✅Neurotransmission Flashcards
Describe axon diameter and propagation speed
• Ion movement is related to cytoplasm concentration • Axon diameter affects action potential speed
• The larger the diameter, the lower the resistance
Describe information
• “Information” travels within the nervous system
• As propagated electrical signals (action potentials)
• The most important information (vision, balance, motor commands)
• Is carried by large-diameter, myelinated axons
What are the three groups of axons?
Type A fibers
Type B fibers
Type C fibers
How are the groups of axons classified?
Diameter
• Myelination
• Speed of action potentials
Describe type A fibres ( high speed 140m/sec)
• Large Myelinated axons, with large diameters (4-20μm)
• Carry rapid information to/from CNS
• For example, position, balance, touch, and motor impulses
Describe type B fibres (medium speed 18m/sec)
• Smaller Myelinated axon, with diameters 2-4 μm.
• Carry intermediate signals
• For example, sensory information, peripheral effectors
Describe type C fibres (slow speed 1m/sec)
• Unmyelinated and less than 2 μm in diameter.
• Carry slower information
• For example, involuntary muscle, gland controls
What are neurons describe as the basic functional units of?
The nervous system
How many different types of neurons are there in the human brain?
10,000
Why must neurons communicate with other neurons and tissues?
To produce coordinated behavioural responses
Communication occurs at specialised sites know as what?
Synapses
One neuron may have between several thousand and 150 thousand what?
Synapses acting on the cell
Where are action potentials transmitted from, where to and how?
Presynaptic neuron to postsynaptic neuron across a synapse
What are the two types of synapses?
Electrical
Chemical
Describe electrical synapses
Pre and post synaptic membranes are locked together at gap junctions (connexons)
• Allow ions to pass between cells
• Produce continuous local current and action potential
propagation
• Are found in areas of brain, eye, ciliary ganglia
• Common in invertebrates and embryos
Describe chemical synapses
• Are found in most synapses between neurons and all synapses between neurons and other cells
• Cells not in direct contact
• Action potential may or may not be propagated to postsynaptic cell, depending on:
• Amount of neurotransmitter released
• Sensitivity of postsynaptic cell (near or far from threshold)
What are the two classes of neurotransmitters?
Excitatory
Inhibitory
What do excitatory neurotransmitters cause?
Depolarisation of postsynaptic membranes
What do excitatory neurotransmitters promote?
Action potentials
What do inhibitory neurotransmitters cause?
Hyperpolarisation of postsynaptic membranes
What do inhibitory neurotransmitters suppress?
Action potentials
What does the effect of a neurotransmitter on a postsynaptic membrane depend on?
The receptor not on the neurotransmitter
What are cholinergic synapses?
Synapses the release ACh
What is ACh
Most widespread neurotransmitter
What are the cholinergic synapses?
- All neuromuscular junctions with skeletal muscle fibers
- ManysynapsesinCNS
- Allneuron-to-neuronsynapsesinPNS
- All neuromuscular and neuroglandular junctions of ANS parasympathetic division
How are cholinergic synapses named?
because the neurotransmitter involved is acetylcholine.
What events happen at a cholinergic synapse?
- Action potential arrives, depolarizes synaptic terminal
- Calcium ions enter synaptic terminal, trigger exocytosis of ACh
- ACh binds to receptors, depolarizes postsynaptic membrane
- ACh removed by AChE
• AChE breaks ACh into acetate and choline
What is the sequence of events at a typical cholinergic synapse?
STEP 1
• An arriving action potential depolarizes the synaptic terminal.
STEP 2
• Calcium ions enter the cytoplasm of the synaptic terminal.
• ACh is released through exocytosis of synaptic vesicles.
• ACh release ceases because calcium ions are removed from the cytoplasm of the synaptic terminal.
STEP 3
• ACh diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.
• Chemically gated sodium channels on the postsynaptic membrane open, producing a graded depolarization.
STEP 4
• The depolarization ends as ACh is broken down into acetate and choline by AChE.
• The synaptic terminal reabsorbs choline from the synaptic cleft and uses it to resynthesize ACh.
When does a synaptic delay of 0.2-0.5 msec occur between?
Arrival of action potential at synaptic terminal
An effect on postsynaptic membrane
What does fewer synapses mean?
Faster response
When does synaptic fatigue occur?
Occurs when neurotransmitter cannot recycle fast enough to meet demands of intense stimuli
In synaptic fatigue, when is the synapse inactive till?
Until ACh is replenished
What are some major categories of neurotransmitters?
- Biogenic amines
* Amino acids
• Neuropeptides
• Dissolved gases
How many neurotransmitters are there other than ACh?
At least 50
What are other important neurotransmitters?
• Norepinephrine (NE)
• Dopamine
• Serotonin
• Gamma aminobutyric acid (GABA)
Describe Norepinephrine?
• Released by adrenergic synapses
• Excitatory and depolarizing effect
• Widely distributed in brain and portions of ANS
Describe dopamine
A CNS neurotransmitter
• May be excitatory or inhibitory
• Involved in Parkinson’s disease and cocaine use
Describe serotonin
• A CNS neurotransmitter
• Affects attention and emotional states
Describe Gamma Aminobutyric Acid (GABA)
Inhibitory effect
• Functions in CNS
• Roughly 20 percent of the synapses in the brain release GABA, but its
functions remain incompletely understood.
What does the nervous system rely on?
A complex form of chemical communication.
Each neuron is continuously exposed to a variety of what?
neurotransmitters. Some usually have excitatory effects, while others usually have inhibitory effects.
In all cases, what do the effects of neurons exposed to neurotransmitters depend on?
the nature of the receptor rather than the structure of the neurotransmitter.
How do many drugs affect the nervous system?
stimulating receptors that respond to
neurotransmitters
• Can have complex effects on perception, motor control, and emotional states
What are neuromodulators?
Other chemicals released by synaptic terminals
What do Neuromodulators have similar a function to?
Neurotransmitters
What are the 4 characteristics of Neuromodulators?
- Effects are long term, slow to appear
• Responses involve multiple steps, intermediary compounds
• Affect presynaptic membrane, postsynaptic membrane, or both - Released alone or with a neurotransmitter
What are Neuropeptides?
Neuromodulators that bind to receptors and activate enzymes
Describe Opioids
- Neuromodulators in the CNS
• Bind to the same receptors as opium or morphine - Relieve pain
What are the 4 classes of opioids?
- Endorphins
- Enkephalins
- Endomorphins
- Dynorphins
How Neurotransmitters and Neuromodulators Work
Direct effects on membrane channels
- For example,ACh,glycine,aspartate
• Indirect effects via G proteins
- For example, E, NE, dopamine, histamine, GABA
• Indirect effects via intracellular enzymes
- For example, lipid-soluble gases (NO, CO)
Why do direct effects occur?
Because these neurotransmitters alter ion movement across the membrane, they
are said to have ionotropic effects.
Describe indirect effect - G proteins?
• Work through second messengers
• Enzyme complex that binds GTP
• Link between neurotransmitter (first messenger) and second messenger
• Activate enzyme adenylyl cyclase
• Whichproducessecondmessengercyclic-AMP(cAMP)
Describe indirect effects - intracellular receptors
• Lipid-soluble gases (NO, CO)
• Bind to enzymes in brain cells
What is the relationship between myelin and the propagation speed of action potentials
The presence of myelin greatly increases the propagation speed of action potentials.
Which of the following axons is myelinated: one that propagates action
potentials at 50 meters per second, or one that carries them at 1 meter per second?
Action potentials travel along myelinated axons at much higher speeds (by saltatory propagation); the axon with a propagation speed of 50 meters per second must be the myelinated axon.
Describe the general structure of a synapse.
The major structural components of a synapse, the site where a neuron communicates with another cell, are a presynaptic cell and a postsynaptic cell, whose plasma membranes are separated by a narrow synaptic cleft.
If a synapse involves direct physical contact between cells, it is termed ________; ; if the synapse involves a neurotransmitter, it is termed ______
Electrical
Chemical
What effect would blocking voltage-gated calcium channels at a cholinergic synapse have on synaptic communication?
If the voltage-gated calcium channels at a cholinergic synapse were blocked, Ca2 could not enter the presynaptic terminal and trigger the release of ACh into the synapse, so no communication would take place across the synapse.
One pathway in the central nervous system consists of three neurons, another of five neurons. If the neurons in the two pathways are identical, which pathway will transmit impulses more rapidly?
Because of synaptic delay, the pathway with fewer neurons (in this case, three) will transmit impulses more rapidly.
Differentiate between a neurotransmitter and a neuromodulator.
Both neurotransmitters and neuromodulators are compounds that are released by one neuron and that affect another neuron. A neurotransmitter alters the transmembrane potential of the other neuron, whereas a neuromodulator alters the other neuron’s response to specific neurotransmitters.
Identify the three functional groups into which neurotransmitters and neuromodulators fall.
• Neurotransmitters and neuromodulators are either
(1) compounds that have a direct effect on membrane potential,
(2) compounds that have an indirect effect on membrane potential,
or (3) lipid-soluble gases that exert their effects inside the cell.
