5. Lectures 12, 13 Flashcards
How do signals travel through neurons?
Signals (voltage changes) typically flow from dendrites (principle synaptic input site) to soma to axon and finally to synapses
What types of channels are found at synaptic sites?
Ligand gated channels
These occur predominantly in the dendrites and somata, but synaptic inputs can also be found in axons
Na+ channels are located in the parts of the neuron that display action potentials, as would delay rectifying K+ channels
How do different neuronal types respond to a continuous depol?
Neuron with fast Na currents and delayed rectifier K currents will repetitively spike (rate of firing regulated by presence of A type K current
Neuron with slow accumulating K current (Ca activated K currents) will display spike frequency adaptation
Neurons can also exhibit rhythmic bursting behaviour by exploiting the interplay between depolarizing and hyperpolarizing currents
Slide 6 lecture 12
What are regenerative signals and non-regenerative signals?
Regenerative signals- action potentials (all or nothing)
Active responses do not decay with distance through axon
Non-regenerative signals (passive responses)- subthreshold potentials (graded potentials) that spread for short distances across cell membranes
Passive responses decay with distance through an axon
Slide 8 lecture 12
What are receptor potentials and post synaptic synaptic potentials?
Receptor potentials- generated during the transduction of sensory stimuli
Postsynaptic synaptic potential- generated by opening of agonist-activates channels
Called graded potential
Response is proportional to stimulus intensity and decays with distance
Lose strength as they move through cell due to current leak and cytoplasmic resistance
What does the spread of electrical correct depend on? (3 things)
Cell geometry
Electrical resistance of the aqueous solutions and cell membrane
Membrane capacitance
What are the 2 types of postsynaptic potentials?
Excitatory postsynaptic potential (EPSP)- depolarization
Inhibitory postsynaptic potential (IPSP)- hyperpolarization
What is signal (postsynaptic potential) summation?
A single neurons may receive inputs from tens of thousands of other neurons
Spatial summation- Excitatory postsynaptic potentials arriving from different dendrites combine
Temporal summation- excitatory postsynaptic potentials arrive rapidly in succession
Slide 10-12 lecture 12
What is the space constant (l)?
Determined the spread of voltage changes in space
Greater specific membrane resistance (Rm) and cable radius (a), greater then length constant and less the loss of signal
Greater the resistance of the internal conductor (Ri) the smaller the length constant and the greater loss of signal
Slide 12-14 lecture 12
What are the voltage gated channels in dendrites?
What are calcium spikes?
Low density of Nav and Kv channels
Some have V-gated Ca channels that boost the signal
Calcium spikes- purkinje cell (Ca-dendritic spikes)
Ca spikes can propagate into the soma (not doen axon)
Slide 15 lecture 12
What are the 3 steps/facts about excitatory postsynaptic potential travelling?
- EPSP attenuated in the soma and the initial segment, but the EPSP is large enough to trigger an action potential at the initial segment
- The threshold is high in regions that have few Nav channels, but falls steeply in the hillock and initial segment
- The density of Nav channels is high only at the initial segment and at each node of ranvier
Slide 16-17 lecture 12
What are glial cells?
Constitute half the volume of the brain and outnumber neurons
Can proliferate throughout life (an injury to the nervous system is the usual stimulus for proliferation) whereas neurons do not
Neural stem cells can transform into neurons or glia
What are the types of glia in CNS?
Central nervous system
Astrocytes- strength support, promote blood brain barrier
Oligodendrocytes- CNS myelin sheath
Microglia- phagocytes, immune cells
Ependymal cells- epithelia, line the ventricle and canal of spinal cord
What are the types of glia in the PNS?
Peripheral nervous system
Schwann cells- PNS myelin sheath
Satellite cells- surround cell bodies of neurons, regulate exchange of materials)
Enteric glia- Schwann cell-like, no myelination
What are the types of glial cells in the CNS?
4 of them
Astrocytes- strength support, promote blood brain barrier, regulate growth migration interconnection, scavengers of K+
Oligodendrocytes- CNS myelin sheath
Microglia- phagocytes, immune cells
Ependymal cells- epithelia, like ventricle and canal of spinal cord, production and circulation of cerebrospinal fluid
What are the types of glial cells in the PNS?
3 of them
Schwann cells- PNS myelin sheath
Satellite cells- surround cell bodies of neurons, regulate exchange of materials
Enteric glia- Schwann cell-like, no myelination
What is myelination?
Oligodendrocytes (CNS) and Schwann cells (PNS)
Leading edge of one of an oligodendrocyte/Schwann cell cytoplasm wraps around the axon many times
Compaction- cytoplasm is then squeezed out if the many cell layers surrounding axon
Myelin- layer on layer of tightly compressed membranes
In CNS, one oligodendrocyte myelinates many axons
In PNS, one schwann cell provides a single myelin segment to a single axon
Slides 3-4 lecture 13
Slide 9 lecture 13
Slide 14 lecture 13
What is nerve degeneration and nerve regeneration?
Nerve degeneration (CNS and PNS) 7 steps remember on slide 5 lecture 12
Nerve regeneration (PNS) Slide 5 lecture 13
What affects signal conduction in axons?
Equation
Slide 6-7 lecture 13
Improve conduction by;
Increasing diameter (increase a, decrease Ri)
Myelin (increase Rm)- myelination speed transmission by increasing membrane resistance (Rm) and decreasing membrane capacitance (Cm)
Slide 11 lecture 13
What are the 5 steps when a local current comes to a membrane?
- Signal causes Vm change
- Cytosol has slight more + charge compared to adjacent inactive regions of the slight - charge cytosol
- Charge imbalance causes current to flow from excited region to adjacent region of cytoplasm
- Current flows in complete circuit along pathways of least resistance and spreads:
A) longitudinally from + to - regions along cyto
B) across membrane conductance pathways (leak channels)
C) along extracellular medium back to site of origin (closes current loop) - Cause of flow if current the region of membrane immediately adjacent to the active region becomes more depolarized
What does intermittent myelination permit?
Saltatory transmission
Slide 9 lecture 13
What is the time constant (τm)?
τm= Rm • Cm
Rm- membrane resistance
Cm- membrane capacitance
Influences the spread of voltage changes in time and this the velocity of signal propagation
Shorter the τm the quicker neighbouring regions if membrane will be brought the threshold
Shorter the τm, the faster the speed of impulse propagation
Slide 12 lecture 13
Why is salutatory conduction faster?
For unmyelinated axons, conduction velocity increases roughly with the square root of the axons diameter, just as length Constance increase with square root of axon diameter or radius
For myelinated axons, conduction velocity is a linear function of diameter and increases ~6 m/s per 1μm increase in outer diameter
A mammalian myelinated axon with an outer diameter of 4μm has roughly the same impulse velocity as a squid giant axon with a diameter of 500 μm!!
Myelination speeds transmission by increasing membrane resistance and decreasing membrane capacitance
Slide 15 lecture 13
What’s the difference between normal conduction and demyelinated conduction?
Normal- slide 16 lecture 13
Demyelinated- slide 17 lecture 13 Decreased conduction velocity Frequency related block Total conduction block Ectopic impulse generation Increase in mechanosensitivity
MS is a result of demyelination, causes impaired conduction of action potentials
What are astrocytes?
Control the immediate environment of neurons
Brain glycogen (energy storage), contain all enzymes required for glucose metabolism
Provide fuel to neurons in the form of lactate (from glycogen or glucose)
Astrocytes buffet excess extracellular K+
Synthesis of glutamate and GABA, provide glutamine to neurons
Removal of glutamate from synaptic cleft
What is astrocyte K+ buffering?
Astrocytes take up K+ in response to elevated [K]o by 3 mechanisms:
Na-K pump
Na/K/Cl cotransporter
Uptake if K and Cl through channels
Astrocyte gap junctions coupling important for spatial buffering
Slide 21 lecture 13
What are microglia?
Represent 20% of the total glial cells within the mature CNS
Rapidly activated by injury to the brain: proliferate, change shape, become phagocytic
Activated microglia releasing substances that are toxic to neurons (free radicals and NO)
