Overview of the Nervous System Flashcards
Current (I)
- Electrical charge flowing through the neuronal membrane per unit time (units: amperes or A)
- Water analogy: how much water (charge) is flowing through the pipe per second
Potential/Voltage (E or V)
- The relative “pressure” on a charge across the membrane (units: volts or V)
- Water analogy: how much pressure is being exerted on the water throughout the pipe?
Conductance (g)
- The ease with which charge flows across the membrane (units: siemens or S)
- Water anology: how wide is the pipe?
What influences conductance (g)?
- The number of open ion channels for a given ion
- E.g. during the rising phase of an action potential, there are many sodium channels open –> gNA is high at this point
Is the intracellular surface of a cell more negative or positive than its extracellular surface?
- The intracellular surface is more negative than the extracellular surface. This is the resting membrane potential of the cell
Resting membrane potential
- A steady-state potential of the cell (when there are no changes occuring)
- The potential across the membrane tends to remain at the resting membrane potential
What charge do many macromolecules in cells have?
Many macromolecules in cells (e.g. nucleic acids, proteins, etc.) have a negative charge
“leak” ion channels
- Open (no gating mechanisms)
- Allow for the free flow of ions
- K+ has the most leak channels, then Cl-, then Na+
- Thus K+ plays the biggest role in creating the resting membrane potential
Sodium-Potassium ATPase
- Uses ATP to counteract the effects of the leak ion channels
- Pumps Na+ and K+ against their electrochemical gradients
- Pumps 3 Na+ ions out for every 2 K+ in
- Helps maintain the resting membrane potential
What is the approximate resting membrane potential in mV?
-65mV
Can neurons excite or inhibit other neurons?
Neurons can both excite or inhibit other neurons
Depolarization
- Making the membrane potential more positive
- Excitation signals depolarize the cell
Hyperpolarization
- Making the membrane potential more negative
- Inhibitory signals hyperpolarize the cell
Repolarization
- Return towards resting membrane potential after depolarization or hyperpolarization
Threshold potential
- Membrane potential at which it is possible for an action potential to occur
- Generally occurs around -55 mV (about 10mV more positive than the resting membrane potential)
General phases of the Action Potential
- Threshold potential met: action potential becomes inevitable (all-or-nothing principle of action potentials)
- Rising/depolarization phase: rapid depolarization of the membrane
- Falling/repolarization phase: slower repolarization of the membrane back towards resting membrane potential (RMP)
- Undershoot phase: After hyperpolarization membrane potential dips under RMP
Voltage-gated ion channels
- Become permeable and impermeable within specific membrane potential ranges. Important examples:
- Voltage-gated Na+ channels
- Voltage-gated K+ channels
Biophysic stages of the action potential
- Voltage-gated Na+ channels open –> Depolarization
- Voltage-gated K+ channels open –> Hyperpolarization
- Voltage-gated Na+ channels close –> K+ channels close slower
- Overshoot phase –> then repolarization
Where are action potentials typically initiated?
What do they propagate down?
Axon Hillock
Axon
Saltatory Conduction
- Action potential jumping from node to node (nodes of Ranvier) for faster signaling
- Nodes of Ranvier have much higher voltage-gated Na+ channel density
How does myelination conserve energy?
- Sodium Potasium ATPase only has to work at nodes of Ranvier
- Fewer axonal ion channels and ATPases needed
General result of demyelinating disorders
- Action potential velocity reduced
- Greater amount of action potentials fail
- Sensory/motor and sometimes autonomic/cognitive symptoms
What is a common demyelinating disorder of the CNS?
Multiple Sclerosis
What is a common demyelinating disorder of the PNS?
Guillain-Barre syndrome
Chemical Synapses
- Presynaptic neuron secretes neurotransmitters into synaptic cleft which bind to postsynaptic neuron
- Can amplify signal (one neuron can synapse on many neurons)
- Are highly plastic (modifiable)
Electrical Synapses
- Direct exchange of ions/molecules at physical junctions between two neurons
- Rapid precise communication
- Useful for large-scale synchronization of neurons
- Can be bidrectional
Neurotransmitters
Molecules synthesized within neurons that mediate signaling at chemical synapses
What are the three main classes of neurotransmitters?
- Amino Acids
- Monoamines
- Neuropeptides
What are three important amino acid neuro transmitters?
- Glutamate (Glu) –> excitatory
- y-amino butyric acid (GABA) –> inhibitory
- Glycine (Gly) –> inhibitory
What are some of the main monoamine neurotransmitters?
- Serotonin (5-HT)
- Epinephrine (Epi)
- Norepinephrine (NE)
- Dopamine (DA)
Note: DA, NE, and Epi are also catecholamines
What classes of neurotransmitters are small-molecule NTs?
- Amino Acid NTs
- Monoamine NTs
Small-molecule neurotransmitters synthesis
- Synthetic enzymes are synthesized on the soma and transported down axon via microtubules
- NT precursors transported into axon terminal
- NTs synthesized by synthetic enzymes and packaged in synaptic vesicles
Neuropeptide neurotransmitter synthesis
- NT precursors and synthetic enzymes are synthesized in the soma and packaged into synaptic vesicles
- These synaptic vesicles are then transported down the axon via microtubules
- Synthetic enzymes cleave precursors into NTs prior to release
Chemical Synapse (Synthesis, Release, and Binding)
- NTs synthesized at presynaptic axon terminal and stored in synaptic vesicles
- Action potential (AP) reaches axon terminal to activate voltage-gated Ca2+ channels –> influx of Ca2+
- Increased [Ca2+] –> promotes vesicle exocytosis via interactions between synaptotagmin and SNARE complexes
What determines the neurotransmitter’s effect(s)?
The NT receptor determiens the NT’s effect(s)
Note: Multiple receptors are associated with most NTs
Ionotropic Receptors
- Have two domains:
- Extracellular: ligand-binding site
- Transmembrane: ion channel
- Can gate specific/nonspecific cation (NA+, K+, CA2+) or anion (Cl-) channels that induce postsynaptic potentials
- Rapid and transient with specific responses in membrane potential
Metabotropic Receptors
- Most are G protein-coupled receptors
- Slow and sustained with many potential responses
- Different types:
- Indirect gating of ion channels
- Activation of 2nd-messenger pathways which can:
- Gating of ion channels
- Induce other changes postsynaptic membrane
- Regulate gene transcription
Excitatory Postsynaptic Potentials (EPSPs)
Depolarize the neuron and make it more likely for an AP to occur
Inhibitory Postsynaptic Potentials (IPSPs)
Hyperpolarize the neuron and make it less likely for an AP to occur (with some exceptions)
Postsynaptic Potentials (PSPs)
- Are graded potentials: their amplitudes (sizes in mV) scale with the sizes of the input(s) that generate them (the number of NT-receptor binding events)
- PSPs can sum together!
Temporal Summation
- Type of graded potential (PSPs)
- Rapid PSPs from the same source within a short period of time accumulate
Spatial Summation
- Type of graded potential (PSPs)
- PSPs from different sources arrive in close succession to sum at the same location
What stops the continued generation of PSPs after presynaptic action potential due to continued neurotransmitter-receptor binding events?
- Neurotransmitters are removed from the synaptic cleft through two mechanisms:
- Reuptake: presynaptic neuron reuptakes NTs via transporters
- Catabolsim: Enzymes degrade NTs
What do reuptake inhibitors do?
What are some common examples?
- Cause NT binding to occur over a greater period of time
- Common examples:
- SSRI
- Psychostimulants (Cocaine)
- Anticonvulsants
