Spread of Nerve Impulses Flashcards
How many neurones are in the brain ?
Roughly 8.6 billion
What makes up the CNS ?
Brain
Spinal cord
(Cranial nerve 2 and retina)
What makes up the PNS ?
Everything that lies outside of the dura mater
-sensory receptors
-peripheral portions of the spinal and cranial nerves
-peripheral portions of the autonomic nervous system
How does the PNS send signals to the CNS ?
Sensory (afferent) nerves
How does the CNS send signals to the PNS ?
Motor (efferent) nerves
What are the cells of the nervous system ?
Excitable cells
Support cells (glial cells)
Name an excitable cell
Neurons
Name the support (glial) cells of the CNS
Oligodendrocytes
Astrocytes
Microglial cells
Epedymal cells
Name the support (glial) cells of the PNS
Schwann cells
Satellite cells
Enteric glial cells
Function of oligodendrocytes
Oligodendrocytes are the myelinating cells of the CNS that allow the fast and efficient transfer of neuronal communication through the myelination of axons.
Function of microglial cells
Microglia regulate brain development primarily through two routes: the release of diffusible factors and phagocytosis.
Function of astrocytes
Largest and most numerous types of glial cells in the CNS.
The broad role of astrocytes is to maintain brain homeostasis and neuronal metabolism.
Function of Schwann cells
A Schwann cell is a type of glial cell in the peripheral nervous system that wraps around nerve fibers, producing the myelin sheath, which insulates and increases the speed of electrical impulses along the axons.
Difference between Oligodendrocytes and Schwann cells
The primary difference is their location. Oligodendrocytes myelinate the central nervous system, while Schwann cells myelinate the peripheral nervous system.
Oligodendrocytes are also capable of myelinating multiple axons, while Schwann cells can only myelinate one axon per cell.
Describe the structure of a neuron involved in signalling
Signal arrives via dendrites/cell body
If signal passes threshold, an action potential is generated and transduced down the length of the axon.
The electrical signal is converted to a chemical signal which is passed to a target cell (muscle, neuron or gland) via nerve terminals
What is the Node of Ranvier ?
The nodes of Ranvier are gaps along the myelin sheath that covers the axon of neuron cells.
They function to recharge the action potential that runs along the axon.
How can neurons be classified ?
By the number of processes that extend from their cell body.
Unipolar
Single projection
e.g. brush cell of the cerebellum
Pseudo-unipolar
2 branches of a single projection
e.g. Sensory neurons of the dorsal root ganglion
Bipolar
2 projections
e.g. Retina, vestibular nerve, spinal ganglia, cerebral cortex
Multipolar
Multiple projections
e.g. most neurons of the CNS
Describe nerve conduction
Signal arrives at dendrites of neuron
Greater than threshold ?
Action potential moves along axon to nerve terminal
Signals to next neuron/target cell
Nerves transmit electrical signals through the movement of ions.
Electrical charge can be conducted both passively and actively.
Passive conduction
Depends on the movement of ions along the two
faces of the plasma membrane; decays with distance.
Active conduction
Depends on the presence and activity of biological molecules such as voltage-gated ion channels; transmit without loss of signal strength.
Generation of action potentials due to opening of ion channels.
How does resistance affect nerve conduction ?
Larger diameter, lower resistance
Faster passive current flow
But there is not enough space to continually increase the size of diameter to maintain speed of conductance over long distances.
How does capacitance affect nerve conduction ?
For current to pass along a nerve, it must overcome the membrane capacitance e.g. stored charge.
Capacitance of the plasma membrane is proportional to surface area.
What is a capacitor ?
2 conducting regions separated by an insulator
e.g. ECF, ICF and cell membrane
Answer involves ions
Property of the cell membrane
Impermeable to charged ions
Name the types of classification of nerve fibres
A alpha
A beta
A gamma
A delta
C nerve
Example of fibre type A alpha function
Motor neurons
Example of fibre type A beta function
Skin touch afferents
Example of fibre type A gamma function
Motor to muscle spindles
Example of fibre type A delta function
Myelinated skin temperature and pain afferents
Example of fibre type C nerve function
Unmyelinated skin pain afferents
Feature of un-myelinated sheaths
Slower conduction
How are electrical currents carried across cell membranes ?
By ions
e.g. Na+, K+, Cl-, Ca2+, HCO3- etc.
How can ions cross the plasma membrane ?
Via membrane proteins
- Ion channels
- Ion transporters
- Ion pumps
Types of ion channels
Passive
Ligand gated
Voltage gated
Passive ion channels
Always open
(aka Leak channels)
Ligand gated ion channels
Require ligand binding to open
Voltage gated ion channels
Require specific membrane voltage to open
What separates the intracellular and extracellular environments ?
The plasma membrane
How are electrochemical gradients generated ?
Asymmetrical distribution of ions on either side of the plasma membrane.
Electrochemical gradient
Concentration
Electrical charge
How is the membrane potential produced ?
Each cell type will express a specific range of ion transport proteins
Na+, K+, Cl-
Extracellular ion concentration
High Na
High Cl
Low K
Intracellular ion concentration
Low Na
Low Cl
High K
What is the resting membrane potential ?
Roughly between -60 to -70 mV
What causes depolarisation ?
If the inside of the cell becomes more positive
What causes hyper polarisation ?
If the inside of the cell becomes more negative
Depolarisation
Excitatory
Hyperpolarisation
Inhibitory
Describe neurons at rest
At rest, voltage gated sodium channels (Na+) are inactive, the leak K+ channels open
Graded potential
Small signals will produce small effects
Graded potentials decay (temporally and spatially)
What happens in graded potential ?
Signal arrives from another nerve terminal.
A larger or multiple signals will cause many Nav channels to open.
Nav in the vicinity open, Na+ enters the cell body (depolarisation)
Cell will depolarise, membrane potential crosses a THRESHOLD.
What does it mean that graded potentials can be additive ?
If multiple small signals arrive at the same time and place.
‘Passive conduction’
Describe the generation of action potential
Neurotransmitter binds to ligand gated channel at a synapse.
Ligand gated channel opens, so Na+ ions diffuse into the neuron.
The Na+ movement causes depolarisation of the plasma membrane.
If the stimulus is large enough to cross the threshold value, rapid depolarisation occurs resulting in opening of the voltage gated channel.
Na+ ions diffuse into the neuron down the electrochemical gradient.
This leads to a wave of electrical excitation along the neurons plasma membrane (Action Potential).
The voltage builds up, so the Na+ channel is inactivated and K+ channel opens.
K+ ions diffuse out of the neuron causing repolarisation.
Because the voltage-gated K+ channels are slow to close, K+ continues to leave cell down its electrochemical gradient causing a hyper polarisation.
Voltage gated K+ channels close and Vm resets due to action of Kleak and Na+/K+ ATPase pumps.
What causes initial depolarisation ?
Rapid opening of Nav (Sodium gated voltage channels)
What causes initial repolarisation ?
Closing of the Nav, opening of the voltage gated K+ channels as well as K+ moving through Kleak channels.
Properties of voltage gated sodium channels
2 gates
- A closed gate
- An inactivation gate
Vm : Rest –> Depolarisation –> Repolarisation –> Hyperpolarisation
Properties of voltage gated potassium channels
One gate
Vm : Rest –> Depolarisation –> Repolarisation –> Hyperpolarisation
Refractory period
An action potential only travels away from a stimulus
Absolute Refractory period
It is not possible to generate a 2nd action potential immediately after the first.
As a result of the inactivation of Nav
Relative refractory period
As the inactivated channels return to the closed state, it takes time.
It is more difficult to elicit a second action potential here.
Propagation of the action potential
Direction
Frequency
Speed
Direction (propagation of action potential)
Action potentials are ALL-OR-NONE in nature
Only when the threshold is reached, will the action potential fire and signal be transmitted along the axon.
Action potentials can only move away from a stimulus.
Frequency (propagation of action potential)
A sustained stimulus which crosses the threshold can produce continuous firing of action potentials.
YOU CAN NEVER HAVE A CONTINUOUS ACTION POTENTIAL.
Repetitive spiking patterns in response to sustained stimuli differ greatly across different cell types, likely related to their function.
Speed (propagation of action potential)
Some axons have a myelin sheath to speed up normal transmission.
Oligodendrocytes (CNS)
Schwann cells (PNS)
Clinical consequences of myelin loss
Demyleinating disease can affect peripheral/central axons resulting in impaired conduction.
Most common form in CNS - Multiple Sclerosis
Multiple Sclerosis
Autoimmune disease affecting oligodendrocytes.
Results in impaired conduction.
Myelination result
Speeds up propagation up to 10 fold
What are Nodes of Ranvier ?
Gaps between insulated sections of myelin.
They are where the voltage gated Na+ channels are found
Where are voltage gated Na+ channels found ?
Nodes of Ranvier
Saltatory conduction
Action potentials jump at high speed from one gap to the next.
