Physiology Quiz 2 (1/2) Flashcards
Types of synapses
Electrical
Chemical
Synapse
Site at which an impulse is transmitted from one cell to another
Ephaptic communication
Two excitable cells communicating by direct passage of electrical current between them (aka electrical synapses)
Chemical synapses
A spike at the presynaptic membrane causes a release of neurotransmitter which attaches to receptors on the post synaptic membrane and causes depolarization
Characteristics of chemical synapses
Synaptic delay of .5-1 msec
Transmission is unidirectional
Pharmacological agents can interfere.
Characteristics of electrical synapses
No synaptic delay
Transmission is bidirectional
Pharmacological interference will not occur
Neuromuscular junction
Chemical synapse between a motor nerve and muscle cell(s)
Benzodiazepines*
Eg. Valium
Anti-anxiety for Pre-surgery
Blocks synaptic transmission*
Overuse can lead to too much transmission along nerve*
What happens when the spike arrives at the alpha motor nerve ending?
- Sudden large increase in calcium conductance at the presynaptic ending
- calcium diffuses inward
- attaches to enzymes, activating them - Activated enzymes cause the vesicles containing neurotransmitter to release into cleft
- Ach is neurotransmitter in neuromuscular junctions - Ach diffuses across cleft
- Ach attaches to cholinergic receptor on post synaptic membrane
- Causes end plate potential (EPP)
- Potentials will sum and when threshold is reached, the cell fires an action potential
Synaptic delay due to…
Time it takes for chemicals to get across synapse
Neurotransmitter at neuromuscular junction
Acetylcholine (Ach)
Two types of cholinergic receptors
Nicotinic
Muscarinic
Nicotinic receptors
Found on muscle, attach to nicotine or Ach
Muscarinic receptors
Found on organ tissue (ie. cardiac tissue)
Atropine
Used to block poisons
Used in surgical settings to interfere with fluid production in lungs
Nicotinic cholinergic type receptors
Cholinergic receptors on muscle
Characteristics of nicotinic cholinergic receptors
Can attach to Ach
Can attach to nicotine
End plate potentials (EPP)
Depolarizations of the postsynaptic membrane surface that are conduced electronically.
- large numbers of EPPs will occur when the nerve fires a spike
- Hesse must sum to cause a spike in the muscle membrane
What happens every time an alpha motor nerve cell fires?
An excess of Ach is released
Is it important to terminate the action of Ach at the neuromuscular junction?
Yes- for normal activity, muscles need to contract and relax
Eg. Lungs-muscles need to both contact and relax in order for you to breathe
How termination of Ach is achieved
Broken down by acetylcholinesterase or by less specific cholinesterases
Where is Ach-ase located?
On surface of postsynaptic membrane
Where are cholinesterases located?
In extra cellular fluid in synaptic cleft
What happens to Ach when it is attached to a receptor and the EPP is initiated?
It detaches
Three possibilities for Ach molecule after it detaches from postsynaptic membrane
- Attaches to another receptor
- Attaches to acetylcholinesterase on the membrane and is destroyed
- Attaches to a cholinesterase in the synaptic cleft and is destroyed
Is an Ach molecule more likely to attach to another receptor or to an esterase as it detaches?
More likely to attach to another receptor (1500 receptors/square micron) than an esterase (2600 esterases/square micron) but as it attaches and detaches, it will eventually run into an esterase
What is Ach broken down into?
Acetyl CoA + choline
What I acetylcholinesterase CoA used for?
Metabolism
What is choline used for?
Actively transported back into nerve terminal and utilized again to manufacture Ach
When Ach is manufactured in the cell body, how does it travel to the nerve terminal?
Antegrade axoplasmic transport by microtubules
Succinylcholine
Attaches to nicotinic cholinergic receptors and initially activates then blocks (does not detach from site)
Used for: Relaxing muscles during surgery or when using a breathing machine (ventilator). It is also used to induce anesthesia or when a tube must be inserted in the windpipe.
Tubocurarine
This attaches to nicotinic cholinergic receptors and blocks them. (No muscle rigidity, just relaxes)
Used for: a diagnosis agent for myasthenia gravis, and also to facilitate the intubation after induction of anesthesia in surgical procedure
Alpha-neurotoxins
Block nicotinic cholinergic
preventing synaptic acetylcholine (ACh) from opening the ion channel.
Loss of inner action of motor end plate
Can happen in trauma where nerve is cut
Can also happen when the nerve has undergone mechanical or toxic insult sufficient to stop the motor nerve from functioning
What happens when the motor nerve input to the motor end plate is lost?
Skeletal muscle develops high sensitivity to Ach
- thought to be due to a spread of receptors beyond the motor end plate
- cause unknown
Fasciculations
Low level uncoordinated contractions “quivering” of skeletal muscle
mEPPs
(Miniature end plate potentials) small depolarizations that occur at the end plates without any activation of the alpha motor neuron
- thought due to spontaneous release of Ach
- some evidence that they are the tonic influence that nerves exert upon muscle to maintain it in a healthy state.
- another idea-they are releasing a chemical compound (probably a combo of both)
Botulism
Caused by ingesting food contaminated with spores of clostridium botulinum which attacks and destroys cholinergic nerve terminals creating acute denervation of motor end plates of muscle.
- respiration stops
- used in Botox injections
Myasthenia gravis
Autoimmune process which destroys nicotinic cholinergic receptors
- most common in females
- facial muscles fatigue when eating is usually first complaint
- long process, patient eventually dies
Treatment for myasthenia gravis
Give anti-cholinesterase drugs
-increases concentration of Ach for more activation of receptors
Osteophytes
Inappropriate growth of bone to stabilize joints
How does compression affect axoplasmic transport?
Slows it down
