the alpha-subunits
– When nAChR binds 2 molecules of ACh, the channel opens almost instantaneously (~20 µs); the closed-to-open transition is associated with a rotation of the TM2 segments
Extracellular recording is achieved by…
Placing 2 electrodes outside of the cell
1 mV (millivolt)
10 x -3 V (volt)
Intracellular recording is achieved by…
Placing the tip of a microelectrode inside of the cell and another outside of the cell
ACh acts on two types of receptors…
- Nicotinic
- Muscarinic
Julius Bernstein (1864-1941)
Membrane Theory of Resting Potential (1902)
K+ ions leaving the cell and equilibrium potential
- The efflux/outflow of K+ causes the inside of the cell to become more negative (hyperpolarized), since the membrane is impermeable to the large anions in the cell, preventing them from following the K+ ions across the membrane
- This leads to an increase in negative force inside the cell, so that at one point the electrostatic attraction of the positively charged K+ ions by the negative membrane potential inside the cell will balance the thermodynamic forces causing the K+ ions to move down the concentration gradient
- At this point, there is no net outflow of K+ ions
- The membrane potential at which this equilibrium is reached is referred to as the equilibrium potential
Nernst equation
Calculates membrane potential at which the ionic species is at equilibrium
Goldman-Hodgkin-Katz equation
- Modification of the Nernst equation
- Takes into account the relative contributions of the various ions to the membrane potential
Two common approaches to measuring resting potentials and action potentials
- Extracellular recordings
- Intracellular recordings
Resting potential / membrane potential
Potential difference between the inside and the outside of the cell, with the inside negative (usually -60 to -80 mV)
Resting potential is caused by…
- The resting potential is caused by a differential distribution of ions across the plasma membrane, particularly of K+, Na+, and Cl- ions –> this unequal distribution is actively maintained by ionic pumps and exchangers
- The thermodynamic forces causing the ions to move down their concentration gradients are balanced by electrostatic attraction, until no net flow of ions is reached –> this equilibrium is referred to as the equilibrium potential
- The equilibrium potential is determined by several factors, including temperature and the relative concentrations of the respective ion species inside and outside the cell, and can be described in quantitative terms by the Nernst equation
Ohm’s law
V = R * I
Ohm’s law states that in an electrical circuit the current I passing through a conductor between two points is directly proportional to the potential difference (i.e., the voltage drop or voltage V) across the two points, and inversely proportional to the resistance R between them
Depolarization
Reduction in negativity of the membrane potential
Hyperpolarization
Increase in negativity of the membrane potential
Phases of an action potential
- Resting state
- Depolarizing phase
- Repolarizing phase
- Undershoot
- Resting state
Separation of inward current and outward current
- Mediated by conductance channels for Na+ and K+
- Inward Na+ current can be eliminated by substituting a larger, impermeant cation (choline-H+) for Na+
Density of voltage-gated Na+ channels at node of Ranvier
Approximately 10 000 per µm^2, compared to approximately 20 per µm^2 at the internodal regions (myelin)
Saltatory conduction
- Action potentials “jump” from one node of Ranvier to the next one –> active triggering of action potentials only at nodal regions
- A new action potential is produced at each node
Receptor cells within sensory organs…
Transduce the energy associated with a stimulus into a receptor potential. Upon transmission to second-order neurons, the graded receptor potential is translated into a series of impulses.
Action potentials are triggered when…
Depolarization reaches a critical level, the so-called threshold
The duration of the action potential is limited by…
A gradual inactivation of Na+ channels and by an opening of K+ channels
Action potentials exhibit ___________ property
All-or-none
Action potential are conducted…
Along the axon by passive flow of positive charge inside the axon –> most of this movement of charge is carried out by K+ ions
In myelinated axons, action potentials travel…
In jumps from one node of Ranvier to the next one, a process referred to as saltatory conduction
Erwin Neher (1944-) and Bert Sakmann (1942-)
- Invented the Patch-clamp
- Patch-clamp technique –> apply suction to get seal of glass pipette electrode to the surface of the cell, insulating that area of the cell from the rest of the cell; Isolates one or a few channels and records charge influx based on the ions entering and exiting the channel
Voltage-gated Na+ channels
- Family with 9 known members
- Integral membrane protein
- Most commonly, complex of 2 protein subunits, one alpha-subunit and one or more auxiliary, smaller beta-subunits
- alpha-subunit forms core of channel
- alpha-subunit alone is able to form channels that conduct Na+ ions, even if beta-subunit(s) is (are) not expressed
Voltage-gated Na+ channel alpha-subunit
- 4 repeat domains (I, II, III, IV); approximately 75% identity
- Each of these domains contains 6 membrane-spanning segments, labeled S1-S6
Voltage-gated Na+ channels alpha-subunit S4 segment
- S4 segment is highly conserved; acts as voltage sensor
- Voltage sensitivity due to positive amino acid residues (arginine, lysine) at every third position
- Shifts in transmembrane voltage lead to conformation changes, allowing channel to become permeable to ions (creating the pore)
- All four S4 segments involved in activation and inactivation of channel
Voltage-gated Na+ channel pore
- Pore: two portions (extracellular, cytoplasmic)
- Extracellular portion formed by P-loops (region between S5 and S6) of the four domains; forms narrow, ion-selective filter
- Cytoplasmic portion formed by combined S5 and S6 regions of the four domains
- Selectivity filter made of negatively charged amino acid residues; attract positive Na+ ions and keep out negatively charged ions
- Pore is 0.3-0.5 nm wide, just enough to allow single Na+ ion with a H2O molecule associated to pass through
Voltage-gated Na+ channels states
- 3 types of states:
– Activated (open)
– Inactivated (closed, but refractory to re-opening) –> due to tethered plug formed by domains III and IV (motif IFM); called “inactivation gate”; after depolarization, channel remains inactivated for a few milliseconds; this state causes “refractory period”
– Deactivated (closed and ready for re-opening) –> channel blocked on its cytoplasmic side by an “activation gate”, which is removed in response to stimulation; channel will then open
