Week 7 Neurology Flashcards
What does the speed of propagation along a membrane depend on?
fibre diameter and myelination
The larger the fibre diameter …
the fast the action potential propagates, since larger diameter = less resistance to local current
Why does myelination increase propagation speeds?
There is less “leakage” of charge across the myelin meaning a local current can spread farther along an axon.
Also, the concentration of Na+ channels in the myelinated region of the axon is low. Therefore, action potentials only occur at the nodes of Ranvier, where myelin coating is interrupted and the concentration of voltage gated Na+ channels is higher.
What is the name for when action potentials jump between nodes of Ranvier?
saltatory conduction
What is the most common disease of the nervous system among young adults?
Multiple sclerosis
What is MS?
Degeneration of myelin and development of scar tissue which in turn disrupts and eventually blocks neurotransmission along myelinated axons
Symptoms of MS
Uncontrolled eye movements/double vision Slurred speech Partial/complete paralysis Tremor Loss of co-ordination Weakness Sensory numbness, prickling, pain
Excitatory vs inhibitory synapse
The membrane potential of a post synaptic neurone is brought closer to threshold (Depolarised) at an excitatory synapse.
The membrane potential of a post synaptic neurone is either driven further from the threshold (hyper polarised) or stabilised at its resting potential at an inhibitory synapse.
2 types of synapse
Electrical
Chemical
Electrical synapse
The plasma membranes of the presynaptic and post synaptic cells are joined by GAP JUNCTIONS which allow local currents resulting in arriving APs to flow directly across the junction through connecting channels
Communication between nerve cells w electrical synapse SPEED
rapid
Chemical synapse
The plasma membranes of pre and post synaptic neurones are joined by the SYNAPTIC CLEFT
The synaptic cleft prevents direct propagation of the current from the presynaptic neurone. Instead, signals are transmitted across be means of NEUROTRANSMITTERS
Communication between nerve cells w electrical synapse SPEED
rapid
What do electrical synapses allow for?
synchronised transmission
Where are electrical synapses found
Brainstem neurones e.g. breathing and hypothalamus (hormone secretion)
What cells cover chemical synapses?
astrocytes (glial cells)
essential for the reuptake of excess neurotransmitter
Neurotransmitter release
Calcium ion channels open when an AP reaches presynaptic neurone.
Ca2+ ions cause vesicles containing neurotransmitters to move to release sites and fuse with presynaptic cell membrane and discharge their contents. Neurotransmitter diffuses across synaptic clef t and attaches to receptor sites.
5 processes of synaptic transmission
- Manufacture (intracellular biochemical process)
- Storage (vesicles)
- Release - AP
- Interact with post-synaptic receptors
- Inactivation
Where is ACh used?
In the brain and NMJs
2 main acetylcholine receptors
muscarinic
nicotinic
What happens once acetylcholine has been bound to the post-synaptic receptor?
acetylcholine esterase breaks it down into choline and acetyl
The choline can then be reabsorbed by the pre-synaptic neurone to be used to make more acetylcholine.
What can post synaptic neurone neurotransmitter receptors take the form of?
transmitter-gated ion channels
When NTs bind to the receptors/channels, it results in DEPOLARISATION or HYPERPOLARISATION depending on the channel type …
Depolarisation will occur in the excitatory channels and cause an excitatory post-synaptic potential EPSP where many Na+ leave and few K+ enter
Hyperpolarisation will occur in inhibitory channels (inhibitory post-synaptic potential IPSP) many K+ leave OR many Cl- enter
The combined effects of many excitatory synapses (since one event by itself is not enough to reach threshold) can be achieved by 2 means …
- TEMPORAL summation : input signals arrive from