11-11-21 - Spread of the Nerve Impulse Flashcards
Learning outcomes
- Define the cells of the nervous system and explain their function.
- Describe the key structural features of a typical neuron.
- Explain the role of (i) nerve diameter and (ii) capacitance of the plasma membrane in the transmission of signal along a neuron.
- Describe the ionic events which underpin the action potential.
- Define how the action potential is propagated in terms of: i. Direction (refractory periods), ii. Frequency (multiple/repeated stimuli) and iii. Speed (myelin sheath).
- Explain the difference between salutatory conduction in a myelinated nerve vs. conduction in an unmyelinated nerve.
What are neurons?
How do they transmit information? What is their structure?
- Neurons are the functional unit of the nervous system (CNS and PNS)
- Neurons are excitable nerve cells that transmit information through electrical signals or action potentials
- A typical neuron has a cell body (soma - red) and neurite(s).
- A neurite can either be an axon or dendrite.
- Axon (blue) is single, can be as long as 1m, covered with a myelin or Schwann sheath.
- Dendrites (green) are multiple, thing, short extensions.
What is the function of glial cells (neuroglia)?
How do they compare in numbers to neurons?
How do they differ to neurons in terms of regeneration?
What are the 4 glial cells of the CNS? What is their function?
What are the 2 glial cells of the PNS?
What is their function?
- Specialized cells called neuroglia support neurons
- Neurons are far outnumbered by glial cells
- Glial cells can regenerate, while neurons hardly generate, if ever.
• The 4 types of glial cells of the CNS:
1) Oligodendrocytes - myelin production in the CNS
2) Astrocytes – reinforce the blood brain barrier between the blood system and substances of the brain. It is a selectively permeable membrane. Brain uses glucose, but cant metabolise proteins, so a barrier is needed
3) Microglia – scavengers that tidy up dead cells and pathogens
4) Ependymal cells – CSF production, which is important in protection, support, and providing nutrition to the CNS
• The 2 types of glial cells of the PNS:
1) Satellite cells – take away excess metabolites and regulate nutrition to cell bodies
2) Schwann cells – Myelin production in the PNS
How are neurons classified?
What are the 4 different types of neurons?
What are their structures like?
Where can they each be found?
• Neurons can be classified by the number of processes that extend from their cell body
• Different types of neurons:
1) Unipolar
• Single branch
• One side becomes dendrites, the other side has nerve terminals
• Brush cell of the cerebellum
2) Pseudo-unipolar
• Still a single branch from the cell body
• Dendrites on one side and nerve terminal on the other side
• Sensory neurons of the dorsal root ganglion
3) Bipolar • 2 branches from the cell body • 1 dendrite side • 1 nerve terminal side • Retina • Vestibular nerve • Spinal ganglia • Cerebral cortex
4) Multipolar
• Lots of dendrites coming out of the cell body
• Single branch (axon) which terminates in nerve terminals
• Make up most neurons of the CNS
Describe the 3 steps of nerve conduction in a neuron.
What can the nerve terminal be a connection to?
1) Signals from the nerve terminals of other neurons synapse at the dendrites of the next neuron
2) If the signal passes the threshold potential, an action potential is generated and transduced down the length of the axon
3) The electrical signal reaches the synapses in the nerve terminals (synaptic boutons) of the axon, where is it converted to a chemical signal and passed to the next neuron/target cell
• The connection at the nerve terminal can be a nerve-to-nerve connection (electrical or chemical synapse), or it can be nerve to gland/nerve to muscle (chemical synapse)
How do nerves transmit electrical signals?
What are the 2 ways electrical charge can be conducted?
- Nerves transmit electrical signals through the movement of ions
- Electrical charge can be conducted both actively and passively
1) Passive conduction
• Electrical activity along a able, like heat along a rod that is not insulated
• Hat will move along the rod, but will start to dissipate, as it can’t maintain the heat
• This is similar to an electrical signal moving along an uninsulated cable
• This is called loss
2) Active conduction
• Generation of action potentials due to the opening of ion channels which regenerate
• Action potential fires, if the threshold potential is reached, another action potential will fire
• There is constant regeneration of the signal
What are 2 factors that affect nerve conduction?
What is a capacitor?
What is needed for current to pass along the nerve?
How does diameter affect both of these factors?
How do different nerve fibres differ in speed of conduction?
1) Resistance
• The larger the diameter of the nerve, the lower the resistance
• The faster the passive current flow
• There is not enough space to continually increase the size of diameter to maintain speed of conductance over long distances
2) Capacitance
• A capacitor is 2 conducting regions separated by an insulator
• E.g ECF, ICF, and cell membrane
• For current to pass along the nerve, it must overcome the membrane capacitance e.g stored d charge along the length of the nerve
• Larger diameter will also increase capacitance, which will make it more difficult for the signal to move along
• Different nerve fibres differ in speed of conduction
What characteristic do neurons exhibit?
What is the resting membrane potential of neurons?
What is depolarisation?
What is repolarization?
What is hyperpolarization?
- Neurons exhibit electrical excitability
- The resting membrane potential (Vm) of neurons is -60 to -70mV
- If the inside of the cell becomes more positive, this is known as depolarization (excitability)
- If the inside of the cell becomes more negative, this is known as repolarization
- If the inside of the cell becomes more negative than the resting membrane potential, this is known as hyperpolarization
How do channels in the neuron exist at rest?
- At rest, voltage gated Na+ (Nav) channels are closed
- Voltage gated K+ (Kleak)channels are open
- The Na+/K+ ATPase is active, and pumping 3 sodium ions out of the cell, and 2 potassium ions into the cell
What are graded potentials?
Describe how they are produced.
How do Graded potentials decay?
How can graded potentials be additive?
What do graded potentials not elicit?
• Graded potentials are what is produced when a small signal arrives from another nerve terminal
• When a small signal arrives from another nerve terminal, Nav channels in the vicinity open, allowing Na+ to enter the cell body
• This results in a slight depolarization which generates a graded potential
• Graded potentials decay temporally (in regards to time) and spatially by Na+ being pumped out by the ATPase
• Graded potentials can be additive if multiple small signals arrive at the same time and place
• Graded potentials don’t elicit an action potential
Describe the 5 steps generation of an action potential.
What does this process cause?
1) A larger or multiple signals cause many voltage gated Nav channels to open, which causes depolarization
2) If this depolarization causes the membrane potential to reach past the threshold potential, rapid depolarization occurs by the opening of many more voltage gated Nav channels
3) Nav channels are rapidly inactivated, and voltage gated K+ channels open, allowing K+ to exit the cell, which causes repolarization
4) Because voltage-gated K+ channels are slow to close, K+ continue to leave down its electrochemical gradient, causing hyperpolarization of the membrane potential (Vm)
5) Voltage gated K+ channels close and the Vm resets due to the action of Kleak channels and the Na+/K+ ATPase
• This process causes the signal to be transmitted along the axon
What is the nature of action potentials?
What direction do action potentials move in?
Why is this?
What is the only way action potentials can be generated?
- Action potentials are all or none in nature – either depolarization passes the threshold potential and generates an action potential, or it doesn’t and a graded potential is generated
- Action potentials can only move in one direction – away from the stimulus
- This is due to refractory period
- Only when the threshold potential is reached, will the action potential generate and the signal be transmitted along the axon
Describe the voltage gated Na+ (Nav) channels during the 4 stages of the action potential
1) Rest
• The voltage gated sodium channel has 2 gates: inactivated gate and closed date
• At rest (-60 - -70mV in a neuron) the closed gate is closed, which prevents any sodium moving through
2) Depolarization
• When the threshold potential is reached, (around -55mv), the closed gate will open and sodium moves into the cell
3) Repolarization
• Very rapidly, the inactivation gate will close, so sodium can no longer move through
• Whilst the inactivation gate is closed, there is no way for sodium to move through, even if the threshold potential is reached again
4) Hyperpolarization
• The closed gate will then close and the inactivation gate will open
• The channel can’t revert to closed gate till the membrane potential returns back to the resting membrane potential
How does the Na+ and K+ voltage gated channel differ?
Describe the voltage gated K+ (Kleak) channels during the last 2 stages of the action potential
- The K+ voltage gated channel only has a closed gate
- The K+ channels are very slow to open and close, whereas the sodium channels are very quick to open and inactivate
1) Repolarization
• Kleak channels open after the slight repolarization after Nav channels closed
• This allows for Repolarization of the membrane potential to occur
2) Hyperpolarization
• The Kleak channels are slow to open and close, which will cause the hyperpolarization of the resting membrane potential
• These channels will then close, and the resting membrane potential is restored by the Na+/K+ ATPase as well as some Kleak channels
What is the absolute refractory period?
What does this cause?
Why is this caused?
What is the relative refractory period?
What does the refractory period mean for the action potential?
- The absolute refractory period is the period in which it is not possible to generate a second action potential immediately after the first
- This means there will always be a delay in between action potentials
- Absolute refractory period is cause by the inactivation of Nav channels
- The relative refractory period is the period when inactivated channels are beginning to return to their closed state, so it is more difficult (but not impossible) to elicit and second action potential
- A bigger signal is required to generate an action potential during this time, as some gates are inactivated, and some are closed
- The refractory period means that an action potential can only travel away from the stimulus
How does injected current affect the generation of action potentials/frequency of action potentials
- If the injected current does not cause the membrane to reach the threshold potential, a graded potential will be generated
- If the injected current causes the membrane potential to pass the threshold potential, an action potential will be generated
- If this current is sustained, multiple action potentials will be generated, forming a repetitive spiking pattern
- If this sustained injected current is increased, then action potentials will be generated more frequently, leading to a more repetitive spiking pattern
How do action potentials vary between cell types?
How does spiking in response to sustained stimuli differ between cell types?
What might cause this?
- Action potentials vary slightly between cell types, but are generally similar
- Repetitive spiking pattern in response to sustained stimuli differ greatly across different cell types
- This is likely related to their function
How is neuronal transmission of the action potential sped up?
What does this consist of?
What does it form?
What are the cells responsible for this in the CNS and PNS?
What are the differences between these cells? What exists between these structures?
What is found here?
What does this allow?
What do action potentials do here?
- Neuronal transmission of the action potential is sped up up to 10x by myelin sheathes covering segments of the axon
- This myelin sheathes consist of many layers of myelin, and form internodes along the axon
- The myelinating cells of the CNS are oligodendrocytes, which can produce myelin for many internodes along the axon
- The myelinating cells of the PNS are Schwann cells, which can produce myelin for 1 internode along the axon
- In between these insulated internodes along the axon, there are nodes of Ranvier
- There are voltage gated Na+ channels on the nodes of Ranvier, which keeps the signal moving down the axon
- Action potentials jump at high speed from one node of Ranvier to the next
- This is known as saltatory (to leap) conduction
What can demyelination diseases affect?
What does this lead to?
What is the most common demyelination disease of the CNS?
What type of disease is this?
- Demyelination diseases can affect peripheral or central axons
- This leads to impaired conduction of action potentials down the axon
- The most common demyelination disease in the CNS is Multiple Sclerosis (MS)
- This is an autoimmune disease affecting oligodendrocytes
