Neurobiology Exam 1

Cellular Components

Cellular Components

Neurons and Glial cells in equivilent numbers (86 x 10^9)

Neural Circuits

Sensory systems bring in information (5 senses, internal environment), associational systems connect higher order functions, motor systems organize and generate movement

Purpose of alternative splicing variations?

As splicing increases, gene variation in expression increases passed the set 14,000 genes of the nervous system.

Reticular Theory by _____ was disproven by the Neuron Doctrine by _____________

Camillo Golgo, Santiago Ramon y Cajal and Charles Sherrington

All about neurons

Neurons are specialized to receive and transmit information, sometimes over long distances via action potentials. Dendrites receive information from other neurons via neurotransmitters. Axons conduct electrical impulses away from a neuron to release a neurotransmitter.

Axons can be one of two types…

Local circuit neurons (interneurons) with short axons or projection neurons with very long axons.

Convergence:

A neuron with a lot of dendrites and a lot of dendritic branching, receives more information from many different neurons.

Integration:

“Making sense of it all”, all of the inputs are summed and contribute to a single output from the axon

Divergence:

An axon may branch and diverge to connect to multiple other neurons (one signal to multiple targets)

Synaptic vesicles:

Membrane-bound structures that contain chemical neurotransmitters

Synaptic cleft:

Extracellular space between pre- and post-synaptic surface.

Myelin Sheath:

Insulating (lipid-rich) wrapping around axons. Comprised os Oligodendroctyes in the CNS and Schwann cells in the PNS

What is abundant in axon terminals?

Mitochondria to provide immense energy output at the terminal.

Nodes of Ranvier:

Patches of uninsulated axon

Microtubules are made of what?

Tubulin (fluoresces green)

What function do microtubules have in axons?

Motor proteins walk down the microtubules.

What does tau do?

(Fluoresces in red) Bundles the microtubules

What cytoskeletal element is made up of actin?

Microfilaments

What is the purpose of actin in growing tips of dendrites and axons?

Changing conformations

Neuromuscular Junction (NMJ):

Synapse between a motor neuron’s axon and a muscle cell.

What neurotransmitter is often at NMJ?

Acetylcholine

Dystrophin:

An intracellular scaffolding protein that links receptors to actin cytoskeleton and localizes them to synapse.

Astrocytes:

Found in CNS, support cells through maintaining ionic and chemical environment for optimal neuronal function, form the blood brain barrier

Oligodendrocytes:

Wrap around axons, found in CNS, form the myelin sheath around CNS axons. Schwann cells are equivilent in the PNS

Microglia:

Brain’s macrophages, scavenger function, cleans up debris via phagocytosis, releases cytokines to regulate inflammation, can lead to an over-action if not regulated, leading to inflammation in the brain.

Types of neural circuits

Afferent (carries sensory information toward the brain or spinal cord), Local (Interneurons that convey information between different regions within the brain and spinal cord), and Efferent (carries motor information away from the brain or spinal cord to elicit a response)

Tracts:

Bundles of axons that connect distinct regions within the brain and/or spinal cord

Commissures:

Bundles of axons that connect neurons on one side of the brain to another

Columns:

Bundles of axons within the spinal cord that convey sensory info to the brain or motor info away from the brain.

Cortex:

A sheet of neuronal cell bodies arranged in layers of CNS.

Somatic Branch of PNS:

Under conscious control; controls skeletal muscle

Autonomic Branch of PNS:

Not under conscious control; controls cardiac muscle, smooth muscle, and glands. Examples include the Parasympathetic (rest and digest), sympathetic (fight or flight), and enteric (gut motility and secretions) systems.

Resting membrane potential:

Ranges from -40 to -90, default conditions.

Events that change Vm:

Photoreceptors (light reception), Cilia on hair cells in cochlea (vibration reception), Olfactory receptors (odorants), Taste cells, mechanoreceptors and thermoreceptors (vibration, pressure, and heat reception).

_____ ___________ responds to indentation of skin, opening stretch-gated Na+ channels, depolarizing the cell.

Pacinian corpuscle.

Neurotransmitters serve as _______ for post synaptic cells

Ligands, producing ligand-gated channels.

Ion flow changes across the post-synaptic membrane, and the resulting membrane potential is referred to as the __________

Synaptic potential

Receptor potentials and synaptic potentials are both considered what?

Generator potentials.

Shape of action potential:

Stimulation of stretch-gated or ligand-gated Na+ channels, reaches threshold, Na+ rushes in during rising phase, depolarizing the cell via voltage-gated Na+ channels, voltage-gated Na+ channels close as voltage-gated K+ channels open, repolarizing the cell, then begin to close, causing hyperpolarization.

Absolute refractory period:

When another action potential cannot be generated at the same spot of membrane (while reaching the peak of AP) due to the closing and locking of voltage gated NA+ channels.

Relative refractory period:

When it is less likely for an action potential to be generated at the same spot of membrane.

Saltatory Conduction:

AP moving in an “all or nothing” fashion, jumping from node to node.

Orthodromic conduction:

Normal AP, initiated at axon hillock and proceeds to axon terminal

Antidromic conduction:

Initiated at free sensory nerve endings and proceeds to the cell body (bypassing the cell body entirely)

Factors influencing rate of conduction

Diameter of axon (wider axons conduct faster), insulation (insulated axons conduct faster)

Patch clamp measurements:

Downward deflection indicate inward flow of ions (voltage-sensitive Na+ channel). Na+ opens quickly, but also closes quickly and lock shut till repolarization. Upward deflection indicate outward flow of ions (voltage-sensitive K+ channel). K+ channels open more slowly and remain open as long as cell is depolarized.

Simple bacterial K+ channel:

4 subunits, each subunit has 2 alpha-helical domains with span the membrane, inner helix forms wall of pore and outer helix interfaces with phospholipids. Helicies are connected toward outside of membrane by a hydrophilic pore loop, which dangles down into top of pore. All components make up a selectivity filter.

Mammalian voltage-gated K+ channel

Same as bacterial except with T1 domain and voltage sensors that stretch out to let ions out.

Voltage-gated Na+ channels;

No separate subunits, spans the membrane 24 times, , folded into 4 domains with 6 alpha helices each, contains 4 pore loops as a selectivity filter between S5 and S6, 4 voltage-sensing domains that open upon depolarization. Each S4 helix is depolarization sensor. Na+ channels twist to open gate.

Neurotransmitter ligand-gated ion channel:

Found on post synaptic sites (dendrites), binding to neurotransmitter released from pre-synaptic axon terminal, open to allow one or a few specific ions to cross. The glutamate receptor allows both Na+ and K+ to cross, but Na+ will cross because of an increased ionic driving force.

H+ ligand-gated ion channel:

Opens in response to increased acidity. Permeable to Na+

Ca2+ ligand-gated ion channel:

Opens in response to increased intracellular Ca2+, binding to neurotransmitter triggers Ca2+ to be released from stores. Ca2+ is the second messenger to neurotransmitters. This is a way to turn the cell off

Cyclic nucleotide ligand-gated ion channel:

Opens in response to increased intracellular cGMP or cAMP, binding to neurotransmitter to receptor triggers synthesis of cGMP or cAMP.

Active Ion Transporters:

Moves ions against their electrochemical gradient, which requires energy. The main examples are the Na+/K+ pump and the Ca2+ pump.

Sodium Potassium Pump Mechanism

Pumps 3 Na+ out and 2 K+ in. ATP lets Na+ bind and K+ release, then ATP phosphorylation occurs to ADP, leaving Na+ bound. Pocket flips and releases Na+ and binds K+, then the pump dephosphorylates, leaving K+ bound. The addition of ATP flips the pocket and releases K+.

Calcium Pump Mechanism

Pumps 2 Ca2+ either out of cell via PMCA or into the ER via SERCA.

Types of synaptic transmissions

Electrical and Chemical synapses

Electrical synapses:

Occur via gap junctions physically attaching a terminal to a dendrite of another neuron, making this connection more rapid. Pores are called connexons comprised of 6 connexins.

Chemical synapses:

Communication via synaptic clefts transmitting neurotransmitters contained synaptic vesicles through exocytosis. Neurotransmitters bind to post-synaptic receptors to alter ion flow into the post-synaptic cell after reception.

Chemical Synapse mechanism:

An action potential causes a depolarization event, leading to an influx of Ca2+ into the cell via voltage-gated Ca2+ channels, which acts as a trigger to cause vesicles to fuse to the pre-synaptic membrane, releasing their neurotransmitters inside. The neurotransmitter binds to a receptor on the post-synaptic membrane, opening/closing post-synaptic channels, which lead to either excitatory or inhibitory potentials. The neurotransmitter is removed by glial cell uptake or enzyme degradation.

Neurotransmitters have two broad classes…

Small neurotransmitters synthesized in the axon terminal with clear-core vesicles and neuropeptides synthesized in the cell body with dense-core vesicles.

NMJ Synapse transmission terms:

EPP: end plate potential (stimulation)

MEPP: Mini end plate potential (absence of stimulation)

Voltage-Gated Ca2+ channels:

Present in high density of pre-synaptic terminal membrane, high density in active zones where clear-core vesicles dock. Open in response to depolarization when Na+ enters terminal from AP.

Vesicle Trafficking Cycle:

• Synapsins: “binds vesicles together”, vesicular proteins that tether vesicles together in a reserve pool

• CaMKII: Ca2+/calmodulin-dependent kinase II

• Ca2+ activates kinase to phosphorylate synapsins, which causes them to release vesicles from the reserve pool.

Docking:

Vesicles become tethered to plasma membrane via GTP-binding proteins and SNAREs

Priming:

Changes within vesicular and plasma membranes that prepare them for fusion.

SNAPs and SNAREs

• SNAPs are soluble NSF attachment proteins that work together with NSF (an ATPase) to reorganize SNAREs

• SNAREs are SNAP receptors. Two subgroups, vesicular (vSNAREs) and target (tSNAREs)

SNAP-25 function

SNAP-25 connects the vesicle to the plasma membrane by binding to both synaptobrevin and syntaxin

Types of post-synaptic receptors:

1. Ionotropic Receptors: Contains an intrinsic ion channel (ligand-gated ion channel) where ligand is neurotransmitter

2. Metatropic Receptors: Do not contain an intrinsic ion channel (G-protein coupled receptor with an effector protein) activating an intracellular signaling cascade that ultimately opens or closes an ion channel.

Excitatory vs inhibitory neurotransmitters:

• Excitatory neurotransmitters cause depolarization through an influx of Na+ or Ca2+, cause excitatory post-synaptic potentials (EPSPs).

• Inhibitory neurotransmitters cause hyperpolarization through an influx of Cl- or an efflux of K+, cause inhibitory post-synaptic potentials (IPSPs)

Acetylcholine (ACh):

A small molecule neurotransmitter capable of excitatory or inhibitory responses. Excitatory at skeletal muscle NMJ and some CNS synapses, inhibitory at cardiac muscle and many CNS synapses.

Excitatory ACh binds to

Nicotinic ACh receptors (nAChR) at NMJ, Muscarinic ACh receptors (mAChR) at CNS synapses and some smooth muscle.

Inhibitory ACh binds to

Muscarinic ACh receptors (mAChR) in cardiac muscle to slow heart contraction.

ACh synthesis:

Acetyl CoA reacts with choline catalyzed by Choline Acetyltransferase (ChAT)

ACh loading into vesicles:

ACh binds to vesicular acetylcholine transporter (vAChT) on vesicle membrane, vAChT exchanges ACh into vesicle for H+ out of vesicle with an anti-porter.

nAChR responce is ______, mAChR responce is ____

Excitatory

Inhibitory or excitatory

nAChR as a receptor:

• Ionotropic receptor (has an intrinsic channel gated by ACh), at NMJ there are 5 subunits (2a, 1b, 1d, 1g/e), at CNS there are 5 subunits (3a, 2b). It takes two ACh to gate the channel. Non-selective, allows Na+ or K+ to cross, but Na+ has higher ionic driving force.

• Also can bind to nicotine as an agonist (same effect on the receptor). Has the same stimulant properties when used as a drug.

• a-bungarotoxin and curare are antagonists (inhibit normal function)

mAChR as a receptor:

• Metatropic receptor (lacks an intrinsic channel, indirectly gates)

• If inhibitory, causes hyperpolarization by opening K+ channels.

• If excitatory, causes depolarization by opening Na+ channels.

• 5 subtypes (M1-M5)

• Also binds to muscarine as an agonist

• Atropine is the antagonist that speeds up heart rate in bradycardia patients

ACh clearance from synapse:

AChE (acetylcholinesterase) degrades ACh into acetate and choline.

Choline reuptake:

Choline binds to ChT (Choline transporter) which co-transports choline along with Na+ into presynaptic terminal.

Glutamate (excitatory) synthesis

Gluta__mine__ is synthesized into gluta__mate__ via glutaminase, then packaged into vesicles via VGLUT (vesicular Glu transporter)

Clearance of Glutamate from cleft:

Glu is transported back into the presynaptic terminal or into glial cells via EAATs (excitatory amino acid transporters). In glial cells, Glutamate is converted back to Glutamine by glutamine synthetase and transported out of the cell via SN1 (system N transporter). Glutamine is transported into the presynaptic terminal by SAT2 (system A transporter 2).

Ionotropic Glu receptors:

• AMPA R: Channel that allows the passage of Na+ and K+, opens quickly upon Glu binding, Na+ enters and depolarizes the postsynaptic cell.

• NMDA R: Channel that allows the passage of Na+, K+, and Ca2+, ligand and voltage-gated, the postsynaptic cell must also be depolarized. Depolarization kicks out Mg2+ that was blocking the channel’s flow of ions.

Metatropic Glu receptors:

• mGluR: Diverse group of receptors, 3 different classes, some excitatory and some inhibitory, unique dimeric GPCR’s.

Major inhibitory neurotransmitters:

GABA (gamma-amino butyric acid) and Glycine

Synthesis of GABA:

Synthesized from Glutamate, catalyzed by GAD (glutamic acid decarboxylase) with pyridoxal phosphate as a cofactor, packaged into vesicles via VIAAT (vesicular inhibitory amino acid transporter). GABA is inhibitory

Receptors of GABA:

GABAa is ionotropic with 5 subunits (2a, 2b, 1g), GABAb is metatropic with 2 subunits (GABAb1 and GABAb2)

Reuptake of GABA:

Removed by GAT (GABA transporter) which is a symporter with Na+ into glial cells of the synaptic terminal. GABA is either repackaged or broken down into succinate within the mitochondria.

Ionotropic GABA receptors:

Antagonists include anxiolytics, hypnotic sedatives, anesthetics, and ethanol. All produce a downing effect.

Synthesis of Glycine:

Synthesized from serine, catalyzed by serine hydroxymethyltransferase, packaged by VIAAT.

Functions of Glycine:

Gly is inhibitory and binds to ionotropic glycine receptors that contain an intrinsic channel specific for Cl-.

Reuptake of glycine:

Taken up by glial cells or presynaptic terminal by glycine transporter, which cotransports Na+

Tyrosine to dopamine:

Tyrosine is synthesized into DOPA by tyrosine hydroxylase (TH), then synthesized into dopamine by DOPA decarboxylase. Dopamine is loaded into vesicles by VMAT (vesicular monoamine transporter), and taken up by glial cells or presynaptic terminal by DAT, degraded intracellularly by monoamine oxidase (MOA) or catechol O-methyltransferase (COMT).