Synapses Flashcards
types of synpases
- electrical
- pre and postsynaptic membranes are bound together by channels
- APs always propagated
- rare : in some vestibular nuclei, eye - chemical
- pre and postsynaptic cells are not bound and require the use of neurotransmitters to communicate
- APs not always propagated (why not??)
- very common
synapse
site of communication between a neuron (pre-synaptic cell )and other cell (post-synaptic cell)
Types of Neurotransmitter
excitatory neurotransmitters cause depolarisation,
inhibitory neurotransmitters cause hyperpolarisation
but for the Structural classes:
acetylcholine, amines (e.g. Dopamine, norepinephrine, epinephrine, serotonin), amino acids (e.g. GABA), neuropeptides (e.g. endorphines), purines, gases, lipids
Neurotransmitter storage and synthesis
Storage
* neurotransmitters are stored in the pre-synaptic terminals
* these terminals also contain mitochondria to fuel metabolism and transport
Synthesis
Synthesis depends on the nature of the neurotransmitter
Small neurotransmitters (e.g. ACh, amines) are synthesised and packaged in the synaptic terminals
Peptide neurotransmitters are synthesised and package in the cell body and transported to synaptic terminals
Describe the steps involved in neurotransmitter release from the axon terminal into the synaptic cleft.
- action potentail depolarises the axon terminal
- depolarisation opens voltage-gated Ca2+ channels and Ca2+ enters the cell
- calcium entry triggers exoctyosis of synaptic vesicle contents
- neurotransmitter diffuses across the synpatic cleft and binds with receptors on the post synpatic cleft
- neurotransmitter binding initiates a response in the postsynpatic cell
Relationship between Frequency and Stimulus Strength/Duration:
The frequency of action potentials is directly related to the strength and duration of a stimulus.
A stronger or more prolonged stimulus is associated with a higher frequency of action potentials.
weak stimulus = fire action potentials at a lower frequency, strong or prolonged stimulus = a higher frequency of action potentials.
Neurotransmitter Receptors types
Two main receptor types:
* Ligand-gated ion channels (ionotropic): fast response,
ion channels open and ions immediately flood into
(or out of) cell
* G protein-coupled receptors (metabotropic):
slow response, ion channel opening relies on an
intermediary ‘second messenger’
neurotransmitter receptors and their mechanisms of post-synaptic signalling
- Ionotropic Receptors:
ligand-gated ion channels.
When a neurotransmitter binds to the ionotropic receptor, the receptor undergoes a conformational change, allowing ions to flow through the channel.
The flow of ions directly affects the membrane potential of the postsynaptic neuron, leading to rapid changes in electrical signaling. - Metabotropic Receptors + ion channel:
GPCR : g protein coupled receptors
When a neurotransmitter binds to the metabotropic receptor, the receptor activates G protein to activate ion channel and let ions in cell - Metabotropic Receptors + signalling cascade:
GPCR : g protein coupled receptors
G proteins activated to create signalling cascade, leading to the modulation of intracellular enzymes or ion channels.
The signaling cascades initiated by metabotropic receptors are slower and can have more prolonged effects compared to ionotropic receptors.
Describe the different ways in which neurotransmission is terminated
- Reuptake
* neurotransmitter can be returned to the axon terminals for resuse or transported into glial cells
* neurotransmitters can be repacakged into vescicles or broken down by enzymes - Enzyme degradation
* enzymes inactivate neurotransmitters
* break down into smaller inactive molecules to be recycled or cleared away - Diffusion
* diffuse away from the synpatic cleft
* move away from synpase and into surrounding ECF (extracellular fluid)
Outline the sequence of events that occur during neurotransmission at the neuromuscular junction at the presynapse
Consists of neuromuscular junction (NMJ) and motor end plate
NMJ : axon terminals, schwann cell sheaths
motor end plate : muscle membrane and ACh receptors
- arriving AP depokarises synaptic knob
- Ca2+ ions enter the cytoplasm and ACh is released through exocytosis of vesicles
- ACh binds to Na channel receptors on post membrane and poduces graded depolarisation
- depolarsation ends as ACh is broken down = ACH –> acetate + choline by AChE
- synpatic knob reabsorbs choline from synpatic cleft to synthesis new ACh molecules
Outline the sequence of events that occur during neurotransmission at the neuromuscular junction at the postsynapse
Consists of neuromuscular junction (NMJ) and motor end plate
NMJ : axon terminals, schwann cell sheaths
motor end plate : muscle membrane and ACh receptors
- Na+ entry and efflux of K+ through ACh receptor/channel initiates muscle action potential
- Action potential in T-tuble causes a change in the conformation of the DHP receptor
- DHP receptor ACTIVATES ryanodine receptors (RyR) on SR
- RyR activated and calcium channels open for and Ca2+ enters cytoplasm
- Ca2+ binds to troponin, allowing actin-myosin binding
- myosin heads execute power stroke
- actin filament slides forward toward centromere
common neurotransmitter : Noradrenaline
Location: Found in the brain and the Autonomic Nervous System (ANS).
Receptor Type: Acts through G Protein-Coupled Receptors (GPCRs).
Function: Typically has excitatory effects on target cells.
common neurotransmitter : Dopamine
Location: Primarily found in the Central Nervous System (CNS).
Receptor Type: Acts through GPCRs.
Function: Can have both excitatory and inhibitory effects depending on the specific receptor subtype and the location in the brain.
common neurotransmitter : Serotonin
Location: Mainly in the CNS.
Receptor Types:
Acts through GPCRs.
Some serotonin receptors function as ligand-gated ion channels.
Function: Exhibits a variety of effects, including both excitatory and inhibitory actions, depending on the specific receptor subtype.
common neurotransmitter : Glutamate
Location: Acts as the primary excitatory neurotransmitter in the Central Nervous System (CNS).
Receptor Types: Acts on ionotropic receptors, including NMDA and AMPA receptors, which are ligand-gated ion channels.
Function: Excites target cells by inducing the influx of ions such as sodium and calcium.
