The Synapse and Neurotransmitters Flashcards
What is a synapse?
The synapse is a junction where 2 neurons come close enough to send chemical signals from one cell to the other.
- The neurons are not connected but separated by a space → synaptic cleft
What is the pre-synaptic neuron? (2)
- Pre-synaptic neuron transmits the signal whereas post-synaptic neuron receives the signal.
- Pre-synaptic neurons have chemical messengers called neurotransmitters enclosed in vesicles.
What happens when pre-synaptic neurons are excited?
When pre-synaptic neurons are excited by an action potential, this causes the vesicles to fuse with the pre-synaptic cell membrane and release neurotransmitters into the synaptic cleft via exocytosis.
Post-synaptic neurons have neurotransmitter receptors, to which these neurotransmitters bind, causing an action in the post-synaptic neuron.
After this, neurotransmitters are removed from the synaptic cleft via: (3)
- Diffusion
- Reuptake: the neurotransmitters return back to the pre-synaptic neuron and are retaken into the vesicles for reuse
- Enzymatic breakdown: neurotransmitters are broken down and then the broken fragments return to the pre-synaptic neuron to stimulate production of neurotransmitters
Electrical activity (the action potential) (via the activation of voltage-gated calcium channels) in the _____ neuron is converted into the release of a chemical called a _______ that binds to receptors in the _____ cell and initiates an AP or secondary pathway.
presynaptic
neurotransmitter
postsynaptic
What does the neurotransmitter initiate?
The neurotransmitter may initiate an electrical response or a secondary messenger pathway that may either excite or inhibit the postsynaptic neuron. In short, a chemical neurotransmitter is released by a presynaptic neuron into the synaptic cleft.
Neurotransmitters and the synapse: (5)
- Neurotransmitters are stored in synaptic vesicles at the ends of axons
- AP reaches synaptic terminal → depolarises the terminal pre-synaptic
membrane opening voltage-gated Ca2+ channels - Calcium triggers exocytosis of neurotransmitters (held in membrane-bound vesicles) into the synapse
- Neurotransmitter diffuses across synaptic cleft → effect either direct or indirect
- Many receptors for neurotransmitters are ligand-gated channels
Synaptic Transmission: (3)
- Action potential arrives; triggers entry of Ca2+ into axon bulb of
presynaptic neuron (voltage-gated Ca2+ channels) - In response to Ca2+, synaptic vesicles fuse with the membrane, release neurotransmitters (synthesized by presynaptic neurons)
- Neurotransmitters reach the postsynaptic membrane by diffusion, chemically/ligand-gated ion channels open when a neurotransmitter binds, causing a change in postsynaptic cell potential (graded potentials)
Ligand-gated ion channels open when bound with the chemical stimulus (neurotransmitter) allowing for: (2)
- Influx in Na+ and K+ ions
This could lead to the formation of graded-potentials
Graded potentials can either be _____ (depolarization) or (hyperpolarization) ______.
excitatory
inhibitory
- Changes in membrane potential that ____ the probability of an action potential are excitatory postsynaptic potentials (EPSPs).
- Changes in membrane potential that _____ the probability of an action potential are inhibitory postsynaptic potentials (IPSPs).
increase
decrease
EPSPs and IPSPs are summed at the___ ____ and may result in an action potential.
axon hillock
Synaptic integration: EPSPs and IPSPs are graded potentials that vary in magnitude but may act together to produce an action potential.
Summation can occur in two ways:
- Temporal summation – repeated EPSPs occur before previous EPSPs decay, summation brings neuron to firing level
- Spatial summation – closely spaced synaptic terminals release neurotransmitters simultaneously on same postsynaptic neuron.
- When action potentials arrive from the same axon close together (temporal) in time or from different axons that have synapses close to one another (spatial), the postsynaptic potentials are _____.
- If excitatory postsynaptic potentials depolarize the axon hillock past a point called the _____, enough ___ channels open to trigger an action potential.
additive
threshold
Na+
What happens to neurotransmitters after transmission of signal? (4)
Removed from cleft
1. Taken back up by presynaptic neurons and repackaged into membrane-bound vesicles
2. Destroyed by enzymes in the synaptic cleft
3. Diffuse away from the synaptic cleft into general circulation where later destroyed
4. Glia take up neurotransmitters and actively use as metabolic fuel
What are neurotransmitters?
Endogenous chemicals that enable neurotransmission i.e., a type of chemical messenger which transmits signals across a chemical synapse, such as a neuromuscular junction, from one neuron to another “target” neuron, muscle cell, or gland cell.
What do neurotransmitters act as?
Neurotransmitters act as triggers for ligand-gated ion channels on postsynaptic neurons alter the flux of ions (mainly Na & K) across the membrane lead to graded potentials/action potentials
Some neurotransmitters can be excitatory or inhibitory depending on _____ type in the postsynaptic membrane.
receptor
Response of postsynaptic cells to neurotransmitters depends on: (3)
- The type of neurotransmitter
- Its concentration in the synaptic cleft
- The types of receptors and chemically sensitive ion channels in the postsynaptic membrane
What is Acetylcholine (ACh)? (3)
- 1st neurotransmitter to be identified and one of most common – muscle simulation, memory, and learning
- Excitatory or inhibitory in CNS depending on the type of receptor
- ACh removed by the enzyme acetylcholinesterase, converts ACh into choline and acetate
What is the function of Acetylcholine (ACh)?
→ Involved in thought, learning and memory. Activates muscle action in the body. Also associated with attention and awakening.
Problems with ACh: (2)
- Alzheimer’s Disease – shortage of ACh, mental impairment/memory
- Botulinum toxin or Botox – neurotoxic protein, produced by the bacterium Clostridium botulinum.
Prevents the vesicle where the ACh is stored from binding to the membrane where the neurotransmitter can be released, prevents muscle contraction for 3-4 months → Block ACh
NB: Some neurons do, however, communicate via _______ synapses through the use of gap junctions, which allow specific ions to pass directly from one cell to another.
electrical
Biogenic Amines
1. Derived from ____ ______.
Amino acids
What are Catecholamines? (3)
– 1 group produced by amino acid tyrosine
- Epinephrine (Adrenaline) & Norepinephrine – excitatory
neurotransmitter in PNS, but also function as hormones
- Dopamine - motivation, reward, addiction
What are Indoleamines? (2)
– derived from amino acid tryptophan
- Serotonin - mood, sleep, appetite, and sexuality
What do biogenic amines affect?
Affect mood, imbalances can result in several mental disorders
What is Norepinephrine/Noradrenalin? (4)
- Neurotransmitter and Hormone
- Excitatory or inhibitory effects in CNS and autonomic division of PNS
- Mood affected by level in brain, since it affects the brain regions where attention and impulsivity are controlled
- In sympathetic nervous system it increases heart rate, metabolises fat and increases muscle readiness
What does concentration affect?
→ affects attention and responding actions in the
brain. Contracts blood vessels, increasing blood flow
What is dopamine? (4)
- Inside the brain, dopamine plays important roles in executive functions, motor control, motivation, arousal, reinforcement, and reward
- Can be both inhibitory/excitatory
- NB in motor function and mood
- PLEASURE → Feelings of pleasure, addiction, movement, and motivation. People repeat behaviours that lead to dopamine release.
ADHD: Attention Deficit Hyperactivity Disorder
Some of the most effective therapeutic agents for ADHD are ______such as methylphenidate (Ritalin, Concerta) and amphetamine (Adderall, Dexedrine), drugs that increase both _____ and _______ levels in the brain.
psychostimulants
dopamine
norepinephrine
Dopamine Disorders: (4)
- Parkinson’s Disease associated with reduced levels and death of dopamine neurons in brain
- Schizophrenia associated with abnormal levels
- Amphetamines & cocaine increase dopamine levels, hence excessive stimulation of the postsynaptic neurons
- LSD and mescaline (psychoactive drugs) – produce hallucinatory effects by binding to brain receptors for serotonin and dopamine
What is serotonin? (4)
- 90% used to regulate intestinal movements
- Remainder in serotonergic neurons of CNS → regulates mood, appetite, and sleep
- Major target for antidepressant drugs
- Many antidepressants are Selective Serotonin Reuptake Inhibitors (SSRIs), increase [serotonin] in synaptic cleft
Example of antidepressants.
E.g., Prozac (Fluoxetine), Zoloft, Fontex → enhance effect of serotonin by inhibiting uptake after release
What is mood?
→ Contributes to well-being and happiness. Helps sleep cycle and digestive system regulation. Affected by exercise and light exposure.
Only 4 amino acids are known to function as neurotransmitters:
gamma aminobutyric acid (GABA)
glycine
glutamate
aspartate
What does GABA do? (2)
- GABA – the chief inhibitory neurotransmitter in the mammalian CNS; increases IPSPs by increasing the permeability of the postsynaptic membrane to Cl-
- Valium and diazepam reduce anxiety by binding to a site on a GABA receptor.
- CALMING → Calming firing nerves in the central nervous system. High levels improve focus, low levels cause ____. Also contributes to _____ control and vision.
anxiety
motor
What are neuropeptides?
– short chains of amino acids
Neuropeptide substance P – excitatory neurotransmitter _____ ______.
mediates pain
What are endorphins?
Endorphins – natural analgesics that decrease pain perception. Additionally, they decrease urine output by stimulating ADH secretion, depress respiration, produce euphoria.
- Heroin – binds to same receptors as _____
- ______- Released during exercise, excitement, and sex, producing well-being and euphoria, reducing pain.
endorphins
EUPHORIA
