06-09-23 – Neurophysiology Revisions Flashcards
Learning outcomes
- Revise and understand how nerves work in both the central and peripheral nervous systems
- Understand the basics of nerve conduction studies, detecting common nerve and neuromuscular junction problems
Label the parts of a neuron (in picture).
Describe the 4 steps in the transmission of information along a neuron.
- Parts of a neuron (in picture).
- 4 steps in the transmission of information along a neuron:
1) Excitable cells
2) Information arrives at the cell body via dendrites where it is assimilated and processed
3) Processed information is then digitised into APs which are transmitted along the axon
4) At the end of the axon the information is passed to the target (muscle or neuron) at boutons
How does diameter and surface area of an axon affect movement of charge along the axon?
What are the characteristics of an unmyelinated axon a balance between?
- The larger the diameter of the axon, the lower the resistance is.
- Passive movement of charge along the axon is easier with less resistance
- Larger axons have faster passive charge movement
- The more surface area there is on an axon, the higher its capacity to store charge across its membrane
- The higher the capacitance the harder it is for charge to cross over the membrane, i.e. to overcome the repellent force of charge accumulated there.
- The characteristics of an unmyelinated axon are a compromise between these two factors
Describe the 4 actions of the Voltage dependent sodium channels during the stages of an action potential
- 4 actions of the Voltage dependent sodium channels during the stages of an action potential:
1) At resting potential Na+ channels are closed - the activation gate is closed
2) Depolarization opens the activation gate and Na+ flows into the cell along its electrochemical gradient
3) A delayed component of voltage dependent activation is the blocking of the channel by the inactivation gate (after about 0.5ms)
4) Repolarization of the cell re-sets the two gates to their equilibrium positions.
How does information transmit along axons?
What is the voltage for resting potential?
What is the threshold voltage for action potentials?
Describe 3 features of action.
Describe the voltage graph for an action potential (in picture)
- Information transmits along axons via action potentials
- The resting membrane potential is -70Mv
- The threshold potential for action potentials is -55mV
- 3 features of action potentials:
1) Self-propagates
2) Travels in one direction
3) No volume control – Just ON and OFF - Voltage graph for an action potential (in picture)
Describe the 6 different stages of an action potential (in picture)
- 6 different stages of an action potential (in picture)
What is myelin?
What is it used for?
What cells provide myelination in the CNS and PNS?
How many axons do these cells provide myelination for?
- Myelin is a fatty sheath
- It is used to insulate axons in order to increase the diameter of axons, therefore increasing the speed of transmission
- Oligodendrocytes provide myelination in the CNS (Multiple axons each)
- Schwann cells provide myelination in the PNS (One axon each)
What are nodes of Ranvier?
What is their purpose?
What is Saltatory conduction?
What is the benefit of saltatory conduction?
- Nodes of Ranvier are the periodic gaps between myelin sheaths along the axon
- They exchange ions in order to regenerate the action potential (action potentials occur at the nodes of Ranvier)
- Saltatory conduction is the propagation of action potentials along myelinated axons
- By saltatory conduction, currents can travel farther and faster with less energy wasted
How does diameter affect conduction in unmyelinated nerve fibres?
How does myelination affect speed of conduction?
What is slowing of conduction consistent with?
Why are most fibres in the nervous system myelinated?
How fast is normal conduction velocity?
How fast is conduction velocity in alpha motor neurons?
- For unmyelinated nerve fibres, larger diameter = faster conduction
- Myelination is responsible for enhancing the speed of conduction, so any slowing of the speed of conduction is consistent with demyelination
- To keep the nervous system compact, most fibres are myelinated, so small diameter myelinated fibres can conduct faster than large unmyelinated ones
- Normal conduction velocity = 50-60 meters/second
- The fastest alpha motor neurons can conduct at 120 m/s
How do neurons communicate between each other?
What is the synaptic cleft?
How large is he synaptic cleft?
What is the bouton of a presynaptic cell?
How are neurotransmitters released from the presynaptic cell membrane?
Where do they diffuse to?
How are neurotransmitters inactivated?
- Neurons communicate via synapse and neurotransmitters
- The synaptic cleft is the gap between the pre and postsynaptic cell
- The synaptic cleft is approximately 20 nanometres
- The bouton of a presynaptic cell is a swelling
- Neurotransmitters are released from the presynaptic cell membrane by exocytosis
- Neurotransmitters diffuse to the postsynaptic membrane where they bind to receptors
- Neurotransmitters are then inactivated by diffusion, re-uptake or enzymal inactivation
What are the 2 ways receptors are classified?
- 2 ways receptors are classified:
1) How they are activated:
* Ionotropic (directly gate ion flow) or
* Metabotropic (indirectly gate ion flow or activate other pathways)
2) Positive/negative effects:
* Excitatory or inhibitory depending on the ions they let into/out of the cell
Describe the 6 steps involved from neurotransmitter release to formation of an action potential (in picture)
What does each receptor type possess?
What are generator potentials used for?
How does strength of external signals affect AP frequency in axons?
- Each receptor type (e.g touch) has a modality and feeds information about modality to the CNS
- Generator potentials are used by receptors to code the duration and magnitude of external signals
- This means stronger external signals result in a higher frequency of APs in the axon
What is a neuromuscular junction? What happens when an AP reaches the neuron terminus?
What is released?
What does this stimulate?
How does botulinum toxin affect this process?
- The neuromuscular junction is a chemical synapse between a motor neuron and a muscle fibre
- When an action potential reaches the neuron terminus, this activates voltage-gated calcium channels
- Acetylcholine is released and attaches to nicotinic- acetylcholine receptors on the postsynaptic surface
- This process stimulates muscle contraction
- Botulinum toxin works by stopping acetylcholine from being released from the end motor neuron, hence stopping muscle contraction
What type of condition is Myasthenia Gravis (MG)?
What is it caused by?
What does MG present with?
What does Mg usually begin with?
What is MG treated with?
How often does Myasthenic crisis occur?
What can this lead to?
- Myasthenia Gravis (MG) is an autoimmune disease of the neuromuscular junction
- It is caused by antibodies blocking / destroying post-synaptic acetylcholine receptors
- MG presents with painless weakness with fatiguability (hallmark of MG)
- MG usually begins with oculomotor weakness in one or both eyes
- Treatment usually acetylcholinesterase inhibitors and immunosuppressants
- Myasthenic crisis can occur in about 20% of patients, which leads to acute respiratory failure
Describe the 3 different portions of spindle fibres (in picture).
What does the sensory afferent to the spinal cord provide?
What is the role of the gamma motor neuron efferent from the spinal cord?
- 3 different portions of spindle fibres (in picture)
- The sensory afferent to the spinal cord provides information on stretch
- The gamma motor neuron efferent from the spinal cord regulates the degree of contraction in a muscle
Describe the mechanisms involved in muscle stretch reflexes
- The mechanism involved in muscle stretch reflexes are feedback loops
- There are positive signals that result in the contraction of the agonistic muscle to protect overstretching (e.g biceps)
- There are negative signals to promote relaxation in the antagonistic muscles (e.g triceps)
Summary of muscle spindle involvement in muscle contraction
Nerve conduction study: motor study.
What do axons and myelin give rise to?
- Nerve conduction study: motor study
- The axons give rise to the amplitude of the action potential (CMAP – Compound Motor Action Potential)
- Myelin gives rise to the conduction velocity (MNCV – Motor Nerve Conduction Velocity)
- Remember speed = distance/ time
Nerve conduction study: sensory study.
What do axons and myelin give rise to?
How can the superficial radial nerve be stimulated?
How can the action potentials be recorded?
- Nerve conduction study: sensory study
- Again, the axon gives rise to the amplitude of the action potential (SNAP – sensory nerve action potential)
- Myelin gives rise to conduction velocity (SNCV – sensory nerve conduction velocity)
- The superficial radial nerve can be stimulated using electrodes placed around the thumb
- The radial nerve sensory action potential can be recorded using surface electrodes at the wrist
What are 3 abnormal patterns that can be seen on nerve conduction studies?
- 3 abnormal patterns that can be seen on nerve conduction studies:
1) Axon loss
* When axons are lost, there are fewer excitable axons, and therefore fewer muscle cells are excited, resulting in a lower compound muscle action potation (CMAP)
2) Diffuse demyelination
* For diffusely disrupted myelin, distal latency is prolonged, and the conduction velocity is slow but the CMAP retains normal amplitude
3) Focal demyelination
* Signals are slowed, with some not getting through at all
What type of condition is Charcot-Marie Tooth Disease?
What type of neuropathy does it cause?
When does it present?
What are 4 symptoms and signs of Charcot-Marie Tooth Disease?
- Charcot-Marie Tooth Disease is a hereditary demyelinating polyneuropathy
- It causes cause sensory and motor peripheral neuropathy
- It typically presents in childhood or early adulthood
- 4 Symptoms and signs of Charcot-Marie Tooth Disease:
1) Difficulty walking
2) Foot drop
3) High arched feet and hammertoe
4) If motor power loss, then legs often resemble an inverted champagne bottle
What type of condition is Guillain-Barré syndrome?
How fatal is this condition?
How do patients usually present with Guillain-Barré syndrome?
What can occur if symptoms are left untreated?
How is it treated?
How long can recovery take?
- Guillain-Barré syndrome is an autoimmune, rapidly progressive demyelinating polyneuropathy
- This condition can be life-threatening
- Patients with Guillain-Barré syndrome usually present with progressive peripheral neuropathy
- If left symptoms can deteriorate until the respiratory muscles are paralysed
- Treatment consists of IVIG (Intravenous immunoglobulin), analgesia, and if needed ventilatory support.
- Guillain-Barré syndrome can have long recovery with need for rehabilitation
